Proton adsorption at an adularia feldspar surface

Abstract

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 (CH 3) 4NCl concentrations, suggesting that (CH 3) 4N + cannot compete with H + for the K + exchange sites. The relatively large diameter of (CH 3) 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.