A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Environment, Resources & Development

Thesis (M.Eng.) - Asian Institute of Technology, 1994

Sequential extraction was performed to find out exchangeable, adsorbed, organic, carbonate and residual forms of the metals in the sludge under consideration. Each metal showed binding characteristics of its own. Zn had distinct affinity to the organic ligands present in the sludge, whereas for Cd, Cr, Cu and Ni, dominant form was residual. Pb and Mn were associated mainly with the organic and carbonate forms. Chemical leaching experiments were carried out for five days by varying the pH to 1.5, 2.5 and 3.5. Chemical leaching of different forms were also dependent on metal characteristics. In general; metals in adsorbed, organic and carbonate forms were solubilized more easily than that of residual form. Exchangeable form increased to a considerable extent after leaching. Different forms of metals remained after leaching for the specified period of time was found out and compared to investigate the chemical leaching process. Formation of stable complexes with humic substances, mineral constituents and buffer action due to release of ligands by H+ from metal-ligand complexes were considered responsible for the remaining of different metal forms even after leaching for 5 days at a very low pH 1.5. Solubilization rate of different heavy metals by the bacterium Thiobacillus ferrooxidans was found very fast compared to chemical leaching at the same pH. Other than sulfides; metals in adsorbed, organic and carbonate forms also were found solubilized due to bacterial activity. This solubilization however, was considered either the result of production of excess H2S04 during bacterial leaching process or due to dissociation of metal-ligand complex by bacteria. Metals in residual form, which was mainly sulfides were not leached completely by bacteria which indicated the presence of very stable metal-mineral complex in this form.