HEAVY-METAL SPECIATION IN ANAEROBIC MUNICIPAL SLUDGE - COMPARISON BETWEEN SINGLE AND SEQUENTIAL EXTRACTION

With the final aim to investigate about the possible ability of munici pal sludge to adsorb heavy metals from industrial wastes, a characteri zation study has been performed for anaerobic sludges. The results are compared with those previously obtained for aerobic sludges. In order to evaluate the possibility to mobilize heavy metals (Cu(II), Zn(II), Cr(III), Mn(II), Ni(II), Pb(II), and Cd(II)) from the biomatrix a seq uential extraction method was employed. The method is based on the con sideration that i) heavy metals are present in the native biomatrix un der complex or association forms with inorganic or organic ligands cha racterized by different hydrophilicity and that ii! the equilibria are likely affected by ionic strenght and pH. As extracting reagents, non polar solvents (like CH2Cl2), polar salt (KNO3), acidic media (CO2 sa turated water pH 4 and HCl pH 2) and coordinating agents in alkaline m edium (EDTA in NaOH pH=9) were used as extraction agents. In order to check for the possible readsorption of metals taking place during the sequential extraction, a comparison was made by using the extracting r eagents sequentially or in a single extraction. In the different extra cts obtained, metal analysis was performed by AAS and ligand character ization by FT-LR and ion chromatography. The method has been applied t o sludges collected in the municipal waste water treatment plant Conso rzio Po-Sangone, serving the town of Torino. The solubilization percen tual amount obtained with the use of the mentioned extractants, has re ached sequential extraction values around 80.3%, referred to the total DS. Partial values obtained for the single extractions are comparable . The results indicate that species characterized by hydrophilic prope rties predominate and that preferential solubilizations take place for complexing competition reactions in alkaline medium. Among the metals present (Cd(II), Cu(II), Ni(II), Mn(II), Zn(II), Cr(III) and Pb(II)) it results that only Cu(II), Mn(II), Cr(III) and Zn(II) could be prese nt in non polar species, while all of them seem to be present in preva lently polar complexes formed with proteins and carbohydrates or under ionic association with chloride, nitrate, sulfate, phosphate and hydr ogenocarbonate.