Surface Interactions of Arsenite and Arsenate on Soil Colloids

ABSTRACT. This work presents a comprehensive picture of As adsorption on a range of soil colloidal particles in comparable conditions of pH and ionic strength to allow a direct comparison of the surface interactions, including surface charging phenomena. The highest adsorption capacity for arsenite [As(III)] was shown by the iron minerals, with a clear shift in the surface charge. Pure ferrihydrite could retain over 5.3 µmol m-2 of As(III); the adsorption density was reduced to 3.36 µmol m-2 when ferrihydrite was a covering film on kaolinite surfaces. Goethite adsorbed up to 2.3 µmol m-2 but this amount was nearly halved on Al-substituted goethite and was much lower for hematite. Arsenite adsorption on a poorly ordered Al hydroxide was lower than on poorly ordered and crystalline Fe (hydr)oxides and its adsorption on gibbsite, kaolinite and calcite was almost negligible. When arsenate [As(V)] was added, a similar adsorption capacity for surface unit was shown by both Fe and Al minerals and, differently from As(III), As(V) was adsorbed also on kaolinite and calcite. The adsorption of arsenate on all adsorbents and of arsenite on Fe minerals were observed to be exothermic reaction. The differences in the affinity for As(III) and As(V) adsorption shown by the different soil components were explained in terms of differences in Lewis hardness/softness of adsorbate and adsorbents. These results provide a comparison between possible adsorbents to be used for remediation, and also to assess the As adsorption capacity of soils from their composition.