Mechanisms of enhanced Cr (VI) removal by waste lignin derived N-doped hierarchical porous carbon (N-HPC).

N-doped hierarchical porous carbon (N-HPC) is made from waste lignin by a one-pot method, and its mechanisms of Cr (VI) removal was investigated. The specific surface area (S_BET) of N-HPC-Fe3 was 1749.8 m²/g, the experimentally determined equilibrium adsorption capacity (q_e) for Cr (VI) was 386.5 mg/g, and the calculated maximum adsorption capacity (q_m) was 627.1 mg/g, which showed excellent adsorption performance. The adsorption process is consistent with the Langmuir model and the pseudo-second-order model. The removal process of Cr (VI) was proposed: the high specific surface area and positively charged surface of N-HPC enhanced the pore filling and electrostatic adsorption effects; and the high content of nitrogen-oxygen functional groups acted as electron donors and adsorption active sites, which reduced Cr (VI) to Cr (III) and complexed it to the N-HPC surface. The contribution of different mechanisms was quantified and 85.1% reduction was the main removal mechanism. The removal efficiency of N-HPC reached 76.5% after 7 cycles and was minimally affected by coexisting ions, showing excellent reusability, stability and selectivity. This study emphasizes the potential of using cost-effective and sustainable biomass waste carbon for Cr (VI) removal, providing a theoretical and practical basis for environmental remediation.