Assessing hydrochar wash-water toxicity: screening strategies for reducing toxicity from food waste-derived hydrochars.

Hydrothermal carbonization (HTC) offers a promising pathway for valorizing food waste into hydrochar for soil amendment applications; however, the release of toxic compounds during post-application leaching remains a key concern. This study evaluated how feedstock composition and HTC conditions influence the ecotoxicity of hydrochar wash waters generated under laboratory-simulated washing conditions designed to represent potential rainfall or irrigation-driven leaching. Wash water composition and toxicity (via Escherichia coli and Aliivibrio fischeri assays) were measured. Food wastes (e.g., fruit, vegetable, grain, meat and dairy/confectionery products) were carbonized at 200-250 °C across multiple residence times. All hydrochars were sequentially washed to simulate rainfall or irrigation events. Across all conditions, cumulative washing was the strongest factor associated with toxicity, with the first wash producing the most toxic leachates and toxicity declining rapidly with subsequent washes, indicating that inhibitory compounds are largely water soluble. Feedstock composition appeared to significantly influence toxicity magnitude, with fruit-derived hydrochars consistently producing the least toxic wash waters, while vegetable- and grain-derived materials exhibited higher and more variable toxicity. A generalized linear model indicated that hydrogen richness was among the most influential feedstock properties associated with toxicity, with lower H:C values associated with reduced E. coli toxicity and higher A. fischeri EC50 values across the evaluated conditions and O:C ratio exerting a comparatively minor influence. Overall, these results provide a screening-level assessment of potential ecotoxicity risks associated with hydrochar-derived leachates.