Proton adsorption at an adularia feldspar surface

The concentration of H⁺ which reacts with an adularia surface, [H_S⁺], was measured with acid-base titrations of adularia powder-water suspensions. Due to the complexity of feldspar surface reactions, it was necessary to calculate a H⁺ mass balance in order to separate the fractions of H⁺ involved in cation exchange reactions, [H_ex⁺]; dissolution reactions, [H_dis⁺]; and adsorption at surface hydroxyl sites, [H_ads⁺]. Reproducibility of acid and base titrations of H_S⁺ was pH-dependent, ranging from ±3 μmol H⁺ m^-2 at pH 4 to ±1.5 μmol H⁺ m^-2 at pH > 6.5. This departure was due to the exchange of K_fsp⁺ for H_aq⁺, which was not completely reversible under the conditions of our experiment. Reproducibility of acid and base titration curves for [H_ads⁺] vs. pH was ± 1.5 μmol m^-2, suggesting the H⁺ adsorption reaction was reversible. Fifteen μmol H⁺ m^-2 reacted with the washed feldspar surface during an acid titration from pH 10 to pH 4, in distilled water. 50-60% of the total is attributed to cation exchange, which is estimated to take place at >3 Å depth within the surface, suggesting the near-surface is porous, and that H⁺ reacts with sites within the surface pores as well as at the external surface. Less than 5% of [H_S⁺] was due to [H_dis⁺], and the remainder to [H_ads⁺]. [H_ex⁺] decreases with increasing concentrations of NaCl, presumably because of competition between the solution ions, H⁺ and Na⁺, for K⁺ exchange sites in the feldspar. [H_ex⁺] is independent of (CH₃)₄NCl concentrations, suggesting that (CH₃)₄N⁺ cannot compete with H⁺ for the K⁺ exchange sites. The relatively large diameter of (CH₃)₄N⁺ probably prohibits it from penetrating the pores of the adularia surface; therefore, it cannot access exchange sites within the pores which are available to the smaller H⁺, Na⁺, and K⁺ ions. Feldspar dissolution rates, often modeled as rate = k_H[H_ads⁺]ⁿ, where k_H = the rate constant, and n = the reaction order, have been observed to decrease with increasing ionic strength. Because we observe an ionic strength dependence in [H_ex⁺], rather than [H_ads⁺], we suggest a rate model where rate = k_H[H_ex⁺]ⁿ. This expression emphasizes that dissolution rates are dependent upon K⁺-H⁺ exchange at the feldspar surface, and that rates decrease with increasing {Na⁺} due to competition between Na⁺ and H⁺ for the surface exchange site.