Supercritical water gasification of hyperaccumulators: Study on hydrogen production, heavy metal immobilization, and environmental risk assessment

Phytoremediation is an effective technology for the removal of heavy metals (HMs) from contaminated soils. However, improper post-treatment of hyperaccumulators can lead to secondary environmental hazards. This study explores the application of supercritical water gasification (SCWG) for the treatment of Cu-enriched hyperaccumulator Elsholtzia splendens ( ES ), with the dual aims of H 2 production and HM immobilization. SCWG process yielded a maximum H 2 output of 16.68 mol/kg after 20 min, demonstrating its potential for waste valorization. Within the temperature range of 500–650°C, the migration and transformation mechanisms of HMs were investigated through detailed biochar characterization. The results demonstrate that SCWG significantly reduces the leaching rate of Cu from 25.18 % to below 0.08 %, with over 99 % of HMs being immobilized in the resultant biochar. Furthermore, higher temperatures and prolonged residence times were found to promote the complexation of HMs with inorganic components in the biochar, resulting in their transformation into stable F4 fractions. Then, HMs in liquid products and solid residues were evaluated in terms of Nemerow index and potential ecological risk. The results indicated that the pollution risks of liquid products were safe while the original hyperaccumulators showed high risk, underscoring the promise of SCWG as an effective method for the concurrent production of H 2 -rich syngas and the immobilization of HMs.