Interactions of arsenate and arsenite with iron oxides: Calorimetry studies and diffuse double layer model

Arsenic is a naturally occurring element with health risks related to its ingestion. The fate of arsenic in the environment is dependent on a variety of complex interactions with soil and sediment constituents. These interactions are further complicated by the existence of numerous species including two inorganic oxidation states, among others. Furthermore, the different oxidation states can coexist due to slow redox kinetics and seasonal changes. Therefore, a better understanding of the fate of arsenic in the environment can be gained by the study of the interactions of arsenic with the individual soil constituents in both single and competitive systems where the different oxidation states can interact simultaneously with the soil fractions.; The interactions of arsenate (As(V)) and arsenite (As(III)) with goethite(α-FeOOH) and hematite (α-Fe₂O₃) were studied both in single sorbate and binary sorbate systems. Differential scanning calorimetry was used to quantify the effects of pH on the interaction energies and the diffuse double layer model (DDLM) was used to predict the competitive adsorption of As(V) and As(III) on the oxides.; Changes in DSC energy of the dehydroxylation of goethite in the presence of sorbed arsenic indicated that pH affects the surface arsenic interactions where monodentate surface complexes may be more favorable with increasing pH for both As(V) and As(III). In addition, single sorbate DDLM derived intrinsic equilibrium constants were successful in predicting As(V) adsorption on goethite in the presence of As(III). However, the accuracy of the predications for As(III) adsorption in the presence of As(V) were dependent on background electrolyte concentration. This indicated that the nature of As(III) surface interactions was affected by the presence of As(V) where the formation of As(III) outer-sphere complexes may have increased. The modeling of competitive As(V) and As(III) adsorption on hematite was not successful due to enhanced dissolution of hematite in the presence of As(III) where single sorbate equilibrium constants for As(III) adsorption on goethite could not be determined. No ferrous iron was detected for the enhanced dissolution. Thus, enhanced dissolution was due to the formation of favorable surface complexes rather than reductive dissolution. Furthermore, no enhanced dissolution occurred for goethite in the presence of As(V) or As(III).