Abstract
•HTC of sewage sludge was performed in various varying feed-water pH situation from 2 to 12 at 270°C.•The accumulation of Pb, Ni, Cd and Zn was significantly affected by feed-water pH.•Alkaline condition benefits HMs transfer from an unstable state to stable.•The lowest pollution level of HMs was obtained at 270°C with pH=11.
In this study, the effect of feed-water pH (pH=2–12) on fate and risk of heavy metals (HMs) in hydrochars (HCs) was investigated. Hydrothermal carbonization (HTC) of sewage sludge (SS) was carried out with different feed-water pH at 270°C. The research results showed that changing feed-water pH had a positive effect on accumulating Pb, Ni, Cd and Zn in HCs. Chemical forms of Cu and Cr converted from an unstable state to stable in the alkaline environment while in the acidic condition was opposite. The effect of feed-water pH on the chemical forms of HMs was variable but not significant. Risk assessments of Igeo, Eri, RAC and RI were applied to evaluate the accumulation levels of individual metal, the potential ecological risks, the bio-availabilities and the comprehensive toxicity and sensitivity of HMs, respectively. The lowest pollution level of HMs was obtained at 270°C with pH=11.