S and N codoping disrupts the electronic symmetry of the lignin-based porous carbon to enhance the removal of Cr(VI): Adsorption-reduction-readsorption mechanism

The reduction and precipitation of Cr during the adsorption process has always been challenging. In this study, S and N codoped lignin-based porous carbon (SN-LPC) was prepared by hydrothermal treatment and K-intercalation activation for the removal of Cr(VI). We discovered that SN-LPC has a new mechanism underlying the adsorption–reduction–readsorption of Cr(VI): First, SN-LPC adsorbs Cr(VI) onto its surface through the hydrogen bonding of -OH and the electrostatic adsorption of N functional group protonation; second, the S and N functional groups act as electron donors to induce the rapid reduction of Cr(VI) to Cr(III). Last, the –C–OR/C N groups on the SN-LPC surface and the S and N functional groups adsorb Cr(III) again, increasing the removal of Cr(VI). DFT calculations have revealed at the molecular structure level that SN-LPC has a more active electronic nature. Doping of S and N increases the affinity of SN-LPC for chromium, which explains why it can efficiently reduce and fix Cr(VI) and adsorb Cr(III). The removal process of Cr(VI) by SN-LPC is mainly multilayer adsorption dominated by chemical adsorption, with a maximum adsorption capacity of 967.45 mg/g (pH 2, 45 °C, and an initial concentration of 800 mg/L), and the removal efficiency is much greater than that of similar activated carbon and biochar. The 2D-FTIR–COS results further elucidated the sequence of functional groups of SN-LPC in the process of reducing Cr(VI), supporting the mechanism hypothesis of adsorption–reduction–readsorption. These findings highlight the significance of sulfur and nitrogen as key groups in reduction and adsorption and provide a reference for the underlying synergistic adsorption reduction mechanism used to repair Cr(VI).