Oxalate adsorbed

M ascorbic acid at pH 4 with various amounts of oxalic acid adsorbed by the goethite (figures at the curves). Insert relationship between the initial dissolution rate k and the amount of oxalate adsorbed by the goethite (StummetaL, 1985, with permission). 

Figure 9.11. Fourier transform infrared (FTIR) spectra of oxalate adsorbed on Ti02 oxalate solution concentration 10 M and pH values between 3.0 and 8.6. Surface complex formation starts below pH 8.6. First, a spectrum with a maximum at 1690 cm is observed. With decreasing pH, the amplitude increases and an additional peak at 1711 cm appears. These bands are assigned to C=0 stretching vibrations. The changing spectral shape is an indication of the formation of at least two different inner-sphere complexes (Hug and Sulzberger, 1994.)...

Yoon, T. H., Johnson, S. B., Musgrave, C. B., Brown, G. E (2004). Adsorption of organic matter at mineral/water interfaces I. ATR-FTIR spectroscopic and quantum chemical study of oxalate adsorbed at boehmite/water and corundum/water interfaces, Geochim. Cosmochim. Acta Vol. 68, No. 22, pp. 4505-4518,0016-7037. 

Sugar Processing. Dispersants are used in the production of cane and beet sugar to increase the time between evaporator clean outs. Typical scales encountered include calcium sulfate, calcium oxalate, calcium carbonate, and silica. Dispersants are fed at various points in the process to prevent scale buildup, which would interfere with efficient heating of the vessels. Only certain dispersants, conforming to food additive regulations, can be used, since a small amount of the dispersant may be adsorbed on the sugar crystals. 

The addition of various Kolbe radicals generated from acetic acid, monochloro-acetic acid, trichloroacetic acid, oxalic acid, methyl adipate and methyl glutarate to acceptors such as ethylene, propylene, fluoroolefins and dimethyl maleate is reported in ref. [213]. Also the influence of reaction conditions (current density, olefin-type, olefin concentration) on the product yield and product ratios is individually discussed therein. The mechanism of the addition to ethylene is deduced from the results of adsorption and rotating ring disc studies. The findings demonstrate that the Kolbe radicals react in the surface layer with adsorbed ethylene [229]. In the oxidation of acetate in the presence of 1-octene at platinum and graphite anodes, products that originate from intermediate radicals and cations are observed [230]. 

The effects of organic molecules and phosphate on the adsorption of acid phosphatase on various minerals, and kaolinite in particular, have been investigated by Huang et al. [97]. The Langmuir affinity constant for AcP adsorption by kaolinite follows the series tartrate (K — 97.8) > phosphate (K= 48.6) > oxalate (K — 35.6) > acetate (K= 13.4). At low concentration, acetate even promoted the adsorption of acid phosphatase. It was considered that competitive interactions between anionic adsorbates can occur directly through competition for surface sites and indirectly through effects of anion adsorption on the surface charge and protonation. 

The scheme in Fig. 5.5 indicates that the ligand, for example, oxalate, is adsorbed very fast in comparison to the dissolution reaction thus, adsorption equilibrium may be assumed. The surface chelate formed is able to weaken the original Al-oxygen bonds on the surface of the crystal lattice. The detachment of the oxalato-aluminum species is the slow and rate-determining step the initial sites are completely regenerated subsequent to the detachment step provided that the concentrations of the reactants are kept constant, steady state conditions with regard to the oxide surface species are established (Table 5.1). If, furthermore, the system is far from dissolution equilibrium, the back reaction can be neglected, and constant dissolution rates occur. 

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. 

Extractable matter should be removed by extraction with organic solvents, e.g., xylene. This is especially important for carbon blacks (25), Oxidized carbon may contain small amounts of oxalic acid. King (33, 34) found 0.002 meq/gm of oxalic acid in oxygen-treated sugar charcoal. More severe is the contamination of the surface with adsorbed gases, mainly carbon dioxide and water. Activated carbon with narrow pores may contain considerable amounts of carbon dioxide (28). The best... 

Consider a system of M two-site molecules (such as oxalic acid) and N solvent molecules (such as water) in a volume V and at temperature T. The system is closed with respect to the adsorbent molecules and to the solvent molecules, but open with respect to the ligands L, maintained at a constant chemical potential p. 

One-Step Activation Process. In a one-step activation process, the sensitizing and nucleating solutions are combined into one solution. It is assumed that when this solution is made up, it contains various Sn-Pd chloride complexes (24). These complexes may subsequently transform into colloidal particles of metallic Pd or a metallic alloy (Sn/Pd) to form a colloidal dispersion (19,28). This dispersion is unstable. It may be stabilized by addition of an excess of Sn ions. In this case, Pd particles adsorbed on the nonconductor surface are surrounded by Sn ions. The latter must be removed by solubilizing before electroless plating so that the catalytic Pd on the surface will become exposed, freely available, to subsequent plating. An example of such a solubilizing solution is a mixture of fluoroboric and oxalic acids in a dilute solution, or just plain NaOH or HCl. 

For iron oxides, IR spectroscopy is useful as a means of identification. Hematite crystals in films that were too thin (<70nm) to be characterized by XRD were shown by IR to be oriented with the c-axis perpendicular to the surface of the film (Yubero et al. 2000). This technique also provides information about crystal morphology, degree of crystallinity and the extent of metal (especially Al) substitution because these properties can induce shifts in some of the IR absorption bands. It is also widely used both to obtain information about the vibrational state of adsorbed molecules (particularly anions) and hence the nature of surface complexes (see Chap. 11) and to investigate the nature of surface hydroxyl groups and adsorbed water (see Chap. 10). Typical IR spectra of the various iron oxides are depicted in Figure 7.1. Impurities arising either from the method of preparation or from adsorption of atmospheric compounds can produce distinct bands in the spectra of these oxides -namely at 1700 cm (oxalate), 1400 cm (nitrate) and 1300 and 1500 cm (carbonate). 

Ambe et aL, 1986) and phosphate adsorbed on lepidocrocite raised the adsorption of zinc (Madrid et aL, 1991a). Adsorption of Cd on goethite is increased by sulphate (Hoins et aL, 1993) and by oxalate (Lamy et aL, 1991). 

Leland and Bard (1987) found that the different iron oxides induced photooxidation of oxalate and sulphite at rates that varied by up to two orders of magnitude. For oxalate, the rate was greater for maghemite than for hematite, but this order was reversed for sulphite. Lepidocrocite (layer structure) induced faster oxidation of both compounds that did the other polymorphs of FeOOH (tunnel structures) the authors considered that the rate differences were probably associated with structural differences between the adsorbents. 

The Fe -oxalate complex then adsorbs on goethite (step 2) where it exchanges an electron with a surface Fe atom to form Fe (step 3) and is itself reoxidized. 

Although 2-line ferrihydrite has been used for dissolution studies, 6-line ferrihydrite has, to date, not been investigated. Fischer (1976) compared the dissolution behaviour of three 2-line ferrihydrites in 0.2 M oxalate and found the slowest dissolution rate for a slowly precipitated sample and faster dissolution for rapidly precipitated samples (hydrolysed by fast addition of NH3 or by bacterial oxidation of Fe citrate). Adsorbed silicate reduced the dissolution rate in oxalate probably by blocking surface Fe sites (Schwertmann Thalmann, 1976). 

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