Iron oxides reduction

Figure 35. Experimental co-MSFB reactor for iron oxide reduction. (Yan, Yao, Wang, Liu, and Kwauk, 1983.)...

Katayama, H., Ohno, T., Yamauchi, M., Matsuo, M., Kawamura, T., and Ibaraki, T., Mechanism of Iron Oxide Reduction and Heat Transfer in the Smelting Reduction Process with a Thick Layer of Slag, ISIJ Int., 32 95 (1992)... 

Rau, M. Rieck, D. Evans, J.W. (1987) Investigation of iron oxide reduction by TEM. Metallurgical Transactions 188 257-278 Raven, K.P. Jain, A. Loeppert, R.H. (1998) Ar-senite and arsenate adsorption on ferrihy-drite Kinetics, equilibrium, and adsorption envelopes. Environ. Sci. Techn. 32 344-349 Rea, B.A. Davis, J.A. Waychunas, G.A. (1994) Studies of the reactivity of the ferrihydrite surface by iron isotopic exchange and Moss-bauer spectroscopy. Clays Clay Min. 42 23-34... 

Oxidation of arsenic-bearing pyrite with adsorption onto iron oxides and/or other metal (oxy)(hydr)oxides Nitrate reduction by pyrite oxidation (note that Appelo and Postma, 1999 referred to pure rather than arsenian pyrite) Manganese oxide reduction and release of sorbed arsenic Fe(lll) reduction on oxide surfaces changes net charge leading to arsenic desorption Iron oxide reductive dissolution and release of sorbed arsenic catalyzed by NOM degradation... 

Fig. 11.14. Exergy vector diagram for an iron oxide reduction, FeO - Fe + 0.5O2, coupled with a heating process at 2800 K [Ref. 15.].

Consider now the dissolution of iron by replacing the Pt WE with an Fe wire and adding 1 M Fe2+ to the solution in chamber B (via dissolution of FeS04, for example), as shown in Fig. 22. Calculation of the reversible potential for iron dissolution indicates that it would be the same as the standard reversible potential, -0.44 V (NHE), as the ferrous ion is at unit activity. We will assume that only iron oxidation/reduction can occur in this cell. Changing the polarity of the variable voltage supply allows removal of electrons from the WE, forcing net oxidation to occur there and net reduction to occur on the Pt CE in chamber C. Figure... 

Let us take the example of iron oxide reduction in solid state by carbon monoxide, as it happens in the sponge iron making process. 

Iron is released by iron oxide reduction and from the weathering of other Fe-containing minerals such as biotite and chlorite. Some Fe-reducing... 

Given this background, it is only possible at present to pose hypotheses and to test them against the available data. The two main hypotheses regarding the source and mobilization of As that have emerged are (i) the pyrite oxidation hypothesis, and (ii) the iron oxide reduction hypothesis. A number of other hypotheses have also been proposed but so far, at least, with less widespread support. We discuss these below. A more detailed discussion is given in Kinniburgh (2001). 

The iron oxide reduction hypothesis was used to explain the high As concentrations in some groundwaters from Ohio, USA by Matisoff et al. (1982) and later more generally by Korte (1991). It has been applied to the West Bengal and Bangladesh situations by Bhattacharya et al. (1997), Nickson et al. (1998) and DPHE/BGS/MML (1999). 

The following factors lend support to some kind of iron oxide reduction hypothesis ... 

There are two schools of thought about the time of the initial As mobilization either (i) it is recent and has been induced by man s activities [there are proponents of this who support both the pyrite oxidation hypothesis and the iron oxide reduction hypothesis (Acharyya et al., 2000)], or (ii) it occurred much earlier and is therefore dominantly a natural process. While we believe that an early release date, (ii) above, is the more likely, this is not to imply that man s recent activities have not had, or will not have, any impact on the extent of the groundwater arsenic problem. For example, recent changes in land use such as irrigation will not only alter the groundwater flow patterns but could also affect the boundary conditions for oxygen diffusion into the aquifer and so could also affect its redox status (Bhattacharya et al., 1997). 

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... 

A highly significant, positive relationship has been shown between iron oxide reduction and organic nitrogen mineralization in tropical wetland soils (Sahrawat, 2004). The following empirical relationships were reported ... 

Besides the chemical attack of oxygen reduction products on the organic layer, it has been discussed by several authors that a change in the oxide thickness due to the high pH of about 12-14 is also possible, and contributes to the delamination process [160, 161]. However, in situ XPS measurements showed that, even in alkaline media, a significant iron oxide reduction is possible only at potentials more negative than —0.8 Vshe [162]. 

---