Properties and Reactivity of Iron-Organic Matter-Arsenic Composites and their Influence on Arsenic Behavior in Microbial Reduction and Oxidation Processes

The biogeochemistry of arsenic in soils is strongly controlled by iron oxides and soil organic matter (SOM). The present study intends to elucidate the behavior of arsenic in Fe-SOM-As composites formed through adsorption or coprecipitation under redox conditions. The X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) showed that crystalline minerals were generated during Fe-HA-As coprecipitation, while other composites exhibited an amorphous structure. In an anoxic environment, iron-reducing bacteria reduced Fe(III) and As(V) to Fe(II) and As(III), respectively, enhancing the mobility of arsenic. The presence of SOM increased the concentrations of dissolved Fe(II) and As(III) through complexation. Notably, elevated As(III) and reduced Fe(II) were observed in the HA-containing coprecipitation group due to the weak adsorption capacity of crystalline minerals, which released As(V) into solution and competed with Fe(III) for electrons. Under oxic conditions, superoxide, hydrogen peroxide, and hydroxyl radical (^•OH) were formed through the oxidation of Fe(II) and reduced SOM. As(III) was subsequently oxidized by superoxide and ^•OH, and the process was dominated by ^•OH. Substantial ^•OH in the HA-containing coprecipitation group mainly oxidized dissolved As(III), while limited ^•OH in other groups contributed greater to adsorbed As(III). These findings contribute substantially to understanding the mechanisms of the coupled transformation of iron and arsenic in soil under fluctuating redox conditions.