Reactions of Organic Acids at the Water-Mineral Interface: Competitive Adsorption and Dissolution Reactions at a Molecular Level

Recent advances in spectroscopic techniques have provided new insights to reactions at the water-mineral interface. With respect to adsorption of organic acids a partly new picture has emerged that shows an intricate relationship between the structure and composition of the acids and the mechanisms of adsorption. This picture highlights the importance of hydrogen bonding interactions and of processes involving comparatively slow kinetics such as dissolution-readsorption reactions. In the present paper we discuss the implications of these findings for competitive ligand adsorption and ligand-promoted dissolution reactions. In both cases main focus will be on the molecular information provided by in-situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) measurements, and a novel technique for collecting the ATR-FTIR spectra will be presented. We will show that reactions between goethite (á-FeOOH) and small carboxylic acids such as oxalate and malonate partially involve a dissolution-readsorption process which results in the formation of a pool of labile ternary Fe(III)-ligand surface complexes. These complexes will also be shown to be the reason behind the synergistic dissolution effect between small organic acids and siderophores. The discussion on competitive adsorption will focus on systems involving phosphate and carboxylic acids. New results will be presented indicating that organic acids forming strong hydrogen bonds are required to significantly out-compete phosphate, which is contrary to the belief that this a competition for inner sphere sites only.