Modeling desorption of arsenate from ferrihydrite-bearing geological materials

Desorption of arsenate from two-line ferrihydrite (HFO) contained in geological materials (flotation tailing, stream-bed sediment, top soil) from an abandoned mine area of Sardinia (Italy) has been studied by means of kinetic batch experiments using solutions with different anionic composition (phosphate, carbonate, sulfate, nitrate and chloride). Modeling of the experimental results with PHREEQC has indicated that carbonate, similarly to phosphate, is able to displace arsenate from HFO surface, acting as a strong competitor owing to inner-sphere type complexation with HFO surface. Simulation totally failswhen the carbonate surface complexes are not included in the model. In the case of the tailing experiments with carbonate solution, the best fit has been obtained in the model which includes dissolution of estimated amounts of scorodite and arsenopyrite as well. Moreover, modeling has stressed the bad quality of the adsorption constants for sulfate that leads to strong overestimation of arsenate desorption at pH 4 and underestimation at pH 7.5.