Towards Sustainable Use of Hydrothermal Carbonised Wastes in Soil: Mitigating Hydrochar-Induced Toxicity by Ageing in Soil and Pyrolysis

Hydrothermal carbonised waste (hydrochar) is increasingly considered a sustainable soil amendment within circular economy frameworks; however, its toxicity to the environment limits its safe application. Ageing and pyrolysis may be prospective treatments to mitigate hydrochar toxicity on soil organisms and plants, but their effectiveness is unresolved since available studies did not investigate the predominantly toxic hydrochars, those produced at high reaction temperatures (>200 °C). In this study, toxic hydrochars produced at hydrothermal carbonisation (HTC) temperature of 260 °C from biosolids, chicken manure and rice straw were post-treated by in situ ageing in soil for 30 days or by pyrolysis at 400 °C for 30 min, and their toxicity was assessed on wheat growth and earthworm ( Eisenia fetida ) avoidance. Untreated hydrochars did not affect wheat ( Triticum aestivum L.) germination, but reduced wheat growth by 23–33%, and caused 82–94% earthworm avoidance. The ageing of hydrochar reduced phytotoxicity in biosolids and chicken manure hydrochar treatments, as indicated by improved wheat biomass (25–71%; compared to untreated hydrochar). Ageing of hydrochar also mitigated toxicity to earthworms, as indicated by the reduced avoidance of 19–31% across all feedstocks. The persistent phytotoxicity with aged rice straw hydrochar was due to toxic organic compounds that could not be fully detoxified after 30 days of ageing. Pyrolysis of hydrochar mitigated phytotoxicity as indicated by increased wheat biomass (43–93%), and reduced toxicity to earthworms (39–51%) across all feedstocks. Findings indicate that while both ageing and pyrolysis are appropriate hydrochar post-production treatments, ageing of hydrochar for only 30 days in soil may not fully alleviate the phytotoxic effect of all hydrochars.