Activation or sequestration of heavy metals during hydrothermal process of swine manure: Interactions among metal species and particulates.

• Interactions among metal species and particulates during HTP of SM were investigated. • Overwhelming changes were observed in terms of K, Ca, Mg and Fe species. • Mg and K dominated in liquid phase to cause release of F2 via ion exchange. • Crystallization of SM resulted in the obvious increase of residual Cu, Zn and Pb. • In contrast to K and Mg, Cu, Zn and Pb were inclined to be immobilized during HTP. Whether the heavy metals in solid biomass is activated or sequestrated during hydrothermal process (HTP) is still debated. Herein, the speciation of light and heavy metals during HTP of swine manure (SM) was investigated to reveal the interactions among these metal species and specific particulates. With increasing temperature, most of exchangeable species and that bound to carbonates were released to liquid phase via ion exchange and acid dissolution. Dissociation of Fe-Mn oxides rarely happened in spite of anoxic atmosphere formed during HTP. Substantial decomposition of lignocelluloses hardly caused significant liberation of fraction bound to organics. Instead, a part of fraction in liquid phase was re-captured by new oxygen-containing functional groups on solid product surface to form fraction bound to organics. Donpeacorite, butschliite and iwakiite were formed as primary minerals, resulting in increase of residual fraction of all metals except for K and Mg at 250 °C. In summary, Cu, Zn and Pb species evolution was affected by speciation of K, Ca, Mg, Fe and Mn significantly. Cu, Zn, Pb, Fe, Mn and Ca were sequestrated whereas K and Mg were activated with enhancing temperature during HTP in terms of their mobility factors. [ABSTRACT FROM AUTHOR]