Effects of Dissolved Calcium On Arsenate Sorption at the Kaolinite-Water Interface.

Arsenic(As) is commonly present in aquatic-terrestrial environments due to indigenous sources and anthropogenic inputs. Understanding the solid-water interfacial reactions of As is one of the most critical information in assessing the environmental risk in the subsurface environment and for the development of remediation strategies. There are a number of studies describing As partitioning processes in soils and geologic components. Arsenate(As(V)) adsorption generally increases with decreasing pH on amorphous Al(OH)₃, a-Al₂O₃, ferrihydrite and hematite. Several spectroscopic studies indicated the inners-sphere adsorption mechanism of As(V) and both inner-sphere and outer-sphere adsorption mechanisms for As(III) on metal oxyhydroxide and phyllosilicate mineral surfaces. While these macroscopic and spectroscopic observations in binary systems (i.e., single adsorbate vs adsorbent) have provided significant insight in As reactivity in geomedia, the results are often difficult to apply to the natural systems, since chemical compositions are generally more complex in soils and aquifer. Of all, alkaline earth metals and transition metals are commonly present at As contaminated sites. Several reports indicate that the presence of Ca increases the adsorption of As(V)/P(V) on iron hydrous oxides. Although some authors have argued that the Ca enhanced As(V) adsorption is attributed to electrostatic forces, others have speculated that these macroscopic observations must be due to ternary complex formation since the solvation energy forces at the interface should enhance the stability of As(V)-Ca ternary adsorbate complexes. To better understand the As reactivity in these complex systems, we investigate the effects of dissolved Ca²⁺ on As(V) adsorption and surface speciation at the kaolinite-water interface using batch adsorption experiments and X-ray Absorption spectroscopy. The results will be compared with the previous hypothesis to explain the enhanced As(V) adsorption mechanisms by Ca ions.