The effect of oxyanions on the oxalate-promoted dissolution of goethite

Organic ligand-promoted dissolution of oxide minerals can be enhanced dr inhibited in the presence of specifically adsorbed oxyanions, It has been proposed that an oxyanion inhibits or enhances dissolution depending on the type of surface complex formed and the strength of the bond, Mononuclear complexes (especially if they are bidentate) accelerate dissolution, while binuclear complexes inhibit dissolution. Recent spectroscopic evidence indicates that chromate and arsenate form different surface complexes depending on surface coverages, This study examined the influence of chromate and arsenate on the oxalate promoted dissolution of goethite, Based on a previous spectroscopic study, oxyanion surface coverages were varied to generate both Mononuclear and binuclear surface complexes. Chromate and arsenate inhibited the oxalate promoted dissolution of goethite at all surface coverages investigated except at pH 6 (Gamma(arsenate) = 1.70 X 10(-6) and Gamma(chromate) = 1.66 X 10(-6) mol m(-2)). It is proposed that chromate and arsenate inhibit goethite dissolution by decreasing oxalate adsorption. This is accomplished because arsenate and chromate are more effective competitors for goethite surface sites than oxalate and upon adsorption increase the negative charge of the goethite surface. At pH 6 the adsorption of chromate and arsenate increases the negative charge Of the goethite surface which in turn increases proton adsorption. Since proton adsorption is a necessary step for oxalate-promoted dissolution:of goethite, and since proton activity at pH 6 is low, an increase in the negative charge of goethite upon adsorption of the oxyanions accelerates dissolution.