Iron oxalate dissolution

Except for phthalic acid, all other carboxylic acids studied induce considerable increases in the light compared to the dark values (the relatively high rate of iron oxide dissolution induced by oxalic acid has been extensively studied (5,8). Phthalic acid actually appears to stabilize the iron oxide against photodissolution despite the solution phase complex exhibiting some photoactivity. 

Various chemical extraction techniques have been introduced in order to selectively remove metals from the different adsorption or complexation sites of natural sediments (e.g., Tessier et al, 1979 Erel et al, 1990 Leleyter et al., 1999). It is, for example, shown by Leleyter et al. (1999) that between 20% and 60% of REE in various suspended river sediments are removed by successive extractions by water, by Mg(N03)2 (exchangeable fraction), sodium actetate (acid-soluble fraction), NH2OH - - HCl (manganese oxide dissolution) ammonium oxalate (iron oxide dissolution) and a mixture of H2O2 + HNO3 (oxidizable fraction). The complexity of... 

Beck et al. used oxalic acid to form a layer of iron oxalate to prevent the dissolution and support the formation of the polymer layer. ° Ferreira etched the steel surface with a dilute nitric acid to form an iron nitride layer, which inhibited the iron dissolution. 

Pathway (d) in Fig. 9.3 provides a possible explanation for the efficiency of a combination of a reductant and a complex former in promoting fast dissolution of Fe(III) (hydr)oxydes. In this pathway, Fe(II) is the reductant. In the absence of a complex former, however, Fe2+ does not transfer electrons to the surface Fe(III) of a Fe(III) (hydr)oxide to any measurable apparent extent. The electron transfer occurs only in the presence of a suitable bridging ligand (e.g., oxalate). As illustrated in Fig. 9.3d, a ternary surface complex is formed and an electron transfer, presumably inner-sphere, occurs between the adsorbed Fe(II) and the surface Fe(III). This is followed by the rate-limiting detachment of the reduced surface iron. In this pathway, the concentration of Fe(U)aq remains constant while the concentration of dissolved Fe(III) increases thus, Fe(II)aq acts as a catalyst to produce Fe(II)(aq) from the dissolution of Fe(III)(hydr)oxides. 

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.)... 

Dissolution of goethite by oxalate in the presence of different concentrations of ferrous iron. The reaction mechanism proposed is that of Fig. 9.3d. The change in the concentration of Fe(III) is given (preconditioning of the surface introduces some incipient Fe(III)). pH = 3.0, goethite 0.46 gIt, oxalate 0.001 M. 

Rate of the photochemical reductive dissolution of hematite, = d[Fe(II)]/dt, in the presence of oxalate as a function of the wavelength at constant incident light intensity (I0 = 1000 peinsteins "1 lr1). The hematite suspensions were deaerated initial oxalate concentration = 3.3 mM pH = 3. (In order to keep the rate of the thermal dissolution constant, a high enough concentration or iron(II), [Fe2+] = 0.15 mM, was added to the suspensions from the beginning. Thus, the rates correspond to dissolution rates due to the surface photoredox process). 

Blesa MA, Marinovich HA, Baumgartner EC, Maroto AJG. 1987. Mechanism of dissolution of magnetite by oxalic acid ferrous iron solutions. Inorganic Chemistry 26 3713-3717. 

Reductive dissolution may be more complex than the two previous mechanisms in that it involves electron transfer processes. Formation of Fe" via reductive dissolution can be effected by adsorption of an electron donor, cathodic polarization of an electrode supporting the iron oxide and by transfer of an electron from within a ternary surface complex to a surface Fe ". Addition of Fe" to a system containing a ligand such as EDTA or oxalate promotes electron transfer via a surface complex and markedly accelerates dissolution. 

In addition to oxalate, malonate and citrate accelerate the dissolution of iron oxides in the presence of Fe (Sulzberger et al., 1989). Fe " also promotes the dissolution of magnetite in sulphuric acid (Bruyere Blesa, 1985). Small amounts of Fe in solution speed up the transformation of ferrihydrite to goethite at 50 °C (see Fig. 14.24) by promoting the dissolution of ferrihydrite (Fischer, 1972). Adsorption... 

There are two principal synthetic routes to dicarboxylate complexes. One of these uses an aqueous solution of the alkali metal dicarboxylate and the corresponding metal halide,93 while the other depends upon the dicarboxylic acid reduction of higher oxidation state metals. This reductive property of oxalic acid results in its ready dissolution of iron oxides and hence a cleaning utility in nuclear power plants.94 Mention must also be made of the successful ligand exchange synthesis of molybdenum dicarboxylates, Mo(dicarboxylate)2 H2 O, from the corresponding acetate complex. Unfortunately the polymeric, amorphous and insoluble nature of these complexes has restricted the study of these systems, which may well provide examples of multiple M—M bonding in dicarboxylate coordination chemistry.95... 

The resulting Fe(III) oxalate complex is an excellent chromophore and will undergo homogeneous photolysis to produce additional Fe(II) species which, in turn, enhance dissolution of the iron oxide. 

Air Spiked air particulate dry and wet ashed dissolution coprecipitation with iron hydroxide and Ca oxalate, purification by solvent extraction and electrodeposition onto platinum a-Spectrometry (isotope quantification) 0.02 dpm/L for in solution No data Singh and Wrenn 1988... 

Cheah S.-F., Kraemer S. M., Cervini-Silva J., and Sposito G. (2003) Steady-state dissolution kinetics of goethite in the presence of desferrioxamine B and oxalate ligands implications for the microbial acquisition of iron. Chem. Geol. 198, 63-75. 

The various elementary steps involved in the surface photoredox reaction, leading to dissolution of hematite in the presence of oxalate, are outlined in Figure 12.10. The two-dimensional stmcture of the surface of an iron(III) hydroxide given in this figure is highly schematic. The charges indicated correspond to relative charges. An important step is the formation of a hypothetical bidentate, mononuclear surface complex. With pressure jump relaxation technique, it has... 

Suter, D., Siffert, C., Sulzberger, B., and Stumm, W. (1988) Catalytic Dissolution of Iron(lII)(Hydr)Oxides by Oxalic Acid in the Presence of Fe(II), Naturwissenschaften 75, 571-573. 

Shi JP, Khan A A, Harrison RM (1999) Measurements of ultrafine particle concentration and size distribution in the urban atmosphere. Sci Total Environ 235 51-64 Siefert RL, Pehkonen SO, Erel Y, Hoffmatm MR (1994) Iron photochemistry of aqueous suspensions of ambient aerosol with added organic-acids. Geochim Cosmochim Acta 58 3271-3279 Sievering H, Boatman J, Gorman E, Kim Y, Anderson L, Ennis G, Luria M, Pandis S (1992) Removal of sulphur from the marine boimdaiy layer by ozone oxidation in sea-salt aerosols. Nature 360 571-573 Siffert C, Sulzberger B (1991) Light-induced dissolution of hematite in the presence of oxalate-A case-study. Langmuir 7 1627-1634... 

The kinetics of the photochemical reductive dissolution of lepidocrocite (y-FeOOH) with oxalate as the reductant depends strongly on pH both the rate and the overall rate constant, k> decrease with increasing pH. This behavior means that the pH dependence of the rate does not simply reflect the pH dependence of oxalate adsorption at the lepidocrocite surface. Between pH 3 and 5, the log k() values can be fitted with a straight line. The dependence of k on the concentration of surface protons, >FeOH2+, can be estimated from the slope of this line and from the protonation curve of lepidocrocite k0 >FeOHf I6. The value of 1.6, which can be considered only a rough estimate, is not too different from the theoretically expected value of 2 for the proton-catalyzed detachment of reduced surface iron centers (i.e., of surface metal centers with the formal oxidation state of II). 

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