Goethite surface reactivity: III. Unifying arsenate adsorption behavior through a variable crystal face - Site density model

We developed a model that describes quantitatively the arsenate adsorption behavior for any goethite preparation as a function of pH and ionic strength, by using one basic surface arsenate stoichiometry, with two affinity constants. The model combines a face distribution-crystallographic site density model for goethite with tenets of the Triple Layer and CD-MUSIC surface complexation models, and is self-consistent with its adsorption behavior towards protons, electrolytes, and other ions investigated previously. Five different systems of published arsenate adsorption data were used to calibrate the model spanning a wide range of chemical conditions, which included adsorption isotherms at different pH values, and adsorption pH-edges at different As(V) loadings, both at different ionic strengths and background electrolytes. Four additional goethite-arsenate systems reported with limited characterization and adsorption data were accurately described by the model developed. The adsorption reaction proposed is: (sic)FeOH + (sic)SOH + AsO43- -> (sic)FeOAsO3[2-]center dot center dot center dot SOH + H2O where (sic)SOH is an adjacent surface site to (sic)FeOH; with log K = 21.6 +/- 0.7 when (sic)SOH is another (sic)FeOH, and log K=18.75 +/- 0.9, when (sic)SOH is (sic)Fe2OH. An additional small contribution of a protonated complex was required to describe data at low pH and very high arsenate loadings. The model considered goethites above 80 m(2)/g as ideally composed of 70% face (1 0 1) and 30% face (0 0 1), resulting in a site density for (sic)FeOH and for Fe3OH of 3.125/nm(2) each. Below 80 m(2)/g surface capacity increases progressively with decreasing area, which was modeled by considering a progressively increasing proportion of faces (0 1 0)/(1 0 1), because face (0 1 0) shows a much higher site density of (sic)FeOH groups. Computation of the specific proportion of faces, and thus of the site densities for the three types of crystallographic surface groups present in goethite, may be performed for each preparation either by experimental determination of site saturation by an index ion (e.g., chromate), or by achieving congruency of proton adsorption data with those of ideal goethites when plotted as percentage of proton-reactive ((sic)FeOH + (sic)Fe3OH) sites occupied. The surface arsenate complexes proposed additionally explained: (1) the higher affinity of goethite for As(V) than for Cr(VI) at high pH, and thus the gentle slope of the arsenate pH adsorption edges; and (2) the lower adsorption capacity for As(V) than for Cr(VI) at low pH on low-surface area goethites, through incomplete (sic)FeOH site occupancy of As(V). The model is very promising as a practical means of predicting the adsorption behavior of arsenate on any goethite preparation, and may extend to predictive capabilities for adsorption behavior of many other relevant oxyanions, as well as for explaining differences in ligand-promoted surface transformation processes on goethite as a function of particle size. (C) 2010 Elsevier Ltd. All rights reserved.