Stability of Cl^-, NO_3^-, ClO_4^-, and SO_4^- Surface Complexes at Synthetic Goethite/Aqueous Electrolyte Interfaces

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We carried out potentiometric titrations of a synthetic goethite in solutions of NaCl, NaNO_3, NaClO_4, and Na_2SO_4 to determine the intrinsic acidity constant (K^<int>_<al>) and intrinsic complexation constants of Cl^-, NO_3^-, ClO_4^-, and SO_4^- surface complexes formed at the goethite/aqueous electrolyte interfaces (*K^<int>_<anion>). The determined pK^<int>_<al> values for the four electrolytes were in a narrow range of 5.9-6.2. the values of p*K^<int>_<anion> were found to be 6.7, 6.5 and 6.2 for Cl^-, NO_3^-, and ClO_4^-, respectively, and p*K^<int>_<SO_4> took the value of 16.0 under the assumption that one SO_4^<2-> occupies tow charged sites. The values of point of zero salt effect (PZSE), which is defined as the pH value at which titration-curves in an electrolyte intersect, were 8.5, 8.6, 8.2 and 8.9 for NaCl, NaNO_3, NaClO_4, and Na_2SO_4, respectively. There was a highly positive correlation between the PZSE and p*K^<int>_<anion>. Selectivity coefficients for stoichiometric Cl-NO_3 and NO_3-SO_4 exchange reactions on goethite were calculated from the K^<int>_<anion> values and exchange isotherms were drawn.
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