Role of MnO2 in controlling iron and arsenic mobilization from illuminated flooded arsenic-enriched soils

Abstract

This study examined the role of intermittent illumination/dark conditions coupled with MnO2-ammendments to regulate the mobility of As and Fe in flooded arsenic-enriched soils. Addition of MnO2 particles with intermittent illumination led to a pronounced increase in the reductive-dissolution of Fe(III) and As(V) from flooded soils compared to a corresponding dark treatments. A higher MnO2 dosage (0.10 vs 0.02 g) demonstrated a greater effect. Over a 49-day incubation, maximum Fe concentrations mobilized from the flooded soils amended with 0.10 and 0.02 g MnO2 particles were 2.39 and 1.85-fold higher than for non-amended soils under dark conditions. The corresponding maximum amounts of mobilized As were at least 92 % and 65 % higher than for non-amended soils under dark conditions, respectively. Scavenging of excited holes by soil humic/fulvic compounds increased mineral photoelectron production and boosted Fe(III)/As(V) reduction in MnO2-amended, illuminated soils. Additionally, MnO2 amendments shifted soil microbial community structure by enriching metal-reducing bacteria (e.g., Anaeromyxobacter, Bacillus and Geobacter) and increasing c-type cytochrome production. This microbial diversity response to MnO2 amendment facilitated direct contact extracellular electron transfer processes, which further enhanced Fe/As reduction. Subsequently, the mobility of released Fe(II) and As(III) was partially attenuated by adsorption, oxidation, complexation and/or coprecipitation on active sites generated on MnO2 surfaces during MnO2 dissolution. These results illustrated the impact of a semiconducting MnO2 mineral in regulating the biogeochemical cycles of As/Fe in soil and demonstrated the potential for MnO2-based bioremediation strategies for arsenic-polluted soils.