Efficient Cadmium(II) Removal from Aqueous Solution Using

Apr 9, 2009 - (1) Commonly, the heavy metal ions from wastewater are removed by the processes of chemical precipitation, ion exchange, and reverse osm...
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Ind. Eng. Chem. Res. 2009, 48, 4688–4696

Efficient Cadmium(II) Removal from Aqueous Solution Using Microwave Synthesized Guar Gum-Graft-Poly(ethylacrylate) Vandana Singh,* Ajit Kumar Sharma, and Sadhana Maurya Department of Chemistry, UniVersity of Allahabad, Allahabad-211002, India

Microwave induced emulsion copolymerization of ethylacrylate and guar gum resulted in copolymer samples of different % grafting (%G) by changing ethylacrylate and guar gum concentrations at fixed microwave power (100%) and exposure time (15 s). The synthesis was done in the absence of any redox initiator/ catalyst, and the adsorption behavior of the copolymer (295%G) was investigated by performing both the kinetics and equilibrium studies in batch conditions. Several experimental parameters such as contact time, initial cadmium concentration, temperature, adsorbent dose, electrolyte amount, and pH of the solution were varied to optimize the adsorption conditions. The most favorable pH for the adsorption was pH 9, and at this pH the adsorption data were modeled using Langmuir and Freundlich isotherms. The data fitted satisfactorily to both the isotherms, indicating that the real heterogeneous nature of the surface sites involved in the metal uptake and overall sorption of Cd(II) on the adsorbent was complex and involved more than one mechanism. On the basis of the Langmuir model, Q0 was calculated to be 714.28 mg/g for microwave synthesized copolymer (mwGG-g-PEA) in comparison to 270.27 for conventionally synthesized copolymer (cvGG-g-PEA), revealing the advantage of using microwaves in the adsorbent synthesis. The sorption by mwGG-g-PEA followed pseudosecond-order kinetics where a linear plot of t/(qt) versus t was obtained, the correlation coefficient (R2) and rate constant at 100 mg/L Cd(II) being 0.9978 and 4.6 × 10-4 g/mg/min, respectively. The adsorbent exhibited high reusability and could be successfully recycled for nine cycles where in the ninth cycle 38% adsorption was feasible. To understand the role of PEA grafts (in the copolymer) in the adsorption process, different %G samples were evaluated as adsorbent under optimized conditions. 1. Introduction Adverse health effects of cadmium are well documented and have been reported to cause renal disturbances, lung insufficiency, bone lesions, cancer, and hypertension in humans.1 Commonly, the heavy metal ions from wastewater are removed by the processes of chemical precipitation, ion exchange, and reverse osmosis; however, these methods have several disadvantages such as unpredictable metal ion removal, high reagent requirements, and generation of toxic sludge which is often difficult to dewater and requires extreme caution in its disposal.2 Selective sorption of lead, cadmium, and zinc ions by a polymeric cation exchanger containing nano-Zr(HPO3S)2 is reported.3 A complexation-ultrafiltration process has been also investigated for mercury and cadmium removal from aqueous solutions by using poly(acrylic acid) sodium salt as a complexing agent.4 Out of the available methods, adsorption is an excellent way to treat industrial waste effluents with advantages like lowcost, availability, profitability, easy of operation, and efficiency.5 Several adsorbents such as aluminum oxide,6 activated carbon prepared from coir pith,7 biosorbents,8,9 and polymeric materials such as dithiocarbamate-anchored polymer/organosmectite10 and poly(vinyl pyridine-poly ethylene glycol methacrylate-ethylene glycol dimethacrylate) beads11 are reported for cadmium removal from the aqueous solutions. Adsorption efficiency of many biomaterials have been suitably tailored by chemical modification, for example, pine cone chemically activated with Fenton reagent,12 surfactant-modified carbon powder from husk and pods of Moringa oleifera,13 pyromellitic dianhydride-grafted s-cyclodextrin,14 and cross-linked carboxymethyl konjac glucomannan.15 Thus several reports are available on the chemical * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +91 532 461518. Fax: +91 532 2540858.

modifications of the biomasses before they are usefully exploited as the adsorbents. Guar gum (GG) is a galactomannan biopolymeric material isolated from the endosperms of Cyamopsis tetragonalobus, which is native to northwestern parts of India.16 GG as such and its derivatives17 find wide commercial applications due to their ability to form high viscosity colloidal dispersions in water at room temperature. An adverse property of the guar gum is its susceptibility to quick biodegradation,18 and therefore it is rarely used in its natural form. Being polyhydroxy biopolymer, the guar gum may be a potentially used as an adsorbent; however, its water solubility does not permit this under aqueous condition. Its stability19 and solubility as well as its sorbing capacity20 can be altered through grafting of vinyl monomers. Grafting can be done using redox initiators,21,22 γ irradiation,23 or microwave irradiation.24,25 Several attempts have been made to modify the properties of guar gum by the grafting of acrylonitrile,25 methylmethacrylate,22 acrylamide,24 methacrylic acid,26 ethyl methacrylate,27 and methacrylamide28 where significant improvement in the properties of the guar gum after incorporation of graft chains has been reported. Water insoluble copolymer of guar gum such as poly(methylmethacrylate) grafted guar gum22 may be potentially used as adsorbent in the aqueous medium successfully. Use of microwave irradiation in the grafting of biopolymeric materials has attracted recent attention.24,25 It not only produces copolymer adsorbents of improved binding potential29,30 when compared to conventionally synthesized copolymers, but also minimizes the usage of chemicals in the copolymer synthesis making the grafting procedure cost-effective. Literature reveals that no attempt so far has been made to graft ethylacrylate (EA) onto guar gum, though the monomer (EA) has been grafted onto various other biopolymeric materials for the favorable modification in the

10.1021/ie801416z CCC: $40.75  2009 American Chemical Society Published on Web 04/09/2009

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properties such as on flax fibers, cellulose, and alginic acid by chemical methods. In view of the undiscovered adsorption behavior of PEA grafted guar gum and the reported advantages of using microwaves in the copolymer synthesis, in the present investigation, a detailed study on the microwave synthesis of GGg-PEA has been undertaken. The microwave synthesized copolymer has been compared to conventionally synthesized copolymer (synthesized using K2S2O8/ascorbic acid) in the removal of Cd(II) from the synthetic solutions. 2. Experimental Details Materials. A Kenstar (model No. OM 20 ESP; 1200 W) domestic microwave oven with a microwave frequency of 2450 MHz and a power output from 0 to 800 W with continuous adjustment was used for the adsorbent synthesis. All the reagents used were of analytical grade. Double distilled deionized water was used throughout the study. A stock Cd(II) solution (1000 mg/L) was prepared in water using Cd(NO3)2 (Merck; Darmstadt, Germany) and all the working solutions were prepared by diluting this stock solution. Commercially available sample of guar gum (BDH; Poole, UK) was used after purification; ethylacrylate (Loba Cheime; India), potassium persulfate (Merck), and ascorbic acid (Merck) were used without further purification. Analysis. Cd(II) concentration was determined by microprocessor-based Systronics single beam visible spectrophotometer (model T-105). Systronics digital pH meter (model 335) was used for pH measurement. The pH values were adjusted by the addition of 5 M H2SO4 or 1 M NaOH. FTIR spectra were recorded on a Bruker Vector-22 infrared spectrophotometer using KBr pellets. For SEM pictures Leo 440 scanning electron microscope was used. Maximum % grafting sample was used for the characterization by spectral studies and also for the sorption study. The percentage and efficiency of grafting were calculated using the following relation:18 % grafting ) % efficiency )

W1 - W 0 × 100 W0 W 1 - W0 × 100 W2

(1) (2)

where W1, W0, and W2 denote, respectively, the weight of the grafted guar gum, the weight of original guar gum, and weight of the monomer used. Methods. Purification of the Guar Gum. Commercial guar gum (GG) was purified through barium complexing24 where an aqueous solution of the gum (2.5% w/v) was precipitated using saturated barium hydroxide solution. The resulting barium complex was separated by centrifugation, washed well with water to remove excess of barium, and dissolved in the minimum required volume of 0.1 N HCl. The resulting solution was finally precipitated with excess of ethanol (4 vol), washed with 70, 80, 90, and 95% ethanol, and dried. Grafting of Ethylacrylate onto GG. Grafting under Microwave Irradiation in Aqueous Medium. To a solution of GG (100 mg in 25 mL) taken in a 150 mL flask, ethylacrylate (0.16 M) was added, and the reaction mixture after vigorous stirring was irradiated in domestic microwave oven at a known microwave power for a definite time period.24 Poly(ethylacrylate) (PEA) grafted GG samples of different % grafting were separated from the respective reaction mixtures by precipitating them in an excess of acetone. The grafted samples were finally extracted with acetone in a Soxhlet apparatus for 4 h to dissolve all the homopolymer. The colorless GG-g-PEA samples were

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dried under vacuum at 40 °C for >24 h to a constant weight and crushed to small flakes (∼0.5 mm thickness) and weighed. At optimum microwave power and exposure time, concentration of the ethylacrylate (0.12-0.20 M) and GG (50-250 mg/25 mL) were varied one at a time keeping the other fixed. Grafting Using K2S2O8/Ascorbic Acid Redox Pair under Thermostatic Water Bath. To a solution of GG (100 mg in 25 mL) taken in a 150 mL reaction flask, ethylacrylate (0.16 M) and ascorbic acid (0.037 M) were added and thermostatted on a water bath at 35 ( 0.2 °C. After 30 min K2S2O8 (0.030 M) was added and this time of addition of persulfate was taken as zero time.18 Graft copolymerization was allowed for 1 h. Then the reaction product (cvGG-g-PEA) was precipitated in acetone to obtain the elastic copolymer that was repeatedly extracted with acetone for the removal of adhered poly(ethylacrylate). The product was crushed finally to small flakes (∼0.5-1 mm) and dried. % G and % E were calculated to be 280% and 69.91%, respectively. Removal of Cd(II) by Microwave Synthesized Copolymer (mwGG-g-PEA). mwGG-g-PEA was evaluated for Cd(II) removal from the aqueous solution and the conditions for the adsorption were optimized. Different adsorption parameters were changed (one at time), keeping the others fixed. Under the optimum adsorption conditions for mwGG-g-PEA, adsorption of Cd(II) by cvGG-g-PEA was also studied for comparison. Sorption Experiments. Stock solution of standard Cd(II) (1000 mg/L) was prepared from cadmium nitrate in double distilled-deionized water. Experiments were carried out on a temperature controlled incubator shaker set at 120 rpm and maintained at 30 ( 2 °C for 16 h in 50 mL conical flasks. Keeping the other parameters fixed, one parameter was varied at a time. For pH studies, 20 mL solutions of 100 mg/L metal ion were adjusted to various pH values ranging from 1 to 10. Different adsorbent doses ranging from 10 to 100 mg were used to study the effect of adsorbent on the removal of Cd(II) at 100 mg/L initial Cd(II) concentration. The range for different initial concentrations of cadmium was 100-1200 mg/L. A 50 mg portion of the copolymer was thoroughly mixed with 20 mL of Cd(II) solution, whose concentration and pH values were previously known. After the flasks were shaken for the desired time, the suspensions were filtered using Whatman 0.45 mm filter paper, and the filtrates after suitable dilutions, were analyzed for Cd(II) concentration spectrophotometrically (at 480 nm wavelength) by developing a violet red color with cadion in basic medium (Triton X-100 method).34 Control experiments showed that no sorption occurred on either glassware or filtration systems. The pH of the reaction mixtures was initially adjusted to 9.0 using sodium hydroxide (0.2 N). The pH, initial concentration of Cd(II), and the electrolyte amount (ionic strength) were varied, one at a time keeping the other parameters fixed. Copolymer samples of different % grafting were used to study the effect of % grafting on the Cd(II) removal under the optimum adsorption condition. The calculation of amount of metal ion adsorbed by GG-g-PEA, after spectrophotometer readings of equilibrium solution, was obtained by calculating the difference using the formula: qe (mg/g) )

Co - Ce(V ⁄ 1000) W

(3)

where qe is the amount of metal ion adsorbed on the adsorbent, C0 is the initial metal ion concentration (mg/L), Ce is the equilibrium concentration of metal ion in solution (mg/L), V is the volume of metal ion solution used (L), and W is the weight of the adsorbent used (g).

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Table 1. Optimization of Microwave Power and Exposure Time at Fixed Monomer Concentration (0.16 M), GG (100 mg), Total Reaction Volume of 25 mL s. no.

microwave power (%)

exposure (s)

yield (mg)

SDEVa

%G

%E

1

20

2

40

3

60

15 25 35 45 55 15 25 35 45 55 05 10 15 25 35 45 55 05 10 15 25 35 45 55 05 10 15 25 35 45 55

108 112 115 124 185 253 285 265 260 240 242 265 325 310 270 250 225 277 305 325 290 255 220 210 294 318 395 360 310 220 190

1.0 1.4 0.7 1.0 0.7 0.7 1.7 1.0 1.2 1.4 1.4 1.0 1.2 1.4 1.4 1.4 1.2 1.0 1.0 1.0 1.4 1.0 1.4 1.4 1.2 1.8 0.7 1.2 1.4 1.4 1.4

8 12 15 24 85 153 185 165 160 140 142 165 225 210 170 150 125 177 205 225 190 155 120 110 194 218 295 260 210 120 90

1.9 2.9 3.7 5.9 21.2 8.2 46.2 41.2 39.9 34.9 35.0 41.2 56.1 52.4 42.5 37.5 31.2 44.3 51.3 56.1 47.5 38.7 29.9 27.5 48.5 54.5 73.7 64.9 52.4 29.9 22.5

4

80

5

100

a

Standard deviation.

Desorption Studies. To determine the reusability of the adsorbent, Cd(II) from the loaded adsorbent was stripped off using 1 M HCl, 1 N H2SO4, 1 M EDTA, and 1 M NaOH and reused. The best results observed were for H2SO4 and, to optimize the concentration of the acid required for the quantitative stripping of the loaded Cd (II), experiments were carried out using various concentrations of H2SO4 ranging from 0.01 to 1.0 M where 0.5 N H2SO4 was found most suitable. Therefore copolymers loaded with cadmium were placed in the 0.5 N H2SO4 and stirred at 120 rpm for 4 h at 30 °C and the final Cd(II) concentration was determined. After each cycle the used copolymer was washed with distilled water and used in the succeeding cycle up to nine cycles. The amount desorbed was calculated from the amount of metal ions loaded on the copolymers and the final cadmium concentration in the stripping medium. For the quantitative stripping 4 h of equilibration was required. 3. Results and Discussions Poly(ethylacrylate) (PEA) was successfully grafted in good yield using persulfate/ascorbic redox initiator18 as well as by microwave induced grafting; however, at the same monomer concentration, higher %G could be achieved under microwave conditions indicating incorporation of more/larger PEA grafts. The optimum %G was obtained by exposing a reaction mixture (25 mL) containing ethylacrylate (EA) (0.16 M) and GG (0.1 g) under 100% microwave power for 15 s (Tables 1 -3). Though the copolymer being a macromolecule cannot show rotation or migration when exposed to an electric field of microwaves, its hydroxyl and ester groups show localized rotation on an essentially immobile copolymer molecule.

Table 2. %G and %E with change in EA Concentration, GG (100 mg), Total Reaction Volume of 25 mL at 100% Microwave Power, 15 s Exposure s. no.

monomer in M

yield ((5.0 mg)

SDEV

%G

%E

1. 2. 3. 4. 5.

0.12 0.14 0.16 0.18 0.20

240 310 395 340 310

1.0 1.2 0.7 1.0 1.2

140 210 295 240 210

46.6 59.9 73.7 53.3 41.9

Table 3. %G and %E with Change in Gum Concentration, EA (0.16 M), Total Reaction Volume of 25 mL at 100% microwave power, 15 s Exposure, GG (100 mg) s. no.

gum in 25 mL

yield ((5.0 mg)

STDEV

%G

%E

1. 2. 3. 4. 5.

50 100 150 200 250

340 395 370 320 280

1.0 0.7 1.4 1.2 1.0

240 295 270 220 180

59.9 73.7 67.4 54.9 44.9

However, such localized rotations25 cannot correspond instantaneously to the rapidly changing direction of the field, which creates friction that manifests itself as heat resulting into bond breaking and the free radical formation which are responsible for the grafting. Further microwaves (MW) are also reported to have special effects on the lowering of Gibbs free-energy of activation of the reactions. In view of the above two effects, grafting of EA under MW conditions is possible in the absence of free radical initiator in a very short time. The representative sample (sample with maximum %G) of mwGG-g-PEA was characterized using FTIR and SEM analysis and was used for the adsorption experiments. Characterization mwGG-g-PEA. In the IR spectrum of pure GG (Figure 1A), a strong O-H stretching peak (3439 cm-1) and a small C-H stretching peak (2924 cm-1) are visible, while in the IR spectrum of mwGG-g-PEA, an additional peak at 1728 cm-1 (ester -CdO group) (Figure 1B) is seen besides O-H stretching (at 3420 cm-1) and C-H stretching (at 2924 and 2854 cm-1) peaks, confirming the grafting of EA onto GG. The IR spectrum of cadmium loaded adsorbent (Figure 1C) showed significantly different spectrum than that of the copolymer. In cadmium loaded copolymer the O-H stretching and carbonyl stretching peaks are seen shifted to higher frequency (at 3435 and 1743 cm-1, respectively) with respect to the peaks in mwGG-g-PEA (3420 and 1728 cm-1) indicating complexation between the cadmium species and the hydroxyl and ester groups at the copolymer. After cadmium adsorption, a change in the shape and the intensity of CH2 absorption is also observed because the -CH2 groups though are not direct participants in the complexation, they are part of the primary alcoholic group participating in the complexation. Moreover the cadmium loaded copolymer showed significantly different absorption pattern in the region of 1000-1500 cm-1. Shift of both O-H stretching and carbonyl stretching peaks confirm that there is more than one mechanism involved in binding. The same was also inferred by successful modeling of the adsorption data to both Langmuir and Freundlich isotherms. The SEM picture showed that graft copolymer before and after Cd(II) loading had significantly different morphologies which evidenced cadmium loading on to the graft copolymer (Figure 2). Ethylacrylate-grafted GG had a flattened flaky thin sheetlike appearance, while in the cadmium-loaded copolymer strandlike small depositions of cadmium are seen on the graftcopolymer surface.

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Figure 1. Infrared spectra of (A) Guar gum, (B) mwGG-g-PEA, and (C) mwGG-g-PEA-Cd.

Figure 2. (A) SEM picture at 1500× of mwGG-g-PEA, (B) mwGG-g-PEA-Cd.

Optimization of Cd(II) Removal. Adsorption conditions were optimized by varying various adsorption parameters, one parameter at a time keeping the others fixed (Figure 3). Effect of pH on Cd(II) Adsorption. The effect of pH on Cd(II) removal using GG-g-PEA was studied, and it was observed that the removal by GG-g-PEA is at a maximum at pH 9.0. As the pH was increased from pH 1 to 9, the extent of Cd(II) removal increased, while further increase in pH decreased the adsorption owing to precipitation of Cd(OH)2. The extent of Cd(II) removal by GG-g-PEA (at 100 mg/L initial Cd(II) concentration) increased from 2% to 94% with the variation of pH from 1.0 to 9.0 (Figure 3a,A). The effect of pH on the sorption indicates polar interaction between copolymer and

cadmium species as was also evident by the infrared spectrum of cadmium-loaded species where participation of the ester and hydroxyl groups is indicated in the complexation. In acidic medium35 the cadmium species exists as hydrated Cd2+ ((Cd(H2O)62+, Cd(H2O)42+), which finds difficult to get adsorbed as compared to the species (Cd(OH)+) existing in the alkaline medium; moreover, in acidic medium H+ ions compete for the binding sites of the adsorbent. Effect of Adsorbent Dose. The effect of copolymer amount on the removal of Cd(II) is presented in Figure 3a,B. It was observed that the percentage removal of Cd(II) increases from 24.6% to 99.9% with the increase in adsorbent dose from 10 mg to 100 mg in 20 mL of 100 mg/L Cd(II) solution at 30 °C,

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Figure 3. (a) Optimization of adsorption conditions: (A) adsorption at various pH values at 100 mg/L Cd(II), adsorbent dose 50 mg, contact time 16 h at 30 °C; (B) adsorption at various adsorbent dose at 100 mg/L initial Cd(II) concentration, pH 9, contact time 16 h at 30 °C; (C) adsorption at various %G at 100 mg/L initial Cd(II) concentration, adsorbent dose 50 mg, pH 9, contact time 16 h at 30 °C; (D) adsorption at various concentrations, adsorbent dose 50 mg, pH 9, contact time 16 h at 30 °C. (b) Effect of (A) temperature at 100 mg/L initial Cd(II) concentration, adsorbent dose 50 mg, contact time 16 h at 30 °C; (B) electrolyte on the adsorption at 1000 mg/L initial Cd(II) concentration, 50 mg adsorption dose, 120 rpm, pH 9 at 30 °C.

Figure 4. Kinetic models for the equilibrium adsorption data: (A) pseudo-second-order model; (B) second order; (C) Lagergren model at 100 mg/L initial Cd(II) concentration.

120 rpm in 16 h. This is explainable because at a higher adsorbent dose extra adsorption sites are available for the adsorption. Effect of %G. With an increase in percent grafting (from 85 to 295%) in the copolymer samples, adsorption increased from 32% to 94.14% at a fixed adsorbent dose (50 mg), pH (9.0), initial Cd(II) (100 mg/L), rpm (120), and contact time (16 h) at 30 °C (Figure 3a,C). Up to 100%G, the graft copolymer samples

were partially water-soluble and hence their efficiency for cadmium removal was quite low. With increase in the %G, the cadmium removal by the copolymer increased which confirmed the participation of ester groups (at the PEA grafts) in the adsorption. Effect of Initial Concentration of Cd(II). The experimental results demonstrating the effect of initial concentration of Cd(II) on the removal is shown in Figure 3a,D. It was observed that

Ind. Eng. Chem. Res., Vol. 48, No. 10, 2009 Table 4. Sorption Kinetics of the Cadmium by mwGG-g-PEA pseudo-second-order model second-order-model R2 qe k′ (g/mg/min) k (g/mg/min) kL (min-1)

0.9978 40.8 4.6 × 10-4

0.9119 69.44 2 × 10-6

Lagergren model 0.9055 1.10 1.15 × 10-6

with the increase in the initial concentration of Cd(II) from 100 to 1200 mg/L, the % removal of Cd(II) decreases from 87.49% to 94.14% at fixed adsorbent dose (50 mg/20 mL) although the amount of Cd(II) adsorbed increases from 94 to 1049 mg/L. With increase in initial Cd(II) concentration, amount of Cd(II) adsorbed increases as more cadmium ions are available for the adsorption, however % adsorption is reduced as ratio of available adsorbing site in the adsorbent (which is fixed) and the available Cd(II) decreases. Effect of Temperature. To understand the effect of temperature on the adsorption, the studies were performed in the temperature range of 10-35 °C where up to 30 °C adsorption continuously increased with the increase in temperature (Figure 3b,A) indicating endothermic nature of the adsorption; however, at 35 °C a marginal decrease in the adsorption was observed indicating some desorption taking place above 30 °C. Effect of Electrolyte. The presence of salts may interfere with the cadmium adsorption. To understand the effect of some interfering ions on Cd binding, NaCl, and Na2SO4 were added to the aqueous synthetic solution of cadmium. With an increase in concentration of both NaCl and Na2SO4 from 0.01 to 1.0 M, removal decreases from 889 to 622 and from 880 to 640 mg/L, respectively, from 20 mL of 1000 mg/L Cd(II) solution at pH 9, temperature 30 °C, rpm 120,and contact time 16 h (Figure 3b,B). The decrease in the removal on increasing the electrolyte concentration may be due to competition between cadmium species and SO4-2/Cl- at the binding sites, where SO4-2/Clion can be electrostatically held. Sorption Kinetics. Cd(II) removal was monitored with time. The kinetics of Cd(II) removal by the copolymer indicated rapid binding of Cd(II) initially, followed by a slow increase until a state of equilibrium at 17 h was reached. No change in the uptake capacity was observed up to 24 h. The initial rapid increase is due to greater number of vacant sites available initially, resulting in a high concentration gradient between

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adsorbate in solution and adsorbate in the adsorbent. Generally when adsorption involves a surface reaction process, the initial adsorption is rapid, then a slower adsorption would follow as the available adsorption sites gradually decrease. Kinetics37 of the adsorption were modeled by the first order Lagergren equation, the pseudo-second-order equation, and the secondorder-rate equation shown below as eqs 4-6, respectively (Figure 4). log

kI t (qe - qt) ) log qe qe 2.303

(4)

t ⁄ qt ) 1 ⁄ k'q2e + t ⁄ qe

(5)

1 ⁄ (qe - qt) ) 1 ⁄ qe + k2t

(6)

where KL is the Lagergren rate constant of adsorption (min-1); k′ is the pseudo-second-order rate constant of adsorption (g/ mg/min) and k2 the second-order rate constant (g/mg/min); qe and qt are the amounts of metal ion adsorbed (mg/g) at equilibrium and at time t, respectively. The kinetic data of Cd(II) adsorption best fit in to the pseudo-second-order kinetic model as reported for other grafted biopolymeric materials38 where the linear plot of t/qt vs t was obtained; the correlation coefficient (R2) and rate constant were 0.9978 and 4.6 × 10-4 g/mg/min, respectively, at 100 mg/L initial Cd(II) concentration. These results suggest that the adsorption kinetics is not diffusion controlled but is chemisorption. Rate constants and R2 values for the different kinetic models for the adsorption have been compared in Table 4. Adsorption Isotherm Studies. Adsorption data were fitted to the Langmuir and Freundlich isotherms.39 The Langmuir isotherm is valid for monolayer sorption due to a surface of a finite number of identical sites and expressed in the linear form as under Ce ⁄ qe ) 1 ⁄ bQo + Ce ⁄ Qo

(7)

where Ce is the equilibrium concentration (mg/L) and qe the amount adsorbed at equilibrium (mg/g). The Langmuir constants Qo (mg/g) represent the monolayer adsorption capacity and b (L/mg) relates the heat of adsorption. The essential feature of the Langmuir adsorption can be expressed by means of RL, a dimensionless constant referred to as separation factor or equilibrium parameter for predicting whether an adsorption

Figure 5. Langmuir isotherms for cvGG-g-PEA at different temperatures for mw GG-g-PEA: (A) 10, (B) 15, (C) 30, and (D) 35 °C.

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Figure 6. Freundlich isotherms for cvGG-g-PEA at different temperatures for mw GG-g-PEA: (A) 10, (B) 15, (C) 30, and (D) 35 °C. Table 5. Langmuir and Freundlich Models Regression Constants for mwGG-g-PEA (300%G) and cvGG-g-PEA (280%G) Langmuir isotherm

Freundlich isotherm

copolymer

temperature (°C)

Qmax

b (L · mg-1)

R2

n

KF (mg · g-1)

R2

mwGG-g-PEA

283 288 303 308 283 303

714.28 714.28 714.28 714.28 270.27 270.27

0.0016 0.0025 0.0085 0.0038 0.0035 0.0040

0.9527 0.9802 0.9709 0.9862 0.9810 0.9758

1.19 1.24 1.37 1.27 1.29 1.57

2.33 3.23 10.40 5.00 7.32 3.74

0.9892 0.9944 0.9987 0.9943 0.9943 0.9800

cvGG-g-PEA

system is favorable or unfavorable. RL is calculated using the following equation. RL ) 1 ⁄ (1 + bC0)

(8)

where C0 is the initial Cd(II) concentration (mg/L). If RL values lies between 0 and 1, the adsorption is favorable. The Freundlich isotherm describes the heterogeneous surface energies by multilayer adsorption and is expressed in linear form as ln qe ) ln Kf + 1 ⁄ n ln Ce

(9)

where Kf indicates adsorption capacity (mg/g) and n is an empirical parameter related to the intensity of adsorption, which varies with the heterogeneity of the adsorbent. The greater the values of the n is, the better is the favorability of the adsorption. Equilibrium adsorption data of both the microwave as well as conventionally synthesized copolymer could be modeled satisfactorily to both the Langmuir and Freundlich isotherms. The isotherms studies for mwGG-g-PEA were performed from 10-35 °C and are shown in Figures 5 and 6, indicating more than one adsorption mechanisms (cf. IR spectrum of Cd(II) loaded polymer) are operating in the adsorption process. Isotherm studies for cvGG-g-PEA were also performed at 10 and 30 °C for comparison; Langmuir and Freundlich constants of the conventionally and microwave synthesized graft copolymers have been summarized in Table 5; however, isotherms of the cvGG-g-PEA are not shown. The results were similar to our previous study onto the microwave synthesized Cassia marginata gum-graft- poly(methylmethacrylate)29 which was used for Cr(VI) removal (Q0 ) 185.119). In the present study Cd(II) adsorption has been undertaken by the GG-g-PEA where Q0 for microwave synthesized copolymer (714.28 mg/g) was higher in comparison to the conventionally synthesized copolymer (270.17) as was also observed in our previous study with GG-g-PMMA. The sorption capacity of the adsorbent has been

Table 6. Sorption Capacity of the Guar-g-PEA in Comparison to Other Reported Adsorbents s. no.

adsorbent

Qmax

1.

poly(vinyl pyridine-poly ethylene glycol methacrylate-ethylene glycol dimethacrylate) beads4 pyromellitic dianhydride-grafted β-cyclodextrin14 Kraft lignin42 Goethite-coated43 sand surface agricultural waste44 inorganic-organic magnesium organosilicate45 modified chitosan46 activated carbon from coconut coir pith7 mwGG-g-PEA (present study) cvGG-g-PEA (present study)

16.50 mg/g

2. 3. 4. 5. 6. 7. 8. 9. 10.

92.85 mg/g 175.36 mg/g 0.70 mg/g 99.4 mg/g 0.35 mmol/g 357.14 mg/g 93.4 mg 714.28 mg/g 270.27 mg/g

compared with other reported adsorbents in Table 6, The Qmax in our current studies is significantly higher than other reported adsorbents. Desorption Studies. Cd(II) loaded copolymer samples were reused in the next cycle after desorption by stirring in 0.5 N H2SO4 for 4 h, and adsorption desorption cycles were repeated for nine times at 500 mg/L initial Cd(II) concentration, 50 mg adsorbent dose, pH 9, 120 rpm, 8 h contact time at 30 °C. It was observed that even in the ninth cycle the adsorbent could adsorb >38% Cd(II). Only 50% loss in the adsorption ability of the copolymer was observed up to the eighth cycle (Figure 7). Adsorption Thermodynamics. The values of thermodynamic parameters are relevant for the practical application of adsorption process.40 Isotherm data related to adsorption of Cd(II) on to the composite at various temperatures ranging from 10 to 35 °C were analyzed to obtain the values of thermodynamic

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at the solid-solution interface during the fixation of the metal ion on the active site of the adsorbent. Conclusion

Figure 7. Adsorption/desorption cycles of Cd(II) adsorption on GG-g-PEA at 500 mg/L Cd(II) concentration, adsorbent dose 50 mg, pH 9, 120 rpm, contact time 8 h.

Using microwave irradiation, GG-g-PEA was successfully synthesized in the absence of any redox initiator or catalyst in much better yield than conventional redox (persulfate/ascorbic acid) initiated synthesis. Moreover the copolymer synthesized using microwaves proved to be a more effective adsorbent for Cd(II) from synthetic solution as compared to conventionally synthesized copolymer. The sorption was found to be pH dependent and was greatest at pH 9.0. It was revealed that more than one adsorption mechanisms are operating in the sorption. The positive ∆H° indicated the endothermic nature of Cd(II) sorption onto mwGG-g-PEA. The sorption followed a pseudosecond-order kinetic model. The temperature dependence of Cd(II) uptake and the pseudo-second-order kinetics of the adsorption indicated that chemisorption is the rate-limiting step that controls the process. The sorbent could be used successfully for nine cycles. Acknowledgment

Figure 8. Thermodynamic simulation of the adsorption of Cd(II) by mwGGg-PEA. Table 7. Thermodynamic Parameters for the Adsorption by mwGG-g-PEA copolymer mwGG-g-PEA

The authors are thankful to the Ministry of Environment & Forests, New Delhi, India, for the financial assistance to carry out this work and the Indian Institute of Technology for providing instrumental facilities. Literature Cited

temp (°K) ∆G0 (KJ/mol) ∆H0 (KJ/mol) ∆S0 (KJ/mol) 283 288 303

-1.1 -2.2 -5.4

+ 59.34

+ 0.2135 + 0.2136 + 0.2136

parameters. The values of thermodynamic function ∆S and ∆H were evaluated using Vant Hoff’s equation, which is given by ln b ) (∆S ⁄ R)-(∆H ⁄ RT)

(10)

∆G ) ∆H - T∆S

(11) -1

where ∆G is the change in Gibbs free energy (J mol ), R is the universal gas constant (8.314 J K-1 mol-1), T is the temperature (Kelvin), ∆H is the change in enthalpy (J mol-1), b is the Langmuir constant at temperature T, and ∆S is the change in entropy (J mol-1 K-1). The values of ∆S and ∆H were calculated from the intercept and slope of a plot between ln b versus 1/T, while the values of ∆G were calculated41 using eq 11. The enthalpy change (∆H) is determined graphically by plotting ln b versus 1/T (Figure 8) which gives a straight line, and the values of ∆G and ∆S computed numerically are presented in Table 7. Negative values of ∆G indicated that the adsorption process was favorable and spontaneous in nature. It may be noted that with the increase in temperature from 10 to 30 °C, the value of ∆G decreased from -1.1 to -5.4 kJ mol-1, thus adsorption of Cd(II) onto the copolymer was increased at a higher temperature. The positive value of enthalpy change (∆H) confirmed the endothermic nature of the adsorption, and also the process may be due to chemical bonding or chemisorption. Also a positive ∆H indicates that the process is irreversible. Positive values of ∆S suggested good affinity of the metal ion toward the adsorbent and increased randomness

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ReceiVed for reView September 19, 2008 ReVised manuscript receiVed March 6, 2009 Accepted March 16, 2009 IE801416Z