Competitive sorption of simple organic acids and sulfate on goethite

Sorption of phthalic acid, chelidamic acid, and sulfate onto goethite (alpha-FeOOH) was examined in single-sorbate and in sulfate-organic acid binary-sorbate systems to determine the extent of competition between the simple organic acids and sulfate. Sorption characteristics of sulfate and the organic acids were similar and resembled those reported for humic substances onto oxides. Sorption data for all three sorbates over a wide range of conditions (pH, I, sorbate/sorbent ratio) were described quantitatively by the generalized two-layer model with a unique set of surface reactions and equilibrium constants for each sorbate. Sorption affinities of sulfate and the organic acids were comparable, and sulfate effectively competed with the organic acids for surface sites on goethite, particularly at low pH. These results suggest that sulfate can significantly influence the sorption of simple organic acids and humic substances in natural aquatic systems. Predictions of sorption in binary-sorbate systems based on single-sorbate data fits represented competitive sorption data reasonably well over a wide range of conditions. However, there were underpredictions of minor-component sorption in the presence of a major component, which may be explained by sorbate-specific surface site heterogeneity and/or by inaccurate representation of Coulombic effects in the model.