Investigation of the Effect of Manganese Oxides on the Reduction of Hexavalent Chromium by Sodium Alginate-Dispersed Nano-Zero-Valent Iron and the Mechanism

Chromium (Cr) is a prevalent soil contaminant, and sodium alginate-modified nano-zero-valent iron (SA-NZVI) has been studied for the remediation of Cr(VI)-contaminated soils. However, the addition of manganese oxides during remediation may result in the re-release of Cr(VI), and the mechanism behind this process remains unclear. Therefore, this paper investigates the effect of different manganese oxides on the oxidation of Cr(III), the impact of manganese oxide on the reduction products of Cr(VI) and SA-NZVI, and the effect of manganese oxide addition on the remediation of Cr(VI)-contaminated soil by SA-NZVI. Solution incubation experiments, XRD characterization, and soil incubation experiments were conducted to analyze the results. Our findings show that acid birnessite (A-Bir) has the lease of Cr(VI) most influence on Cr(III) oxidation and affects the stability of the SA-NZVI and Cr(VI) reduction products. In the remediation of Cr(VI)-contaminated soils, the presence of A-Bir increases the soil pH and available Mn content and reduces the available Fe content, resulting in the re-release of Cr(VI), but the Cr converted to residue form by SA-NZVI is difficult to oxidize by A-Bir. This study suggests that A-Bir plays a crucial role in the re-release during the remediation of Cr(VI)-contaminated soils by SA-NZVI.