Immobilization of metal(loid)s in hydrochars produced from digested swine and dairy manures

Abstract Anaerobic digestion technology is widely used for treatment of swine and dairy manures in livestock farms, but the digested swine and dairy manures (SD-S, SD-D) must be properly disposed. In this study, hydrothermal carbonization (HTC) was used to deal with SD-S and SD-D. The resulting hydrochars (HC-S and HC-D) were investigated for the migration, speciation and potential environmental risk of metal(loid)s therein. The results showed that about 20%−50% of metal(loid)s in SD-S and 11%−36% in SD-D lost through the dissolution of the metal(loid)s in solution during HTC process. The remaining metal(loid)s were more concentrated in HC-D compared to HC-S. The concentrations of water-extractable metal(loid)s showed clear decrease trend in HC-S and HC-D. The bioavailable metal(loid) fraction (acid soluble/exchangeable fraction and reducible fraction) were transformed into the stable fraction (residual fraction) during HTC process. The results indicated that HTC process could immobilize most metal(loid)s leaching from HC-S and HC-D, except for Zn and Cd in HC-S. The maximum leaching concentrations of all metal(loid)s happened at pH of 2; meanwhile less fraction of metal(loid)s can be leached out from HC-D into water. The environmental risk assessment values suggested that HC-D was more environment-friendly than HC-S. This study provides a useful support for reuse of HC-S and HC-D as pollution remediation and soil amendment with very low leaching toxicity and potential ecological risk of metal(loid)s.