Competitive adsorption of phosphate and carboxylate with natural organic matter on hydrous iron oxides as investigated by chemical force microscopy

The behaviour of phosphate and natural organic matter (NOM) in soils and aqueous media is strongly influenced by their association with surfaces of colloidal mineral oxide and oxyhydroxide particles of iron. Here we investigate the interaction of atomic force microscope (AFM) tips terminated with bis(11-thioundecanoic)phosphate and 16-thiohexadecanoic acid as a function of pH in the context of competitive phosphate-organic matter adsorption interactions at the surface of hydrous iron oxide colloids. Experiments were carried out on unmodified colloids as well as colloids coated with gallic acid, tannic acid or peat-derived humic material. The colloids were also examined by infrared spectroscopy (IR) and zeta potentiometry. Force titrations on a gallic acid control surface revealed that the main mode of interaction of this compound with the tips was hydrogen bonding. A strong interaction in the pH range 4-8 observed for the PO2H probe on the unmodified iron (hydroxy)oxide was attributed to a specific adsorption reaction with an A-type Fe-OH surface site. The CO2H tip displayed a strong but less specific interaction that extended over the pH range 4-10. Force titrations of probes against colloids post-precipitated with gallic acid, tannic acid or peat-derived humic material were significantly different than for the unmodified surfaces. Surface-bound organic molecules reduced the specific mid pH range interactions and gave rise to two new force titration features with maxima about pH 4 and 8, which were assigned to H-bonding between the probes and benzoic and phenolic groups of surface-bound organic acids, respectively. (C) 2002 Elsevier Science B.V. All rights reserved.