Ion Adsorption and Desorption at the CaF₂‐Water Interface Probed by Flow Experiments and Vibrational Spectroscopy

Abstract

The dissolution of minerals in contact with water plays a crucial role in geochemistry. However, obtaining molecular insight into interfacial chemistry is challenging. Dissolution typically involves the release of ions from the surface, giving rise to a charged mineral surface. This charge affects the interfacial water arrangement, which can be investigated by surface-specific vibrational Sum Frequency Generation (v-SFG) spectroscopy. For the fluorite-water interface, recent spectroscopic studies concluded that fluoride adsorption/desorption determines the surface charge, which contrasts zeta potential measurements assigning this role to the calcium ion. By combining v-SFG spectroscopy and flow experiments with systematically suppressed dissolution, we uncover the interplay of dominant fluoride and weak calcium adsorption/desorption, resolving the controversy in the literature. We infer the calcium contribution to be orders of magnitude smaller, emphasizing the sensitivity of our approach.

Ion-Specific Interactions at a Mineral-Water Interface Revealed by Surface-Sensitive Spectroscopy under Flow Conditions

“…We also comment in S3 of the SI on the small effect of the switch between ions on the spectral shape. As shown and discussed in previous studies, − the spectrum obtained under flow-on conditions is substantially increased in its spectral intensity. As shown in Figure b, we quantify this increase by showing the v-SFG intensity (integral over the OH stretch region between 2500 and 4000 cm –1 ) as a function of time over one flow-on/off cycle.…”

“…Due to the excess of adsorbing anions, the dilution of dissolved fluoride anions upon flow becomes negligible. This has also been shown recently by varying the added amount of fluoride . With an excess of adsorbing anions (here X = I), the equilibrium from eq does not change upon flow.…”

“…This observation has been explained in our previous work by an increased surface charge upon flow 19 since flow reduces the interfacial concentrations of the dissolved fluoride and calcium ions due to dilution. 17,18 This concentration changes shift the adsorption/desorption equilibria (eqs 3 and 4) to the righthand side, where ≡ denotes the solid CaF 2 subphase.…”

“…Due to the dominance of the fluoride adsorption/desorption reaction under the investigated conditions, those observations can be mainly assigned to enhanced fluoride dissolution upon the reduced fluoride concentration during the flow-on period. 17 Consequently, the surface charge is increased upon flow, which gives rise to higher v-SFG intensities.…”

“…In some experiments, the v-SFG intensity shows an unspecific drift, on top of which the flow-induced changes occur. To extract the flow-induced changes, the data were corrected as described in ref . The relative changes were calculated based on averaging the corrected intensities of the last minute before switching between flow on/off, where an equilibration of the concentration and surface charges can be assumed.…”