Characteristics of Fe oxide minerals and their roles in arsenic hydrogeochemistry in high arsenic aquifer sediments

Characteristics and reactivity of iron-bearing minerals control arsenic transfer and transformation in aquifer systems. In this study, aquifer sediments obtained from Inner Mongolia were investigated by kinetic experiments to study the connection between characteristics and reactivity of Fe-bearing minerals in aquifer sediments and As reactivity. Results showed that lithology controls the amount of total reactive Fe(Ⅲ) oxides (m0) in sediments. The value of m0 was 5.2 μmol/g in fine sand and 45 μmol/g in silty clay. The initial rate k′ values for both silty clay and fine sand were within the order of magnitude of 10-5 s-1. The value of parameter γ that characterizes heterogeneity of reactivity lay between synthetic Fe(Ⅲ) oxide minerals and shallow sediments. There may be two different Fe(Ⅲ) oxides in terms of reactivity in sediments: highly reactive Fe(Ⅲ) oxides and lowly reactive Fe(Ⅲ) oxides. Arsenic prefers to be adsorbed on the Fe(Ⅲ) oxides with high reactivity. In anoxic conditions, the reductive dissolution of Fe(Ⅲ) oxides with high reactivity in sediments led to arsenic release into groundwater.