Critical assessment of biowaste-derived hydrochars for remediating contaminated soils

Hydrothermal carbonization (HTC) has emerged as a promising method for managing wet biodegradable organic wastes while producing carbon-rich materials for soil improvement. This study examined five common waste types: organic fraction of municipal solid waste, green waste, grape bagasse, wood waste, and cheese whey. They were processed by pilot-scale HTC at 195 °C for 3 hours, with steam from a clinical waste incineration plant providing heat. The resulting hydrochars were evaluated for yield and physicochemical properties and compared to a typical biochar. Pot trials conducted over 4 months showed the behavior of hydrochars as amendment (5 wt%) on two different contaminated soils from mining sites. The overall patterns observed between hydrochar characteristics and soil properties (organic carbon fractions, pH, cation exchange capacity, P and N concentrations, etc.) were generally positive, with the exception of Ryegrass growth. The strengths of hydrochars were further supported by the reduction of Cu and Pb availability in treated soils. However, the slight increase in As availability suggests that they may not be entirely suitable for soil remediation, indicating the need for supplementary amendments.