Exogenous arsenic redox and methylation transformation in anoxic-oxic alternating acidic paddy soil: Quantitative insights into chemical and biological processes

Both chemical and biological processes can mediate arsenic (As) transformation. However, quantitative evaluation of key processes remains lacking during As redox and methylation transformation in soil. Here, the chemical and biological processes of exogenous iAs(III) or iAs(V) transformation in a paddy soil under anoxic-oxic alternating conditions were evaluated via establishing a kinetic model to fit the temporal changes of As, Mn/Fe/S/C/H 2 O 2 , and As redox/methylation genes. For exogenous iAs(III), 8.4% of total iAs(III) was oxidized dominantly by Mn(IV) during the anoxic stage. For exogenous iAs(V), 24.3% of total iAs(V) was reduced by bacteria. Among them, dissolved iAs(V) reduction via both dissimilatory and cytoplasmic iAs(V) reductases showed the highest reaction rates, controlling anoxic iAs transformation in both cases. For both cases, the oxidation of adsorbed iAs(III) by ·OH dominated during the oxic stage. Methylated As (Met-As) accounted for 4.8%–7.2% of total As during anoxic-oxic incubation, in which the MMA-to-DMA transformation was faster than other methylation processes, resulting in DMA as the dominant Met-As species. Additionally, all the As methylation processes were accelerated under oxic conditions. Geobacter and Hoeflea were identified as potentially key iAs(V)-reducing bacteria, while Rhodopseudomonas and Neotoma mediated anaerobic and aerobic As methylation, respectively. These findings provide a quantitative and deeper understanding of exogenous iAs transformation in a redox-oscillating environment.