Comparison of different oxides

As stated before, there is no fixed dissolution rate for a given mineral-specific structure, because rate-determining factors can vary significantly for different samples of the same oxide. Nevertheless, some consistent mineral-specific differences from studies comparing different oxides have evolved. 

Mineral Dissolution rate k Activation energy eJ Frequency factor A  

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). 

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). 

In conclusion, it appears necessary to study more extensively those properties of the various oxides, which determine their specific dissolution behaviour. As pointed out by Postma (1993), the variation in reactivity, a solid phase parameter, may, in some cases, be twice as high as the effect of the type of dissolution (protonation, complexation, reduction). 

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Figure 15 presents a qualitative comparison of different oxidation types on histidine ACW. Hence, the ACP cannot be seen to be characteristic for antioxidant defense, but more likely it is a feature of prehistory connected with the free-radical processes, reflecting the degree of oxidative stress. 

Table 2 Comparison of different oxidative treatment strategies with respect to the energy demand for 50% degradation of PFOA resp. PFOS...

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