Effects of Arsenic–iron–dissolved Organic Matter Interactions on Arsenic Mobilization: Insight from Column Experiments

Arsenic (As)-iron (Fe)-organic matter (OM) compound is critical for the mobilization of As under irrigation conditions. We explore the interaction mechanism among As-Fe-OM components for different reaction pathways in column experiments under irrigation conditions using ferrihydrite-coated sand, sodium humate (HA), and As (III). Arsenic mobilization is strongly controlled by the adsorption of As on Fe minerals, which in turn is affected by the presence of OM. In the pure ferrihydrite-coated sand column, outlet As and HA concentrations gradually increased and reached a stable state. In the ferrihydrite-As-coated sand columns, the release of As occurred with HA infiltration and the HA concentrations influenced the release rate of As from the ferrihydrite. The results suggest OM interacts with As and Fe minerals to promote As mobility, with the concentration OM influencing the amount of As released. Although competitive adsorption occurred in the presence of OM, ferrihydrite acts as bridges between As and OM to form As-Fe-OM ternary complexes and further affected the mobilization of As. These results are important for evaluating the migration of As under agricultural activities using high-As groundwater for irrigation and indicate managing OM and As-contaminated water for agricultural activities is required to limit impacts on human health.