Distribution and transformation behaviors of heavy metals during liquefaction process of sewage sludge in ethanol-water mixed solvents

Abstract

Liquefaction of sewage sludge (SS) in ethanol-water cosolvents is a promising process for the preparation of bio-oil/biochar products. Effect of the combined use of ethanol and water on the distribution/transformation behaviors of heavy metals (HMs) contained in raw SS is a key issue on the safety and cleanness of above liquefaction process, which is explored in this study. The results show that pure ethanol facilitates the migration of HMs into biochar products. Pure water yields lower percentages of HMs in mobile/bioavailable speciation. Compared with sole solvent treatment, ethanol-water cosolvent causes a random/average effect on the distribution/transformation behaviors of HMs. After liquefaction of SS in pure water, the contamination degree of HMs is mitigated from high level (25.8 (contamination factor)) in raw SS to considerable grade (13.4) in biochar and the ecological risk is mitigated from moderate risk (164.5 (risk index)) to low risk (78.8). Liquefaction of SS in pure ethanol makes no difference to the pollution characteristics of HMs. The combined use of ethanol and water presents similar immobilization effects on HMs to pure water treatment. The contamination factor and risk index of HMs in biochars obtained in ethanol-water cosolvent treatment are 13.1−14.6 (considerable grade) and 79.3−101.0 (low risk), respectively. In order to further control the pollution of HMs, it is preferentially suggested to improve the liquefaction process of SS in ethanol-water mixed solvents by introducing conventional lignocellulosic/algal biomass, also known as co-liquefaction treatment.