Bioavailability of Iron Oxides

AG values were adopted from Stumm and Morgan (1996). For Fe(OH)3 and FeOOH a range of free energy is given as the solubility products 

The variability of FeOOH thermodynamic properties is a result of the metastability of freshly precipitated ferric iron phases (chapter 7.4.2.1, Fig. 7.10). 

Once a framework for the availability of iron oxides is established, the kinetics of individual reactions provides insight into reaction rates and rate limiting steps for the overall reactivity of iron. Here, the kinetics of microbial iron oxide reduction is explored and in section 7.4.4.1 analog information are provided for the reduction by sulfide and ligands. Building on previous experimental results demonstrating the control of mineral surface area for the degree of iron reduction (Roden and Zachara 1996 Fig. 7.13), it was shown, that also the rate of microbial iron reduction in natural sediments is of first-order and controlled by the mineral surface area (Roden and Wetzel 

Kinetic experiments with synthetic iron oxyhydroxides have shown that the initial microbial reduction rate increases with increasing initial ferric iron concentration up to a given maximum reduction rate (Bonneville et al. 2004). This observation was explained by a saturation of active membrane sites with Fe(III) centers. The respective reaction was best described with a Michaelis-Menten rate expression with the maximum reduction rate per cell positively correlating with the solubility of the iron oxyhydroxides (Bonneville et al. 2004). Kinetic studies involving iron are not only inherently important to describe reaction pathways and to derive rate constants, which can be used in models. Kinetic studies also increasingly focus on iron isotopic fractionation to better understand the iron isotopic composition of ancient sediments, which may assist in the reconstruction of paleo-environments. Importantly, iron isotope fractionation occurs in abiotic and biotic processes the degree of isotopic fractionation depends on individual reaction rates and the environmental conditions, e.g. whether reactions take place within an open or closed system (Johnson et al. 2004). 

The (microbial) dissimilatoiy iron reduction was shown in the previous section. In this section the reactions with major oxidants and reductants will be introduced. Additionally, the interactions between iron and phosphorus, as well as the formation of siderite and iron-bearing sheet silicates will be pointed out briefly to show to the variety of reactions in marine sediments coupled the reactivity of iron. 

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