Quantitative evaluation of heavy metals in solid residues from sub- and super-critical water gasification of sewage sludge

Solid residues (SRs) are important byproducts of sub- and super-critical water gasification of sewage sludge (SS). In this study, the quantitative evaluation of heavy metals (HMs) in SRs, compared with SS, is applied in terms of potential ecological risks, pollution levels, and both bioavailability and eco-toxicity. The results show the bioavailability and eco-toxicity of HMs in SRs decrease, although the total concentration of HMs increased, particularly in the bioavailable fraction of Cu, which decreased nearly 97%. The geo-accumulation and potential ecological risk index indicated that the gasification process increased contamination by two levels (to the maximum), while the overall risk was in keeping with SS. However, based on the risk assessment code, each tested HM exhibited lower environmental risk after gasification, especially for Cd, which drastically dropped from 66.67 (very high risk) in SS to 0.71 (no risk) in SRs, with a reaction temperature of 375 °C for 60 min.