Hydroxamate Complexes in Solution and at the Goethite−Water Interface:  A Cylindrical Internal Reflection Fourier Transform Infrared Spectroscopy Study

Abstract

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.

Iron oxide dissolution and solubility in the presence of siderophores

“…Holmen et al (1997) have elucidated the structure of the acetohydroxamic acid (aHA) surface complex on goethite by FTIR-spectroscopy. aHA adsorbs on the goethite-water interface by a ligand exchange reaction as shown in Figure 6.…”

Coupled biogeochemical cycling of iron and manganese as mediated by microbial siderophores

“…Hydroxamate siderophore-promoted dissolution rates, compiled from a number of sources (Hersman et al 1995;Lloyd 1999;Cocozza et al 2002;Neubauer et al 2002;Yoshida et al 2002;Cheah et al 2003;Carrasco et al 2007;Wolff-Boenisch and Traina 2007;Carrasco et al 2008) and spanning a large range of pH (3-9), concentration (1.3 9 10 -5 -10 -3 M), and solid phases [a-FeOOH, a-Fe 2 O 3 , and a poorly crystalline Fe(III)-hydroxide] vary by less than a factor of 20 (R = 10 -11.5 -10 -12.8 mol m -2 s -1 ), strongly suggesting commonalities among the mechanisms of hydroxamate siderophore-promoted dissolution. Desferrioxamine B promoted-dissolution of goethite appears to be surface-controlled (Cheah et al 2003) and possibly mediated by the formation of dissolutionactive bidentate mononuclear surface structures (Holmen et al 1997;Cocozza et al 2002). Interestingly, a number of low-molecular mass organic acids (LMMOAs; e.g., oxalate, citrate, malonate, succinate, and fumarate) have been shown to augment siderophore-promoted dissolution rates (Cheah et al 2003;Reichard et al 2007;Wolff-Boenisch and Traina 2007).…”

Temperature dependence of goethite dissolution promoted by trihydroxamate siderophores

“…Liermann et al (2000) observed an approximately square-root dependence on DFO-B concentration for the Fe release rate from hornblende, leading them to conclude that DFO-B adsorption is necessary to mineral dissolution. Casey (1996, 1998) and Holmén et al (1997) have added mechanistic details to this picture in their careful investigations of the dissolution of goethite promoted by acetohydroxamic acid [CH 3 COHNOH,aHA], a simple monohydroxamate ligand. They concluded that the aHA hydroxamate group coordinates to a surface Fe(III) center on the mineral in much the same way as in aqueous solution (i.e., bidentate ligation), but does so with the greater impact of vicinal atoms in the mineral, as compared with solvating water molecules in aqueous solution, on the distribution of electron density within the surface complex.…”