modificacion de celulosa

8/12/2019 Modificacion de Celulosa

1/10

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

0141-8130/ 2014 Elsevier B.V.All rightsreserved.
http://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.ijbiomac.2014.01.068http://www.sciencedirect.com/science/journal/01418130http://www.elsevier.com/locate/ijbiomacmailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.ijbiomac.2014.01.068http://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.ijbiomac.2014.01.068mailto:[email protected]://crossmark.crossref.org/dialog/?doi=10.1016/j.ijbiomac.2014.01.068&domain=pdfhttp://www.elsevier.com/locate/ijbiomachttp://www.sciencedirect.com/science/journal/01418130http://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.ijbiomac.2014.01.068


2/10

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).


3/10

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.


4/10


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.


5/10


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.


6/10


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


7/10


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


8/10


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).


9/10


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

[1] R. Farrauto, Y. Liu, W. Ruettinger, O. Ilinich, L. Shore, T. Giroux, Catal. Rev.-Sci.Eng. 49 (2007) 141196.
http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005http://refhub.elsevier.com/S0141-8130(14)00079-8/sbref0005


10/10

modificacion de celulosa

Documents