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International Journal of Environmental Research, Vol. 4, No. 3, July-September, 2010, pp. 427-432 Article Adsorption of heavy metals by Salvinia Biomass and agricultural residues Dhir, B. 1* and Kumar, R . 2 1 Department of Environmental Biology, University of Delhi, Delhi-110007, India Date of Submission: 10-Feb-2009 Code Number: er10046 Abstract Batch adsorption experiments were performed to study adsorption potential of agricultural residues viz. rice straw, wheat straw and Salvinia plant biomass for removal of heavy metals such as Cr, Ni, and Cd. Plant materials were used in different combinations. Heavy metal removal efficiency was more at low metal concentration (35 mg/L). Salvinia biomass possessed higher efficiency for removing heavy metals such as Cr, Ni and Cd followed by a combination where three materials (rice straw, wheat straw, Salvinia biomass) were taken together in comparison to other combinations. The adsorption data fitted in Langmuir and Freundlich isotherm models.The present investigations revealed that agricultural residues such as rice straw and wheat straw along with Salvinia biomass can serve as low-cost alternative for removal of heavy metals from wastewaters.Keywords: Adsorption, Heavy metals, Rice straw, Salvinia, Wheat straw, Biomass Introduction Heavy metal released during different industrial and mining processes pose threat to living organisms, there-fore it becomes important to develop technologies that result in effective removal of heavy metals from waste-waters. Various methods for heavy metal removal in-cluding chemical precipitation, membrane process, ion exchange, liquid extraction and electrodialysis are non-economical and have many disadvantages such as high reagent and energy requirements, generation of toxic sludge or other waste products that require disposal or treatment (Demirbas, 2008). In contrast, adsorption tech-nique can has been proved to be an excellent method to treat industrial waste effluents, offering significant advantages like low-cost, availability, profitability, easy operation and efficiency (Li et al., 2007). Biosorption of heavy metals from aqueous solutions is a relatively new process that has proven very promising in the removal of contaminants from aqueous effluents. In recent years, various natural adsorbents such as agri-cultural wastes including sunflower stalks, Eucalyp-tus bark, maize bran, coconut shell, waste tea, rice straw, tree leaves, peanut and walnut husks have been tried to achieve effective removal of various heavy metals (Sun and Shi, 1998; Karthikeyan et al., 2005; Sarin and Pant, 2006; Singh et al., 2006; Hashem et al., 2007; Demirbas, 2008; Kahraman et al. 2008; Nameni et al., 2008). Biosorption of heavy metal ions by non-living biomass of aquatic plant species Potamogeton lucens, Salvinia herzogii, Eichhornia crassipes, Ludwigia stolonifera, Ceratophyllum demersum, Hydrilla verticillata have been reported in literature (Schneider and Rubio, 1999; Elifantz and Tel-Or et al., 2002; Keskinkan et al., 2004, 2007; Bunluesin et al., 2007). In present study, an attempt was made to I) investigate efficiency of Salvinia biomass for its metal sorption potential; ii) compare the metal sorption potential of Salvinia biomass along with other agricultural resi-dues such as wheat and rice straw; iii) assessment of adsorption potential using Langmuir and Freundlich isotherms; iv) suggest best suitable combination for use as low-cost adsorbent for the removal of toxic heavy metals. Material and Methods The wheat straw, rice straw and Salvinia biomass was ground and passed through standard steel sieves (72 mm mesh size). The fine powder was used without washing or any other physical or chemical treatments. Different combinations namely A, B, C, D, E, F, G were tried. The details are included in [Table - 1]. The amount of adsorbent was kept constant as one gram in all the combinations. Adsorbent was added to 100ml metal solutions with concentrations 35, 50 and 100 mg/L pre-pared by using K2Cr2O7, Cd(NO 3 ) 2 .4H 2 O and Ni(NO 3 ) 2 .6H 2 O respectively. Samples were agitated at 250 rpm for 2h (pH 5.5). The sorption studies were carried out at 25±1ºC. A metal and biomass free blank was used as control. The mixtures were filtered through filter paper (pore diameter 11΅m) and metal concentra-tions were determined in the filtrate using Flame Atomic Absorption Spectrophotometer (AA 6300, Shimadzu, Japan). Metal removal (%) at any instant of time was determined by the following equation: Heavy metal removal (%) = (C i - C f )/ C i X 100 Where, C i and C f represent initial and final metal con-centration (mg/L) at any instant of time, respectively (Argun et al. 2007). The adsorption data obtained was analyzed using Langmuir and Freundlich isotherm models. Freundlich and Langmuir isotherms are the earliest and simplest known relationships describing the adsorption equa-tion (Jalali et al. 2002; Verma et al., 2008).The Langmuir isotherm represents the equilibrium distribution of metal ions between the solid and liquid phases. The linearlized equation allows the calculation of adsorp-tion capacities and the Langmuir constants and is equated by the following equation: C eq /q = 1/q max. b + C eq /q max where q is metal accumulated by biosorbent mate-rial (mg/g); C eq is the metal residual concentration in solution (mg/L); qmax is the maximum specific uptake corresponding to the site saturation (mg/g) and b is Langmuir equation constant which represents ratio of adsorption and desorption rates (Langmuir, 1918; Igwe and Abia, 2007). The Freundlich isotherm model was chosen to es-timate the adsorption intensity of the sorbent towards the adsorbent. The linearized form of the Freundlich equation was used for analysis and it is given as: log Q e = log K f + 1/n log C e Where, C e is the equilibrium concentration of the adsorbate (mg/L) and Q e is the amount adsorbed per unit mass of adsorbent (mg/g), K f and n are Freundlich equilibrium coefficients (Freundlich, 1907). Standard errors on means and slopes were calculated using the standard Microsoft Excel spreadsheet routines. The data was statistically analyzed using software newMSTAT-C (version 2.1, Michigan State University) (Dhir et al., 2009). Results and Discussion Batch adsorption studies showed that Cr, Ni and Cd removal (%) was more at lower metal concentration in all the combinations (A-G), irrespective of heavy metal [Figure - 1]. Nickel removal efficiency followed the order G > A = F > B = C > D = E, Cr removal efficiency followed the order G = F= A > C > E = B > D and Cd removal efficiency followed the order G > A = D > B = E > F = C at 35 mg/L concentration [Table - 2]. Salvinia biomass showed highest metal removal efficiency (G) in all three metal treatments. The adsorption data fit-ted in both (Langmuir and Freundlich) adsorption iso-therms in all treatments. Adsorption was described well by Langmuir model as correlation coefficient (R 2 ) was greater than 0.9 in all cases (R 2 > 0.9), but for Freundlich model correlation coefficient (R 2 ) the values ranged between 0.84 to 0.99 [Table - 3]. Langmuir and Freundlich parameters are listed in [Table - 3]. The values of Langmuir and Freundlich constants indicate favorable conditions for adsorption. Out of all combinations tested, maxi-mum Cd, Cr and Ni adsorption capacity (q max ) (mg/g) was noted in A (three materials taken together). Maxi-mum adsorption capacity calculated from the Langmuir isotherm was 61 mg/g Cr, 46.9 mg/g Ni and 44 mg/g Cd (pH 5) in combination A. The Freundlich isotherm pa-rameter n and K f for different metals have been listed in [Table - 3]. The value of n was more than 1 in all combi-nations irrespective of metal treatment. Higher values of K f represent greater adsorption intensity. Salvinia biomass showed higher potential for Cr, Ni and Cd removal in comparison to agricultural resi-dues. Adsorption isotherms give the capacity of the adsorbent based on the ratio between the quantity adsorbed and the remaining in solution at fixed tem-perature at equilibrium. Salvinia biomass possesses good capacity to adsorb Cr, Ni and Cd. Metal adsorp-tion capacity of Salvinia is comparable or somewhat higher than those reported for other aquatic plant spe-cies (Keskinkan et al., 2004, 2007; Bunluesin et al., 2007). The adsorption capacity of Hydrilla verticillata was found to be 15.0 mg/g for Cd (Bunluesin et al., 2007), Myriophyllum spicatum was found to be 10.37 mg/g for Cu, 15.59 mg/g for Zn, 46.49 mg/g for Pb and Ceratophyllum demersum was found to be 6.17 mg/g for Cu, 13.98 mg/g for Zn and 44.8 mg/g for Pb (Keskinkan et al., 2007) and Vallisneria spiralis was found to be 53.4 mg/g for Ni, 3.48 mg/g for Cr and 4.38 mg/g for Cd, Pistia stratiotes was found to be 5.10 mg/ g for Ni, 2.96 mg/g for Cr and 4.49 mg/g for Cd (Verma et al. 2008). Agricultural residues showed low heavy metal adsorption capacity in comparison to Salvinia biom-ass. Among agricultural residues, rice straw possessed higher metal adsorption capacity. Earlier, isotherm stud-ies showed adsorption of Pb, Cu, Zn, Cr on rice husk, wheat bran and data fitted well in Langmuir and Freundlich models (Ajmal et al., 2003; Mohan et al. 2008; Nameni et al., 2008). Both Langmuir and Freundlich isotherm models explained adsorption of heavy metals. The data fitted well in Langmuir adsorption isotherm as compared to Freundlich isotherm. Better fitting of the experimen-tal data to Langmuir isotherm indicate mono-layered tion of Cd, Ni and Cr in all combinations. Langmuir isotherm parameter q max indicates maximum adsorp-tion capacity. Irrespective of metal, maximum adsorp-tion capacity (q max) was noted in combination A where three plant materials were used. The Freundlich equa-tion gives an adequate description of adsorption data over a restricted range of concentration and suggests heterogenity of the adsorption sites on biomass (Juang et al. 1996 ). The Freundlich isotherm parameter K indi-cates adsorption intensity when concentration of metal ion in equilibrium is unitary. Higher the values of K f and n, higher is the affinity for adsorption of metals (Jalali et al. 2002; Igwe and Abia, 2007). The values of 1/n< 1 indicate favorable adsorption of all metals irre-spective of treatment and the values of n within the range of 2-10 represent good adsorption (Teng and Hsieh, 1998; Ozer and Pirincci, 2006). Higher values of K f indicate high adsorption intensity. Previous find-ings showed that dry biomass of plant species such as Eichhornia crassipes, Vallisneria spiralis, Nymphaea sp and Pistia stratiotes possessed good metal sorption capacity and data fitted well into Freundlich is otherm equation (Elangovan et al., 2008; Verma et al., 2008). Conclusion The present study demonstrated that plant resi-dues can be used effectively in the removal of Cr, Ni and Cd from aqueous solution. Salvinia biomass can be utilized as a biological resource with immense po-tential for heavy metal adsorption. The equilibrium data fitted well with Langmuir isotherm model. Use of agro waste materials and plant biomass can offer as inex-pensive and effective metal ion adsorbents as replace-ments for existing commercial materials. Acknowledgements The financial assistance from Department of Sci-ence and Technology, New Delhi to Bhupinder Dhir is gratefully acknowledged.[28] References
Copyright 2010 - International Journal of Environmental Research The following images related to this document are available:Photo images[er10046t1.jpg] [er10046t3.jpg] [er10046f1.jpg] [er10046t2.jpg] |
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