Mechanisms of goethite dissolution in the presence of desferrioxamine B and Suwannee River fulvic acid at pH 6.5

Siderophores are Fe3+ specific low MW chelating ligands secreted by microorganisms in response to Fe stress. Low MW organic acids such as oxalate have been shown to enhance siderophore mediated dissolution of Fe3+ oxides. However, the effect of fulvic acid presence on siderophore function remains unknown. We used batch dissolution experiments to investigate Fe release from goethite in the goethite-fulvic acid desferrioxamine B (goethite-SRFA-DFOB) ternary system. Experiments were conducted at pH 6.5 while varying reagent addition sequence. FTIR and UV-Vis spectroscopy were employed to characterise the Fe-DFOB, Fe-SRFA and DFOB–SRFA complexes. Iron released from goethite in the presence of SRFA alone was below detection limit. In the presence of both SRFA and DFOB, dissolved Fe increased with reaction time, presence of the DFOB-SRFA complex, and where SRFA was introduced prior to DFOB. FTIR data show that in the ternary system, Fe3+ is complexed primarily to oxygen of the DFOB hydroxamate group, whilst the carboxylate C=O of SRFA forms an electrostatic association with the terminal NH3+ of DFOB. We propose that SRFA sorbed to goethite lowers the net positive charge of the oxide surface, thus facilitating adsorption of cationic DFOB and subsequent Fe3+ chelation and release. Furthermore, the sorbed SRFA weakens Fe-O bonds at the goethite surface, increasing the population of kinetically labile Fe. This work demonstrates the positive, though indirect role of SRFA in increasing the bioavailability of Fe3+

Defects and impurities in jarosite: A computer simulation study

Computer modelling techniques involving a rigid ion model have been used to investigate the defect structure and impurity site preferences in end-member K-jarosite. Calculated intrinsic vacancy energies show that the K2SO4 neutral cluster, with an energy per species of 1.34 eV, will be the most common defect in the pure phase. Defect reactions leading to vacancies on the Fe site have high energies, in excess of 4.0 eV per species, and are thus unlikely to occur in great numbers. However, the calculations show that divalent metal cations can be incorporated onto the Fe site via solution reactions with oxides leading to the formation of goethite. Calculated solution reactions are exothermic and thus predicted to be highly favourable. At K sites substitutions occur in the order Cd > Zn > Cu, but will be limited due to endothermic solution energies and structural considerations

Effect of desferrioxamine B and Suwannee River fulvic acid on Fe(III) release and Cr(III) desorption from goethite

Siderophores are biogenic chelating ligands that facilitate the solubilization of Fe(III) and form stable complexes with a range of contaminant metals and therefore may significantly affect their biogeochemical cycling. Desferrioxamine B (DFOB) is a trihydroxamate siderophore that acts synergistically with fulvic acid and low molecular weight organic ligands to release Fe from Fe(III) oxides. We report the results of batch dissolution experiments in which we determine the rates of Cr(III) desorption and Fe(III) release from Cr(III)-treated synthetic goethite as influenced by DFOB, by fulvic acid, and by the two compounds in combination. We observed that adsorbed Cr(III) at 3% surface coverage significantly reduced Fe(III) release from goethite for all combinations of DFOB and fulvic acid. When DFOB (270 µM) was the only ligand present, dissolved Fe(III) and Cr(III) increased approximately 1000-fold and 16-fold, respectively, as compared to the ligand-free system, a difference we attribute to the slow rate of water exchange of Cr(III). Suwannee River fuvic acid (SRFA) acts synergistically with DFOB by (i) reducing the goethite surface charge leading to increased HDFOB+ surface excess and by (ii) forming aqueous Fe(III)-SRFA species whose Fe(III) is subsequently removed by DFOB to yield aqueous Fe(III)-DFOB complexes. These observations shed new light on the synergistic relationship between DFOB and fulvic acid and reveal the mechanisms of Fe(III) acquisition available to plants and micro-organisms in Cr(III) contaminated environments

Comparison of the structures of natural and synthetic Pb-Cu-jarosite-type compounds

Jarosite minerals are effective scavengers of potentially toxic elements such as Pb and Cu, and are abundant in acid rock drainage systems, acid sulfate soils and metallurgical wastes. We used XRD, SEM, and infrared, Raman and X-ray absorption (EXAFS and XANES) spectroscopy to determine the structural differences between natural and synthetic Pb-Cu-jarosites. Differences in the a0 unit cell dimensions for the natural and synthetic samples (7.2288(27) Å and 7.32088(26) Å, respectively), and c0 unit cell values (34.407(14) Å and 17.0336(7) Å, respectively) are attributed to different proportions of H3O, Fe and Pb in the jarosite structures. The synthetic Pb-Cu-jarosite has sharper Raman and IR spectra, with narrower and more intense bands, suggesting that it is more crystalline than the natural sample. EXAFS fitting of the Fe and Pb data for the natural and synthetic Pb-Cu-jarosite samples are similar to each other, and are also similar to previously reported EXAFS data for jarosites, suggesting that Fe occupies the B sites, and Pb the A sites. The natural sample's Pb EXAFS data are only fitted over a short k-range, however, and the comparison is based on only the first two shells around Pb. Our Cu fits for the natural Pb-Cu-jarosite are similar to the first and third shell fittings for Fe, except that the Cu occupies a tetrahedral rather than an octahedral site (4 O in first shell at r = 1.94-1.95 Å; 4 Fe in third shell at r = 3.59-3.60 Å). In the synthetic Pb-Cu-jarosite, Cu also shares the B site with Fe, but fits of 4 Cu around the Cu atom at a bond distance of 2.60 Å and XANES evidence also suggest the presence of metallic Cu within the structure. Variations in the structures of the natural and synthetic samples are likely due to their differing chemistries and conditions of formation

Simultaneous Release of Fe and As during the Reductive Dissolution of Pb–As Jarosite by <i>Shewanella putrefaciens</i> CN32

Jarosites are produced during metallurgical processing, on oxidized sulfide deposits, and in acid mine drainage environments. Despite the environmental relevance of jarosites, few studies have examined their biogeochemical stability. This study demonstrates the simultaneous reduction of structural Fe­(III) and aqueous As­(V) during the dissolution of synthetic Pb–As jarosite (PbFe₃(SO₄,AsO₄)₂(OH)₆) by <i>Shewanella putrefaciens</i> using batch experiments under anaerobic circumneutral conditions. Fe­(III) reduction occurred immediately in inoculated samples while As­(V) reduction was observed after 72 h. XANES spectra showed As­(III) (14.7%) in the solid phase at 168 h coincident with decreased aqueous As­(V). At 336 h, XANES spectra and aqueous speciation analysis demonstrated 20.2% and 3.0% of total As was present as As­(III) in the solid and aqueous phase, respectively. In contrast, 12.4% of total Fe was present as aqueous Fe­(II) and was below the detection limits of XANES in the solid phase. TEM-EDS analysis at 336 h showed secondary precipitates enriched in Fe and O with minor amounts of As and Pb. Based on experimental data and thermodynamic modeling, we suggest that structural Fe­(III) reduction was thermodynamically driven while aqueous As­(V) reduction was triggered by detoxification induced to offset the high As­(V) (328 μM) concentrations released during dissolution