environments
Article
Bioremoval of Phenol from Aqueous Solutions Using
Native Caribbean Seaweed
Abel E. Navarro 1, *, Anibal Hernandez-Vega 2 , Md Emran Masud 1 , Loretta M. Roberson 3
and Liz M. Diaz-Vázquez 2
1
2
3
*
Science Department, Borough of Manhattan Community College, The City University of New York,
New York, NY 10007, USA; [email protected]
Department of Chemistry, University of Puerto Rico at Rio Piedras, San Juan, PR 00931, USA;
[email protected] (A.H.-V.); [email protected] (L.M.D.-V.)
Department of Environmental Sciences, University of Puerto Rico at Rio Piedras, San Juan, PR 00931, USA;
[email protected]
Correspondence: [email protected]; Tel.: +1-212-220-8000; Fax: +1-212-748-7471
Academic Editor: Yu-Pin Lin
Received: 8 July 2016; Accepted: 7 December 2016; Published: 22 December 2016
Abstract: Among several Puerto Rican algae, Sargassum sp. (SG) and Chaetomorpha (CM) showed the
highest phenol adsorption capacity from aqueous solutions and were used in optimized adsorption
batch experiments at room temperature. The effects of pH, adsorbent dose, phenol concentration,
salinity and presence of interfering substances were evaluated. Initial solution pH exhibited a strong
effect, mainly on the phenol aqueous chemistry; showing the maximum adsorption at pH 10. Sorption
isotherm results were modelled according to the Langmuir, Tempkin and Freundlich equations.
Isotherm modelling indicated a maximum adsorption capacity (qmax ) of 82.10 and 17.7 mg of phenol
per gram of SG and CM, respectively. Salinity and presence of detergent in the matrix solution
showed a positive effect on the adsorption, suggesting that adsorption of phenol was mostly driven
by polar forces and not by ionic exchange. On the other hand, presence of heavy metals like copper,
lead and cobalt had a negative effect on the adsorption. According to these results, the potential
formation of hydrogen bonds between the algae and phenol is proposed as the main adsorption
mechanism. These results provide further insight into the adsorption mechanism of phenol and their
use as inexpensive adsorbents for the treatment of phenol-containing wastewaters.
Keywords: adsorption; marine algae; phenol; pH; isotherm
1. Introduction
Phenol and its derivatives are mainly generated by petroleum industries. Due to their toxicity,
these compounds are considered as priority pollutants by the Environmental Protection Agency (EPA)
and the World Health Organization (WHO), who have established the maximum level of phenol
concentration allowable in drinking water at 1 µg·L−1 [1,2]. When phenols are released into the
water, some of them remain in solution or react, producing even more toxic substances and therefore
threatening the aquatic life by entering the food chain. The treatment processes for phenolic wastewater
have been classified into two principal categories [3]: (i) recuperative process such as adsorption into
porous solids, osmosis and solvent extraction and; (ii) destructive process by their oxidation with
hydrogen peroxide, manganese oxides and ozone. Recent studies utilized enzymatic degradation [4]
and naturally occurring adsorbents [5–8] to control the contamination of liquid effluents by phenol
and derivatives.
Among these techniques, adsorption has reported high phenol removal, low operative costs and
innocuous side effects caused to the environment. The high surface area per unit mass of activated
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carbon converts it into one of the most efficient adsorbents in the removal of organic pollutants from
aqueous solutions [9–11]. Nevertheless, due to the relatively high cost of activated carbons there have
been attempts to utilize low cost adsorbents to remove trace organic and inorganic contaminants
from wastewaters.
In developing countries, industries cannot afford to use conventional wastewater treatments
such as activated sludge, because they are not cost-effective, require constant monitoring, energy
consumption and special technical training. On the other hand, marine seaweeds line up the shores
of Caribbean islands (such as Puerto Rico) causing more detriment than asset to the surrounding
community. Bioremediation as an area of biotechnology has explored the potential use of these
seaweeds as adsorbents of pollutants. Different types of algae have been studied for the removal
of heavy metals [12–14], phenolic compounds [15,16] and dyes [17,18] yielding positive results.
Other alternative materials have also been studied, for example natural clays reported a high affinity
towards inorganic and organic pollutants [19–22] and are extracted from natural mines at minimal cost.
Previous studies [14] have determined that cross-linkage of brown seaweeds with calcium
chloride enhances their mechanical properties and affinity towards heavy metals. Rubin et al. [16]
have previously observed that raw algae were easily decomposed by phenolic solutions and that
decomposition products interfere in the determination of phenols in aqueous solutions. It was
discovered that a cross-linkage with calcium chloride resolved this issue. This improvement was
attributed to the stabilization of alginate molecules by the formation of the characteristic alginate
arrangement known as the egg-box structure [14,23].
Several phenol adsorption studies have been conducted using different types of marine algae,
but few of them have established a mechanism corresponding to the results or optimized equilibrium
parameters [21]. Furthermore, a detailed discussion on the relationship of the chemical characterization
of the algal biomass and its adsorption performance has not been clearly established in the literature.
This study reports a comparison into the adsorption behavior of phenol across native marine algae
from a structural, mechanistic and equilibrium point of view.
2. Materials and Methods
2.1. Algal Biomass
The marine algae Chaetomorpha (CM), Sargassum sp. (SG), Chlorella vulgaris (CV), Osmundaria (OD)
and Ulva lactuca (UL) were collected from Puerto Rican beaches during the Spring 2015. Algal samples
were vigorously and sequentially rinsed with tap, distilled and deionized water, to assure the
elimination of soluble substances and adhered particles. Then, algae were sun-dried and crushed with
a mortar and pestle. Finally, the algae were sieved to a homogenous size (particle size between 100 and
180 µm) and stored in glass containers on the bench at room temperature until their use. Algal samples
were classified as green algae (CM, CV and UL) and brown algae (OD and SG).
2.2. Reagents and Solutions
Stock solutions of 1000 ppm (mg of phenol per liter of solution) were prepared by dissolving
Phenol (Analytical grade, Fisher Scientific) in deionized water. Solutions were stored in amber glass
bottles under refrigeration. Solution of varying concentrations were produced by dilution of stock
solutions until the desired concentration was reached. No buffer was used in the preparation of the
solution to minimize salt effects in the experiments. This protocol has been previously used with other
inorganic and organic pollutants [12–18]. The initial pH of each solutions was adjusted to the required
values by adding aliquots of diluted HCl and NaOH prior to contact with the adsorbents.
Finally, stability of phenol in solution was monitored by UV spectrophotometry under the same
working conditions, showing a stability of more than 24 h.
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2.3. Characterization of Adsorbents
Preliminary adsorption experiments indicated that SG and CM had the most efficient adsorption
amongst the five algae samples. Therefore, the biomass characterization focused on these two samples
to evaluate the chemical, textural, and thermal properties of both adsorbents. The presence of
organic functional groups of SG and CM were analysed by Fourier transform infrared spectroscopy,
and recorded on a Frontier FTIR spectrometer (Perkin Elmer, Waltham, MA, USA), which provides
counts using an Attenuated Total Reflectance (ATR) accessory for measurement in solid state. FTIR
allowed us to identify the functional groups that could be potentially involved in phenol adsorption.
Morphology and textural analysis of both adsorbents was performed under a scanning electron
microscope, by using a Tabletop microscope, model TM3000 (Hitachi, New York, NY, USA).
Both samples were directly measured in the instrument without gold coating. Simultaneously,
elemental analysis of SG and CM was conducted by energy dispersive X-ray spectrophotometry
(EDS) (Bruker, Billerica, MA, USA) with a spectrometer, model Quantax 70. This technique allowed us
to determine the percent abundance of key elements that were present on the algae surface.
Finally, the thermal resistance and mechanical properties of both adsorbents were determined by
thermogravimetric analysis (TGA) by using a Simultaneous thermal analyser (Perkin Elmer, USA),
model STA 6000. The measurements were performed under a nitrogen atmosphere from room
temperature to 600 ◦ C at a heating rate of 5 ◦ C/min.
2.4. Adsorption Studies
Batch adsorption studies were performed at room temperature (25 ± 1 ◦ C) in polyethylene
centrifuge tubes by shaking a variable mass of algae with 50 mL of phenol solution. Preliminary
studies indicate that phenol does not adsorb or interact with the polyethylene tubes. The effect of pH
on the adsorption was investigated at different whole values within the range 2 to 10 for all the samples
and placed in a temperature-regulated shaker for at least 12 h. After agitation, samples were filtered
and the supernatant was separated. UV-visible spectrometry was used for phenol determination by
using the Gales and Booth method [24] with a spectrometer at λ = 510 nm, in which antipyrine forms a
red/orange complex with phenol.
The pH effect was used as a screening test to identify the best performing algae. Based on this
experiment, algae SG and CM showed the highest adsorption degree. Therefore, these two algae
were selected for all other batch experiments. Other effects such as adsorbent dose and initial phenol
concentration were then explored to maximize the adsorption capacity. For these experiments, series
of tubes containing 50 mL of phenol solution of different concentrations were mixed with varying
amounts of adsorbents. Concentrations ranged from 40 to 300 ppm, and masses from 40 to 200 mg.
The next experiments included the study of the presence of interfering substances on the adsorption of
phenol. These substances included salinity (NaCl, Na2 SO4 and CaCl2 ), detergents (sodium dodecyl
sulfonate, SDS) and heavy metal ions (Co+2 , Cu+2 , and Pb+2 ) in their nitrate form. For the effect of
heavy metals in solution, the pH of the solution was reduced to pH 6 to assure complete solubility of
the heavy metal ions, the adsorption was compared to a control adsorption at the same pH.
2.5. Data Analysis
The amount of adsorbed phenol is expressed as Adsorption Percentage (%ADS) and Adsorption
Capacity (q, mg·g−1 ) calculated as shown in Equations (1) and (2):
Ci − Ceq × 100
%ADS =
(1)
Ci
Ci − Ceq × V
q=
(2)
m
where m is the mass of the adsorbent expressed in g, V is the volume of the solution in L and Ci and
Ceq are the initial and at the equilibrium concentrations of phenol, expressed in mg/L, respectively.
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Plots and linear regression were carried out using the statistical software Origin v8.0 (OriginLab,
Plots and linear regression were carried out using the statistical software Origin v8.0 (OriginLab,
Northampton, MA, USA), obtaining errors between 2.5% and 4%. Error bars were not added to avoid
Northampton,
MA,
USA),Linear
obtaining
errors between
4%. Error
bars were not
added tovalues,
avoid
crowding in the
curves.
regression
reported2.5%
dataand
includes
R22 correlation
coefficient
crowding
in the
curves.
regression reported
includes
R correlation
coefficientwith
values,
and p-value
defined
asLinear
the significance
relevancedata
of the
modeling
that is associated
the
and
p-value
defined
as
the
significance
relevance
of
the
modeling
that
is
associated
with
the
possibility
possibility of an observed result that a null hypothesis is true.
of an observed result that a null hypothesis is true.
3. Results and Discussion
3. Results and Discussion
3.1. Characterization
Characterization of
of the
the Adsorbents
Adsorbents
3.1.
Good adsorbent
adsorbent candidates
candidates should
should possess
possess optimum
optimum chemical,
chemical, thermal,
thermal, morphological
morphological and
and
Good
textural
properties
not
only
to
increase
the
affinity
towards
a
given
pollutant,
but
also
to
face
harsh
textural properties not only to increase the affinity towards a given pollutant, but also to face harsh
conditions in
in which
which those
those pollutants
pollutants exist.
exist. FTIR
FTIR consists
consists in
in the
the absorption
absorption of
of infrared
infrared radiation
radiation by
by
conditions
chemical
functional
groups
with
a
dipole
moment.
The
FTIR
of
SG
and
CM
revealed
the
presence
of
chemical functional groups with a dipole moment. The FTIR of SG and CM revealed the presence
the
typical
functional
groups
in
algae
(Figure
1).
According
to
previously
reported
data
[25],
algae
of the typical functional groups in algae (Figure 1). According to previously reported data [25],
are biomass
rich inrich
fattyinacids,
and carbohydrates
such as such
alginate
and fucoidans
(sulfated
algae
are biomass
fatty proteins
acids, proteins
and carbohydrates
as alginate
and fucoidans
polysaccharide).
As
observed
in
the
FTIR
spectra,
both
algae
SG
and
CM
display
peaks
3280,
(sulfated polysaccharide). As observed in the FTIR spectra, both algae SG and CM displayatpeaks
−1
1680
and1680
1425and
and1425
at 1100
attributed
the presence
hydroxyl,ofcarbonyl
andcarbonyl
hemiacetals,
at
3280,
andcm
at 1100
cm−1 to
attributed
to theofpresence
hydroxyl,
and
respectively.
These
functional
groups
agree
with
the
presence
of
polyalginate
and
fucoidan
groups
hemiacetals, respectively. These functional groups agree with the presence of polyalginate
and
in both algae.
However,
it is important
the higher
relative
of the peaks
of SG,
fucoidan
groups
in both algae.
However,toithighlight
is important
to highlight
theintensity
higher relative
intensity
of
compared
to
CM.
As
previously
mentioned,
SG
and
CM
are
brown
and
green
algae,
respectively;
the peaks of SG, compared to CM. As previously mentioned, SG and CM are brown and green algae,
therefore main
structural
andstructural
chemical composition
are differences
expected. Brown
algae are Brown
known
respectively;
therefore
main
and chemicaldifferences
composition
are expected.
to
contain
a
higher
concentration
of
polyalginic
acid,
which
is
believed
to
contribute
to
the
adsorption
algae are known to contain a higher concentration of polyalginic acid, which is believed to contribute to
−1
of pollutants
This hypothesis
is corroborated
a weak peak
1680 cm
the absence
of
the
adsorption[14,15]
of pollutants
[14,15] This
hypothesis is by
corroborated
byat
a weak
peakand
at 1680
cm−1 and
−1 for CM. These peaks are associated with the presence of carbonyl groups, main
the
peak
at
1425
cm
−
1
the absence of the peak at 1425 cm for CM. These peaks are associated with the presence of carbonyl
components
alginates. Lastly,
the spectra
displays
a smaller
CM at 3280
cmat−1
groups,
main of
components
of alginates.
Lastly, also
the spectra
also
displaysintensity
a smallerinintensity
in CM
(hydroxyl
groups), demonstrating
a lower aamount
of polysaccharides,
whenwhen
compared
to the
3280
cm−1 (hydroxyl
groups), demonstrating
lower amount
of polysaccharides,
compared
to
SG
sample.
the SG sample.
100
% Transmittance
90
80
70
60
4000
SG
CM
3500
3000
2500
2000
1500
1000
-1
Wavenumber (cm )
Figure1.1.Fourier
Fouriertransformed
transformed infrared
infrared spectra
spectra spectra
spectra of
of Sargassum
Sargassum sp.
sp. (SG)
(SG) and
and Chaetomorpha
Chaetomorpha (CM)
(CM)
Figure
samplesbefore
beforephenol
phenoladsorption.
adsorption.
samples
Scanning electron micrographs were used to elucidate the morphological and textural properties
Scanning electron micrographs were used to elucidate the morphological and textural properties
of both algae. The micrographs are shown in Figure 2. According to the images, both adsorbents show
of both algae. The micrographs are shown in Figure 2. According to the images, both adsorbents
a highly heterogeneous surface, with valleys, pockets and protrusions, properties of biological
show a highly heterogeneous surface, with valleys, pockets and protrusions, properties of biological
materials. These surface features are positive properties for adsorbent candidates. It is always ideal
materials. These surface features are positive properties for adsorbent candidates. It is always ideal to
to have surface imperfection where the pollutant can be housed and trapped, to facilitate the
interaction with the adsorption sites of the biomass. Both adsorbent show similar morphological
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have
surface2016,
imperfection
where the pollutant can be housed and trapped, to facilitate the interaction
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with the adsorption sites of the biomass. Both adsorbent show similar morphological properties,
however
a higher
surface
randomness
is observed in
demonstrating
this adsorbent
could
have a
properties,
however
a higher
surface randomness
is SG,
observed
in SG, demonstrating
this
adsorbent
could adsorption
have a higher
due toofthe
complexity
of its
surface.
results have
higher
due adsorption
to the complexity
its surface.
Similar
results
haveSimilar
been observed
with been
other
observed with
biomasses
such[26,27]
as spent
teaother
leaves
[26,27]
and other
types
of marine algae [15,17].
biomasses
suchother
as spent
tea leaves
and
types
of marine
algae
[15,17].
A
B
C
D
Figure2.2.Scanning
Scanning Electron
Electron Micrographs
Micrographs of
of the
the adsorbents:
adsorbents: Chaetomorpha
Figure
Chaetomorpha(A,
(A,B),
B),and
andSargassum
Sargassumsp.
sp.
(C,
D)
at
different
magnifications.
(C, D) at different magnifications.
SEM analysis was coupled with elemental analysis by EDS to determine the presence of key
SEM analysis was coupled with elemental analysis by EDS to determine the presence of key
elements that are important in the adsorption of pollutants. The results are shown in Figure 3,
elements that are important in the adsorption of pollutants. The results are shown in Figure 3, indicating
indicating the qualitative presence of carbon, oxygen and sulfur. Carbon and oxygen are expected
the qualitative presence of carbon, oxygen and sulfur. Carbon and oxygen are expected elements due to
elements due to the biological nature of both adsorbents. However, as observed in the spectra, SG
the biological nature of both adsorbents. However, as observed in the spectra, SG shows the presence
shows the presence of calcium and magnesium ions. Although these species are not structural
of calcium and magnesium ions. Although these species are not structural components of the biomass,
components of the biomass, their presence suggests a higher content of carboxyl groups in SG.
their
presence suggests a higher content of carboxyl groups in SG. Divalent ions have a special affinity
Divalent ions have a special affinity towards carboxyl groups [12–14]. These carboxyl groups were
towards
carboxyl
groups
[12–14].
groups
confirmed
FTIR
(Figure
1) and
+2 and
+2 on the
confirmed
by FTIR
(Figure
1) andThese
now carboxyl
elucidated
by thewere
presence
of Caby
Mg
raw now
SG
+2 and Mg+2 on the raw SG adsorbent. Algae CA does not show
elucidated
by
the
presence
of
Ca
adsorbent. Algae CA does not show substantial amounts of these ions, supporting the idea of the
substantial
amounts
of thesefunctional
ions, supporting
ideagroups
of the lower
number
of carboxyl
functional
lower number
of carboxyl
groups.the
These
are able
to form
strong dipole
and
groups.
These
groups
are
able
to
form
strong
dipole
and
hydrogen
bonding
with
the
charged
and
hydrogen bonding with the charged and uncharged phenol pollutant and lead us to the adsorption
uncharged
phenol
pollutant
and
lead
us
to
the
adsorption
mechanism
on
algal
biomass.
mechanism on algal biomass.
Finally,
by thermogravimetric
thermogravimetricanalysis
analysis
Finally,the
thethermal
thermalproperties
properties of
of the
the adsorbents
adsorbents were
were studied
studied by
(TGA).
to be
be used
used at
at high
hightemperature,
temperature,the
theresults
resultscan
can
(TGA).Even
Eventhough
thoughthese
theseadsorbent
adsorbent are
are not
not expected
expected to
be
translated
into
thermal,
physical
and
mechanical
resistance
that
these
adsorbent
possess.
be translated into thermal, physical and mechanical resistance that these adsorbent possess.
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A
cps/eV
2.4
2.2
2.0
1.8
1.6
1.4
S
K
Cl C
1.2
Na
O
Mg
Al
S
Cl
Ca
K
Ca
1.0
0.8
ELEMENT
CARBON
OXYGEN
SULFUR
POTASSIUM
CHLORINE
SODIUM
ATOM %
52.75
41.40
1.49
1.46
1.12
0.89
ELEMENT
CARBON
OXYGEN
CALCIUM
POTASSIUM
SULFUR
MAGNESIUM
ATOM %
55.09
42.35
0.77
0.57
0.47
0.38
0.6
0.4
0.2
0.0
0.5
2.5
2.0
keV
1.5
1.0
4.0
3.5
3.0
B
cps/eV
6
5
4
S
3
K
C
N
O
Na
Mg
Al
Si
P
S
K
Ca
Ca
2
1
0
0.5
1.0
1.5
2.0
keV
2.5
3.0
3.5
4.0
Figure
Figure 3.
3. Energy
EnergyDispersive
DispersiveX-ray
X-rayspectra
spectraand
andAtom
AtomPercent
Percent Abundance
Abundance of
of the
the most
most prevalent
prevalent elements
elements
for
both
adsorbents
Chaetomorpha
(A)
and
Sargassum
sp.
(B).
Other
elements
that
are
labeled
for both adsorbents Chaetomorpha (A) and Sargassum sp. (B). Other elements that are labeled on
on the
the
spectra,
have
%ATOM
smaller
than
0.89%
and
0.38%
for
Chaetomorpha
and
Sargassum
sp.,
respectively.
spectra, have %ATOM smaller than 0.89% and 0.38% for Chaetomorpha and Sargassum sp., respectively.
It is expected that materials that resist thermal decomposition at higher temperature, are
It is expected that materials that resist thermal decomposition at higher temperature, are expected
expected to resist more adverse conditions such as oxidation, light degradation and other adverse
to resist more adverse conditions such as oxidation, light degradation and other adverse environmental
environmental conditions that might be present in real wastewaters. Both TGA analyses under
conditions that might be present in real wastewaters. Both TGA analyses under nitrogen atmosphere
nitrogen atmosphere (non-oxidizing conditions) are shown in Figure 4. According to the curves, both
(non-oxidizing conditions) are shown in Figure 4. According to the curves, both samples show an initial
samples show an initial
mass loss above 100 °C corresponding to loss of moisture. Then, there is a
mass loss above 100 ◦ C corresponding to loss of moisture. Then, there is a slow thermal decomposition
slow thermal decomposition
for both samples until 230 °C. This mass drop could be attributed to one
for both samples until 230 ◦ C. This mass drop could be attributed to one type of components, most
type of components, most likely volatile or weakly bonded molecules. It is important to highlight that
likely volatile or weakly bonded molecules. It is important to highlight that SG has a higher mass loss
SG has a higher mass loss than CA within this temperature
range (100–230 °C). Then, a drastic mass
than CA within this temperature range (100–230 ◦ C). Then, a drastic mass loss is observed between
loss is
observed between 230 °C and 375 °C. Within this temperature range, the decomposition of CA
230 ◦ C and 375 ◦ C. Within this temperature range, the decomposition of CA reflects the loss of at
reflects the loss of at least 4 types of compounds (demonstrated by the number of inflections points),
least 4 types of compounds (demonstrated by the number of inflections points), whereas SG only
whereas SG only shows 2. This results agrees with the chemical composition of green algae (CA),
shows 2. This results agrees with the chemical composition of green algae (CA), which are rich in
which are rich in proteins, lipids and carbohydrates, as described elsewhere [7,18]. On the other hand,
proteins, lipids and carbohydrates, as described elsewhere [7,18]. On the other hand, brown algae,
brown algae, such as SG, are richer in carbohydrates such as polyalginate and fucoidans. Lastly, a
such as SG, are richer in carbohydrates such as polyalginate and fucoidans. Lastly, a final and slow
final and slow mass loss is observed
above 375 °C, representing stronger biopolymers and fiber.
mass loss is observed above 375 ◦ C, representing stronger biopolymers and fiber. In conclusion,
In conclusion, both adsorbents display optimum thermal resistance and are highly stable at
both adsorbents display optimum thermal resistance and are highly stable at temperatures up to
temperatures
up to 230 °C (maximum mass loss of 10%). However, CA algae shows a better thermal
230 ◦ C (maximum mass loss of 10%). However, CA algae shows a better thermal resistance when
resistance when compared to SG. This could be explained by the more heterogeneous composition of
compared to SG. This could be explained by the more heterogeneous composition of green algae,
green algae, including nucleic acids, lipids, proteins and carbohydrates.
including nucleic acids, lipids, proteins and carbohydrates.
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77 of
70
SG
CM
60
% Mass Loss
50
40
30
20
10
0
100
200
300
400
500
600
0
Temperature ( C)
Figure 4. Thermogravimetric analysis of both adsorbents under nitrogen atmosphere at a heating rate
Figure
4. Thermogravimetric analysis of both adsorbents under nitrogen atmosphere at a heating rate
of
5 ◦ C/min.
of 5 °C/min.
3.2. Adsorption Tests
3.2. Adsorption Tests
3.2.1. Effect of Initial pH Value
3.2.1. Effect of Initial pH Value
pH is one of the most important factors in the adsorption of environmental contaminants in
pH solutions
is one of since
the most
important
the adsorption
of environmental
in
aqueous
it influences
the factors
solutioninchemistry
of phenols
(i.e., hydrolysis,contaminants
redox reactions,
aqueous solutions
it influences
the has
solution
chemistry
(i.e., hydrolysis,
redox
polymerization
and since
complexation).
pH also
a strong
influenceofonphenols
the speciation
and the sorption
reactions,
polymerization
and
complexation).
pH
also
has
a
strong
influence
on
the
speciation
and
availability of phenolic compounds as well as the ionic state of functional groups on the surface
of
the sorption[7,8,28].
availability of phenolic compounds as well as the ionic state of functional groups on the
adsorbents
surface
of adsorbents
[7,8,28].
Figure
5 shows the
effect of solution pH on the adsorption of phenol at room temperature.
Figure
5
shows
the
solution pH
on theis adsorption
at room
temperature.
For all algae samples, the effect
highestofadsorption
capacity
reported at of
pHphenol
10. These
phenomena
can be
For all algae
samples,
theofhighest
adsorption
capacity is reported
at pH
These phenomena
can be
explained
as the
product
electrostatic
repulsion/attraction
between
the10.
adsorbent
and the adsorbate.
explained
as
the
product
of
electrostatic
repulsion/attraction
between
the
adsorbent
and
Recent studies [29] demonstrated that marine algae have apparent ionization constants around the
3.0
adsorbate.
[29] the
demonstrated
thatare
marine
algae charged.
have apparent
constants
whereas
at Recent
higher studies
pH values,
algal surfaces
negatively
At theionization
same time,
phenol
around
3.0 whereas at higher pH values, the algal surfaces are negatively charged. At the same time,
(pK
a = 9.9) becomes negatively charged with increasing pH. Consequently, the higher concentration of
phenol
(pKa =ions
9.9)inbecomes
negatively
with increasing
Consequently,
the higher
hydroxonium
the solution,
promotecharged
the prevalence
of neutralpH.
phenol,
and the deprotonation
concentration
of hydroxonium
ions
in thefavor
solution,
promoteSurprisingly,
the prevalence
neutral phenol,
and
of
the algal surface
at higher pH
values,
adsorption.
theofadsorption
is greatly
the
deprotonation
of
the
algal
surface
at
higher
pH
values,
favor
adsorption.
Surprisingly,
the
decreased at low pH values, indicating that an efficient adsorption interaction only occurs between
adsorption
is
greatly
decreased
at
low
pH
values,
indicating
that
an
efficient
adsorption
interaction
the adsorbent and the negatively charged phenol. Moreover, no substantial changes were observed at
onlyapparent
occurs between
and the that
negatively
chargedmechanism
phenol. Moreover,
substantial
the
pKa valuethe
of adsorbent
alginate, indicating
the adsorption
is mainlyno
driven
by the
changes
were
observed
at
the
apparent
pK
a value of alginate, indicating that the adsorption
chemistry of phenol and not by the ionization of the adsorbent. Higher pH values were not studied
mechanism
is mainly driven
by the chemistry
of phenol
and not by the ionization of the adsorbent.
due
to the impractical
use of solutions
at extremely
basic ranges.
Higher
pH values
were
not studied
to data,
the impractical
useaofpolar
solutions
at extremely
basiconto
ranges.
Based
on these
results
and thedue
FTIR
we propose
adsorption
of phenol
SG
Based
on
these
results
and
the
FTIR
data,
we
propose
a
polar
adsorption
of
phenol
onto
SG
and
and CM. The adsorption interactions should occur between the hydroxyl groups of the algae and
CM.phenoxide
The adsorption
interactions
should occur
betweenbonds.
the hydroxyl
the
algae and
the
the
ion, through
the formation
of hydrogen
Alginategroups
shouldof
not
intervene
in the
phenoxide
ion,
through
the
formation
of
hydrogen
bonds.
Alginate
should
not
intervene
in
the
removal of phenol due to electrostatic repulsion of alginate anions and phenoxide at the highest
removal
of
phenol
due
to
electrostatic
repulsion
of
alginate
anions
and
phenoxide
at
the
highest
adsorption pH. Marine algae are mainly composed of a polysaccharide named polyalginic acid, which
adsorption
pH. Marine
algae are(M)
mainly
composed
a polysaccharide
named polyalginic[25,29].
acid,
contains
mixtures
of mannuronic
and guluronic
(G)ofacid
(carboxylated monosaccharides)
which
contains
mixtures
of
mannuronic
(M)
and
guluronic
(G)
acid
(carboxylated
monosaccharides)
These polysaccharides are rich in hydroxyl groups and observed in higher percentages in brown algae
[25,29].
polysaccharides
richreported
in hydroxyl
and removal
observedofinother
higher
percentages
in
(SG
and These
OD). Although
alginatesare
have
a highgroups
adsorptive
pollutants
[12–18],
brown
algae (SG
and OD).
Although
alginates have
reported
a high
adsorptive
removal
of other
their
adsorption
efficiency
relies
on the appropriate
M and
G sequence
in the
polysaccharide.
According
pollutants
[12–18],
their
adsorption
efficiency
relies
on
the
appropriate
M
and
G
sequence
in the
to the results, OD, also a brown algae, shows a low adsorption, which could be tentatively explained
polysaccharide.
According
to the
OD, of
also
brown
algae,
shows
a lowThis
adsorption,
which
by
the unfavorable
proportion
andresults,
sequencing
M aand
G units
in its
structure.
important
role
could be tentatively explained by the unfavorable proportion and sequencing of M and G units in its
Environments 2017, 4, 1
Environments 2016, 4, 1
8 of 14
Environments 2016, 4, 1
8 of 14
8 of 14
important
role
has alsoby
been
explained
and observed
by Rubin
et al. [30]. More studies
hasstructure.
also beenThis
explained
and
observed
Rubin
et al. [30].
More studies
on monosaccharides
content
on
monosaccharides
content
are needed
support by
thisRubin
hypothesis.
structure.
Thisare
important
role
has sequencing
also this
beenhypothesis.
explained
andtoobserved
et al. [30]. More studies
and
sequencing
needed
to and
support
on monosaccharides content and sequencing are needed to support this hypothesis.
80
SG
CM
SG
CV
CM
OD
CV
UL
OD
UL
Adsorption
Percentage
(%ADS)
Adsorption
Percentage
(%ADS)
80
60
60
40
40
20
20
0
0
2
4
6
8
10
pH
6
8
10
pH
Figure
pH
effectononthe
theadsorption
adsorptionof
ofphenol
phenol on
on marine
marine algae.
mg
and
Phenol
Figure
5. 5.
pH
effect
algae.Mass
Massofofadsorbent:
adsorbent:5050
mg
and
Phenol
concentration:
50
mg/L.
Figure
5.
pH
effect
on
the
adsorption
of
phenol
on
marine
algae.
Mass
of
adsorbent:
50
mg
and
Phenol
concentration: 50 mg/L.
2
4
concentration: 50 mg/L.
3.2.2. Effect of Adsorbent Dose
3.2.2. Effect of Adsorbent Dose
3.2.2. Effect of Adsorbent Dose
A sustainable and efficient bioremediation technique should not only be “green” but also
A
sustainable
and efficient
bioremediation
should
not
only be “green”
but
also
minimize
minimize
the amount
of biomass
be used in technique
the process.
Figure
6 displays
the be
role“green”
of
the
adsorbent
A sustainable
and
efficienttobioremediation
technique
should
not only
but also
thedose
amount
of
biomass
bephenol
used to
in
theused
process.
6 displays
theresults,
role the
of higher
the
dose
on the
adsorption
at be
the
optimum
pH.
According
the
of
CMon
minimize
the
amount to
ofofbiomass
in theFigure
process.
Figureto
6 displays
roleadsorbent
of masses
the adsorbent
theincrease
adsorption
of
phenol
at
the
optimum
pH.
According
to
the
results,
higher
masses
of
CM
increase
the adsorption
adsorption,ofreaching
a the
steady
value around
100 mg.toThis
indicates
that masses
dose on the
phenol at
optimum
pH. According
the results,
higher
masses higher
of CM
thethan
adsorption,
reaching
steady
value
around
100
mg.
This
masses
higher
100 mg
100the
mgadsorption,
have no asubstantial
on
thearound
adsorption.
Conversely,
SG shows
a than
decreased
increase
reaching
aimpact
steady
value
100indicates
mg.
Thisthat
indicates
that
masses
higher
have
no100
substantial
impact
on
the adsorption.
Conversely,
shows
a decreased
adsorption
with
adsorption
with
increasing
masses.
This can
explained
bySG
the
formation
of aggregates
with SG.
than
mg
have
no substantial
impact
onbethe
adsorption.
Conversely,
SG
shows
a decreased
increasing
masses.
This
can
be
explained
by
the
formation
of
aggregates
with
SG.
In
solution,
SG
tends
In
solution, with
SG tends
to formmasses.
clumpsThis
(clearly
in thebyadsorption
tube),ofdecreasing
surface
adsorption
increasing
can observed
be explained
the formation
aggregatesitswith
SG.
to form
clumps
observed
inThis
the
adsorption
tube),
decreasing
itstube),
surface
area in
that
exposed
area
that
is exposed
the
solution.
phenomenon
was
not
experimentally
observed
CM,
where
In solution,
SG(clearly
tendstoto
form
clumps
(clearly
observed
in the
adsorption
decreasing
itsissurface
aarea
homogeneous
particle
observed throughout
the solution.
Therefore,
optimum
to the
solution.
This
phenomenon
not
experimentally
observed
in CM,observed
where
ain
homogeneous
that is exposed
to the distribution
solution.was
Thiswas
phenomenon
was not
experimentally
CM,
where
masses
of 40 mg and
100observed
mg
werethroughout
takenwas
for observed
SGthe
andsolution.
CM,
respectively.
adsorption
a homogeneous
particle
distribution
throughout
theSimilar
solution.
Therefore,
particle
distribution
was
Therefore,
optimum
massesaggregation
ofoptimum
40 mg and
reported
by
Rubin
al. [16,30]
a different
type ofaggregation
brown
algae.
masses
ofwas
40
mg
and
100
mg
wereet
taken
for SGfor
and
CM,adsorption
respectively.
Similar
adsorption
100behavior
mg were
taken
for SG
and
CM,
respectively.
Similar
behavioraggregation
was reported
reported
Rubin et al.
[16,30]
for a different
by behavior
Rubin et was
al. [16,30]
forby
a different
type
of brown
algae. type of brown algae.
35
Adsorption
Percentage
(%ADS)
Adsorption
Percentage
(%ADS)
35
30
30
25
25
20
SG
CM
SG
CM
20
15
15
10
10
5
5
0
0
40
60
80
100
140
160
180
200
40
60
80
Mass
algae (mg)
100 of 120
140
120
160
180
200
Mass of algae (mg)
Figure 6. Effect of the adsorbent dose on the adsorption of phenol on marine algae. Initial solution
pH:
10
Phenol
concentration:
100 mg/L.
Figure
6.
Effect
the
adsorbentdose
dose
onthe
theadsorption
adsorption of
of phenol
phenol on
Figure
6. and
Effect
ofof
the
adsorbent
on
on marine
marinealgae.
algae.Initial
Initialsolution
solution
pH:
10
and
Phenol
concentration:
100
mg/L.
pH: 10 and Phenol concentration: 100 mg/L.
Environments 2017, 4, 1
9 of 14
3.2.3. Equilibrium Modelling
Environments 2016, 4, 1
9 of 14
Adsorption isotherms have been widely used to describe the distribution of a substance
(adsorbate)
from oneModelling
phase onto a solid surface (adsorbent). This adsorbate transport is driven
3.2.3. Equilibrium
by thermodynamic and equilibrium factors that are expressed in terms of mathematical equations and
Adsorption isotherms have been widely used to describe the distribution of a substance
based on different
assumptions [7,18]. The Langmuir theory assumes that (i) the adsorbent’s surface has
(adsorbate) from one phase onto a solid surface (adsorbent). This adsorbate transport is driven by
a defined number of active sites that are energetically uniform; (ii) adsorbed species do not interact on
thermodynamic and equilibrium factors that are expressed in terms of mathematical equations and
the adsorbent’s surface, meaning that the adsorption of an adsorbate does not promote nor inhibit the
based on different assumptions [7,18]. The Langmuir theory assumes that (i) the adsorbent’s surface
adsorption
a second
adsorbate
species;
(iii)
adsorptionuniform;
saturation
reached when
monolayer
has a defined
number
of active
sitesand
that
arethe
energetically
(ii)isadsorbed
speciesa do
not
of adsorbate
has
been
formed
on
the
surface
of
the
adsorbent.
On
the
other
hand,
as
opposed
interact on the adsorbent’s surface, meaning that the adsorption of an adsorbate does not promote to
Langmuir’s
theory,
Freundlich
equation
assumes
a logarithmic
decrease
in the
adsorption
enthalpy
nor inhibit
the adsorption
a second
adsorbate
species;
and (iii) the
adsorption
saturation
is reached
with when
an increasing
number
of occupied
sites
of aofrather
energetically
a monolayer
of adsorbate
has been adsorption
formed on the
surface
the adsorbent.
On the heterogeneous
other hand,
as opposed
to Both
Langmuir’s
theory,
Freundlich
equation
assumes
decrease
in and
the the
adsorbent
surface.
adsorption
isotherm
equations
can be
writtena inlogarithmic
their linearized
forms
adsorption
enthalpy
with
an
increasing
number
of
occupied
adsorption
sites
of
a
rather
energetically
Langmuir and Freundlich constants can be calculated, according to Equations (3) and (4), respectively.
heterogeneous adsorbent surface. Both adsorption isotherm equations can be written in their
1
1 Freundlich constants
1
linearized forms and the Langmuir
and
can be calculated, according to
=
+
(3)
Equations (3) and (4), respectively.
q
qmax
b × qmax × Ceq
1
1
1
=
+
1
𝑞𝑞ln q 𝑞𝑞=𝑚𝑚𝑚𝑚𝑚𝑚
× ln×C𝐶𝐶eq𝑒𝑒𝑒𝑒
ln k F 𝑏𝑏+× 𝑞𝑞𝑚𝑚𝑚𝑚𝑚𝑚
(3)
n
(4)
1
where qmax (mg·g−1 ) and b (L/mg) are the
(4)
ln 𝑞𝑞 Langmuir
= ln 𝑘𝑘𝐹𝐹 + constants
× ln 𝐶𝐶𝑒𝑒𝑒𝑒 related to the maximum adsorption
𝑛𝑛
capacity and to the adsorption energy, respectively. The parameters kF and 1/n are the Freundlich
−1) and b (L/mg) are the Langmuir constants related to the maximum adsorption
whererelated
qmax (mg·g
constants
to the
adsorption capacity and the adsorption intensity, respectively. In addition
capacity
and
to
the
adsorption
energy,
respectively.
The based
parameters
kF and
1/n are thethat
Freundlich
to these models, Tempkin,
proposed
a different
theory
on the
assumption
the heat of
constants related to the adsorption capacity and the adsorption intensity, respectively. In addition to
adsorption would decrease linearly with the increase of coverage of adsorbent. The mathematical
these models, Tempkin, proposed a different theory based on the assumption that the heat of
equations of Tempkin’s theory is expressed with Equation (5):
adsorption would decrease linearly with the increase of coverage of adsorbent. The mathematical
equations of Tempkin’s theory is expressed
with
R ×
T Equation (5): q=
𝑞𝑞 =
ln a
× C
eq
T
𝑅𝑅bT× 𝑇𝑇
ln(𝑎𝑎 𝑇𝑇 × 𝐶𝐶𝑒𝑒𝑒𝑒 )
𝑏𝑏𝑇𝑇
(5)
(5)
in which R is the gas constant, T is the absolute temperature in Kelvin, bT is a constant related to the
in which R is the gas constant, T is the absolute temperature in Kelvin, bT is a constant related to the
heat of adsorption and aT is the Tempkin isotherm constant. The Tempkin isotherm equation has been
heat of adsorption and aT is the Tempkin isotherm constant. The Tempkin isotherm equation has been
widely
applied
to describe
thethe
adsorption
surfaces.
widely
applied
to describe
adsorptionon
onheterogeneous
heterogeneous surfaces.
The The
isotherms
obtained
at
room
temperature
for
the
adsorptionofof
phenol
at the
optimum
isotherms obtained at room temperature for the adsorption
phenol
at the
optimum
adsorption
pH and
adsorbent
doses
7,whereas
whereasTable
Table
1 presents
corresponding
adsorption
pH and
adsorbent
dosesare
aregiven
givenin
in Figure
Figure 7,
1 presents
thethe
corresponding
constants
along
with
thethe
coefficients
ofoflinear
(R22))associated
associatedwith
with
each
linearized
model.
constants
along
with
coefficients
linearcorrelation
correlation (R
each
linearized
model.
Adsorption Capacity, q (mg/g)
30
SG
CM
25
20
15
10
5
0
0
30
60
90
120
150
180
Phenol Concentration at Equilibrium (mg/L)
Figure
7. Concentration
effect
isotherm
curves
adsorption
of phenol
marine
algae.
Figure
7. Concentration
effect
andand
isotherm
curves
for for
the the
adsorption
of phenol
on on
marine
algae.
Initial
Initial solution pH: 10 and masses of SG and CM are 40 mg and 100 mg, respectively.
solution pH: 10 and masses of SG and CM are 40 mg and 100 mg, respectively.
Environments 2017, 4, 1
10 of 14
Table 1. Calculated Constants and Parameters of the isotherm models for the adsorption of phenol
onto Sargassum sp. (SG) and Chaetomorpha (CM) algae.
Isotherm Model
SG
CM
82.10
0.0038
0.98
5.1 × 10−5
<0.0001
17.69
0.0064
0.98
9.64 × 10−4
<0.0001
0.579
0.787
0.99
5.1 × 10−3
<0.0001
0.199
0.767
0.98
9.22 × 10−3
<0.0001
0.0711
209.42
0.962
3.36
<0.0001
0.083
696.201
0.95
0.575
<0.0001
Langmuir
qmax (mg·g−1 )
b (L/mg)
R2
SD2
p
Freundlich
kF
1/n
R2
SD2
p
Tempkin
aT
bT
R2
SD2
p
As observed in Table 1, the maximum value for the adsorption capacity for phenol was obtained
by the marine seaweed SG (qmax = 82.10 mg·g−1 ), followed by CM (qmax = 17.7 mg·g−1 ). As for the
coefficients of correlation, we can postulate that the three models are adequate for modelling the
isotherm of the removal of phenol by both adsorbents. Therefore, a mixed adsorptive mechanism
under these experimental conditions can be suggested. The modelling of the adsorption process
also indicates that although SG has a higher adsorption capacity than CM, CM reports a higher
adsorbate/adsorbent affinity, as indicated by the b Langmuir constant [26,27]. This higher affinity of
CM is corroborated by the higher bT Tempkin constant for CM, which is associated with the heat of
adsorption. Lastly, the Freundlich theory shows a spontaneous adsorption as denoted by the 1/n value,
with values around 0 and 1 for both adsorbents. The isotherm results demonstrate that these marine
algae are competitive adsorbent when compared to other previously studied seaweed [15,16].
3.2.4. Effect of Salinity
The adsorption capacity is strongly associated with different types of electrostatic interactions
(complexation, ionic exchange, and electrostatic forces) between the adsorbate and the adsorbent.
These coulombic interactions strongly depend on the electrostatic environment which determines the
existence of the interaction. Other ions in solution compete for the active sites [5,6]. Figure 8 displays
the effect of different salts on the adsorption of phenol. Competition between the phenol and the salt
for the active sites was expected. However, as shown in Figure 8, SG increases its phenol affinity at
increasing salt concentrations. This could be explained by the non-electrostatic adsorption mechanism
of phenol. The presence of salt makes the solution more polar and therefore phenol (with its aromatic
ring) tends escape from this unfriendly polar environment and promote the adsorption onto the surface
and pores of the algae. On the other hand, CM shows a completely different profile. The adsorption
improves at low salt concentration, and dramatically decreases at relatively higher salt concentrations.
These results could be attributed to the different types of actives sites on SG and CM. Apparently, in SG,
the adsorption sites for phenol are not shared by the adsorption sites for the salt ions, and therefore SG
is able to tolerate salts.
Environments 2017, 4, 1
Environments 2016, 4, 1
11 of 14
11 of 14
60
NaCl
CaCl2
Na2SO4
50
40
30
NaCl
CaCl2
Na2SO4
Adsorption Percentage (%ADS)
Adsorption Percentage (%ADS)
60
50
40
30
20
10
20
0
50
100
150
Salt concentration (mmol/L)
(a)
200
0
50
100
150
200
Salt Concentration (mmol/L)
(b)
Figure 8. Salinity effect on the adsorption of phenol onto Sargassum sp. (a) and Chaetomorpha (b).
Figure 8. Salinity effect on the adsorption of phenol onto Sargassum sp. (a) and Chaetomorpha (b). Phenol
Phenol concentration 100 mg/L; Initial solution pH: 10; masses of Sargassum sp. and Chaetomorpha: 40
concentration 100 mg/L; Initial solution pH: 10; masses of Sargassum sp. and Chaetomorpha: 40 mg and
mg and 100 mg, respectively.
100 mg, respectively.
Conversely, the adsorption sites of CM are shared by phenol and salt ions. Although the
Conversely,
the adsorption
of CM
are shared by
and decreases,
salt ions. especially
Althoughwith
the
adsorption
is initially
increased atsites
low salt
concentrations,
the phenol
adsorption
adsorption
is initially
increased at low salt concentrations, the adsorption decreases, especially with
NaCl
and CaCl
2. This demonstrates an interfering effect of chloride ions on the adsorption of phenol
NaCl
and
CaCl
.
This
an interfering
of chloride
ions
on the adsorption
phenol
2
onto CM. This negativedemonstrates
effect is not observed
in Na2effect
SO4, where
a slight
adsorption
increase isofactually
onto
CM.
This
negative
effect
is
not
observed
in
Na
SO
,
where
a
slight
adsorption
increase
is
actually
observed. The small increase of adsorption at low2 salt4 concentration could be explained due to the
observed.coefficient
The smallofincrease
adsorption
at low
salt
concentration
be explained
duesites
to the
partition
phenol of
between
the polar
bulk
solution
and the could
solvent-isolated
active
of
partition
coefficient
of
phenol
between
the
polar
bulk
solution
and
the
solvent-isolated
active
sites
of
CM. However, when the salt concentration increases, the competition between chloride and phenol
CM.
However,
when
the
salt
concentration
increases,
the
competition
between
chloride
and
phenol
ions decreases and the adsorption of phenol decreases. This effect is not observed with sulfate ions,
ions decreases
and
the adsorption
phenol
decreases.and
Thistherefore
effect is do
notnot
observed
with
because
they are
known
to be veryofpoor
nucleophiles
compete
forsulfate
active ions,
sites
because
they
are
known
to
be
very
poor
nucleophiles
and
therefore
do
not
compete
for
active
sites
(Lewis bases).
(Lewis bases).
3.2.5. Effect of the Presence of Detergent
3.2.5. Effect of the Presence of Detergent
The real challenge of wastewaters resides in the complexity of the solution, which is expected to
The real challenge of wastewaters resides in the complexity of the solution, which is expected
contain a broad diversity of inorganic and organic species that may prevent the binding of the target
to contain a broad diversity of inorganic and organic species that may prevent the binding of the
pollutants to the active adsorption sites (steric hindrance). To explore the role steric factors on the
target pollutants to the active adsorption sites (steric hindrance). To explore the role steric factors
adsorption, phenol solutions were mixed with an anionic detergent, namely sodium dodecyl
on the adsorption, phenol solutions were mixed with an anionic detergent, namely sodium dodecyl
sulfonate (SDS), which is an inert molecule and highly soluble in water. Preliminary tests with visible
sulfonate (SDS), which is an inert molecule and highly soluble in water. Preliminary tests with
spectrophotometry with and without SDS showed no difference in the absorbance for phenol
visible spectrophotometry with and without SDS showed no difference in the absorbance for phenol
quantification, demonstrating that SDS does not interact with phenol or the adsorbents. It is expected
quantification, demonstrating that SDS does not interact with phenol or the adsorbents. It is expected
that SDS will sterically interfere with the appropriate interaction between the adsorbent and the
that SDS will sterically interfere with the appropriate interaction between the adsorbent and the
phenol at the solution level. Results are shown in Figure 9, demonstrating that SDS promotes the
phenol at the solution level. Results are shown in Figure 9, demonstrating that SDS promotes the
adsorption of phenol for both adsorbents with similar intensities. This effect could be attributed to a
adsorption of phenol for both adsorbents with similar intensities. This effect could be attributed to a
change in the phenol availability in the solution, and not to the type of adsorbent. Although SDS is
change in the phenol availability in the solution, and not to the type of adsorbent. Although SDS is
an amphipathic molecule, it is not able to trap phenol in a micelle. At pH 10, most of the phenol is
an amphipathic molecule, it is not able to trap phenol in a micelle. At pH 10, most of the phenol is
ionized at the level of the phenoxide ion, but is also hydrophobic due to the presence of the aromatic
ionized at the level of the phenoxide ion, but is also hydrophobic due to the presence of the aromatic
ring. Our explanation for this adsorption increase could be understood as the stabilization of the
ring. Our explanation for this adsorption increase could be understood as the stabilization of the
aromatic ring of phenoxide in solution by the SDS hydrophobic side chains. This could potentially
aromatic ring of phenoxide in solution by the SDS hydrophobic side chains. This could potentially
make phenoxide ions more accessible and chemically available in the solution to be adsorbed by the
make phenoxide ions more accessible and chemically available in the solution to be adsorbed by the
algae. Consequently, SDS promotes a more amphipathic environment and enhances the transport of
algae. Consequently, SDS promotes a more amphipathic environment and enhances the transport of
phenol from the bulk solution into the algal surface.
phenol from the bulk solution into the algal surface.
Environments
2017,2016,
4, 1 4, 1
Environments
12 of 14
12 of 14
Environments 2016, 4, 1
Adsorption
Percentage
Adsorption
Percentage
(%ADS) (%ADS)
12 of 14
40
SG
CM
40
30
SG
CM
30
20
20
10
10
0
0.0
0.1
0.2
0.3
0.4
0.5
0.4
0.5
SDS Concentration (mmol/L)
0
0.0
0.1
0.2
0.3
Figure 9. Effect of the presence of detergentSDS
Sodium
dodecyl
Concentration
(mmol/L)sulphonate (SDS) on the adsorption of
Figure 9. Effect of the presence of detergent Sodium dodecyl sulphonate (SDS) on the adsorption of
phenol onto Sargassum sp. (SG) and Chaetomorpha (CM). Phenol concentration 100 mg/L; Initial solution
phenol onto Sargassum sp. (SG) and Chaetomorpha (CM). Phenol concentration 100 mg/L; Initial
pH: 10;
massesEffect
of10;
SG
and
CM:
40and
mg
and40100
mg,
solution
masses
of SG
CM:
mgSodium
andrespectively.
100dodecyl
mg, respectively.
Figure 9.pH:
of
the presence
of detergent
sulphonate (SDS) on the adsorption of
phenol onto Sargassum sp. (SG) and Chaetomorpha (CM). Phenol concentration 100 mg/L; Initial
Effect
of
the
Heavy
Metal
Ions
3.2.6.3.2.6.
Effect
of the
Presence
ofofof
Heavy
solution
pH:
10;Presence
masses
SG
and Metal
CM:
40 Ions
mg
and 100 mg, respectively.
Industrial
contamination
comprises inorganic
inorganic and
andand
heavy
metals
are aare a
Industrial
contamination
comprises
andorganic
organicpollutants
pollutants
heavy
metals
3.2.6. Effect of the Presence of Heavy Metal Ions
large
component
of
this
pollution.
Heavy
metal
ions
are
introduced
into
industrial
wastewaters
by by
large component of this pollution. Heavy metal ions are introduced into industrial wastewaters
Industrial
contamination
inorganic and
and
heavy metalsa are
a
several
processes
and becomecomprises
a strong competitor
for organic
phenol. pollutants
Algae have
demonstrated
high
several processes and become a strong competitor for phenol. Algae have demonstrated a high affinity
large component
of thismetals
pollution.
Heavy
metal
are introduced
into
industrialthe
wastewaters
by
affinity
towards heavy
as well
[12–14],
andions
therefore
it is crucial
to evaluate
effect of these
towards heavy metals as well [12–14], and therefore it is crucial to evaluate the effect of these species
several on
processes
and become
a strong
forfor
phenol.
Algae
a high
species
the adsorption
of phenol.
Thecompetitor
solution pH
this test
hadhave
to bedemonstrated
adjusted to pH
6 to
on the
adsorption
ofheavy
phenol.
Theinsolution
pH
for
this test
had
to10,
bedivalent
adjusted
to pH
6 toand
maintain
the
maintain
the heavy
metal
ions as
solution.
As displayed
in Figure
copper,
affinity towards
metals
well [12–14],
and therefore
it is crucial
to evaluate
thelead
effect
ofcobalt
these
heavyions
metal
ions
inadsorption
solution.
Asphenol.
displayed
Figure
10,at
divalent
copper,
and cobalt
ions
have a
have
athe
positive
effect on
the
adsorption
of phenol
low
ppm)
both
species
on
of
The in
solution
pH
for
thisconcentrations
test
had tolead
be(5–100
adjusted
to for
pH
6 to
positive
effect
on
the
adsorption
of
phenol
at
low
concentrations
(5–100
ppm)
for
both
algae.
However,
maintain
the heavyasmetal
ions inconcentration
solution. As displayed
in the
Figure
10, divalent
copper, lead
cobalt
algae.
However,
the metal
increases,
adsorption
decreases.
Thisand
could
be
as theexplained
metal
concentration
increases,
the
adsorption
decreases.
could
be explained
by the
coverage
ions
have
abypositive
effect
on
of phenol
at low
concentrations
(5–100 ppm)
forwhich
both
the coverage
of the
the adsorption
surface
of the
adsorption
atThis
higher
metal
concentrations,
algae.
However,
as the
metal
concentration
increases,
the adsorption
decreases.
could
the
binding
of phenol,
even onmetal
different
actives sites.
At
low metal
concentration,
it can
be
of theprevents
surface
of the
adsorption
at higher
concentrations,
which
prevents
the This
binding
of phenol,
explained
by
theadsorbed
coverage
of
surface
ofconcentration,
the
adsorption
higher
metal
concentrations,
which
that
metal
ions
serve
as bridges
between
the adsorbent
and phenol
even postulated
on different
actives
sites.heavy
Atthe
low
metal
itatcan
be postulated
that adsorbed
heavy
prevents
the
binding
of
phenol,
even
on
different
actives
sites.
At
low
metal
concentration,
it
can
be
(due
to
their
positive
charge
and
Lewis
acidity).
Nevertheless,
with
an
increasing
heavy
metal
ions
metal ions serve as bridges between the adsorbent and phenol (due to their positive charge and Lewis
concentration,
increasing
concentration
the as
counter
ions
such asthe
nitrate
is alsoand
observed,
postulated thatan
adsorbed
metal ions ofserve
bridges
between
phenol
acidity).
Nevertheless,
with anheavy
increasing
heavy metal
ions
concentration,
anadsorbent
increasing
concentration
competing
the active
sites.
conclusion,
metal ions
a negative
effect
on ions
the
(due to theirfor
positive
charge
and In
Lewis
acidity). heavy
Nevertheless,
with have
an increasing
heavy
metal
of the counter ions such as nitrate is also observed, competing for the active sites. In conclusion, heavy
concentration,
an increasing
of the
counter
ions
such
is also algae/lead
observed,
adsorption
of phenol
for both concentration
algae, with a more
intense
effect
with
Pb, as
duenitrate
to a stronger
metal(II)
ions
have
a negative
effect
onare
the
adsorption
of strongly
phenol
both
algae,
with[13]
a more
intense
competing
for
the
active
sites.
In
conclusion,
metalfor
ions
have
a algae
negative
effecttherefore
on theeffect
ions
affinity.
Cobalt
(II)
ions
not
known toheavy
be
adsorbed
onto
and
with do
Pb,
due
to
a
stronger
algae/lead
(II)
ions
affinity.
Cobalt
(II)
ions
are
not
known
to
be
strongly
adsorption
of phenol
for both algae,
with
a more
intense
with Pb,
due to a stronger algae/lead
not represent
an interfering
species
with
respect
to theeffect
adsorption
of phenol.
adsorbed
onto
algaeCobalt
[13] and
therefore
not to
represent
anadsorbed
interfering
with
(II) ions
affinity.
(II) ions
are not do
known
be strongly
ontospecies
algae [13]
andrespect
thereforeto the
adsorption
of phenol.an interfering species with respect to the adsorption of phenol.
do not represent
Co
Cu
Pb
Co
Cu
Pb
50
50
40
40
30
Co
Cu
Pb
30
20
0
20
40
60
80
Heavy metal ion concentration (mg/L)
20
0
20
(a)
40
60
80
Heavy metal ion concentration (mg/L)
(a)
Co
100
Cu
Pb
100
Percentage
AdsorptionAdsorption
Percentage
(%ADS) (%ADS)
Percentage
AdsorptionAdsorption
Percentage
(%ADS) (%ADS)
50
50
40
40
30
30
20
20
10
0
20
40
60
80
100
Heavy Metal ion concentration (mg/L)
10
0
20
(b)
40
60
80
100
Heavy Metal ion concentration (mg/L)
(b)
Figure 10. Effect of the presence of heavy metal ions on the adsorption of phenol onto Sargassum sp.
(a) and Chaetomorpha (b). Phenol concentration 100 mg/L; Initial solution pH: 6; masses of Sargassum sp.
and Chaetomorpha: 40 mg and 100 mg, respectively.
Environments 2017, 4, 1
13 of 14
4. Conclusions
The development of new alternatives for the decontamination of industrial runoffs with high
level of phenol and its derivatives is still a scientific concern. Alternative and eco-friendly adsorbents
that do not produce byproducts are also priority materials that need to be studied. This study
proposes native marine algae from Puerto Rico as candidates for the adsorption of phenol from
solutions. Results indicate that pH, salinity and the presence of interfering substances have an
important impact on the adsorption. Experimental data indicates that phenol prefers to adsorb
onto Sargassum (SG). It is hypothesized that hydroxyl groups from the structural polysaccharides
are responsible for this preference due to the formation of hydrogen bonds and dipole interactions.
Langmuir, Tempkin and Freundlich isotherm models were fitted to the results, suggesting a combined
adsorption mechanism, with maximum adsorption capacities of 82.1, and 17.7 mg·g−1 for SG and CM,
respectively. Equilibrium experimental data indicate that pH has a strong effect on the adsorption
and allow high adsorption at pH 10 and using a minimum mass of 40 mg and 100 mg for SG and
CM, respectively. Finally, the adsorbent and the mechanism were characterized by instrumental
analysis. FTIR studies demonstrated that carboxyl, hydroxyl and carbonyl groups could be associated
with the adsorption mechanism of phenol. SEM/EDS experiments suggest that these algae have
morphological and chemical properties that are appropriate for good adsorbents. SEM showed pockets
and protrusions that are essential to house and seclude pollutants. EDS indicates the presence of key
elements that could be associated with specific functional groups that are associated with adsorption
of phenol. Finally, TGA demonstrated that both adsorbents can tolerate temperatures as high as
230 ◦ C, indicating that these materials have mechanical and thermal properties that make them good
candidates as adsorbers of phenol under more adverse conditions. The use of renewable and naturally
occurring materials as adsorbents of pollutants is a new area in bioremediation that needs to be further
explored. Marine algae have demonstrated that they can be used to produce biofuels as well as to
decontaminate inorganic and organic pollutants from contaminates wastewaters.
Acknowledgments: This work was supported by the CRSP and CSTEP programs and the Science Department at
BMCC/CUNY. This work was supported by the C3IRG grant #1224 (to A.E.N.) the National Science Foundation
grant # EEC-1263250 to Rutgers University, and a grant from the US Department of Education and the Center for
Renewable Energy and Sustainability to the UPR Río Piedras.
Author Contributions: Abel E. Navarro and Anibal Hernandez-Vega prepared the adsorbents and conducted
the adsorption experiments; Loretta Roberson contributed to the algae characterization and data analysis;
Md. Emran Masud performed the material characterization experiments; Abel E. Navarro and Liz Diaz-Vasquez
designed the research and equally contributed to the writing and editing of the manuscript. All authors read and
approved the final manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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