Transformation of heavy metals from sewage sludge during bioleaching

Abstract

Bioleaching experiment was carried out in 80L SBR reactor with sewage sludge collected from each treatment step of the wastewater treatment plant (WWTP) system, to investigate the transformation of heavy metals during bioleaching. The corresponding changes in pH, oxidation-reduction potential (ORP) and the concentration of Zn, Cu and Pb in extracellular polymeric substances (EPS) were also studied. The results showed that the physical and chemical properties of sewage sludge could affect the sludge acidification and raise the ORP by oxidizing ferrous sulfate. The highest total concentration of Zn-EPS, Cu-EPS, and Pb-EPS was found in secondary sludge (SS), primary sludge (PS) and aerobic sludge (OS), respectively. Content of Pb in LB-EPS was more than in TB-EPS during 9 days bioleaching. Concentration of Cu and Zn in LB-EPS was better in the early leaching period while that in TB-EPS was at the end. Most of Zn existed in their forms of Fe-Mn oxides-bound in raw sludge (50.87%-61.79% of the total), followed by the organic oxide bound. The organic oxide bound was the predominant fraction of Cu (50.14-70.15% of the total) and Pb (41.28-61.94% of the total) in all original sludge samples. After bioleaching, Pb in primary sludge (PS) remained as the residual fraction, while the Fe-Mn oxides bound fraction in aerobic sludge (OS), anaerobic sludge (AS) and secondary sludge (SS). Zn existed as a high available fraction (F 1+2 ) in primary sludge (PS), aerobic sludge (OS) and anaerobic sludge (AS) but the residual fraction in secondary sludge (SS) after bioleaching. For Cu in all bioleached sludge, the predominant fraction was the residual fraction (average 32.71% of the total).Significant differences of the formation of heavy metals were observed in different sludge system during bioleaching, suggesting that the species of sludge significantly affected the distribution of various fractions of the metals in bioleaching.