Highly efficient adsorption of chromium on N, S-codoped porous carbon materials derived from paper sludge

The synergistic effect of heteroatoms is a viable method to enhance the adsorption performance of heavy metal onto carbon-based materials. However, the high cost, complex operation and a lot of pollution from the synthesis process have limited its development. Herein, a facile two-step pyrolysis method is used to prepare in situ N and S doped porous biochar from paper mill sludge for the removal of Cr(VI) from aqueous environment. The NSC-450 sample prepared under the optimum conditions has a large specific surface area of 3336.7 m² g^-1, an average pore size of 2.56 nm and a total pore volume of 2.10 cm³ g^-1, manifesting the excellent adsorption capacity of 356.25 mg g^-1 for Cr(VI). The adsorption of Cr(VI) by NSC-450 is consistent with the Langmuir isotherm and pseudo-second-order model, suggesting a spontaneous and endothermic chemisorption process. The analysis results show that the NH, graphitic nitrogen and thiophene structures have a positive effect on converting a large amount of Cr(VI) to Cr(III) by synergistic reduction, indicating obviously facilitating Cr(VI) removal compared to other sites. Therefore, in this material, the strong adsorption mechanism is mainly reductive complexation. Moreover, the effects of real water quality, anions, cations and fulvic acid on the adsorption behavior of Cr(VI) onto the NSC-450 were further investigated. The results demonstrate that the chromium removal rate remains above 82% even in actual electroplating wastewater, suggesting NSC-450 has great practical application prospect. This work offered a feasible method for high-value utilization of sludge, but also provided a novel perspective for the future design of heteroatom-doped carbon materials for promoting to eliminate hexavalent chromium from water environment.