Molecular perspectives on goethite dissolution in the presence of oxalate and desferrioxamine-B

Iron, an essential nutrient, is primarily present in soils in the form of iron-bearing minerals characterized with low solubilities. Under iron deficient conditions, some plants and microorganisms exude a mixture of iron-complexing agents, including carboxylates and siderophores, that can cause minerals to dissolve and increase iron solubility. Siderophores are chelating agents with functional groups such as hydroxamate, catecholate, or α-hydroxycarboxylate, that have high selectivity and specificity for Fe(III). This thesis is focused on adsorption/dissolution processes at the surface of a common soil mineral, goethite(α-FeOOH), in the presence of oxalate and a trihydroxamate siderophore, desferrioxamine-B (DFOB) at pH 4 and/or 6 in the absence of visible light. In order to characterize these processes at a molecular level and to understand the reaction mechanisms, a combination of attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy and quantitative solution phase measurements were applied. In the oxalate-goethite system, four surface species were detected: 1) an electrostatically attracted outer-sphere complex, 2) a hydrogen bonded outer-sphere complex, 3) an inner-sphere oxalate coordinated to surface iron and 4) a ternary type A complex formed during a dissolution-readsorption process. Addition of Al(C 2O 4 ) 3 3-or Ga(C 2 O 4 ) 3 3- to a goethite suspension resulted in the formation of an additional surface complex - oxalate coordinated to Al or Ga in a ternary type A complex. In the DFOB-goethite system, DFOB is subjected to surface-mediated hydrolysis followed by the reduction of Fe(III) as evidenced by the release of acetate and a nitroso-DFOB fragment into the aqueous phase. It is postulated that Fe(II) is not detected in the solution phase due to its adsorption at the surface. At low surface coverage, a small fraction of dissolved FeHDFOB + complex is also likely to form ternary surface complexes and hydrolyze. These observations suggest that DFOB-promoted dissolution of goethite may proceed not only via purely ligand-exchange reactions, but also through reductive pathways. In the oxalate-DFOB-goethite system, the dissolution rates are greater than the sum of the dissolution rates in the single-ligand systems. Results presented demonstrate that this synergistic effect is due to the formation of the above mentioned ternary oxalate surface complex via dissolution and readsorption. Iron in this ternary complex is more labile than iron in the crystal lattice and thus more readily accessible for other complexing agents, e.g. siderophores. The results presented in this thesis provide a molecular-level view of ligand-promoted mineral dissolution in the presence of small carboxylates and/or siderophores, which improves our fundamental understanding of the role of surface complexation in mineral dissolution and iron bioavailability

Time-Resolved Investigation of Cobalt Oxidation by Mn(III)-Rich δ‑MnO₂ Using Quick X‑ray Absorption Spectroscopy

Manganese oxides are important environmental oxidants that control the fate of many organic and inorganic species including cobalt. We applied ex situ quick X-ray absorption spectroscopy (QXAS) to determine the time evolution of Co­(II) and Co­(III) surface loadings and their respective average surface speciation in Mn­(III)-rich δ-MnO₂ samples at pH 6.5 and loadings of 0.01–0.20 mol Co mol^–1 Mn. In this Mn oxide, which contained few unoccupied vacancies but abundant Mn­(III) at edge and interlayer sites, Co­(II) sorption and oxidation started at the particle edges. We found no evidence for Co­(II) oxidation by interlayer Mn­(III) or Mn­(III, IV) adjacent to vacancy sites at <10 min. After 10 min, basal surface sites were implicated due to slow Co oxidation by interlayer Mn­(III) and reactive sites formed upon removal of interlayer Mn­(III), such that 50–60% of the sorbed Co was incorporated into the MnO₂ sheets or adsorbed at vacancy sites by 12 h. Our findings indicate that the redox reactivity of surface sites depends on Mn valence and crystallographic location, with Mn­(III) at the edges being the most effective oxidant at short reaction times and Mn­(III,IV) in the MnO₂ sheet contributing at longer reaction times