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Refinery Produced Wastewater Treatment by PVDF Composite Hollow Fiber Membrane
E. Yuliwatia,b,* , A.F. Ismailb,c
a
Department of Industrial Engineering, Faculty of Engineering, Universitas Bina Darma, 30251 Palembang,
Indonesia
Tel:+62(711)515-579; Fax: +62(711)515-581
b
Advanced Membrane Technology Research Centre Universiti Teknologi Malaysia
c
Faculty of Petroleum and Renewable Energy Engineering,
Universiti Teknologi Malaysia, 81310 UTM, Skudai Johor, Malaysia
Tel. +60 (7) 553-5592; Fax: +60 (7) 558-1463
*Corresponding author: [email protected]
Abstract
The aim of this study is to investigate the effect of surface modified of PVDF membranes
by adding the hydrophilic additives for refinery produced wastewater treatment. This paper
presents the results of a research on direct clean water treatment using hollow fiber
ultrafiltration. The source of water is the synthetic refinery wastewater with mixed liquor
suspended solids (MLSS) concentration of 3 g/l. All experiments were conducted at 25oC and 0.5
bar absolute. The morphological and performance tests were conducted on PVDF ultrafiltration
membranes prepared from different additives concentrations. The cross sectional area of the
hollow fiber membranes was observed using a field emission scanning electron microscope
(FESEM). The surface wettability of porous membranes was determined by measurement of
contact angle. Mean pore size and surface porosity were calculated based on the permeate flux.
The results also indicated that the PVDF composite membranes with lower additives
concentration loading possessed smaller mean pore size, more apertures inside the membranes
with enhanced membrane hydrophilicity. The flux and rejection of refinery wastewater using
PVDF composite membrane achieved 140.82 L/m2h and 98.8 % , respectively.
Keywords: surface modified membrane; composite; inorganic additives; average pore size;
hydrophilicity.
1. Introduction
Waterborne outbreaks of enteric diseases are a major public health concern, yet monitoring
and identifying the disease-causing microorganism from water samples remain difficult.
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Produced water is by far the largest contaminated stream resulting from thermal heavy oil
recovery operations and its treatment and reuse is essential for the sustainability of oil sands
processing [1]. Organic contaminants in produced waters are toxic and corrosive leading to
environmental and operational problems. From an environmental sustainability and perspective,
it is necessary to recycle produced water and thus it must undergo proper treatment in order to
avoid potentially negative impacts on drinking water supplies and aquatic organisms [2,3]. From
an industrial standpoint, the different contaminants in the produced water may adversely affect
equipment leading to scaling and corrosion [4-6].
Many studies have been documented on the use of UF membranes for treating oily
wastewater [7-13]. These membranes were prepared from polymeric materials such as cellulose
acetate (CA), polysulfone (PSf), polyethersulfone (PES) incorporated with inorganic material
such as alumina (Al2O3) and titanium dioxide (TiO2). As these membranes were quite
hydrophilic and displayed relatively smaller pores, water which was free of oil or with reduced
oil content were recovered as permeate.
Polyvinylidene fluoride (PVDF) is one of the most extensively applied membrane material
in UF system due to its outstanding antioxidation activity, excellent chemical resistance and
thermal stability, highly organic selectivity, as well as good mechanical and membrane forming
properties. However, its hydrophobic nature, which often resulted in severe membrane fouling
and declined permeability, has been a barrier to its application in water and wastewater treatment
[14]. In general, PVDF shows a good solubility in many common organic solvents such as N,Ndimethylacetamide (DMAc), N,N-dimethylformamide (DMF), N-methyl pyrrolydone (NMP)
and dimethylsulfoxide (DMSO). As a semi-crystalline polymer, PVDF generally exhibits more
complicated phase separation behaviour than amorphous polymer [15]. These advantageous
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properties, coupled with its hydrophobicity, make it an outstanding membrane material
particularly for industrial wastewater treatment applications involving oily emulsion [16],
organic/water separations [17,18], gas absorption and stripping [19,20], and membrane
distillation [21,22]. Although its hydrophobicity is favorable in promoting the transport of the
organic component of an organic/water feed solution, the neat PVDF membrane is liable to be
contaminated and resulted in a dramatic decreased of membrane water flux [23]. Many attempts
have been carried out to improve the hydrophilicity of PVDF membranes through various
methods for instance such as physical blending, chemical grafting, and surface modifications
[24]. The effect of hydrophilic additives, i.e. LiCl and PVP, on the thermodynamic/kinetic
relations during the phase inversion process in the preparation of PVDF-based membranes was
investigated by Fontananova et al. [25].
In this study, the PVDF UF membranes have been fabricated by addition of LiCl.H2O and
TiO2 in various concentrations to modify membrane surface properties and filtration
performance. Surface wettability of membranes is usually expressed in terms of contact angle for
a liquid drop on the membrane surface to measure the tendency for liquid to wet of the
membrane surface. Pore size, porosity, and elemental composition analysis of the PVDF UF
membranes were investigated. The surface and inner structures of the sample membranes were
studied using field emission scanning electron microscope (FESEM) and energy dispersive x-ray
(EDX) apparatus. The pretreatment used phytoremediation is an emerging cleanup technology
for contaminated wastewater. The performance for refinery wastewater treatment was
characterized by pure water flux and rejection efficiency of refinery wastewater.
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2.
Experimental
2.1. Materials
Ultrafiltration membranes have been prepared using Kynar®740 PVDF polymer pellets
were purchased from Arkema Inc. Philadelphia, USA. The solvent N,N-dimethylacetamide
(DMAc, Aldrich Chemical) (Synthesis Grade, Merck, >99%) was used as polymer solvent
without further purification.
Lithium chloride monohydrate (LiOH.H2O) and nanoparticles
titanium dioxide (TiO2) were used as inorganic additives. Both chemical additives were
purchased from Sigma-Aldrich and used as received. Glycerol was purchased from MERCK
(Germany) and used as non solvent for post treatment of membrane. In all cases, tap water was
used as the external coagulation bath medium in the spinning process.
2.2. Preparation of PVDF spinning dopes
An amount of pre-dried (24 h oven dried at 50 oC) PVDF pellets was weighed and poured
into pre-weighed DMAc solvent. The mixture was stirred to ensure thorough wetting of polymer
pellets, prior to the addition of appropriate amounts of LiCl.H2O at 50 oC. TiO2 was then added
to the polymer dope mixtures which were continuously stirred for 48 h (IKA-20-W) at 500 rpm
until a homogenous solution was formed. The polymer solution was kept in a glass bottle and air
bubbles formed in the dope were removed using water aspirator for several hours. The fully
dissolved polymer solution was transferred to a stainless steel reservoir, allowed to stand and
degassed for 24 h at room temperature prior to spinning process. Solution viscosity was
measured using rheometer (Bohlin Instrument Ltd.) at various temperatures between 25 and 50
o
C.
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2.3. Membrane preparation
PVDF hollow fiber UF membranes were spun at room temperature by a dry-jet wet
spinning method. The spinning solutions were divided into two batches. First batch consisted of
different PVDF concentration ranging from 16 to 22 wt.%. The second one was prepared from
19 wt.% PVDF in DMAc at different TiO2 concentration (0, 5, 10, 15, 20 wt.%) and LiCl.H2O
was maintained at 5.2 wt % of the weight of PVDF, as shown in Table 1 respectively.
Table 1. Membrane composition
Sample
PVDF wt. %
TiO2 wt. %
LiCl.H2O wt. %
PTL-0
19
0
0.98
PTL-5
19
1
0.98
PTL-10
19
1.95
0.98
PTL-15
19
2.85
0.98
PTL-20
19
3.8
0.98
The hollow fiber spinning process by dry-jet wet phase inversion was explained elsewhere.
The detailed spinning parameters are listed in Table 2.
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Table 2. The detailed spinning condition
Dope extrusion rate (ml/min)
Bore fluid
Bore fluid flow rate (ml/min)
External coagulant
Air gap distance (cm)
Spinneret o.d./i.d. (mm)
Coagulation temperature (oC)
4.20
H2O
1.40
Tap water
1 cm
1.10/0.55
25
In general, the polymer solution was pressurized through spinneret with controlled extrusion
rate, while internal coagulant was adjusted at 1.4 ml/min. The hollow fiber emerged from the tip
of the spinneret was guided through the two water baths at a take up velocity 13.7 cm/s, carefully
adjusted to match free falling velocity before landed in a final collection bath to complete the
solidification process. The spun hollow fibers were immersed in the water bath for a period of 3
days, with daily change of the water, to remove the residual DMAc and the additives. The
hollow fibers were then post-treated using 10 wt.% glycerol aqueous solution as non solvent
exchange for 1 day in order to minimize fiber shrinkage and pores collapse. After the fibers were
dried for 3 days, they were ready for making hollow fiber test modules.
2.4. Membrane characterizations
The morphology of the membrane was observed by field emission scanning electron
microscope (FESEM) (JEOL JSM-6700F). The FESEM micrographs were taken at certain
magnifications. It produced photographs at the analytical working distance of 10 nm. Surface
composition analysis was carried out on energy dispersive x-ray (EDX) (JEOL JSM-6380LA).
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The static contact angle of membrane was measured by the sessile drop method using a
DropMeter A-100 contact angle system (Maist Vision Inspection & Measurement Co. Ltd.) to
characterize the membrane wetting behaviour. A water droplet at 3 µL was deposited on the dry
membrane using a microsyringe. A microscope with a long working distance 6.5x objectives was
used to capture micrographs.
2.5 Permeation flux and rejection of refinery wastewater measurements
The permeation flux and rejection of PVDF hollow fiber membranes were measured by
submerged ultrafiltration experimental equipment as shown in Fig. 1. An in-house produced Ushape hollow fiber bundle, with a filtration area of 11.23 dm2, was submerged in prepared
suspension in membrane reservoir with volume of 14 L. A cross-flow stream was produced by
air bubbling generated by a diffuser situated underneath the submerged membrane module for
mechanical cleaning of the membrane bundle. The air bubbling flow rates per unit projection
membrane area was set constantly at 1.8 L/min in order to maintain proper turbulence. The
filtration pressure was supplied by a vacuum pump. Permeate flow rates were continually
recorded using flow meter respectively.
The rejection test was carried out with distilled water and synthetic refinery wastewater
with mixed liquor suspended solid (MLSS) concentration of 3 g/L. All experiments were
conducted at 25 OC. Firstly, the pure water permeation flux (Jw) was measured using prepared
PVDF submerged membrane under reduced pressure (0.5 bar absolute) on the permeate side.
Finally, the permeation measurement with refinery wastewater (JR) and rejection (R) were
measured under reduced pressure on the permeate side.
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Figure 1. Schematic diagram of submerged ultrafiltration
Pure water flux was measured after the flux was steady, then calculated as
F=
V
At
(1)
where F is the pure water flux (L/m2 h), V is the permeate volume (L), A is the membrane
surface area (m2), and t is the time (h).
Rejection (R) was characterized with a synthetic refinery wastewater after the membrane
was previously filtered with pure water until the flux became steady. The synthetic refinery
wastewater was an in-house synthesized and consisted of fresh water, hydraulic oil, diesel fuel,
surfactant, and carbon black in proper composition, based on mixed liquor suspended solid
(MLSS) measurement 3 g/L and UV wavelength 2.6 cm-1. It was calculated as
R = (1-
cp
cf
) x 100
(2)
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where R is the rejection ultrafiltration process (%), cp is the concentration of the permeate (%)
and cf is the concentration of the feed (%).
3.
Results and discussions
3.1. Effect of additives concentration on the structural and physical properties of PVDF
membranes
3.1.1. Morphological studies of PVDF membranes
Fig. 2 shows the FESEM micrographs of the PVDF ultrafiltration membranes prepared
using different concentrations of TiO2 at a constant concentration of LiCl.H2O. Improvement of
membrane morphology occurs with small amount addition of TiO2 nanoparticles. TiO2
nanoparticles have high specific areas and good hydrophilicity, which will affect the mass
transfer during the spinning process.
The cross-section morphology of PVDF UF membranes indicated that the finger-like
macrovoids extended from both inside and outside of the membranes and caused suppression in
an intermediate spongy substructure at lower TiO2 concentration, as illustrated in Fig. 2(b-c).
However, with further increasing TiO2 concentration it was observed that the intermediate and
the outer and inner membrane layer have been changed significantly. The long finger-like
structure became shorter at the both outer and inner layer of membrane and the intermediate
layer presented a thicker sponge-like structure, whereas in Fig. 2(d-e). These results can be
explained on the basis of the delayed liquid-liquid demixing process, which could be attributed
to the higher viscosity and lower phase-inversion rate of the spinning dope. Therefore, the kinetic
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hindrance due to viscosity overcomes the thermodynamic factor and thus resulted in the
formation of thick sponge-like layer. Moreover, formation of few drop cavities in the sponge-like
layer was also associated to the slow solidification process during phase inversion.
The higher TiO2 concentration induced also an aggregate phenomenon and absorbed into
the substructure of PVDF UF membrane. Those aggregates blocked the pores and caused the
decreased of the average pore size. This result was attributed to the porous structure and possible
hydrophilicity of the TiO2 nanoparticles. It indicates that hydrophilicity of nanoparticles TiO2
was directly correlated with porosity and might be responsible for the higher liquid uptake.
(a)
(b)
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(c)
(d)
(e)
Figure 2. FESEM images of prepared PVDF membranes with maintained LiCl.H2O of 0.98 wt.%
and TiO2 of a) 0 b) 1c)1.95 d) 2.85 e) 3.8 wt.%.
3.1.2. Porosity and surface wettability studies of PVDF UF membranes
The membranes were characterized in terms of surface wettability measurement. The
results are shown in Fig. 3. Surface wettability is one of the important membranes properties
which could affect the flux and antifouling ability of membranes. As presented in Fig. 3, it was
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found that contact angle of PVDF UF membranes decreased significantly with increasing TiO2
concentration above 1.95 wt.%, then increased with further increasing TiO2 content. The
decreased contact angle indicates the decrease in effective hydrophilic area and hydroxyl group
number. The hydrophilic TiO2 particles, which contained hydroxyl groups and adsorbed on the
membrane surface, were responsible for increased hydrophilicity. Thus, it might be considered
that hydrophilicity was the most important factor among the membrane performances [26].
Contact angle (o)
100
80
60
40
20
0
0
1
1.95
2.85
3.8
TiO2 content (wt. %)
Figure 3. Contact angle with water of the PVDF composite membrane and standard
deviation on measurement in different regions of the membrane surface
3.2. Effect of additives concentration on the performance of PVDF membranes
As shown in Fig. 4, PTL-10 membranes showed the flux peak value of 140.82 L/m2 h
when TiO2 concentration were at 1.95 wt.% and decreased with further increasing TiO2
concentration. The values of rejection demonstrated the similar trend to the flux, which increased
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to the peak value 98.8 % at 1.95 wt.% TiO2 concentration then decreased with further increasing
TiO2 concentration. The hydrophilicity TiO2 particles on the membrane surface reduced the
interaction between contaminants and the membrane surface. The increased membrane
hydrophilicity and membrane pore size with lower TiO2 concentration (≤1.95 wt.%) could attract
water molecules inside the composite membrane; facilitated their penetration through the
membrane, enhancing the flux and rejection. However, higher TiO2 concentration (> 1.95 wt.%)
resulted in the formation of a highly viscous dope. This slowed down the formation process of
PVDF UF membranes and produced a compact sublayer, as shown in Fig. 4. Moreover, the
enhanced flux and rejection values were also developed due to air bubbling flow rate per unit
projection membrane area. This was set constantly in order to maintain turbulence, increase mass
transfer coefficient, decrease oil droplets and suspended solid concentrating on the membrane
surface, and weakens the effect of polarization.
100
140
80
100
60
80
60
40
Jw
40
R, (JR /Jw) %
J (L/m 2 h)
120
JR
20
R
20
JR/ Jw
0
0
0.95
1.95
2.8
0
3.85
TiO2 wt %
Figure 4. Effect of the TiO2 concentration on permeation flux and rejection
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The antifouling properties of PVDF ultrafiltration membranes could be evaluated by the
ratio of refinery wastewater flux (JR) and pure water flux (JW). For the higher antifouling
submerged UF membrane, the feed of refinery wastewater would cause a small flux loss and the
ratio (JR/JW) would be higher. Fig. 4 also shows that initially, the ratio (JR/JW) is increased
sharply and reached the highest peak of 1.95 wt.% TiO2 concentration. However, higher TiO2
concentration, namely, 2.8 and 3.95 wt.% TiO2, resulted the decreased value of the ratio (JR/JW).
The membrane surface hydrophilicity was improved significantly and reduced the interaction
between the contaminants and the membrane surface, then effectively improved the antifouling
properties. However, the ratio (JR/JW) of PVDF UF membranes decreased slightly at >1.95 wt.%
TiO2 concentration until the fouling phenomenon disappeared at 3.85 wt.% TiO2 concentration.
As the hydrophilic additives content in the membrane increased the decrement of ratio (J R/JW)
became gradually less, but it still indicated that the antifouling properties of the PVDF UF
membranes were promising by adding of the inorganic additives LiCl.H2O and TiO2.
4. Conclusions
PVDF UF membranes were fabricated via a dry-jet wet spinning method. Various
concentrations of TiO2 at constant value of LiCl.H2O were used as inorganic additives in the
spinning dopes in order to improve the phase-inversion rate and provide porous asymmetric
membranes with advanced structure for refinery produced wastewater treatment. Several
characterizations and measurement techniques such as membrane structure, surface wettability,
porosity, average pore size, and permeability were utilized to evaluate fine structural details of
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the membrane and membrane performance. Refinery produced wastewater filtration was
conducted through in-house prepared PVDF hollow fiber ultrafiltration membranes. FESEM
analysis indicated that 19 wt.% PVDF concentration had suppressed both inner and outer
membrane surface finger-like macrovoids with slightly denser skin layer which decreased mass
transport resistance. Addition of 1.95 wt.% TiO2 nanoparticles resulted in smaller nanoparticles
which in turn achieved higher hydrophilicity, small pore size, and high porosity. Permeability
test results illustrated that LiCl.H2O and TiO2 nanoparticles affected the PVDF UF membranes
performance remarkably. Significantly higher flux and rejection of refinery wastewater were
observed. Furthermore, the achieved filtration performance was indicated that approximately
18.3 % flux reduction was gradually occurred within the first 18 h before became constant until
the end of the filtration operation.
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