Iron oxyhydroxide system

Fig. 15-5 Comparative adsorption of several metals onto amorphous iron oxyhydroxide systems containing 10 M Fej and 0.1 m NaNOs. (a) Effect of solution pH on sorption of uncomplexed metals, (b) Comparison of binding constants for formation of soluble Me-OH complexes and formation of surface Me-O-Si complexes i.e. sorption onto Si02 particles, (c) Effect of solution pH on sorption of oxyanionic metals. (Figures (a), (c) reprinted with permission from Manzione, M. A. and Merrill, D. T. (1989). "Trace Metal Removal by Iron Coprecipitation Field Evaluation," EPRI report GS-6438, Electric Power Research Institute, California. Figure (b) reprinted with permission from Balistrieri, L. et al. (1981). Scavenging residence times of trace metals and surface chemistry of sinking particles in the deep ocean, Deep-Sea Res. 28A 101-121, Pergamon Press.)...

Aryanpour, M., van Duin, A.C.T., and Kubicki, J.D. Development of a reactive force field for iron-oxyhydroxide systems. Journal of Physical Chemistry A, 114, 6298-6307, 2010. 

This subsection has been divided into two parts. The first will present results of two rather recent contributions from which considerable insight into the electrochemical behavior of the iron oxyhydroxide system has been obtained, whereas the second part will address studies of the passive film in borate buffer media. 

Research conducted at Washington State University, as well as in situ applications by commercial entities, has indicated that stabilization of hydrogen peroxide is necessary for effective subsurface injection [39]. Without stabilization, added peroxide decomposes rapidly through interaction with iron oxyhydroxides, manganese oxyhydroxides, dissolved metals, and enzymes (e.g., peroxidase and catalase). Some of these peroxide decay pathways involve nonhydroxyl radical-forming mechanisms, and therefore are especially detrimental to Fenton oxidation systems. 

Aqueous Fe2+ and many of its coordination complexes serve as excellent catalysts for the formation of hydroxyl radical from hydrogen peroxide. Iron oxyhydroxides have also been found to catalyze the formation of hydroxyl radical [45], although at a much slower rate than dissolved iron. Consequently, a number of researchers have investigated the potential for using soil minerals as catalyst to avoid the need for the addition of soluble iron to the system. 

Coupled Iron-phosphorus Cycling. The affinity of phosphate for sorptive association with ferric oxide and oxyhydroxide phases, well documented in soil and freshwater systems (see Sections 8.13.3.1 and 8.13.3.2), is also a well-studied process in marine systems. Three distinct marine environments where coupled iron-phosphorus cycling has been identified as an important process are MOR systems, estuaries, and continental margin sediments. The purely physicochemical process of sorption is essentially the same in these three distinct environments, where an initial, rapid surface sorption phase is followed, given enough time, by a redistribution of adsorbed phosphate into the interior of iron oxyhydroxides by solid-state diffusion (Bolan et al., 1985 ... 

The fi O-Pj system has also recently been applied to phosphates associated with ferric iron oxyhydroxide precipitates in submarine ocean ridge sediments (Blake et al., 2000, 2001). The gi O-Pj signature of phosphate associated with these authigenic Fe-oxyhydroxide precipitates indicates microbial phosphate turnover at elevated temperatures. The latter observation suggests that phosphate oxygen isotopes may be useful biomarkers for fossil hydrothermal vent systems. On the basis of this work, Blake et al. (2001) also hypothesize that authigenic phases extant on other planets may retain imprints of primitive biospheres, in the form of detectable and diagnostic fi O-Pj composition, imparted by biochemical, enzymatic processes. 

Duringpumping, ferrous iron is sorbed to 0.19mmol/l exchange sites, and furthermore to 0.19 mmol/1 precipitated iron-oxyhydroxide. The amount sorbed to iron-oxyhydroxide was calculated to be 0.2 mol Fe "" per mol iron-oxyhydroxide for the water composition of PP8 (discussed later). The total iron sorbed during the first cycle is thus 0.19 + (0.2 x 0.19) = 0.23 mmol/1. The expected efficiency is therefore, from eqn. 9, E = Rre/ROi = (1 + 0.23/0.1)/ 1.17 = 2.8. Note that it is the dynamics of the system, expressed by the retardations of the ions, that makes the efficiency deaease to a much smaller value than was estimated earlier when assuming simply that all O2 was available for oxidation of iron. 

First, it is remarkable that the As concentration was quite high already when injected water was backpumped. It may be that As was sorbed to colloidal iron-oxyhydroxide particles formed during oxidation, but were too small to be removed by filtration over 0.45 pm before analysis. This mechanism was suggested by Rott et al. (1996) who observed similar As peaks during the first cycles of an in situ iron removal system. When samples were analyzed from a later cycle in Schuwacht, the As concentrations had decreased to about 2 pg As/1 and they were similar in unfiltered and 0.1 pm filtered subsamples. Thus, sorption to colloidal iron cannot be mled out as a mechanism and it should be investigated thoroughly in the incipient cycles of another system. 

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). 

Further development of these methods will allow the examination of structural effects of systems, such as iron oxyhydroxides or natural organics, on filtration behaviour. In this thesis it was shown that there are structural effects, but their quantification for these systems (that is, oxyhydroxide floes and organic-calcium aggregates) was not possible. 

Several important points have emerged from the structural analyses performed on single crystals of ferritins. The identification of the ferroxidase centers is one, the similarities and differences between ferritins from different parts of the same organism another, and the differences between, for example, mammalian and bacterioferritins a third. Another relevant consideration is the fact that very little is really known about the iron sites in vivo, especially the construction of the core. Most of what has been suggested about the coordination environments of the iron centers has been inferred from a combination of mutagenesis studies, comparison with better-characterized systems, e.g., diiron protein sites to compare with the ferroxidase center or iron oxyhydroxide minerals for the core, and some model compound studies. 

Phosphorus solubility is also directly affected by the changes in redox potential (Patrick, 1964). In well-drained mineral soils, some of the inorganic P is bound to oxidized forms of iron such as iron oxyhydroxides. Ferric phosphate is a common phosphate compound in these systems. Similarly, in oxidized portions of mineral wetland soils, some of the inorganic phosphorus is also present as ferric phosphate. The mineral form of FeP04 is called strengite. 

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