Heavy metal speciation in anaerobic municipal sludge. comparison between single and sequential extraction

Abstract

With the final aim to investigate about the possible ability of municipal sludge to adsorb heavy metals from industrial wastes, a characterization 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 sequential extraction method was employed.

The method is based on the consideration that i) heavy metals are present in the native biomatrix under complex or association forms with inorganic or organic ligands characterized by different hydrophilicity and that ii) the equilibria are likely affected by ionic strength and pH. As extracting reagents, non polar solvents (like CH₂Cl₂), polar salt (KNO₃), acidic media (CO₂ saturated water pH 4 and HCl pH 2) and coordinating agents in alkaline medium (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 reagents sequentially or in a single extraction. In the different extracts obtained, metal analysis was performed by AAS and ligand characterization by FT‐IR and ion chromatography.

The method has been applied to sludges collected in the municipal waste water treatment plant Consorzio Po‐Sangone, serving the town of Torino.

The solubilization percentual amount obtained with the use of the mentioned extractants, has reached 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 properties 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 present in non polar species, while all of them seem to be present in prevalently polar complexes formed with proteins and carbohydrates or under ionic association with chloride, nitrate, sulfate, phosphate and hydrogenocarbonate.

Key words:

• anaerobic municipal sludge
• heavy metal speciation
• sequential extraction