Understanding the migration and transformation characteristics of Cr3+ in the hydrothermal cracking of biomass model components

Hydrothermal cracking, a thermochemical process operating at 180–220 °C and saturated pressure, offers a promising way to valorize Cr-contaminated agricultural wastes ( e.g. , maize straw, manure) into stable organic fertilizers. Using model compounds (cellulose, xylan, lignin) and three Cr salts (CrCl 3 , Cr(NO 3 ) 3 , Cr(CH 3 COO) 3 ), experiments at 180–220 °C and 30–90 min revealed that hydrothermal cracking achieved >63 % Cr immobilization in solid products, predominantly as stable oxidizable (OF) and residual (RESI) fractions. Nitrate additives (Cr(NO 3 ) 3 ) outperformed chloride and acetate salts, yielding 57.4 % RESI through redox cycling and complexation. Risk assessment (RAC <10 %, RI < 80) confirmed low environmental hazards, with prolonged reaction times (90 min) further reducing bioavailability. Mechanistic studies revealed that Cr immobilization primarily occurs through the interaction between organic matter and Cr, with surface functional group complexation as an additional immobilization pathway. The work provides actionable strategies for tailoring hydrothermal processes to address heavy metal contamination in circular agricultural systems, emphasizing anion-specific optimization for scalable remediation.