Bioleaching of heavy metals from sewage sludge using indigenous iron-oxidizing microorganisms

Abstract

The objective of this study was to investigate the bioleaching of Cr, Cu, Pb, and Zn from sewage sludge using iron-oxidizing microorganisms. These conditions include the solid concentration, initial pH, ferrous iron concentration, inoculum concentration as well as the kinetics of solubilization of metals from sewage sludge to determine whether they impact on bioleaching efficiency. The sludge sample containing bacteria used in this study was collected from Fuzhou Jingshan sewage treatment plant. Indigenous iron-oxidizing bacteria were enriched from the sludge. Conditions affecting the bioleaching and application were conducted using batch experiments. The analysis of Cr, Cu, Pb, and Zn was carried out with an atomic absorption spectrophotometer, and the pH and ORP were measured using pH meter and ORP meter. The data show that 88.5 % of Zn, 79.9 % of Cu, 50.1 % of Pb, and 33.2 % of Cr can be removed from the sludge after 12 days of bioleaching at 30 °C, while only 80.2 % of Zn, 21.8 % of Cu, 10.9 % of Pb, and 10.5 % of Cr were leached out in the control without iron-oxidizing microorganisms. The leaching kinetics study shows that the rate of metal solubilization in bioleaching using iron-oxidizing microorganisms was more effective compared to chemical leaching. The results suggest that the leaching of metals from sludge can be attributed to two leaching approaches: firstly, chemical leaching; and secondly, bioleaching. However, their effectiveness depends on metal species because of their different bindings in sludge. For example, the leaching of Zn from the sludge was dominated by chemical leaching while the removal of Cu, Pb, and Cr was dominated by bioleaching.