Immobilization of heavy metals in sewage sludge by using subcritical water technology

•Subcritical water technology was used for heavy metals immobilization in sludge.•Heavy metals were transformed from weakly bounded to stable states after SCWT.•Direct toxicity and bioavailability of heavy metals decreased after SCWT.•Leaching toxicity and leachability of heavy metals was declined after SCWT. Heavy metals (HMs) immobilization in sewage sludge was investigated by using subcritical water technology (SCWT) in this study. The characteristics of sludge and toxicity of HMs were analyzed after SCWT process. The results showed that besides large reduction in sludge volume, SCWT had some positive effect on HMs dissolution into liquid phase, while the majority of HMs was still accumulated in solid phase. The direct toxicity and bioavailability of HMs in sludge was greatly decreased with no toxicity fractions of HMs highly increased. Pb was always at low risk level and the risk of other HMs was greatly reduced from low risk to no risk after SCWT treatment. Moreover, the leaching toxicity of HMs declined after SCWT and the best result was obtained at 280°C with the metal concentrations in leachate decreased by 97.46%, 93.91%, 86.14%, 73.67%, 71.93% and 10.71% for Cu, Cd, Zn, Cr, Ni and Pb, respectively.