Surface complexation modelling of arsenate and copper adsorbed at the goethite/water interface

The co-adsorption of Cu(II) and arsenate onto the surface of goethite has been studied by performing adsorption experiments and potentiometric titrations, and a surface complexation model has been developed to describe the experimental results. Models for the binary systems, Cu-goethite and arsenate-goethite, were acquired separately and the model parameters were then included in the ternary system, together with the solubility products of solid Cu(II) arsenates reported in the literature. The adsorption of Cu(II) was described applying a model in which Cu(II) forms bidentate bridging mono- and binuclear surface complexes. According to recent interpretations of ATR-FTIR and EXAFS data the arsenate ions are assumed to be coordinated in a monodentate fashion to singly coordinated hydroxyl groups at the surface, and hydrogen-bonded to neighbouring triply coordinated surface oxide sites. In the case of co-adsorption of Cu(II) and arsenate, the adsorption could not be predicted by applying the combined model from the two binary systems. Two ternary Cu(II)-arsenate-goethite surface complexes must be included, one complex in which an arsenate ion is coordinating to surface Fe(III) (FeOAsO3Cu0.5−) and one complex in which arsenate is bound to the surface by coordinating to an adsorbed Cu(II) ion ((Fe3OFeOH)Cu2(OH)2HAsO41-). No solid Cu(II) arsenate phases were formed under the experimental conditions in the present study. From constructed predominance area diagrams, the significance of adsorption and precipitation processes are discussed. Furthermore, calculated solubility of Cu(II) and As(V) is used to indicate optimum conditions for the cleaning of contaminated natural waters.