Competitive Interaction between Phosphate and Citrate on Goethite

Interaction between various species in natural systems may affect the adsorption of these species and thereby their mobility and bioavailability. Recently an ion adsorption model has been developed, based on the surface structure of metal (hydr)oxides and the structure of surface complexes identified by spectroscopy. This model has been applied to the adsorption of phosphate and weak organic acids on goethite in single anion systems. We studied the interaction between phosphate and citrate on goethite, which is important for a better understanding of the competitive binding of phosphate and organic matter in natural systems. Citrate adsorption in the presence of phosphate decreased strongly over a wide pH range, whereas phosphate adsorption was only affected at pH values below 7, with the largest interaction around pH 5. Competition for binding sites appeared to be an important factor in determining the competitive adsorption of phosphate and citrate when taking into account that a larger number of surface groups is involved in the complexation of citrate compared with phosphate. Competitive adsorption was predicted satisfactorily by applying model parameters from the single anion experiments, using surface complexes coordinated to three or four surface groups for citrate and to one or two surface groups for phosphate.