Hydroxamate complexes in solution and at the goethite-water interface: A cylindrical internal reflection Fourier transform infrared spectroscopy study

The infrared spectra of aqueous solutions of acetohydroxamic acid (aHA), an analogue for important iron(III) chelating ligands in soils and groundwaters, and of suspensions of goethite with adsorbed aHA were measured using a cylindrical internal reflectance cell. Using molecular orbital theory and data from the literature on the infrared spectra of solid hydroxamates and metal-hydroxamate complexes, we identify the spectral changes occurring upon complexation with both aqueous Fe(III) and Fe atoms on the surface of goethite. The effect of pH upon the solution and surface spectra and the possibility of aHA. hydrolysis at the goethite surface at pH = 3 were also examined. There was no significant difference in adsorbed aHA spectra between pH = 3 and pH = 6, indicating identical surface complexes form over the pH range where the solution phase complex changes from the bis- to the tris(hydroxamato)iron(III) complex. The infrared technique was not capable of detecting the low concentrations of hydrolysis products that form at the mineral surface over a week-long incubation.