Ferrihydrite Reduction Increases Arsenic and Uranium Bioavailability in Unsaturated Soil

Abstract

Redox driven mobilization and plant uptake of contaminants under transiently saturated soil conditions need to be clarified to ensure food and water quality across different irrigation systems. We postulate that solid-phase iron reduction in anoxic microsites present in the rhizosphere of unsaturated soil is a key driver for mobilization and bioavailability of contaminants under nonflooded irrigation. To clarify this, two major crops, corn and soybean differing in iron uptake strategies, were grown in irrigated synthetic soil under semiarid conditions with gravimetric moisture content ∼12.5 ± 2.4%. 2-line ferrihydrite, which was coprecipitated with uranium and arsenic, served as the only iron source in soil. Irrespective of crop type, reduced iron was detected in pore water and postexperiment rhizosphere soil confirming ferrihydrite reduction. These results support the presence of localized anoxic microsites in the otherwise aerobic porous bulk soil causing reduction of ferrihydrite and concomitant increase in plant uptake of comobilized contaminants. Our findings indicate that reactive iron minerals undergo reductive dissolution inside anoxic microsites of primarily unsaturated soil, which may have implications on the mobility of trace element contaminants such as arsenic and uranium in irrigated unsaturated soils, accounting for 55% of the irrigated area in the US.

Effects and Mechanisms of Copper Oxide Nanoparticles with Regard to Arsenic Availability in Soil–Rice Systems: Adsorption Behavior and Microbial Response

“…A high dose of NPs could enhance the reduction of Fe­(III) by promoting long-distance electron transport in flooded soil . Iron plaques decreased as a result of the decrease in multiple anaerobic sites and ferrihydrite in unsaturated soil, thereby increasing As availability. , Therefore, a high dose of CuO NPs promoted the decrease of Fe plaques during flooding and flooding–drying cycles.…”

“…39 Iron plaques decreased as a result of the decrease in multiple anaerobic sites and ferrihydrite in unsaturated soil, thereby increasing As availability. 36,57 Therefore, a high dose of CuO NPs promoted the decrease of Fe plaques during flooding and flooding−drying cycles.…”

Microbial iron reduction under oxic conditions: when microfluidics meets geochemistry

“…This phenomenon drives contaminant spreading and nutrient cycling in anoxic environments such as ooded soils 68 , deeper groundwater systems 69,70 , and anoxic microsites. 71 Our ndings suggest that microbially induced ferrihydrite reduction could play an unaccounted role in oxic environments. For example, microbial dissolution of ferrihydrite under oxic conditions could be the culprit of arsenate and chromate contamination observed in different oxic aquifers 72 .…”

Consider the Anoxic Microsite: Acknowledging and Appreciating Spatiotemporal Redox Heterogeneity in Soils and Sediments

“…130 Similarly, rhizosphere-associated anoxic microsites have been shown to enhance ferrihydrite dissolution and mobilize adsorbed As and U, facilitating plant uptake of these contaminants. 131 Redox gradients within anoxic microsites control the colocation of Fe, Mn, and other reactants, thereby controlling which Fe and Mn transformations occur within or near anoxic microsites. Several Fe and Mn redox reactions are facilitated at redox interfaces.…”

“…For example, in agricultural soils, anoxic microsites in a compacted plough layer drove reduction of Fe–oxides and Mn–oxides and comobilized P into soil water . Similarly, rhizosphere-associated anoxic microsites have been shown to enhance ferrihydrite dissolution and mobilize adsorbed As and U, facilitating plant uptake of these contaminants …”