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 mu mol H+ m(-2) at pH 4 to +/-1.5 mu mol 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 mu mol m(-2), suggesting the H+ adsorption reaction was reversible. Fifteen mu mol 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 Angstrom 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)(4)NCl concentrations, suggesting that (CH3N+ cannot compete with H+ for the K+ exchange sites. The relatively large diameter of (CH3N+ 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.