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Highly enhanced adsorption for decontamination of lead ions from battery wastewaters using chitosan functionalized with xanthate
Bioresource Technology 99 (2008) 9021–9024
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Highly enhanced adsorption for decontamination of lead ions from battery wastewaters using chitosan functionalized with xanthate Divya Chauhan, Nalini Sankararamakrishnan * Facility for Ecological and Analytical Testing, 302 Southern Laboratories, India Institute of Technology, Kanpur, UP 208016, India
a r t i c l e
i n f o
Article history: Received 15 February 2008 Received in revised form 9 April 2008 Accepted 9 April 2008 Available online 19 May 2008 Keywords: Lead Decontamination Battery wastewater Chitosan
a b s t r a c t Decontamination of lead ions from aqueous media has been investigated using cross linked xanthated chitosan (CMC) as an adsorbent. Various physico-chemical parameters such as contact time, amount of adsorbent, concentration of adsorbate were optimized to simulate the best conditions which can be used to decontaminate lead from aqueous media using CMC as an adsorbent. The atomic absorption spectrometric technique was used to determine the distribution of lead. Maximum adsorption was observed at both pH 4 and 5. The adsorption data followed both Freundlich and Langmuir isotherms. Langmuir isotherm gave a saturated capacity of 322.6 ± 1.2 mg/g at pH 4. From the FTIR spectra analysis, it was concluded that xanthate and amino group participate in the adsorption process. The developed procedure was successfully applied for the removal of lead ions from real battery wastewater samples. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Lead is suitable for batteries, because of its specific characteristics (conductivity, resistance to corrosion and the special reversible reaction between lead oxide and sulfuric acid). Due to its toxicity the US Environmental Protection Agency (EPA) has set the maximum contaminant level (MCL) of lead ions in drinking water to be 0.015 mg/L (Primary Drinking Water Rules, 1992). Lead removal from acidic wastewaters is typically accomplished by neutralization and precipitation (Wallace and Singer, 1981; Patterson, 1985). However, this process suffers from the disadvantage of incomplete metal removal and generation of toxic sludge or other waste products that require safe disposal. Chitosan, a derivative from N-deacetylation of chitin-a naturally occurring and abundant biopolymer has been found to be capable of adsorbing various heavy metal ions, including copper, lead, mercury, cadmium, chromium, and so on; this is largely attributed to the presence of the amine groups of chitosan that have strong affinity to and can form complexes with various heavy metal ions (Guibal, 2004; Varma et al., 2004). In the past decade, the rapid development of chemical modification technologies used as popular methods to provide materials with improved or desirable properties for practical applications has been observed (Uyama et al., 1998). Xanthation has been used previously on cellulose and sawdust (Bailey et al., 1999), brown marine algae (Kim et al., 1999) and chitin (Kim et al., 2006) to increase the adsorption the adsorption capacity.
In this study, chemical modification of cross linked chitosan through introduction of xanthate was investigated in order to achieve highly enhanced adsorption performance for lead ions under acidic solution conditions. The choice of xanthate group is due to the presence of sulfur atoms and it is well known that sulfur groups has a very strong affinity for most heavy metals, the metal–sulfur complex is very stable in basic condition. A series of batch adsorption experiments were conducted in the laboratory to evaluate the adsorption and desorption performance of lead ions on the chemically modified chitosan and Fourier transform infrared spectroscopy was used to elucidate the mechanisms of chemical modification and lead ion adsorption in the study. 2. Methods 2.1. Materials Chitosan flakes was acquired from India Sea Food, Cochin, India and used in the present study without any further purification. The degree of deacetylation was reported to be 85% by the manufacturer. Glutaraldehyde and carbon disulfide were purchased from Sigma–Aldrich and used without further purification. Stock solution of Pb(II) was prepared using Pb(NO3)2 (BDH chemicals). All the inorganic chemicals used were anular grade and all the reagents were prepared in Millipore milli-Q deionised water. 2.2. Chemical modification of the chitosan (CMC)
* Corresponding author. Tel./fax: +91 512 2597844. E-mail address: [email protected] (N. Sankararamakrishnan). 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.04.024
Chitosan flakes were cross linked with glutaraldehyde, chemically modified and characterized as described earlier
D. Chauhan, N. Sankararamakrishnan / Bioresource Technology 99 (2008) 9021–9024
(Sankararamakrishnan et al., 2006). Chitosan flakes (ca. 0.5 g) were suspended in methanol (100 ml), and a 25% aqueous glutaraldehyde solution (0.046 ml, 0.12 mmol) was added. After stirring at room temperature for 6 h, the product was filtered. Cross linked chitosan flakes (0.5 g) was treated with 25 ml of 14% NaOH and 1 ml of CS2. The mixture was stirred at room temperature for 24 h. The obtained orange product, cross linked chemically modified chitosan flakes (CMC) were washed thoroughly with water, air dried and used for further experiments. The adsorbents used were all sieved through a sieve of 0.4–0.6 mm particle size range. 2.3. Metal concentration analysis Dissolved lead was determined by Analyst 400 Perkin Elmer Atomic Absorption Spectrophotometer using an air-acetylene flame at wavelength 283.3 nm using a slit width of 0.7 nm. Experimental 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 filtration systems. All assays were carried out in triplicate and only mean values are presented. 2.4. Lead adsorption batch experiments Batch experiments were carried out with synthetic solutions of Pb2+ in 100 mL flasks with stopper at 100 rpm of orbital stirring in an incubator shaker, at room temperature and 4 h of contact time with the adsorbent. Samples were then filtered with Whatman No. 42 filter paper, diluted and analyzed for lead. Unless otherwise stated the parameters with synthetic water were: sample volume 20 ml, sorbent dose 2.5 g/l, initial metal ion concentration 100 mg/l, pH 5, equilibration time 4 h. For pH studies the pH was varied from 2 to 6 keeping the other conditions the same. Experiments were not conducted at higher pH due to the precipitation of lead hydroxide. For kinetic studies the sample volume was maintained at 20 ml and at every 30 min. sample was withdrawn for analysis. For equilibrium studies the adsorbent dose was maintained at 2.5 g/l varying the initial Pb2+ concentration from 100 to 1500 mg/l. 2.5. Desorption experiments To examine the desorption behaviors of lead ions from CMC, adsorption experiments were first conducted by placing a 0.10 g amount of CMC in 20 mL of a lead ion solution (with an initial concentration of 500 mg/L and a solution pH value of 5) for 4 h, and the final lead ion concentration in the solution was analyzed. The CMC adsorbed with lead ions were separated from the solution by filtration and then added into 20 mL of a 0.01 M EDTA, 0.01 N HCl, and 0.1 N NaOH solution in a flask for lead ion desorption. The contents of the flask were stirred at 200 rpm at room temperature for a time period of up to 2 h, and the lead ion concentrations in the solution were analyzed. The adsorbents were finally collected from the solution by filtration, washed with DI water, and then reused in the next cycle of adsorption experiments. The adsorption–desorption experiments were conducted for three cycles. 2.6. Studies with battery wastewater The samples were acquired from a local battery manufacturer located in Kanpur City, UP, India during September 2007. The wastewater samples were analyzed promptly after collection using standard analytical methods (APHA, 1998). The characteristics of battery wastewater were color: yellow, pH: <1, ionic conductivity: 120.5 mS, TDS: 55.292 g/l, TSS: 5397 mg/l, Pb: 93.89 mg/l.
3. Results and discussion 3.1. Adsorption performance at different initial pH values A pH effect test is performed to determine the pH of adsorption at which maximum uptake of metal occurs. The pH from 2 to 6 was adjusted initially with either hydrochloric acid or sodium hydroxide (0.1 M). No efforts were made to maintain the pH throughout the adsorption procedure. Fig. 1 shows the adsorption of Pb2+ by xanthated chitosan for various initial pH conditions. The hydrogen ion concentration of the heavy metal solution is a crucial factor in lead uptake by xanthated chitosan as shown in Fig. 1. Any study on the uptake capacity beyond pH 6.0 was avoided because insoluble lead hydroxide precipitates were observed in the solution. The maximum uptake capacity decreased with decreasing initial pH values as shown in Fig. 1. There are two possible explanations, first, hydrogen ions compete with lead ions to the same binding sites on the adsorbent. Second, the xanthate group is known to be unstable in strong acid solution and to be able to dissociate from the chitosan back bone. Decomposition is a two-stage process in which the first is the protonation of the hydroxyl group and the second is the elimination of carbon disulfide (Hulanicki, 1967). Since lead ions are bound to the sulfur atoms of xanthate groups, any loss of sulfur could lead to a reduction in the uptake capacity. The amount adsorbed increased from 20 to 39.8 mg/g when the pH was increased from 2 to 5 and thereafter a slight decline in amount adsorbed was observed with increase in pH to 6. The optimum pH for the removal of Pb2+ by chitosan flakes was found to be 5 and that of CMC was found to be at both pH 4.0 and 5.0. The pKa of xanthate-xanthic acid dissociation constant is reported to be 1.70 (Iwasaki and Cooke, 1958). Thus, in the pH range used in the present study, the characteristics of surface group of the adsorbent are unlikely to change. 3.2. Adsorption mechanisms of xanthated chitosan for Pb(II) For the xanthated chitosan used in this study, it may be expected that the adsorption sites for lead are at the nitrogen atoms of the amino groups in chitosan, sulfur atoms of attached xanthate groups and oxygen atoms of the hydroxyl groups in chitosan. The complexation of the lead ions with the xanthate group is shown in reaction (1). Both nitrogen and oxygen atoms have a lone pair or lone pairs of electrons that can bind a proton or a metal ion through an electron pair sharing to form a complex. Because of
50
40
Lead uptake (mg/g)
9022
30
20 CMC
10
Plain Flakes
0 2
3
4
5
Initial pH Fig. 1. Effect of pH on adsorption of lead.
6
9023
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the stronger attraction of the lone pair of electrons to the nucleus in an oxygen atom than in a nitrogen atom, the nitrogen atoms would have a greater tendency to donate the lone pair of electrons for sharing with a metal ion to form a metal complex than the oxygen atoms. The reaction scheme of lead ions with amino and xanthate groups are shown in reactions (1)–(4)
soft bases tend to form stable complexes with metals such as Cd2+, Pb2+ and Cu2+ (Winter, 1994). Since xanthate groups can be classified as soft bases xanthated chitosan will have a much higher affinity and sorption capacity when compared to that of plain chitosan. The strong affinity between sulfur and lead may result in high affinity constant and hence high uptake capacity.
ð1Þ
2þ
R—NH2 þ Pb
2þ
! R—NH2 Pb
R—NH2 þ Hþ ! R—NHþ 3 2þ
R—NHþ 3 þ Pb
2þ
! RNH2 Pb
þ Hþ
ð2Þ
3.4. Sorption kinetics
ð3Þ
Rapid interaction of the metal ions to be removed with the adsorbent is desirable and beneficial for practical adsorption applications. The kinetic results of lead ion adsorption on CMC shown in Fig. 2. It can be observed that lead uptake on CMC is a fast process. The amount of adsorption increased rapidly in the first 3 h, contributing to about 70% of the ultimate adsorption amount, and then augmented slowly. The adsorption equilibrium was achieved within about 4 h in this case. The pseudo-second-order kinetic model has often been used to fit the experimental kinetic adsorption data and determine whether an adsorption process is dominated by the chemical adsorption phenomenon. The linearized pseudo-second-order kinetic equation usually takes the following form:
ð4Þ 2+
To further prove the possible sites of Pb bonding to the CMC, FTIR spectra were obtained for the CMC before and after Pb2+ adsorption. The sulfur in CMC was evident by a shoulder due to thiol group at 2676 cm1. The diminution of the thiol peak after adsorption is indicative of the role of thiol group in the complexation with lead ions. Disappearance of the absorption bands at 1070 cm1, associated with the stretching of the secondary –C–OH and the bands at 1152 cm1 corresponding to the stretching of C–N bond weaken indicated the formation of N–Pb bond in the adsorption process. Formation N–Pb bond was further evident from the diminution of carbonyl stretching vibration of amide I band at 1654 cm1 and disappearance of –N–H bending vibration of amide II band. The IR absorption band corresponding to vibration of the C–S bond occured at the frequency 1015 cm1 and did not change significantly due to adsorption. The band at about 1200 cm1 was assigned to the stretching vibrations of the C–O bond with significant participation of the adjacent C–S bond. The absorption band, for the lead xanthate complex was observed at the wave number 1250 cm1. Due to the presence of some residual carbonate in the chitosan even after processing, formation of lead carbonate was evident by the doublet peak at 1320 cm1 and 1280 cm1 From the FTIR spectra analysis, it could be concluded that both amino and xanthate groups are involved in the adsorption process.
Table 1 Isotherm constants and correlation coefficients for adsorption of lead(II) ions from aqueous solutions pH
4 5
Langmuir
Freundlich 2
qmax (mg/g)
b (ml/mg)
R
n
kf
R2
322.6 ± 1.5 303.0 ± 1.9
0.014 0.017
0.97 0.98
3.27 3.42
43.1 42.7
0.93 0.86
3.3. Sorption equilibrium Sorption isotherms were modeled using linearized Langmuir and Freundlich model represented by Eqs. (5) and (6), respectively qe ¼ qmax bC e =ð1 þ bC e Þ
ð5Þ
ln qe ¼ n ln C e þ ln kF
ð6Þ
where qe is the equilibrium adsorbate loading on the adsorbent (mg/g), Ce the equilibrium concentration of the adsorbate (mg/l), qmax the ultimate capacity (mg/g), b the relative energy (intensity) of adsorption, also known as binding constant (L/mg), n (dimensionless) is a constant depicting adsorption intensity and kF (mg/ g)(L/mg) is Freundlich constant characterizing the adsorption capacity. The values obtained for the various parameters of these two models are given in Table 1. CMC exhibited approximately three times the capacity for uptake compared to the plain chitosan flakes (Ng et al., 2003). An adsorption capacity of 322.6 mg Pb/g was observed for modified chitosan flakes compared to 100.5 mg/g for plain flakes as reported earlier (Ng et al., 2003). The value obtained is considerably higher than those obtained on activated carbon (Issabayeva et al., 2006) and natural materials (Pavasant et al., 2007; Schneegurt et al., 2001) sorption capacities in these cases are less than 101 mg Pb/g of the sorbent. According to HSAB theory,
Fig. 2. Kinetic adsorption results of lead ions on CMC.
9024
D. Chauhan, N. Sankararamakrishnan / Bioresource Technology 99 (2008) 9021–9024 0
t=qt ¼ 1=ðk q2e Þ þ t=qe
ð7Þ 0
where qt (mg/g) is the adsorption amount at time t (min), k (g/mg/ min) is the rate constant of the pseudo-second-order kinetic adsorption. The values of k0 and qe can be obtained from the intercept and slope of the plot of the experimental t/qt versus t. The fitting of Eq. (7) to the experimental kinetic data from the study is given in the insertion of Fig. 2. It is observed that the adsorption kinetic data of lead ions on CMC are indeed well-represented by the pseudo-second-order kinetic model, with the correlation coefficient R2 being almost unity (R2 = 0.9853). The parameter values of qe and k2 determined from the fitting of the kinetic model in Eq. (2) to the experimental data are found to be 335.5 mg/g and 1.07 104 g/ mg/min, respectively. The maximum adsorption capacity of 335.5 mg/g obtained from the pseudo-second-order kinetic model is very close to that obtained from the Langmuir model earlier (322.6 mg/g). 3.5. Desorption studies Among the various stripping solutions it was observed only 26% of the loaded Pb(II) stripped in 0.5 N NaOH and 48% stripped with 0.01 N HCl and 0.01 M EDTA. Addition of NaOH might lead to the precipitation of Pb(OH)2 on the surface of the adsorbent which eventually blocks the adsorption sites for further adsorption. Hence, further experiments were carried out only with 0.01 N HCl solutions. Though both EDTA and 0.01 N HCl desorbed 48% of the lead ions, equilibrium achieved was faster in HCl (30 min) compared to EDTA (4 h) The adsorbent/desorption cycles were repeated for three cycles using 0.1 g of the adsorbent and 20 ml of 500 mg/l of lead ions for adsorption and desorption was carried out with 20 ml of 0.01 N HCl. Lead adsorption declined from 100% to 46% in the second cycle and subsequently decreased to 36% in the III cycle. This observation was similar to earlier reports (Jin and Bai, 2002; Singh et al., 2007) where the percent adsorbed reduced in subsequent cycles. 3.6. Effect of adsorbent dose and adsorption in battery wastewater Experiments were conducted with varying adsorbent dose of CMC on battery wastewater. Since the battery wastewater is highly acidic the effluent was diluted twice and used. It was observed (results not shown) that even at a very low dose rate of 0.5 g/l the adsorbent was effective in complete removal of lead from the battery effluent. 4. Conclusions Chemical modification of the chitosan flakes with xanthate group increased the adsorption capacity to three times compared to the plain flakes. Adsorption studies were modeled by both Langmuir and Fruendlich isotherms. From the FTIR spectra analysis, it
was concluded that xanthate and amino groups are primarily responsible for the adsorption process. Adsorbent–adsorbate kinetics exhibited pseudo-second-order. High adsorption capacity of xanthated chitosan towards lead makes it an attractive adsorbent for the removal of lead ions from battery wastewater. This method is very promising compared to other conventional and generally more expensive processes. Acknowledgements The authors are thankful for the funding provided by International Foundation for Science (IFS) to carry out this work. References APHA, 1998. Standard Methods for Water and Wastewater, 20. American Public Health Association, Washington, DC. Bailey, S., Olin, T., Bricka, M., Adrian, A., 1999. A review of potentially low cost adsorbents for heavy metals. Water Res. 33, 2469–2479. Guibal, E., 2004. Iterations of metal ions with chitosan-based sorbents: a review. Sep. Purif. Technol. 38, 43–74. Hulanicki, A., 1967. Complexation reactions of dithiocarbamates. Talanta 14, 1371– 1392. Issabayeva, G., Kheireddine Aroua, M., Sulaiman, N.M.N., 2006. Removal of lead from aqueous solutions on palm shell activated carbon. Bioresour. Technol. 97, 2350–2355. Iwasaki, I., Cooke, S.R.B., 1958. The decomposition of xanthate in acid solutions. J. Am. Chem. Soc. 80, 285–288. Jin, L., Bai, R., 2002. Mechanisms of lead adsorption on chitosan/PVA hydrogel beads. Langmuir 18, 9765–9770. Kim, Y.H., Park, J.Y., Yoo, Y.J., Kwak, J.W., 1999. Removal of lead using xanthated marine algae, Undaria piinatifida. Process Biochem. 34, 647–652. Kim, S.H., Song, H., Nisola, G.M., Ahn, J., Galera, M.M., Lee, C., Chung, W.J., 2006. Adsorption of lead(II) ions using surface-modified chitins. J. Ind. Eng. Chem. 12, 469–475. Ng, J.C.Y., Cheung, W.H., McKay, G., 2003. Equilibrium studies for the sorption of lead from effluents using chitosan. Chemosphere 52, 1021–1030. Patterson, J.W., 1985. Industrial Wastewater Treatment Technology, second ed. Butterworth, Stoneham, Mass. pp. 175–189. Pavasant, P., Apiratikul, R., Sungkhum, V., Suthiparinyanont, P., Wattanachira, S., 2007. Biosorption of Cu2+, Cd2+, Pb2+, and Zn2+ using dried marine green macroalga Caulerpa lentillifera. Bioresour. Technol. 97, 2321–2329. Primary Drinking Water Rules; sec. 141. 32 (e) (20), 1992, Federal Regulations. The Bureau of National Affairs Inc., Washington, DC. Sankararamakrishnan, N., Dixit, A., Iyengar, L., Sanghi, R., 2006. Removal of hexavalent chromium using a novel cross linked xanthated chitosan. Bioresour. Technol. 97, 2377–2382. Schneegurt, M.A., Jain, J.C., Menicucci, J.A., Brown, S.A., Kemner, K.M., Garofalo, D.F., Quallick, M.R., Neal, C.R., Kulpa, C.F., 2001. Biomass byproducts for the remediation of wastewaters contaminated with toxic metals. Environ. Sci. Technol. 35, 3786–3791. Singh, V., Tiwari, S., Sharma, A.K., Sanghi, R., 2007. Removal of lead from aqueous solutions using Cassia grandis seed gum-graftpoly (methylmethacrylate). J. Colloid Interf. Sci. 316, 224–232. Uyama, Y., Kato, K., Ikada, Y., 1998. Surface modification of polymers by grafting. Adv. Polym. Sci. 137, 1–39. Varma, A.J., Deshpandey, S.V., Kennedy, J.F., 2004. Metal complexation by chitosan and its derivatives: a review. Carbohyd. Polym. 55, 77–93. Wallace, J.R., Singer, P.C., 1981. Precipitation of lead from a storage battery manufacturing wastewater. In: Proceedings of the 35th Industrial Waste Conference, Purdue University. pp. 702–717. Winter, M.J., 1994. In: d-Block Chemistry. Oxford University Press, New York.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Highly enhanced adsorption for decontamination of lead ions from battery wastewaters using chitosan functionalized with xanthate Divya Chauhan, Nalini Sankararamakrishnan * Facility for Ecological and Analytical Testing, 302 Southern Laboratories, India Institute of Technology, Kanpur, UP 208016, India
a r t i c l e
i n f o
Article history: Received 15 February 2008 Received in revised form 9 April 2008 Accepted 9 April 2008 Available online 19 May 2008 Keywords: Lead Decontamination Battery wastewater Chitosan
a b s t r a c t Decontamination of lead ions from aqueous media has been investigated using cross linked xanthated chitosan (CMC) as an adsorbent. Various physico-chemical parameters such as contact time, amount of adsorbent, concentration of adsorbate were optimized to simulate the best conditions which can be used to decontaminate lead from aqueous media using CMC as an adsorbent. The atomic absorption spectrometric technique was used to determine the distribution of lead. Maximum adsorption was observed at both pH 4 and 5. The adsorption data followed both Freundlich and Langmuir isotherms. Langmuir isotherm gave a saturated capacity of 322.6 ± 1.2 mg/g at pH 4. From the FTIR spectra analysis, it was concluded that xanthate and amino group participate in the adsorption process. The developed procedure was successfully applied for the removal of lead ions from real battery wastewater samples. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Lead is suitable for batteries, because of its specific characteristics (conductivity, resistance to corrosion and the special reversible reaction between lead oxide and sulfuric acid). Due to its toxicity the US Environmental Protection Agency (EPA) has set the maximum contaminant level (MCL) of lead ions in drinking water to be 0.015 mg/L (Primary Drinking Water Rules, 1992). Lead removal from acidic wastewaters is typically accomplished by neutralization and precipitation (Wallace and Singer, 1981; Patterson, 1985). However, this process suffers from the disadvantage of incomplete metal removal and generation of toxic sludge or other waste products that require safe disposal. Chitosan, a derivative from N-deacetylation of chitin-a naturally occurring and abundant biopolymer has been found to be capable of adsorbing various heavy metal ions, including copper, lead, mercury, cadmium, chromium, and so on; this is largely attributed to the presence of the amine groups of chitosan that have strong affinity to and can form complexes with various heavy metal ions (Guibal, 2004; Varma et al., 2004). In the past decade, the rapid development of chemical modification technologies used as popular methods to provide materials with improved or desirable properties for practical applications has been observed (Uyama et al., 1998). Xanthation has been used previously on cellulose and sawdust (Bailey et al., 1999), brown marine algae (Kim et al., 1999) and chitin (Kim et al., 2006) to increase the adsorption the adsorption capacity.
In this study, chemical modification of cross linked chitosan through introduction of xanthate was investigated in order to achieve highly enhanced adsorption performance for lead ions under acidic solution conditions. The choice of xanthate group is due to the presence of sulfur atoms and it is well known that sulfur groups has a very strong affinity for most heavy metals, the metal–sulfur complex is very stable in basic condition. A series of batch adsorption experiments were conducted in the laboratory to evaluate the adsorption and desorption performance of lead ions on the chemically modified chitosan and Fourier transform infrared spectroscopy was used to elucidate the mechanisms of chemical modification and lead ion adsorption in the study. 2. Methods 2.1. Materials Chitosan flakes was acquired from India Sea Food, Cochin, India and used in the present study without any further purification. The degree of deacetylation was reported to be 85% by the manufacturer. Glutaraldehyde and carbon disulfide were purchased from Sigma–Aldrich and used without further purification. Stock solution of Pb(II) was prepared using Pb(NO3)2 (BDH chemicals). All the inorganic chemicals used were anular grade and all the reagents were prepared in Millipore milli-Q deionised water. 2.2. Chemical modification of the chitosan (CMC)
* Corresponding author. Tel./fax: +91 512 2597844. E-mail address: [email protected] (N. Sankararamakrishnan). 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.04.024
Chitosan flakes were cross linked with glutaraldehyde, chemically modified and characterized as described earlier
D. Chauhan, N. Sankararamakrishnan / Bioresource Technology 99 (2008) 9021–9024
(Sankararamakrishnan et al., 2006). Chitosan flakes (ca. 0.5 g) were suspended in methanol (100 ml), and a 25% aqueous glutaraldehyde solution (0.046 ml, 0.12 mmol) was added. After stirring at room temperature for 6 h, the product was filtered. Cross linked chitosan flakes (0.5 g) was treated with 25 ml of 14% NaOH and 1 ml of CS2. The mixture was stirred at room temperature for 24 h. The obtained orange product, cross linked chemically modified chitosan flakes (CMC) were washed thoroughly with water, air dried and used for further experiments. The adsorbents used were all sieved through a sieve of 0.4–0.6 mm particle size range. 2.3. Metal concentration analysis Dissolved lead was determined by Analyst 400 Perkin Elmer Atomic Absorption Spectrophotometer using an air-acetylene flame at wavelength 283.3 nm using a slit width of 0.7 nm. Experimental 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 filtration systems. All assays were carried out in triplicate and only mean values are presented. 2.4. Lead adsorption batch experiments Batch experiments were carried out with synthetic solutions of Pb2+ in 100 mL flasks with stopper at 100 rpm of orbital stirring in an incubator shaker, at room temperature and 4 h of contact time with the adsorbent. Samples were then filtered with Whatman No. 42 filter paper, diluted and analyzed for lead. Unless otherwise stated the parameters with synthetic water were: sample volume 20 ml, sorbent dose 2.5 g/l, initial metal ion concentration 100 mg/l, pH 5, equilibration time 4 h. For pH studies the pH was varied from 2 to 6 keeping the other conditions the same. Experiments were not conducted at higher pH due to the precipitation of lead hydroxide. For kinetic studies the sample volume was maintained at 20 ml and at every 30 min. sample was withdrawn for analysis. For equilibrium studies the adsorbent dose was maintained at 2.5 g/l varying the initial Pb2+ concentration from 100 to 1500 mg/l. 2.5. Desorption experiments To examine the desorption behaviors of lead ions from CMC, adsorption experiments were first conducted by placing a 0.10 g amount of CMC in 20 mL of a lead ion solution (with an initial concentration of 500 mg/L and a solution pH value of 5) for 4 h, and the final lead ion concentration in the solution was analyzed. The CMC adsorbed with lead ions were separated from the solution by filtration and then added into 20 mL of a 0.01 M EDTA, 0.01 N HCl, and 0.1 N NaOH solution in a flask for lead ion desorption. The contents of the flask were stirred at 200 rpm at room temperature for a time period of up to 2 h, and the lead ion concentrations in the solution were analyzed. The adsorbents were finally collected from the solution by filtration, washed with DI water, and then reused in the next cycle of adsorption experiments. The adsorption–desorption experiments were conducted for three cycles. 2.6. Studies with battery wastewater The samples were acquired from a local battery manufacturer located in Kanpur City, UP, India during September 2007. The wastewater samples were analyzed promptly after collection using standard analytical methods (APHA, 1998). The characteristics of battery wastewater were color: yellow, pH: <1, ionic conductivity: 120.5 mS, TDS: 55.292 g/l, TSS: 5397 mg/l, Pb: 93.89 mg/l.
3. Results and discussion 3.1. Adsorption performance at different initial pH values A pH effect test is performed to determine the pH of adsorption at which maximum uptake of metal occurs. The pH from 2 to 6 was adjusted initially with either hydrochloric acid or sodium hydroxide (0.1 M). No efforts were made to maintain the pH throughout the adsorption procedure. Fig. 1 shows the adsorption of Pb2+ by xanthated chitosan for various initial pH conditions. The hydrogen ion concentration of the heavy metal solution is a crucial factor in lead uptake by xanthated chitosan as shown in Fig. 1. Any study on the uptake capacity beyond pH 6.0 was avoided because insoluble lead hydroxide precipitates were observed in the solution. The maximum uptake capacity decreased with decreasing initial pH values as shown in Fig. 1. There are two possible explanations, first, hydrogen ions compete with lead ions to the same binding sites on the adsorbent. Second, the xanthate group is known to be unstable in strong acid solution and to be able to dissociate from the chitosan back bone. Decomposition is a two-stage process in which the first is the protonation of the hydroxyl group and the second is the elimination of carbon disulfide (Hulanicki, 1967). Since lead ions are bound to the sulfur atoms of xanthate groups, any loss of sulfur could lead to a reduction in the uptake capacity. The amount adsorbed increased from 20 to 39.8 mg/g when the pH was increased from 2 to 5 and thereafter a slight decline in amount adsorbed was observed with increase in pH to 6. The optimum pH for the removal of Pb2+ by chitosan flakes was found to be 5 and that of CMC was found to be at both pH 4.0 and 5.0. The pKa of xanthate-xanthic acid dissociation constant is reported to be 1.70 (Iwasaki and Cooke, 1958). Thus, in the pH range used in the present study, the characteristics of surface group of the adsorbent are unlikely to change. 3.2. Adsorption mechanisms of xanthated chitosan for Pb(II) For the xanthated chitosan used in this study, it may be expected that the adsorption sites for lead are at the nitrogen atoms of the amino groups in chitosan, sulfur atoms of attached xanthate groups and oxygen atoms of the hydroxyl groups in chitosan. The complexation of the lead ions with the xanthate group is shown in reaction (1). Both nitrogen and oxygen atoms have a lone pair or lone pairs of electrons that can bind a proton or a metal ion through an electron pair sharing to form a complex. Because of
50
40
Lead uptake (mg/g)
9022
30
20 CMC
10
Plain Flakes
0 2
3
4
5
Initial pH Fig. 1. Effect of pH on adsorption of lead.
6
9023
D. Chauhan, N. Sankararamakrishnan / Bioresource Technology 99 (2008) 9021–9024
the stronger attraction of the lone pair of electrons to the nucleus in an oxygen atom than in a nitrogen atom, the nitrogen atoms would have a greater tendency to donate the lone pair of electrons for sharing with a metal ion to form a metal complex than the oxygen atoms. The reaction scheme of lead ions with amino and xanthate groups are shown in reactions (1)–(4)
soft bases tend to form stable complexes with metals such as Cd2+, Pb2+ and Cu2+ (Winter, 1994). Since xanthate groups can be classified as soft bases xanthated chitosan will have a much higher affinity and sorption capacity when compared to that of plain chitosan. The strong affinity between sulfur and lead may result in high affinity constant and hence high uptake capacity.
ð1Þ
2þ
R—NH2 þ Pb
2þ
! R—NH2 Pb
R—NH2 þ Hþ ! R—NHþ 3 2þ
R—NHþ 3 þ Pb
2þ
! RNH2 Pb
þ Hþ
ð2Þ
3.4. Sorption kinetics
ð3Þ
Rapid interaction of the metal ions to be removed with the adsorbent is desirable and beneficial for practical adsorption applications. The kinetic results of lead ion adsorption on CMC shown in Fig. 2. It can be observed that lead uptake on CMC is a fast process. The amount of adsorption increased rapidly in the first 3 h, contributing to about 70% of the ultimate adsorption amount, and then augmented slowly. The adsorption equilibrium was achieved within about 4 h in this case. The pseudo-second-order kinetic model has often been used to fit the experimental kinetic adsorption data and determine whether an adsorption process is dominated by the chemical adsorption phenomenon. The linearized pseudo-second-order kinetic equation usually takes the following form:
ð4Þ 2+
To further prove the possible sites of Pb bonding to the CMC, FTIR spectra were obtained for the CMC before and after Pb2+ adsorption. The sulfur in CMC was evident by a shoulder due to thiol group at 2676 cm1. The diminution of the thiol peak after adsorption is indicative of the role of thiol group in the complexation with lead ions. Disappearance of the absorption bands at 1070 cm1, associated with the stretching of the secondary –C–OH and the bands at 1152 cm1 corresponding to the stretching of C–N bond weaken indicated the formation of N–Pb bond in the adsorption process. Formation N–Pb bond was further evident from the diminution of carbonyl stretching vibration of amide I band at 1654 cm1 and disappearance of –N–H bending vibration of amide II band. The IR absorption band corresponding to vibration of the C–S bond occured at the frequency 1015 cm1 and did not change significantly due to adsorption. The band at about 1200 cm1 was assigned to the stretching vibrations of the C–O bond with significant participation of the adjacent C–S bond. The absorption band, for the lead xanthate complex was observed at the wave number 1250 cm1. Due to the presence of some residual carbonate in the chitosan even after processing, formation of lead carbonate was evident by the doublet peak at 1320 cm1 and 1280 cm1 From the FTIR spectra analysis, it could be concluded that both amino and xanthate groups are involved in the adsorption process.
Table 1 Isotherm constants and correlation coefficients for adsorption of lead(II) ions from aqueous solutions pH
4 5
Langmuir
Freundlich 2
qmax (mg/g)
b (ml/mg)
R
n
kf
R2
322.6 ± 1.5 303.0 ± 1.9
0.014 0.017
0.97 0.98
3.27 3.42
43.1 42.7
0.93 0.86
3.3. Sorption equilibrium Sorption isotherms were modeled using linearized Langmuir and Freundlich model represented by Eqs. (5) and (6), respectively qe ¼ qmax bC e =ð1 þ bC e Þ
ð5Þ
ln qe ¼ n ln C e þ ln kF
ð6Þ
where qe is the equilibrium adsorbate loading on the adsorbent (mg/g), Ce the equilibrium concentration of the adsorbate (mg/l), qmax the ultimate capacity (mg/g), b the relative energy (intensity) of adsorption, also known as binding constant (L/mg), n (dimensionless) is a constant depicting adsorption intensity and kF (mg/ g)(L/mg) is Freundlich constant characterizing the adsorption capacity. The values obtained for the various parameters of these two models are given in Table 1. CMC exhibited approximately three times the capacity for uptake compared to the plain chitosan flakes (Ng et al., 2003). An adsorption capacity of 322.6 mg Pb/g was observed for modified chitosan flakes compared to 100.5 mg/g for plain flakes as reported earlier (Ng et al., 2003). The value obtained is considerably higher than those obtained on activated carbon (Issabayeva et al., 2006) and natural materials (Pavasant et al., 2007; Schneegurt et al., 2001) sorption capacities in these cases are less than 101 mg Pb/g of the sorbent. According to HSAB theory,
Fig. 2. Kinetic adsorption results of lead ions on CMC.
9024
D. Chauhan, N. Sankararamakrishnan / Bioresource Technology 99 (2008) 9021–9024 0
t=qt ¼ 1=ðk q2e Þ þ t=qe
ð7Þ 0
where qt (mg/g) is the adsorption amount at time t (min), k (g/mg/ min) is the rate constant of the pseudo-second-order kinetic adsorption. The values of k0 and qe can be obtained from the intercept and slope of the plot of the experimental t/qt versus t. The fitting of Eq. (7) to the experimental kinetic data from the study is given in the insertion of Fig. 2. It is observed that the adsorption kinetic data of lead ions on CMC are indeed well-represented by the pseudo-second-order kinetic model, with the correlation coefficient R2 being almost unity (R2 = 0.9853). The parameter values of qe and k2 determined from the fitting of the kinetic model in Eq. (2) to the experimental data are found to be 335.5 mg/g and 1.07 104 g/ mg/min, respectively. The maximum adsorption capacity of 335.5 mg/g obtained from the pseudo-second-order kinetic model is very close to that obtained from the Langmuir model earlier (322.6 mg/g). 3.5. Desorption studies Among the various stripping solutions it was observed only 26% of the loaded Pb(II) stripped in 0.5 N NaOH and 48% stripped with 0.01 N HCl and 0.01 M EDTA. Addition of NaOH might lead to the precipitation of Pb(OH)2 on the surface of the adsorbent which eventually blocks the adsorption sites for further adsorption. Hence, further experiments were carried out only with 0.01 N HCl solutions. Though both EDTA and 0.01 N HCl desorbed 48% of the lead ions, equilibrium achieved was faster in HCl (30 min) compared to EDTA (4 h) The adsorbent/desorption cycles were repeated for three cycles using 0.1 g of the adsorbent and 20 ml of 500 mg/l of lead ions for adsorption and desorption was carried out with 20 ml of 0.01 N HCl. Lead adsorption declined from 100% to 46% in the second cycle and subsequently decreased to 36% in the III cycle. This observation was similar to earlier reports (Jin and Bai, 2002; Singh et al., 2007) where the percent adsorbed reduced in subsequent cycles. 3.6. Effect of adsorbent dose and adsorption in battery wastewater Experiments were conducted with varying adsorbent dose of CMC on battery wastewater. Since the battery wastewater is highly acidic the effluent was diluted twice and used. It was observed (results not shown) that even at a very low dose rate of 0.5 g/l the adsorbent was effective in complete removal of lead from the battery effluent. 4. Conclusions Chemical modification of the chitosan flakes with xanthate group increased the adsorption capacity to three times compared to the plain flakes. Adsorption studies were modeled by both Langmuir and Fruendlich isotherms. From the FTIR spectra analysis, it
was concluded that xanthate and amino groups are primarily responsible for the adsorption process. Adsorbent–adsorbate kinetics exhibited pseudo-second-order. High adsorption capacity of xanthated chitosan towards lead makes it an attractive adsorbent for the removal of lead ions from battery wastewater. This method is very promising compared to other conventional and generally more expensive processes. Acknowledgements The authors are thankful for the funding provided by International Foundation for Science (IFS) to carry out this work. References APHA, 1998. Standard Methods for Water and Wastewater, 20. American Public Health Association, Washington, DC. Bailey, S., Olin, T., Bricka, M., Adrian, A., 1999. A review of potentially low cost adsorbents for heavy metals. Water Res. 33, 2469–2479. Guibal, E., 2004. Iterations of metal ions with chitosan-based sorbents: a review. Sep. Purif. Technol. 38, 43–74. Hulanicki, A., 1967. Complexation reactions of dithiocarbamates. Talanta 14, 1371– 1392. Issabayeva, G., Kheireddine Aroua, M., Sulaiman, N.M.N., 2006. Removal of lead from aqueous solutions on palm shell activated carbon. Bioresour. Technol. 97, 2350–2355. Iwasaki, I., Cooke, S.R.B., 1958. The decomposition of xanthate in acid solutions. J. Am. Chem. Soc. 80, 285–288. Jin, L., Bai, R., 2002. Mechanisms of lead adsorption on chitosan/PVA hydrogel beads. Langmuir 18, 9765–9770. Kim, Y.H., Park, J.Y., Yoo, Y.J., Kwak, J.W., 1999. Removal of lead using xanthated marine algae, Undaria piinatifida. Process Biochem. 34, 647–652. Kim, S.H., Song, H., Nisola, G.M., Ahn, J., Galera, M.M., Lee, C., Chung, W.J., 2006. Adsorption of lead(II) ions using surface-modified chitins. J. Ind. Eng. Chem. 12, 469–475. Ng, J.C.Y., Cheung, W.H., McKay, G., 2003. Equilibrium studies for the sorption of lead from effluents using chitosan. Chemosphere 52, 1021–1030. Patterson, J.W., 1985. Industrial Wastewater Treatment Technology, second ed. Butterworth, Stoneham, Mass. pp. 175–189. Pavasant, P., Apiratikul, R., Sungkhum, V., Suthiparinyanont, P., Wattanachira, S., 2007. Biosorption of Cu2+, Cd2+, Pb2+, and Zn2+ using dried marine green macroalga Caulerpa lentillifera. Bioresour. Technol. 97, 2321–2329. Primary Drinking Water Rules; sec. 141. 32 (e) (20), 1992, Federal Regulations. The Bureau of National Affairs Inc., Washington, DC. Sankararamakrishnan, N., Dixit, A., Iyengar, L., Sanghi, R., 2006. Removal of hexavalent chromium using a novel cross linked xanthated chitosan. Bioresour. Technol. 97, 2377–2382. Schneegurt, M.A., Jain, J.C., Menicucci, J.A., Brown, S.A., Kemner, K.M., Garofalo, D.F., Quallick, M.R., Neal, C.R., Kulpa, C.F., 2001. Biomass byproducts for the remediation of wastewaters contaminated with toxic metals. Environ. Sci. Technol. 35, 3786–3791. Singh, V., Tiwari, S., Sharma, A.K., Sanghi, R., 2007. Removal of lead from aqueous solutions using Cassia grandis seed gum-graftpoly (methylmethacrylate). J. Colloid Interf. Sci. 316, 224–232. Uyama, Y., Kato, K., Ikada, Y., 1998. Surface modification of polymers by grafting. Adv. Polym. Sci. 137, 1–39. Varma, A.J., Deshpandey, S.V., Kennedy, J.F., 2004. Metal complexation by chitosan and its derivatives: a review. Carbohyd. Polym. 55, 77–93. Wallace, J.R., Singer, P.C., 1981. Precipitation of lead from a storage battery manufacturing wastewater. In: Proceedings of the 35th Industrial Waste Conference, Purdue University. pp. 702–717. Winter, M.J., 1994. In: d-Block Chemistry. Oxford University Press, New York.