Adsorption and desorption of arsenic on an oxisol and its constituents.

The present work investigates the adsorption and mobility (desorption) of As(III) and As(V) on an oxisol, and its main mineral constituents, as part of a broader project aimed at selecting a soil liner to be used in tailings dams at a sulfidic gold ore plant. Emphasis was given to a quantitative comparison of As mobility-here assessed by the amount of As leached from the loaded samples-under different experimental conditions. From among the soil constituents, goethite was the most efficient adsorbent with regard to arsenic adsorption, 12.4 mg x g(-1) for As(V) and 7.5 mg x g(-1) for As(III), respectively. Gibbsite also presented a relevant adsorption capacity (4.6 mg x g(-1) for As(V) and 3.3 mg x g(-1) for As(III)); adsorption on kaolinite was negligible (<0.23 mg x g(-1) for As(V) and As(III)). Desorption of the arsenic was shown to vary largely with the arsenic oxidation state, the adsorbents and the leaching solutions. While only 1-2% max. of As(V) was released from the loaded samples, leaching the A(III) reached 32%, the highest values corresponding to the solutions containing sulfate ions. Oxisol and goethite were superior to gibbsite with respect to As immobilization. Adsorption and mobility were also discussed with the help of electrophoretic mobility and isoelectric points (IEP) determined prior and following arsenic adsorption on goethite and gibbsite. The results indicated that As(V) is mainly adsorbed as an inner sphere complex. As(III) may be adsorbed as an inner or an outer neutral complex.