Reductant, immobilized

Chemical treatment (oxidation, incineration, reduction, immobilization, chelating, transformation)... 

Of greater recent interest are metals that exist predominantly as soluble, hazardous oxyanions in oxic groundwaters, but that become relatively insoluble species when reduced, making them candidates for remediation by reductive immobilization. These metals include As(V), Cr(VI), Se(VI), Tc(VII), U(VI), and a few others [51,63,64], In general, a complex and variable mixture of processes is responsible for sequestration of these contaminants by Fe°. For example, Cr(VI) is at least partially reduced to Cr(III), which is then precipitated as a mixed oxyhydroxide [65-67]. 

Redox reactions. Two prolinol derivatives and 6 with polyfluorinated a-substituents have been developed for use in the CBS-type reduction. Immobilization of 6 in hydrofluoroether perhaps contributes to the attainment of high ee of the reduction and it facilitates recovery of the adjuvant (for reuse). 

Xu YH, Zhao DY (2007) Reductive immobilization of chromate in water and soil using stabilized iron nanoparticles. Water Res 41 2101-2110... 

Unloaded silica does not recover HPA from aqueous solution. The surface of silica gel modified with quarternary ammonium salts (QAS) gets anion-exchange properties. The aim of the work is the elaboration of solid-phase reagents on the base of ion associate of HPA with QAS immobilized onto silica surface for the determination of phosphoms and organic reductants. Heterocyclic (safranine and lucigenine) and aliphatic (trinonyloctadecyl ammonium iodide and tetradecyl ammonium nitrate) compounds have been examined as QAS. 

The Zincke reaction has also been adapted for the solid phase. Dupas et al. prepared NADH-model precursors 58, immobilized on silica, by reaction of bound amino functions 57 with Zincke salt 8 (Scheme 8.4.19) for subsequent reduction to the 1,4-dihydropyridines with sodium dithionite. Earlier, Ise and co-workers utilized the Zincke reaction to prepare catalytic polyelectrolytes, starting from poly(4-vinylpyridine). Formation of Zincke salts at pyridine positions within the polymer was achieved by reaction with 2,4-dinitrochlorobenzene, and these sites were then functionalized with various amines. The resulting polymers showed catalytic activity in ester hydrolysis. ... 

Similar results were reported in deforested Amazonian rainforests (66). Within three years following forest clearing and burning, nutrient concentrations of soil leachates had returned to levels typical of primary forests of the area. A combination of high rates of immobilization and storage by successional vegetation, coupled with a decline in easily decomposable substrates, was attributed to the reduction in leaching losses. 

O Water absorbing polymer Figure 8.26 Asymmetric reduction of ketones in CO2 by Geotrichum candidum immobilized whole cell [20], (a) Time course for the reduction of o-fluoroacetophenone (b) substrate specificity (c) apparatus for C. candidum-cata yzed reduction with semiflow process using scC02. 

Ionic liquid [bmimJPFg can be used as a solvent in yeast reduction [21]. The reduction ofketones with immobilized baker s yeast (alginate) in a 100 10 2 [bmimjPFfi ionic liquid water MeOH mix affords chiral alcohols (Figure 8.28). 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

FIGURE 3 Scanning electron micrograph (1200x magnification) of the surface of a porous alumina particle coated with poly(diphenoxy-phosphazene). Surface nitration, reduction, and glutaric dialdehyde coupling immobilized enzyme molecules to the surface. (From Ref. 23.)... 

Probably the first non-covalent immobilization of a chiral complex with diazaligands was the adsorption of a rhodium-diphenylethylenediamine complex on different supports [71]. These solids were used for the hydride-transfer reduction of prochiral ketones (Scheme 2) in a continuous flow reactor. The inorganic support plays a crucial role. The chiral complex was easily... 

The advantages of microreactors, for example, well-defined control of the gas-liquid distributions, also hold for photocatalytic conversions. Furthermore, the distance between the light source and the catalyst is small, with the catalyst immobilized on the walls of the microchannels. It was demonstrated for the photodegradation of 4-chlorophenol in a microreactor that the reaction was truly kinetically controlled, and performed with high efficiency [32]. The latter was explained by the illuminated area, which exceeds conventional reactor types by a factor of 4-400, depending on the reactor type. Even further reduction of the distance between the light source and the catalytically active site might be possible by the use of electroluminescent materials [19]. The benefits of this concept have still to be proven. 

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