Hydrous ferric oxide: evaluation of Cd–HFO surface complexation models combining Cd K EXAFS data, potentiometric titration results, and surface site structures identified from mineralogical knowledge

Abstract

The surface properties of ferrihydrite were studied by combining wet chemical data, Cd K EXAFS data, and a surface structure and protonation model of the ferrihydrite surface. Acid–base titration experiments and Cd(II)–ferrihydrite sorption experiments were performed within 3<−log[H +]<10.5 and 0.5<[Cd t]<12 mM in 0.3 M NaClO 4 at 25 °C, where [Cd t] refers to total Cd concentration. Measurements at −5.5⩽log[Cd t]⩽−1.4 at fixed pH completed the wet chemical data set. The acid–base titration data could be adequately modeled by FeOH 2 +1/2− H +↔ FeOH −1/2, logk ( int) =−8.29, assuming the existence of a unique intrinsic microscopic constant, log k (int), and consequently the existence of a single significant type of acid–base reactive functional groups. The surface structure model indicates that these groups are terminal water groups. The Cd(II) data were modeled assuming the existence of a single reactive site. The model fits the data set at low Cd(II) concentration and up to 50% surface coverage. At high coverage more Cd(II) ions than predicted are adsorbed, which is indicative of the existence of a second type of site of lower affinity. This agrees with the surface structure and protonation model developed, which indicates comparable concentrations of high- and low-affinity sites. The model further shows that for each class of low- and high-affinity sites there exists a variety of corresponding Cd surface complex structure, depending on the model crystal faces on which the complexes develop. Generally, high-affinity surface structures have surface coordinations of 3 and 4, as compared to 1 and 2 for low-affinity surface structures.