Results from enriched (57)Fe isotope tracer experiments have shown that atom exchange can occur between structural Fe in Fe(III) oxides and aqueous Fe(II) with no formation of secondary minerals or change in particle size or shape. Here we derive a mass balance model to quantify the extent of Fe atom exchange between goethite and aqueous Fe(II) that accounts for different Fe pool sizes. We use this model to reinterpret our previous work and to quantify the influence of particle size and pH on extent of goethite exchange with aqueous Fe(II). Consistent with our previous interpretation, substantial exchange of goethite occurred at pH 7.5 (≈ 90%) and we observed little effect of particle size between nanogoethite (average size of 81 × 11 nm; ≈ 110 m(2)/g) and microgoethite (average size of 590 × 42 nm; ≈ 40 m(2)/g). Despite ≈ 90% of the bulk goethite exchanging at pH 7.5, we found no change in mineral phase, average particle size, crystallinity, or reactivity after reaction with aqueous Fe(II). At a lower pH of 5.0, no net sorption of Fe(II) was observed and significantly less exchange occurred accounting for less than the estimated proportion of surface Fe atoms in the particles. Particle size appears to influence the amount of exchange at pH 5.0 and we suggest that aggregation and surface area may play a role. Results from sequential chemical extractions indicate that (57)Fe accumulates in extracted Fe(III) goethite components. Isotopic compositions of the extracts indicate that a gradient of (57)Fe develops within the goethite with more accumulation of (57)Fe occurring in the more easily extracted Fe(III) that may be nearer to the surface.

Preparation of Nickel-substituted Goethite and Hematite Ni(II)-substituted goethite and hematite samples used for kinetic release experiments were prepared using modified previously reported methods (Schwertmann and Cornell, 2000). Ni(II)-goethite was synthesized by slowly adding 125 mL of a solution containing 0.98 M ferric nitrate and 0.02 M Ni(II) chloride to 225 mL of 5 M NaOH. The slurry was then diluted to 1 L and placed in an oven at 70 °C for 5 days. Ni(II)-hematite was prepared by slowly adding 600 mL of a solution containing 0.198 M ferric nitrate and 0.002 M Ni(II) chloride to 360 mL of 1 M NaOH. The pH of this slurry was then adjusted to 8.5 by dropwise addition of 1.0 or 0.1 M NaOH. The suspension was then placed in an oven set to 98 °C for 11 days. Once removed from the oven and cooled to room temperature, both materials were treated with 0.25 M HCl for 2 hr using a solid to solution ratio of 1:100. This acid treatment was employed to remove residual adsorbed cations and amorphous Ni-containing iron oxides. The materials were then washed with DI water by centrifugation until a pH of >5 was achieved. The solids were resuspened into 250 mL of DI water and stored as a suspension until further use. An aliquot was removed and oven dried at 70 °C to obtain powder that was used for sample characterization. Mineral surface area (Table DR1) was determined by collecting nitrogen B.E.T. adsorption isotherms at 77 K using a Quantachrome instruments Autosorb-1. X-ray diffraction (Rigaku Geigerflex D-MAX/A diffractometer using Cu-Kα radiation) confirmed that the Ni-substituted Fe oxides consisted of goethite or hematite and contained no other crystalline impurities. Ni content (Table DR1) was determined by inductively-coupled plasma optical emission spectrometry (ICP-OES, Perkin-Elmer Optima 7300DV) after digestion of the solid in a 20% HNO 3 :5% HCl (trace metal grade, Fisher Scientific) mixture. The digested sample liquid was diluted as necessary before analysis. Fe(II)-catalyzed Nickel Incorporation into Goethite and Hematite All experiments were conducted under anoxic conditions in an anaerobic chamber (Coy Laboratory Products, Inc.) containing a 3% H 2 /97% N 2 atmosphere with a Pd catalyst to eliminate residual O 2. Doubly deionized water (18.2 MΩ cm) was sparged with ultra-high purity N 2 gas prior to being brought into the anaerobic chamber. Trace oxygen levels in the chamber atmosphere were further lowered by a secondary oxygen trap; the chamber atmosphere was bubbled in sequence through a 15% pyrogallol/ 50% KOH solution and deionized water. The KOH solution also served to remove trace amounts of CO 2 from the chamber. The filtered gas stream was used to re-sparge and further deoxygenate the deionized water and any solution not prepared in the chamber. Dissolved oxygen content of the deionized water was estimated colorimetrically (CHEMetrics test kit K-7511) and all analyses were below the detection limit of 1 μg/L. Fe(II) stock solutions were prepared from deoxygenated deionized water and reagentgrade ...

Electron transfer and atom exchange (ETAE) between aqueous Fe(II) and Fe(III) oxides induces surface growth and dissolution that affects trace element fate and transport. We have recently demonstrated Ni(II) cycling through goethite and hematite (adsorbed Ni incorporates into the mineral structure and preincorporated Ni releases to solution) during Fe(II)-Fe(III) ETAE. However, the chemical parameters affecting net trace element release remain unknown. Here, we examine the chemical controls on Ni(II) and Zn(II) release from Ni- and Zn-substituted goethite and hematite during reaction with Fe(II). Release follows a rate law consistent with surface reaction limited mineral dissolution and suggests that release occurs near sites of Fe(III) reductive dissolution during Fe(II)-Fe(III) ETAE. Metal substituent type affects reactivity; Zn release is more pronounced from hematite than goethite, whereas the opposite trend occurs for Ni. Buildup of Ni or Zn in solution inhibits further release but this resumes upon fluid exchange, suggesting that sustained release is possible under flow conditions. Mineral and aqueous Fe(II) concentrations as well as pH strongly affect sorbed Fe(II) concentrations, which directly control the reaction rates and final metal concentrations. Our results demonstrate that structurally incorporated trace elements are mobilized from iron oxides into fluids without abiotic or microbial net iron reduction. Such release may affect micronutrient availability, contaminant transport, and the distribution of redox-inactive trace elements in natural and engineered systems.

Aqueous Fe(II) has been shown to exchange with structural Fe(III) in goethite without any significant phase transformation. It remains unclear, however, whether aqueous Fe(II) undergoes similar exchange reactions with structural Fe(III) in hematite, a ubiquitous iron oxide mineral. Here, we use an enriched (57)Fe tracer to show that aqueous Fe(II) exchanges with structural Fe(III) in hematite at room temperature, and that the amount of exchange is influenced by particle size, pH, and Fe(II) concentration. Reaction of 80 nm-hematite (27 m(2) g(-1)) with aqueous Fe(II) at pH 7.0 for 30 days results in ∼5% of its structural Fe(III) atoms exchanging with Fe(II) in solution, which equates to about one surface iron layer. Smaller, 50 nm-hematite particles (54 m(2) g(-1)) undergo about 25% exchange (∼3× surface iron) with aqueous Fe(II), demonstrating that structural Fe(III) in hematite is accessible to the fluid in the presence of Fe(II). The extent of exchange in hematite increases with pH up to 7.5 and then begins to decrease as the pH progresses to 8.0, likely due to surface site saturation by sorbed Fe(II). Similarly, when we vary the initial amount of added Fe(II), we observe decreasing amounts of exchange when aqueous Fe(II) is increased beyond surface saturation. This work shows that Fe(II) can catalyze iron atom exchange between bulk hematite and aqueous Fe(II), despite hematite being the most thermodynamically stable iron oxide.

Aqueous Fe(II) reacts with Fe(III) oxides by coupled electron transfer and atom exchange (ETAE) resulting in mineral recrystallization, contaminant reduction, and trace element cycling. Previous studies of Fe(II)-Fe(III) ETAE have explored the reactivity of either pure iron oxide phases or those containing small quantities of soluble trace elements. Naturally occurring iron oxides, however, contain substantial quantities of insoluble impurities (e.g., Al) which are known to affect the chemical properties of such minerals. Here we explore the effect of Al(III), Cr(III), and Sn(IV) substitution (1-8 mol %) on trace element release from Ni(II)-substituted goethite and Zn(II)-substituted hematite during reaction with aqueous Fe(II). Fe(II)-activated trace element release is substantially inhibited from both minerals when an insoluble element is cosubstituted into the structure, and the total amount of release decreases exponentially with increasing cosubstituent. The limited changes in surface composition that occur following reaction with Fe(II) indicate that Al, Cr, and Sn do not exsolve from the structure and that Ni and Zn released to solution originate primarily from the bulk rather than the particle exterior (upper ~3 nm). Incorporation of Al into goethite substantially decreases the amount of iron atom exchange with aqueous Fe(II) and, consequently, the amount of Ni release from the structure. This implies that trace element release inhibition caused by substituting insoluble elements results from a decrease in the amount of mineral recrystallization. These results suggest that naturally occurring iron oxides containing insoluble elements are less susceptible to Fe(II)-activated recrystallization and exhibit a greater retention of trace elements and contaminants than pure mineral phases.

The reduction of trace elements and contaminants by Fe(II) at Fe(III) oxide surfaces is well documented. However, the effect of aqueous Fe(II) on the fate of redox-active trace elements structurally incorporated into iron oxides is unknown. Here, we investigate the fate of redox-active elements during Fe(II)-activated recrystallization of Cu-, Co-, and Mn-substituted goethite and hematite. Enhanced release of Cu, Co, and Mn to solution occurs upon exposure of all materials to aqueous Fe(II) relative to reactions in Fe(II)-free fluids. The quantity of trace element release increases with pH when Fe(II) is present but decreases with increasing pH in the absence of Fe(II). Co and Mn release from goethite is predicted well using a second-order kinetic model, consistent with the release of redox-inactive elements such as Ni and Zn. However, Cu release and Co and Mn release from hematite require the sum of two rates to adequately model the kinetic data. Greater uptake of Fe(II) by Cu-, Co-, and Mn-substituted iron oxides relative to analogues containing only redox-inactive elements suggests that net Fe(II) oxidation occurs. Reduction of Cu, Co, and Mn in all materials following reaction with Fe(II) at pHs 7.0-7.5 is confirmed by X-ray absorption near-edge structure spectroscopy. This work shows that redox-sensitive elements structurally incorporated within iron oxides are reduced and repartitioned into fluids during Fe(II)-mediated recrystallization. Such abiotic reactions likely operate in tandem with partial microbial and abiotic iron reduction or during the migration of Fe(II)-containing fluids, mobilizing structurally bound contaminants and micronutrients in aquatic systems.

Goethite (α-FeOOH) is a source and sink for trace elements in surficial environments. Its elemental and isotopic composition may be perturbed during recrystallization, particularly when accelerated by aqueous Fe(II), but the factors that control such reactivity and the extent to which it occurs are poorly understood. Here we react goethite samples of varying crystallinity in 57 Fe-enriched Fe(II) solutions and detail the temporal distribution of the tracer isotope and the evolution of goethite crystallites. Consistent with earlier work, isotope exchange occurs between dissolved Fe(II) and goethite. By completely dissolving Fe(II)-reacted goethite in sequential steps while measuring the tracer isotope, we reconstructed the goethite recrystallized with time. Initially, the tracer isotope is enriched at the goethite surface. With continued reaction, however, the 57 Fe tracer-isotope becomes distributed throughout the bulk goethite with an isotopic composition equal to that of Fe(II) dissolved in solution. Crystallite size increased by 7−45% after a 30 day reaction period with the largest increase occurring for goethite samples with poor initial crystallinity, small particle sizes, and large specific surface areas. These results suggest that substantial exchange of Fe in goethite occurs in the presence of dissolved Fe(II) but such reactivity decreases with increasing crystallinity. Crystallite size may be a predictive feature of the potential reactivity of iron oxides in the environment, and hence the mobility of associated metal ions.

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