Investigation of the effect of manganese oxides on the reduction of Hexavalent chromium by sodium alginate dispersed nano zero-valent iron and the mechanism

Abstract

Abstract One of the most typical pollutants in soil is chromium (Cr). The remediation of Hexavalent chromium [Cr(VI)] contaminated soils by sodium alginate modified nano zero-valent iron (SA-NZVI) has been extensively studied, while naturally occurring oxidants (manganese oxides) have been found to affect its effectiveness. Therefore, in this paper, by exploring the effect of manganese minerals on chromium reduction, it was found that acid birnessite (A-Bir) has the greatest effect on the oxidation of Cr(III) and also finds that A-Bir affects the stability of SA-NZVI and Cr(VI) reduction products. In the remediation of Cr(VI)-contaminated soils, A-Bir significantly promoted the removal of Cr(VI) by SA-NZVI in the first 7 d and inhibited the reduction of Cr(VI) or re-oxidizes the reduced Cr(VI) and releases it into the environment after 14 d. This process caused a rise in soil pH, which in turn led to a decreased in the effectiveness of Fe in the soil and a significant increased in the effective state of Mn. The investigation also revealed that the re-rise in Cr(VI) content could be due to a reaction between dissolved Cr(III) in the reduction products and the solid A-Bir surface, or it could be due to a reaction between the Mn dissolved out of the A-Bir in the soil and the reduction products. This study shows that A-Bir is a key factor influencing the remediation of Cr(VI)-contaminated soils by SA-NZVI.