Hydrophobic polymer XAD

Figure 8.23 Reduction of ketones with Geotrichum candidum in the presence of hydrophobic polymer XAD [19a],...

In the reduction of benzoyloxypropanone, the hydrophobic polymer XAD-7 was used to prevent product inhibition and increase substrate concentration [19b]. Thus, the reduction proceeded in 70 gl substrate concentration and afforded 87% (12.4 g) of (S)-l-benzoyloxy-2-propanol in >99% ee (Figure 8.24). Thebutanone derivative could be reduced with the same method and afforded (S)-alcohol in 72% yield and >99% ee, but the pentanone derivative could not be reduced. 

Hydrophobic polymer (XAD, etc.) can be an additive for the reaction in aqueous buffer, but the role can be considered to be same as that for solvent As shown in Figure 3.3, it promotes solubilization of hydrophobic substrates by adsorbing them reversibly onto the surface of the polymer. The substrate, after desorption from the surface of the polymer, is converted enzymatically to product that is then adsorbed on the surface of the polymer. In so doing, it reduces the subsUate and product concenUations in the aqueous layer that often prevents subsUate and product inhibition and/or enzyme inactivation. 

Humic and flilvic acids are traditionally extracted from soils and sediment samples as the sodium salts by using sodium hydroxide solution. The material that remains contains the insoluble humin fraction (Figure 3). The alkaline supernatant is acidified to pH 2 with HCl. The humic acid precipitates and the fulvic acid remains in solution with other small molecules such as simple sugars and amino acids. These molecules can be separated by passing the solution through a hydrophobic resin, such as the methacrylate cross-linked polymer, XAD-8. The fulvic acids will sorb to the resin while the more hydrophilic molecules pass through the column. The fulvic acid can be removed with dilute base. 

Rohm and Flaas Company has developed Amberlite polymeric adsorbent resins that can be used to remove organic compounds from contaminated groundwater, aqueous wastes, and vapor-phase wastes. Amberlite XAD-4 is a crosslinked polystyrene-type polymer. It is hydrophobic and has no ionic functional groups incorporated into its resin structure. The material is most useful in removing low-molecular-weight organic substances from aqueous systems. Amberlite XAD-4 has been commercially available since the 1970s. 

The sorbents, Chromosorb 102 and XAD-2, which are styrene-divinyl benzene cross-linked porous polymers, proved to be most useful in our studies. Capacity of the sorbent sampling tubes was not a problem with the pesticides we studied since most were not extremely volatile. Sampling humid atmospheres of the pesticides also did not affect the sorbent capacity since these porous polymers are hydrophobic. 

High-surface-area polymers can be used to accumulate large amounts of relatively insoluble organic compounds from very large volumes of water. In the adsorption step of the accumulation, the more soluble components are not recovered efficiently so that the accumulated solutes do not accurately reflect the proportions of different compounds present at trace levels in the water. Nevertheless, the very simple polymer approach can be used for many studies because the mutagenicity appears to reside in the hydrophobic fraction that the polymers accumulate efficiently. This conclusion is based on favorable bioassay comparison of the extracts accumulated by the XAD-2 method and the extracts collected by the more complicated and expensive freeze concentration method (216, 233). 

Comparisons of commonly used XAD resins have been published for the isolation of both fulvic acid (Aiken et al., 1979) and humic acid (Cheng, 1977) from water. These resins differ in pore size, surface area, polymer composition, and polarity (Table 5) (Kunin, 1977). As with anion-exchange resins, hydrophobic styrene-divinylbenzene resins (XAD-1, XAD-2, XAD-4) were found more difficult to elute than hydrophilic acrylic-ester resins (Table 6). This is due to hydrophobic interactions, and possible tt-tt interactions with the aromatic resin matrix of styrene-divinylbenzene resins. In addition, ki-... 

Although original water samples can apply for some techniques, the isolation procedure is essential for better HS characterization with most analytical techniques. The traditional HS isolation method probably is XAD adsorption, which has been widely used for decades in the HS isolation from waters, soils, and organisms. This method can not only isolate HS, but also further separate HS into two fractions (humic acid and fulvic acid). Detailed isolation procedures have been reviewed in previous articles.XAD resins are styrene-divinylbenzene or methyl methacrylate polymer with various hydrophobicities and cross linkages. The resins adsorb dissolved organic matter mainly by hydrophobic binding or weak interactions such as Van Der Waals force, but the exact mechanism of adsorption is still unknown. 

Previous experience with polystyrene-type adsorbent materials (Amberhte XAD-4) in clinical hemoperfusion was characterized by significant bioincompatible responses such as complement activation and sharp drop of white blood cell (WBC) and platelet counts (neutropenia and thrombocytopenia). This was believed to be due to the hydrophobic nature of the polystyrene surface. Similar problems were also noted with activated carbon adsorbents. The biocompatibility of both materials needed to be improved by applying more hydrophilic polymers as surface... 

Humic substances can be isolated from waters by using column adsorption techniques and concentrated by methods such as vacuum distillation, lyophilization, freezing concentration, ultraflltration, reverse osmosis. The IHSS has adopted the XAD-8 resin adsorption method to isolate humic and fulvic acids from natural waters. In this method, after filtration through a 0.45 pm silver or polymer membrane Alter, the dissolved organic matter (DOM) is fractionated initially into hydrophobic and hydrophilic fractions through preferential adsorption of the hydrophobic fraction on the XAD-8 resin (water sample preacidifled at pH 2... 

Removal of organics can also be accomplished by adsorbent synthetic polymers. Such polymers as Amberlite XAD-4, a copolymer of polystyrene and divinylbenzene, have hydrophobic surfaces and strongly attract relatively insoluble organic compounds such as chlorinated pesticides. The porosity of these polymers is up to 50% by volume, and the surface area may be as high as 850 m2/g. They are readily regenerated by solvents such as isopropanol and acetone. Under appropriate operating conditions, these polymers remove virtually all nonionic organic solutes for example, phenol at 250 mg/L is reduced to less than 0.1 mg/L by appropriate treatment with Amberlite XAD-4. However, the use of adsorbent polymers is more expensive than that of activated carbon. 

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