The migration and transformation behavior of heavy metals during the liquefaction process of sewage sludge

Bio-oils and bio-chars were obtained from sewage sludge (SS) by liquefaction with ethanol (or acetone) as the solvent at the temperature of 280, 320 and 360 °C. The migration and transformation of HMs as Pb, Zn, Cu and Ni during liquefaction were thoroughly investigated. Meanwhile, the environmental risk of HMs in the bio-oils and bio-chars was assessed according to the risk assessment code (RAC). The results showed that the liquefaction solvent and temperature significantly affected the redistribution of HMs. HMs distributed mainly into the bio-chars, with less than 10% into the bio-oils. Increasing liquefaction temperature would promote a higher HM content in bio-oils. The environmental risk of HMs in bio-chars was mitigated compared to SS, especially for Ni. However, the environmental risk of Zn and Ni in bio-oils was undesirably high in comparison with bio-chars. It was suggested that the bio-oil should be pretreated before utilization.