Effect of ozonation treatment on the chemical speciation distributions of heavy metals in sewage sludge and subsequent bioleaching process

Effect of ozonation treatment on the chemical form distributions of heavy metals (Cr, Mn, Ni, Cu, Zn, As, Cd, and Pb) in sewage sludge (SS) and bioleaching efficiency was investigated. The results showed that the portions of heavy metals present in the oxidizable fraction decreased in the ozonized sewage sludge (OSS). Meanwhile, the proportions of most heavy metals in the reducible fraction were increased (Cr, Ni, Cu, As, and Pb) or remained constant (Mn and Cd) after ozonation treatment. Slight increase of the heavy metals (except As) in the residual fraction was also found. The OSS showed lower final pH value after bioleaching and faster increase of oxidation-reduction potential (ORP) during bioleaching process. As a result, the bioleaching rate of all the heavy metals was improved, and the solubilization efficiency of Cr, Ni, Zn, As, and Pb was increased in OSS. The comparisons of the chemical fractions in the bioleached SS and OSS indicated that the higher solubilization in OSS might be mainly due to the decrease of the oxidizable fraction and increase of the unstable fraction of these heavy metals after ozonation treatment.