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Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 5158
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Colloids and Surfaces A: Physicochemical andEngineering Aspects
journa l homepage: www.elsevier .com/ locate /colsur fa
pH dependence and thermodynamics ofHg(II) adsorption onto
chitosan-poly(vinyl alcohol) hydrogel adsorbent
Xiaohuan Wang a,1, Ruzhong Sun b,, Chuanyi Wang a,
a Key Laboratory of Functional Materials andDevices for Special Environments of CAS, Xinjiang TechnicalInstitute of Physics & Chemistryof CAS, 40-1
South Beijing Road, Urumqi 830011, PR Chinab College of Chemistryand Pharmacy Engineering, Nanyang NormalUniversity, 1638 WolongRoad, Nanyang 473061, PR China
h i g h l i g h t s
CTS-PVA hydrogel adsorbent showed
superior adsorption properties for
Hg(II) ions. pH influence and theromodynamics
of Hg(II) adsorption on the hydrogel
adsorbent was studied. Functional groups responsible for
Hg(II) adsorption changed with the
pH ofsolutions. The binding force between Hg(II) and
functional groups was strengthened
at high temperatures. Results obtained in this study may
provide a scientific and engineering
basis for Hg(II) removal.
g r a p h i c a l a b s t r a c t
a r t i c l e i n f o
Article history:
Received 2 June 2013
Received in revised form 25 August 2013
Accepted 29 August 2013
Available online 6 September 2013
Keywords:
pH dependence
Thermodynamics
Chitosan/poly(vinyl alcohol) hydrogel
adsorbent
Hg(II)
Adsorption
a b s t r a c t
The chitosan/poly(vinyl alcohol) (CTS-PVA) hydrogel adsorbent with three-dimensional network struc
ture showed superior adsorption properties for Hg(II) ions. The pH influence study showed Hg(II
adsorption on the hydrogel adsorbent is strongly pH-dependent. The adsorption capacity in pH 2.0
solution (200.20 mg/g) is higher than that in pH 11.00 solution (140.27 mg/g), which indicated that Hg(II
adsorption on the hydrogel adsorbent was not only by electrostatic interactions. FT-IRspectral analysi
before and after Hg(II) adsorption represented that the functional groups responsible for Hg(II) adsorp
tion changed with the pH of solutions. Thermodynamic studies revealed that Hg(II) adsorption on th
hydrogel adsorbent is a favorable, spontaneous, and endothermic chemisorption process. The bindin
force between Hg(II) ions and functional groups was strengthened at high temperatures, which mad
the adsorption more favorable at high temperatures. The adsorption process fitted well by the Langmui
and the Freundlich isotherm models. The Langmuir maximum adsorption capacity increased from 697.7
to 950.62 mg/g with increasing temperature from 20to 60 C. This study will contribute to an in-deptunderstanding ofadsorption phenomena, and provide a scientific and engineering basis for the practica
application ofthe CTS-PVA hydrogel adsorbent.
2013 Elsevier B.V. All rights reserved
Corresponding author. Tel.: +86 377 63525056. Corresponding author. Tel.: +86 991 3835879; fax: +86 991 3838957.
E-mail addresses: [email protected](X. Wang), [email protected] (R. Sun),
[email protected] (C. Wang).1 Tel.: +86991 3835879; fax: +86991 3838957.
1. Introduction
Allmercury compoundsare known to be highlytoxic chemicals
Inorganic mercury has been reported to produce harmful effect
in culture medium at 5g/L. It exists in two ionic states: Hg(Iand Hg(II). Hg(II) or mercuric salts, are much more common i
the environment than Hg(I) or mercurous salts [1]. Hg(II) can b
0927-7757/$ see front matter 2013 Elsevier B.V. All rights reserved.
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52 X.Wang et al. / Colloids and Surfaces A: Physicochem.Eng. Aspects 441 (2014) 5158
methylated by reducing bacteria in anoxic habitats and trans-
formed into methylmercury (MeHg+ orMe2Hg). So, it is considered
as the most toxic form of mercury [2]. Recently, great efforts have
been made to develop methods that can effectively remove Hg(II)
ions from wastewater, andat thesame time, recover itscompounds
in a more concentrated form to avoid the second pollution after
removing them from the water [3]. The use of adsorbents pos-
sessing selectivity toward Hg(II) ions makes good contributions
to achieving this target. Polymer adsorbents containing ligandssuch as nitrogen-containing functional groupsare often used in the
selective removal of Hg(II) from aqueous solutions [46].
Chitosan (CTS), a copolymer that is primarily composed
of(14) linked 2-amino-2-deoxy-d-glucopyranose units, andresidual 2-acetamido-2-deoxy-d-glucopyranose units, is a biopoly-
mer of the most widely studies on heavy metal removal. It shows
outstanding adsorption capacity for Hg(II) ions ranging from 430
to 1127mg/g especially in pH close to neutral, due to the presence
of large number of functional groups (acetamido, primary amino
and/or hydroxyl groups) with high chemical reactivity in its chem-
ical structure [5,7,8]. However, the protonation of amino groups
results in CTS dissolution in acidic solutions, which limits CTS to be
used as an adsorbent in acidic media. In addition, the amine groups
of CTS do not have good adsorption selectivity for different metal
ions [9]. All of these determine CTS is not a satisfying adsorbent for
the selective removal of Hg(II) ions from wastewater [10].
Poly(vinyl alcohol) (PVA), a semicrystalline hydrophilic poly-
mer, is usually used to blend with CTS to obtain a variety of
materials with good mechanical and chemical properties, such as
hydrogels [11,12], blend foams [13], blend/composite membranes
or films [14], blend beads [15,16], etc. The addition of PVA can
improve the mechanical property and reduce the brittleness of the
dried sample. Meanwhile, hydroxyl groups in PVA polymer struc-
ture may contribute to the adsorption for heavy metal ions. These
materials have been widely studied in the removal of heavy metal
ions such as lead, copper, silver and cadmium ions from water
or wastewater [11,12]. However, little attention has been paid to
investigating their adsorption characteristics for mercury ions.
Our previous studies showed that the CTS-PVA hydrogel adsor-bent prepared by a glutaraldehyde cross-linking in combination
with an alternate freeze-thawed process demonstrated superior
adsorption properties for Hg(II) ions [17]. The hydrogel adsor-
bent contains NH2, NHCOCH3 and C N groups as well as OH
groups. It can selectively adsorb Hg(II) ions from multi-component
solutions with high efficiency. The CTS-PVA hydrogel adsorbent
containing 30wt.% PVA showed moderate mechanical property,
excellent adsorption properties, and the most important is, it had
no leaching in solutions. So, it was thereby selected as an objective
adsorbent for typical adsorption experiments in the present work.
The present work focused on the pH dependence and adsorption
mechanisms of the CTS-PVA hydrogel adsorbent for Hg(II) ions in
differentpH solutions,as well as the detailed adsorption thermody-
namics. This study will contribute to an in-depth understanding ofadsorption phenomena and open up an effective way for mercury
removal from water.
2. Experimental
2.1. Materials
CTS (industrial grade, the degree of deacetylation was 75%,
and the average molecular weight was 3105 g/mol) was pur-
chased from Zhejiang Golden-Shell Biochemical Co. Ltd. (Zhejiang,
China). PVA(industrialgrade, the degree of hydrolysis was99% and
the average degree of polymerization was 1700) was provided by
Lanzhou Xinxibu Vinylon Company Ltd (Gansu, China). Glutaralde-
hyde with a concentration of 25%, mercuric acetate and other
chemicalswere of analyticalgrade andcommerciallyobtainedfrom
Tianjin Kermel Chemical Reagent Co., Ltd (Tianjin, China). Aqueous
solutions were all prepared with Millipore water (18.25 M/cm).
2.2. Preparation
The CTS-PVA hydrogel adsorbent with a three-dimensional net-
work structure waspreparedaccording to our previous report[17].
Its schematic network structure and the digital photo of the dried
samplewere shown in Fig.1. The sampleusedfor adsorption exper-
iments was milled and sieved through an 80-mesh screen.
2.3. Adsorption experiments
The adsorption experiments were carried out by adding certain
amount of the driedsampleinto a simulatedwastewater containing
Hg(II) ions at a desirable concentration and pH value, and then the
mixture was shaken in a thermostatic shaker bath at an appropri-
ate temperature until the adsorptiondesorption equilibrium was
established. The pH of solutions was adjusted by the dilute acetic
acid or sodium hydroxide solution and measured by a pH meter
(Mettler Toledo 320).Forinvestigating theeffectof theinitial pH (pH0) of solutions on
adsorption, a pH range from 2.00 to 11.00 was selected, and a con-
centrationof 85.56mg/L wasselected as the initial concentration of
Hg(II) solution to avoid precipitation of Hg(II) ions. For equilibrium
adsorption experiments at different temperatures (20, 30, 40, 50
and 60C), Hg(II) solutions with a concentration range from about
500mg/L to 2500mg/L were used, and the mixture was shaken in
a thermostatic shaker bath for 24h to ensure the equilibrium to be
established.
The concentration of mercury in solutions was determined by
an atomic fluorescencespectrophotometer (AF-640,Beijing Beifen-
gruili Analytical Instrument Co., China). The adsorption capacity of
the adsorbent sample for Hg(II) ions at a given time tor at equilib-
rium can be derived using the following equation:
q =(C0 C) V
m , (A.1)
where q is the amountof Hg(II) adsorbed at time tor at equilibrium
(mg/g), C0 is the initial Hg(II) concentration (mg/L), Cis the Hg(II)
concentration at time tor at equilibrium (mg/L), m is the mass of
the adsorbent used (g) and Vis the volume of Hg(II) solution used
(L).
2.4. Characterization
2.4.1. The determination of the isoelectric point (pI)
Theisoelectricpoint(pI)of theCTS-PVA hydrogel adsorbentwasdetermined by the solid addition method [18]. 0.1mol/L NaCl solu-
tion was used as electrolytic solution in this study. The pH value
of the solution was adjusted with 0.1 mol/L NaOH or 0.1 mol/L HCl
solutions to cover a range from 2 to 12 in approx 2pH unit incre-
ments. To 25mLNaCl solutions with the desired initial pH value
(pH0), 0.1000 g CTS-PVA hydrogel adsorbent samples were added.
Then theconical flasks were securely cappedimmediately. The sus-
pensions were manually shaken andallowed to equilibrate for48 h
with intermittent manual shaking at room temperature. The final
pH value (pHf) of each supernatant fluid was determined again.
By plotting the difference between the pH0and the pHf(i.e., pH,which equals pH0minus pHf) versus pH0, a curve can be obtained.
The pH value which is corresponding to pH= 0 in the resulting
curve is the pI of the sample.
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X. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 5158 5
Fig. 1. Schematic structure(A) and digital photo (B)of theCTS-PVA hydrogel adsorbent.
2.4.2. Fourier transform infrared spectroscopy (FTIR)
The FTIR spectra of the CTS-PVA hydrogel adsorbent before and
after Hg(II) adsorption were recorded using a Fourier transform
infrared spectrometer (FTS-165, BIO-RAD, USA). All the sampleswere prepared as potassium bromide tablets, and the range of the
scanning wavenumbers was 5004000cm1.
2.4.3. Surface area analysis
The surface area of the CTS-PVA hydrogel adsorbent were ana-
lyzed by a nitrogen adsorptiondesorption method at 77K using
a surface area analyzer (Micromeritics, ASAP 2020). The pretreat-
ment conditions are 105 C and vacuum for 3h. The surface area
was calculated by the BrunauerEmmertTeller (BET) equation.
3. Results and discussion
3.1. pH dependence and adsorption mechanisms of the hydrogel
adsorbent for Hg(II) ions
3.1.1. Effect of the pH0of solution on adsorption
The determination of the isoelectric point (pI) of an adsorbent is
necessary for further elucidating the adsorption mechanism. So, a
preliminary test of pI wasdetermined by thesolid addition method
[18]. Result showed that the pI of the sample was 7.85. Therefore,
in the solution of pH value below 7.85, the particle surface of the
hydrogel adsorbent is positively charged. Otherwise, the surface is
negatively charged [18].
The effect of the pH0 of solution on Hg(II) adsorption was
shown in Fig. 2. The pH-dependent curve showed two peaks
and a minimum, which revealed the strong pH-dependence ofHg(II) adsorption on the hydrogel adsorbent. Previous study about
lead (II) adsorption on CTS-PVA hydrogel bead also showed a
similar phenomena [12]. The adsorption capacity in pH 2.00
solution (200.20mg/g) is higher than that in pH 11.00 solution
(140.27mg/g), whichindicatedthat Hg(II) adsorption on the hydro-
gel adsorbent was not only by electrostatic interactions, because
that, the positively charged surface of the adsorbent in pH 2.00
solution was not as favorable as the negatively charged surface
in pH 11.00 solution for the adsorption of Hg(II) cations. The high
adsorption capacity of the adsorbent in pH 2.00 solution also indi-
cated that there must be more than one mechanismresponsiblefor
Hg(II) adsorption on the hydrogel adsorbent. Our previous study
has revealed that both chelation and ion exchange contributed to
Hg(II) adsorption on the CTS-PVA hydrogel adsorbent [17].
3.1.2. Analysis of adsorption mechanisms under different pH
conditions
In order to further illustrate the nature of Hg(II) adsorption o
the hydrogel adsorbent under different pH conditions, FT-IR spectra of the dried sample before and after adsorption in different pH
solution were compared.
Firstly, we compared FT-IR spectra of the hydrogel adsorben
before and after adsorption in pH 2.00 acetic acid and mercuri
acetate solutions, respectively (Fig. 3). It was found that, com
pared with the FT-IR spectrum of the unloaded sample (Fig. 3a)
the FT-IR spectrum of the sample after adsorption in pH 2.00 aceti
acid solution (Fig. 3b) showed several obvious spectral changes
The characteristic peak of the N H stretching vibration situate
at about 3372cm1 obviously shifted to the higher wavenum
ber (3403 cm1). Meanwhile, the characteristic peaksof the C O
stretching vibration (shifted from 1651 to 1635cm1), the N H
deformation vibration (shifted from 1598 to 1557cm1) and
the C N stretching vibration ( shifted from 1420 to 1384 cm
1
strongly shifted to the lower wavenumbers. These changes may b
caused by the binding of NH2 and NHCOCH3 groups with pro
tons to form complexes (Eqs. (B.1) and (B.2)), and thereby leading
the surface of the sample to be positively charged [12].
R NH2 +H+ R NH3
+ (B.1
Fig. 2. pH0 effect on the adsorption of the CTS-PVA hydrogel adsorbent for Hg(I
ions. Adsorption experimentsC0 : 85.56mg/L; sample dose: 0.0100g/50mL; th
range ofpH0: 2.0011.00; temperature: 30C; contact time: 24h.
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54 X.Wang et al. / Colloids and Surfaces A: Physicochem.Eng. Aspects 441 (2014) 5158
Fig. 3. FT-IR spectra of the unloaded CTS-PVA hydrogel adsorbent (a), the sample
after being dipped in pH 2.00 acetic acid solution (b) and the sample after Hg(II)
adsorption in pH 2.00 mercuric acetate solution (c), respectively.
NH CCH3
O
+ H+ CH3C
O
NH2+
R
R
(B.2)
Compared with FT-IR spectra of the unloaded sample (Fig. 3a)
and the sample after adsorption in pH 2.00 acetic acid solution
(Fig. 3b), the FT-IR spectrum of the adsorbent after adsorption
in pH 2.00 mercuric acetate solution (Fig. 3c) showed several
new changes. The characteristic peaks of the saccharide structure
stronglyshifted to the lowerwavenumbers.Meanwhile, the charac-
teristic peak of the C H stretching vibration shifted tothe higher
wavenumbers (2935 cm1).These indicated thatfreependant OH
groups in the polymer structure should interact with Hg(II) ions in
pH 2.00 mercuric acetate solution. The possible chemical reaction
was given as follows:
R OH + Hg2+ (R OH)Hg2+ or (R OH)2Hg2+ (B.3)
Despite that, several other spectral changes were observed inFig, 3c when compared with Fig. 3a. The characteristic peak corre-
sponding to the C O and C N stretching vibration shifted from
1651cm1 to 1641cm1, which is not as obvious as that observed
in Fig.3b. Thismaybeindicatethe interaction of NHCOCH3groups
and C N groups with Hg(II) ions, because that the C N group is a
milder basic ligand than NH2 or OH groups and it can interact
with a verysoft acidsuch asHg(II) ions. The reaction between C N
groups and Hg(II) ions has been confirmed by our previous study
[17] and other related researches [19]. It was given as follows:
RCH N R' + (CH3COO)2Hg RCH HgOOCCH3
R'
N
(B.4)
The characteristic peak of the N H deformation vibration in
Fig.3c nearly disappeared completely when compared withFig. 3b.
This may be due to the reaction between NHCOCH3 groups and
Hg(II) ions, which was given as follows:
R NH CCH3+(CH3COO)2Hg CH3C
O
N
R
HgOOCCH3orCH3C N Hg N CCH3R
O OO
R (B.5)
Secondly, we compared FT-IR spectra of the hydrogel adsorbent
before and after adsorption in different pH mercuric acetate solu-
tions (shown in Fig. 4). Itcan be seen that, compared with the FT-IR
spectrum after adsorption in pH 2.00 mercuric acetate solution
Fig. 4. FT-IR spectra of the unloaded CTS-PVA hydrogel adsorbent (a) and samples
after Hg(II)adsorptionin differentpH solutions(b: pH 2.00; c: pH 5.00; d: pH 7.00;
e: pH9.00; f: pH11.00).
(Fig. 4b), the FT-IR spectrum after adsorption in pH 5.00 mercuric
acetate solution (Fig. 4c) showed several more obvious changes.
Except for the obvious changes of the characteristic peaks of thesaccharide structure, C N groups and NHCOCH3 groups, the
characteristic peaks of the N H stretching vibration obviously
shifted to the higher wavenumber (from 3398 to 3428 cm1).
Meanwhile, the characteristic peaks of the C H stretching
vibration near 2935 cm1 and the C H symmetrical deformation
vibration at 1417cm1 and 1381cm1 evidently shifted to the
lower wavenumbers. These indicated that free pendant NH2groups may also interact with Hg(II) ions when the sample was
dipped in pH 5.00 mercuric acetate solution. The possible chemical
reaction was given as follows:
2R NH2 +Hg2+ (R NH2)2Hg
2+ (B.6)
The FT-IR spectra after Hg(II) adsorption in pH 7.00 and 9.00
mercuric acetate solutions were very similar (Fig. 4d and e). Com-
pared with the FT-IR spectrum of the unloaded sample (Fig. 4a),
the obvious shifts of the FT-IR spectra after Hg(II) adsorption in
pH 7.00 and 9.00 mercuric acetate solutions mainly concentrated
on the characteristic peaks of the N H stretching vibration, the
C O stretching vibration, the N H deformation vibration and
the C N stretching vibration.These indicated NH2, NHCOCH3,
and C N groups may allparticipatein Hg(II)adsorption under such
pH conditions.
Compared with Fig. 4a, the FT-IR spectrum after Hg(II) adsorp-
tion in pH 11.00 (Fig. 4f) only showed an obvious shift of the
characteristic peak of the N H stretching vibration in NH2groups. This implied NH2 groups may be the main functional
groups responsible for Hg(II) adsorption in pH 11.00 mercuric
acetate solutions. According to the research of Li and Bai [12], OH
ions can be adsorbed onto the surface of the CTS-PVA hydrogeladsorbent through hydrogen bonds when the pH of solutions is
higher than its pI, and thereby lead to the surface of the adsorbent
negatively charged (Eq. (B.7)) [12].
R NH2 +OH R NH2. . .OH
(B.7)
So, in solutions which pH value is higher than the pI of the
adsorbent, the negative charged surface of the adsorbent will
facilitate Hg(II) ions being adsorbing by the electrostatic interac-
tion.
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Table 1
Parameters associated with Hg(II) adsorption on the CTS-PVA hydrogel adsorbent at different temperatures. (T: K; qm,exp: mg/g;KL: L/g; aL: L/mg;qm,cal: mg/g; S:m2/g; G
kJ/mol; H: kJ/mol; S: kJ/(mol K);KF:L/mg.).
T qm,exp The Langmuir model S RL G H S The Freundlich model
KL aL qm,cal R2 KF n R2
293 697.70 7.0215 0.0092 763.36 0.9973 87.056 0.16300.0429 4.748 94.5606 0.2925 0.9979
303 723.87 10.8378 0.0141 769.23 0.9979 87.726 0.11280.0284 6.003 134.227 0.2492 0.9970
313 741.31 15.6397 0.0202 775.19 0.9984 88.405 0.08160.0200 7.156 48.183 0.179 173.1929 0.2174 0.9972
323 837.25 41.4079 0.0484 854.70 0.9987 97.473 0.03570.0084 9.999 307.7797 0.1532 0.9972
333 950.62 71.1238 0.0754 943.40 0.9976 107.590 0.02320.0054 11.806 401.7723 0.1319 0.9892
3.2. Adsorption thermodynamics
3.2.1. Adsorption isotherms
The adsorption isotherms of Hg(II) adsorption on the CTS-PVA
hydrogel adsorbent at different temperature were presented in
Fig. 5. The isotherm shapes are consistent with the favorable
adsorption equilibrium patterns proposed by Weber et al. [20].
This suggested the adsorption process of Hg(II) adsorption on the
hydrogel adsorbent should be a favorable process. Such anticipa-
tionwas also confirmed by the equilibrium parameterRLcalculated
in the following section. The increase of the adsorption capacity
with increasing temperature indicated that the adsorption processof the CTS-PVA hydrogel adsorbentfor Hg(II) ionswas endothermic.
3.2.2. Adsorption isotherm analysis
The analysis of isothermdata is veryimportant for developingan
equation which canaccurately representthe results andbe used for
design purposes. The adsorption data obtained in this study were
analyzed by the well-known and widely applied isotherm models,
namely, the Langmuir and the Freundlich isotherm models.
The Langmuir model. The Langmuir model is perhaps the most
straightforward non-linear isotherm model. It is predicated on
the assumptions that the energy of sorption for each molecule is
the same and independent of surface coverage. And that sorption
occurs only on localizedsites and involves no interactions between
adsorbed molecules [20]. The Langmuir model is usually expressedas follows [21]:
qe =x
m =
KLCe(1+ aLCe)
, (A.2)
wherex is theamount of Hg(II) ions adsorbed (mg),m is theamount
of adsorbent used (g),Ce(mg/L) andqe(mg/g) are the liquid phase
Fig. 5. Adsorption isotherms of Hg(II) adsorption on the CTS-PVA hydrogel
adsorbent at different temperatures. Adsorption experimentsthe range of C0:
02500 mol/L; sample dose: 0.0500 g/25mL; pH0: 5.50; the range of temperature:
2060 C; contact time: 24h.
concentration and solid phase concentration of Hg(II) ions at equi
librium, respectively.
At low surface coverage, the Langmuir isotherm reduces to
linear relationship. The most common linear form of the Langmui
model is formulated as:
Ceqe
=1
KL+ (
aLKL
)Ce, (A.3
where KL(L/g) andaL(L/mg) are the Langmuir isotherm constants
respectively, and aLrelates to the energy of adsorption. When Ce/q
is plotted against Ce, a straight line will be obtained. The value ofK
can be obtained from the intercept which is 1/KL, and the value oaL can be obtained from the slope which is aL/KL. The maximum
adsorption capacity of the adsorbent, qm,cal, i.e., the equilibrium
monolayer capacity or saturation capacity, is numerically equal t
KL/aL. The Langmuir parameters of Hg(II) adsorption on the CTS
PVA hydrogel adsorbent at different temperatures were listed i
Table 1.
From Table 1, it can be seen that the linear correlation
coefficients (R2) at different temperatures are all close to 1. Thi
indicatedthat the adsorption process fitted well with the Langmui
model. So, it can be considered that Hg(II) ions were adsorbed with
a monolayer on the particle surface of the adsorbent. The Langmui
maximum adsorption capacity (qm,cal) increased from 697.70 to
950.62mg/g with the temperature of solutions increasing from 2
to 60C, which reflected the endothermic nature of the adsorption
process again. Compared with other adsorbents (Listed in Table 2
[2229], the Langmuir maximum adsorption capacity of the CTS
PVA hydrogel adsorbent are quite high. This indicated the CTS-PVA
hydrogel adsorbent is a superior adsorbent that can be used for th
removal of Hg(II) from aqueous solutions. The value ofaLincreased
from0.0092L/mg to 0.0754 L/mgwith increasingtemperature from
20 to 60 C, which indicated the binding force between Hg(II) ion
Table 2
A comparison of themaximumadsorption capacityof theadsorbents forHg(II) ion
Adsorbent Maximum adsorption
capacity (mg/g)
Source
Chitosan adsorbent (which is
crosslinked with glutaraldehyde)
145 (25C) [22]
Modified magnetic chitosan
adsorbents (which is crosslinked with
glutaraldehyde and functionalized
with magnetic nanoparticles (Fe3O4))
152 (25C) [22]
The polyaniline/attapulgite (PANI/ATP)
composite
800 (25C) [23]
A chitosanthioglyceraldehyde Schiffs
base cross-linked magnetic resin
(CSTG)
982 [24]
The cross-linked magnetic
chitosan-phenylthiourea (CSTU) resin
1353 [25]
The chitosanECH matrix 621.83 [26]
The polyaniline/humic acidcomposite 671 [27]
The PPyRGO composite 980 [28]
The ethylenediamine-modified
magnetic crosslinking chitosan
microspheres (EMCR)
539.59 [29]
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56 X.Wang et al. / Colloids and Surfaces A: Physicochem.Eng. Aspects 441 (2014) 5158
Fig. 6. Plot ofRLagainst C0at different temperatures. Adsorption experimentsthe
range ofC0: 02500mol/L; sample dose: 0.0500g/25mL; pH0: 5.50; the range of
temperature: 2060C; contact time: 24h.
and functional groups of the hydrogel adsorbent was strengthened
at a higher temperature. This further substantiated the endother-
mic nature of the adsorption process.
The dimensionless constant separation factor or equilibrium
parameter, RL, is another factor that can express the essential fea-
tures of the Langmuir isotherm. It is defined by the following
relationship [30]:
RL =1
1+ (KL/qm)C0 . (A.4)
For favorable adsorption process, the value of RL should be
ranged from 0 to 1. If the value of R L is beyond 1, it implies that
the adsorption process is unfavorable. The value of RLof Hg(II)
adsorption on the CTS-PVA hydrogel adsorbent changed with C0was shown in Fig. 6, and the values of R L at different temper-
atures were given in Table 1. Values of RL were in the range of
01, which indicatedthe adsorption process of the hydrogel adsor-
bent for Hg(II) ions is favorable again. Values of RLdecreased with
the increase of the temperature suggested the adsorption is more
favorable at high temperatures.
The Freundlich model. The Freundlich model is an empirical
equation and perhaps the most widely used nonlinear sorption
equilibrium model. This model applies to adsorption onto hetero-
geneous surfaces with a uniform energydistribution and reversible
adsorption. Moreover, this model is not restricted to the formation
of the monolayer. Thats to say, the amount of adsorbate adsorbed
on the adsorbent will increase in case of increasing the adsorbate
concentration in the solution.The Freundlichmodel has the generalform as follows [31]:
qe = KFCne , (A.5)
whereKFis the Freundlichconstantwhich relates to sorption capac-
ity(L/mg),n is the heterogeneity factor whichrelatesto the sorption
intensity. KFand n are empirical constants which is dependent on
several environmental factors. The value ofn ranges from 0 to 1,
andindicates thedegree of non-linearity between thesolutioncon-
centration and the adsorption. If the value ofn is equal to 1, the
adsorption is linear. If the value is below 1, the adsorption process
is chemical. If the value is above 1, the adsorption is a favorable
physical process. The more heterogeneous the surface, the closer
the value ofn is to 0 [12,32].
For determination of these empirically derived constants, the
Freundlich model is usually expressed the logarithmic form as fol-
lows:
log(qe) = n log(Ce) + log KF. (A.6)
KFand n can be determined from the slope and intercept of the
linear plot of log(qe) versus log(Ce). The Freundlich parameters of
Hg(II) adsorption on the CTS-PVA hydrogel adsorbent at different
temperatures were listed in Table 1.From Table 1, it can be seen that the linear correlation
coefficients (R2) at different temperatures are close to 1, which
indicated that the Freundlich model can also be used to describe
the adsorption process of the CTS-PVA hydrogel adsorbent for
Hg(II) ions. The values ofn decreased from 0.2925 to 0.1318 with
increasing temperature from 20 to 60C. This suggested that the
adsorption process of Hg(II) ions on the hydrogel adsorbent is
chemical, and the surface of the adsorbent is more heterogeneous
at high temperatures.
3.3. The specific surface area of the hydrogel adsorbent toward
Hg(II) ions binding
The Langmuir monolayer saturation capacity (qm,cal, mg/g) cal-culated from the Langmuir equation (Eq. (A.3)), i.e., the ultimate
adsorption capacity at high concentrations, can be used to esti-
mate the specific surface area (S, m2/g) of the CTS-PVA hydrogel
adsorbenttoward Hg(II) binding, usingthe following equation[33]:
S =qm,calNA
1000M , (A.7)
where N is Avogadro constant, 6.021023 mol1, A is the cross-
sectional area of metal ion (m2), M is the molar weight of
Hg(II) ions, 200.59g/mol. According to the radius of Hg(II) ion
(1.101010 m), the cross-sectional area of Hg(II) ion was cal-
culated to be 3.801020 m2. The specific surface areas of the
hydrogel adsorbenttowardHg(II) binding at differenttemperatures
were calculated and listed in Table 1.From Table 1, it can be seen that the specific surface area of
the hydrogel adsorbent toward Hg(II) ions binding increased with
increasingtemperature.This indicatedthat Hg(II) adsorptionon the
hydrogel adsorbent is chemisorption. The BET specific surface area
of the sample was determined to be 14.724m2/g, which is much
smaller than the binding specific surface area calculated. This fur-
ther confirmed the chemisorption nature of the adsorption process.
3.4. Thermodynamic parameters
Withthe helpof equilibrium dataobtainedat different tempera-
tures, thermodynamicparameters such as Gibbs freeenergy change
(G, J/mol), enthalpy change (H, J/mol) and entropy change (S,
J/(mol K)) of the adsorption process of the CTS-PVA hydrogel adsor-bent for Hg(II) ions can be determined by Gibbs equation and Vant
Hoff equation listed as follows:
G = RT ln KL, (A.8)
ln KL =H
(RT) +
S
R , (A.9)
where KLis the equilibrium constant obtained from the Langmuir
model (L/g),Tis the absolute temperature (K), R is the universalgas
constant (8.314J/(mol K)). The values ofHand Scan be deter-
mined from the slope and the intercept of the plot of ln(KL) versus
1/T. The plotand thermodynamic parametersof the adsorption pro-
cess of the hydrogel adsorbent for Hg(II) ions were shown in Fig. 7
and Table 1, respectively.
-
5/25/2018 hidrogel
7/8
X. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 5158 5
Fig. 7. The plot of ln(KL) versus1/T.
The linear correlation coefficient (R2) obtained from Fig. 7 was
0.9807. It is very close to 1. Such a good linear relationship made
the calculation of the values ofHand Saccording to Eq. (A.9)
possible.The values ofG are all negative at all temperatures. This indi-
cated that the adsorption process was spontaneous. The reduced
values ofG with increasing the temperature suggested the spon-taneous nature is improved at higher temperature. The positive
value ofHsuggested the adsorption process was an endothermicprocess. The positive value ofSindicated the entropy of the sys-
temincreased after adsorption, that is to say, therandomness at the
solidliquid interface increased. This unusual phenomenon may be
owing to the fact that the desolvation disturbs the structure of the
reaction medium and promotes the disorganization of the system
during the formation process of complex. It was also observed by
Weber et al. [20] and Guerra et al. [34].
4. Conclusions
Hg(II) adsorption on the CTS-PVA hydrogel adsorbent is a favor-
able, spontaneous, and endothermic chemisorption process. The
pH of solution had a great effect on Hg(II) adsorption on the hydro-
gel adsorbent. The adsorption mechanisms and functional groups
responsible for Hg(II) adsorption changed with the pH of solutions.
In lower pH solutions, chelation between functional groups and
Hg(II)ionswas the mainmechanism.Free pendant OHgroupsand
C N g roups a s w ell a s NHCOCH3groupswere themain functional
groups responsible for Hg(II) adsorption. When pH of solution was
close to thepI ofthe hydrogel adsorbent,freependant OH, NH2and NHCOCH3groups aswell asC N groupsallinvolved inHg(II)
adsorption. While in solutions which pH was much higher than the
pI of the hydrogel adsorbent, electrostatic interaction played animportant role. Both chelation and electrostatic interaction made
contributions to Hg(II) adsorption, and free pendant NH2groups
became the main functional groups responsible for Hg(II) adsorp-
tion. The binding force between Hg(II) ions and functional groups
was strengthened at higher temperature, and thereby the adsorp-
tion was more favorable at high temperature. Hg(II) adsorption on
thehydrogeladsorbent canbe well explainedby both theLangmuir
and the Freundlich isotherm models.
Acknowledgements
This work is financial supported by the Western Light Pro-
gram of the Chinese Academe of Sciences (XBBS201116), the
National Natural Science Foundation of China (21107133), and the
One Hundred Talents program of Chinese Academy of Science
(1029471301).
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Desalination 258 (2010) 4147.
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