Master’s dissertation submitted in partial fulfilment of the requirements for the joint degree of
International Master of Science
in Environmental Technology and Engineering
an Erasmus+: Erasmus Mundus Master Course jointly organized by
Ghent University, Belgium
University of Chemistry and Technology, Prague, Czech Republic
UNESCO-IHE Institute for Water Education, Delft, the Netherlands
Academic year 2015 – 2016
Treatment of acid mine drainage by forward
osmosis: Rejection of selected metals
Host University:
University of Chemistry and Technology, Prague, Czech Republic
Barbara Vital
Promoter: Associate Prof. Jan Bartacek
Co-promoter: Prof. David Jeison
This thesis was elaborated at University of Chemistry and Technology, Prague, Czech Republic and
defended at University of Chemistry and Technology, Prague, Czech Republic within the framework
of the European Erasmus Mundus Programme “Erasmus Mundus International Master of Science in
Environmental Technology and Engineering " (Course N° 2011-0172)
© 2015 Prague, Barbara Vital, Ghent University, all rights
reserved.
ii
DECLARATION
This thesis was written in the department of Water Technology and Environmental Engineering
of the University of Chemistry and Technology, Prague from February to September of 2016
I hereby declare that this thesis is my own work. Where other sources of information have
been used, they have been acknowledged and referenced in the list of used literature and
other sources.
I have been informed that the rights and obligation implied by Act No. 121/2000 Coll. On
Copyright, Rights Related to Copyright and on the Amendment of Certain Laws (Copyright
Act) apply to my work. In particular I am aware of the fact that the University of Chemistry and
Technology, Prague has a right to sign a licence agreement for use of these work as
schoolwork under §60 paragraph 1of the Copyright Act. I have also been informed that in the
case this work will be used by myself or that a license will be granted for its usage by another
entity, the University of Chemistry and Technology, Prague is entitled to require from me a
reasonable contribution to cover the cost incurred in the creation of the work, according to the
circumstances up to the full amount.
I agree to the publication of my work in accordance with the Act No. 111/1998 Coll. On Higher
Education and the amendment of the related laws (Higher Education Act)
In Prague on the 18 st of August of 2016
ii
ACKNOWLEDGMENTS
I would like to thank my supervisors Dr. David Jeison and Dr. Juan Carlos Ortega Bravo from
Universidad de La Frontera (UFRO), Temuco, Chile first for receiving me as a part of their
own department and second for the support, guidance and assistance in all the questions
related to the thesis work. I am very grateful for their help and for putting me in the right
direction when I was lost. I would like to thank also the promoter of this thesis Dr. Jan Bartacek,
for his assistance and support, which was of crucial importance to conclude this project.
I would like to thank all the team from BIOREN - UFRO (Scientific and Technological Bio
Resource Nucleus) who motivated and helped me when I needed a “light”: Noelia Sepulveda,
Carolina Beltran, Javier Pavez, Francisco Cabrera, Alvaro Torres, Carla Duarte, Juan Diaz
and Karina Godoy.
I would like to acknowledge the financial support offered by European Commission through
the Erasmus + Program of International Master of Science in Environmental Technology and
Engineering, making it possible to develop my studies in Europe.
Finally, I must express my very profound gratitude to my grandmother, Luiza Vital, who lead
my family to pursue their studies and made the first step for my achievements now. For my
mother, Fatima, for providing me support in all stages of my education and her encouragement
and love, helping me to complete this master. For my loved Brazilians who were actively part
of this 2 years abroad: Ana, Mayra, Sara and Patricia, thank you for the motivation in the hard
times and long skype sessions telling me that everything was going to be fine.
iii
ABSTRACT
Forward osmosis is a novel technology which has been investigated for application in a variety
of water treatments in the last decade. It has shown great potential for rejection of compounds,
and has advantages over other membrane processes, such as being driven by an osmotic
gradient and not pressure, consuming a depreciable amount of energy and presenting less
fouling.
Acid mine drainage is a residue of concern mainly for environmental contamination by metals,
and historically it has not been treated, posing a risk to the environment. The aim of this work
was to evaluate the rejection of ions by a forward osmosis membrane when recovering water
from acid mine drainage. The results showed that forward osmosis can reach rejection rates
higher than 98% for most metals of the acid mine drainage, with a comparable performance
with reverse osmosis. Water fluxes were as high as 11.2 L/m2.h for forward osmosis with feed
of acid mine drainage and draw solution of 1 M of NaCl, so the thin-film composite membrane
from Porifera appears as a better choice than other commercial membranes. Reverse diffusion
of ammonium ions from NH3-CO2 draw solution were as low as 0.3 mol/m2.h, which is
considered very low compared to other values reported before. With these results, forward
osmosis can be seen as an economically and environmentally sustainable solution for treating
acid mine drainage.
Keywords: Forward osmosis, metals, acid mine drainage, water recovery, NH3-CO2 draw
solution
iv
TABLE OF CONTENTS
LIST OF ABBREVIATIONS .................................................................................................. vii
1
2
INTRODUCTION ........................................................................................................... 1
1.1
Research objectives................................................................................................ 2
1.2
Hypothesis .............................................................................................................. 2
LITERATURE REVIEW .................................................................................................. 4
2.1
Challenges on water scarcity and reuse.................................................................. 4
2.2
Mining industry........................................................................................................ 4
2.2.1
2.3
Forward Osmosis .................................................................................................... 7
2.3.1
Concentration polarization ............................................................................... 8
2.3.2
Fouling behaviour .......................................................................................... 10
2.3.3
Draw solution: NaCl and NH3-CO2 ................................................................. 10
2.3.4
Forward osmosis membranes ........................................................................ 12
2.4
Comparison of membrane technologies ................................................................ 13
2.5
Transport of ions in FO membranes ...................................................................... 13
2.5.1
3
Acid mine drainage .......................................................................................... 5
Rejection of ions ............................................................................................ 15
MATERIALS AND METHODS ..................................................................................... 16
3.1
Forward osmosis membranes ............................................................................... 16
3.2
Feed and draw solutions used .............................................................................. 17
3.3
Determination of water flux ................................................................................... 18
3.3.1
Clean water flux ............................................................................................. 18
3.3.2
Osmotic pressure of feed and correspondent water flux ................................ 19
3.4
Filtration experiments for ions rejection ................................................................. 20
3.5
Evaluation of reverse flux of solutes from draw solution ........................................ 22
3.6
Description of the procedure for rejection calculation ............................................ 22
3.7 Analytical methods ................................................................................................... 23
4
RESULTS AND DISCUSSION ..................................................................................... 25
4.1
Characterization of AMD sample........................................................................... 25
v
4.2
Determination of initial water flux .......................................................................... 26
4.3
Rejection experiments with NaCl draw solution ..................................................... 28
4.3.1
Osmotic pressure and water flux .................................................................... 28
4.3.2
Comparison of membranes for ions rejection ................................................. 30
4.3.3
Forward ions rejection with synthetic solutions............................................... 31
4.3.4
Mass balance analysis of experiments with synthetic solutions ...................... 33
4.3.5
Model of ions transport .................................................................................. 34
4.3.6
Forward ions rejection with acid mine drainage.............................................. 35
4.3.7
Mass balance analysis of experiment with acid mine drainage ...................... 37
4.3.8
Reverse diffusion with NaCl draw solution ..................................................... 38
4.4
Rejection experiments with NH3-CO2 draw solution .............................................. 39
4.4.1
4.5
Reverse diffusion with NH3-CO2 draw solution ............................................... 42
Recommendations ................................................................................................ 46
5
CONCLUSIONS........................................................................................................... 48
6
BIBLIOGRAPHY .......................................................................................................... 50
7
ANNEX ........................................................................................................................ 54
7.1
Specification sheet of each membrane used for flux experiments, as provided by
the fabricant .................................................................................................................... 54
7.1.1
Aquaporin Inside ............................................................................................ 54
7.1.2
Hydration Technology Innovations ................................................................. 55
7.1.3
Porifera Inc. ................................................................................................... 57
7.2
Speciation of ions and molecules according to software Visual Minteq ®.............. 58
7.3
Relation of pH and zeta potential .......................................................................... 60
7.4
FT-IR analysis of precipitate formed during NH3-CO2 experiments ....................... 61
7.5
Thermogravimetric analysis (TGA) of precipitate formed during NH3-CO2
experiments ..................................................................................................................... 62
vi
LIST OF ABBREVIATIONS
AMD
Acid Mine Drainage
CEOP
Cake-Enhanced Osmotic Pressure
CP
Concentration Polarization
CTA
Cellulose Triacetate
ECP
External Concentration Polarization
ED
Electro-Dialysis
FO
Forward Osmosis
ICP
Internal Concentration Polarization
LMH
L.m-2.h-1
NF
Nanofiltration
RO
Reverse Osmosis
TFC
Thin-Film Composite
vii
viii
1
INTRODUCTION
Water is an essential resource for life on Earth, being necessary in diverse human activities
and it is present in the form of fresh water mostly all around the world. Its availability for society
depends on various aspects, such as natural abundance, climate of the region, natural and
human-made pollution, improper management, etc. Due to its multiple uses is considered a
valuable resource, however not always treated as such. It is often wasted and contaminated
when used in human’s activities, and then can became scarce in some areas, despite areas
of dry weather that suffer from lack of water anyhow. The growing population and the
expansion of metropolis rise the demand of water around the world and so, combined with
scarcity problems, we see a rising need for a planned and sustainable use of water. In order
to achieve this, reuse of water has been an important asset, since the majority of wastewater
is still water that just need to be separated from its contaminants to be usable again. In order
to do so, one needs an appropriate treatment, which should be efficient in the separation of
contaminants, but also economically viable and environmental friendly.
Industrial activities are still expanding in developing countries and they are responsible for
producing a large amount of the products we consume nowadays, providing goods, tools and
some comfort for our daily lives. In the other hand, they also produce great quantities of rejects
and play an important role in contamination of water, soil and air, leading to the many
environmental problems we know, such as climate change, global warming, water scarcity, air
pollution and environment contamination, for instance.
One industry that has been related to environmental contamination is the mine industry. It
produces a lot of residues, which are contaminated with metals and if released to the
environment without further treatment can pose a serious environmental risk on the area. One
specific type of reject formed on a mining site is known as acid mine drainage, which is formed
when the excavated ore is exposed to rain and other natural erosion factors (Masindi, 2016).
There are various treatments options available for water contaminated with metals, such as
ion exchange, chemical precipitation, adsorption, flotation, electrochemical deposition and
membrane filtration (Cui et al., 2014; Simate and Ndlovu, 2014). In the last decades, the use
of membranes as treatment increased, and so, their participation on the market share became
more relevant, since it has been proved that membranes are effective at reasonable costs.
They became known for being relatively easy to operate and scale up, for the long range of
size particles/solutes separation, and high selectivity (rejection) of the desired molecule or
compound (Zhu et al., 2015). Moreover, membrane process is considered one of the less
harmful technologies for the environment, since the use of chemicals is minimum compared
to others technologies. Besides, energy consumption can be low or use alternatives sources.
1
For removal of metals, the membranes technologies that have been more extensively used
so far, have been electro-dialysis (ED), nanofiltration (NF) and reverse osmosis (RO). Forward
osmosis has been getting increasing attention during the last years (Fu and Wang, 2011). It
uses the osmotic pressure difference between two fluids to move one of them through a semipermeable membrane, concentrating one and diluting the other (Cath et al., 2006). So, the
biggest advantage of forward osmosis process is that it is based only in osmotic pressure
gradient between the two fluids and being so, does not require additional input of energy. This
can result in less capital and operational costs and also a more environmental friendly
technology, as it is not dependent of energy from fossil fuels (Chekli et al., 2016; Cui et al.,
2014).
In this research, we evaluated the efficiency of forward osmosis with a thin-film composite
(TFC) membrane when treating acid mine drainage. A sample of acid mine drainage from a
mining site in Chile was used for the evaluation. The general aim was to extract maximum
amount of water to be reused for the mining activity and to concentrate the solution of metals
in order to facilitate the final treatment. A low cost and simple technology was proposed which
can reduce the amount of residue generated and still provide water for reuse within the
industry.
1.1
Research objectives
The overall objective of this research is to investigate the performance of rejection of a thinfilm composite (TFC) membrane when treating acid mine drainage using the principle of
forward osmosis.
1. Determine the rejection of ions with a synthetic solution of metal sulphate and with a
real sample of acid mine drainage when performing forward osmosis
2. Determine the transport of ions by reverse diffusion in forward osmosis when using
NaCl and ammonia carbonate (NH3-CO2) as draw solutions
1.2 Hypothesis
Previous considerations:

The recovery of water from mine industry residue is of crucial importance as there is a
rising demand for water in the industry and nearby communities.

The sustainable use of water resources in mining activities is a rising need, in order to
reduce the impact of mine industry in the ecosystem around it.
2

Forward osmosis is a novel technology for water treatment, with potential to reject a
large range of contaminants at low cost and low energy consumption, being considered
an environmental friendly technology.
Taking into account the above considerations, the following hypothesis is proposed:
It is possible to recover water from acid mine drainage by forward osmosis process,
with a high rejection factor for metals.
3
2
LITERATURE REVIEW
2.1 Challenges on water scarcity and reuse
With more frequent dry periods and water shortages, it has become clear to society that water
is a valuable resource and if we do not use it wisely, we will face serious problems of lack of
water in the near future. As human beings we use water for various activities and the
expanding population, rising population density in cities, pollution of water sources and
extraction of great amount of water for irrigation purposes can lead to water scarcity. Besides,
new challenges are arising for proper management of water, such as climate change altering
the patterns of precipitation and seasonal fluctuations in population due to tourism. (Angelakis
and Durham, 2008). Reuse of water has been reported as an important strategy in water
management strategies and can help to reduce the supply demand, the abstraction licenses
and reduce the costs, since it reduces the cost of supply and of disposal of waste in form of
effluent (Casani et al., 2005; Coday et al., 2014). One of the main reasons that have increased
the interest about reusing water is the raising of environmental constraints about the quality of
wastewater discharges, but also the obvious benefit of increase of water as a resource
available in a region (Angelakis and Durham, 2008; Casani et al., 2005).
Recently, another key point for interest in reuse and recycling of water has been the
improvement in the available technologies for treatment, allied with more feasible technologies
on the economic point of view as well (Casani et al., 2005). Besides, the rising prices for water
can make in a near future the reuse technologies compete on the economic side with
traditional water supply (Casani et al., 2005)
Also, it is important to notice that problems of water availability are normally related to energy
production as well, since production of potable water requires energy, but also to produce
energy, normally one needs water available on the region (Chekli et al., 2016).
Considering all that, it is clear that reuse of water within the mine industry or reintegration on
the ecosystem will have a beneficial impact for themselves, since there would be reduction of
waste and reduce the source of pollution being released to the environment, besides the
increment of water availability on the region (Machado and Silva, 2012)
2.2 Mining industry
According to Fuisz-Kehrbach (2015) the mining industry faces a dilemma with public
perception of its activities: they are perceived as a source of pollution and unsustainable due
to the kind of product extracted, that are non-renewable and can pollute and have an impact
on the surroundings. However, they are also seen as an asset for economic development of
4
a region, mostly in developing countries, and important for the supply of resources needed for
many electronic gadgets with rising demand nowadays. So, attempts to make this industry
“greener” are emerging and proper water management is seen as a key to achieve this.
In the context of mining industry, water is critical for the development of various activities, such
as griding, flotation, gravity concentration and hydrometallurgical processes (Wessman et al.,
2014). Process like smelting and leaching for extraction of metals in ores with low
concentrations of metals can consume even more water, being of great concern when
addressing water management in mining industry (Wessman et al., 2014).
2.2.1
Acid mine drainage
As any other industrial activity, mining produces a considerable amount of waste and even
though the composition is highly variable, it is certain to have metals, which can pose a great
risk of environmental contamination (Johnson, 2003). A particular type of residue generated
at mines is known as acid mine drainage, which can be defined as acid wastewater generated
in an open ore due to drainage of water along the ore, on which a series of reactions can occur
releasing different amounts of metals and metalloids to the water (Johnson, 2003; Machado
and Silva, 2012). Acid mine drainage is formed due to an oxidation process of pyrite (Fe2S)
and other sulphidic compounds, which is accelerated on the presence of oxygen and water.
This leads to the formation of ferrous iron and sulfuric acid, which are dissociated on the
solution, decreasing the pH and incorporating soluble metals cations (Johnson and Hallberg,
2005; Nleya et al., 2016). The metal composition of an AMD depends on the mineral being
explored, local conditions and interactions with the environment (Machado and Silva, 2012).
The low pH is favourable for keeping metals on the solution, what makes this water very
corrosive and toxic (Nleya et al., 2016). High content of sulphate is also a characteristic of this
waters. There is no precise estimation of amount of AMD being generated in the world, but it
is believed that one mine can lead to the formation of hundreds to thousands of cubic meters
of AMD (Buzzi et al., 2013). The production of AMD often happens on abandoned mines, after
their operation is finalized and the ore is left open, without treatment or immobilization of
metals (Johnson, 2003). The main reasons for the formation of this residue is that during
operation of the mine, usually there are pumping activities keeping the groundwater table low
in the area. However, after the mine finishes it operation, pumping is not done anymore,
leading to a possibility of rise on the groundwater table and allowing water to be in contact
with the ore. It can also happen due to rain or melting of ice on nearby mountains (Johnson
and Hallberg, 2005; Johnson, 2003). These contaminated waters are released to the
environment, where they become a source of pollution and can be incorporated on the food
web, leading to biomagnification and posing a risk for safety, environment and human health
5
(Machado and Silva, 2012). It is important to notice that this process of pollution can go on for
many years if a treatment solution or a stabilization method is not applied (Johnson and
Hallberg, 2005).
The contamination of the environment with AMD is of concern because metals are persistent
compounds, that hardly will decompose in non-toxic substances without an appropriate
treatment (Machado and Silva, 2012).
Following the logical chain of residue’s treatment, the first step should be preventing the
generation of the acid mine drainage, applying methods to avoid the formation of such residue.
But in cases where the residue already exists, some treatment options suggested in literature
are shown on Table 1. These alternatives could be implemented before or after the
concentration of acid mine drainage by forward osmosis process being studied in this project.
For most of the process listed below, a high concentration of metals is desired for better
performance (Simate and Ndlovu, 2014).
Table 1. Options for acid mine drainage treatment. Sources: (Fu and Wang, 2011; Johnson and Hallberg,
2005; Nleya et al., 2016; Simate and Ndlovu, 2014)
Alternative
Explanation
Add an alkaline solution in order to raise the pH and precipitate some
Addition of chemical
neutralising agent
metals, with carbonates and hydroxides. Can be efficient but high
operational cost and disposal of bulky sludge. Limestone is the most
used. Loss of possible extractable components, such as metals and
acidity.
Preventing oxidation of iron with anoxic conditions and adding alkali to
Anoxic limestone
drains
the AMD. Inside a drain made of plastic and clay, the partial pressure of
CO2 is increased in order to raise the concentration of alkalinity. It can be
low cost and is considered a passive treatment, but it is not suitable to
all AMD waters and may require an anoxic pond prior to treatment.
Include various treatments, such as wetlands and bioreactors, but are
fundamentally based on the activity of microorganisms that during a
Biological
reduction process are able to generate alkalinity and immobilise the
remediation
metals. They present low operational costs, but in the other hand have
high capital cost, requires large areas for installation and efficiency on
the long term is not certain.
Sulphidogenic
The principle is based on the production of H2S to generate alkalinity and
bioreactors
precipitate metals sulphides. The bioreactors avoid direct contact
6
between sulphate reducing bacteria (SRB) and the AMD inflow,
improving the production of H2S. They are more reliable than biological
treatments, allow the recovery of selected metals and reduce
concentration of sulphate, besides generating less and more stables
metal compounds. However, capital and operational costs are high.
Is one of the most used methods for metals removal from contaminated
waters, using low cost and highly porous materials. Nanomaterials have
emerged recently, providing high adsorption efficiency and removal of
Adsorption
metals, besides being highly regenerated and it is easy to separate the
metals from the adsorbent. Adsorbent should be selected according to
the water characteristic and budget available, as a big range of
adsorbents have been studied.
Most process cited above uses variable pH or are pH dependent, which can influence on water
recovery (Simate and Ndlovu, 2014). Membrane process do not depend on pH and this is a
benefit in comparison to other technologies for extraction of water from AMD (Fu and Wang,
2011). In general, they are considered highly efficient on the separation process and
environmental friendly due to not using chemicals, but present high capital and operational
costs and fouling of the membrane can be a drawback (Nleya et al., 2016).
It is important to notice that AMD should be viewed as a resource, from which valuables byproducts can be extracted (Nleya et al., 2016). So, the ideal technology for treatment would
reduce the environment impact and would also be able to recover water, metals and sulfuric
acid from the residue, turning it into an economic profitable activity (Nleya et al., 2016; Simate
and Ndlovu, 2014). Also in the context of an abandoned mine, a low cost solution with no need
of external energy input is preferred (Simate and Ndlovu, 2014).
2.3 Forward Osmosis
Osmosis or forward osmosis can be understood as an engineered process of diffusion across
a selectively permeable membrane. Movement of water is based on the osmotic pressure
gradient created on the different sides of the membrane and the water flux is from the solution
that is less concentrated (higher water chemical potential) to the one that is more concentrated
(lower water chemical potential), as nature tries to make the concentrations equal (Cath et al.,
2006). So, the driving force of the process is the difference of osmotic pressure across the
membrane and the synthetic membrane work as a barrier to ions and molecules, but allows
the passage of water molecules (Cath et al., 2006). Figure 1 shows the working principle of
FO compared to RO.
7
Figure 1. Forward osmosis (FO) and reverse osmosis (RO) principle. Source: Cath et al., 2006
Recently forward osmosis has been used in diverse fields of application, as wastewater
treatment, food processing, seawater desalination, power generation, pharmaceutical
industry, among others, and has shown great performance, making room to further
developments and implementations (Cath et al., 2006; Zhao et al., 2012). Previous researches
show that FO process has a good potential to reject metals and can be an option to treat the
type of residue in question (Cui et al., 2014).
At a first thought, FO is known for not demanding additional input of energy, since there is no
need to generate mechanical pressure for osmosis to happen. But one has to be careful when
affirming that an entire FO process is not energy dependent (Lutchmiah et al., 2014). In cases
that the final product is the water that passed to the draw solution side, it will always be
necessary an additional step for separation of the draw solute and water and this one can be
energy intensive (Chekli et al., 2016). Due to this fact it is important to choose an adequate
draw solute for the type of energy available on the region where the project is thought for
implementation.
Some considerations about forward osmosis like concentration polarization, fouling behaviour,
draw solutions and membrane type have an important role on water recovery process, and
are discussed below.
2.3.1
Concentration polarization
A phenomenon that occurs in all osmotic driven process is concentration polarization, which
can be understood as the movement of solute particles to get near the membrane surface or
support layer and reduces the concentration difference across the membrane to values much
lower than expected with the known concentrations of the bulk feed and draw solution. It can
be divided into external and internal polarization and will be discussed below:
a. External concentration polarization (ECP): it is a phenomenon that occurs externally on
both sides of the membrane and causes the reduction of the osmotic gradient across the
8
active layer. There are two ways it can happen, the first is the solutes on the feed side
accumulate near the membrane’s active layer and this is called concentrative external CP
(Cath et al., 2006; Zhao et al., 2012). The other way is the area near the membrane on the
draw side gets lower concentrations of draw solutes due to presence of permeate water.
This is called dilutive external CP. Increasing the cross flow velocity and generating
turbulence near the membrane can reduce significantly external CP and it is not
considered the main cause for a lower flux than expected in FO (Cath et al., 2006; Zhao
et al., 2012).
b. Internal concentration polarization (ICP): this phenomenon occurs within an asymmetric
membrane and plays an important role in the reduction of osmotic potential. Draw solutes
tends to diffuse in the support layer mesh and due to its concentration an osmotic potential
is created. When water permeates through the membrane from the feed side to the draw
side, dilution of these solutes happens, reducing the osmotic potential and characterizing
internal dilutive CP. As this occurs inside the membrane, internal CP cannot be avoided
by changing the hydrodynamics conditions of the experiment and its intensity will depend
on the draw solutes and membranes characteristics (Cath et al., 2006; Zhao et al., 2012).
Internal dilutive CP is known to be the main cause for a flux lower than the theoretical flux
calculated only with osmotic potential difference across the membrane (Cath et al., 2006).
Figure 2 illustrates dilutive ICP.
Figure 2. Illustration of dilutive ICP in FO process. Source: Zhao et al. (2012)
9
2.3.2
Fouling behaviour
In respect of a lower tendency to fouling, Lee et al. (2010) conducted a study to compare the
fouling behaviour between reverse osmosis and forward osmosis technologies. Since during
FO there is no applied hydraulic pressure, the fouling layer was found to be sparse and large,
with feed particles accumulating loosely around the membrane layer. On the other hand, the
fouling layer in RO was compact and thin. Besides, they found out that the cake-enhanced
osmotic pressure (CEOP) is of greater importance for driving flux decline in FO than in RO.
This is attributed to reverse salt diffusion across the membrane, since when salt goes to the
feed side of the membrane, it gets trapped on the thicker cake layer formed on FO process
and diminishes the osmotic gradient that drives the flux. Although the flux decline in FO is of
concern, Lee et al. (2010) also found out that the fouling is reversible by changing
hydrodynamic conditions (cross-flow velocity) or physical cleaning (back washing). This was
found to not be true for RO system, where the fouling could only be reverse with chemical
cleaning. Therefore, it is possible to affirm that a FO system has a lower tendency for
irreversible fouling than a RO set-up, leading to less use of chemicals in the cleaning process
and consequently lower operational cost as well.
2.3.3
Draw solution: NaCl and NH3-CO2
The draw solution is a highly concentrated solution, which will draw water to its side of the
membrane by osmotic gradient. It is normally chosen considering the properties of the feed
and the conditions of the particular application (Cath et al., 2006; Ge et al., 2013; Zhao et al.,
2012). Draw solution selection represents one of the main challenges when considering
forward osmosis technology. It needs to adapt to different types of membranes and provide
some characteristics that are essential to a good performance of the system (Cath et al., 2006;
Ge et al., 2013; Zhao et al., 2012). Important characteristics for a good draw solute according
to Ge et al. (2013) and Jeffrey R. et al. (2006) are:

to have a high solubility in water and a high degree of dissociation, leading to a high
osmotic potential and consequently a high water flux;

small molecular weight and low viscosity to reduce internal concentration polarization;

reasonable costs for production and separation process from water;

minimal reverse draw solute diffusion;

a non-toxic compound;

simple process of recovery that does not demand great quantity of energy.
Among modern applications of FO, we can identify two main types, the ones where the goal
is to concentrate the feed solution, and the other that aims to recover water from the feed.
10
When the goal is to produce clean water is very important that the draw solute can be easily
separated from water, in order to make a sustainable project. Different draw solutions have
been proposed aiming to fulfil all the requirements for a suitable draw solution (Cath et al.,
2006; Ge et al., 2013; Zhao et al., 2012). Some of the typical compounds used nowadays are:
sodium chloride (NaCl), magnesium chloride (MgCl2), potassium chloride (KCl), ammonium
bicarbonate (NH4HCO3), CaCl2, sucrose, sodium formate, polyglycol copolymer, etc (Ge et al.,
2013).
Seawater has been considered for some forward osmosis applications, due to its high osmotic
potential and easy and cheap availability in coastal areas. It does comply with some of the
characteristics for a good draw solution, as high solubility in water and degree of dissociation,
low viscosity and small molecular weight, and low cost of production. But as a drawback for
FO, the separation process of water and solutes can demand a considerable amount of
energy, since the main procedures for that are based on thermal process or reverse osmosis.
If some waste heat is available to perform a thermal distillation, seawater can still be
considered adequate for the case. So, NaCl will be used as draw solution in this study, due to
its easy preparation and great osmotic power, besides being a typical draw solution used in
scientific investigations, making possible a fair comparison of results.
Ammonium bicarbonate (NH4HCO3) is a compound which decomposes into ammonia and
carbon dioxide, and have been used as draw solution since the beginning of exploitation of
forward osmosis system (Ge et al., 2013). It has some qualities that make it feasible to use as
draw solution, but the main drawback associated with it is the low solubility in water, which
leads to a low osmotic potential (Ge et al., 2013). This problem was addressed by McCutcheon
et al. (2006) and they were able to increase the solubility of a mixture of ammonia and carbon
dioxide by adding an adequate proportion of ammonium hydroxide (NH4OH). The solution
provided a high osmotic driving force for a forward osmosis system, with concentrations of
ammonia-carbon dioxide solution varying from 1M to 6M. These solutes are considered easy
to remove from water, since when heated up to around 60°C, ammonia and carbon dioxide
become gases that can be separated from water and recovered by simple processes (Jeffrey
R. et al., 2006). For this reason, the ammonia-carbon dioxide solution (NH3-CO2) was elected
as a draw solution for this project, as we want to investigate its efficiency when combined with
a feed containing metals. As disadvantage this draw solute can present a high reverse
diffusion of NH4+- NH3, mostly when a strong electrolyte is used as feed solution (Arena et al.,
2014; Chekli et al., 2016). The recovery of the solutes present in the draw solution can be
done by solar distillation or using a membrane technology, such as membrane distillation (Ge
et al., 2013; Kim et al., 2016).
11
The scheme of the proposed treatment is shown below, on Figure 3. In case of NaCl as draw
solution, it is possible that the step of solar/membrane distillation is substituted by a reverse
osmosis process.
Figure 3. Scheme of treatment process idealized for acid mine drainage.
2.3.4
Forward osmosis membranes
Forward osmosis studies have been historically focusing more on performance of cellulose
triacetate (CTA) membranes, even though thin-film composite (TFC) membranes can achieve
better performance, including rejection and water flux (Coday et al., 2014). Higher rejection of
compounds can be related to the presence of higher amount of charge on the membrane
surface when compared to the CTA and also to a smaller B value, which is a parameter related
to solute permeability (Coday et al., 2014). TFC membranes are also known for handling better
extreme pHs conditions of feed and draw solitions. For all of these reasons, it is believed that
the present study can have a better performance with TFC membranes.
Other type of membranes present in the market are the protein membranes. They are known
for being made of biological material and have good water flux characteristics, since in their
composition there are very small water channels called Aquaporins, which try to reproduce
mechanisms of water transport of the human body (Kumar et al., 2007). Although, according
to the fabricants they also cannot handle extreme pHs for a long time of operation, what can
be a problem when treating acid mine drainage.
12
2.4 Comparison of membrane technologies
The membrane process used traditionally for treatment of acid mine drainage are NF, RO and
ED, and they all have different working principles. In nanofiltration, the process is based on
applying pressure on one side of the membrane to force the liquid through the membrane,
which will select the desired molecules (Al-Zoubi et al., 2010; Zhu et al., 2015). In this case
the mechanism reigning nanofiltration is physical sieving, also known as steric hindrance, and
will be discussed further, in section 2.5.1 (Al-Zoubi et al., 2010).
RO principle is to apply external pressure to overcome the osmotic potential of a solution and
force water through the membrane, going to the opposite side it would naturally do (Cath et
al., 2006). As the membrane does not allow most of the contaminants to pass, the water is
purified. In both technologies, NF and RO the driving force is the external applied pressure,
and besides costly, it can increase the concentration polarization and fouling propensities and
demands a thick support layer to handle the pressure (Cath et al., 2006; Zhao et al., 2012).
Electro-dialysis uses an electric field to move ions through charged membranes (Fu and
Wang, 2011). So the mechanism for rejection is mostly electrostatic repulsion, better explained
on section 2.5.1. It can achieve rejection higher than 97% and water recovery up to 80%, but
can demand a prior treatment to remove iron ions to avoid scaling on the membrane and
demands a lot of energy (Fu and Wang, 2011; Simate and Ndlovu, 2014). With ED it is possible
to recover sulfuric acid and metals as well, with another step of separation (Nleya et al., 2016)
RO, ED and NF are highly efficient rejecting particles and solutes, but they can consume
considerable amounts of energy (Li et al., 2014). This energy supply may not be available in
some parts of the world or it may come with a high economic and environmental cost. As said
before forward osmosis has the advantage of using chemical energy as driving force (Cath et
al., 2006) . Other advantages when compared to RO or NF are a more simple apparatus for
supporting the membrane (module), thinner membrane support layer, lower tendency to
fouling, easier and more effective membrane cleaning and/or back-washing (Chekli et al.,
2016; Cui et al., 2014; Lee et al., 2010). Besides, the rejection rate of contaminants are
comparable among all these technologies, even though they vary depending on the compound
and other factors (Coday et al., 2014; Cui et al., 2014; Haese et al., 2013).
2.5 Transport of ions in FO membranes
“The most important property of membranes is their ability to control the rate of permeation
of different species” (Baker, 2004)
13
In process involving membranes there are two base models for transport of permeants: poreflow model and solution-diffusion model. Basically, the pore-flow model says that permeants
are transported as a result of the pressure applied to the system, flowing through tiny pores
on the membrane. This implies that transport can be explained by a convective flow and it is
similar to a filtering process, excluding the larger particles than the pore from the permeate.
On the other hand, solution-diffusion model says that “permeants dissolve in the membrane
material and then diffuse through the membrane down a concentration gradient” (Baker,
2004). In this case separation occurs due to different solubility of permeants in the membrane
and different rates of diffusion through the membrane. Membranes that are classified as
nonporous dense membranes, which are composed of a dense film, normally have a better fit
to the solution-diffusion model of transport. That is the case for reverse and forward osmosis
membranes. This model is based on the principle of diffusion and an important remark is that
the transport depends only of the solubility of ions in the membrane material (Baker, 2004).
The equation governing this process is known as Fick’s law of diffusion, described as:
𝐽𝑖 = −𝐷𝑖 ∗
𝑑𝐶𝑖
𝑑𝑥
Equation I
with Ji (mol/m2.s) being the rate of transfer of a compound i (also known as flux), dCi being
the concentration gradient of component i, dx (m) is the difference in position (or length) of the
gradient being measured and Di is the diffusion coefficient (m2/s), which translates into the
mobility of individual molecules (Baker, 2004). The minus sign is adequate because the
movement of particles is down the gradient, from the more concentrated to the less
concentrated.
According to Haese et al. (2013) the transport in forward osmosis is very likely to be similar to
pressure-driven process such as nanofiltration and RO, because of the similarity of pore size
and membrane material. Although, they emphasize that due to the presence of a draw solution
and the possibility of reverse solute diffusion, the mechanisms of transport can be altered, as
well the rejection for some compounds (Haese et al., 2013). Reverse diffusion is the
movement of solutes from the draw solution to the feed side, against the water flux, governed
by difference of concentration (Cath et al., 2006).
Factors affecting transport in a forward osmosis system are: solute properties (like molecular
weight and size, partitioning and diffusion rate through the membrane, etc.), water flux and
membrane material (which will influence on electrostatic forces of attraction and repulsion). A
combination of these factors are important in the determination of how the transport occurs.
14
2.5.1
Rejection of ions
Previous studies show that FO has potential for rejection of components from wastewater or
seawater, with similar levels of rejection achieved as RO (Cath et al., 2006; Coday et al., 2014;
Li et al., 2014).
Rejection of compounds by a membrane is influenced by different types of interactions and
characteristics of the compounds. The most relevant characteristics are the electric charge
and molecular size (Coday et al., 2014). The most important interactions between membrane
and ions reported so far are described below:

Electrostatic repulsion: polymeric membranes present electric charge on their surface,
and these charges interact with solute ions to increase or decrease rejection, either by
repulsion or attraction or by Donnan Exclusion (Haese et al., 2013)

Steric hindrance: depends on size of solute and membrane pore, besides the actual
format and conformation of the solute molecule (Haese et al., 2013). The polymer
chains on the membrane surface can be influenced by electric charge of the functional
groups, altering their conformation (Haese et al., 2013).

Partitioning and diffusion trough the membrane: depends on solute and membrane
affinity, determining the rate which a compound can diffuse in the membrane matrix
(Haese et al., 2013).
Previous studies show that electrostatic repulsion play a big role (Lu et al., 2014). Depending
on the zeta potential of a membrane, positive or negatively charged ions can be attracted or
repulsed by the membrane, leading to smaller or higher rejection, respectively (Coday et al.,
2014). The zeta potential of a membrane is a measurement of electrical potential at the
membrane surface and determines the movement of charged compounds around the
membrane (Kim et al., 1996). Most membranes with a polyamide composition have negative
zeta potential at neutral pH. (Coday et al., 2014; Haese et al., 2013).
Rejection of compounds is also affected by the duration of an experiment, since it has been
reported that initial rejection levels are high, due to absorption of compounds to the membrane
(Haese et al., 2013). But this is followed by an increase in difference of concentrations of ions
between the membrane and draw solution, which leads to a higher permeation rate of
compounds to the draw side after the membrane becomes saturated, decreasing the rejection
for the compound in question (Coday et al., 2014; Haese et al., 2013).
15
3
MATERIALS AND METHODS
The materials and methods used to comply with the objectives cited above are described
below.
3.1 Forward osmosis membranes
For initial flux experiments, three different membranes were considered on this study: one
made of protein from Aquaporin Inside™ (AIM FO - Sterlitech Co, USA), and other two of thinfilm composite (TFC): one from Hydration Technology Innovations (HTI OsMem - Albany, OR,
USA) and the last from Porifera Inc. (FOMEM-0415 - Hayward, CA, USA), specification of
each is in Annex. Properties of each of those membranes are shown on Table 2. Water
permeability (A0) of a membrane is a measurement of how permeable it is and will influence
on the water flux. The salt permeability (B0) is a measurement of how much salt the membrane
allows to pass and it will impact on the rejection of compounds. The structural parameter (S0)
is related to the internal structure of the membrane, its width, and influences on how the
membrane will present internal concentration polarization (ICP) (Blandin et al., 2016). The
zeta potential (ζ) is a measurement of the surface charge of the membrane and will impact on
the rejection mechanism by electrostatic repulsion (Kim et al., 1996). Lastly, the contact angle
(θ) is measured between a drop of water and the membrane surface, and indicates the
hydrophilicity of a membrane and it is very related to fouling and water flux.
Table 2. Membrane and active layer properties. Modified table from Blandin et al. (2016)
Membrane characterization
Membrane
A0
B0
2
a
Active layer properties
S0
-7
(x10 m/s)
b
(µm)
a
Zeta Potential
Contact Angle
(pH=7) ζ(mV)
θ°
supplier
(L/m .h.bar)
HTI
1.3
0.3
1227
-9 ± 0.1
47 ± 18
Porifera
2.1
1.2
344
-13.7 ± 1.9
36 ± 6
Aquaporin
~4.0c
-
-
-
-
a
Parameters determined using feed of DI water and draw of dry red sea salts.
determined with feed of 100 mM dry red salts and NaCl and draw of dry red sea salts.
c Value obtained from Tang et al. (2013), conditions not available
b Parameter
After initial flux experiments, the membrane from Porifera was chosen for further experiments
of rejection. A more detailed characterization of this membrane was carried out by doing a
SEM-EDX image of the surface of a clean membrane. Results are shown on Figure 4, where
we see that the width of the membrane is around of 40-45 µm.
16
A)
B)
C)
Figure 4. Membrane characterization by SEM-EDX. (A) Membrane cross section, width around 4045 µm. (B) Membrane active layer image. (C) Graph of membrane composition.
3.2 Feed and draw solutions used
A real acid mine drainage (AMD) sample was obtained from a mine in North Chile. Analysis
of the compounds present in the sample were made, in order to characterize the residue in
question. Copper and sulphate were present at high concentrations and become of great
interest in this study.
Two types of solutions were selected to be on the feed side of the membrane: first, synthetic
metal solution and later, acid mine drainage sample. Therefore, synthetic solutions of copper
sulphate, with formula CuSO4 (Merck, Germany), were prepared and the range of
concentration used includes the concentrations found on AMD characterization. Three
concentrations were chosen, variating among them with a factor of 10, to represent extreme
conditions. The initial concentrations of ions are shown on Table 3. The pH was adjusted for
3.5 on the feed solution, the same as in the AMD sample, by adding hydrochloric acid (HCl).
17
Table 3. Initial concentration of ions and compound on each experiment
Designation of Cu+2 concentration SO4-2 concentration
CuSO4 concentration
experiment
(mM)
(g/L)
(g/L)
1st
0.10
0.15
1.6
nd
2
1.00
1.51
15.8
3rd
10.00
15.11
158.3
0.61
4.33
-
AMD
For draw solutions, two types of solutions were elected with intention of comparison of
performance achieved by both. Sodium chloride (NaCl) and an ammonia bicarbonate solution,
described here as NH3-CO2 solution, were used with concentration of 1 M. As both salts
dissociated similarly in water, the same osmotic pressure was achieved, of 48.72 bar. The
NH3-CO2 solution was prepared based on the proportion suggested by Lu et al. (2014): 0.9 M
of ammonium bicarbonate (NH4HCO3) and 0.1 M of ammoniac solution (NH4OH), reaching 1
M of ammonium ions in solution. For correction of salinity on the draw side, it was used a
brine, or a very concentrated solution of the desired compounds. Maintaining the proportions,
a 5 M solution of NaCl and a 2.5 M solution of NH3-CO2 were used. Theoretical calculations
and trials experiments were made to determine the volume and frequency that need to be
added in order to maintain the concentration on the draw side at 1 M.
3.3 Determination of water flux
3.3.1
Clean water flux
Clean or initial water flux experiments were made in order to test the membranes with the
conditions provided by the setup. Clean water flux measurements represent the highest flux it
can be expected from a membrane, since it is done with ultrapure water as feed, so nearly
zero osmotic pressure on the feed side. Initial fluxes were measured using 2 draw solutions:
NaCl and NH3-CO2, at different concentrations. For comparison, the pH of the feed of some
experiments was adjusted for 3.5, using a few drops of hydrochloric acid (HCl).
Some important parameters of membranes and active layer that are related to water flux are
shown on Table 2.
The setup used for this experiment is shown in Figure 5. A graduated tube was connected to
the draw solution chamber. Permeation of water to the draw solution produced a continuous
increase in the liquid level inside the graduated tube, which was used to determine the flow of
water crossing the membrane.
18
Figure 5. Experimental setup for determination of initial flux of three membranes, Aquaporin, HTI
and Porifera.
With the area of the tube and with measurements of height made in time, it was possible
calculate the volume of permeation and hence the flux can be determined. Initial flux was
calculated as:
𝐽𝑤 =
∆ℎ ∗ 𝐴𝑡
𝐴𝑚 ∗ ∆𝑡
Equation II
Where Δh (m) is the increase of height on the draw solution read in the tube in a determined
Δt of time (h). At (m2) is the area of the tube and Am (m2) is the effective area of membrane.
3.3.2
Osmotic pressure of feed and correspondent water flux
Osmotic pressure of the acid mine drainage sample was measured by Cryoscopic Osmometer
(OSMOMAT 030), after filtration with 0.2 µm filter. Other osmotic pressures were calculated
from concentration of ions, using the Van Hoff’s formula:
𝜋 = 𝑖 ∗ 𝐶𝑠 ∗ 𝑅 ∗ 𝑇
Equation III
Where 𝜋 (bar) is the osmotic pressure, i is the factor of dissociation of the compound, Cs (M)
is the concentration of solutes, R (L.bar/(K.mol)) is the constant of gases and T (K) is the
temperature.
Experimental water flux (Jw) data was obtained from experiments and calculated as:
𝐽𝑤 =
𝑉𝑝
𝐴𝑚 ∗ ∆𝑡
19
Equation IV
Where Vp is the volume of permeate (L), Am is the effective are of membrane (m 2) and Δt (h)
is the difference in time from previous measurement, which was set at 20 minutes.
Theoretical water flux (Jw theoretical) was calculated for comparison with the experimental
flux. Loeb’s equation was used:
𝐽𝑤 𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 =
𝐷
𝑆
∗ 𝑙𝑛
𝐴 ∗ 𝜋𝐷𝑆 + 𝐵
𝐴 ∗ 𝜋𝐹𝑆 +𝐽𝑤+𝐵
Equation V
Where D (m2/h) is the dilute diffusion coefficient, S (m) is the structural parameter of the
membrane, A (m3/m2.h.bar) is the water permeability parameter, πDS (bar) is the osmotic
pressure of the draw solution, B (m3/m2.h is the salt permeability parameter and πFS (bar) is
the osmotic pressure of the feed solution. This equation is iterative and was calculated used
Microsoft Excel.
The structural parameter of the membrane (S) could be calculated with the same Equation V,
considering the conditions of the setup. Using the results of water flux (Jw) from experiments
with clean water, S was made the unknown variable and the average value for 3 different
concentration of the draw solute was found to be 5.6 * 10 -4 ± 5.5 * 10 -5.
3.4 Filtration experiments for ions rejection
The setup for experiments of ions rejection was built as shown on Figure 6, using acrylic
modules. The setup was on flat sheet mode with dead-end filtration, with available membrane
area of 0.00135 m2 (13.5 cm2) for each module. Each module could accommodate up to 180
mL of solution on each compartment. Experiments were done in triplicate, at room
temperature. All experiments were made in FO mode, meaning that the active layer was facing
the feed solution and no external pressure was applied.
20
Figure 6. Scheme of forward osmosis setup for rejection experiments.
A stirring plate and magnets on both sides of modules were used to keep concentrations on
the module equally distributed. Storage for feed and draw were graduated beakers, where it
was possible to measure the volume that permeates and then calculate the flux provided in
each test.
Two peristaltic pumps were used for maintaining concentration constant on the draw side. The
first, a Master Flex/Cole-Parmer (7524-45), working at 95 mL/min, was used to keep
concentrations evenly distributed between the module and storage. The second pump, a
Master Flex/Cole-Parmer (7551-10), was programmed to add a brine at regular intervals in
order to keep the draw solution concentration in contact with the membrane constant. That
was necessary to counteract the dilution effect on the draw side, caused by the water flux.
Experiments were performed until a minimum concentration factor of 2 was reached, so
permeation of 190 mL of feed solution was stablished, determining the duration of the
experiment. Experiments done with Porifera membrane lasted between 6h and 15h, and the
one done with HTI lasted up to 31h.
21
3.5 Evaluation of reverse flux of solutes from draw solution
The reverse flux of NH3 and inorganic carbon was studied. The presence of precipitates in
previous experiments suggested that components of the draw solution was permeating to the
feed compartment. In order to determine such levels of permeation, filtration experiments were
performed, with deionized water and a NaCl solution of 0.1 M as feed. Both pH of the feeds
was adjusted to 3.5, by adding HCl.
Draw solution was the same NH3-CO2 used before. A similar setup to the one showed on
Figure 6 was used, but in this case no brine was added on the draw side. There was
recirculation on the draw and feed side. Same modules and pumps were used and
experiments run for 6 hours and one sample was withdrawn in the middle of experiment, at 3
hours.
3.6 Description of the procedure for rejection calculation
The removal efficiency of the membrane for each ion present on the feed side, also known as
forward ions rejection, is measured as a percentage of ions that are on the feed side compared
to the amount of ions that permeate to the draw side. By the end of the experiment, the feed
and draw solutions were stored in separated bottles and sent for analysis to identify ions
concentrations. So, forward ions rejection R (%) was calculated, using the equation:
𝑅 =1−(
𝐶𝑝
) ∗ 100
𝐶𝑓
Equation VI
Where Cf (mg/L) is the concentration of the ion on the feed side by the end of the experiment
and Cp (mg/L) is the concentration of the ion on water that permeates to the draw side,
calculated with equation (3):
𝐶𝑝 =
𝐶𝑑 ∗ 𝑉𝑑
𝑉𝑝
Equation VII
Where Cd (mg/L) is the concentration of the ion on the draw side in the end of experiment, Vd
(L) is the volume of draw in the end of experiment and Vp (L) is the volume of water that
permeates from the feed side to the draw side.
Reverse diffusion is known as the unwanted diffusion of ions from the draw side to the feed
side, which occurs in the opposite direction of the water flux. Reverse ions flux Js (mol/m2.h)
was calculated as follow:
𝐽𝑠 =
𝐶𝑓 ∗ (𝑉𝑓 − 𝑉𝑝)
𝐴𝑚 ∗ ∆𝑡
22
Equation VIII
Where Cf (mol/L) is the concentration of the ion on the feed side, Vf (L) is the initial volume of
the feed, Vp (L) is the volume that permeated to the draw side after a Δt (h) of time and Am
(m2) is the effective area of membrane.
Forward ions flux (Js) was calculated as:
𝐽𝑠 =
𝐶𝑑 ∗ (𝑉𝑑 + 𝑉𝑝)
𝐴𝑚 ∗ ∆𝑡
Equation IX
Where Cd (mol/L) is the concentration of the ion on the draw side, Vd (L) is the initial volume
of the draw and other variables are the same as defined for Equation VIII.
3.7 Analytical methods
Measurements of concentration of ions were made to calculate rejection. Measurements of all
metals in question in this study (Aluminium, Calcium, Cobalt, Chromium, Copper, Iron,
Magnesium, Manganese, Nickel, Sodium, Silicon and Zinc) were made with Inductively
Coupled Plasma Mass Spectrometry (ICP-MS), both on the feed side and on the draw side.
For ions sulphate (SO4-2) measurements on the feed side were made by gravimetric
calcination and on the draw side by ion chromatography with chemical suppression of eluent
conductivity. For Chloride (Cl-) on the feed side it was used the method of Mohr titration and
on the draw side it was ion chromatography with chemical suppression of eluent conductivity.
For experiments with NH3-CO2 draw solution, total ammonia nitrogen (TAN) was measured by
spectrophotometry with Hatch (DR2800) colorimetric kits. Carbon was measured with a Total
Organic Carbon analyser (TOC) from Shimadzu and results were used to calculate amount of
carbonate and bicarbonate ions present in solutions. In these experiment, Sodium was
measured by ion chromatography (Metrohm 882IC Plus). Adjustment of pH solution was made
by measurement with pHmeter from Thermo Scientific, Singapore.
The control for concentration of NaCl draw solution, made manually during the filtration
experiments, was done with a salinity measurer (OAKTON, Salt 6+) on regular intervals. For
control of concentration of NH3-CO2 draw solution samples were withdrawn from the
experiment on regular intervals and Hatch kits were used as well.
Scanning Electron Microscopy/Energy dispersive X-ray spectroscopy (SEM-EDX) was used
for characterization of the membrane and composition of precipitates on the membrane
surface. It was used a SEM-EDX from HITACHI (SU3500).
FT-IR (Agilent, Cary 630) analysis was done to identify functional groups present in the
precipitated formed during the experiments. Thermogravimetric analysis (Perkin Elmer - DSC
STA 6000) was also used as one of the procedures to identify precipitates.
23
4
RESULTS AND DISCUSSION
4.1 Characterization of AMD sample
Results of the analysis of the acid mine drainage can be found on Table 4. A characterization
from literature is also present, as comparison. The concentration of sulphate ions is
remarkable, as it is the substance present by far in higher concentration. After sulphates,
copper ions are second on the rank of concentration. Figure 7 shows the percentage of each
element, making it easy to identify the substances presents in higher content.
Table 4. Acid mine drainage sample characterization and literature values of concentration.
Parameter
AMD sample – pH 3.5
Literature values for AMD
(mg/L)
– pH 2.5 (mg/L)*
Sulphate (SO4-2)
4327
14337
Aluminium (Al)
293
1139.0
Calcium (Ca)
313
325.9
Cobalt (Co)
2.33
-
Chromium (Cr)
0.04
-
Copper (Cu)
615
2298.0
Iron (Fe)
13.4
627.5
Magnesium (Mg)
436
630.6
Manganese (Mn)
203
224.5
Nickel (Ni)
0.56
-
Potassium (K)
1.91
4.31
Silicon (Si)
15.3
-
Zinc (Zn)
68.5
-
* Source: Al-Zoubi et al. (2010)
25
Figure 7. Mass distribution of the species present in sample of acid mine drainage from Chile.
Category of other ions include concentrations of Cobalt (Co), Iron (Fe), Silicon (Si), Chromium (Cr),
Nickel (Ni) and Potassium (K).
4.2 Determination of initial water flux
Initial flux experiments were made in order to determine the maximum flux that the membranes
considered for the study could provide, at the hydrodynamics conditions provided by the setup.
By using ultrapure water as feed, concentration polarization and fouling do not play a role on
the water flux. The pH on the feed side was not corrected, and its average value was 6.0.
Results of initial flux experiments are shown in Figure 8:
26
Figure 8. Initial water flux for three different membranes tested: protein by Aquaporin, TFC by HTI
and TFC by Porifera. Determination of flux with sodium chloride (NaCl) as draw solution and ultrapure
water as feed.
It is remarkable that the flux for the TFC Porifera membrane is much higher than the other
two, reaching values up to 4 times higher than the protein membrane and more than double
of HTI. According to a compilation made by Wei et al. (2011), TFC membranes in FO mode
and flat-sheet configuration can provide water fluxes varying between 20.5 and 5.5 L/m2h
when using water as feed and NaCl 1M as draw solution. The value obtained for Porifera
membrane are on the upper part of the above range (around 16 L/m2h). The fabricant claim
that the membrane can reach values between 31 and 35 L/m2h in the same feed and draw
conditions. So, values found in this work are around half of the flux claimed by the fabricant.
The same happen for the other two membranes evaluated in this study. The possible reasons
for this difference can be the use of a cross-flow setup by membrane provider, which is known
to increase water flux. Moreover, experiments done at temperatures higher than 25 °C have
been related to lead to an increase on flux (Al-Zoubi et al., 2010). Experiments of this study
were done at room temperature, which is below 25 °C.
The higher flux of Porifera membrane is based on the fact that its structure can use the osmotic
pressure in a more efficient way, and this is demonstrated by a high water permeability
coefficient (A0) and a small structural parameter (S0) (Blandin et al., 2016). As smaller is the
S0 value, the thinner is the membrane, it has lower tortuosity and it presents more porosity.
This affects ICP behaviour of the membrane, affecting the water flux the membrane can
achieve (Blandin et al., 2016; Wei et al., 2011).
The lower flux achieved by HTI membrane is attributed to a lower value for A0 and a higher
one for S0.
27
The TFC membrane from Porifera was the one chosen for further experiments of rejection, as
with the high flux it can achieve is the one that can present a better performance and obtain
better results on an economical evaluation.
The selected membrane was tested at different pH, in order to determine its water flux. This
was done, considering that AMD normally present low pHs. The membrane was also tested
with the second draw solution used in the study, NH3-CO2. Results are presented in Figure 9.
Figure 9. Initial water flux of TFC membrane provided by Porifera related to draw solution
concentration. Feed solution of ultrapure water (UPW) and pH and draw solution as indicated on the
legend.
Results show that is not possible to visualize a pattern and it is noticeable that variations on
water flux due to pH change are minimal and as well for the two draw solutions in question.
Coday et al. (2014) and Li et al. (2014) also studied the influence of pH in initial water flux and
obtained comparable results, with no relevant difference on flux with acid pH. For the two
different draw solutions, it was expected no relevant difference on water flux, since
concentrations were chosen equally and both solutions could provide same osmotic pressure,
due to similar dissociation mechanisms.
4.3 Rejection experiments with NaCl draw solution
4.3.1
Osmotic pressure and water flux
Table 5 shows a comparison of osmotic pressure (𝜋 feed) generated by each feed solution and
the osmotic pressure difference across the membrane (Δ 𝜋). Also, the experimental water flux
achieved in filtration experiments is compared with theoretical water flux (Jw), calculated with
Equation V.
28
Table 5. Relation of osmotic pressure and the average water flux provided by draw solution of
NaCl at concentration of 1 M (Jw experimental) and theoretical water flux (Jw theoretical),
calculated with Equation V.
𝝅 feed
Δ𝝅
Jw experimental
Jw theoretical
(bar)
(bar)
(L/m2h)
(L/m2h)
CuSO4 at 1.6 mM
0.077
48.64
16.36 ± 0.14
15.00
CuSO4 at 15.8 mM
0.771
47.95
15.49 ± 0.57
14.51
AMD
1.795
46.93
11.15 ± 0.19
13.83
CuSO4 at 158.3 mM
7.713
41.01
9.85 ± 0.25
10.56
Feed solution
The comparison between experimental and theoretical flux shows very similar values, proving
that experimental flux is in accordance with the model. The obtained results for water flux were
satisfactory, since other TFC membranes could not reach similar water fluxes as Porifera
membrane did. In a similar study done by Cui et al., (2014), where metals rejection was
evaluated using a TFC membrane in FO, the water flux obtained with draw solution
concentration of 1M and feed solution concentration of 2.0 g/L was between 10.5 and 12.5
L/m2h. In comparison, Porifera achieved a higher flux of 15.5 ± 0.6 L/m2h at similar conditions
(feed
concentration
of
2.5
g/L,
even
higher).
The
draw
solution
used
was
Na4[Co(C6H4O7)2]*2H2O (described as Na-Co-CAs solution) and the osmotic pressure
generated by it is not reported. However, a comparison of fluxes provided by NaCl and NaCo-CA as draw solution is made, and authors affirm that the second can achieve higher fluxes
at same concentration. The membrane they used was fabricated on the laboratory using as
substrate the commercial available membrane Matrimid ® 5218 (Vantico Inc). However, even
with the Na-Co-CA draw solution, the water flux achieved by the membrane is lower than the
flux achieved by Porifera with NaCl as draw solution.
Blandin et al. (2016) also found that Porifera membrane could obtain higher fluxes than the
TFC membrane manufactured by HTI. Unfortunately, on this study, different solutions and
hydrodynamic conditions were used, not allowing comparison of water flux values. On the
other hand, they reported a relevant decreased on water flux of Porifera membrane during the
first batch of experiments, which was not observed with HTI membrane. In our experiments,
a decrease of around 43% on water flux was observed after 10h of operation with acid mine
drainage as feed solution. Then, it seems that Porifera membrane may be more affected by
fouling over time. However, it still presents an average water flux considerable higher than HTI
and others TFC membranes. So, it shows great potential for pilot scale applications.
29
Al-Zoubi et al. (2010) report values of water flux for NF and RO. Such values can be useful
to compare performances observed for FO with other traditional membrane processes. To
reach a similar water flux of 11 L/m2h with the membranes tested, it would be necessary to
apply a pressure of around 7 bars on a NF process and almost 10 bars on an RO process.
Of course, they can reach higher fluxes, up to 40 L/m2h, but at very high pressure.
To compensate the lower flux of FO, it is possible to use more membrane area for treatment,
and reach a similar duration of a process with NF or RO when treating the same amount of
residue. In cases where the time is not an important factor for the treatment, what can be the
situation of abandoned mines producing AMD, the lower flux is not an issue.
4.3.2
Comparison of membranes for ions rejection
Figure 10 presents copper (Cu+2) and sulphate (SO4-2) rejections provided by Porifera and HTI
TFC membranes. Tests were made with synthetic solution of copper sulphate (CuSO4) as
feed, at a concentration of 1.6 mM and pH 3.5, and draw solution of NaCl at concentration of
2 M. Rejection was calculated according to Equation VI.
Figure 10. Copper and sulphate ions rejection by TFC membranes from Porifera and HTI.
Experiments made with feed of CuSO4 at 1.6 mM with pH 3.5 and NaCl at 2 M as draw solution.
Scale starts at 50% of rejection. Error bars represent one standard deviation.
Results show that levels of copper rejection are about the same for both membranes. Porifera
had a slightly higher rejection, reaching 99.6 ± 0.1%. Sulphate rejection presents lower values
than copper, and Porifera has a better performance, with 94.7 ± 0.4 % rejection over 91.7 ±
0.6 % of HTI membrane. Sodium reverse flux was also evaluated on this experiment. Porifera
membrane presented average value of 0.34 ± 0.08 mol/m2.h and HTI with 0.26 ± 0.10
mol/m2.h. Water flux for Porifera was 22.9 ± 0.4 L/m2h and 5.3 ± 0.3 L/m2h for HTI, which
reveals that experimental water flux of Porifera can be up to 4 times higher than HTI. Due to
the large difference of water flux, duration of experiments also varied a lot, with Porifera
30
experiments lasting less than 7 hours and HTI over than 30 hours. Despite the better
performance of HTI membrane for reverse ions flux, Porifera membrane presented better
performance in terms of initial water flux and forward ions rejection. So that membrane was
chosen for further tests.
4.3.3
Forward ions rejection with synthetic solutions
Experiments were performed in order to test if observed rejection levels changes when
modifying feed concentration, using the concentrations described on Table 3. Results are
shown on Figure 11:
Figure 11. Levels of rejection for each initial molecular concentration of CuSO4. Scale starts at 50%
of rejection. Experiments were made with NaCl 1 M as draw solution. Error bars show one standard
deviation.
Rejection levels for copper are extremely high, varying from 99.1% up to 99.5%. For sulphate
levels are a bit lower, varying from 94.9% to 98.3%. Rejection of copper can be considered
almost constant, not depending on initial concentration of ions. In the other hand, rejection of
sulphate increases with higher initial concentration of the feed. Sulphate has a negative
charge, the increase of negative charge around the membrane leads to an increase on
rejection levels of sulphate The influence of initial ions concentrations is more relevant for
sulphate, as there is an increase on rejection levels as feed concentration increases. It is
expected that dissociate forms can cross the membrane more easily. According to a model of
speciation done by the software Visual Minteq ®, which results can be found on Annex, the
species as ions Cu+2 and SO4-2 are present at higher percentage of total content when the
molecular concentration of the feed solution is low. In the case of a concentration of 1.6 mM
of CuSO4 (low molecular concentration) the presence of sulphate as ions SO4-2 was 83% of
total sulphate. With the solution of 158.3 mM of CuSO4 (high molecular concentration) only
31
40% of sulphate was dissociated as ions SO4-2, meaning that a high percentage of these ions
were present as CuSO4
(aq).
So this fact can be one reason for a higher rejection of sulphate
at higher concentration of the feed, as the molecule is more easily reject by the membrane.
Also, sulphate has a negative charge, and due to interactions between ions and membrane,
the increase of negative charge around the membrane can lead to an increase on rejection
levels of sulphate.
The results suggest that the membrane has the ability to achieve higher levels of rejection for
the compounds with positive charge than for the ones with negative charge. This ability is
closely related to electrostatic repulsion and will depend on the excess charge on the
membrane surface. According to the literature, Blandin et al. (2016), most TFC membranes
have a negative zeta potential, which is a parameter for identification of the charge on the
membrane surface, and that is also the case for Porifera membrane, as shown on Table 2.
However, the zeta potential varies with the pH of the feed solution and the zeta potential values
reported are usually near neutral pH. On the study of Lu et al. (2014) a graph, which is
presented in the Annex, shows the relation of pH feed solution and zeta potential for a TFC
membrane from Oasys Water (Oasys Water Inc, Boston, MA). This membrane has a similar
zeta potential as Porifera membrane at pH 7.0, both around -14 mV, so a similar variance with
pH is expected. At the pH which experiments were carried out (i.e. 3.5) the zeta potential of
Oasys membrane is 0, as 3.5 is the turning point where the membrane starts to have more
positive charge on its surface. So, it is believed that the same happened for Porifera
membrane, and that the turning point is between pH 4.0 and 3.5, resulting that at pH 3.5 the
membrane is positively charged. So, the higher rejection of positively charged contaminants
is attributed to electrostatic repulsion between the membrane and compound, and the lower
rejection of negatively charged ones is explained by electric attraction between the membrane
and them.
According to Jeffrey R. et al. (2006) the rejection of ions in forward osmosis by a cellulose
acetate membrane can increase with higher water flux, since there is a “dilution effect” when
the water flux is increased without increasing the salt flux. The results of this experiments do
not show clear relation between a higher flux and a higher rejection. It does happen for
sulphate and later on it will be shown for NH4+ and HCO3-, but in all cases the increase in
rejection with a higher flux it is believe to be the result of charge interaction among ions across
the membrane and with the membrane itself. Also, the type of membrane cited on the work of
Jeffrey R. et al. (2006) is not the same as used on this work, so it might be the reason for a
different type of transport. More on this matter will be discussed on section 4.4.1.
32
Other metals are expected to reach, moreover, same levels of rejection as copper, because
the similarity of behaviour due to influence of initial ions concentrations, as they also have
positive charge and the affinity for the membrane should be about the same.
4.3.4
Mass balance analysis of experiments with synthetic solutions
In order to see if the total mass present in the system was maintained during the experiment,
mass balance calculations were made for copper and sulphate ions. Results are shown on
Table 6. The error E (%) showed on the table was calculated as:
𝐸 (%) =
|(𝑇𝑜𝑡𝑎𝑙 𝑓𝑖𝑛𝑎𝑙 𝑐𝑜𝑛𝑡𝑒𝑛𝑡− (𝐹𝑒𝑒𝑑 𝑖𝑛𝑖𝑡𝑖𝑎𝑙+𝐷𝑟𝑎𝑤 𝑖𝑛𝑖𝑡𝑖𝑎𝑙))|
∗ 100
(𝐹𝑒𝑒𝑑 𝑖𝑛𝑖𝑡𝑖𝑎𝑙+𝐷𝑟𝑎𝑤 𝑖𝑛𝑖𝑡𝑖𝑎𝑙)
Equation X
where Feed initial (mg) is the mass on the feed side on the beginning of the experiment, which
was obtained by multiplying the concentration by the volume. All other masses were obtained
in the same way. Draw initial (mg) is the mass on the draw side when the experiment start,
which was defined as zero. Total final content (mg) is the sum of feed final and draw final
mass, measured in the end of the experiment.
Table 6. A) Mass balance for copper, all quantities in mg. B) Mass balance for sulphate, all quantities in mg.
Experiments done with synthetic feed solution of CuSO4 and draw solution of NaCl, with varying
concentrations. Total final content is the sum of feed final with draw final.
Copper mass balance
A)
Experiment
HTIa
Error
Feed initial
Feed final
Draw
Draw
Total final
(mg)
(mg)
initial (mg)
final (mg)
content (mg)
(%)
39.8 ± 0.5
38.7 ± 0.2
0.0 ± 0.0
0.4 ± 0.1
39.1± 0.3
1.7
a
Porifera 1
37.7 ± 0.2
41.2 ± 1.0
0.0 ± 0.0
0.2 ± 0.0
41.4 ± 1.0
9.9
Porifera 2b
37.0 ± 0.0
40.1 ± 0.9
0.0 ± 0.0
0.2 ± 0.0
40.3 ± 0.9
8.9
Porifera 3c
370.5 ± 0.4
387.5 ± 0.6
0.0 ± 0.0
2.8 ± 0.9
390.3 ± 0.5
5.3
Porifera 4d
3706.0 ± 6.0
3921.7 ± 21.9 0.0 ± 0.0
37.8 ± 5.6
3959.4 ± 24.2
6.8
33
Sulphate mass balance
B)
Experiment
Feed initial
Feed final
(mg)
(mg)
HTIa
63.6 ± 0.8
53.3 ± 0.6
Porifera 1a
64.8 ± 0.4
Porifera 2b
Draw
Error
Draw final
Total final
(mg)
content (mg)
0.0 ± 0.0
4.9 ± 0.4
58.1 ± 1.0
8.6
60.7 ± 6.5
0.0 ± 0.0
3.6 ± 0.6
64.3 ± 7.1
0.8
63.6 ± 0.0
68.5 ± 6.8
0.0 ± 0.0
3.7 ± 0.5
72.2 ± 7.3
13.4
Porifera 3
653.1 ± 0.8
728.3.5 ± 17.1
0.0 ± 0.0
29.1 ± 4.2
757.4 ± 17.9
13.7
Porifera 4d
7019.8 ± 12.3
6976.6 ± 276.1
0.0 ± 0.0
125.6 ± 12.1
7102.2 ± 269.7
1.2
c
initial
(mg)
a
Feed solution of 1.6 mM and draw solution of 2 M
Feed solution of 1.6 mM and draw solution of 1 M
c Feed solution of 15.8 mM and draw solution of 1 M
d Feed solution of 158.3 mM and draw solution of 1 M
b
Table 6 show that most errors were below 10%, and are considered acceptable for this study.
Some errors for the mass balance of sulphate are a bit higher than 10%, and that can be due
to different methods used to measure the concentrations on the draw and feed side. It was
not possible to do ion chromatography on the feed side because the concentration of sulphate
was too high.
4.3.5
Model of ions transport
A model of ions transport was idealized with the results obtained on the previous section. Later
on, in section 4.4.1, it will be discussed about the nature of the ions transport on forward
osmosis technology. For now, it was assumed that the transport occurs mainly due to diffusion
and convection played a minor role. So using Fick’s law of diffusion (Equation I) it was possible
to calculate the diffusion coefficient (D) for copper. Results are show on Table 7. The copper
flux was calculated using Equation VIII, the length Δx is the width of the membrane, and the
difference of concentration is the concentration on the feed side minus the concentration on
the draw side, which were obtained by directly measurements. The error was calculated as
the difference of each value from the average value.
34
(%)
Table 7. Model transport for copper diffusion. The diffusion coefficient (D) was calculated in order to
make a model of transport for copper ions. Error was calculated as difference from the average value.
Feed
Copper flux -
Concentration
Membrane
Diffusion
Error
concentration
Js (mol/m2.s)
difference - ΔC
width - Δx
coefficient
(%)
(µm)
2
3
(mol/m )
(m /s)
CuSO4 1.6 mM
5.10*10(-6)
3.50
45
-6.73*10(-11)
3.25
CuSO4 15.8 mM
5.84*10(-5)
33.77
45
-6.53*10(-11)
0.49
CuSO4 158.3 mM
5.13*10(-4)
342.99
45
-6.27*10(-11)
3.74
-6.52*10(-11)
2.49
Average
It is interesting that even with 3 totally different concentrations the diffusion coefficient was
maintained and the difference among the values calculated is minimal. That shows that the
transport is really diffusive, and occurs in the same rate independent on the initial
concentration of ions on the feed side. Of course this value of diffusion coefficient is only valid
for the TFC membrane of Porifera, and unfortunately no other papers were found in order to
compare values.
This is the behaviour of transport found for copper, but it is likely that other metals have a
similar behaviour. Other metals are expected to have similar values of diffusion coefficient.
Unfortunately, this diffusion coefficient found for copper could not be validate with results from
acid mine drainage experiment, as the process of rejection in that case involves interaction of
more ions present in the solution, and that will alter the constant.
4.3.6
Forward ions rejection with acid mine drainage
After experiments conducted with copper sulphate, the same setup used before and shown
on Figure 6, was used to perform an experiment with a real sample of acid mine drainage.
Levels of rejection were calculated for all the components that were identified on the
characterization of the sample and results are show in Figure 12.
35
Figure 12. Levels of rejection for each component identified on the AMD sample. Experiments
performed with NaCl at 1M of draw solution. Scale starting at 50% of rejection. Error bars represent
one standard deviation
As we can see, rejection of metals is extremely high, with most average values above 98%.
Besides, it is remarkable that for most compounds, the standard deviation is quite low,
showing a good reproducibility of the experiment. The metal chromium (Cr) is the only one
with a rejection lower than 90%, but this could have been the result of a very low concentration
on the draw side, below detection limits. Indeed, initial quantities were already very low on the
feed side. Concentration of iron (Fe) on the draw side was also lower than detection limits.
However, as initial concentrations were considerable higher than chromium, rejection could
be determined using the detection limit and it actually represents one of the highest rejections
of the series (99.97%).
Copper rejection in this experiment is lower than the ones found with copper sulphate synthetic
solutions. This can be due to molecular and charge interactions among ions present in the
AMD and enhanced ions exchange mechanism with the draw solution.
Results of the sulphate mass balance were not consistent in this experiment and because of
that are not reported.
Most traditional methods for removal of metals from water present high variability of levels of
removal, because a lot of factors can impact on it (Li et al., 2014). In the other hand,
membranes processes are more stable, as they are not dependent on chemical reaction and
because of that have more similar values of rejection. Li et al. (2014) show in their study a
comparison of levels of rejection and one method that presents less variation high levels of
36
removal is bioretention/biofiltration. The range of rejection goes from 43% to 97% for copper,
zinc and lead. It is remarkable that a membrane process like FO can provided rejection for a
broader range of metals at rates higher than 98% reaching up to 99.9% of rejection.
Another interesting comparison is made with values reported by Al-Zoubi et al. (2010) for
rejection of acid mine drainage components by NF and RO membranes. Both have presented
high rejection levels for metals (Cu, Ca, Fe, Mg, Mn and Al) reaching more than 97% with a
similar water flux of the present study. These results show the suitability of FO for the treatment
of acid mine drainage and that this technology can compete with the more traditional
technologies used up to date for treatment of this residue.
4.3.7
Mass balance analysis of experiment with acid mine drainage
The mass balance was also done for all the compounds which were evaluated for rejection.
Table 8 show the results. Error was calculated according to Equation X.
Table 8. Mass balance from AMD experiment, all metals considered in this study. Total final content is the sum of
feed final with draw final.
Metals mass balance
Metal
Feed initial
Feed final
(mg)
(mg)
Aluminium
108.3 ± 0.1
89.1 ± 1.4
Calcium
115.7 ± 0.1
Chromium
Draw
Error
Draw final
Total final
(mg)
content (mg)
0.0 ± 0.0
0.5 ± 0.1
89.6 ± 1.3
17.27
91.9 ± 1.9
0.0 ± 0.0
0.6 ± 0.1
92.5 ± 1.8
20.05
1.4*10-2± 0.0
1.5*10-2± 0.0
0.0 ± 0.0
2.2*10-3± 0.0
1.7*10-2± 0.0
18.37
Cobalt
0.9 ± 0.0
0.7 ± 0.0
0.0 ± 0.0
0.0 ± 0.0
0.7 ± 0.0
13.85
Copper
227.3 ± 0.1
191.7± 3.1
0.0 ± 0.0
2.7 ± 0.3
194.4 ± 2.8
14.51
Iron
5.0 ± 0.0
3.0 ± 0.1
0.0 ± 0.0
0.0 ± 0.0
3.0 ± 0.1
39.06
Magnesium
161.2 ± 0.1
105.8 ± 1.3
0.0 ± .0
1.2 ± 0.2
107.0 ± 1.4
33.59
Manganese
75.0 ± 0.0
58.7 ± 1.2
0.0 ± 0.0
0.2 ± 0.0
58.9 ± 1.2
21.47
Silicon
5.7 ± 0.0
4.1 ± 0.3
0.0 ± 0.0
0.1 ± 0.0
4.2 ± 0.3
25.82
Zinc
25.3 ± 0.0
30.6 ± 0.8
0.0 ± 0.0
0.1 ± 0.0
30.7 ± 0.8
21.41
initial
(mg)
(%)
In this experiment with acid mine drainage, the majority of errors was due to less content by
the end of experiment than originally, meaning that there was some loss of mass during the
experiment. Even though this loss is relevant, it does not invalidate the results obtained and it
can be explained for two reasons: the first is that the metals could have precipitate during
experiment, due to reactions and changes of pH. The second is that there were sediments on
the AMD effluent, which would deposit after some time. So, there was deposition in the storage
37
beaker and inside the module, and a large part of these sediments could not be recovered for
analysis, since the only way to extract them was by adding water and that would change
concentrations. These sediments could have been bonded with some ions and that would be
a reason for a lower concentrations of ions by the end of the experiment.
4.3.8
Reverse diffusion with NaCl draw solution
Measurements of sodium (Na+) and chloride (Cl-) ions were made on the feed side of the
module in order to quantify the reverse flux of these ions. The reverse diffusion of ions from
the draw solution to the feed solution is not desired, because in long time operation it means
the loss of draw solutes. This represents a need of addition of the compound to the setup,
which will have a cost. Experiments conditions were the same as the ones performed for
metals rejection. The results are shown on Figure 13.
Figure 13. Reverse diffusion of ions Na+ and Cl-, classified by the initial concentrations used on the
feed side and on the real sample of acid mine drainage (AMD). The draw solution was NaCl with
concentration of 1 M and errors bars show one standard deviation.
Sodium and chloride diffusion have very similar values on the experiments done with synthetic
solutions, but with the AMD experiment chloride reverse diffusion reaches values almost two
times higher than sodium. The higher diffusion of chloride is attributed to the negative charge
of the compound being more attracted by the membrane, with the mechanism of electrostatic
repulsion working as explained before. The lower diffusion of sodium in the AMD experiment
is attributed to the fact that in the feed solution there were a lot of positively charged
compounds, what prevented more positive charges passing to the feed side, following the
principle of electroneutrality. So, the ions exchange mechanism did not enhance the reverse
diffusion of sodium, and then, the main mechanism for diffusion was the difference in
concentration on both sides of the membrane only. Also, it is remarkable that there is a
38
considerable variation on the data set, as shown by the standard deviation bars, implying that
the reverse diffusion can present a different behaviour even among similar conditions.
4.4 Rejection experiments with NH3-CO2 draw solution
Following the proposed objectives of this work, the experiments were repeated with NH3-CO2
solution. First experiment was made with 1.6 mM of CuSO4 on the feed side and although
there was change of colour on both solutions, the experiment could be carried out without any
problems and was complete successfully. However, the chemicals reactions occurring on both
feed and draw were intriguing. The feed side acquired a strong intense blue coloration. A
similar colour also appeared on the draw side, just less intense. A complex can be naturally
formed on the presence of ions NH4+, SO4-2 and Cu+2. This complex is known as
Tetraamminecopper (II) sulfate - [Cu(NH3)4]SO4, which has the unique characteristic of intense
blue colour. Complex was formed on the draw solution as a result of the transport of Cu+2 and
SO4-2. On the other hand, as the reverse diffusion of ions NH4+ also occurs, same complex
was also formed on the feed side.
The next experiment was carried with 15.8 mM of CuSO4 and at some point, on the feed side,
a blueish colour with milky aspect started to appear. This was the result of the formation of
precipitated on the feed solution, which deposited on the bottom of the module. Moreover,
again the intense blue colour on the draw appeared, as seen on Figure 14.
Figure 14. Experiment of 15.8 mM of CuSO4 on the feed and draw solution of NH 3-CO2 with
concentration of 1 M.
The precipitate was filtered and dried, in order to prepare for a rough analysis and FT-IR. The
precipitate was diluted in deionized water and hydrochloric acid (HCl) was added to the
solution in order to observe the reaction. Bubbles were observed (formation of CO2), so it was
concluded that precipitates were carbonates. But in other to analyse if other functional groups
were present, a FT-IR (Agilent, Cary 630) analysis was carried and results are shown on
39
Annex. The main conclusions withdrawn from the analysis was that there was presence of
functional group OH on the compound and possibility of carbonate and sulphate.
Next, a Thermogravimetric analysis (TGA) was carried out and the obtained curve was
compared with literature for basic copper carbonate, with formula: [Cu2(OH)2CO3] (copper
carbonate hydroxide) (Frost et al., 2002; Shaheen and Maksod, 2009). Results are shown on
Annex and the curve obtained for the precipitate is very similar to the ones present on the
paper. At the same time SEM-EDX analysis was made with the precipitate. Element
composition is shown on Table 9 and percentage of each element is compared with copper
carbonate hydroxide. Hydrogen cannot be detected on SEM analysis, due to its small
molecular weight. For this reason, the composition of the sample is lacking the percentage of
hydrogen. Also, this is a semi quantitative analysis, so it is not a precise percentage and will
not sum up to 100%, but it is a good approximation.
Table 9. Element composition of precipitate sample measured with SEM-EDX analysis and the
correspondent standard deviation. Comparison of percentages based on the chemical formula of
Cu2(OH)2CO3
Element
Precipitate (%)
Cu2(OH)2CO3 (%)
Carbon
4.77 ± 0.59
5.43
Oxygen
11.68 ± 1.46
5.43
Copper
60.59 ± 0.32
57.48
With all these results in hand we can say that there was formation of copper carbonate
hydroxide [Cu2(OH)2CO3]. At first this precipitate was seen as an improvement of the initial
goals of the project, since besides recovering the water we would already be precipitating the
metal on the feed, leaving it in a more stable state. However, it was noticed that it would have
an impact on the measurements of concentration of metals in solution on the feed side and
that would impact on the calculation of rejection.
Following, the next experiment was done with AMD sample on the feed. Unfortunately, the
experiment using NH3-CO2 as draw solution and acid mine drainage as feed could not be
completed, as precipitates were formed on the membrane surface at a very initial stage of the
experiment, blocking the flow of water through the membrane. The experiment was done in
triplicate and later repeated and exactly the same happened, leaving no doubt that with these
conditions the experiment could not be carried out. Figure 15 shows three membranes after
the experiment, the precipitate on its surface and SEM-EDX analysis of the membrane
surface.
40
A)
B)
C)
Figure 15. A) Membranes after experiment with AMD as feed and draw of NH3-CO2 at 1 M. B) Detail of precipitation
on the membrane by SEM image, scale of 100 mm and C) Percentage of compounds on membrane surface after
same experiments. Average of 3 membranes, measurements made by SEM-EDX
The analysis revealed a very diverse composition, with some compounds presenting
substantial differences on each measurement, as shown on Table 10. That happens due to
the diverse composition of the feed, containing all sorts of elements. It is clear that precipitation
occurred on the membrane surface rather than in the feed solution because the acid mine
drainage has a much more diverse composition than only the CuSO4 solution. Maybe one
specific ion is responsible for the precipitation, but it is hard to know without specific
experiments. Analysing the composition of the precipitated salts it is remarkable the amount
of oxygen present. It is very likely that sources of oxygen are mainly sulphate and carbonates.
Carbonates can precipitate quite easily and with reverse diffusion of ions bicarbonate and
ammonium the pH would rise and favour the reaction. Also, the amount of ions Aluminium is
more than double of its presence on the initial AMD sample solution, which can be an
indication that this element is playing a role on the precipitation as well.
41
Table 10. Composition of membrane surface after experiments with AMD as feed solution and draw
of NH3-CO2 at 1 M. Membranes were weighted and percentages of each element was found with
SEM-EDX analysis. Average values and correspondent standard deviation for measurements of 3
membranes are shown.
4.4.1
Element
Weight (mg)
Carbon
8.9 ± 0.6
Nitrogen
3.8 ± 1.4
Magnesium
1.0 ± 0.2
Oxygen
54.9 + 3.4
Aluminium
15.1 ± 1.6
Silicon
0.9 ± 0.1
Sulphur
6.8 ± 1.6
Chlorine
0.3 ± 0.2
Calcium
7.2 ± 1.4
Manganese
4.9 ± 1.5
Iron
0.8 ± 0.3
Copper
8.2 ± 1.4
Zinc
2.8 ± 0.7
Iodine
2.5 ± 1.7
Reverse diffusion with NH3-CO2 draw solution
Enhanced transport of NH4+ ions with TFC membranes was reported before with other
membrane fabricants (Jeffrey R. et al., 2006; Lu et al., 2014). For this reason, to quantify the
reverse diffusion of NH4+ is of great importance when studying a FO process which uses a
draw solution with this ion. Transport of HCO3- ions was also expected to happen, as it was
seen a higher transport of Cl- ions compared to Na+ on previous experiments done with NaCl.
The transport of these ions can have a big influence on the feed solution, as they have
potential to change the pH of the solution and as seen on the previous section they can react
with metals and precipitate on form of stable salts. These changes were observed visually on
the experiments of rejection but could not be quantified exactly, because there was formation
of precipitate and measurements would be affected. So, a new setup was made, as described
in 3.5. The purpose was to observe the migration of ions from draw solution to the feed
chamber. Equation VIII and Equation IX were used for the evaluation of the migration of ions.
Results are shown on Figure 16. Graph titles show the dominant species of each solution, but
ammonia was present as NH4+ and NH3, and carbon as HCO3- and CO3-2.
42
H C O 3- O N T H E F E E D
N H 4+ O N T H E F E E D
Feed:
0.004
DI Water
DI water
NaCl 0.1 M
180 min
360 min
0.003
0.003
0.002
0.001
0.002
0.001
0.000
0.000
0 min
180 min
0 min
360 min
N H 4 + O N T H E D R AW
H C O 3 - O N T H E D R AW
0.3
Feed:
DI Water
0.3
NaCl 0.1 M
0.2
Amount (mol)
Amount (mol)
Feed:
NaCl 0.1 M
Amount (mol)
Amount (mol)
0.004
0.1
0.0
Feed:
DI Water
NaCl 0.1 M
0.2
0.1
0.0
0 min
180 min
360 min
0 min
180 min
360 min
Figure 16. Measurements of amount of ions HCO3- and NH4+ (mol) on both sides of the membrane, according to time elapsed
of experiment.
As show on Figure 16, there is a tendency of both ammonium and bicarbonate ions to move
from the draw to the feed side, at a quite constant rate. Initial amount of HCO3- on the feed is
due to presence of carbon species even on deionized water.
It is remarkable that transport of ammonia and bicarbonate from the draw solution to the feed
side is enhanced on the presence of an electrolyte, in this case NaCl, even though water flux
is smaller due to a reduced difference of osmotic pressure. This makes it clear that there is a
mechanism of ion exchange on the process. It is also noticeable that the transport of NH4+
occurs at higher rates than HCO3-. This is an important fact, since on the measurements of
reverse diffusion of NaCl there was lower transport of the compound with positive charge. This
shows that transport of ammonia is not driven only by electrostatic repulsion. Arena et al.
(2014) attributed this different transport to two facts, the first is the mechanism of ion exchange
between the two sides of the membrane, that is based on the principle of electroneutrality.
43
That means that when a positive charge crosses the membrane from the draw to the feed, a
potential of diffusion is created. With this potential or a negative charge moves in the same
direction (draw to feed) or a positive charge will move on the opposite direction (feed to draw)
to balance the charges. The second reason is that the molecule of ammonia (NH3) is
uncharged and have similar characteristics to the water molecule, such as the size, being
polar and is able to form hydrogen bonds. All this allows ammonia to cross the membrane
more easily than other ions, as the membrane have difficulties to select this type of molecule.
Although, after reaching the feed side it can speciate as NH4+, creating the potential for
diffusion.
To have a better understanding of the ion exchange mechanism, the concentrations of ions
Na+ and Cl- were also measured on the draw side and the forward flux regarding these ions
could be calculated. Results comparing these solutes flux in mol/m2.h is presented on Figure
17.
Figure 17. Water flux (Jw) as parameter for comparison of experiments. Reverse ions flux of
bicarbonate and ammonium compared with sodium and chloride forward ions flux at 3h of
experiment. Errors bars represent one standard deviation.
Similar studies were performed by Arena et al. (2014) and Lu et al. (2014), with NH3-CO2 draw
solution at 2 M and feed solution of 0.2 M and 0.25 M respectively of NaCl. They propose a
similar model of transport of ions, dominated by a bidirectional diffusion of cations and anions.
Lu et al. (2014) and Arena et al. (2014) believe that ammonia (in form of NH3) cross the
membrane to the feed side more easily, due to molecule characteristics, and as it is not
charged at first does not change the electroneutrality. When it gets to the feed solution, which
is at a low pH, it speciate into ammonium ions (NH4+) and then creates the potential for other
charge movements. This potential can be fulfilled by a negative charge going on the same
direction (draw to feed) or by a positive charge going on the opposite direction (feed to draw).
44
As in these experiments, the only positive charge present on the feed is Na+ and the negative
charge on the draw is HCO3-, these ions will start to diffuse to the opposite side of the
membrane. This crossing will create another potential, now or for a positive charge to go from
the draw to the feed (in this case NH4+ again) or for a negative charge to go from the feed to
the draw, which in this case will be Cl-. And as so the process will be continued, enhancing
transport of ions across the membrane.
Figure 18 illustrate the mechanism.
Figure 18. Ion exchange mechanism, adapted from Lu et al. (2014) for a TFC membrane for FO. FS
is the feed solution, AL is the membrane active layer, SL is the membrane support layer and DS is
the draw solution A) Initial diffusion of NH 3. (B) Potential diffusions for solution charge balance. (C)
Ion exchange and bidirectional diffusion of ions phenomenon.
In the case of the present study, it is believed that a similar behaviour was observed. The only
difference is that in the case of Na+ transport to the draw side, electrostatic repulsion happens,
so the transport occurs at low rates. Because of that, the transport of HCO3- to the feed solution
is enhanced, compensating the lower transport of Na+. Transport of Cl- is also enhanced with
this mechanism, allied with attraction to the membrane. So, the difference on the mechanism
cited by Lu et al. (2014) is that in their case the membrane attracts the positive charge, and in
our work it repels.
It is remarkable that when comparing the values of transport in those works with the ones
obtained on the present study, the membrane from Porifera seems to be able to have
considerable lower transport of all the ions involved. For instance, Lu et al. (2014) reported
values of NH4+ transport between 5.5 and 1.5 mol/m2.h, depending on the membrane used
and pH of the feed solution. Arena et al. (2014) reported values for the same ion between 0.9
and 0.75 mol/m2.h, while in this work the value does not even reach 0.3 mol/m2.h. Similar
proportions of difference are found for HCO3- and Na+, just not for Cl-, which the values are
quite similar, around 0.2 and 0.1 mol/m2.h. This finding is of great importance, showing that
the membrane from Porifera has good potential for application with this draw solution.
45
4.5 Recommendations
As recommendation for further studies, we would suggest to keep investigating why the
precipitation of salts happens on the membrane surface when using the NH3-CO2 draw
solution. That would be relevant for choosing the measures that could be applied for solving
this issue and allow the use of this technology for recovery of water from AMD. According to
the results shown on Figure 15 and compared to the initial composition of acid mine drainage
(Table 4), it was noticed a relevant increment on the percentage of Aluminum in total
composition of the precipitates on membrane surface. That could indicate that ions Al+3 are
interacting with the membrane and causing the precipitation on the surface. In order to prove
that, experiments should be made with aluminum solutions alone and observe the results. If
the precipitation does not happen on the membrane surface, other ions such as Zinc and
Manganese, for instance, should be added to the solution, one by one, until finding which one
is responsible for the precipitation on the membrane.
If a solution for the problem is found, and the NH3-CO2 solution can be used on the forward
osmosis process, it would be interesting to study the recovery of NH3 and CO2 by solar
distillation. The resultant water could be analyzed and compared with guidelines for reuse of
water, aiming on irrigation purposes or within the mine industry itself.
Improvement of the hydrodynamics conditions of this experiments should be investigated as
well, such as cross flow configuration and increased shear stress on membrane surface.
In the scheme of the proposed project showed on Figure 3 it could be inserted a filtration step,
before the FO process. That would be interesting to remove sediments from the acid mine
drainage, since they can impact on the performance of the forward osmosis and adsorb metal
ions, leading to mistakes on calculation of rejection levels.
Another line of research would be to test the forward osmosis process with other draw
solutions. One type that have appeared recently on papers and studies are known as hydro
acids complexes (Cui et al., 2014). According to Ge and Chung (2013) these are materials
composed of metals and ligands, that can be chosen for a better suitability of the desired
process. Their advantages are an expanded configuration of the structure, which leads to a
small reverse salt flux, and the formation of multi-ionic species in a water solution, leading to
a great osmotic potential (Cui et al., 2014; Ge and Chung, 2013). As the recovery step, the
same study reported indicate possibility of nanofiltration and membrane distillation. In the case
of South American countries, the low cost and simple operation is of great importance,
46
otherwise it could be hard to implement a technology that does not show a great benefit for
the mining industry.
As rejection levels on the experiments made with NaCl draw solution were really high, it would
be interesting to take the research to a higher level, doing experiments at pilot scale. It would
be important to consider a higher membrane area and longer time of operation, as these
variables can have an impact on rejection levels. Besides, in pilot scale, other parameters can
be controlled, such as pressure, temperature, and water velocity.
47
5
CONCLUSIONS
Forward osmosis is a technology suitable for recovering water from acid mine drainage, since
it presents high levels of rejection for all evaluated compounds and a competitive water flux is
reached. The proposed technology with NH3-CO2 solution needs improvements, as with the
conditions tested it was not possible to perform the water recovery. Recommendations that
can be followed in order to solve the issue of precipitation of salts on the membrane surface
are cited on the section above.
A wide range of compounds was covered during this study. All presented a very high
percentage of rejection by the tested FO membrane. This indicates that a membrane process
can be more adequate than other alternatives for removal of metals when the goal is to recover
water from a contaminated feed.
In comparison to other membrane technologies that are more stablished for acid mine
drainage treatment, forward osmosis performed very well, reaching comparable or higher
levels of rejection, showing its efficiency and suitability for the case. Water fluxes reached by
the technology are lower than many used on applications of nanofiltration and reverse
osmosis, but the benefit of not using pressure on the process can have a great impact on
lowering operational cost and making it very attractive to implementation on developing
countries.
Although FO water flux is reduced when compared to RO or NF, the flux reached by the TFC
membrane from Porifera outperformed most published fluxes of forward osmosis membranes
in similar conditions. Even in experiments with AMD, the water fluxes were considered high
for FO membranes, and improvements of hydrodynamic conditions can lead to achieve better
performance yet.
During this work, the rejection of ions contained in AMD were evaluated, using a real sample.
This certainly adds value to the study, since a lot of other studies could only perform tests with
synthetic solution. Results showed remarkable differences of performance between a real
sample and the synthetic solutions, as a result of a much more complex composition.
It is also important to highlight that the proposed steps for water recovery in this project are in
accordance with sustainable principles. It is proposed a technology using alternative sources
of energy, that can recover water and prevent its extraction. Finally, the solution is treating
and removing a hazardous residue from the environment, which will be concentrated,
facilitating the final step of stabilization or metal recovery.
48
49
6
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7
ANNEX
7.1 Specification sheet of each membrane used for flux experiments, as provided by
the fabricant
7.1.1
Aquaporin Inside
54
7.1.2
Hydration Technology Innovations
55
56
7.1.3
Porifera Inc.
57
7.2 Speciation of ions and molecules according to software Visual Minteq ®
Figure 19. Percentage distribution of ionic and molecular species present in the feed synthetic
solution of CuSO4 at concentration of 1.6 mM by the end of experiment with 1M NaCl draw solution.
Conditions used was the measured pH average of 4.8, temperature of 20ºC and concentrations of
each component determined with specific analysis.
Figure 20. Percentage distribution of ionic and molecular species present in the feed synthetic
solution of CuSO4 at concentration of 158.3 mM by the end of experiment with 1M NaCl draw solution.
Conditions used was the measured pH average of 4.2, temperature of 20ºC and concentrations of
each component determined with specific analysis.
58
Figure 21. Percentage distribution of ionic and molecular species present in the draw solution of
NH3-CO2 by the end of experiment with feed solution of deionized water. Conditions used was the
measured pH average of 8.8, temperature of 20ºC and concentrations of each component
determined with specific analysis.
Figure 22. Percentage distribution of ionic and molecular species present in the draw solution of
NH3-CO2 by the end of experiment with feed solution of 0.1M of NaCl. Conditions used was the
measured pH average of 9.0, temperature of 20 ºC and concentrations of each component
determined with specific analysis.
59
7.3 Relation of pH and zeta potential
Figure 23. Relation of zeta potential of pristine and modified TFC membranes from Oasys
Water (Oasys Water Inc., Boston, MA) and pH of feed solution. pH adjusted with HCl and KOH,
measurements made at room temperature 23 ºC. Source: (Lu et al., 2014)
60
7.4 FT-IR analysis of precipitate formed during NH3-CO2 experiments
75
1098.717 51.842
817.609 228.046
80
1383.660 1038.471
85
1047.682 438.503
90
1498.217 469.153
%Transmittance
95
70
3900
3800
3700
3600
3500
3400
3300
3200
3100
3000
2900
2800
2700
2600
2500
2400 2300 2200
Wavenumber
2100
2000
1900
1800
1700
1600
1500
1400
1300
1200
1100
Figure 24. FT-IR analysis of precipitated formed during experiment of 15.8 mM concentration of CuSO 4 as feed and draw solution of NH3-CO2. Prior to analysis the
precipitated was washed with deionized water and filtrated.
61
875.160 263.623
fs3b_2016-05-25t12-02-15(1)
3315.927 4.831
100
7.5 Thermogravimetric analysis (TGA) of precipitate formed during NH3-CO2
experiments
A)
)
B)
)
Figure 25. A) Thermogravimetric analysis of malachite. Source: (Frost et al., 2002). B) Result of
thermogravimetric analysis of the unknown precipitate. Similarities in the curve can be observed,
such as percentage of weight loss and temperature of the loss.
62