Oxalate (C(2)O(4)(2-)) acts synergistically on the dissolution of
goethite (alpha-FeOOH) in the presence of siderophores that are secreted
by plants and microorganisms to sequester iron. We report here the first
in situ molecular-scale observations of synergistic ligand-promoted
dissolution processes. We show that there are conditions under which
oxalate promotes goethite dissolution, but dissolved Fe(III)
concentrations do not increase because Fe(III)-oxalate complexes
readsorb to the mineral surface. We demonstrate that these readsorbed
Fe(III)-oxalate complexes are highly mobile, extremely reactive in the
presence of uncomplexed siderophores, and responsible for the
synergistic effects on the dissolution of goethite.