Phytic acid-mediated hydrothermal valorization of woody biomass containing heavy metals into functional hydrochar through mechanistic insights and safe utilization strategies

Phytoremediation-derived biomass may cause secondary pollution due to heavy metals (HMs) retention in conventional hydrothermal carbonization (HTC). Here, a phytic acid (PA)-enhanced HTC system was developed using willow biomass harvested from HMs-contaminated soils. PA promoted biomass decomposition and carbonization, yielding 37.04%–54.42% hydrochar with high heating values of 23.73–27.75 MJ/kg. Importantly, PA addition significantly promoted HM migration from the solid to the liquid phase, achieving removal efficiencies of 99.61% for Cd and 92.45% for Zn at 0.75 wt% PA, which further increased to 99.95% and 99.65% at 5 wt%, respectively. 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 (Q_max) of 22.82 mg/g for Cd²⁺ and 87.78 mg/g for Cu²⁺. These adsorption behaviors fit well with pseudo-second-order kinetic model and the Langmuir isotherm model. The adsorption process is governed by multiple mechanisms, including surface complexation, cation exchange, and cation-π interaction. Application of 3% PAHC improved soil porosity by 16.8%, increased cation exchange capacity to 97.61–102.57 cmol/kg, and promoted transformation of HMs to stable forms (Cd: 0.59%; Zn: 0.29% at week 10). Ecological risk was reduced (PAHC: RI = 224.81) compared with WHC (250.44) and control (301.48). Overall, the PA-HTC system enables safe valorization of hazardous biomass and supports a closed-loop remediation strategy for HMs-contaminated soils.