Easily oxidizable fractions

Solvent refining to adjust the viscosity index (VI) and improve the base stock s response to oxidation. This solvent extraction separates the low VI and easily oxidizable fraction from the more desirable high VI and oxidation-resistant components. 

The battle for precise, accurately determined data, has been most severe in analysing of the DOC. Depending on the method, differences by a factor 2 to 3 have been observed for samples from the same ocean areas. As is gradually becoming clear, each method is determining something different with sometimes great accuracy. Some differences depend on the composition of volatile and easily or less easily oxidizable fractions. This may even differ from area to area as well as with depth. 

In this work a novel five-step leaching scheme for HM has been developed addressing exchangeable, acid soluble (carbonate), easily reducible (bound to Mn oxides), easily oxidizable (bound to humic and fulvic acids), and moderately reducible (bound to amorphous Ee oxides) fractions extractable by 0.05 M Ca(N03), 0.43 M CH3COOH, 0.1 M NH,OH-HCl (non-acidified), 0.1 M K/,03 (pH 11), and 0.1 M (NH4),C,04 (pH 3), respectively. The sequence of extractants was chosen according to recent studies on the selectivity of leachants toward dissolved phases of soils. 

In general, TEs in exchangeable and acid-soluble forms are considered to be easily bioavailable (see Table 12.1). The metal-organic complex-bound form is also relatively mobile (Krishnamurti et al., 1995). The reducible and oxidizable forms are relatively stable under standard soil conditions, yet easily reducible and oxidizable fractions may be readily mobilized, as detailed in the ensuing sections. 

More aromatic fractions under the same conditions demonstrated immediate oxidation (no induction pause) and relatively slow oxygen uptake rates that were approximately constant throughout the oxidation period, unlike the paraffinic samples. These, too, eventually slowed down with increased oxygen consumption. This type of oxidation in which the rate slows as the reaction progresses is termed autoretardant. In these cases, oxidation begins immediately due to the presence of easily oxidizable materials (e.g., aromatics with alkyl side chains) and eventually slows again due to the presence or generation of inhibitors/antioxidants. 

Shuman (1982, 1985), working on the speciation of Cu, Mn, Fe and Zn, modified the sequence of the extraction as exchangeable, oxidizable (organically bound), easily reducible (Mn oxides-bound), moderately reducible (amorphous Fe oxides-bound), strongly reducible (crystalline Fe oxides-bound), and residual. The low percentages of the oxidizable fractions obtained following the sequential extraction scheme of Shuman (1985) as compared to those obtained following the scheme of Tessier et al. (1979) were attributed to the lack of selectivity of the method used (Charlatchka et al, 1997). 

TABLE 1 The ratio of sums of the more easily oxidizable to the more poorly oxidizable fractions of PL (XEOPL/XPOPL) and ratio of phosphatidyl choline to phosphatidylethanolamine (PC/PE) in the initial lipids of organs and formed from them liposomes in the different experiments. 

In connection with the problems arising from the disposal of solid wastes, particularly of dredged materials, extraction sequences have been applied which are designed to differentiate between the exchangeable, carbonatic, reducible (hydrous Fe/Mn oxides), oxidizable (sulfides and organic phases) and residual fractions (Engler et al., 1977). One of the more widely applied extraction sequences of Tessier and coworkers (1979) has been modified by various authors a version of Ker-sten Forstner (1986) differentiates easily and moderately reducible components (Table 3-4). 

Copper and Cr are prevalently held in the oxidizable (-68%) and residual (-24%) fractions, with other forms making up much less than 10%. Pb is primarily held in the residual mineral fraction (77%) with smaller amounts held in the oxidizable (18%) and reducible (3%) forms. Zn is uniformly distributed between easily extractable (23%), reducible (24%) and residual (49%) fractions, with very small percentage occurring in oxidizable forms (4%). Thus, only Zn appears to occur in the studied soils in consistent amounts as bioavailable and leachable forms. Copper and Cr, which are mainly associated with organic matter, for which both metals have high affinity are characterised by a lower mobility, unless the occurrence of oxidising conditions may induce their release in solution. Lead, and in lower proportion also Cu and Cr, extracted mainly by the fourth step, result relatively immobile. 

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