Towards building a unified adsorption model for goethite based on variable crystal face contributions: II. Pb(II), Zn(II) and phosphate adsorption

Following the first series of this work, where the acidity constants of the two surface proton-reactive sites of goethite were found, as well as the capacitance values for each of the four previously well-characterized goethites, in this second part we report the successful simulation of the adsorption of two heavy metal cations [Pb(II) and Zn(II)] and an oxyanion (phosphate) of high affinity for the goethite surface. All experimental data were freshly generated with the same four previously characterized goethites with independent crystal surface composition determinations and calculations of the corresponding surface site densities, to maintain complete self-consistency and avoid unwanted interfering variables. Three experimental pH adsorption edges at different total concentrations were constructed for each cation on each goethite, and four adsorption isotherms at different pH values for phosphate. Two surface complexes bound to singly-coordinated surface oxygens were found for both types of ions to optimally describe the experimental data: In the case of the cations, a bidentate complex with the unhydrolized cation and a monodentate complex with the hydroxylated cation. For phosphate a bidentate complex with the deprotonated phosphate moiety and a monodentate complex with a mono-protonated phosphate yielded the optimal results (a small contribution of a protonated bidentate complex was also found necessary to describe the data at pH 4). The values of the optimized goethite affinity constants for Pb(II) were ca. two orders of magnitude higher than those of Zn(II), and the optimized values of Charge Distributions were different for each metal cation, but most of the charge for the bidentate complexes occurred at the 1-plane, while that for the monodentate hydroxylated complexes occurred mostly at the 0-plane. In the case of phosphate, the same three stoichiometries as those found previously for arsenate described all adsorption isotherm data, but their affinity constants were found to be higher (less than one order of magnitude) than those for arsenate; and their charge distributions were slightly different as well.

The results of this work contribute to progressively build a unified model that describes the geochemical behavior of goethite towards adsorption of species of environmental interest. This endeavor may eventually be of aide to describe more complex systems and the interactions occurring among different component species as related to the presence of goethite.