Removal of Cr⁶⁺ in water by superoxide anion-mediated redox reaction assisted by lignin-rich kiwifruit twig biochar: Application of DFT calculation

This research aims to investigate the role of reactive oxygen species (ROS) in the adsorption and reduction of Cr⁶⁺ on lignin-rich biochar under dark conditions and under various oxygen treatment conditions. The research found that under aerobic conditions, the reduction content of Cr⁶⁺ (0.38 mg) and the production content of ·O₂^- (20.36 × 10^-6 mg·L^-1) are the highest, followed by untreated conditions (0.32 mg, 15.03 × 10^-6 mg·L^-1), and the lowest under anaerobic conditions (0.21 mg, 5.14 × 10^-6 mg·L^-1). Compared with anaerobic conditions, the reduction content of Cr⁶⁺ increased by 1.52 times under untreated conditions. Meanwhile, under anaerobic conditions, ·O₂^- disappeared, indicating that ·O₂^- had played an important role in the reduction of Cr⁶⁺. Kinetic results showed that the role of ·O₂^- in the reduction of Cr⁶⁺ mainly occurred in liquid solution. DFT calculations confirmed that C-OH was the main electron supplier in the reduction process of Cr⁶⁺, and there was a positive correlation between the production content of ·O₂^- and the content of C-OH in liquid solution. The present research is expected to provide a scientific basis for the transformation of Cr⁶⁺ on lignin-rich biochar in liquid solution.