STUDY ON BIOSORPTION OF SELECTED HEAVY METALS FROM AQUEOUS SOLUTIONS USING MODIFIED AND UNMODIFIED COW HOOVES

Abstract

The feasibility of using modified and unmodified cow hooves to sequester cadmium (II), chromium VI), copper (II), nickel (II), lead (II) and zinc (II) ions from aqueous solutions was studied. Cow hoof powder in four different forms was used in this study namely: unmodified cow hoof (CH), citric acid modified (CACH), hydrochloric acid modified (HCH) and sodium hydroxide modified cow hoof (SCH). Morphological characterization of these biosorbents before metal removal was investigated using Scanning Electron Microscope (SEM) while the quantitative and qualitative elemental composition of the hoof samples were determined using Energy-Dispersive X-ray Spectroscopy (EDX) before the adsorption studies. Batch experiments were conducted to investigate the effect of the significant process parameters such as initial pH, contact time, adsorbent amount, temperature and the initial metal concentration. The optimum conditions established for each metal-adsorbent system was applied to sequester the same set of metals from real industrial wastewaters. The scanning electron micrographs obtained showed that there were significant changes in the morphological appearance on the surface of cow hoof powder after different chemical modifications had been made. The EDX analysis gave the following information about the prepared biosorbents: CH contained C (48.69%), N (18.70%), O (28.50%), S (3.37%), K (0.18%) and Ca (0.56%); CACH contained C (39.15%), N (15.18%)’ O (43.84%), S (1.83%), HCH contained C (45.39%), N (17.27%), O (33.19%), S (2.67%), Si (0.18%), Cl (1.30%) and SCH contained C (35.78%), N (25.89%), O (3.05%), S (2.79%), Na (0.79%), and Ca (0.7%). Kinetic data obtained for contact time at 298 K was regressed against the pseudo-first-order, pseudo-second-order kinetic and Elovich equations in their non-linear forms. It was found that biosorption of metals by CH and CACH generally followed pseudo-second order equation with few exceptions. Biosortption of metals by HCH majorly followed Elovich equation. For metals biosorbed on SCH, pseudo-first order fitted the kinetic data for cadmium, copper and zinc while pseudo-second order model fitted the kinetic data for the remaining three metals. The equilibrium data were modelled using the non-linear forms of Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models. This showed that the removal of metals by CH seemed to follow D-R model while the removal of metals by CACH, HCH and SCH generally followed Langmuir model with few exceptions. The values of the separation factors RL evaluated from Langmuir constant KL revealed that the biosorption of all the six metals by all the biosorbents used was favourable. The thermodynamic parameters showed that the biosorption of these metals was spontaneous. The values of enthalpy change obtained revealed the process to be endothermic for some metal-biosorbent systems and exothermic for others. The results obtained through the application of the biosorbents in this study to remove heavy metals from real industrial wastewaters showed that metal uptake from industrial effluents was a little lower than what was obtainable from simulated wastewaters. However, judging from the favourability and feasibility of the adsorption process, these biosorbents are promising environmentally friendly low-cost adsorbents which can be applied industrially either alone or in combination with other proven biosorbents to treat industrial wastewaters.