Iron oxides reduction rates

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... 

Reaction rates have been usually measured by suspending a solid pellet within a controlled temperature environment (e.g., a furnace) where it is reacted with a moving gas stream. The reaction products may be all gaseous (e.g., in coal gasification) or a solid product may also be produced (e.g., in iron oxide reduction, roasting of sulfides). 

The iron carbide process is alow temperature, gas-based, fluidized-bed process. Sized iron oxide fines (0.1—1.0 mm) are preheated in cyclones or a rotary kiln to 500°C and reduced to iron carbide in a single-stage, fluidized-bed reactor system at about 590°C in a process gas consisting primarily of methane, hydrogen, and some carbon monoxide. Reduction time is up to 18 hours owing to the low reduction temperature and slow rate of carburization. The product has the consistency of sand, is very britde, and contains approximately 6% carbon, mostly in the form of Ee C. 

Reduction of BaSO appears to begin about 900°C (15). The presence of iron or iron oxide can cataly2e the barium (9) and also strontium reduction reaction rates. However, iron impurity can also increase the acid-soluble content of the black ash (9). 

Sonoelectrochemistry has also been used for the efficient employment of porous electrodes, such as carbon nanofiber-ceramic composites electrodes in the reduction of colloidal hydrous iron oxide [59], In this kind of systems, the electrode reactions proceed with slow rate or require several collisions between reactant and electrode surface. Mass transport to and into the porous electrode is enhanced and extremely fast at only modest ultrasound intensity. This same approach was checked in the hydrogen peroxide sonoelectrosynthesis using RVC three-dimensional electrodes [58]. 

Figure 36. Comparison of rate of reduction of iron oxide with and without co-MSFB (Tan, Yao, Wang, Liu, andKwauk, 1983).

For a particular iron(III) oxidant, the rate constant (log kpe) for electron transfer is strongly correlated with the ionization potential Ip of the various alkylmetal donors in Figure 4 (left) (6). The same correlation extends to the oxidation of alkyl radicals, as shown in Figure 4 (right) (2). [The cause of the bend (curvature) in the correlation is described in a subsequent section.] Similarly, for a particular alkylmetal donor, the rate constant (log kpe) for electron transfer in eq 1 varies linearly with the standard reduction potentials E° of the series of iron(III) complexes FeL33+, with L = substituted phenanthroline ligands (6). 

One of the most ingenious ways in which corrosion is inhibited is to strap a power pack to each leg (just above the level of the sea) and apply a continuous reductive current. An electrode couple would form when a small portion of the iron oxidizes. The couple would itself set up a small voltage, itself promoting further dissolution. The reductive current coming from the power pack reduces any ferric ions back to iron metal, which significantly decreases the rate at which the rig leg corrodes. 

Based upon thermodynamic data given in Table I, oxidant strength decreases in the order NijO > Mn02 > MnOOH > CoOOH > FeOOH. Rates of reductive dissolution in natural waters and sediments appear to follow a similar trend. When the reductant flux is increased and conditions turn anoxic, manganese oxides are reduced and dissolved earlier and more quickly than iron oxides (12, 13). No comparable information is available on release of dissolved cobalt and nickel. 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

This variation in the efficiency of reduction, with pse of time, is clearly illustrated in the graph. Fig. g, hich shows the carbon monoxide and carbon dioxide jntent of the water gas after passing at the rate of 300 cubic feet per hour over 4 2 tons of iron oxide, sated to 750° C. 

Fig. 16.9 Change in first-order rate constant k for the reduction of CgCl NO as a result of varying goethite content in media with 473 iM Fe(II) and 200mM NaCl (pH 6.96). Error bars to indicate 95% confidence intervals would be smaller than symbols. Reprinted with permission from Klupinski TP, Chin YP, Traina S J (2004) Abiotic degradation of pentachloronitrobenzene by Fe(II) Reactions on goethite and iron oxide nanoparticles. Environ Sci Technol 38 4353-4360. Copyright 2004 American Chemical Society...

Reduction of these PHMs took place in experiments containing both Fe(ll) and iron oxide minerals, under anoxic conditions. The transformation of PHMs by surface-bound Fe(ll) generally follows a pseudo-first-order kinetic rate law, expressed by... 

Table 16.3 Names, abbreviations, pseudo-first-order rate constants, and half-lives of polyhalo-genated alkanes in Fe(II)/goethite suspension. Experimental conditions 25 m L" goethite, pH 7.2, tgq>24 h. Fe(II) = 1 mM. b Standard deviation, c number of replicates, d t =5 h. Reprinted with permission from Pecher K, Haderline SB, Schwarzenbach RP (2002) Reduction of polyhalo-genated methanes by surface-bound Fe(II) in aqueous suspensions of iron oxides. Environ Sci Technol 36 1734-1741. Copyright 2002 American Chemical Society...

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