This paper reports an investigation of the effects of a trihydroxamate siderophore, desferrioxamine B (DFO-B), and a common biological ligand, oxalate, on the steady-state dissolution of goethite at pH 5 and 25 jC. The main goal of our study was to quantify the adsorption of the ligands and the dissolution of goethite they promote in a two-ligand system. In systems with one ligand only, the adsorption of oxalate and DFO-B each followed an L-type isotherm. The surface excess of oxalate was approximately 40 mmol kg À 1 at solution concentrations above 80 AM, whereas the surface excess of DFO-B was only 1.2 mmol kg À 1 at 80 AM solution concentration. In the two-ligand systems, oxalate decreased DFO-B adsorption quite significantly, but not vice versa. For example, in solutions containing 40 AM DFO-B and 40 AM oxalate, 30% of the DFO-B adsorbed in the absence of oxalate was displaced. The mass-normalized dissolution rate of goethite in the presence of DFO-B alone increased as the surface excess of the ligand increased, suggesting a ligand-promoted dissolution mechanism. In systems containing oxalate only, mass-normalized goethite dissolution rates were very low at concentrations below 200 AM, despite maximal adsorption of the ligand. At higher oxalate concentrations (up to 8 mM), the steady-state dissolution rate continued to increase, even though the surface excess of adsorbed ligand was essentially constant. Chemical affinity calculations and dissolution experiments with variation of the reactor flow rate showed that far-from-equilibrium conditions did not obtain in systems containing oxalate at concentrations below 5 mM. The dissolution rate in the presence of DFO-B at solution concentrations between 1 and 80 AM was approximately doubled when oxalate was also present at 40 AM solution concentration. The dissolution rate in the presence of oxalate at solution concentrations between 0 and 200 AM was increased by more than an order of magnitude when DFO-B was also present at 40 AM solution concentration. Chemical affinity calculations showed that, in systems containing DFO-B, goethite dissolution was always under far-from-equilibrium conditions, irrespective of the presence of oxalate. These results were described quantitatively by a model rate law containing a term proportional to the surface excess of DFO-B and a term proportional to that of oxalate, with both surface excesses being determined in the twoligand system. The pseudo first-order rate coefficient in the DFO-B term has the same value as measured for goethite dissolution in the presence of DFO-B only, while the rate coefficient in the oxalate term must be measured in the two-ligand system, since it is only in this system that far-from-equilibrium conditions obtain. These latter conditions do not exist in the system containing oxalate only, but they do exist in the DFO-B/oxalate system because the siderophore is able to remove Fe(III) from all Fe -oxalate complexes rapidly, leaving the uncomplexed oxalate ligand in solution free to react again wit...

Abstract-Hydroxamate siderophores are biologically-synthesized, Fe(III)-specific ligands which are common in soil environments. In this paper, we report an investigation of their adsorption by the iron oxyhydroxide, goethite; their influence on goethite dissolution kinetics; and their ability to affect Pb(II) adsorption by the goethite surface. The siderophores used were desferrioxamine B (DFO-B), a fungal siderophore, and desferrioxamine D 1 , an acetyl derivative of DFO-B (DFO-D1). Siderophore adsorption isotherms yielded maximum surface concentrations of 1.5 (DFO-B) or 3.5 (DFO-D1) mol/g at pH 6.6, whereas adsorption envelopes showed either cation-like (DFO-B) or ligand-like (DFO-D1) behavior. Above pH 8, the adsorbed concentrations of both siderophores were similar. The dissolution rate of goethite in the presence of 240 M DFO-B or DFO-D1 was 0.02 or 0.17 mol/g hr, respectively. Comparison of these results with related literature data on the reactions between goethite and acetohydroxamic acid, a monohydroxamate ligand, suggested that the three hydroxamate groups in DFO-D1 coordinate to Fe(III) surface sites relatively independently. The results also demonstrated a significant depleting effect of 240 M DFO-B or DFO-D1 on Pb(II) adsorption by goethite at pH Ͼ 6.5, but there was no effect of adsorbed Pb(II) on the goethite dissolution rate.

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Abstract-This article reports an investigation of the temperature dependence of goethite dissolution kinetics in the presence of desferrioxamine B (DFO-B), a trihydroxamate siderophore, and its acetyl derivative, desferrioxamine D1 (DFO-D1). At 25 and 40°C, DFO-D1 dissolved goethite at twice the rate of DFO-B, whereas at 55°C, the behavior of the two ligands was almost the same. Increasing the temperature from 25 to 55°C caused little or no significant change in DFO-B or DFO-D1 adsorption by goethite. A pseudo-first-order rate coefficient for dissolution, calculated as the ratio of the mass-normalized dissolution rate coefficient to the surface excess of siderophore, was approximately the same at 25 and 40°C for both siderophores. At 55°C, however, this rate coefficient for DFO-D1 was about half that for DFO-B. Analysis of the temperature dependence of the mass-normalized dissolution rate coefficient via the Arrhenius equation led to an apparent activation energy that was larger for DFO-B than for DFO-D1, but much smaller than that reported for the proton-promoted dissolution of goethite. A compensation law was found to relate the pre-exponential factor to the apparent activation energy in the Arrhenius equation, in agreement with what has been noted for the proton-promoted dissolution of oxide minerals and for the complexation of Fe 3ϩ by DFO-B and simple hydroxamate ligands in aqueous solution. Analysis of these results suggested that the siderophores adsorb on goethite with a only single hydroxamate group in bidentate ligation with an Fe(III) center.

The primary objective of this study is to determine the effect of substrate type on the coordination environments of Cu 2+ adsorbed on amorphous SiO 2 , γ-Al 2 O 3 , and anatase at a surface coverage of approximately 1 µmol/m 2 . We also collected X-ray absorption fine structure (XAFS) data for several Cu 2+ -containing model compounds, including tenorite ( VI CuO), spertiniite [ VI Cu(OH) 2 ], dioptase ( VI CuSiO 2 ·H 2 O), shattuckite [ VI Cu 5 (SiO 3 ) 4 (OH) 2 ], chrysocolla [ VI (Cu,Al) 2 H 2 Si 2 O 5 (OH) 4 ·nH 2 O], and Cu 2+ acetate monohydrate [ VI Cu(CH 3 CO 2 ) 2 ·H 2 O], for comparison with the sorption sample data. Detailed analysis of these model compounds indicates that the bonding of second neighbors surrounding a central Cu absorber determines whether these second neighbors can be detected by XAFS. The XAFS results of Cu 2+ sorption samples are consistent with the presence of Jahn-Teller distorted Cu 2+ (O,OH) 6 octahedra, with four equatorial Cu-O bonds (1.95 Å) and two longer axial bonds; the axial Cu-O bonds are difficult to characterize quantitatively by XAFS spectroscopy. Cu 2+ sorbed on amorphous SiO 2 was found to have Cu second and third neighbors at 2.95 Å, 3.30 Å, and 5.72 Å, but no Cu-Si correlation was detected for these sorption products associated with amorphous SiO 2 . Based on XAFS and wet chemical results, it seems likely that a Cu(OH) 2 precipitate has formed in the Cu 2+ /amorphous SiO 2 system. Cu 2+ sorbed on γ-Al 2 O 3 is present as a mixture of monomeric, dimeric, and perhaps a small number of oligomeric hydroxo-bridged Cu(O,OH) 6 species with a Cu-Cu distance of approximately 2.95 Å. Sorbed Cu 2+ on anatase is present predominantly as hydroxo-bridged Cu dimers. At similar sorption densities, Cu 2+ cluster sizes on amorphous SiO 2 are significantly larger than those on γ-Al 2 O 3 or anatase, indicating that the substrate has an important effect on the type of Cu 2+ sorption complex or precipitates formed.