Goethite-hematite reaction

So far the discussion of the goethite/hematite equilibrium refers to aqueous systems in which the water activitY (i. e. relative humidity), anp, is unity. In many cases, however, the water activity may be <1. This applies to soils and sediments where can be lowered by the binding of water in pores. When considering the dehy-droxylation reaction. 

Langmuir, D. (1971) Particle size effect on the reaction goethite = hematite + water Amer. J. Sci. 271 147-151. 

In a second example, we calculate how pH affects sorption onto hydrous ferric oxide, expanding on our discussion (Section 10.4) of Dzombak and Morel s (1990) surface complexation model. We start as before, setting the dataset of surface reactions, suppressing the ferric minerals hematite (Fe203) and goethite (FeOOH), and specifying the amount of ferric oxide [represented in the calculation by Fe(OH)3 precipitate] in the system... 

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 three most abundant minerals forms are Fe203 (hematite), Fe(OH)j (hydrous ferric oxide or ferrihydrite), and FeO(OH) (goethite). The chemical reactions describing their dissolution and K in slightly acidic water at 25°C, 1 atm, and the ionic strength of... 

Fig. 8.3 Particle size effect on the Gibbs free energy of the reaction 2 goethite (Gt) hematite (Hm) + H2O at 25 °C (Langmuir, 1971 with permission).

Various phenolic reductants have been used to dissolve synthetic goethite and hematite (LaKind Stone, 1989). In hydroquinone, goethite dissolved according to the following reaction ... 

Dos Santos Alfonso and Stumm (1992) suggested that the rate of reductive dissolution by H2S of the common oxides is a function of the formation rate of the two surface complexes =FeS and =FeSH. The rate (10 mol m min ) followed the order lepidocrocite (20) > magnetite (14) > goethite (5.2) > hematite (1.1), and except for magnetite, it was linearly related to free energy, AG, of the reduction reactions of these oxides (see eq. 9.24). A factor of 75 was found for the reductive dissolution by H2S and Fe sulphide formation between ferrihydrite and goethite which could only be explained to a small extent by the difference in specific surface area (Pyzik Sommer, 1981). 

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