Proton adsorption at an adularia feldspar surface

Lisa L. Stillings, Susan L. Brantley, Michael L. Machesky

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Abstract

The concentration of H+ which reacts with an adularia surface, [HS+], 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, [Hex+]; dissolution reactions, [Hdis+]; and adsorption at surface hydroxyl sites, [Hads+]. Reproducibility of acid and base titrations of HS+ 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 Kfsp+ for Haq+, which was not completely reversible under the conditions of our experiment. Reproducibility of acid and base titration curves for [Hads+] 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 [HS+] was due to [Hdis+], and the remainder to [Hads+]. [Hex+] decreases with increasing concentrations of NaCl, presumably because of competition between the solution ions, H+ and Na+, for K+ exchange sites in the feldspar. [Hex+] 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 = kH[Hads+]n, where kH = 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 [Hex+], rather than [Hads+], we suggest a rate model where rate = kH[Hex+]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.

Original languageEnglish (US)
Pages (from-to)1473-1482
Number of pages10
JournalGeochimica et Cosmochimica Acta
Volume59
Issue number8
DOIs
StatePublished - Apr 1995

All Science Journal Classification (ASJC) codes

  • Geochemistry and Petrology

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