fulltext1adsorption of cd(ii) from aqueous solutions by cellulose modified with maleic anhydride...

8/13/2019 fulltext1Adsorption of Cd(II) from Aqueous Solutions by Cellulose Modified with Maleic Anhydride and Thiourea

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583

Adsorption of Cd(II) from Aqueous Solutions by Cellulose Modified with

Maleic Anhydride and Thiourea

Yanmei Zhou*

, Xiaoyi Hu, Qiang Jin, Xinhai Wang and Tongsen Ma Institute of Environmentand Analytical Science, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, P.R.

China.

(Received 8 July 2012; revised form accepted 25 June 2013)

ABSTRACT: In this work, cellulose modified with maleic anhydride and

thiourea (CMT) was synthesized as a novel type of adsorbent to remove heavy-

metal ion. The synthesized adsorbent was characterized by Fourier transform

infrared spectroscopy, elemental analysis, scanning electron microscopy,

thermogravimetric analysis and X-ray diffraction. Batch experiments were

performed to investigate the influence of different factors on the adsorption of

Cd(II) from aqueous solution. Based on the adsorption data, the adsorption

isotherm model was confirmed and the maximum adsorption capacity of Cd(II)

from the Langmuir model was found to be 94.47 mg g1. The adsorption kinetics

indicated that the adsorption process followed pseudo-second-order model.

Adsorption/desorption experiments for more than six cycles showed the

possibility of repeated use of CMT for the adsorption of Cd(II) from aqueous

solutions.

1. INTRODUCTION

Water pollution is one of the most serious environmental problems facing industrial and economic

development, and identifying methods to remove toxic pollutions effectively from the aqueous

solution is one of the topics of great interest in the present pollution-control research. Heavy-metal

ions constitute a serious environment problem because these substances are non-biodegradable

and are also highly toxic to living organisms. Besides the toxic and harmful effects to organisms

living in water, heavy-metal ions can also get accumulated through the food chain and eventually

cause harm to human beings (Volesky 2001; Martins et al. 2004; Junior et al. 2009). Various

methods, such as ion exchange, neutralization, reverse osmosis, precipitation, solvent extractionand adsorption, were used to remove toxic metals from aqueous solution. Compared with other

methods, adsorption could be an economical and versatile choice for the removal of different

pollutions owing to its easy handling and high efficiency in removing heavy-metal ions, especially

at medium or low ion concentrations from wastewater (Reichert and Binner 1996;

Trakulsujaritchok et al. 2011).

Cadmium is a highly toxic environmental pollutant, which is extensively used in automotive

industries, metal finishing, electroplating, battery manufacturing, mining, electric-cable

manufacturing, tannery, textile and steel industries. Therefore, there is a high possibility of

cadmium ions getting easily accumulated in living tissues, especially in human beings, through

the food chain. According to the FAOWHO guidelines, the provisional maximum tolerable daily

*Author to whom all correspondence should be addressed. E-mail: [email protected] (Y. Zhou).


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2 hours with constant magnetic stirring (Gurgel et al. 2008). Thiourea (CM:thiourea = 1:10) was

then added to this reaction mixture. The media and the materials were allowed to react overnight

in order to obtain the adsorbent (CMT). The products were filtered in a sand core funnel, washed

with distilled water and ethanol, dried at 80 C overnight and then the adsorbent (CMT) was

obtained.

2.3. Degree of Carboxyl Groups

The degree of carboxyl groups present in CM was determined by titrating the NaOH solution

treated with CM against HCl (Gurgel et al. 2008, 2009). The experiment was carried out three

times in parallel in order to obtain exact results. The concentration of carboxylic groups [CCOOH(mmol g1)] was calculated as follows:

(1)

where CNaOH is the concentration of NaOH solution (mol L1), CHCl is the concentration of HCl

solution (mol L1), VNaOH is the volume of NaOH solution (L), VHCl is the volume of HCl spent in

the titration of excessive non-reacted NaOH solution (L) and mCM (g) is the mass of the CM in

this experiment.

2.4. Characterization of the New Materials Obtained

The IR spectrum of the samples prepared by mixing 1 mg of each material with 100 mg of

spectroscopy-grade KBr was recorded from 4000 to 400 cm1 using a Thermo Nicolet Avatar 360

Fourier transform infrared spectroscopy (FTIR) spectrometer.

C C V C Vm

COOHNaOH NaOH HCl HCl

CM

= ( ) 4 1000

Adsorption of Cd(II) from Aqueous Solutions 585

MCCM

OH OH

OH

OHO

O O

O O O

O

O

O

OO

O

OOO

O

OO

HO HO

HO

H2N NH2

OH

DIC

Thiourea

Pyridine

Maleic anhydride

CMT

NH NH2

S

S

n

n

n

Figure 1. Preparation of the CMT adsorbent.


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Scanning electron micrographs (SEMs) of samples were recorded using a JEOL JSM5600LV

scanning electron microscope. The elemental analysis of intermediate product (CM) and the

adsorbent (CMT), which were dried previously, was carried out using a Perkin-Elmer 2400II

equipment. Thermogravimetric analysis (TGA) and the differential thermal analysis (DTA) were

recorded using a Mettler-Toledo DTA/TGA instrument in the temperature range of 50500 C ata rate of 10 C minute1 under nitrogen flow. Wide-angle X-ray diffraction (XRD) measurement

was carried out on an X-ray diffractometer (X-Perpro; Philips, Holland, the Netherlands) in

symmetric reflection mode.

2.5. Batch Adsorption Studies

Bath adsorption studies were carried out by shaking a certain amount of CMT with 25 mL of

simulated wastewater in different conical flasks using a temperature-controlled shaker at 180 rpm

for some time. The mixtures were then separated and the supernatant fluid was analyzed for its

residual concentration. The concentration of Cd(II) ions were determined by standardethylenediaminetetraacetic acid titration, with 0.1% xylenol orange chosen as the indicator and

20% hexamethylenetetramine used as the buffer solution (Li et al. 2008; L et al. 2010).

The adsorption capacity (q) is expressed as follows:

(2)

where q is the adsorption capacity of the adsorbate (mg g1), m is the weight of adsorbent (g), V

is the volume of solution (L), and C0 (mg L1) and Ce (mg L1) are initial and equilibriumconcentrations of adsorbate in solution, respectively.

2.6. Desorption Study

A glass column was used to pack 60 mg of adsorbent. A 25 mL sample solution containing

appropriate amount of Cd(II), after adjusting the pH to 4.8, was passed through the column at a

flow rate of 0.5 mL minute1. After washing with distilled water, the adsorbed Cd(II) was stripped

from the adsorbent using 25 mL NaOH [at a certain pre-specified concentration (0.010.3 mol L1)]

at a rate of 0.5 ml minute1.

3. RESULTS AND DISCUSSION

3.1. Degree of Carboxyl Groups

Based on the results of the titration method the degree of carboxyl groups in CM was determined.

Using equation (1), the concentration of carboxyl groups on CM was calculated to be 2.70 mmol g1.

3.2. Characterization of Adsorbent

The FTIR spectra of MC, CM and CMT are presented in Figure 2. As depicted in Figure 2(b),

the most relevant change observed in the FTIR spectra of CM in relation to MC is

qC C V

m

e=

( )0

586 Y. Zhou et al./Adsorption Science & Technology Vol. 31 No. 7 2013


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appearance of bands at 1730 and 1637 cm1. The bands at 1730 cm1 is due to the stretching

of the carbonyl group (C = O). The bands at 1162 and 1062 cm1 are attributed to the

increase in the number of ether bonds. The band at 1637 cm1 corresponds to deformationvibration of the vinyl groups (C = C) (Chang and Chang 2001; Liu et al. 2007; Stenstad

et al. 2008). These spectra suggest that the maleic anhydride has successfully been grafted

onto the cellulose surface. As shown in Figure 2(c), the band at 1430 cm1 corresponds to

the stretch of CN (Karnitz et al. 2007), and the evident band at 1120 cm1 is attributed to

the vibration of C = S, which indicates that the thiourea has also been successfully grafted

onto the surface of CM. A wide absorption peak at 3400 cm1 is due to the stretching of the

hydroxyl group (OH), whereas the peak at 2900 cm1 is related to the CH vibration of the

CH2 groups.

Figure 3 shows the SEM images of MC and CMT. These images show the surfaces of cellulose

and CMT. As compared with cellulose, the surface of CMT is more irregular and rough, which is

due to the swelling effect of cellulose as well as the surface of cellulose being grafted with maleic

anhydride and thiourea. Although the specific surface area was not determined, the SEM images

suggest that the specific surface area of CMT is larger than MC because of the heterogeneous

surface. Obviously, this situation will be helpful to adsorb the pollutants.

The constituents of CM and CMT were determined by elemental analysis method. The

percentage of carbon and hydrogen present in CM was 41.11% and 6.28%, respectively.

Compared with MC, the percentage of carbon and hydrogen in CMT was 45.58% and 5.59%,

respectively. In addition, the percentage of nitrogen in CMT was 1.87%, which is due to the

thiourea that was grafted onto the surface of CM.The TGA and DTA curves of CM and CMT are presented in Figure 4. The differences in the

thermal behaviour of three samples are due to (1) the temperatures of initial weight loss;

(2) the rate of weight loss; (3) the magnitude of the enthalpy change and (4) the temperature


1650

17301637

1730

3500 3000 2500 2000 1500 1000 500

1637

1430

1120

Wave numbers (cm1)

a

b

c

Figure 2. FTIR spectra of (a) MC, (b) CM and (c) CMT.


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corresponding to the values of the peak of the DTA curve (Yang and Kokot 1996). When exposed

to a nitrogen atmosphere at 120 C, the first weight loss of all the samples is less than 5.0% in the

TGA curves and this is attributed to a significant amount of water released from CM and CMT,

which corresponds to an endothermic peak in the DTA curves around 60 C (Aggour 2000; Bari

and Begum 2009). The second slow weight loss of CM, which commences at about 270 C, is

represented by a shoulder and this is the initial stage of thermal degradation (Huang and Li 1998).

As for CMT, the second weight loss of CMT, which commences at about 160 C is attributed to

thermal degradation of the thiourea groups. The rapid weight loss, from 280 to 370 C, reflects

major thermal degradation, which is attributed to the thermal cleavage of the glucosidic units and

split of other CO bonds (Espert et al. 2003; Sailaja and Seetharamu 2009), corresponding to an

endothermic peak at approximately 360 C (Evans et al. 1996). As shown in Figure 4(a), the

weight loss of CMT is more than 90% at 360 C, whereas that of the CM is 80% at 370 C, whichis attributed to the thiourea grafted onto the surface of CM.

The diffraction patterns with Cu-K radiation ( = 0.15406 nm) at 40 kV and 40 mA were

collected in the 2 range from 10 to 40 at a rate of 0.08 second1. The degree of crystallinity


20 kV X1, 000 22 28 SEI10 m

20 kV X1, 000 21 20 SEI10 m

(a)

(b)

Figure 3. SEM morphology of (a) MC (magnification 1000) and (b) CMT (magnification 1000).


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c (%) of the cellulose and modified cellulose was estimated by Rabeks method as follows

(Tang et al. 2011):

(3)cc

c a

S

S S=

+ 100


100

80

60

40

20

0

0 100 200 300 400 500

Temperature (C)

100 200 300 400 500

Temperature (C)

Weight(%)

a

a

b

b

c

c

a

b

c

CMT

CM

MC

a

b

c

CMT

CM

MC

1.0

0.5

0.0

0.5

1.0

1.5

2.0

2.5

Differentialtemperature(C)

(a)

(b)

Figure 4. (a) TGA curves of MC, CM and CMT; (b) DTA curves of MC, CM and CMT.


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22.7

16.514.9

a

b

c

10 15 20 25 30 35

2 ()

Figure 5. X-ray diffraction patterns of (a) cellulose, (b) CM and (c) CMT.

where Sc and Sa are the area of crystal and amorphous diffraction peaks of samples, respectively.

Figure 5 shows the XRD pattern of cellulose, CM and CMT adsorbent. The native cellulose had

typical diffraction peaks at 2 = 14.9, 16.5 and 22.7 (Liu and Zhang 2009; Krishnaveni andThambidurai 2011). As shown in Figure 5, the change in diffraction patterns is not obvious in the

chemically modified cellulose (Melo et al. 2009), but there is an increase in the degree of

crystallinity of cellulose after the modification. The degree of crystallinity of CM (c = 68.14%)

is lower than that of native cellulose (c = 77.22%), indicating that the crystal structure of the

native cellulose has been destroyed after the modification with maleic anhydride. Compared with

the XRD pattern of CM, the values of CMT (c= 63.09%) are lower than that of CM after thiourea

is grafted onto the surface of CM, indicating that the structure of the CMT has been destroyed

because of the addition of thiourea on the surface of CM.

3.3. Effect of pH on Adsorption

The removal of pollutants from wastewaters by adsorption is highly dependent on the pH of

solution. Any variation in solution pH can affect the surface charge of the adsorbent, the degree

of ionization and speciation of the adsorbate (Elliott and Huang 1981). To obtain optimal pH, a

series of experiments is performed with different pH values. The results are presented in Figure 6.

As shown in Figure 6, at low pH values, the concentration of H+ is high, which competes with

Cd(II) ions for surface active sites, and the adsorption rate of Cd(II) ions is low (Aydin et al.

2008). The adsorption capacity of Cd(II) ion increases as the pH value increases from 2.0 to 4.8.

The optimal pH ranges from 4.0 to 5.0, and the adsorption rate of Cd(II) decreases beyond theoptimal pH range (i.e. at larger pH values). The rate of adsorption is reduced because the

competitive hydroxyl ions react with the Cd(II) ions when pH > 5.0. Therefore, the optimal initial

pH for the removal of Cd(II) ions was determined to be 4.8.


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3.4. Effect of Contact Time on Adsorption

The adsorption capacities were measured as a function of time to determine the optimal contact

time for the adsorption of Cd(II) ions on CMT. The effect of contact time on adsorption is shown

in Figure 7.As depicted in Figure 7, the adsorption of Cd(II) ions was fast up to 35 minutes and the

equilibrium time is observed to be around 40 minutes, beyond which the adsorption capacity

changed inconspicuously. Thus, the optimal contact time for Cd(II) adsorption was determined to

be 40 minutes in the subsequent studies.


50

45

40

35

30

25

20

152 3 4 5 6 7

pH

Adsorptioncapacity(mg

g1)

Figure 6. Effect of pH on adsorption.

120100806040200

Contact time (min)

42

40

38

36

34

32

30

Adsorptioncapacity(mgg1)

Figure 7. Effect of contact time on adsorption.


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7

6

5

4

3

2

1

0

10 20 30 40 50 60 70 80 90

Adsorbent amount (mg)

Ads

orptionpercentageofcadmium(%)

Figure 8. Effect of adsorbent amount on adsorption.

3.5. Effect of Adsorbent Amount on Adsorption

The effect of adsorbent amount on Cd(II) adsorption was studied by changing the amount of

adsorbent from 10 to 80 mg, while the concentration of Cd(II) ions was fixed. The results are

presented in Figure 8.The adsorption percentage of Cd(II) ions increases with the adsorbent amount up to an optimum

dosage, beyond which the adsorption percentage does not significantly change. It could be explained

as follows: the increasing adsorbent dosage provides large surface area (or more adsorption sites),

which enhanced the adsorption capacity. Moreover, the concentration of initial adsorbate was fixed

and the adsorbent is excessive when the adsorbent amount overran the optimum dosage. As shown

in Figure 8, the optimal dosage for Cd(II) adsorption was determined to be 60 mg.

3.6. Kinetics Study

The kinetics and equilibrium of adsorption, which are two important physicalchemical aspectsof the process, were studied to evaluate the process of adsorption. Two rate equations were used

to analyze the adsorption kinetics data, namely, pseudo-first-order kinetics and pseudo-second-

order reaction kinetics (OConnell et al. 2006; Ai et al. 2011).

Lagergrens pseudo-first-order kineticsequation (4)can be represented in a non-linear

formequation (5)and a linear formequation (6). Pseudo-second-order kinetics equation (7)

can be used to assess the concentration of Cd(II) ions absorbed. The linear form of pseudo-second-

order kinetics is shown in equation (8).

(4)

(5)q q et ek t

= ( )1 1

d

d k q q

q

te t

t=

1( )


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(6)

(7)

(8)

where qe (mg g1) is the equilibrium amount of adsorbate, and qt (mg g

1) is the amount of

adsorbate at any time t; k1 (g mg1 minute1) and k2 (dm

3 mg1 minute1) are the pseudo-first-order

and pseudo-second-order rate constants, respectively.

The records of the kinetics study are presented in Table 1. The R

2

value of the pseudo-second-order model is closer to 1. Therefore, the kinetic adsorption for Cd(II) is in accordance with the

pseudo-second-order kinetics. The plot showed an exact coefficient (R2), which is coherent with

the pseudo-second-order equation proposed Figure 9 shows the pseudo-second-order adsorption

kinetics of adsorption on the CMT adsorbent.

t

q k q qt

t e e

= +1 1

2

2

d

d

k q qq

t

e tt= 2

2( )

ln( ) lnq q q k te t e = 1


TABLE 1. Pseudo-First-Order and Pseudo-Second-Order Models for Adsorption of Adsorbate

Parameters

Kinetics models qe (mg g1) k1 (g mg

1 minute1) k2 (g mg1 minute1) R2

Pseudo-first-order 9.92 4.46 102 0.9575

Pseudo-second-order 42.08 1.21 102 0.9995

3.0

2.5

2.0

1.5

1.0

0.5

0.0

0 20 40 60 80 100 120

Time (min)

t/qt(mingmg1)

Figure 9. Pseudo-second-order adsorption kinetics of adsorption on the CMT adsorbent.


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3.7. Adsorption Isotherms

An adsorption isotherm can be used to characterize the interaction of adsorbate with adsorbents. The

isotherm provides a relationship between the concentration of adsorbate in solution and the amount

of adsorbate adsorbed on the solid phase when both phases are in equilibrium (Kara et al. 2004). Inthis study, the temperature was maintained at 30 C, and the adsorption isotherms were analyzed by

the three most commonly used models, namely, the Langmuir, Freundlich and Temkin isotherm

models. The adsorption isotherm models of Langmuir and Freundlich are shown in equations (9) and

(11), respectively, while they can also be represented in the linear form as shown in equations (10)

and (12), respectively. The Temkin isotherm model is shown in equation (13). The parameters

calculated from the three models are presented in Table 2. In addition, Figure 10 shows the linear

plot of Langmuir adsorption isotherm of the adsorbate at 30 C.


TABLE 2. Isotherm Models for Adsorption of Cd(II) Ions

Models Parameters Cd(II)

Q0 (mg g1) 94.97

Langmuir isotherm bL (L mg1) 2.4041

R2 0.9910

KF (L g1) 63.37

Freundlich isotherm nF 2.2730

R2 0.9386

AT (L mg1) 19.84

Temkin isotherm bT (J mol1) 113.41

R2 0.9492

0.030

0.025

0.020

0.015

0.010

0.005

0.0 0.5 1.0 1.5 2.0 2.5

Ce(mg L1)

Ce

/qe

(gl1)

Figure 10. Linear plots of Langmuir adsorption isotherm at 30 C. Experimental conditions: pH: 4.8; contact time: 40

minutes; dosage amount: 60 mg.


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(9)

(10)

(11)

(12)

(13)

where Ce (mg L1) and qe (mg g

1) are the concentration and adsorption capacity at equilibrium,

respectively. Q0 (mg g1) and bL (L mg1) are adsorption capacity and binding energy of adsorption

of Langmuir, respectively. KF and nF are Freundlich constants measuring the adsorption capacity

and the adsorption intensity, respectively. AT (L mg1) and bT (J mol

1) are the Temkin constants.

R is the universal gas constant (8.314 J mol1 K1) and T is the absolute temperature (303 K).

As depicted in Table 2, the correlation coefficient (R2) of Langmuir isotherm model is closest

to 1, indicating that the adsorption process is better described by the Langmuir isotherm model.

The maximum adsorption capacity of the Cd(II) ions was calculated to be 94.47 mg g1.

Adsorption capacities of CMT are compared with some materials in Table 3.

qRT

bA

RT

bCe

T

T

T

e= +ln ln

ln ln lnq Kn

Ce FF

e= +1

q K Ce F e1/nF=

C

q

C

Q Q b

e

e

e

L= +0 0

1

qQ b C

b Ce

L e

L e

=+

0

1


TABLE 3. Comparison of Adsorption Capacities of Various Adsorbents for Cd(II) Ions

Adsorbent pH Reaction time Adsorption capacity Reference

CMT 4.8 40 minutes 94.97 mg g1 This work

Corn stalk 7.0 6 hours 22.17 mg g1 Zheng et al. (2010)

NaY zeolite 108 mg g1 Britto et al. (2007)

Rice husk 6.8 20.24 mg g1 Kumar and Bandyopadhyay (2006)

Saw dust 24 hours 168 mg g1 Gaey et al. (2000)

Walnut sawdust 1 hours 4.39 mg g1 Bulut and Tez (2007)

Clinoptilolite 8.0 10 hours 0.392 mmol g1 Faghihian and Nejati-Yazdinejad (2009)

Bentonite 8.0 10 hours 0.381 mmol g1

Bentonite 5.0 61.35 mg g1 Mockovciakov et al. (2010)

Modified bentonite 7.0 6 hours 16.00 mg g1 Olu-Owolabi et al. (2010)

HA/Ca-Mont 5.0 14.15 mg g1 Wu et al. (2011)

3.8. Desorption

Regeneration and reuse of the adsorbent are important. In this work, NaOH solution was selected

to be a desorption agent in the Cd(II) desorption experiment. The relationship between thedesorption percentage and the concentration of NaOH solution was studied and the results are

presented in Figure 11. According to the experimental results, the optimum concentration of

NaOH solution was determined to be 0.20 mol L1.


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The CMT adsorbent was repeatedly used for six times in the adsorption experiment and the results

are shown in Figure 12. The adsorption capacity of the adsorbent slowly decreased with increasing

cycle number. At the end of the sixth regeneration cycle, the adsorption capacity of Cd(II) was found


0.00

75

80

85

90

95

100

0.05 0.10 0.15

Concentration of NaOH (mol L1)

Desorptionpercentage(%

)

0.20 0.25 0.30

Figure 11. Effect of concentration of NaOH solution on Cd(II) desorption.

1

50

60

70

80

90

100

2 3 4

N

Ad

sorptioncapacity(mgg1)

5 6

Figure 12. Effect of recycling adsorbents after Cd(II) adsorption.


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to be 70 mg g1. These results show that the CMT adsorbent has excellent regeneration capability

and thus can be reused for Cd(II) uptake for up to six times with 0.20 mol L1 NaOH solution.

4. CONCLUSIONS

A novel adsorbent was synthesized, characterized and applied to uptake Cd(II) ions in an aqueous

solution. Bath adsorption experiments were performed to evaluate the efficiency of CMT

adsorbent towards Cd(II) ions, and the effects of adsorption conditions on adsorptive performance

were investigated. The adsorption process fits the assumptions of the Langmuir isotherm and

pseudo-second-order kinetics model. In the desorption experiments, the adsorbent shows

regeneration and reuse capabilities with 0.20 mol L1 NaOH solution as the desorption agent. The

CMT adsorbent has thus been proved as one of the efficient adsorbents for removing Cd(II) ions

from aqueous solutions.

ACKNOWLEDGEMENT

The authors are grateful for the Key Scientific and Technological Project of Henan province

(112102310360, 122300410260), the natural science research project of Henan province Education

Department (2011A610005) and the financial support of the Foundation of International Scientific

and Technological Cooperation of Henan province (124300510012) in China.

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fulltext1adsorption of cd(ii) from aqueous solutions by cellulose modified with maleic anhydride...

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