Distribution, risk assessment and stabilization of heavy metals in supercritical water gasification of oily sludge

Supercritical water gasification (SCWG) of oily sludge is a potential handling method for resource utilization and pollution reduction. Heavy metals (HMs) in oily sludge were assessed due to their threat to human health and environmental safety. This work investigated the distribution of four HMs (Cu, Zn, Ni and Cr) under different reaction conditions (550–700 °C, 1–60 min). The concentrations of HMs in liquid residues (LRs) decreased continuously at higher temperatures and longer residence times except for Ni. Besides, the concentrations of HMs in solid residues (SRs) increased after SCWG except for Zn, and they varied slightly under different conditions. Then, HMs in LRs and SRs were evaluated in terms of Nemerow index, geo-accumulation index and potential ecological risk. The results indicated that the pollution risks of HMs in LRs were minimum at 650 °C and 30 min, while that in SRs changed little. When Na₂CO₃ was added, pollution risks changed slightly, but nearly complete stabilizations of Cu, Zn and Cr (96.2%, 84.2% and 98.6%) were achieved at 600 °C. Adding Na₂CO₃ promoted the formation of aluminosilicate to combine with HMs and enhanced their stabilization notably. This work may demonstrate a promising clean way for oily sludge utilization and HMs stabilizing.