Valorizing lignocellulosic biomass-derived hydrothermal carbonization aqueous phase for heavy metal removal from...

Valorization of hydrothermal carbonization aqueous phase (HTC-AP) remains a key challenge for the sustainable deployment of hydrothermal carbonization technology. Herein, lignocellulosic biomass-derived HTC-AP was employed as a low-cost soil washing agent for heavy metal removal, with emphasis on linking HTC conditions, molecular characteristics, and remediation performance. Results showed that HTC conditions had limited effects on bulk physicochemical properties except total acidity, but significantly altered HTC-AP molecular composition. HTC-AP produced at 210 °C exhibited lower molecular weight, higher aromaticity, and greater carboxyl abundance than those obtained at 180 and 240 °C. Correspondingly, removal efficiencies for Cu (17.8-31.4%), Zn (45.1-71.5%), and Cd (17.7-56.0%) increased with HTC severity and plateaued above 210 °C. Metal removal resulted from the combined effects of acid activation (ion exchange and mineral dissolution) and metal-specific complexation. Specifically, Cu preferentially interacted with O-rich non-aromatic compounds (200-400  Da) and polycyclic aromatics (400-500  Da), whereas Zn and Cd were mainly associated with aromatic fractions (200-400  Da) and non-aromatic compounds (400-500  Da) in HTC-AP. The proposed metal-specific mechanisms were supported by Fourier transform ion cyclotron resonance mass spectrometry, correlation analysis, and two-dimensional correlation spectroscopy. These findings highlight HTC-AP as an effective and sustainable soil washing agent and advance understanding of the process-structure-performance relationships underlying HTC-AP-mediated heavy metal remediation.