RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Sulfate, a major anion in nature, can affect the mobility of coexisting dissolved toxic trace elements by influencing the adsorption properties of the species on mineral surfaces. Ferrihydrite is an important scavenger for trace elements in natural water because of its adsorption capacity and its ubiquitous formation in surface conditions. To elucidate the adsorption behavior of the trace elements in natural water, it is important to construct a predictive model of sulfate adsorption on ferrihydrite that can predict the adsorption behavior of sulfate quantitatively under widely various environmental conditions based on the spectroscopic information of sulfate adsorption.

In situ infrared spectra for sulfate adsorption on ferrihydrite as a function of pH (3–7), ionic strength (I = 0.01 and 0.1) and sulfate loading ([SO₄^2-] = 0.1 and 0.2 mM) were obtained to constrain the surface speciation of sulfate on ferrihydrite. The shape of the spectra was pH-dependent. The degree of ν₃ band splitting decreases with pH. Little difference of the spectra was found between different ionic strengths and sulfate loadings for the same pH. The little influence of ionic strength on the IR spectra reflects that the inner sphere and outer sphere species most likely do not exist simultaneously in the same pH conditions. Based on the IR spectra obtained from lower pH showing the splitting of ν₃ band to two peaks with activation of ν₁ band, the surface species is identified to be single inner sphere monodentate sulfate. The changes of spectra with pH are most likely attributable to the changes of the electric field strength posed to the sulfate on ferrihydrite surface, which is strongly pH-dependent.

The predictive model for sulfate adsorption was constructed using an extended triple-layer model (ETLM). The pH adsorption edges and proton surface charges in the presence of sulfate as a function of ionic strength and sulfate concentration were obtained, respectively, from batch adsorption and acid–base titration experiments. The sulfate adsorption on ferrihydrite increases continuously with decreasing pH and ionic strength. These macroscopic adsorption data were analyzed using ETLM to retrieve the sulfate adsorption reaction and the equilibrium constant. Results of ETLM analyses showed that adsorption of sulfate on ferrihydrite is a single monodentate inner sphere process that is consistent with in situ infrared spectroscopic observation, as expressed by the following complexation reaction:

2>FeOH + H⁺ + SO₄^2- = (>FeOH)(>FeOSO₃^-) + H₂O ,

where >FeOH denotes a surface hydroxyl. Batch adsorption data from earlier studies of sulfates on ferrihydrite were reasonably reproduced using ETLM with the same adsorption reaction and equilibrium constant.

Prediction of the effect of sulfate on trace dissolved anionic species adsorption on ferrihydrite was conducted using previously reported adsorption parameters for iodine and arsenic. The predictions showed that the adsorption of I⁻ and As(V) on ferrihydrite were diminished because of the competition with sulfate, although the adsorption of IO₃^- and As(III) were enhanced in the presence of sulfate. The sulfate surface species ((>FeOH)(>FeOSO₃^-)) possesses net negative charge. To compensate the negative surface charge, the formations of net positive surface species, which are dominant IO₃^- and As(III) surface species, are enhanced. The predictions demonstrated that sulfate strongly influences the adsorption behavior of trace anionic species on a ferrihydrite surface.