Desferrioxamine-B (DFOB) is a bacterial trihydroxamate siderophore and
probably the most studied to date. However, the manner in which DFOB
adsorbs at mineral surfaces and promotes dissolution is still under
discussion. Here we investigated the adsorption and dissolution
reactions in the goethite-DFOB system using both in situ infrared
spectroscopic and quantitative analytical methods. Experiments were
carried out at a total DFOB concentration of 1 mu mol/m(2), at pH 6, and
in the absence of visible light. Our infrared spectroscopic results
indicated that the adsorption of DFOB was nearly complete after a 4-h
reaction time. In an attempt to determine the coordination mode at the
goethite surface, we compared the spectrum of adsorbed DFOB after a 4-h
reaction time to the spectra of model aqueous species. However, this
approach proved too simplistic in the case of such a complex ligand as
DFOB, and we suggest that a more detailed investigation (IR in D(2)O,
EXAFS of adsorbed model complexes) is needed to elucidate the structure
of the adsorbed siderophore. Between a 4-h and 4-day reaction time, we
observed the growth of carboxylate stretching bands at 1548 and 1404
cm(-1), which are indicators of DFOB hydrolysis. Acetate, a product of
DFOB hydrolysis at its terminal hydroxamate group, was quantified by ion
chromatography. Its rate of formation was linear and nearly the same as
the rate of Fe(III) dissolution. The larger hydrolysis product, a
hydroxylamine fragment, was not detected by LC MS. However, a signal due
to the oxidized form of this fragment, a nitroso compound, was found to
increase linearly with time, which is an indirect indication for Fe(III)
reduction. Based on these findings, we propose that DFOB undergoes
metal-enhanced hydrolysis at the mineral surface followed by the
reduction of surface Fe(III). While Fe(II) was not detected in solution,
this is likely because it remains adsorbed at the goethite surface or
becomes buried in the goethite crystal by electron conduction. Taking
into account the extent and similarity between the rates of hydrolysis
and dissolution, we suggest that a reductive mechanism could play an
important part in the dissolution of goethite by DFOB. This possibility
has not been considered previously in the absence of light and at
circumneutral pH. Published by Elsevier Ltd.