Dissolution of oxide

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. 

The dissolution of oxides and hydroxides can be presented schematically as follows ... 

Phillips, J. F. Dissolution of Oxide-Coated Zirconium and Zirconium Alloys, U.S. AEC Report BNWL-600, Pacific Northwest Laboratory, Richland, WA, 1968. 

Different approaches are needed for the dissolution of oxides, silicates, and sulfides.2 Generally, the processes dealing with sulfides require oxidizing conditions, and much work has been described99,100 aimed at generating sulfur as a by-product using reactions such as... 

Stumm, W. and R. Wollast, 1990, Coordination chemistry of weathering, kinetics of the surface-controlled dissolution of oxide minerals. Reviews of Geophysics 28, 53-69. 

The formation condition for PS can be best characterized by i-V curves. Figure 2 shows a typical i-V curve of silicon in a HF solution.56 At small anodic overpotentials the current increases exponentially with electrode potential. As the potential is increased, the current exhibits a peak and then remains at a relatively constant value. At potentials more positive than the current peak the surface is completely covered with an oxide film and the anodic reaction proceeds through the formation and dissolution of oxide, the rate of which depends strongly on HF concentration. Hydrogen evolution simultaneously occurs in the exponential region and its rate decreases with potential and almost ceases above the peak value. 

Natural Systems Dissolution of Oxides, Silicates and other minerals... 

The Kinetics of Surface Controlled Dissolution of Oxide Minerals an Introduction to Weathering... 

Surface Reactions. As we have seen from the dissolution of oxides the surface-controlled dissolution mechanism would have to be interpreted in terms of surface reactions in other words, the reactants become attached at or interact with surface sites the critical crystal bonds at the surface of the mineral have to be weakened, so that a detachment of Ca2+ and C03 ions of the surface into the solution (the decomposition of an activated surface complex) can occur. 

Stumm, W., and E. Wieland (1990), "Dissolution of Oxide and Silicate Minerals Rates Depend on Surface Speciation", in W. Stumm, Ed., Aquatic Chemical Kinetics, John Wiley and Sons, New York, 367-400. 

In Goulded RF, Equilibrium concepts in natural water systems. Adv Chem Ser 67 161-172 Stumm W, Furrer G, Wieland E, Zinder B (1985) The effects of complex-forming ligands on the dissolution of oxides and aluminosilicates In Drever JI (ed) The chemistry of weathering. Reidel, Dordrecht, pp 55-74... 

Bloesch, P.M. Bell, L.C. Hughes, J.D. (1987) Adsorption and desorption of boron by goethite. Aust. J. Soil Res. 25 377-390 Blomiley, E.R. Seebauer, E.G. (1999) New approach to manipulating and characterising powdered photo adsorbents. NO on Cl treated Ee20j. Langmuir 15 5970-5976 Bloom, P.R. Nater, E.A. (1991) Kinetics of dissolution of oxide and primary silicate minerals. In Sparks, D.L. Suarez, D.L. (eds.) Rates of soil chemical processes. Soil Sci. 

Casey, W.H. (1995) Surface chemistry during the dissolution of oxide and silicate materials. In Vaughan, D.J. Pattrick, R.A.D. (eds.) Mineral surfaces. Min. Soc. Series 5, Chapman Hall, London, 185-217 Catalyst Handbook (1970) Wolfe Scientific Books, 231 p. 

Kraemer, S.M. Hering, J.G. (1997) Influence of solution saturation state on the kinetics of ligand-controlled dissolution of oxide phases. Geochim. Cosmochim. Acta 61 2855-2866 Kraemer, S.M., Xu, J., Raymond, K.N. Spo-sito, G. (2002) Adsorption of Pb(II) and Eu(III) by oxide minerals in the presence of natural and synthetic hydroxamate sidero-phores. Environ. Sd. Technol. 36 1287-1291 Kraemer, S.M. Cheah, S.-F. Zapf, R. Xu, J. Raymond, KN. Sposito, G. (1999) Effect of hydroxamate siderophores on Fe release and Pb(II) adsorption by goefhite. Geochim. Cosmochim. Acta 63 3003—3008 Kratohvil, S. Matijevic, E. (1987) Preparation and properties of coated uniform colloidal partides. I. Aluminum (hydrous) oxide on hematite, diromia, and titania. Adv Ceram. Mater. 2 798-803... 

Pryor, M.J. Evans, U.R. (1950) The reductive dissolution of ferric oxide in acid. I. The reductive dissolution of oxide films present on iron. J. Chem. Soc., 1259-1266 Puchelt, H. (1973) Recent iron sediment formation at the Kameni Islands, Santorini (Greece). In Amstutz, G.C. Bernard, A.J. (eds.) Ores in sediments. Springer Berlin, 227-245... 

Rates of sorption and desorption of phosphate. Eur. J. Soil Sd. 48 101-114 Strens, R.G.S. Wood, B.J. (1979) Diffuse reflectance spectra and optical properties of some iron and titanium oxides and oxyhydr-oxides. Min. Mag. 43 347—354 Stumm, W. Eurrer, G. (1987) The dissolution of oxides and aluminum silicates Examples of surface-coordination-controlled kinetics. 

Often fractional orders best describe soil chemical processes. For example, the reaction order for dissolution of oxides, calcite, feldspars, and ferromagnesian minerals is often <1 (Stumm et al., 1985 Bloom and Erich, 1987). 

Several studies have been conducted on the rates of dissolution of oxides. The work of Stumm and coworkers is noteworthy in this area (Stumm et al., 1983, 1985 Zutic and Stumm, 1984 Stumm, 1986). They have studied the effects of H+ and various complex-forming anions on oxide dissolution rates and found that dissolution rate (v) depends strongly on the relative concentrations of proton surface groups -OH2 and ligand surface complexes -L such that (Stumm et al., 1985)... 

Dissolution of oxides and hydroxides as well as several other minerals in acids is usually of fractional order (Table 7.4). 

As mentioned earlier, the dissolution of oxides and hydroxides, like feldspars and ferromagnesian minerals, appears to be a surface-controlled reaction. One indication of this is the high E values found by several investigators. Bloom and Erich (1987) obtained E values ranging from 59 4.3 to 67 0.6 kj mol"1 for gibbsite dissolution in acid solutions (pH 1.5-4.0). These values are much higher than for diffusion-controlled reactions reported earlier. 

Stumm, W., Furrer, G., Wieland, E., and Zinder, B. (1985). The effects of complex-forming ligands on the dissolution of oxides and aluminosilicates. In The Chemistry of Weathering (J. I. Drever, ed.), pp. 55-74. Reidel Publ., Dordrecht, The Netherlands. 

Reductive Dissolution of Oxides by Organic Reductants 164 Reaction Scheme and Mechanism 164 Specific Studies 166... 

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