Control of arsenic methylation in paddy soils by iron nanoparticles

Rice, as the most essential food grain, is frequently exposed to high concentrations of arsenic. Among the arsenic species, dimethylarsenate (DMAs(V)) is preferentially translocated from paddy soils to rice grains, posing serious threats to food safety and yield. Herein, we report an efficient strategy for DMAs(V) mitigation in paddy soils with nanoscale Zero-Valent Iron (nZVI). Species and concentrations of arsenic in paddy porewater were monitored during a 28-d soil-water incubation. Effects of nZVI dose towards microbial sulfate reduction and methane generation potential in paddy soils, which are crucial for arsenic methylation and demethylation, were analyzed via metagenomic sequencing. Results demonstrated that the maximal DMAs(V) concentration in paddy porewater decreased from 0.37 to 0.04 μM in arsenic-contaminated paddy soils with nZVI dose increasing from 0 to 5.0 g/kg. Accordingly, the maximal concentration of inorganic arsenite (iAs(III)), which is the precursor of DMAs(V), decreased from 1.39 to 0.23 μM. Furthermore, the application of nZVI reshaped the structure of microbial community in paddy soils. Specifically, the relative abundance of δ-proteobacteria involved in sulfate reduction, which is crucial for iAs(III) methylation, waned from 7.62 % to 3.17 %, while that of Methanomicrobia for DMAs(V) demethylation and methanogenesis proliferated from 7.03 % to 13.62 %, with nZVI dose increasing from 0 to 5.0 g/kg. Via simultaneous inhibition of DMAs(V) formation and acceleration of DMAs(V) transformation, nZVI efficiently controls the accumulation of DMAs(V) in paddy porewater. In conclusion, these findings prove the efficient performance for DMAs(V) mitigation with nZVI and uncover its underlying mechanisms.