Adsorption of oxalate and malonate at the water-goethite interface: molecular surface speciation from IR spectroscopy

The adsorption of oxalate and malonate at the water-geothite interface was studied as a function of pH and total ligand concentrations by means of quantitative adsorption measurements and attenuated total reflectance Fourier transform infrared spectroscopy. The obtained results conclusively showed that oxalate and malonate both form outer-sphere and inner-sphere surface complexes on goethite, and that these complexes coexist over a broad pH interval. The inner-sphere complexes were favored by low pH, while, the relative, concentrations of the outer-sphere species increase with increasing pH. Based on comparisons with model complexes characterized by Extended X-Ray Adsorption Fine Structure (EXAFS) and results from theoretical frequency calculations, the structures of the inner-sphere complexes of oxalate and malonate were best described as mononuclear five- and six-membered ring chelate structures, respectively. The stability of the inner-sphere complexes followed the trend expected from solutions studies, with the oxalate five-membered ring yielding the more stable complexes compared to the six-membered ring of malonate. The increased stability of the inner-sphere complex of oxalate was manifested in a greater extent of adsorption at acidic pH values. Despite the fact that significant amounts of oxalate and malonate inner-sphere surface complexes were formed, no ligand-promoted dissolution was observed at the experimental conditions in the study. Copyright (C) 2005 Elsevier Ltd.