Removal Mechanisms of Cr(VI) By Redox-Active Moieties on HNO3 Modified Biochar Under Different pH and O2 Conditions

Abstract

Nitric acid (HNO3) modified biochar (NBC) has been demonstrated to be a promising sorbent. However, the roles of their redox-active moieties (RAMs, i.e., environmentally persistent free radicals (EPFRs) and oxygen-containing function groups) in Cr(VI) removal under varying pH and O2 conditions remain poorly understood. In this study, HNO3 oxidation caused an obvious increase in specific surface area, porous volume, RAMs content, and surface potential of the biochar, leading to the more effective removal of Cr(VI) (with the removal rate reached 100% at pH 2.0) than that of the untreated biochar. Kinetics experiments revealed that O2 and pH are of great importance for the reduction efficiency and rate of Cr(VI). RAMs on NBC can either directly reduce Cr(VI)(predominant pathway) or activate O2 to produce •O2− for indirect Cr(VI) reduction. In addition, we examined the changes in the compositions of RAMs during the reaction by tuning the RAMs compositions using methanol and hydrogen peroxide. The results of electron paramagnetic resonance and X-ray photoelectron spectroscopy analysis demonstrated that the main electron donors on NBC were different at different pH values: oxygen-containing groups, e.g., –OH and C–O–C, played a dominant role in reducing Cr(VI) under acidic conditions while the neutral condition was beneficial to EPFRs-dominated reduction. This study investigated the roles of the EPFRs and oxygen-containing function groups on HNO3 modified biochar, which may provide new insights into the promoted reduction of Cr(VI) by applications of biochar.