modificacion de celulosa
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International Journal of Biological Macromolecules 66 (2014) 125134
Contents lists available at ScienceDirect
InternationalJournal ofBiological Macromolecules
j ournal homepage: www.elsevier .com/ locate / i jb iomac
Modification and characterization ofcellulose cotton fibers for fastextraction ofsome precious metal ions
M. Monier, M.A. Akl, Wael M. Ali
ChemistryDepartment, Faculty of Science, Mansoura University, Mansoura, Egypt
a r t i c l e i n f o
Article history:
Received 3 December 2013
Received in revised form 19 January 2014Accepted 28 January 2014
Available online 11 February 2014
Keywords:
Cotton fibers
Grafting
Acrylonitrile
Phenyl thiosemicarbazide
Precious metals
a b s t r a c t
In this work, native cellulose cotton fibers were first modified through graft copolymerization ofpoly-
acrylonitrile (PAN) and then by insertion ofphenyl thiosemicarbazide moieties to finally produce C-PTS
chelating fibers, which were fully characterized using various instrumental techniques such as SEM,
FTIR, EDX and XRD spectra. The obtained C-PTS were employed in removal and extraction ofAu3+, Pd2+
and Ag+ precious metal ions from their aqueous solutions using batch experiments. The kinetic studies
showed that the pseudo-second-order model exhibited the best fit for the experimental data. In addi-
tion, the adsorption isotherm studies indicated that the adsorption follows the Langmuir model and the
maximum adsorption capacities for Au3+, Pd2+ and Ag+ were 198.31, 87.43 and 71.14 mg/g respectively.
2014 Elsevier B.V. All rights reserved.
1. Introduction
As a result of their unique chemical and physical properties,
precious metals like gold,platinum, silverand palladium are exten-
sively utilized in many applications such as catalysis, electronics,
anti-corrosion materials and jewelry. In addition, with in the next
few years, precious metals are expected to be employed in some
new environmental, biotechnological and therapeutic uses [13].
For these reasons, many eco-friendly methods were developed for
efficientextraction andrecoveryof these valuable metals fromtheir
limited sources and from industrial effluents. Among the efficient
process for the separationand isolationof these precious metal ions
from industrial wastewater, sorption onto active adsorbents such
as activated carbon [4,5], modified silicates [6,7], chelating poly-
meric resin [8,9], biopolymers and biomass [1012] considered as
a common and successful method.
Between all of the previously mentioned sorbents, chelating
materials derived from inexpensive natural or synthetic sources
considered of a great importance particularly from the economic
point of view [1318]. Usually, the chelating materials are uti-
lized in form of beads, resin or membranes [19,23]. However,
recently many studies had been focused on preparation and
Corresponding author at: Chemistry Department, Faculty of Science, Mansoura
University, Mansoura, 35516, Egypt. Tel.: +20 1003975988.
E-mail address:[email protected] (M. Monier).
application of fibrous chelatingmaterials which derived fromeither
natural fibers such as wool fibers [24] or synthetic fibers such asPET and polypropylene fibers [21,22,25]. Actually the chelating
fibers provides many advantages when compared to the conven-
tional chelating materials in form of beads or membranes, first of
all, the ease of preparation, extraction and modification, also, the
ability to be applied as felts or fabrics which provides a high effi-
ciencyand high surface area during the contact with the media and
subsequently enhance the rate of both reaction and regeneration
process.
In the past few years various studies focused on evaluation of
modifiedcellulosicmaterials as an efficientand inexpensive biosor-
bents. In addition to the great advantages of cellulose as a highly
abundant, cheap and biodegradable material, cellulose can also be
easily modified due to the high availability of the active hydroxyl
groups which play an important role in various types of reactions
like oxidation, ether formation, esterification and free radical graft
co-polymerization by which many cellulose derivatives are pre-
pared [2628].
In this article, brand new chelating fibers based on cellulosic
cotton fabrics modified by insertion of phenyl thiosemicarbazide
moieties (C-PTS) had been prepared for quick elimination of Au(III),
Pd(II) and Ag(I) precious metal ions from aqueous solution. The
prepared C-PTS chelating fibers were fully characterized using var-
ious instrumental techniques such as elemental analysis, scanning
electron microscope (SEM), FTIR and wide angle X-ray spectra.
Also, various parameters such as pH, temperature, kinetics and
http://dx.doi.org/10.1016/j.ijbiomac.2014.01.068
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126 M. Monier et al./ International Journal of Biological Macromolecules 66 (2014) 125134
adsorption isotherms were investigated in order to evaluate the
optimum adsorption conditions and maximum adsorption capac-
ity.
2. Materials and methods
2.1. Materials
Cotton fibers were collected from the high agriculture schoolfarm in Mansoura, Egypt and treated by desizing in 1% (v/v) H2SO4and scouring in 1% (w/v) NaOH. The fibers were then washed
with distilled water and absolute ethanol and finally the cleaned
fibers were dried in oven at 50 C till constant weight. Acryloni-
trile(AN) (SigmaAldrich) was purified by treatmentwith 3% (w/w)
NaOH solutionand then washedwithdistilled water until free from
alkali. Potassium persulphate(KPS) (SigmaAldrich), Thiourea (TU)
(BDH-England); hydrazine hydrate (Adwic); phenylisothiocyanate
(SigmaAldrich), AuCl3, PdCl2 and AgNO3 were purchased from
Sigma Aldrich. All chemicals were used as received.
2.2. Synthesis of C-PTS chelating fibers
The actual synthetic reaction had beenaccomplished within thenext few steps. Initially the native cotton fibers were modified by
graft copolymerization with polyacrylonitrile (PAN) as in the fol-
lowing. 0.1g of thecotton fibers was immersed into 100 mLconical
flask containing the combined redox initiator system composed of
25mL2mmol KPS and 25mL2mmol TU and to which, 0.5 mL1%
H2SO4was added. After which, the mixture was vigorously shaken
for 5 min then, the AN monomer (2mL) was added and the shaking
was continued up to 3h at 80 C. In order to terminate the graft
copolymerization reaction, 5 mLof 3% hydroquinone solution was
added.The PAN grafted cotton(C-g-PAN) fibers were then removed
from the reaction medium, washed with DMF to extract the PAN
homopolymer and finally dried at 50 C till constant weight.
Grafting percentage (GP) was evaluatedaccording to the follow-
ing mathematical expression:
Graftingpercentage (GP) =
A B
B
100 (1)
where A and B are the weight of grafted product and native cotton
fibers, respectively.
The resulted C-g-PAN from the previous step, were then treated
with 100 mL10% (v/v) alcoholic hydrazine hydrate solution and the
mixture was refluxed for 4h at 80C. The obtained modified C-g-
PAH fibers were then eliminated from the mixture, washed with
ethanol and dried at 50C.
The modified C-PTS chelating fibers were finally manufactured
by refluxing the previously prepared C-g-PAN with 100 mL10%
(v/v) alcoholic phenylisothiocyanate solution at 80 C for 4 h. The
fibers were then removed, washed with ethanol and dried at 50
C.Schematic presentation for the synthetic steps of the C-PTS is
shown in Scheme 1.
2.3. Characterization of the polymer samples
The elemental analysis (E.A.) of the native cotton fibers, C-g-
PAN, C-g-PAH and C-PTS was obtained from a Perkin-Elmer 240 C
Elemental Analytical Instrument (USA).
FTIR spectra were performed using a Perkin-Elmer spectrum.
Thefiber samples weredried overnightat 60 Cunderreducedpres-
sure andpressurizedwith a glass slide on topof thequartz window
of the ATR instrument.
Thesurface morphologies of thenative andmodifiedfibers were
observed usinga FEI Quanta-200 scanningelectronmicroscope (FEI
Company, The Netherlands) equipped with Oxford energy disper-
sive X-ray system (EDX) operating at 20kV.
ASAP 2010 Micrometrics instrument was utilized to anticipate
the specific surface area of the studied samples by N2 adsorption
isotherm and by BrunauerEmmettTeller (BET) method.
Crystallinity of the polymeric samples was determined
using X-ray powder diffractometer (Japanese Dmax-rA, wave-
length=1.54A, CuK radiation). Generator intensity was 40kV,
generator current was 50mA. The sample was then scanned from
2= 570, in step of 0.02. The resultant graphs were printed out
on the Origin graph plotting package.
2.4. Metal ion uptake experiments using batch method
2.4.1. Instrumentation
A Perkin-Elmer Model 5000 atomic absorption spectrometer
(Perkin-Elmer, Shelton, CT-USA) was utilized for detection of the
precious metal ion concentrations. The instrument was set at Ag
328.068nm, Au 242.795 nm and Pd 324.270 nm.
2.4.2. Adsorption and desorption experiments
In all adsorptionstudies,the experiments were performedusing
batch method. 0.03g of the studied fiber samples were placed in a
small glass-stoppered bottles containing 30mLmetal ion solution
with main concentration 100mg/L (except for adsorptionisotherm
studies in which the concentration ranged from 10 to 400 mg/L),
at 30 C (except for thermodynamic studies in which the temper-
ature ranged between 20 and 40C), pH 5 (except in pH studies
in which the pH ranged between 1 and 5 using KCl/HCl for pH 1,
2, and 3; CH3COOH/CH3COONa for pH 4 and 5) and contact time
180min (except for the kinetic studies in which the reaction time
ranged between 10 and 120min). The bottles were equilibrated
on a thermostated shaker at 150rpm. The percent removal and
the amount adsorbed can be estimated according to the following
mathematical expressions:
Percent removal (%) =(Ci Ce) 100
Ci(2)
qe =(Ci Ce)V
W (3)
where Ci (mg/L) and Ce (mg/L) initial and equilibrated metal ion
concentrations, respectively; qe (mg/g) adsorption capacity; V(L)
volume of addedsolutionandW(g)the mass of the adsorbent(dry).
The desorption experiments were performed as in the follow-
ing: initially the precious metal ions loaded C-PTS chelating fibers
were prepared bysoaking 0.1 g of the fibers into100mL(100mg/L)
metal ion solution at pH 5.0 for 3 h and at 30 C. The batch was
equilibrated on a thermostated shaker adjusted at 150rpm. Then,
the metal ion loaded fibers were removed, washed with distilled
water to get rid of the unadsorbed ions and then agitated with
100mL0.1 N HNO3 solution for 60min. The concentrations of the
desorped metal ions were estimated utilizing atomic absorption
techniques. The reusability of the chelating fibers were examined
by repeating the above adsorptiondesorption cycles for five times
and the desorption percentage (D%) was calculated as Eq. (4).
D% =CHNO3
Cad 100 (4)
where CHNO3isthemetaliondesorbedtotheHNO3 solutions (mg/L)
and Cadis the metal ion adsorbed onto the resin (mg/L).
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M. Monier et al./ International Journal of Biological Macromolecules 66 (2014) 125134 127
Scheme 1. Synthesis of C-PTS chelating fibers.
Table 1
Elemental analysis of cotton, C-g-PAN, C-g-PAH andC-PTS.
Fibers C(%) H(%) O(%) N(%) S(%)
Cotton 42.2 6.01 51.79 0 0
C-g-PAN 57.2 5.8 21.5 15.41 0
C-g-PAH 42.9 6.1 16.1 34.7 0C-PTS 52.7 5.5 7.6 21.7 12.4
3. Results and discussion
3.1. Characterization
Table 1 presents the E.A. results for native cotton, C-g-PAN,
C-g-PAH, C-PTS.As canbe seen, the nitrogen content presents a sig-
nificant increase after grafting and hydrazine hydrate modification,
whichgivesan evidence forthe insertionof PANchains andconver-
sionof the vastmajority ofthe CNgroupsintoH2N C NH NH2.
Also, the appearance of sulfur in C-PTS may confirm the for-
mation of the functional phenyl thiosemicarbazide moieties on
the modified fibers. The approximate estimation of the phenylthiosemicarbazide content on the C-PTS is 3.88 mmol g1.
The surface morphologies of native and modified fibers were
examined using scanning electron microscope (SEM) and the
photos of native cotton, C-g-PAN and C-PTS are shown in Fig. 1.
As can be noticed, the observed size increase of C-g-PAN could be
attributed to the insertion of the grafted PAN chains onto the main
cellulose fibers backbone. In addition, the relatively rough surface
observed in C-PTS compared to both native and grafted fibers may
be due to the further treatment steps during the insertion of the
active chelating phenyl thiosemicarbazide moieties. According to
the BET surface area measurements, both native cotton and C-PTS
fibers exhibited a surface area 2.432 m2/g and 6.235m2/g, respec-
tively. This relatively low surface area gives an indication that the
heavy metal ion adsorption is mainly due to the coordination with
the active functional groups inserted onto the fibers.
The FTIR spectra of the modified and unmodified cotton fiberswere presented in Fig. 2. The IR spectrum of the native cot-
ton fibers (Fig. 2a) exhibited the main characteristic cellulose
peaks at approximately 10701150 cm1 due to C O stretching,
12601410cm1 due to O H bending, and 36003100cm1 due
to O H stretching [29]. On the other hand, the spectrum of the
grafted cottonfibersC-g-PAN (Fig. 2b) confirms theinsertion of the
PAN onto the polysaccharidecellulose backbone by the appearance
ofthe CNspecific peakat2350cm1. However, the further modifi-
cationby turning the majorityof CNgroups into H2N N C NH2groups (Fig. 2c), was obviously cleared by the observed lowering
of the CN characteristic peak at 2350 cm1 and appearance of
peaks at about 1660cm1 and 3200cm1 corresponding to the
azomethine (C N) and NH2 groups respectively, in addition to
the clear N N characteristic peak at about 1030cm1. Moreover,
the spectrum of the obtained chelating C-PTS fibers (Fig. 2d) gives
an obvious evidence for the creation of the active phenyl thiosemi-
carbazide moieties by the appearance of the diagnostic C S peaks
at 1300cm1 and 870 cm1, in addition to the peaks at 1580cm1
and 770 cm1 which are relatedtothe C C and C H of the inserted
aromatic units.
Usually the thiosemicarbazide derivatives can exhibit
thionthiol tautomerism (Structure 1) in solution as a result
of thioamide (NH C S) functional group [30]. The absence of the
thiol (S H) and C S bands at about 2300 cm1 and 1200cm1,
Fig. 1. SEM photosof modified and unmodified cotton fibers. (a)Native unmodified cottonfibers, (b) C-g-PAN, and (c) C-PTS.
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128 M. Monier et al./ International Journal of Biological Macromolecules 66 (2014) 125134
Fig. 2. FTIR spectra of (a) native cotton fibers, (b) C-g-PAN, (c) C-g-PAH and (d) C-PTS.
Structure1. Molecular modeling of thechelating fibers active center(a) thioneform and (b)thiol form.
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M. Monier et al./ International Journal of Biological Macromolecules 66 (2014) 125134 129
respectively in addition to the presence of the sharp C S peaks
at 1330 c m1 and 870 c m1 suggests that the active phenyl
thiosemicarbazide moieties exist mainly in the thion form.
Geometry optimization of the PTC derivative moieties inserted
onto the modified cotton fibers was performed using MM+ force-
field in HyperChem software version 8.03 implemented on a Dell
core i5 personal computer and the molecularly modeled structure
was presented in Structure 1.
The changes in the crystalline structure resulted from the
chemical modifications were examined using wide angle X-ray
diffraction (XRD). As can be seen in Fig. 3a the XRD pattern of the
native cotton fibers exhibited crystalline peaks at approximately
15, 16 in addition to a sharp intense peak at about 23, which
is in accordance with previous report [31]. On the other hand,
the crystalline pattern of the modified C-g-PAN and C-PTS (Fig. 3b
and c) present a sharp intensive peak at approximately 17 which
attributed to the crystalline pattern of the grafted PAN chains [32],
this may give an evidence that during the graft copolymerization,
the PAN grown to relatively long chains and form a well organized
crystalline pattern. In addition, the intensities of the characteristic
cottonfibers peaks at 15, 16, and 23 present an obvious decrease
upongrafting andmodificationwhichmay indicate the crystallinity
decrease.
Fig. 4ac presents the EDX spectra of native unmodified cot-
ton fibers, C-g-PAN and C-PTS chelating fibers, respectively. It is
obvious that the spectrum of C-g-PAN (Fig. 4b) showed an addi-
tional nitrogen peak in addition to the original carbon and oxygen
peaks observed in thenative cottonfibers spectrum (Fig.4a), which
confirm thesuccessfulgrafting reaction.In addition, theC-PTS spec-
trum (Fig. 4c) exhibited the characteristic sulfur peak at 2.3 keV.
This may gives an evidence for the further modification of the
grafted chains via insertion of the functional phenyl thiosemicar-
bazide chelating groups.
3.2. Metal ions uptake studies
3.2.1. Effect of functionalization
The influence of cotton functionalization on the precious metal
ions removal capabilities was explored at 30C, while maintaining
all other circumstances fixed. The outcomes are demonstrated in
Fig. 3. X-ray diffraction pattern of (a)native unmodifiedcotton fibers,(b) C-g-PAN,
and (c) C-PTS.
Table 2. As can be observed, the original unmodified cotton fibers
as well as C-g-PAN fibers did not show any characteristic potential
for the studied precious metal ion extraction, which caused by
the lack of functional groups that may possibly bind with these
metal ions. Alternatively, C-g-PAH displayed a relatively elevated
potential for metal ion removal. For the phenyl thiosemicarbazide
functionalized C-PTS fibers, the percent removal of the three
metal ions Au3+, Pd2+ and Ag+ presented a noticeable increase and
these can be resulting from a greater surface area with abundant
Fig. 4. EDX analysis of (a) native cotton fibers, (b) C-g-PAN, and (c) C-PTS.
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130 M. Monier et al./ International Journal of Biological Macromolecules 66 (2014) 125134
Table 2
Effectof thefunctionalization on the percent removal of Au3+ , Pd2+ and Ag2+ .
Fibers Percent removal (%)
Au3+ Pd2+ Ag+
Cotton 1.8 1.5 1.1
C-g-PAN 6.7 5.6 3.2
C-g-PAH 70.2 66.3 37.5
C-PTS 98.4 76.3 62.5
1 2 3 4 5
0
20
40
60
80
100
Percentremoval
pH
Au3+
Pd2+
Ag+
Fig. 5. Effect ofpH onthe uptakeof Au3+ , Pd2+ , and Ag+ ions by C-PTS (initial con-
centration 100mg/L; C-PTS 1 g/L; contact time 3 h; shaking rate 150rpm, 30 C).
accessibility to considerably more active sites to which the heavy
metal ions can possibly bind.
3.2.2. Influence of pH
Fig. 5 presents the pH influence on the percent removal of Au3+
,Pd2+ and Ag+ by the modified C-PTS chelating fibers. As can be
noticed,the adsorption experiments were performedin a pH range
15 prior to the precipitation limits of the studied metal ions. As
expected, the percent removal of the metal ions exhibited a signif-
icant increase at high pH value; this could be attributed to the low
H+ concentration which may compete with the metal ion for the
coordination with active phenyl thiosemicarbazide moieties and
subsequently lower the percent removal at low pH values.
In order to understand the mechanism through which the stud-
ied metal ions coordinated to the C-PTS fibers, the FTIR spectra of
the C-PTS loadedwithAu3+, Pd2+ andAg+ wereperformed andcom-
pared with the spectra of the metal free fibers to investigate the
active sites involved in the coordination. The characteristic peaks
of the inserted phenyl thiosemicarbazide moieties are expected tochange upon coordinationwith the studied metal ions. Table 3 lists
these guide peaks before and after the coordination. For both Au3+
and Pd2+, the absence ofv(S H), v(C S), appearance ofv(C S) and
v(C N)b, in addition to the lower shift of the v(C N)a stretching
Table 3
Assignments of characteristic IR spectral peaks (cm1) of C-PTS and M-C-PTS.
Fibers v(C N)a v(C N)b v/(C S) v/(C S) v(N N)
C-PTS 1660 1300, 870 1030
Au-C-PTS 1610 1630 1150, 650 1095
Pd-C-PTS 1614 1635 1173, 670 1112
Ag-C-PTS 1622 1220, 805 1055
a Azomethine.b
New.
0.00320 0.00325 0.00330 0.00335 0.00340 0.00345
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
y = -0.3665 + 1451.8074xR
2= 0.9975
y = -6.07442 + 3411.41086x
R2= 0.9998
y = -22.41883 + 8660.82896x
R2= 0.9987
ln
KC
1/T (K-1)
Au3+
Pd2+
Ag+
Fig.6. Plot ofln KCas a functionof reciprocalof temperature (1/T) forthe adsorption
ofAu3+ , Pd2+ , and Ag+ by C-PTS fibers.
vibration of the complexes, suggested the coordination through
the deprotonated thiol and azomethine nitrogen atom and forma-
tion of five membered ring as shown in the molecularly modeled
Structures 2 and 3. This suggestion supported by the marked pH
lowering after the Au3+ and Pd2+ adsorption from 5 to about 3. On
the other hand,the spectrum of Ag-C-PTS exhibited a lower shift of
both v(C S) and v(C N)a in addition to the absence of both v(C S)
and v(C N)b, whichsuggestthe coordinationof the Ag+ through the
thione sulfur and azomethine nitrogen atoms in a five membered
chelation mode as presented in Structure 4.
3.2.3. Effect of temperature
The thermodynamic parameters for the adsorption of Au3+, Pd2+
and Ag+ were anticipated by equilibrating 0.03g C-PTS chelating
fibers with 30mLof the studied metal ion solution with concen-
tration 30mg/L at 293, 303 and 313 K. In all studied cases, raisingtemperature lowers the adsorption, which may give anevidencefor
the exothermic nature of the adsorption. In addition, we can esti-
mate the thermodynamic parameters (free energy (G), enthalpy(H) and entropy (S)) according to the following mathematicalequations:
KC=CadCe
(5)
where Cad is the concentration of solute adsorbed on the fibers at
equilibrium(mg/L) andCe is the equilibrium concentrationof metal
ion in the solution (mg/L).
Free energy of the adsorption (Gads) can be calculated using
the following equation:
Gads= RTln KC (6)
The adsorption standard enthalpy (Hads) and entropy (Sads
)
can be evaluated using the following equation:
ln KC=Sads
R
Hads
RT (7)
where R (8.314 J/molK) is the gas constant.
By plotting lnKCvs 1/T(Fig.6), the valuesof theslopeHads/R
and the intercept Sads/R were employed in calculating both
Hads and Sads
for Au3+, Pd2+ and Ag+ adsorption onto the mod-
ified C-PTS chelating fibers.
The calculated thermodynamic parameters were collected in
Table 4, and as can be noticed in all cases, the Gads exhibited
negative values, which means that the adsorption is a spontaneous
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Structure2. Molecular modeling of Au-C-PTS.
Structure 3. Molecular modeling of Pd-C-PTS.
Structure 4. Molecular modeling of Ag-C-PTS.
Table 4
Thermodynamic parameters for the adsorption of Au3+ , Pd2+ and Ag+ on C-PTS fibers.
Metal ion KC Gads (kJ/mol) Hads
(J/mol K) Sads (J/mol K)
293 K 303 K 313 K 293 K 303 K 313 K
Au3+ 1499 332.33 229.76 17.812 14.626 14.148 72.006 186.39
Pd2+ 271.7 165.66 129.43 13.651 12.872 12.654 28.362 50.502
Ag+ 99.01 82.33 72.17 11.193 11.111 11.009 12.070 3.047
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0 20 40 60 80 100 120
30
40
50
60
70
80
90
100
qt
(m
gg
-1)
Time (min-1)
Au3+
Pd2+
Ag+
Fig. 7. Effect of contact time on the uptake of Au3+ , Pd2+ , and Ag+ ions by C-PTS
(initial concentration 100mg/L, C-PTS 1 g/L, pH 5.0, shaking rate 150rpm, 30 C).
process. In addition both Hads and Sads are also negative indi-cating that the process is an exothermic process with a decrease
in entropy, which could be attributed to the aggregation of the
metal ion onto the surface of the chelating fibers. This is a common
observation during the metal ion removal process [23].
3.2.4. Kinetic studies
TheinfluenceoftheadsorptiontimeontheremovalofAu3+, Pd2+
and Ag+ by modified C-PTS chelating fibers is presentedin Fig. 7. As
can be seen, the removal exhibited a rapid rate for the first 20min
where the percent removal reached about 85%, 70% and 55% with
initial rate of approximately 5.5, 4.0 and 2.8mg g1 min1 for Au3+,
Pd2+ and Ag+, respectively.
For better evaluation of the kinetic mechanism which governs
the whole removal process, the resulted experimental data were
fitted with the well known kinetic pseudo-first-order and pseudo-
second-order models according to the following equations [20,21]:
1
qt=
k1qet
+1
qe(8)
where k1is the pseudo-first-order rate constant (min1) of adsorp-
tion and qe and qt (mg/g) are the amounts of metal ion adsorbed
at equilibrium and time t(min), respectively. The value of 1/qtwas
calculated from the experimental results and plotted against 1/t
(min1). The linear form of pseudo-second-order equation can be
written as
t
qt=
1
k2q2e+ 1
qe
t (9)
where k2 is the pseudo-second-order rate constant of adsorption
(g/(mg min)).
The kinetic parameters of the removal process were summa-
rized in Table 5. According to the resulted correlation coefficients
(R2), the experimental date exhibited the best fit with the pseudo-
second-order equation model, which means that the chemical
coordination step considered as the rate determining step without
involvement of a mass transfer in solution [33,34]. Generally, C-PTS
is characterized by its relatively high nitrogen and sulfur percent-
age related totheazomethines (C N)andthione/thiol(C S/C SH)
of the inserted phenyl thiosemicarbazide moieties, which are able
to coordinate with the precious metal ions as previously presented
in Structures 14.
Table 5
Kinetic parameters forAu3+ , Pd2+ and Ag+ ions adsorption by C-PTS fibers.
Metals First-order model
k1 (min1) qe1 (mg/g) R
2
Au3+ 8.743 98 3 0.9324
Pd2+ 6.983 75 4 0.9221
Ag+ 7.349 61 3 0.8998
Metals Second-order modelk2 (g/(mgmin)) qe2 (mg/g) R
2
Au3+ 7.1103 99 1 0.9998
Pd2+ 5.2103 76 1 0.9999
Ag+ 4.9103 62 1 0.9988
The pseudo-second-order kinetic model also showed the best
fit with the experimental data in the studies carried out by Monier
and Abdel-Latif[22] on adsorption of Hg2+, Cu2+ and Co2+ onto che-
lating fibers based on PET; by Hajeethet al. [27] on removal of Cu2+
and Ni2+ ions onto cellulose-g-acrylic acid copolymer; by Jia and
Demopoulos [4] on adsorption of Ag+ ions onto activated carbon;
Kangetal. [6] onremovalofPt2+ andPd2+ using thiol-functionalized
mesoporoussilica which werein agreementwith thekinetic resultsfound in our study.
3.2.5. Adsorption isotherms
The adsorption isotherm studies are essential for anticipa-
tion of the characteristic relationship between the adsorbate and
the adsorbent [35] and subsequently, provide valuable informa-
tion about the adsorption mechanism, which is very important
in designing the adsorption system. The adsorption isotherms of
Au3+, Pd2+ andAg+ ions were performed under the optimum condi-
tions of pH,temperature, contact time and a range of concentration
between 10 and 400mg/L. As can be seen in Fig. 8, the adsorption
of the three studied precious metals was increased gradually by
raising the initial concentration until the C-PTS reached the max-
imum saturation capacity at high concentration. Both Langmuirand Freundlich models were employed to evaluate the adsorption
parameters. The Langmuir adsorption isotherm model assumes
that the adsorbate form monolayer on the adsorbent surface, the
surface is homogeneous with energetically equivalent adsorption
sites and there are no intermolecular forces between the adsorbed
0 50 100 150 200 250 300 350
0
50
100
150
200
qe
(mgg
-1)
Ce(mg L
-1 )
Au3+
Pd2+
Ag+
Fig. 8. Adsorptionisotherms of Au3+ , Pd2+ , and Ag+ ionsby C-PTS (initial concentra-
tion 10400mg/L, C-PTS 1g/L,pH 5.0, shaking rate 150rpm,30
C).
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M. Monier et al./ International Journal of Biological Macromolecules 66 (2014) 125134 133
Table 6
Parameters for Au3+ , Pd2+ and Ag+ ions adsorption by C-PTS fibers according to
different equilibrium models.
Metals Langmuir isotherm constants
KL (L/g) qm(mg/g) R2
Au3+ 29.7102 198.31 0.9997
Pd2+ 25.2102 87.43 0.9999
Ag+ 15.0102 71.14 0.9999
Metals Freundlich isotherm constants
KF n R2
Au3+ 29.057 3.423 0.8997
Pd2+ 20.253 3.783 0.9012
Ag+ 18.287 4.002 0.9112
species [35]. The mathematical expression of Langmuir adsorption
model is represented according to the following equation:
Ceqe
=
1
KLqm
+
Ceqm
(10)
where qe is the amount of metal ion adsorbed on one gram of the
adsorbent (mg/g) at equilibrium, Ce the equilibrium concentration
in the solution(mg/L),qm themaximumadsorption in monolayeredadsorption systems (mg/g) and KL is the adsorption equilibrium
constant related to adsorption energy (Lmg1). The adsorption
parameters (qmand KL) were evaluated from theslop and intercept
ofCe/qm vs Ce plot.
For Freundlich isotherm model, the adsorbate assumed to form
a multilayer on energetically nonequivalent heterogeneous adsor-
bent surface. The model can be mathematically expressed as in the
following equation:
ln qe = ln KF+1
n(ln Ce) (11)
where KFis a constant related to the adsorption capacity and 1/n is
an empirical parameter related to the adsorption intensity, which
depend on the material heterogeneity. Both KFand n can be esti-
mated by plotting lnqe vs lnCe.Both Langmuir and Freundlich parameters were collected in
Table 6. According to the correlation coefficients, the experimen-
tal results exhibited the best fit with Langmuir model suggesting
a monolayer homogeneous adsorption of the three precious metal
ions onto the modified C-PTS chelating fibers. As can be seen in
Table 6, the maximum adsorption capacities for Au3+, Pd2+ and Ag+
were 198.31, 87.43 and 71.14mg/g, respectively, indicating a high
potential of the prepared fibers for removal of these metal ions
from aqueous solution. In addition, the higher tendency for Au3+
compared to Pd2+ and Ag+ could be attributed to the fact that triva-
lent ions have a higher coordination power than both divalent and
monovalent ions. Also, gold is considered as a soft Lewis acid due
to its high polarizability. It forms strong covalent bonds with soft
Lewis bases, notably with reduced sulfur [22].The suitability of the adsorption process could be evaluated
by calculating the separation factor constant (RL): RL > 1.0, unsuit-
able; RL =1, linear; 0< RL < 1, suitable; RL = 0, irreversible [21]. The
RLvalue can be estimated according to the following equation:
RL =1
1 + C0KL(12)
where KL is the Langmuir equilibrium constant and C0(10400 mg/L) is the initial concentration of the metal ions.
The values of RL lie between 0.252 and 0.0083, indicating the
suitability of the synthesized chelating fibers as adsorbents for
Au(III), Pd(II) and Ag(I) from aqueous solution.
Table 7 lists the maximum adsorption capacities of Au3+, Pd2+
and Ag+
adsorbed onto some previously prepared adsorbents. As
Table 7
Maximum adsorption capacities forthe adsorption of Au3+ , Pd2+ and Ag+ onto vari-
ous adsorbents.
Adsorbent Maximum adsorption
capacity
Reference
Au3+ Pd2+ Ag+
Modified rice husks 250.2 [36]
Glycine-chitosan resin 169.9 120.3 [37]
Basic anion exchange resins 121.5 [38]
Thiourea modified chitosan 112.4 [39]
Pd-ion-imprinted resin 38.9 [40]
Thiourea modified chitosan 235.2 [41]
2-Mercaptobenzimidazole-TiO2 128.2 [35]
Present study 198.31 87.43 71.14
Table 8
Repeated adsorption of Au3+ , Pd2+ , and Ag+ ions by C-PTS (initial concentration
100mg/L, C-PTS 1 g/L, pH 5.0, contact time 3h, shaking rate 150rpm,30 C).
Cycle n umber Adsorption c apacity ( %)
Au3+ Pd2+ Ag+
1 100 100 100
2 99.7 98.5 98.8
3 98.1 97.7 98.2
4 96.8 95.7 96.45 94.9 94.2 94.8
canbe noticed,by comparison,the prepared C-PTS exhibiteda good
capacity for the studied precious metal ions.
3.2.6. Desorption and regeneration of C-PTS chelating fibers
Desorption experiments are significant to evaluate both metal
ions recovery and adsorbent regeneration for reusability require-
ments. In the current study 0.1 N HNO3 solution was used as
eluent for desorption experiments, the process was repeated for
five adsorptiondesorption cycles and the results were presented
in Table 8. As can be seen, desorption efficiency did not exhibit a
significant decrease, after the fifth cycle, the fibers still maintain
more than 94% of its original capacity. The obtained results con-firmed that there is no appreciable loss in activity over at least five
cycles.
4. Conclusions
Removal of Au3+, Pd2+ and Ag+ was carried out using a novel
phenyl thiosemicarbazide modified cotton fibers (C-PTS) as adsor-
bent. The adsorption capacity follows the order Au3+ > Pd2+ > Ag+.
The adsorption kinetic studies indicated that the adsorption pro-
cess fits with the pseudo-second-order model. On the other hand,
the adsorption isotherm studies confirmed that the experimental
results follow theLangmuir model. Various techniquessuch as SEM,
FTIR, SEM, XRD and EDX were utilized for characterization of the
modified C-PTS chelating fibers. Also, FTIR spectra were performedto understand the mechanism of the precious metal ion coordina-
tion with the active phenyl thiosemicarbazide moieties inserted
onto the modified chelating cotton fibers.
Acknowledgements
The authors are grateful toDr. S. Das for supplying us withsome
chemicals.Also, theauthorsappreciate theefforts of Dr.M. Shamma
for the assistance in performing the instrumental analysis.
References
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