Ind. Eng. Chem. Res. 2008, 47, 5267–5276
5267
APPLIED CHEMISTRY Efficient Chromium(VI) Adsorption by Cassia marginata Seed Gum Functionalized 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 attracted attention toward the use of biosorbents for Cr(VI) removal. The present study evaluates a novel biosorbent derived 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 catalyst in good yield. Adsorbents of different performances could be obtained by varying the amount of the methylmethacrylate, microwave power, and exposure time. A representative sample of microwave synthesized adsorbent was characterized using FTIR, XRD, TGA, and SEM analysis. Cr(VI) sorption was optimized using the copolymer sample of highest grafting ratio and efficiency (270% G and 59.65% E) where the removal was found to be pH and concentration dependent, pH 1.0 being the optimum value at which from 20 mL of 100 ppm Cr(VI) solution, 16.94 mg/g Cr(VI), could be removed using 5 g/L adsorbent dose at 30 °C. The adsorption data followed both Langmuir (R2 ) 0.9703) and Freundlich isotherms (R2 ) 0.8957) probably due to the real heterogeneous nature of the surface sites involved in the metal uptake, and overall sorption of Cr(VI) on the biosorbent is complex and involves more than one mechanisms. The adsorption followed second order kinetics, the rate constant being 0.10 × 10-5 g/(mg min) at 100 mg/L Cr(VI) concentration. The adsorbent was also found efficient in Cr(VI) removal from real industrial wastewater. Used copolymer was recycled after stripping off the adsorbed chromium with 2 M NaOH where after each cycle a successive decrease in the 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 of microwave synthesized copolymer. Introduction Cr(VI) is a known highly toxic metal, and its removal1 is considered as a priority. Ion exchange techniques are used for Cr(VI) removal where generation of volumetric sludge increases the cost.2 Adsorption using commercial activated carbon (CAC)1 can remove chromium from wastewater,3 but CAC remains an expensive material for the removal. Polymeric materials such as poly(4-vinyl pyridine) beads,4 amidoxime chelating resin,5 cross-linked poly(glycidylmethacrylate-co-methylmethacrylate),6 and aminated polyacrylonitrile fibers7 have been reported for efficient removal of Cr(VI). However, environmental issues led to the development of several biosorbents for Cr(VI) removal. Many polysaccharide materials in their native form as well as after modification have been exploited as biosorbents. Chitin and chitosan have been widely used for Cr(VI) removal in their native/modified form.8 Chitosan beads,9 chitosan cross-linked with glutaraldehyde,10 chitosan microspheres cross-linked with epichlorohydrin,11 chitosan-based polymeric surfactants,12 and cross-linked xanthated chitosan13 are some examples. Other polysaccharides such as Kendu fruit gum,14 mucilaginous seeds of 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 adsorbent materials 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 The amine-modified polyacrylamide-grafted coconut coir pith carrying the -NH3+Cl- functional group at the chain end was Table 1. % G and % E with Different Microwave Powers and Exposure Times at 16 × 10-2 M Monomer, 4 g/L Gum Concentration, and 25 mL Total Reaction Volume sample no.
microwave power (%)
exposure time
%G
%E
1
20
2
40
3
60
4
80
5
100
10 20 30 40 50 10 20 30 40 50 10 20 30 40 50 10 20 30 40 50 10 20 30 40 50
18 32 70 80 93 26 70 120 190 145 30 50 90 160 100 35 70 140 80 55 50 110 90 43 12
4.44 7.89 17.26 19.72 22.93 6.41 17.26 29.59 46.86 35.76 7.39 12.33 22.20 39.46 24.66 8.63 17.26 34.53 19.73 13.76 12.33 27.13 22.2 10.60 2.96
* To whom correspondence should be addressed. E-mail:
[email protected]. Tel: +91 532 2461518. Fax: +91 532 2540858. 10.1021/ie070467j CCC: $40.75 2008 American Chemical Society Published on Web 07/09/2008
5268 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 Scheme 1. Synthesis of the Adsorbent
Table 2. % G and % E at Different Monomer Concentrations at Fixed Gum Concentration (4g/L), Microwave Power (40%), Exposure Time (40 s), and Total Reaction Volume (25 mL) sample no. 1 2 3 4 5
monomer concentration (M) -2
12 × 10 14 × 10-2 16 × 10-2 18 × 10-2 20 × 10-2
Table 3. % G and % E at Different Gum Concentrations at Fixed Amount of Monomer (18 × 10-2 M, Microwave Power (40%), Exposure Time (40 s), and Total Reaction Volume (25 mL)
%G
%E
sample no.
gum (mg/25 mL)
%G
%E
80 130 190 270 240
26.51 37.25 46.85 59.65 48.02
1 2 3 4 5
50 100 150 200 250
120 270 173.3 125 108
13.25 59.65 57.44 55.23 59.65
investigated as an adsorbent for its possible application in the removal of chromium(VI) from aqueous solution and wastewater.19 Oxyanionic contaminants, specifically selenium and Cr(VI), were removed from solution by sorption onto gel beads formed by pretreating the biopolymer alginic acid with calcium and iron(III).20 Vinyl modified polysaccharides show good adsorption properties, and the use of microwaves in their synthesis has been reported to furnish even better adsorbent materials.21 No attempt so far has been made in the direction of exploiting microwave synthesized polymer modified biomasses for removing the metal ions from aqueous solution. Cassia plants22 are reported to contain a substantial amount of seed gums, and potentially these gums may find use as a renewable biosorbent material for Cr(VI) removal. Since these seed gums are soluble in water, they require some modification before they can be exploited for such use. Their stability and solubility 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 and microwave irradiation,29,30 and the properties of the obtained graft copolymer are found to be dependent on the method used. Cassia marginata syn: Cassia roxburghii31 is a large sized Indian tree having cylindrical and indehiscent long pods (with many seeds) containing a black cathartic pulp, used as a horse medicine. Analysis of its seed oil has been published while no attempt so far has been made for the commercial exploitation of its seed gum though found in substantial amount. The main objective of the present study is the microwave induced synthesis Cassia marginata seed gum-graft-poly(methylmethacrylate) and its evaluation as biosorbent in the removal of Cr(VI) from the aqueous solution and wastewater. In conventional grafting procedures, surface adsorption of the cations (furnished by redox initiators/catalyst) by the copolymer may possibly reduce its adsorption potential. To understand the advantage of using microwaves in adsorbent synthesis, the conventionally synthesized graft copolymer synthesized at identical monomer concentration was also tested for the Cr(VI) removal, and the results
Table 4. % G and % E with Changing Total Reaction Volume at Fixed Monomer Concentration (18 × 10-2 M), Microwave Power (40%), and Exposure Time (40 s) sample no.
total reaction volume (mL)
%G
%E
2 3 4 5
15 20 25 30 35
120 200 270 220 140
26.51 44.19 59.65 48.60 30.93
Table 5. Solubility of the Grafted Gums of Different % G in Water sample no.
%G
% solubility in water
1 2 3 4 5 6 7 9 10 11
18 30 50 70 80 100 162 200 240 270
100 82.38 61.13 40.62 17.75 3.02 0 0 0 0
of the two were compared to understand the advantage of using microwaves in the adsorbent synthesis. Materials Cassia marginata seeds were supplied by Himani seed stores, Deheradun, and identified by systematic botanist at Botanical Survey of India, Allahabad. Methylmethacrylate (Loba Cheme) was washed with 5% aqueous alkali to remove phenolic inhibitor and then distilled before use. Temperature controlled flask shaker was used for batch experiments. For Cr(VI) removal, all the chemicals used are of analytical reagent grade and were utilized as received without further purification. Deionized water is used throughout the study. Aqueous solutions of K2CrO4, HCl, H2SO4, and NaOH were prepared from K2CrO4 (Merck), respective acids, and NaOH (Merck), respectively. 1,5-Diphenyl carbazide (Merck), ascorbic acid (Merck), and potassium persulfate (Merck) were used without further purification. The sample with maximum grafting ratio was used for the charac-
Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5269 32,33
method after suitable dilution. Total chromium in solution was assessed by atomic absorption spectrophotometer (Electronics Corporation of India Limited model-AAS 4141) at λ 357.9 nm, slit width 1 nm, using an air-acetylene flame. Systronics Digital pH meter model 335 was used for pH measurement. The pH values are adjusted by the addition of 5 M H2SO4 or 1 M NaOH. TDS and the conductivity of the waste water were measured by microprocessor based Systronics Conductivity/TDS meter model 1601 with cell constant 1 cm-1. Samples were filtered using Whatman 0.45 mm filter paper, and the filtrates after suitable dilutions were analyzed. Control experiments showed that no sorption occurred on either glassware or the filtration 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 FTIR spectrophotometer using a KBr pellet, and X-ray diffraction (XRD) was carried out on Bruker D8. Thermogravimetric analysis (TGA) analysis was done on Mettler Toledo STARe in N2 atmosphere. Methods
Figure 1. IR spectra of CM gum (A), CM-g-PMMA (B), and (C) CM-gPMMA Cr(VI) loaded.
Isolation and Purification of the Seed Gum. Powdered seeds (1 kg) of Cassia marginata were exhaustively extracted with light petroleum followed by EtOH to remove fatty and coloring materials from them. The seeds were then suspended in 1% aqueous acetic acid overnight, stirred mechanically, and filtered. The filtrate (mucilage) was precipitated with 95% EtOH to give a white amorphous seed gum.22 The crude gum was collected, washed with ethanol, and dried. The sample was finally purified by dialysis and filtration through Millipore membranes. The pure seed gum was a nonreducing, white, fibrous material with ash content 0.28% and [R]D25 +67.140 (water). Graft Copolymerization under Microwave Irradiation. The calculated amount of the CM gum and methylmethacrylate were taken in an 150 mL open necked flask and irradiated in a domestic microwave oven to definite microwave power for different time periods in the monomer concentration range of 12-20 × 10-2 M (Table 1). Poly(methylmethacrylate) (PMMA) grafted CM gum samples (Scheme 1) of different grafting extents were separated26 from the respective reaction mixtures by pouring them into excess of acetone. The grafted samples were finally extracted with acetone in a Soxhlet apparatus for
Figure 2. XRD of CM gum (A) and CM-g-PMMA (B).
terization by the spectral studies. The wastewater was procured from a local electroplating industry in Kanpur, Uttar Pradesh, India (color: greenish yellow, pH ) 1.6, TDS ) 6.67 ppt, and conductivity was 10.38 m S-1). Analysis 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. The concentration of Cr(VI) was measured using a visible spectrophotometer (Systronics model 105) at 540 nm using the diphenyl carbazide
Figure 3. TGA of CM gum (A) and CM-g-PMMA (B).
5270 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008
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 7. Adsorption of Cr(VI) with changing grafting ratio at fixed adsorbent dose 5 g/L, pH 1.0, [Cr(VI)] 100 mg/L, temperature 30 °C, rpm 150. and time 18 h.
Figure 6. Adsorption of Cr(VI) with changing Cr(VI) concentration at fixed pH 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.
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 contact time 18 h.
Table 6. Adsorption of Different Chromium Species from 20 mL of 100 ppm Cr(VI) Solution Using 100 mg of Adsorbent at Different pH’s, Temperature of 30°C, rpm 150, and Contact Time of 18 h sample no. 1 2 3
pH
total Cr (mg)
Cr(VI) (mg)
Cr(III)
adsorbed Cr(VI) + Cr(III) (mg)
1 6 10
0.796 1.972 1.985
0.306 1.694 1.984
0.490 0.278 0.001
1.204 0.028 0.015
4 h to dissolve all the homopolymer. The colorless graft copolymer samples were dried under vacuum at 50 °C for >24 h to a constant weight, % G and % E were calculated as below,24,25 and the results are recorded in Tables 1 to 4. Grafting ratio )
W1-W0 × 100 W0
(1)
Grafting efficiency )
W1-W0 × 100 W2
(2)
where W1, W0, and W2 denote respectively the weight of the grafted seed gum, the weight of original seed gum, and weight of the monomer used. Grafting Using K2S2O8/Ascorbic Acid Redox Initiator. K2S2O8/ascorbic acid redox initiator24,25 was used to graft PMMA on to the CM seed gum at the same monomer and seed gum concentration at which optimum grafting was observed under microwave conditions. To a solution of CM (0.1 g in 25 mL of water), methylmethacrylate (18 × 10-2 M) and ascorbic acid (15.4 × 10-3 M) were added, and the reaction mixture was thermostatted on thermostatic water bath at 35 ( 0.2 °C. After 30 min K2S2O8 (15.09 × 10-3 M) was added, and this time of addition of
Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5271
Calculation of amount of metal ion adsorbed by Cassia marginata-graft-poly(methylmethacrylate) (CM-g-PMMA) after spectrophotometer readings of the equilibrium solution was obtained by calculating the difference using the formula qe ) (C0 - Ce) × L⁄W
Figure 9. Adsorption with changing concentration of electrolyte at fixed 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 7. Adsorption of Cr(VI) with Time Using Different Adsorption Doses at pH 1.0, Total Volume of 20 mL, Cr(VI) Concentration of 100 mg/L, Temperature of 30 °C, rpm 150, and Contact Time of 6 h amount of chromium (mg) in 20 mL of solution
adsorbed (mg) (adsorbent dose 0.1 g)
adsorbed (mg) (adsorbent dose 0.2 g)
adsorbed (mg) (adsorbent dose 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 2 3 4 5 6 7
Rhizopus arrhius Rhizopus nigrificans Chlorella Vulgaris Scenedesmus obliquus Synechocystis sp. Bengal gram husk CM-g-PMMA
10.99 12.06 0.48 0.68 1.54 2.815 3.24
0.18 3.24 1.26 1.42 1.40 1.814 1.72
41 42 43 43 43 44 this study
persulfate was taken as zero time. Graft copolymerization was allowed for 1 h. Grafted CM gum was precipitated26 and purified as described above and dried under vacuum at 50 °C for >24 h to a constant weight. Sorption Experiments. Stock solutions of 1000 mg/L each of standardized Cr(VI) were prepared from K2CrO4 in distilled-deionized water. Experiments were carried out on a temperature controlled incubator shaker set at 150 rpm and maintained at 30 ( 2 °C for 18 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 20 to 140 mg were used to study the effect of adsorbent on the removal of Cr(VI) at 100 mg/L Cr(VI) concentration. The range for different initial concentrations of chromium was 100 to 1200 mg/L. A total of 100 mg of the copolymer was thoroughly mixed with 20 mL of Cr(VI) solutions, whose concentration and pH 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 Cr(VI) concentration spectrophotometrically (at 540 nm wavelength) by developing a purple violet color with 1,5diphenyl carbazide32 in an acidic medium. Control experiments showed that no sorption occurred on either glassware or filtration systems. The pH of the reaction mixture was initially adjusted to 1.0 using either sulfuric acid or sodium hydroxide (0.2 N). The pH, initial concentration of Cr(VI), and the electrolyte amount (ionic strength) were varied, one at a time, keeping the other parameters fixed. Copolymer samples of different grafting extents were used to study the effect of grafting ratio on the Cr(VI) removal under the optimum sorption condition.
(3)
where qe is the amount of metal ion adsorbed on the adsorbent, C0 the initial metal ion concentration (mg/L), Ce the equilibrium concentration of metal ion in solution (mg/L), V the volume of metal ion solution used (L), and W the weight of the adsorbent used (g). Desorption Studies. To determine the reusability of the adsorbent, after use it was stripped off with distilled water, 1 M HCl, 1 M EDTA, and 1 M NaOH and reused, where the best results observed were for NaOH. To optimize the concentration of the alkali required for the quantitative stripping of the loaded Cr(VI), experiments were carried out with varying concentrations of NaOH ranging from 0.01 to 2 M. Copolymers loaded with chromium were placed in the 2 M NaOH and stirred at 150 rpm for 15 h at 30 °C, and the final Cr(VI) concentration was determined. After each cycle the used copolymer was washed with distilled water and used in the succeeding cycle. The amount desorbed was calculated from the amount of metal ions loaded on the copolymers and the final chromium concentration in the stripping medium. For the quantitative stripping, 15 h of equilibration was required. Each sample after successive leaching was used thrice using 0.1 g of the copolymer and 100 mg/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 for 16 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 seed gum using microwave (MW) irradiation in the absence of any radical initiator in aqueous medium. Because water is polar, it absorbs microwave energy and results in dielectric heating of the reaction medium. Dielectric heating is also contributed by the localized rotation34 of pendant hydroxyl groups of the seed gum molecule as if they are attached to an immobile raft, the polysaccharide macromolecule. The dielectric heating results in the breaking of bonds, furnishing the free radicals which initiate the grafting reaction. Further MWs are also reported to have the special effect35 of lowering of Gibbs energy of activation of the reactions. In view of these two effects more efficient grafting under microwave can be explained even though no catalyst or initiators were used. Optimization of the Grafting Conditions. Changing the various reaction parameters, we obtained grafted samples of different performances, and the optimum grafting was observed at [CM] 4 g/L, [MMA] 18.0 × 10-2 M, exposure time 0.66 min, 40% power (Tables 1–4). Water solubility of the samples decreased with increasing % grafting (Table 5), and the samples obtained under optimum grafting condition were fully water insoluble; this sample was evaluated for Cr(VI) removal from the aqueous solution. The solubility was checked by weighing the undissolved copolymer in the solution. Samples of different % grafting were evaluated for Cr(VI) binding under the optimum conditions to study the effect of grafting on metal binding ability (Figure 3). It was found that the efficacy of the graft copolymer for Cr(VI) removal increases with increase in grafting ratio; this is due to the availability of the additional binding sites at the
5272 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 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
sample no.
[Cr(VI)] (ppm)
R2
kL (min-1)
R2
1 2 3
100 200 400
0.9820 0.9830 0.9842
0.10 × 10-5 0.04 × 10-5 0.02 × 10-5
0.9601 0.9602 0.9601
k′ (g/(mg min)) 10.5 × 10-5 5.21 × 10-5 2.63 × 10-5
second-order plot R2
k2 (g/(mg min))
0.9844 0.9844 0.9844
0.10 × 10-5 0.06 × 10-5 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 sample no.
polymer
1
CM-g-PMMA (microwave synthesized)
2
CM-g-PMMA (conventionally synthesized)
wastewater
Cr(VI) (ppm) concentration
removal (mg/g)
Cr(VI) (ppm) concentration
removal (mg/g)
10 100 1000 10 100 1000
1.99 16.94 124.40 1.02 5.46 13.06
9.09 90.95 909.5 9.09 90.95 909.5
1.68 14.17 104.41 0.58 2.91 8.91
PMMA grafts in the copolymer. Conventionally synthesized copolymer sample was also evaluated for the removal under optimum adsorption conditions. Since the conventionally synthesized sample had lesser % G (smaller/lesser PMMA grafts), it was less efficient in Cr(VI) removal. Moreover, potassium ions furnished from the potassium persulfate may get partially adsorbed on the surface of biosorbent, thereby adversely effecting its sorbing potential. Samples of lower grafting ratio (
