COMPETITIVE INTERACTION BETWEEN PHOSPHATE AND CITRATE ON GOETHITE

Interaction between various species in natural systems may aff ect the adsorption of these spec ies a nd there by their mobility and bioavai lability. Recently an ion adsorption model has been developed, based o n the surface structure of metal (hydr)oxides and the structure of sur face 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 sy stems. Citrate adsorption in the presence of phosphate decreased stron gly over a wide pH range, whereas phosphate adsorption was only affect ed at pH values below 7, with the largest interaction around pH 5. Com petition for binding sites appeared to be an important factor in deter mining the competitive adsorption of phosphate and citrate when taking into account that a larger number of surface groups is involved in th e complexation of citrate compared with phosphate. Competitive adsorpt ion was predicted satisfactorily by applying model parameters from the single anion experiments, using surface complexes coordinated to thre e or four surface groups for citrate and to one or two surface groups for phosphate.