Exothermic adsorption of oxo-anions by goethite

Column and field tests related to ATES found significantly elevated Mo, V, and other oxo-anions that could not be explained by reductive dissolution of Fe oxyhydroxides. A common hypothesis levied was that the oxo-anion mobilization appears to be related to a thermal desorption process. However, for an accurate prediction of the concentration changes, there is a lack of thermodynamic parameters to prove that hypothesis which was the aim of this study.

Batch equilibrium adsorption experiments with oxo-anions such as molybdate and vanadate were performed using goethite suspensions with different concentrations, ionic strengths, pH values, and at four temperatures between 10 and 75 °C. The results of this large number (>500) of individual batch equilibrium experiments showed that the amount of an oxo-anion adsorbed decreased with increasing temperature. The experimental data were fitted using the CD-MUSIC surface complexation model framework. Temperature variations in the complexation constants were in turn fitted using the two-term van’t Hoff equation to obtain molar enthalpies and entropies. The enthalpies were negative, indicating that the adsorption of the oxo-anions is exothermic and therefore the adsorption affinity decreases with increasing temperature. The entropies could be correlated to the adsorbate molecule volumes together with those previously determined for other oxo-anions, which can be used to extrapolate the adsorption entropies of many other oxo-anions for which EXAFS-based adsorbate structures are available, but for which such data are not yet available. Hydration differences across the bivalent oxo-anion molecule series apparently affect the derived enthalpies and hence the adsorption energies for oxo-anions.