Biosorption of Cr (III) from aqueous solution by the leaf biomass of Calotropis procera

The biosorption of Cr (III) onto the leaf biomass of Calotropis procera popularly known as ‘bom bom’ in western Nigeria, over a wide range of reaction conditions were studied. The batch experiments showed that the biosorption of Cr (III) onto Calotropis procera leaf biomass is a rapid process reaching equilibrium within 10 minutes at an optimum pH value of 5. Other reaction conditions such as biomass dosage, initial metal ion concentration and temperature were also found to influence the biosorption process. Both Langmuir and Freundlich isotherms were employed to describe the biosorption process and both proved to be applicable. However, Langmuir gave a better fit with an R-Squared value of 0.967 (closer to unity than that of freundlich), Langmuir constant, KL of 0.0188 and monolayer adsorption capacity, qm of 32.26 whereas the Rsquared value for the freundlich plot was 0.948 with adsorption capacity Kf and adsorption intensity, n of 1.156 and 1.146 respectively. The biosorption process followed the pseudo-second order kinetic model evident by an Rsquared value of 0.999 and the pseudo second order rate constant, K2 of 0.3668 gmg min. Thermodynamic studies revealed negative value of change in free energy, ∆G (4.046KJmol) as an indicator of feasibility and spontaneity of the Cr (III) biosorption process. A positive value of enthalpy, ∆H (26.099 kJmol) was obtained which indicated the endothermic nature of the biosorption process. FT-IR studies of the biosorbent before and after the biosorption process indicated that carboxylate, amino and nitro functional groups were involved in the sorption of Cr (III) onto Calotropis procera leaf biomass. These findings indicate that the leaf of biomass of Calotropis procera could be employed in the removal of Cr (III) from aqueous solutions and industrial effluents. @ JASEM Chromium is one of the most abundant elements on earth and is of considerable environmental concern as it is widely used in leather tanning, electroplating, metal finishing, chromate preparation, wood preservation and manufacture of dyes and pigments (Krishna et al., 2004; Shali and Indu 2005). Chromium occurs in aqueous systems in trivalent and hexavalent forms. Chromium (III) is used in tanneries as chromium sulphate which may be converted to Chromium (VI) in the effluent. Cr (VI) may be converted to Cr (III) under reduced environment, which is much less toxic and less soluble by several microorganisms which possess chromate reductase and thus reduction by these enzymes affords a means of chromate bioremediation (Shaili and Indu, 2005). Airborne emissions from chemical plants and incineration facilities are common sources of chromium. They are also derived from effluents of chemical plants, contaminated landfill, topsoils and rocks. Cr (III) often accumulates in aquatic life adding to the danger of eating fish that may have been exposed to high levels of Cr (Greaney, 2005). Chromium (III) proves to be biologically essential to mammals as it maintains effective glucose, lipid and protein metabolism (Krishna et al., 2004). However, long time contact causes skin allergy and cancer (Yun et al., 2001). Cr (III) can also be oxidized to the more carcinogenic and mutagenic Cr (VI) by MnO2 in the environment or by some bacteria in soil under proper conditions (Ahmet et al., 2008). Cr (III) is also toxic to fish when its concentration in water exceeds 5.0 mg/L. It therefore becomes imperative that the level of Cr (III) in the environment be kept as minimal as possible. The hexavalent chromium has higher toxicity and leads to liver damage, pulmonary congestion, skin irritation, resulting in ulcer formation and is carcinogenic (Parke et al., 2000). The maximum permissible levels of Cr (VI) in potable and industrial waste water are 0.05 and 1.0 mg/L respectively (Goyal et al., 2003), yet levels as high as 80 ppm have been observed in paper mill effluents (Sudhakar et al., 1991). Chemical precipitation methods are commonly employed for the removal of chromium but this leads to formation of chrome-bearing solid wastes plus the fact that it is uneconomical when the concentration of the chromium in the effluent is low (Onyancha et al., 2008). Therefore, various kinds of biomaterials have been investigated for chromium removal and have been found efficient under various conditions such as pH, biosorbent dose, agitation time and initial metal ion concentration. Cr (III) ions are found to bind more strongly as the pH is increased from 3 to 5. The pH dependence occurs when metal ions and protons compete for the same active metal binding sites such as carboxylate or amino groups on the biomass surface (Loukidou et al., 2004). Above pH 5, a decrease in Cr adsorption capacity of Spirogyra condensata was observed. For Rhizoclonium heiroglyphicum adsorption capacity increased from pH 3-4 but decreased above pH 4. The highest adsorption capacity values were observed at pH 4 and 5 for Rhizoclonium heiroglyphicum and Spirogyra condensata respectively (Onyancha et al., 2008). The effect of