Efficient removal of low-concentration Cd(II) and Cu(II) from aqueous solution by deep eutectic solvent-assisted nitrogen-doped hydrochar

A novel approach for the synthesis of nitrogen-doped hydrochar from raw biomass was developed with the assistance of urea-metal chloride deep eutectic solvent (DES). The results indicated that the urea/ZnCl₂ based DES-functionalized hydrochar (ZnU-HC) could efficiently remove of low-concentration (0–50 mg·L −1 ) Cd 2+ and Cu 2+ from aqueous solutions. Nonlinear Langmuir isotherm fitting yielded maximum adsorption capacities of 44.9 ± 1.1 mg·g −1 for Cd 2+ and 46.8 ± 1.4 mg·g −1 for Cu 2+ , representing increases of 121.2% and 167.4%, respectively, compared with unmodified hydrochar. XPS analysis indicated that DES modification increased the contents of pyrrolic-N and pyridinic-N and was associated with changes in Zn-N-related surface environments. These observations suggest that nitrogen-containing and Zn-associated surface environments may be involved in metal binding together with other nitrogen- and oxygen-containing surface functionalities. Molecular dynamics simulations indicated that Cd 2+ exhibited stronger local association near confined interfaces, whereas Cu 2+ remained more spatially dispersed, suggesting greater accessibility to spatially distributed interaction environments. The removal mechanisms of Cd 2+ and Cu 2+ were primarily attributed to surface complexation, cation-π interactions, electrostatic forces, and precipitation. Finally, life cycle assessment (LCA) confirmed that ZnU-HC exhibited the lowest climate-change impact among the evaluated hydrochars, with an emission reduction of 3.2 kg CO₂ eq per functional unit relative to the direct-mixing ZnUHC control.