Heavy metals speciation and its bioavailability in sewage sludge pyrolysis residue

The presence of unstable heavy metals can influence the safe recycling of sewage sludge. Since the bioleaching effects of heavy metals from pyrolysis residue were sparely studied, heavy metals speciation and its bioavailability in sewage sludge pyrolysis residue (SSPR) were investigated in this study. Pyrolysis of sewage sludge, as well as bioleaching of SSPR, was carried out successively. Besides, a four-stage sequential extraction, Community Bureau of Reference, was adopted to analyze the heavy metals speciation in SSPR and its bioleaching residue. Results indicated that pyrolysis can enrich the concentrations of Cu, Ni, Zn, and Pb in sewage sludge except for Cd. Different physical properties of heavy metals, as well as the decrease in organics and moisture in sewage sludge, would result in the enrichment phenomena of heavy metals in SSPR. Thus, the concentrations of Cu, Zn, and Ni in SSPR were higher than those of Cu, Zn, and Ni in sludge standard for agricultural use. In addition, heavy metals in SSPR could be bioleached by Acidithiobacillus ferrooxidans which is commonly grown in nature. Most of the remained heavy metals after bioleaching were distributed in F3 (oxidizable) and F4 (residual) fractions, which meant the heavy metals in SSPR were further stabilized by Acidithiobacillus ferrooxidans. Hence, the combination of pyrolysis and bioleaching can be used to stabilize heavy metals in sewage sludge, and the concentrations of heavy metals in bioleaching residue can satisfy the sludge standard for agricultural use.