Phytic Acid-Mediated Hydrothermal Valorization of Woody Biomass Containing Heavy Metals into Functional Hydrochar: Mechanistic Insights and Sustainable Resource Pathways

Phytoremediation-derived biomass poses risk of secondary pollution due to the retention of heavy metals (HMs) in conventional hydrothermal carbonization (HTC). In this study, a novel phytic acid (PA)-enhanced HTC system was developed to address the challenge using willow biomass harvested from HMs-contaminated soils. The results showed that PA significantly enhanced biomass decomposition and carbonization, yielding 37.04–54.42% hydrochar and modifying the surface structure of the resulting hydrochar. This process produced energy-dense hydrochar with high heating values ranging from 23.73 to 27.75 MJ/kg, demonstrating promising potential as a biofuel. Importantly, the addition of PA promoted the substantial migration of HMs from the solid phase into the liquid phase, with transfer rates of 99.95% for Cd and 99.65% for Zn at 5 wt% PA. The hydrochars exhibited excellent HMs adsorption capacities (1.57 ± 0.08–6.40 ± 0.47 mg/g for Cd²⁺; 4.80 ± 0.46–15.30 ± 0.06 mg/g for Cu²⁺), with PAHC-40 achieving the highest maximum adsorption capacities (Qₘₐₓ) of 22.82 mg/g for Cd²⁺ and 87.78mg/g for Cu²⁺. These adsorption behaviors fit well with Pseudo-second kinetics and the Langmuir isotherm model. The adsorption mechanisms were governed by synergistic interactions, including surface complexation, cation exchange, and cation–π interactions. Application of PAHCs at a 3% rate significantly improved soil properties, increasing porosity by 16.8% and enhancing cation exchange capacity to 97.61–102.57 cmol/kg in HMs-contaminated soils. Further, the addition of 3% PAHC can promote the transformation of HMs in the soil from active to stable states (Cd: 0.59%, Zn: 0.29%, 10th week). In terms of ecological risk reduction, PAHC is superior to WHC (PAHC: RI = 224.81 vs WHC: 250.44 vs control: 301.48). The PA-HTC system demonstrates potential for closing the remediation cycle by converting hazardous biomass into functional materials for soil HMs immobilization. It shed light on establishing a sustainable circular mode for phytoremediation biomass management, simultaneously enabling energy recovery and environmental decontamination through engineered hydrochar applications.