Migration/transformation characteristics of heavy metals and polycyclic aromatic hydrocarbons in the co-liquefaction treatment of pig manure and lignocellulosic biomass

Abstract

The migration/transformation characteristics of heavy metals and polycyclic aromatic hydrocarbons (PAHs) during the co-liquefaction of pig manure and rice straw/wood sawdust were explored in this study. More than 87% of the heavy metals in feedstocks were enriched in hydrochars. The decreased proportion of active heavy metals in the hydrochars suggested partial passivation of the heavy metals. The pollution degree and risk of heavy metals were significantly mitigated from high and considerable levels in pig manure to moderate and low levels in hydrochar, respectively. Compared with commercial diesel, bio-oil products still contained an undesirable amount of heavy metals. PAHs were re-synthesized during liquefaction, with a net synthesis amount of 29.65–73.98 mg·kg⁻¹. Moreover, the PAHs mainly existed in bio-oils, with a content of 57.32–132.33 mg·kg⁻¹ and a toxicity equivalent of 3.25–8.19 mg·kg⁻¹. Compared to pig manure, the hydrochars presented a lower content of PAHs (1.76–3.53 mg·kg⁻¹ versus 3.73 mg·kg⁻¹) and a smaller toxicity equivalent (0.14–0.22 mg·kg⁻¹ versus 0.26 mg·kg⁻¹). Interestingly, introducing lignocellulose (especially for rice straw) during the liquefaction of pig manure further mitigated the pollution degree/risk of heavy metals and PAHs. Overall, hydrochar reached a safe utilization level, while bio-oil products needed further clarification.