Magnetite reductive dissolution

In general there does not appear to be any direct correlation between the rate of the chemical dissolution of oxides and the rate of scale removal, although most work on oxide dissolution has concentrated on magnetite. For example, Gorichev and co-workers have studied the kinetics and mechanisms of dissolution of magnetite in acids and found that it is faster in phosphoric acid than in hydrochloric, whereas scale removal is slower. Also, ferrous ions accelerate the dissolution of magnetite in sulphuric, phosphoric and hydrochloric acid , whereas the scale removal rate is reduced by the addition of ferrous ions. These observations appear to emphasise the importance of reductive dissolution and undermining in scale removal, as opposed to direct chemical dissolution. 

Few comparative studies have been made on the reductive dissolution of different mineral phases. In one such study, the order of reaction with seven organic and transition metal reductants was found to be the same hematite (a-Fe203)>magnetite (FejO,/,)>nickel ferrite (NiFe204) (43). Magnetite is an interesting case, since both Fe(III) and Fe(II) are present in the lattice prior to reaction. Evidence indicates that Fe(IIl) sites reduced to Fe(II) sites by redox reaction dissolve more quickly than Fe(II) sites originally present in the mineral lattice (6). 

Biological reductive dissolution by Shewanella putrifaciens of Fe oxides in material from four Atlantic pleistocene sediments (ca. 1.5-41 g/kg Fe oxides) was compared with that of the synthetic analogues (ferrihydrite, goethite, hematite) (Zachara et al. 1998). In the presence of AQDS as an electron shuttle, the percentage of bio-reduc-tion of the three oxides was increased from 13.3 %(fh) 9.2%(gt) and 0.6%(hm) to 94.6% 32.8% and 9.9% with part of the Fe formed being precipitated as vivianite and siderite, but not as magnetite. The quinone was reduced to hydroquinone which in turn, and in agreement with thermodynamics, reduced the Fe as it had much better access to the oxide surface than did the bacteria themselves. 

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). 

Hematite Magnetite Reduction Reduction-dissolution reprecipitation Reducing gas Alkaline solution with N2H4... 

Bruyere.V.I.E. Blesa, M.A. (1985) Acidic and reductive dissolution of magnetite in aqueous sulphuric add. Site-binding model and experimental results. J. Electroanal. Chem. 182 141-156... 

Buxton, G.V. Rhodes,T. Sellers, R.M. (1983) Radiation chemistry of colloidal hematite and magnetite in water reductive dissolution by 1-mefhylefhanol radicals (EDTA) iron(ll). J. Chem. Soc. Faraday Trans. 1. 79 2961-2974 Bye, G.C. Howard, C.R. (1971) An examination by nitrogen adsorption of the thermal decomposition of pure and silica doped goefhite. J. Appl. Chem. Biotechnol. 21 324-329... 

Dos Santos Afonso, M. Di Risio, C.D. Roit-berg. A. Marques R.O. Blesa, M.A. (1990) Reductive dissolution of neutron- and gamma-irradiated magnetite. Radiat. Phys. Chem. 36 457-460... 

The reductive dissolution of solid compounds in anaerobic soils, sediments, and waters begins with the reduction of prominent cations within the compounds. Many Fe(III) (oxy)(hydr)oxide compounds are especially susceptible to reductive dissolution. The reduction process converts Fe(III) into more water-soluble Fe(II). The formation of Fe(II) causes the (oxy)(hydr)oxides to decompose in water. In some cases, the Fe(II) rapidly precipitates as new solid compounds, such as siderite (FeCCT) or magnetite (FesCL). 

Redox reactions are of importance in the dissolution of Fe-bearing minerals. Reductive dissolution of Fe(III)(hydr)oxides can be accomplished with many reductants especially organic and inorganic reductants, such as ascorbate, phenols, dithionite, and HS. Fe(II) in the presence of complex formers can readily dissolve Fe(III)(hydr)oxides. The Fe(II) bound in magnetite and silicate and adsorbed to oxides can reduce O2 (White, 1990 White and Yee, 1985). 

These observations indicate that reduction of As(V) to As(III) does not, in itself, result in the mobilization of arsenic. This conclusion is supported by laboratory adsorption studies showing similar affinities of As(III) and As(V) for hydrous ferric oxide, goethite, and magnetite.16 However, outstanding questions remain regarding the factors that control the rate and extent of the reductive dissolution of iron in these sediments and whether the arsenic (and iron) that is released into the porewater is (re)sorbed onto the residual iron oxyhydroxides in... 

The major cause of removal of oxide films on iron is the reductive dissolution of the oxides. The anodic current for the process may be supplied by an outside source or by areas of the metal at which iron is dissolved. In both cases the ferric oxide is reduced to Fe". The solubility of Fe(0H>2 is much higher than either Fe203, FesOi, or the FeOOH s and it dissolves. The magnetite component of the oxide is reduced in part to Fe and in part to Fe metal. 

Equation 5 describes the oxidation of magnetite to form maghemite and equation 6 describes the reductive dissolution of magnetite (22). The decoupled half cell reactions describing ilmenite oxidation (equation 3) are... 

Sharp current minimums in Figure 2 for magnetite (400 mV) and ilmenite (-40 mV) correspond to conditions in which no current is applied and the rates for the anodic and cathodic half cell reactions are equivalent and equal to equation 1. Polarization of the magnetite electrode to potentials less than 400 mV results in the dominance of the reductive dissolution of magnetite as described by equation 6. This reaction consumes electrons by reducing ferric atoms in the magnetite structure and releasing Fe(II) to solution. 

Figure 2. Potentiodynamic scans plotted as functions of applied potential E and measured current i for natural magnetite (A) and ilmenite (B) electrodes in anoxic solutions at pH 3. Inset is a detail of the current peak defining reductive dissolution of magnetite as a function of pH (adapted from ref. 16)...

The reductive dissolution of the outer y-FejOj layer exposes the inner magnetite layer of the oxide him. In acid solutions (pH less than 4) the magnetite layer rapidly dissolves but in near neutral solution it may be stable and protective, depending on the nature of the anion present and its concentration The magnetite layer is stable in inhibitive solutions of anions, e.g. benzoate , carbonate ", hydroxide ", borate (though not bicarbonate ). The stability of the magnetite layer controls the inhibition of corrosion of iron when coupled to electronegative metals such as aluminium, zinc or cadmium . 

Haruyama and Masamura stated that the reductive dissolution of magnetite occurs with 100% efficiency In the potential range from 0-900 mV (vs. SHE), as follows ... 

Shoesmith et al. has made an extensive study of oxide-covered iron electrodes in EDTA and citric acid solutions. Three distinct potential regions were observed. In Region I ( > -100 mV vs. SCE), little Fe + was released, and there was only minor oxide dissolution. This is considered the induction period for pore formation. In Region II (-450 mV < E < -100 mV), potential values were between those of magnetite reduction (reductive dissolution) and metal dissolution, suggesting that autoreduction. 

Other work on the mechanism of magnetite dissolution in chelants was recently reported. Hausler looked at magnetite dissolution in ammonium EDTA at pH values from 4.2 to 7.0, and found that hydrazine accelerated the dissolution rate at a pH of 7 but not at 4.2. Instead of invoking the accepted reductive dissolution mechanism, he proposed an unusual N2H4-Fe(lil)EDTA complex to explain his results. 

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