Effect of low molecular weight organic acids on the oxidation of Cr(III) by nano-MnO2.

The oxidation of Cr(III) to Cr(VI), a prevalent environmental process, poses significant challenges for pollution control due to the heightened toxicity and mobility of Cr(VI). However, the influence of environmental co-factors, such as low molecular weight organic acids (LMWOAs), on the oxidation process of Cr(III) remains to be verified. In this study, laboratory experiments were conducted to investigate how LMWOAs would affect the Cr(III) oxidation by MnO2. Two types of MnO2 were synthesized via hydrothermal methods and characterized. Three LMWOAs (citric acid (CA), oxalic acid (OA), and malic acid (MA)) were added to the Cr(III)-MnO2 system to exam their influences. The results revealed that Cr(III) oxidation by MnO2 were suppressed by LMWOAs, following the order of CA > MA > OA. LMWOAs primarily inhibited Cr(III) oxidation through complexation and competitive adsorption site occupation, while their reductive capacity minimized the accumulation of the oxidation product Cr(VI). Furthermore, LMWOAs altered the physical characteristics of MnO2, including surface charge and specific surface area, which affected Cr(III) adsorption. LMWOAs, as a reducing agent, additionally consumed the oxidation ability of MnO2, thereby creating a competitive relationship with Cr(III) in redox reactions. These findings elucidate the critical roles of LMWOAs in regulating Cr speciation in MnO2-active systems. By bridging the knowledge gap on ligand-mediated Cr transformation mechanisms, this work provides insights into environmental transformation of Cr and advances the development of LMWOAs-associated strategies for Cr pollution control.