Adsorption (AsIII,V) and oxidation (AsIII) of arsenic by pedogenic Fe–Mn nodules

Determining the nature of As adsorption and oxidation by naturally occurring iron and manganese oxides in soil is critical for understanding the fate of contaminant As in soils and formulating effective remediation strategies. In batch experiments, oxidation of As(III) and adsorption of As(III) and As(V) by pedogenic Fe–Mn nodule material were examined at various solution pH and As concentrations, and after pre-treatment with humic acid. An experiment using a factorial design showed that most of the variability in As adsorption could be explained by varying solution pH and As concentrations. A semi-mechanistic model was used to fit the adsorption capacity for As(III) and As(V) over a pH range from 6 to 8. Arsenic speciation analysis and reductive dissolution of metal oxides proved that almost all the As(III) added to untreated Fe–Mn nodule material was oxidized by manganese oxides. The adsorption and oxidation of As(III) by Fe–Mn nodule material was greatly reduced by a pre-adsorbed coating of humic acid. These results are useful in predicting the chemical reactions and fate of arsenic in soil.