Hydroxamate ligands, surface chemistry, and the mechanism of ligand-promoted dissolution of goethite [α-FeOOH(s)]

Effect of hydroxamate siderophores on Fe release and Pb(II) adsorption by goethite

“…No evidence for multinuclear surface complexes or unidentate ligation under the conditions of their experiments was found by Holmén et al (1997). Holmén and Casey (1996; noted also that the adsorption envelope (graph of amount adsorbed versus pH) for acetohydroxamic acid on goethite was ligand-like (Stumm, 1992), as expected. The rate of goethite dissolution was much less than the rate of surface complex formation, leading therefore to the well-known linear correlation between the dissolution rate coefficient and adsorbed ligand concentration (Stumm et al, 1990).…”

“…3). This result can be compared directly with the surface concentration of acetohydroxamic acid (aHA) measured on goethite under comparable conditions by (Holmén and Casey, 1996), 10 mol/g. Iron(III) coordination by aHA has been investigated by CIR-FTIR spectroscopy (Holmén et al, 1997), which shows that this bidentate ligand forms only mononuclear complexes both in solution and at the goethite surface.…”

“…Iron(III) coordination by aHA has been investigated by CIR-FTIR spectroscopy (Holmén et al, 1997), which shows that this bidentate ligand forms only mononuclear complexes both in solution and at the goethite surface. If excess ligand is present (i.e., L diss /Fe diss Ͼ3), Fe(aHA) 3 is the dominant solution complex near neutral pH (Holmén and Casey, 1996) with a formation constant of 10 28.3 . Compared to the dissolved tris aHA complex, Fe(III) complexation by DFO-B or DFO-D1 in solution exhibits only a small chelate effect, with a Fe(III) complex formation constant of 10 31 (Albrecht-Gary and Crumbliss, 1998).…”

“…Noting that the Fe solubilized after 28 days accounted for only about 8% of the complexing capacity of the added desferrioxamine B, Watteau and Berthelin (1994) suggested that further research was needed on the accessibility of this siderophore to the goethite surface. Holmén and Casey (1996; and Holmén et al (1997) have explored the mechanisms of hydroxamate-promoted dissolution of goethite in studies utilizing a model compound, acetohydroxamic acid ( Fig. 1), which is a small ligand possessing a single hydroxamate functional group.…”

“…1), which is a small ligand possessing a single hydroxamate functional group. They concluded from dissolution kinetics data, as well as from FTIR spectroscopic measurements, that adsorbed acetohydroxamic acid forms a surface chelate with exposed Fe 3ϩ by an associative ligand exchange mechanism (Stumm et al, 1990;Holmén and Casey, 1996). The metal-organic bonding in the surface chelate formed with this bidentate ligand is very similar to that in the soluble Fe(III) chelate, which is a five-membered ring including the N-O bond (Holmén et al, 1997).…”