Adsorption of Benzoic Acid and Related Carboxylic Acids onto FeOOH(Goethite): The Low Ionic Strength Regime
2015
Whitehead, Charles F. | Carbonaro, Richard F. | Stone, Alan T.
In the absence of added electrolyte, at ionic strengths below 0.5 mM, maximum extents on adsorption of 100 μM ligand onto 1.06 g L⁻¹ FeOOH(goethite) increased in the order (2-phenylglycine) < phenylacetate < 5-methyl-2-anthranilate < anthranilate < 4-aminobenzoate ~ N-phenylglycine < benzoate < 4-hydroxybenzoate < 4-nitrobenzoate ~ salicylate. With salicylate, the maximum extent of adsorption occurred at a pH 0.60 log units higher than pKₐ(0/−), which corresponds to conversion of the monoanion to the protonated, neutral species. With the other eight ligands, maximum extents of adsorption occurred within 0.40 log units of pKₐ(0/−). Capillary electrophoresis enabled us to quantify extents of adsorption and confirm that added ligand had not undergone chemical alteration. The computer program DLIM performed the usual tasks of computer equilibrium speciation models, but also made it possible to integrate concentrations of ionic species from the surface outward toward bulk solution, yielding the nonspecific contribution to adsorption. DLIM enabled us to track changes to ionic strength as a function of reagents added and measured pH. In the low ionic strength regime, the adsorption stoichiometry [Formula: see text] provided a better fit to experimental data for most ligands than the conventional stoichiometry [Formula: see text]. Partial loss of waters of adsorption accompanying adsorption impedes the adsorption of strongly hydrated anions over less strongly hydrated ones.
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