Reductants, reductive dissolution

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

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. 

Density Separation Storage Volume Required Size Modification or Separation Size Reduction Dissolution... 

From these two examples, which as will be seen subsequently, present a very oversimplified picture of the actual situation, it is evident that macroheterogeneities can lead to localised attack by forming a large cathode/small anode corrosion cell. For localised attack to proceed, an ample and continuous supply of the electron acceptor (dissolved oxygen in the example, but other species such as the ion and Cu can act in a similar manner) must be present at the cathode surface, and the anodic reaction must not be stifled by the formation of protective films of corrosion products. In general, localised attack is more prevalent in near-neutral solutions in which dissolved oxygen is the cathode reactant thus in a strongly acid solution the millscale would be removed by reductive dissolution see Section 11.2) and attack would become uniform. 

The electrochemical effects of slowly and erratically thickening oxide films on iron cathodes are, of course, eliminated when the film is destroyed by reductive dissolution and the iron is maintained in the film-free condition. Such conditions are obtained when iron is coupled to uncontrolled magnesium anodes in high-conductivity electrolytes and when iron is coupled to aluminium in high-conductivity solutions of pH less than 4-0 or more than 12 0 . In these cases, the primary cathodic reaction (after reduction of the oxide film) is the evolution of hydrogen. 

The mechanisms of oxide dissolution and scale removal have been widely studied in recent years. This work has been thoroughly reviewed by Frenier and Growcock who concluded, in agreement with others", that oxide removal from the surface of steel occurs predominantly by a process of reductive dissolution, rather than by chemical dissolution, which is slow in mineral acids. 

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. 

A mechanism such as that given above provides explanations for the known effects of many process variables ". The reductive dissolution and undermining processes require access of the acid to the metal surface, hence the benefits obtained by the deliberate introduction of cracks in the oxide by cold-working prior to pickling. Also the increase in pickling rate with agitation or strip velocity can be explained in terms of the avoidance of acid depletion at the oxide-solution interface. 

A similar study was conducted by Dimitrijevi6 et al. with neutral solutions of a-Fe203. The yield of Fe was found to be very low. However, a large Fe yield was found after dissolution of the colloid by hydrochloric acid under an argon atmosphere. This showed that electrons donated by the free radicals penetrated deep into the colloidal particles to reduce iron to F, . Buxton et al. observol in a study on the reductive dissolution of colloidal Fe304 that Fe ions in this material are less readily released into the aqueous phase than reduced Fe ions. 

Dissolved arsenic is correlated with ammonia (Fig. 4), consistent with a release mechanism associated with the oxidation of organic carbon. Other chemical data not shown here provide clear evidence of iron, manganese and sulfate reduction and abundant methane in some samples indicates that methanogenesis is also occurring. It is not clear however if arsenic is released primarily by a desorption process associated with reduction of sorbed arsenic or by release after the reductive dissolution of the iron oxide sorbent. Phreeqc analysis shows PC02 between 10"12 and 10"° bars and that high arsenic waters are supersaturated with both siderite and vivianite. 

With the emplacement of the cover, the atmospheric oxygen that fuelled the precipitation of secondary As phases was essentially eliminated. Secondary phases such as jarosite, scorodite and amorphous iron sulfo-arsenates became unstable in the present conditions in the ARS (Salzsauler et at. 2005). Reductive dissolution of the secondary phases and residual arsenopyrite gives rise to 100 mg/L As in pore water at the base of the residue pile (Salzsauler et al. 2005). 

Redox reactions are of importance in the dissolution of Fe-bearing minerals (Table 9.1). Reductive dissolution of Fe(III)(hydr)oxides can be accomplished with many reductants, especially organic and inorganic reductants, such as ascorbate, phenols, dithionite, HS, etc. Fe(II) in presence of complex formers can readily dissolve... 

We exemplify the reductive dissolution of minerals by illustrating the reductive dissolution of hydrous ferric oxide. With this oxide (Sulzberger et al. 1989, Suter et al.,... 

The first two pathways (a) and (b) show, respectively, the influence of H+ and of surface complex forming ligands on the non-reductive dissolution. These pathways were discussed in Chapter 5. Reductive dissolution mechanisms are illustrated in pathways (c) - (e) (Fig. 9.3). Reductants adsorbed to the hydrous oxide surface can readily exchange electrons with an Fe(III) surface center. Those reductants, such as ascorbate, that form inner-sphere surface complexes are especially efficient. The electron transfer leads to an oxidized reactant (often a radical) and a surface Fe(II) atom. The Fe(II)-0 bond in the surface of the crystalline lattice is more labile than the Fe(III)-0 bond and thus, the reduced metal center is more easily detached from the surface than the original oxidized metal center (see Eqs. 9.4a - 9.4c). 

Although thermodynamically favorable, reductive dissolution of Fe(III)(hydr)oxides by some metastable ligands (even those, such as oxalate, that can form surface complexes) does not occur in the absence of light. The photochemical pathway is depicted in Fig. 9.3e. In the presence of light, surface complex formation is followed by electron transfer via an excited state (indicated by ) either of the iron oxide bulk phase or of the surface complex. (Light-induced reactions will be discussed in Chapter 10.)... 

The following reaction sequence in reductive dissolution is plausible and is exemplified (Suter et al., 1991) here for the reaction of Fe(III)(hydr)oxide with ascorbate. It follows the general scheme given in Eqs. (9.4a) - (9.4c). 

Although the experiments (Suter et al., 1991) were carried out at relatively low pH, there are experimental results at pH = 7 and pH = 8 which clearly demonstrate reductive dissolution in the neutral pH-range. 

The Rate of reductive Dissolution of Hematite by H2S as observed between pH 4 and 7 is given in Fig. 9.6 (dos Santos Afonso and Stumm, in preparation). The HS" is oxidized to SO. The experiments were carried out at different pH values (pH-stat) and using constant PH2s- 1.8 - 2.0 H+ ions are consumed per Fe(II) released into solution, as long as the solubility product of FeS is not exceeded, the product of the reaction is Fe2+. The reaction proceeds through the formation of inner-sphere =Fe-S. The dissolution rate, R, is given by... 

It is remarkable that this reductive dissolution, a heterogeneous multi-electron transfer, is so fast. 

Reactivity of Fe(III)(hydr)oxide as measured by the reductive dissolution with ascorbate. "Fe(OH)3" is prepared from Fe(II) (10 4 M) and HCO3 (3 10 4 M) by oxygenation (po2 = 0.2 atm) in presence of a buffer imidazd pH = 6.7 (Fig. a) and in presence of TRIS and imidazol pH = 7.7 (Fig. b). After the formation of Fe(III)(hydr)oxide the solution is deaerated by N2, and ascorbate (4.8 10 2 M) is added. The reactivity of "Fe(OH)3 differs markedly depending on its preparation. In presence of imidazole (Fig. a) the hydrous oxide has properties similar to lepidocrocite (i.e., upon filtration of the suspension the solid phase is identified as lepidocrocite). In presence of TRIS, outer-sphere surface complexes with the native mononuclear Fe(OH)3 are probably formed which retard the polymerization to polynuclear "Fe(OH)3" (von Gunten and Schneider, 1991). 

This equation, containing the Langmuir expression for the adsorption (surface complex formation) of phenol on the Mn(III)(hydr)oxide, is similar to the principles discussed for reductive dissolution of Fe(IH)(hydr)oxide. 

Rates of reductive dissolution of amorphous manganese (111,1V) oxide particles decrease as the electrode half-wave potentials of the substituted phenols (as reported by Suatoni et al., 1961) increase (4.8 x 10 5 M total manganese, pH 4.4). 

In soils the organic matter (range 2 in Fig. 9.12) is a significant pH and pe buffer because it represents a reservoir of bound H+ and e When organic matter is mineralized alkalinity and [NO3], [SO ] increase and Fe(II) and Mn(II) become mobilized. Phosphate, incipiently bound to Fe(III)(hydr)oxides, is released as a consequence of the partial reductive dissolution of the Fe(III) solid phases. At lower pe values (range 3 in Fig. 9.12) the concentration of Fe(II) and Mn(II) further... 

The reductive dissolution of Fe(III)(hydr)oxides is also of importance in the iron uptake by higher plants. According to Brown and Ambler (1964), iron defiency causes a release of reducing exudates from the roots. These substances cause the reductive dissolution of particulate Fe(III) in the proximity of the roots. This reduction is followed by uptake of Fe(Il) into the root ceils. 

Hering, J., and W. Stumm (1990), "Oxidative and Reductive Dissolution of Minerals," in M. F. Hochella Jr. and A. F. White, Eds., Reviews in Mineralogy 23 Mineral-Water Interface Geochemistry, pp. 427-465, Mineralogical Society of America. 

In heterogeneous photoredox systems also a surface complex may act as the chromophore. This means that in this case not a bimolecular but a unimolecular photoredox reaction takes place, since electron transfer occurs within the lightabsorbing species, i.e. through a ligand-to-metal charge-transfer transition within the surface complex. This has been suggested for instance for the photochemical reductive dissolution of iron(III)(hydr)oxides (Waite and Morel, 1984 Siffert and Sulzberger, 1991). For continuous irradiation the quantum yield is then ... 

Photocatalytic Reductive Dissolution of Hematite in the Presence of Oxalate... 

In the absence of oxygen the photocatalytic reductive dissolution of hematite in the presence of oxalate occurs according to the following overall stoichiometry (Siffert and Sulzberger, 1991) ... 

In aerated suspensions no measurable (in the time-frame of typical experiments) reductive dissolution takes place and hematite acts as a photocatalyst for the oxidation of oxalate by 02 ... 

Faust and Hoffmann (1986) and Litter and Blesa (1988) who investigated the wavelength-dependence of the rate of photochemical reductive dissolution of iron(III)(hydr)oxides using hematite-bisulfite and maghemite-EDTA as model systems, respectively. 

---