Dual-nitrogen-source anchored biochar achieves efficient removal of Cr(VI) from wastewater through an adsorption-reduction coupling mechanism

In this study, a dual-nitrogen-source modified biochar material (N₄₀₀-BC/P₁₀) was developed via an ultrasonic-assisted hydrothermal method for the efficient removal of hexavalent chromium (Cr(VI)). Skeleton nitrogen structures such as pyridinic nitrogen and pyrrolic nitrogen were introduced through urea doping, while amino groups were grafted on the surface using polyethyleneimine (PEI). This modification strategy achieved the synergistic enhancement of “skeleton nitrogen” and “functional group nitrogen”. N₄₀₀-BC/P₁₀ exhibited a high affinity toward Cr(VI). At pH 2, the adsorption capacity of the material progressively increased with the augmentation of nitrogen content. At an initial concentration of 200 mg·L⁻¹, N₄₀₀-BC/P₁₀ exhibited an order-of-magnitude increase for Cr(VI) equilibrium adsorption capacity (340.02 mg·g⁻¹) compared with that of single-nitrogen-source modified counterpart (251.36 mg·g⁻¹). Even in the presence of interfering coexisting anions and cations, N₄₀₀-BC/P₁₀ maintained an adsorption capacity of at least 296.68 mg·g⁻¹ and a removal efficiency of at least 74 %. The adsorption process followed the pseudo-second-order kinetics and the Langmuir isotherm model, indicating that chemical adsorption was the predominant mechanism. XPS analysis revealed that the removal of Cr(VI) by N₄₀₀-BC/P₁₀ followed a coupled mechanism of “adsorption-reduction-stabilization”. Moreover, N₄₀₀-BC/P₁₀ demonstrated excellent practical wastewater treatment capability, highlighting its significant potential for industrial wastewater treatment. The dual-nitrogen-source anchoring strategy proposed in this study provides a new paradigm for the design of biochar-based heavy metal removal materials with high performance, low cost, and low environmental load. Furthermore, the detailed adsorption-reduction coupling mechanism also provides a theoretical basis for the precision remediation of heavy metal pollution.