Abstract

The removal and chemical speciation changes of heavy metals in the sewage sludge during the single bioleaching and combined bioleaching/Fenton-like processes were compared in this study. The improvement in the dewaterability of the treated sludge was also investigated. The single bioleaching led to a removal of Zn, Cu, Cd, Cr, Mn, Ni, As and Pb of 67.28%, 50.78%, 64.86%, 6.32%, 56.15%, 49.83%, 20.78% and 10.52% in 10 days, respectively. The chemical speciation analysis showed that the solubilization of heavy metals in mobile forms (exchangeable/acid soluble and reducible forms) and oxidizable form was the main reason for their removal. Subsequent Fenton-like treatment was carried out at different bioleaching stages when the bioleached sludge dropped to certain pH values (4.5, 4.0 and 3.0), by adding H2O2 at different dosages. The highest removal ratio of Zn, Cu, Cd, Cr, Mn and Ni could reach 75.53%, 52.17%, 71.91%, 11.63%, 66.29% and 65.19% after combined bioleaching/Fenton-like process, respectively, with appropriate pH and H2O2 dosages in less than 6 days. The solubilization efficiencies of these heavy metals in mobile forms were further improved by Fenton-like treatment. The removal efficiencies of As and Pb decreased due to their transformation into insoluble forms (mostly residual fraction) after Fenton treatment. The capillary suction times (CST) of the raw sludge (98.7 s) decreased by 79.43% after bioleaching and 87.44% after combined process, respectively.

Highlights

  • The removal and chemical speciation changes of heavy metals in the sewage sludge during the single bioleaching and combined bioleaching/Fenton-like processes were compared in this study

  • The rate of microbial sulfur and ferrous ion oxidation is the limiting step governing the efficiency of bioleaching process, and it has been reported that long time length (6–12 days) is usually required to obtain high solubilization efficiency and makes this process non-economically v­ iable[11,12]

  • The efficiency of Fenton oxidation is mainly depended on the reaction condition, it is crucial to investigate the influence of reaction pH and ­H2O2 dosage in the combined bioleaching/Fenton-like process, which has been neglected in previous studies

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Summary

Introduction

The removal and chemical speciation changes of heavy metals in the sewage sludge during the single bioleaching and combined bioleaching/Fenton-like processes were compared in this study. Fontmorin and Sillanpaaand investigated the efficiency of combined bioleaching/Fenton-like process for heavy metals removal and dewaterability improvement of the ­sludge[10]. The chemical form distributions of heavy metals in sewage sludge during the combined process of bioleaching and Fenton oxidation are still poorly understood. The efficiency of Fenton oxidation is mainly depended on the reaction condition, it is crucial to investigate the influence of reaction pH and ­H2O2 dosage in the combined bioleaching/Fenton-like process, which has been neglected in previous studies

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