Hydrothermal liquefaction of sewage sludge and model compound: Heavy metals distribution and behaviors

Hydrothermal liquefaction (HTL) is an attractive thermochemical method for the conversion of wet sewage sludge (SS) into bio-oil. However, the bio-oil suffers from the contamination of heavy metals (HMs). In this study, the effects of reaction conditions on the HMs (Cu, Zn, Cd, Cr and Pb) migration during HTL of SS was investigated. Soybean protein was used as the typical model compound of SS to reveal the interactions between HMs and organic matters in the HTL process. Results showed that higher temperatures and longer reaction times promoted HMs enriched into the solid residues with more stable forms, while the proportion of HMs in the bio-oil was also increased. XPS analysis showed that the coordinate bond was formed between HMs and N during HTL of soybean protein at 180 °C. At higher temperature of 280 °C, metal oxides appeared in residues, while Cu and Cd led to the formation of nitrates. GC–MS analysis showed that Cu reduced the aromatic content by 52.8 %, while other HMs (Zn, Cd, Cr and Pb) reduced the oxygenated compounds content by 21.6 % at 180 ℃. In addition, the HMs had little effects on the bio-oil compositions at 280 °C due to the more stable forms of HMs at higher temperatures. [Display omitted] •Both raw sludge and model compound were used as feedstocks for HTL.•High temperatures contributed to the enrichment and curing of HMs in the residues.•The chelate interaction between HMs ions and organics occurred at 180 ℃.•The HMs oxides and nitrates were formed in residues at 280 °C.•Zn, Cr, Pb and Cd contributed to bio-oil deoxygenation at 180 °C.