Computer Science
Scientific paper
Apr 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59.1473s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 8, pp.1473-1482
Computer Science
4
Scientific paper
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 mol H + m -2 at pH 4 to ±1.5 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 mol m -2 , suggesting the H + adsorption reaction was reversible. Fifteen 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 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 ) 4 NCl concentrations, suggesting that (CH 3 ) 4 N + cannot compete with H + for the K + exchange sites. The relatively large diameter of (CH 3 ) 4 N + 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.
Brantley Susan L.
Machesky Michael L.
Stillings Lisa L.
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