Distribution behavior and risk assessment of metals in bio-oils produced by liquefaction/pyrolysis of sewage sludge

The distribution behaviors of metals in bio-oils derived from sewage sludge (SS) by liquefaction with different solvents (ethanol, methanol, or acetone) and by pyrolysis at different temperatures (550-850 °C) were investigated. The concentrations of crust metals (K, Na, Ca, Mg, Fe, and Al) in bio-oils were much higher than those of the anthropogenic metals (Cu, Zn, Pb, Cd, Cr, Ni, V, Mn, Ba, Co, Ti, Sn, As, and Hg), but the anthropogenic metals were more inclined to distribute in bio-oil phase compared with crust metals. The anthropogenic metals in bio-oils can be divided in three groups in terms of the distribution similarities according to Cluster analysis: (A) Cu, Co, Ni, V, and Sn; (B) Cr, Ti, Mn, and Ba; (C) Pb, Cd, As, Hg, and Zn. Cu, Cr, Hg, Cd, V, Co, and Sn distributed in the liquefaction/pyrolysis bio-oils accounted for as high as 5-20% of the metals in SS and were evaluated "moderate enrichment" by the enrichment factors method. According to the potential ecological risk index (PERI) method, Hg presented very high risk, Cu presented moderate risk, and Cd presented low to moderate risk; and the overall risk levels of these bio-oils were very high risk (except P550, presented considerable risk).