Stability of Cl⁻, NO₃⁻, ClO₄⁻, and S0₄⁻ surface complexes at synthetic goethite/aqueous electrolyte interfaces

We carried out potentiometric titrations of a synthetic goethite in solutions of NaCl, NaNO₃, NaC1O₄, and Na₂SO₄ to determine the intrinsic acidity constant K^int_aj and intrinsic complexation constants of Cl⁻, NO₃⁻, ClO₄⁻, and S0₄⁻ surface complexes formed at the goethite/aqueous electrolyte interfaces *K^int_aj. The determined K^int_aj values for the four electrolytes were in a narrow range of 5.9-6.2. The values of K^int_aj were found to be 6.7, 6.5, and 6.2 for Cl⁻, NO₃⁻, ClO₄⁻, and S0₄⁻, respectively, and K^int_aj took the value of 16.0 under the assumption that one SO₄²⁻ occupies two 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, NaNO₃, NaC1O₄, and Na₂SO₄, respectively. There was a highly positive correlation between the PZSE and K^int_aj. Selectivity coefficients for stoichiometric Cl-NO₃ and NO₃-SO₄ exchange reactions on goethite were calculated from the K^int_aj values and exchange isotherms were drawn.