The effect of desferrioxamine B, enterobactin, oxalic acid, and Na-alginate on the dissolution of uranyl-treatedgoethite at pH 6 and 25 °C

Organic acids are essential in the cycling and environmental fate of many elements. Their ability to solubilize and complex key elements confer them a major role in the bioavailability of otherwise insoluble nutrients. The ligand-promoted dissolution of goethite as a primary source for Fe(III) has been investigated in the presence of adsorbed uranyl for four different organic ligands.With respect to the polysaccharide alginate goethite dissolution lingers on a low level until a second ligand enters the solution. The addition of oxalate and DFO-B, respectively significantly enhances goethite dissolution. This synergistic effect on goethite dissolution in the 2-ligand systems alginate/oxalate and alginate/DFO-B is similar in magnitude and can be explained by a considerable increase in the chemical affinity of the dissolution reaction in the presence of the second ligand. Besides an increase in iron solubility alginate polymers may also adsorb on the goethite surface weakening thus its structure while the second ligand induces the shift away from solution equilibrium.For oxalate, the ligand-promoted effect becomes only noticeable when the amount of goethite and/or oxalate in solution overcomes certain concentration thresholds, regardless of the presence of adsorbed uranyl. The free oxalate concentration at circum-neutral pH in soils is limited by Ca- and Mg-oxalate precipitates and the speciation with many polyvalent cations. Model calculations in the presence of these cations yield uncomplexed oxalate concentrations in the lower nM range indicating that bioavailability of iron mediated through oxalate-promoted dissolution of goethite is heavily restrained.As to the siderophores, the presence of adsorbed uranium does not influence the ligand-promoted goethite dissolution. On an equimolar basis, far-from-equilibrium goethite dissolution rates in the presence of enterobactin are 5 times higher than analogous DFO-B-promoted rates although both chelators show low fractional adsorption < 6%.Altogether, the findings demonstrate that iron availability is substantially improved in the presence of 2 ligands inducing a synergetic effect on pure goethite dissolution. Likewise, the amount of goethite present positively influences its solubilization under constant ligand concentrations. Furthermore, results from the siderophore dissolution studies indicate that increased iron solubility and the related shift away from solution equilibrium is a more compelling explanation of the driving force behind goethite dissolution than surface complexation of the ligands.