APPLIED CHEMISTRY

Efficient Chromium(VI) Adsorption by Cassia marginata Seed GumFunctionalized with Poly(methylmethacrylate) Using Microwave Irradiation

Vandana Singh,* Ajit Kumar Sharma, Premlata Kumari, and Stuti Tiwari

Department of Chemistry, UniVersity of Allahabad, Allahabad-211001, India

Increasing cost of the chemicals and production of toxic sludge in the Cr(VI) treatment methods have attractedattention toward the use of biosorbents for Cr(VI) removal. The present study evaluates a novel biosorbentderived from Cassia marginata seed gum in the removal of Cr(VI) from the aqueous solution and wastewater.The adsorbent was synthesized using microwave irradiation in the absence of any radical initiator or catalystin good yield. Adsorbents of different performances could be obtained by varying the amount of themethylmethacrylate, microwave power, and exposure time. A representative sample of microwave synthesizedadsorbent was characterized using FTIR, XRD, TGA, and SEM analysis. Cr(VI) sorption was optimizedusing the copolymer sample of highest grafting ratio and efficiency (270% G and 59.65% E) where the removalwas found to be pH and concentration dependent, pH 1.0 being the optimum value at which from 20 mL of100 ppm Cr(VI) solution, 16.94 mg/g Cr(VI), could be removed using 5 g/L adsorbent dose at 30 °C. Theadsorption data followed both Langmuir (R2 ) 0.9703) and Freundlich isotherms (R2 ) 0.8957) probablydue to the real heterogeneous nature of the surface sites involved in the metal uptake, and overall sorption ofCr(VI) on the biosorbent is complex and involves more than one mechanisms. The adsorption followed secondorder kinetics, the rate constant being 0.10 × 10-5 g/(mg min) at 100 mg/L Cr(VI) concentration. The adsorbentwas also found efficient in Cr(VI) removal from real industrial wastewater. Used copolymer was recycledafter stripping off the adsorbed chromium with 2 M NaOH where after each cycle a successive decrease inthe binding capacity was observed. To understand the advantage of using microwaves in the adsorbent synthesis,the copolymer synthesized using a K2S2O8/ascorbic acid redox pair at identical monomer concentrations (220%G and 48.6% E) was also evaluated as Cr(VI) sorbent, and the results obtained were compared with that ofmicrowave synthesized copolymer.

Introduction

Cr(VI) is a known highly toxic metal, and its removal1 isconsidered as a priority. Ion exchange techniques are used forCr(VI) removal where generation of volumetric sludge increasesthe cost.2 Adsorption using commercial activated carbon (CAC)1

can remove chromium from wastewater,3 but CAC remains anexpensive material for the removal. Polymeric materials suchas poly(4-vinyl pyridine) beads,4 amidoxime chelating resin,5

cross-linked poly(glycidylmethacrylate-co-methylmethacrylate),6

and aminated polyacrylonitrile fibers7 have been reported forefficient removal of Cr(VI). However, environmental issues ledto the development of several biosorbents for Cr(VI) removal.Many polysaccharide materials in their native form as well asafter modification have been exploited as biosorbents. Chitinand chitosan have been widely used for Cr(VI) removal in theirnative/modified form.8 Chitosan beads,9 chitosan cross-linkedwith glutaraldehyde,10 chitosan microspheres cross-linked withepichlorohydrin,11 chitosan-based polymeric surfactants,12 andcross-linked xanthated chitosan13 are some examples. Otherpolysaccharides such as Kendu fruit gum,14 mucilaginous seedsof Ocimum basilicum,15 and poly(acrylamide)-grafted saw dust16

have also been reported for Cr(VI) removal. Cellulose, starch,and alginic acid derivatives17 have been used as adsorbentmaterials after chemical modifications or grafting; for example,cellulose acetate and sulfonated poly(ether ether ketone) blend

ultrafiltration membranes are reported for Cr(VI) removal.18 Theamine-modified polyacrylamide-grafted coconut coir pith car-rying the -NH3

+Cl- functional group at the chain end was

* To whom correspondence should be addressed. E-mail:singhvandanasingh@rediffmail.com. Tel: +91 532 2461518. Fax: +91532 2540858.

Table 1. % G and % E with Different Microwave Powers andExposure Times at 16 × 10-2 M Monomer, 4 g/L GumConcentration, and 25 mL Total Reaction Volume

sample no. microwave power (%) exposure time % G % E

1 20 10 18 4.4420 32 7.8930 70 17.2640 80 19.7250 93 22.93

2 40 10 26 6.4120 70 17.2630 120 29.5940 190 46.8650 145 35.76

3 60 10 30 7.3920 50 12.3330 90 22.2040 160 39.4650 100 24.66

4 80 10 35 8.6320 70 17.2630 140 34.5340 80 19.7350 55 13.76

5 100 10 50 12.3320 110 27.1330 90 22.240 43 10.6050 12 2.96

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10.1021/ie070467j CCC: $40.75 2008 American Chemical SocietyPublished on Web 07/09/2008

investigated as an adsorbent for its possible application in theremoval of chromium(VI) from aqueous solution and waste-water.19 Oxyanionic contaminants, specifically selenium andCr(VI), were removed from solution by sorption onto gel beadsformed by pretreating the biopolymer alginic acid with calciumand iron(III).20 Vinyl modified polysaccharides show goodadsorption properties, and the use of microwaves in theirsynthesis has been reported to furnish even better adsorbentmaterials.21 No attempt so far has been made in the directionof exploiting microwave synthesized polymer modified biom-asses for removing the metal ions from aqueous solution.

Cassia plants22 are reported to contain a substantial amountof seed gums, and potentially these gums may find use as arenewable biosorbent material for Cr(VI) removal. Since theseseed gums are soluble in water, they require some modificationbefore they can be exploited for such use. Their stability andsolubility as well as their sorbing capacity can be altered23

through their graft copolymerization with vinyl monomers.Grafting is done using redox initiators,24–27 γ irradiation,28 andmicrowave irradiation,29,30 and the properties of the obtainedgraft copolymer are found to be dependent on the method used.Cassia marginata syn: Cassia roxburghii31 is a large sizedIndian tree having cylindrical and indehiscent long pods (withmany seeds) containing a black cathartic pulp, used as a horsemedicine. Analysis of its seed oil has been published while noattempt so far has been made for the commercial exploitationof its seed gum though found in substantial amount. The mainobjective of the present study is the microwave induced synthesisCassia marginata seed gum-graft-poly(methylmethacrylate) andits evaluation as biosorbent in the removal of Cr(VI) from theaqueous solution and wastewater. In conventional graftingprocedures, surface adsorption of the cations (furnished by redoxinitiators/catalyst) by the copolymer may possibly reduce itsadsorption potential. To understand the advantage of usingmicrowaves in adsorbent synthesis, the conventionally synthe-sized graft copolymer synthesized at identical monomer con-centration was also tested for the Cr(VI) removal, and the results

of the two were compared to understand the advantage of usingmicrowaves in the adsorbent synthesis.

Materials

Cassia marginata seeds were supplied by Himani seed stores,Deheradun, and identified by systematic botanist at BotanicalSurvey of India, Allahabad. Methylmethacrylate (Loba Cheme)was washed with 5% aqueous alkali to remove phenolic inhibitorand then distilled before use. Temperature controlled flask shakerwas used for batch experiments. For Cr(VI) removal, all thechemicals used are of analytical reagent grade and were utilizedas received without further purification. Deionized water is usedthroughout the study. Aqueous solutions of K2CrO4, HCl,H2SO4, and NaOH were prepared from K2CrO4 (Merck),respective acids, and NaOH (Merck), respectively. 1,5-Diphenylcarbazide (Merck), ascorbic acid (Merck), and potassiumpersulfate (Merck) were used without further purification. Thesample with maximum grafting ratio was used for the charac-

Scheme 1. Synthesis of the Adsorbent

Table 2. % G and % E at Different Monomer Concentrations atFixed Gum Concentration (4g/L), Microwave Power (40%),Exposure Time (40 s), and Total Reaction Volume (25 mL)

sample no. monomer concentration (M) % G % E

1 12 × 10-2 80 26.512 14 × 10-2 130 37.253 16 × 10-2 190 46.854 18 × 10-2 270 59.655 20 × 10-2 240 48.02

Table 3. % G and % E at Different Gum Concentrations at FixedAmount of Monomer (18 × 10-2 M, Microwave Power (40%),Exposure Time (40 s), and Total Reaction Volume (25 mL)

sample no. gum (mg/25 mL) % G % E

1 50 120 13.252 100 270 59.653 150 173.3 57.444 200 125 55.235 250 108 59.65

Table 4. % G and % E with Changing Total Reaction Volume atFixed Monomer Concentration (18 × 10-2 M), Microwave Power(40%), and Exposure Time (40 s)

sample no.total reactionvolume (mL) % G % E

15 120 26.512 20 200 44.193 25 270 59.654 30 220 48.605 35 140 30.93

Table 5. Solubility of the Grafted Gums of Different % G in Water

sample no. % G % solubility in water

1 18 1002 30 82.383 50 61.134 70 40.625 80 17.756 100 3.027 162 09 200 0

10 240 011 270 0

5268 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008

terization by the spectral studies. The wastewater was procuredfrom a local electroplating industry in Kanpur, Uttar Pradesh,India (color: greenish yellow, pH ) 1.6, TDS ) 6.67 ppt, andconductivity was 10.38 m S-1).

Analysis

A Kenstar (model no. OM 20 ESP; 1200 W) domesticmicrowave oven with a microwave frequency of 2450 MHzand a power output from 0 to 800 W with continuous adjustmentwas used for the adsorbent synthesis. The concentration ofCr(VI) was measured using a visible spectrophotometer (Systron-ics model 105) at 540 nm using the diphenyl carbazide

method32,33 after suitable dilution. Total chromium in solutionwas assessed by atomic absorption spectrophotometer (Electron-ics Corporation of India Limited model-AAS 4141) at λ 357.9nm, slit width 1 nm, using an air-acetylene flame. SystronicsDigital pH meter model 335 was used for pH measurement.The pH values are adjusted by the addition of 5 M H2SO4 or 1M NaOH. TDS and the conductivity of the waste water weremeasured by microprocessor based Systronics Conductivity/TDSmeter model 1601 with cell constant 1 cm-1. Samples werefiltered using Whatman 0.45 mm filter paper, and the filtratesafter suitable dilutions were analyzed. Control experimentsshowed that no sorption occurred on either glassware or thefiltration system. For scanning electron microscopy (SEM)pictures a Leo 440 scanning electron microscope was used.Infrared (IR) spectra were recorded on a Nicolet 5700 of FTIRspectrophotometer using a KBr pellet, and X-ray diffraction(XRD) was carried out on Bruker D8. Thermogravimetricanalysis (TGA) analysis was done on Mettler Toledo STARein N2 atmosphere.

Methods

Isolation and Purification of the Seed Gum. Powderedseeds (1 kg) of Cassia marginata were exhaustively extractedwith light petroleum followed by EtOH to remove fatty andcoloring materials from them. The seeds were then suspendedin 1% aqueous acetic acid overnight, stirred mechanically, andfiltered. The filtrate (mucilage) was precipitated with 95% EtOHto give a white amorphous seed gum.22 The crude gum wascollected, washed with ethanol, and dried.

The sample was finally purified by dialysis and filtrationthrough Millipore membranes. The pure seed gum was anonreducing, white, fibrous material with ash content 0.28%and [R]D

25 +67.140 (water).Graft Copolymerization under Microwave Irradiation.

The calculated amount of the CM gum and methylmethacrylatewere taken in an 150 mL open necked flask and irradiated in adomestic microwave oven to definite microwave power fordifferent time periods in the monomer concentration range of12-20 × 10-2 M (Table 1). Poly(methylmethacrylate) (PMMA)grafted CM gum samples (Scheme 1) of different graftingextents were separated26 from the respective reaction mixturesby pouring them into excess of acetone. The grafted sampleswere finally extracted with acetone in a Soxhlet apparatus for

Figure 1. IR spectra of CM gum (A), CM-g-PMMA (B), and (C) CM-g-PMMA Cr(VI) loaded.

Figure 2. XRD of CM gum (A) and CM-g-PMMA (B).

Figure 3. TGA of CM gum (A) and CM-g-PMMA (B).

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4 h to dissolve all the homopolymer. The colorless graftcopolymer samples were dried under vacuum at 50 °C for >24h to a constant weight, % G and % E were calculated asbelow,24,25 and the results are recorded in Tables 1 to 4.

Grafting ratio)W1-W0

W0× 100 (1)

Grafting efficiency)W1-W0

W2× 100 (2)

where W1, W0, and W2 denote respectively the weight of thegrafted seed gum, the weight of original seed gum, and weightof the monomer used.

Grafting Using K2S2O8/Ascorbic Acid Redox Initiator.K2S2O8/ascorbic acid redox initiator24,25 was used to graftPMMA on to the CM seed gum at the same monomer and seedgum concentration at which optimum grafting was observedunder microwave conditions.

To a solution of CM (0.1 g in 25 mL of water), methyl-methacrylate (18 × 10-2 M) and ascorbic acid (15.4 × 10-3

M) were added, and the reaction mixture was thermostatted onthermostatic water bath at 35 ( 0.2 °C. After 30 min K2S2O8

(15.09 × 10-3 M) was added, and this time of addition of

Figure 4. SEM picture of CM gum (A), CM-g-PMMA (B), and CM-g-PMMA Cr(VI) loaded.

Figure 5. Adsorption with changing pH at fixed adsorbent dose 5 g/L,temperature 30 °C, [Cr(VI)] 100 mg/L, rpm 150, and time 18 h.

Figure 6. Adsorption of Cr(VI) with changing Cr(VI) concentration at fixedpH 1.0, copolymer dose 5 g/L pH 1.0, 100 ppm Cr(VI), temperature 30°C, contact volume 20 mL, rpm 150, and contact time 18 h.

Table 6. Adsorption of Different Chromium Species from 20 mL of100 ppm Cr(VI) Solution Using 100 mg of Adsorbent at DifferentpH’s, Temperature of 30°C, rpm 150, and Contact Time of 18 h

sampleno. pH

totalCr (mg)

Cr(VI)(mg) Cr(III)

adsorbedCr(VI) +

Cr(III) (mg)

1 1 0.796 0.306 0.490 1.2042 6 1.972 1.694 0.278 0.0283 10 1.985 1.984 0.001 0.015

Figure 7. Adsorption of Cr(VI) with changing grafting ratio at fixedadsorbent dose 5 g/L, pH 1.0, [Cr(VI)] 100 mg/L, temperature 30 °C, rpm150. and time 18 h.

Figure 8. Adsorption with changing adsorbent dose at fixed pH 1.0, [Cr(VI)]100 mg/L, temperature 30 °C, rpm 150, contact volume 20 mL, and contacttime 18 h.

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persulfate was taken as zero time. Graft copolymerization wasallowed for 1 h. Grafted CM gum was precipitated26 and purifiedas described above and dried under vacuum at 50 °C for >24h to a constant weight.

Sorption Experiments. Stock solutions of 1000 mg/L eachof standardized Cr(VI) were prepared from K2CrO4 indistilled-deionized water. Experiments were carried out on atemperature controlled incubator shaker set at 150 rpm andmaintained at 30 ( 2 °C for 18 h in 50 mL conical flasks.Keeping the other parameters fixed, one parameter was variedat a time. For pH studies, 20 mL solutions of 100 mg/L metalion were adjusted to various pH values ranging from 1 to 10.Different adsorbent doses ranging from 20 to 140 mg were usedto study the effect of adsorbent on the removal of Cr(VI) at100 mg/L Cr(VI) concentration. The range for different initialconcentrations of chromium was 100 to 1200 mg/L.

A total of 100 mg of the copolymer was thoroughly mixedwith 20 mL of Cr(VI) solutions, whose concentration and pHwere previously known. After the flasks were shaken for thedesired time, the suspensions were filtered using Whatman 0.45mm filter paper, and the filtrates, after suitable dilutions, wereanalyzed for Cr(VI) concentration spectrophotometrically (at 540nm wavelength) by developing a purple violet color with 1,5-diphenyl carbazide32 in an acidic medium. Control experimentsshowed that no sorption occurred on either glassware or filtrationsystems. The pH of the reaction mixture was initially adjustedto 1.0 using either sulfuric acid or sodium hydroxide (0.2 N).

The pH, initial concentration of Cr(VI), and the electrolyteamount (ionic strength) were varied, one at a time, keeping theother parameters fixed. Copolymer samples of different graftingextents were used to study the effect of grafting ratio on theCr(VI) removal under the optimum sorption condition.

Calculation of amount of metal ion adsorbed by Cassiamarginata-graft-poly(methylmethacrylate) (CM-g-PMMA) afterspectrophotometer readings of the equilibrium solution wasobtained by calculating the difference using the formula

qe ) (C0 -Ce) × L⁄W (3)

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

Desorption Studies. To determine the reusability of theadsorbent, after use it was stripped off with distilled water, 1M HCl, 1 M EDTA, and 1 M NaOH and reused, where thebest results observed were for NaOH. To optimize the concen-tration of the alkali required for the quantitative stripping ofthe loaded Cr(VI), experiments were carried out with varyingconcentrations of NaOH ranging from 0.01 to 2 M. Copolymersloaded with chromium were placed in the 2 M NaOH and stirredat 150 rpm for 15 h at 30 °C, and the final Cr(VI) concentrationwas determined. After each cycle the used copolymer waswashed with distilled water and used in the succeeding cycle.The amount desorbed was calculated from the amount of metalions loaded on the copolymers and the final chromium concen-tration in the stripping medium. For the quantitative stripping,15 h of equilibration was required. Each sample after successiveleaching was used thrice using 0.1 g of the copolymer and 100mg/L of Cr(VI) solution in total volume of 20 mL.

Hydrolysis of the Used Adsorbent. A total of 100 mg Cr(VI)loaded adsorbent was refluxed22 with 10 mL of 2 N H2SO4 for16 h. The insoluble PMMA graft was filtered and weighed.Cr(VI) was determined in the hydrolyzate.

Results and Discussion

Methylmethacrylate was efficiently grafted onto CM seedgum using microwave (MW) irradiation in the absence of anyradical initiator in aqueous medium. Because water is polar, itabsorbs microwave energy and results in dielectric heating ofthe reaction medium. Dielectric heating is also contributed bythe localized rotation34 of pendant hydroxyl groups of the seedgum molecule as if they are attached to an immobile raft, thepolysaccharide macromolecule. The dielectric heating resultsin the breaking of bonds, furnishing the free radicals whichinitiate the grafting reaction. Further MWs are also reported tohave the special effect35 of lowering of Gibbs energy ofactivation of the reactions.

In view of these two effects more efficient grafting undermicrowave can be explained even though no catalyst or initiatorswere used.

Optimization of the Grafting Conditions. Changing thevarious reaction parameters, we obtained grafted samples ofdifferent performances, and the optimum grafting was observedat [CM] 4 g/L, [MMA] 18.0 × 10-2 M, exposure time 0.66min, 40% power (Tables 1–4). Water solubility of the samplesdecreased with increasing % grafting (Table 5), and the samplesobtained under optimum grafting condition were fully waterinsoluble; this sample was evaluated for Cr(VI) removal fromthe aqueous solution. The solubility was checked by weighingthe undissolved copolymer in the solution. Samples of different% grafting were evaluated for Cr(VI) binding under the optimumconditions to study the effect of grafting on metal binding ability(Figure 3). It was found that the efficacy of the graft copolymerfor Cr(VI) removal increases with increase in grafting ratio; thisis due to the availability of the additional binding sites at the

Figure 9. Adsorption with changing concentration of electrolyte at fixedcopolymer dose 5 g/L pH 1.0, 100 ppm Cr(VI), temperature 30 °C, contactvolume 20 mL, rpm 150, and contact time 18 h.

Table 7. Adsorption of Cr(VI) with Time Using DifferentAdsorption Doses at pH 1.0, Total Volume of 20 mL, Cr(VI)Concentration of 100 mg/L, Temperature of 30 °C, rpm 150, andContact Time of 6 h

amount ofchromium (mg)

in 20 mL of solution

adsorbed (mg)(adsorbentdose 0.1 g)

adsorbed (mg)(adsorbentdose 0.2 g)

adsorbed (mg)(adsorbentdose 0.3 g)

2 0.98 1.496 1.766

Table 8. Comparison of Different Biosorbents, regarding Cr(VI)Removal Capacity (Feundlich Model)

sample no. adsorbents Kf 1/n reference

1 Rhizopus arrhius 10.99 0.18 412 Rhizopus nigrificans 12.06 3.24 423 Chlorella Vulgaris 0.48 1.26 434 Scenedesmus obliquus 0.68 1.42 435 Synechocystis sp. 1.54 1.40 436 Bengal gram husk 2.815 1.814 447 CM-g-PMMA 3.24 1.72 this study

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PMMA grafts in the copolymer. Conventionally synthesizedcopolymer sample was also evaluated for the removal underoptimum adsorption conditions. Since the conventionally syn-thesized sample had lesser % G (smaller/lesser PMMA grafts),it was less efficient in Cr(VI) removal. Moreover, potassiumions furnished from the potassium persulfate may get partiallyadsorbed on the surface of biosorbent, thereby adverselyeffecting its sorbing potential. Samples of lower grafting ratio(<100% G) were partially water soluble, had poor efficiencyfor Cr(VI) removal, and were not useful as adsorbents.

Characterization of the Graft Copolymer. The graftcopolymer sample with maximum % G and % E from thesample with optimized sorption of Cr(VI) was characterizedusing infrared spectroscopy, XRD, TGA, and SEM analysis.

The IR spectrum of pure CM gum has a broad strong bandat 3411 cm-1 due to O-H stretching and at 2924 cm-1

indicating C-H linkages while IR spectra of CM-g-PMMA(Figure 1) had additional absorption peaks at 1731 cm-1 and2951 cm-1 (due to carbonyl stretching and symmetrical stretch-ing of the methyl group, respectively). This result providessubstantial evidence of grafting onto CM seed gum. Moreover,the O-H stretching peak’s intensity decreases after chromiuminteraction, and its position is seen shifted from 3420 cm-1 to3404 cm-1 in chromium loaded samples indicating complex-ation. Ester C-O stretching has been shifted from 1149 cm-1

to 1172 cm-1, indicating the involvement of ester oxygen inbinding. The IR spectra of chromium loaded graft copolymeralso showed two new peaks at 777 and 901 cm-1 which areattributed to the CrsO and CrdO bonds from the Cr(VI)species, suggesting that Cr(VI) was adsorbed.

XRD of the CM and CM-g-PMMA further supports grafting(Figure 2). CM gum is galactomannan, and like guar gum,30 itis amorphous and shows a broad hallow; however due to graftingof PMMA it develops crystalline zones. Small crystalline peaksare observed at 2θ ) 8.3 (d ) 10.58), 14.49 (d ) 6.1), 20.35(d ) 4.34), 22.5 (d ) 3.945), 29.5 (d ) 3.03), and 41.98° (d )2.148), which further confirmed the grafting.

TGA of CM (Figure 3) showed that decomposition onsets at188 °C. Thereafter, the weight loss occurs slowly, and 60%weight loss is seen up to 600 °C while in CM-g-PMMA weightloss onsets at 400 °C and only 46% loss in weight up to 600°C is observed, indicating the grafting has taken place and thegrafted gum had more thermal stability.

After the adsorption of the Cr(VI) there are structural as wellmorphological changes in the copolymer. Structural changes arereflected by the IR spectrum of the Cr(VI) loaded copolymer;however, the SEM picture of the loaded copolymer shows

alteration in the surface morphology, where adsorbed chromiumis clearly seen as the small deposition on the copolymer surface(Figure 4).

Optimization of Cr(VI) Removal. Effect of pH onCr(VI) Adsorption. The effect of pH on Cr(VI) removal byCM-g-PMMA is shown in Figure 5 where maximum removalwas observed at pH 1.0. The extent of Cr(VI) removal by CM-g-PMMA decreased from 16.94 mg/g to 0.16 mg/g with thevariation of pH 1.0-10.0 at 100 mg/L initial Cr(VI) concentration.

Total chromium in the equilibrium solution (20 mL) afterthe adsorption (at 100 mg/L Cr(VI) concentration and pH 1.0)was found to be 0.796 mg, whereas Cr(VI) as detected by theDiphenylcarbazide (DPC) method was 0.306 mg, indicating that0.490 mg of Cr(VI) gets converted to Cr(III) during theadsorption and remains in the equilibrium solution along withthe remaining Cr(VI); however, Cr(III) could not be detectedby the used DPC method. A similar finding has been reportedby Krishnani et al.36 for the adsorption of chromium onlignocellulosic substrates where also the adsorption was greaterat low pH and was found to decrease with increasing pH. Atlow pH, lignin reportedly reduced hexavalent chromium intoCr(III), which was subsequently adsorbed contrary to the presentstudy where the copolymer adsorbed chromium mainly asCr(VI), while Cr(III), generated due to reduction of the Cr(VI)at acidic pH, still remained in solution. Out of 2 mg of Cr(VI)present in 20 mL of 100 ppm solution, 1.204 mg of Cr(VI)could be actually adsorbed. Therefore, the adsorbent was foundefficient in Cr(VI) removal though Cr(III) is not adsorbed whichhas been formed by the reduction of Cr(VI) at pH 1.0.

However, at pH 10, after the adsorptions, total chromium andCr(VI) were found to be 1.985 mg and 1.984 mg, respectively,indicating negligible conversion of Cr(VI) to Cr(III), that is,0.001 mg (Table 6).

Table 9. Kinetic Data of Cr(VI) Adsorption on CM-g-PMMA at Different Concentrations, pH 1.0, Temperature 30 °C, and rpm 150

Lagergren plot pseudosecond-order plot second-order plot

sample no. [Cr(VI)] (ppm) R2 kL (min-1) R2 k′ (g/(mg min)) R2 k2 (g/(mg min))

1 100 0.9820 0.10 × 10-5 0.9601 10.5 × 10-5 0.9844 0.10 × 10-5

2 200 0.9830 0.04 × 10-5 0.9602 5.21 × 10-5 0.9844 0.06 × 10-5

3 400 0.9842 0.02 × 10-5 0.9601 2.63 × 10-5 0.9844 0.03 × 10-5

Table 10. Cr(VI) Removal by Microwave and Conventionally Synthesized Graft Copolymer Using 0.1 g/20 mL Adsorbent Dose at pH 1.0,Temperature 30 °C, rpm 150, and Contact Time 18 h

standard solution wastewater

sample no. polymer Cr(VI) (ppm) concentration removal (mg/g) Cr(VI) (ppm) concentration removal (mg/g)

1 CM-g-PMMA (microwave synthesized) 10 1.99 9.09 1.68100 16.94 90.95 14.17

1000 124.40 909.5 104.412 CM-g-PMMA (conventionally synthesized) 10 1.02 9.09 0.58

100 5.46 90.95 2.911000 13.06 909.5 8.91

Figure 10. Adsorption/desorption studies. Adsorption conditions: [Cr(VI)]) 100 ppm, pH ) 1, equilibration time )18 h, total volume ) 20 mL.Desorption conditions: stripping solution ) N NaOH, total volume ) 20mL, equilibration time ) 15 h.

5272 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008

The effect of pH on the removal of Cr(VI) onto CM-g-PMMAcan be interpreted with the help of the surface charge presentin the PMMA grafts in an acidic/basic medium. In an acidicmedium,37 positive surface charge is developed due to proto-nation of oxygen lone pairs of PMMA grafts, which increaseswith the increase in H+ (decrease in pH). So, due to increasein electrostatic attraction between positive surface and HCrO4

-/(Cr2O7

-2, dominating species in acidic medium) adsorptionincreases with decrease in pH. At such low pH, conversion ofsome of Cr(VI) to Cr(III) is indicated as total chromium comesto be higher than the detected Cr(VI) in the equilibrium solutionafter the adsorption.

Cr3+ is not adsorbed due to repulsion between the samecharges and remains in the solution; this again evidenced theinvolvement of PMMA grafts in the adsorption.

In acidic medium,

But in basic medium,38 base abstracts ∝ -H+ of thecarbonylgroup, which makes the surface of CM-g-PMMA negative. So,due to electrostatic repulsion, percentage removal decreases withincrease in pH and almost negligible removal was observed atpH 10.0. The total chromium at basic pH indicates that there isnearly no conversion of Cr(VI) to Cr(III) (Table 6).

In basic medium,

Effect of Initial Concentration of Cr(VI). The experimentalresults demonstrating the effect of initial concentration of Cr(VI)on the Cr(VI) removal by CM-g-PMMA are shown in Figure6. With the increase in the initial concentration of Cr(VI) from100-1200 mg/L, the removal of Cr(VI) in 20 mL of solutionincreases from 16.94 to 130.01 mg/g. At higher initial Cr(VI)concentration, more Cr(VI) is available for the adsorption. At10 ppm Cr(VI) concentration, the same adsorption dose underidentical conditions could remove 1.99 mg/g chromium.

Effect of % G. The experimental results demonstrating theeffect of % G on the removal of Cr(VI) by CM-g-PMMA areshown in Figure 7. With increase in grafting ratio from 50 to270, adsorption increases from 8.34 to 16.94 mg/g at fixedcopolymer dose 5 g/L, sample volume 20 mL, pH 1.0, Cr(VI)100 mg/L, temperature 30 °C, rpm 150, and contact time 18 h.Up to 100% G, the graft copolymer samples were partially watersoluble, and hence their efficiency for chromium removal wasquite low and thus were not suitable to be used as adsorbents.The increase in the removal with increase in grafting ratio isdue to increase in binding sites.

Effect of Adsorbent Dose. The experimental results dem-onstrating the effect of copolymer amount on the removal ofCr(VI) is shown in Figure 8. The percentage removal of Cr(VI)increases from 6.48 to 19.99 mg/g by increasing the adsorbentdose from 20 mg to 140 mg in 20 mL of 100 mg/L Cr(VI)concentration at 30 °C, rpm 150, and contact time 18 h. This isbecause more binding sites are available at higher dose ofcopolymer due to increased surface area. Contact time for theadsorption may be reduced significantly from 18 to 6 h onincreasing the adsorbent dose. Higher adsorbent doses, 0.2 and0.3 g, could result in 1.496 and 1.766 mg Cr(VI) removal,respectively, from 20 mL of 100 mg/L Cr(VI) solution at pH1.0, temperature 30 °C, rpm 150, and contact time 6 h (Table9) in comparison to 0.98 mg of adsorption when 0.1 g ofadsorbent was used.

Effect of Electrolyte. The experimental results demonstratingthe effect of electrolyte (NaCl and NaSO4) on the Cr(VI)removal by CM-g-PMMA are shown in Figure 9. With anincrease in concentration of both NaCl and NaSO4 from 0.01to 1.0 M, % removal decreases from 14.32 mg/g to 1.652 mg/gand from 11.846 mg/g to 0.6 mg/g, respectively, from 20 mLof 100 mg/L Cr(VI) solution at pH 1.0, temperature 30 °C, rpm150, and contact time 18 h. The decrease in the removal onincreasing the electrolyte concentration may be due to competi-tion between Cr2O7

-2 and SO4-2/Cl- at the binding sites of

the copolymer.Adsorption Isotherm Studies. Adsorption data were fitted

to the Langmuir and Freundlich isotherms.The Langmuir isotherm is valid for monolayer sorption due

to a surface of a finite number of identical sites and is expressedin the linear form as

Ce ⁄ qe ) b ⁄ Q0 +Ce ⁄ Q0 (4)

where Ce is the equilibrium concentration (mg/L) and qe theamount adsorbed at equilibrium (mg/g). The Langmuir constantQ0 (mg/g) represents the monolayer adsorption capacity, and b

Figure 11. (A) Langmuir isotherm (B) Freundlich isotherm of Cr(VI) adsorption by CM-g-PMMA (Cr(VI)).

Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5273

(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 toas separation factor or equilibrium parameter for predictingwhether an adsorption system is favorable or unfavorable. RL

is calculated using the following equation

RL ) 1 ⁄ (1+ bC0) (5)

where C0 is the initial Cr(VI) concentration (mg/L). If the RL

values lie between 0 and 1, the adsorption is favorable.The Freundlich isotherm describes the heterogeneous surface

energies by multilayer adsorption and is expressed in linear formas

ln qe ) ln Kf + (1 ⁄ n) ln Ce (6)

where Kf indicates adsorption capacity (mg/g), and n is anempirical parameter related to the intensity of adsorption, whichvaries with the heterogeneity of the adsorbent. The greater thevalues of the n, the better the favorability of the adsorption.

The adsorption isotherms obtained for Cr(VI) ions uptake bymicrowave synthesized CM-g-PMMA were found to follow ata satisfactory extent both the Freundlich (R2 ) 0.9703) andLangmuir (R2 ) 0.8957) predictions. Yet, the correlationcoefficient of the Freundlich curve was distinctly higher. Thisobservation implies that monolayer biosorption, as well asheterogeneous surface conditions, may coexist under the appliedexperimental conditions. Hence, the overall sorption of Cr(VI)on the biosorbent is complex, involving more than one mech-anism, such as ion exchange, surface complexation, andelectrostatic attraction as evident by the IR spectrum of Cr(VI)loaded biosorbent, where two adsorption sites are evident;oneis the PMMA grafts, and the other is the complexation throughsugar hydroxyl groups. On the basis of the Langmuir isotherm,Q0 was calculated to be 185.19, indicating that the adsorbenthad a high capacity to remove Cr(VI) ions. At 100 ppm Cr(VI),RL was calculated to be 0.7299; thus, adsorption is favorable.The Freundlich constant (Kf) and 1/n were calculated to be 3.240mg/g and 1.72, respectively from the Freundlich isotherm.Different biosorbents have been compared regarding Cr(VI)removal capacity (Freundlich model) in Table 10. SorptionKinetics % Removal was monitored with time. The kinetics ofCr(VI) removal by CM-g-PMMA indicated rapid binding ofCr(VI) to the sorbent initially, followed by a slow increase untila state of equilibrium at 24 h was reached. No change in theuptake capacity was observed up to 28 h. These observationswere in agreement with the work reported earlier with the othermetal ion biomaterial systems.39,40 Kinetics of heavy metaladsorption13 was modeled by the first order Lagergren equation,

the pseudosecond-order equation and the second-order rateequation shown below as eqs 7–9, respectively.

log(qe - qt)

qe) log qe -

kL

2.303(7)

t ⁄ qt ) 1 ⁄ (k ′ qe2)+ t ⁄ qe (8)

1 ⁄ (qe - qt)) (1 ⁄ qe)+ (t ⁄ qe) (9)

where KL is the Lagergren rate constant of adsorption (min-1),k′ the pseudosecond-order rate constant of adsorption (g/(mgmin)), and k2 the second-order rate constant (g/(mg min)); qe

and qt are the amounts of metal ion sorbed (mg/g) at equilibriumand at time t, respectively. Kinetic data of Cr(VI) adsorptionby CM-g-PMMA were modeled by Lagergren, pseudosecond-order, and second-order equations. Data were best fitted to thesecond order kinetic equation, where the linear plot of 1/(qe -qt) vs t was obtained with the correlation coefficient (R2) being0.9844 and rate constant 0.01 × 10-4 (g/(mg min)), at 100 ppmCr(VI) concentration. Kinetics was studied at three differentCr(VI) concentrations, and the R2 values and the rate constantsfor all three models are listed in Table 7. The values of the rateconstants, k2, were found to decrease41 with increase in Cr(VI)concentration from 100-400 mg/L. Comparison of Adsorptionby MW and Conventionally Synthesized Copolymer. At 10 ppm,100 ppm, and 1000 ppm Cr(VI) solution, 0.1 g of microwavesynthesized copolymer at pH 1 could remove 1.99 mg/g, 16.94mg/g, and 124.4 mg/g Cr(VI), respectively, in 20 mL solutionat 30 °C (Figure 2). The grafting ratio had significant effect onthe removal. Adsorption was observed to increase with theincrease in % G. From 20 mL of 100 ppm Cr(VI) solution,MW grafted CM gum samples with 270%, 250%, and 200%grafting ratios showed 16.94 mg/g, 12.84 mg/g, 10.92 mg/gsorption, respectively (Figure 3), while conventionally graftedCM gum with 220% G showed 13.06 mg/g, 5.46 mg/g, and1.02 mg/g sorption in 20 mL of solution of 1000 ppm, 100 ppm,and 10 ppm chromium(VI) concentrations, respectively (Table8).

The utility of the microwave synthesized CM-g-PMMA wasdemonstrated by treating it with real industrial wastewater,which was found to contain a very high Cr(VI) concentration(9095 mg/L). After optimizing removal, removal of Cr(VI) wasundertaken from 10, 100, and 1000 times diluted industrialwastewater (having Cr(VI) concentration 909.5 ppm, 90.95 ppm,and 9.095 ppm, respectively) where 104.41 mg/g, 14.17 mg/g,and 1.68 mg/g removal was observed at pH 1.0 using 0.1 g ofcopolymer dose in 20 mL of wastewater. Conventionallysynthesized copolymer under identical conditions could remove8.91 mg/g, 2.91 mg/g, and 0.58 mg/g Cr(VI) from 10, 100, and1000 times diluted industrial wastewater. Thus, the microwavesynthesized copolymer is found to be an efficient Cr(VI)adsorbent both in standard solutions and in real industrial wastes.

Conventionally grafted CM gum was observed to be lessefficient on Cr(VI) in comparison to that of MW grafted CMgum; this may be partly explained on the basis of the surfacesorption of the ions furnished by the redox initiator (used inthe conventional grafting procedure) which can potentiallydecrease the available binding sites. Moreover in microwavesynthesis we obtained copolymer with higher % grafting, wheremore PMMA grafts are available for binding, confirming therole of PMMA grafts in Cr(VI) binding.

Desorption Studies. A total of 84.7% (16.94 mg/g) of theCr(VI) was removed in the first cycle. The used graft copolymerwas treated with 2 M NaOH where 46.4% (9.28 mg/g) strippingof Cr(VI) could be done. In the second cycle the material could

Figure 12. Second order plots of Cr(VI) sorption by CM-g-PMMA.

5274 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008

now remove 63.3% (12.66 mg/g) Cr(VI) that could be desorbedup to 29.3% (5.86 mg/g). In the third cycle this could removeonly 54.6% (10.92 mg/g) Cr(VI) where desorption up to 25.2%(5.04 mg/g) was possible (Figure 10). The removal decreasedper cycle, suggesting that the copolymer’s efficiency goes ondecreasing up to the third cycle continuously. In the third cyclethe copolymer could remove nearly 50% of its original capacity.Hence, it appeared that during desorption only the metal ions,which were electrostatically adsorbed, were desorbed. It waslikely that both the electrostatic and complexation reactionsoccurred between the sorbent and the metal ion; therefore,complete desorption was not possible.

The Cr(VI) from the used adsorbent could be easily recoveredby refluxing the used adsorbent with 2 N H2SO4 whereuponthe glycosidic linkages in the polysaccharide part get dissociatedand PMMA grafts get separated (10 mg) leaving the watersoluble sugars and Cr(VI) in solution. In the hydrolyzate, Cr(VI)concentration was found to be 57.18 ppm, indicating a majorpart of adsorbed Cr(VI) is recovered in the hydrolyzate. Therecovered PMMA may be recycled for the grafting of thebiopolymer.

Conclusions

Microwave synthesis of Cassia marginata seed gum-graft-poly(methylmethacrylate) not only involved minimum use ofchemicals (therefore minimum possibility of chemical contami-nation in the product) but also yielded more efficient bioadsor-bent material for Cr(VI) removal than the conventional synthesisusing K2S2O8/ascorbic acid as redox initiator. The adsorptionwas found to be pH and concentration dependent, pH 1.0 beingthe optimum value. Though the equilibrium data were success-fully modeled by both Langmuir and Freundlich models, datawere fitted better to the Freundlich model, indicating surfaceheterogeneity and multilayer adsorption. The adsorption showedsecond order kinetics. The removal from the industrial waste-water was also significant. The loaded copolymer can beregenerated by alkali treatment and reused for up to three cyclesbut with some loss in the adsorption capacity. The adsorbentwas found to be easily disposable.

Acknowledgment

Authors are thankful to Ministry of Forests and Environmentfor the financial support to carry out this work and NationalPhysical Laboratory, New Delhi, for providing the instrumentalfacility.

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ReceiVed for reView April 1, 2007ReVised manuscript receiVed May 1, 2008

Accepted May 7, 2008

IE070467J

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