Do Fe-oxides Control the Adsorption of Arsenic in Aquifers of the Red River Floodplain, Vietnam?☆

Abstract Adsorption of As(III) to aquifer sediments from the Red River floodplain, Vietnam, has been studied with the aim of identifying controlling parameters for the adsorption. The adsorption capacity differs between similar sediments, indicating that it is not related to bulk components of the sediment but rather that certain properties of the sediments are important for the adsorption. Normalizing the adsorption of As(III) to the content of Fe-oxides in the sediments results in almost identical adsorption isotherms, indicating that Fe-oxides are the main adsorbents for As(III) in these three reduced aquifer sediments. This is further strengthened by adsorption experiments carried out with sediments leached for Fe-oxides, as this resulted in reduced adsorption capacity and despite the difference in adsorption capacity of the untreated sediments, the adsorption capacity became identical after Fe-oxide leaching. The importance of Fe-oxides for As(III) adsorption to reduced aquifer sediments has implications for the mobility of As(III) in the groundwater aquifers, as it will vary spatially and also over time, as the Fe-oxides in the sediment are reduced.