Temperature-dependent adsorption by goethite of molybdate and vanadate

Abstract

Goethite, a common Fe(III) oxyhydroxide found in soils and aquifers, is employed in water purification technology due to its high oxo-anion adsorption capacity. In a series of batch equilibrium experiments, goethite suspensions were subjected to varying ionic strengths, concentrations of vanadate and molybdate, and pH values ranging from 3 to 10. These experiments were also carried out at four distinct temperatures, ranging from 10 to 75 ◦C. The results from over 1400 individual batch equilibrium experiments revealed that the adsorption of vanadate and molybdate generally decreases as the pH increases, as is typically observed for anion adsorption envelopes. However, this decrease was found to be less pronounced for vanadate compared to molybdate, with only the latter showing a sharp adsorption envelope decline at circumneutral pH values near the point of zero charge of the goethite surface. To interpret these findings, the Charge Distribution Multisite Surface Complexation (CD-MUSIC) model framework was employed. According to the model fit, the adsorption of molybdate is primarily governed by an outer-sphere surface complex at circumneutral pH conditions, which remains relatively unaffected by temperature variations, indicating a nearly zero enthalpy of reaction. Similarly, for vanadate, there was also no significant temperature effect, but the reasons differed. For the two inner-sphere vanadate surface complexes, a linear decrease in logK values with temperature was observed, suggesting an exothermic adsorption reaction with negative enthalpy as typical for inner-sphere binding of oxyanions. This was counterbalanced by an increase in logK with temperature for the strong dioxovanadium(V) cation surface complex, which exhibited a positive enthalpy and thus an endothermic reaction typical for cations. As a result, the overall uptake of vanadate by goethite was found to be almost temperature independent as well.