Reduction of Goethite

Sedimentary Fe(lll) oxyhydroxides, of which goethite is the most common, are the source of iron for most precipitated Fe(II) sulfides. Pyzik and Sommer (1981) (see also Morse et al. 1987) have proposed a number of possible reaction mechanisms for the reduction of goethite to FeS. All involve initial reactant HS , with production of Fe % and S°, polysulfide ions or thousulfate. These species then react to form the Fe(II) sulfides. Their initiating reactions are... 

The results of Wielers and co-workers have shown that Fe(II) is stabilized by some interaction with alumina. The stabilization may be due to the formation of FeAl204 or, more probably, to an interaction between ferrous oxide and an alumina or iron(II) aluminate surface. Kock and co-workers demonstrated the stabilization of Fe(II) by alumina using magnetic measurements. The authors studied the reduction of goethite (FeOOH), hematite (Fe203), a physical mix of FeOOH and alumina, and FeOOH deposited on alumina with carbon monoxide. They raised the temperature of their samples held in a flow of 5% carbon monoxide in helium, with a heating rate of 4.8 K min up to a temperature of 770 K and... 

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 ... 

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...

The rate of disproportionation increases with decreasing pH and rising temperature and these conditions also favour oxide reduction. To achieve a reasonable rate of dissolution, one has to compromise on the pH. A pH of 3 is used in kaolin bleaching (Jepson, 1988), whereas in soil analysis the system is usually buffered with citrate and bicarbonate at ca. pH 7 (Mehra Jackson, 1960). Citrate also complexes the dissolved Fe " and prevents its precipitation as Fe" sulphide. For the dithionite/EDTA system, Rueda et al. (1992) found a maximum efficiency at pH 5-6 and an activation energy for goethite dissolution of 70 kj mol". They stressed the importance of adsorption of S2O4 on the surface to ensure reduction of Fe ". 

Fig. 12.15 Di ssolution rate of goethite by protonation, complexation with oxalate and reduction by ascorbic acid as a function of pH (Stumm, Furrer, 1987, with permission).

The rate of the biotic reduction of Fe oxides by a strain of Corynehacterium under 02-free conditions followed the order natural ferrihydrite > synthetic goethite > hematite (Fischer (1988) (Fig. 12.29) in accordance with the sequence in reducibility by Fe-reducing bacteria isolated from a eutrophic lake sediment (Jones et al., 1983). Iron from ferrihydrite reduced by Shewandla alga was found to be isotopically lighter than that of the ferrihydrite Fe by a 5 ( Fe/ " Fe) of 1.3 %o This difference may be used to trace the distribution of microorganisms in modern and ancient earth (Beard etal. 1999). 

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