Goethite reduction

Iron frequently has been postulated to be an important electron acceptor for oxidation of sulfide (58, 84,119, 142, 152). Experimental and theoretical studies have demonstrated that Fe(III) will oxidize pyrite (153-157). Reductive dissolution of iron oxides by sulfide also is well documented. Progressive depletion of iron oxides often is coincident with increases in iron sulfides in marine sediments (94, 158, 159). Low concentrations of sulfide even in zones of rapid sulfide formation were attributed to reactions with iron oxides (94). Pyzik and Sommer (160) and Rickard (161) studied the kinetics of goethite reduction by sulfide thiosulfate and elemental S were the oxidized S species identified. Recent investigations of reductive dissolution of hematite and lepidocrocite found polysulfides, thiosulfate, sulfite, and sulfate as end products (162, 163). 

Again, aU reactions are thermodynamically feasible, with the reduction of iron (III) hydroxide the most favourable. In contrast, goethite reduction is only just thermodynamically feasible. Such data are useful in considering the various redox pairs, and their relative favourability, avaUable to iron-reducing micro-organisms. 

In the goethite process, the precipitation of iron from solution occurs in the form of hydrated ferric oxide, FeOOH. The commercial development of the process was due to Societe de La Vielle Montagne. The process basically involves the reduction of iron to the ferrous state, and this is followed by oxidation by air at a temperature of around 90 °C and at a pH controlled at around 3.0. The reaction can chemically be shown as ... 

The principal differences between the goethite and the jarosite processes take place following the hot acid leaching of the zinc ferrite residues. In the goethite process, the liquor from hot acid leaching, holding (in g l-1) 100 Zn, 25-30 Fe3+ and 50-60 H2S04, is initially subjected to a reduction step, where the ferric iron is reduced to the ferrous form by reaction with unroasted zinc sulfide concentrate at 90 °C ... 

This discrepancy might be explained if after about an hour the reaction approached equilibrium and slowed due to a diminishing thermodynamic drive. If the Fe+++ produced did not precipitate on the hematite surface, and did not form either hematite or goethite (FeOOH), it would accumulate in solution and weaken the drive for uranyl reduction. As the saturation index for hematite reached about 1.7, or about 1.25 for goethite, reaction would cease. 

The standard calorimetric reaction of tris(hydroxymethyl) aminomethane (THAM) neutralization with HC1 was used in several initial experiments to determine both precision and accuracy for the data acquisition and reduction process. Three to five minutes were allowed between acid additions, since this same time frame was used for all later suspension titrations in order to minimize the effects of slow surface reactions which occur during a titration (9,29,30). The amount of acid added in each experiment was varied to generate heat changes of 40-400 mj (typical heat changes observed in our adsorption studies with goethite suspensions). 

In the sorption experiments of Icopini et al. (2004), the measured isotopic contrast between Fe(II)aq and the goethite starting material was -0.8%o after Fe(II) had sorbed to the surface over 24 hours in this case, the isotopic fractionation between sorbed Fe(II) and Fe(II)aq is not the 0.8%o measured difference, but is approximately +2.1%o based on an inferred 8 Fe value for the sorbed component as calculated from Fe mass balance (Fig. 4), as was noted in that study. Measured differences in Fe isotope compositions between ferric oxide/hydroxide and Fe(II)aq during dissimilatory Fe(III) reduction and photosynthetic Fe(II) oxidation have been proposed to reflect fractionation between soluble Fe(II) and Fe(III) species, where the soluble Fe(III) component is postulated to be bound to the cell and is not directly measured (Beard et al. 2003a Croal et al. 2004). In the case of dissimilatory Fe(III) reduction, assuming a static model simply for purposes of illustration, if 50% of the Fe in a pool that is open to... 

Larson O, Postma D (2001) Kinetics of reductive bulk dissolution of lepidocrocite, ferrihydrite, and goethite. 

Wielinga et al. (2001) demonstrated this process by incubating goethite anaerobically at pH 7 with lactate and an iron-reducing bacterium, and introducing Cr(VI) after commencement of Fe(III) reduction (Figure 7.7). In treatments without Cr(VI), accumulation of Fe(II) in solution continued, but in the treatments with Cr(VI) it was reversed in abiotic controls there was no accumulation of Fe(II). Chromate can also be reduced abiotically by sulfide. 

Fig. 16.9 Change in first-order rate constant k for the reduction of CgCl NO as a result of varying goethite content in media with 473 iM Fe(II) and 200mM NaCl (pH 6.96). Error bars to indicate 95% confidence intervals would be smaller than symbols. Reprinted with permission from Klupinski TP, Chin YP, Traina S J (2004) Abiotic degradation of pentachloronitrobenzene by Fe(II) Reactions on goethite and iron oxide nanoparticles. Environ Sci Technol 38 4353-4360. Copyright 2004 American Chemical Society...

Table 16.3 Names, abbreviations, pseudo-first-order rate constants, and half-lives of polyhalo-genated alkanes in Fe(II)/goethite suspension. Experimental conditions 25 m L" goethite, pH 7.2, tgq>24 h. Fe(II) = 1 mM. b Standard deviation, c number of replicates, d t =5 h. Reprinted with permission from Pecher K, Haderline SB, Schwarzenbach RP (2002) Reduction of polyhalo-genated methanes by surface-bound Fe(II) in aqueous suspensions of iron oxides. Environ Sci Technol 36 1734-1741. Copyright 2002 American Chemical Society...

XANES spectra provide information about contaminants present at too low a level to produce EXAFS spectra. It has been used to investigate the oxidation of As on Mn-goethite (Sun et al. 1999). This technique has also provided information about the valence of Fe in Fe oxide films during cathodic reduction in a borate buffer (Schmicki et al. 1996), about the dissolution of Fe oxide films in acidic solutions (Vir-tanen et al. 1997) and about the orientation of styrene molecules adsorbed on FeO (111) and Fe304 (111) (Wuehn et al. 2000). 

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