THE SORPTION OF NATURALLY OCCURRING ORGANIC ACIDS AND THEIR INFLUENCE ON OXIDE REACTIVITY

The charging characteristics, solubility and stability of metal (hydr)oxides in aquatic systems depend to a large extent upon interactions with naturally-occurring organic acids. Knowledge of these interactions is also needed to understand the mobility of contaminants such as heavy metals and nonpolar organics that react with both metal oxides and organic acids. Although the bulk of the organic acid ligands in aquatic systems are associated with heterogeneous natural organic matter (NOM), numerous low-molecular-weight (LMW) acids (e.g., oxalic acid, citric acid, and phthalic acid) are also present in significant concentrations. The reactive nature of these LWM acids like NOM is dominated by carboxylic and phenolic functional groups and these functional groups react with the surface hydroxyl groups located on the metal oxide surfaces to form adsorbed surface complexes. The adsorption behavior of both macromolecular NOM and LWM and their subsequent influence on metal hydroxide and co-solute behavior in the system depends on the physical and chemical characteristics of the acid and oxide surface as well as an array of other factors, including but not limited to solution composition, pH and time.

In this contribution we evaluate the mechanisms responsible for the adsorption of organic acids to mineral surfaces, focusing on the study of simple LMW acids adsorbed to metal (hydr)oxides, and discuss the impact these adsorbed organic acids have on the interfacial behavior of the combined oxide-acid surface. Results are presented for a variety of systems studied using traditional wet chemical techniques, infrared spectroscopy, molecular modeling and surface complexation modeling. Our results suggest outer-sphere modes dominate adsorption in most systems. Secondary modes of adsorption, likely inner-sphere in coordination, become important at low pH and shift in relative importance as a function of surface coverage. Interestingly, the adsorption modes exhibit differences as a function of the metal oxide sorbent, which is somewhat counter to conventional wisdom. Finally, we discuss how the adsorbed acids influence the oxide surfaces charging characteristics and ability to adsorb co-solutes.