Reactive migration mechanism of Fe2+ and As3+ during groundwater discharge into river water: Insight from sand column experiments

Abstract

During groundwater discharge into river water, the Fe2+ and As3+ in anoxic groundwater are brought into the oxygen-bearing interaction zone. However, the reactive migration and transformation mechanism of Fe2+ and As3+ in the interaction zone are poorly understood. This study is based on sand column experiments to simulate the process of groundwater discharge into river water. Results show that the chemical oxidation and precipitation of Fe2+ in the interaction zone mediated the migration and transformation of Fe2+ and As3+. Fe2+ is oxidized by dissolved oxygen in the pore water of the interaction zone and hydrolyzed to form precipitated Fe oxides, whose 90% were poorly crystalline. The formation of Fe oxides seriously hindered the migration of Fe2+ and As3+ to river water, creating a “Fe and As enriched area” near the side edge of groundwater flow in the interaction zone. About 70% of the As3+ was converted to As(V), and it is primarily retained through adsorption on the surface of Fe oxides or coprecipitation with Fe into the poorly crystalline Fe oxides. At the same time, As3+ significantly promoted the migration of Fe2+ by inhibiting the accumulation of Fe(II) on the surface of secondary minerals and the transformation of poorly crystalline Fe oxides into iron minerals with higher crystallinity. Fe2+ and As3+ concentrations and groundwater velocity impacted the comigration of Fe2+ and As3+ in riparian sediment. This study reflects that Fe2+ controls the fate of As3+ in groundwater during groundwater discharge into river water, which is significant for water quality protection.