Hemicelles

The effect that hydrophobic organic substances can become sorbed to organic coatings can be used to make sorbents by coating inorganic colloids such as ferri-hydrite with surfactants. Such surfactant-coated surfaces contain films of hemicelles (see 4.5) which are able to remove hydrophobic solutes, such as toluene or chloro-hydrocarbons, from solution (Holsen, et al., 1991). 

Suitable collectors can render hydrophilic minerals such as silicas or hydroxides hydrophobic. An ideal collector is a substance that attaches with the help of a functional group to the solid (mineral) surface often by ligand exchange or electrostatic interaction, and exposes hydrophobic groups toward the water. Thus, amphi-patic substances (see Chapter 4.5), such as alkyl compounds with C to C18 chains are widely used with carboxylates, or amine polar heads. Surfactants that form hemicelles on the surface are also suitable. For sulfide minerals mercaptanes, monothiocarbonates and dithiophosphates are used as collectors. Xanthates or their oxidation products, dixanthogen (R - O - C - S -)2 are used as collectors for... 

The alternative pathway is the biochemical route. It processes starches/sugars into ethanol, a standard technology with installations world-wide, but in a biorefinery the start is the whole-plant material or biomass residues containing hemicel-lulose, which is broken into sugars that then can be fermented to ethanol and/or other alcohols such as butanol. As mentioned before, there is the need to develop novel and/or improved biocatalysts for alternative organic fuels, such as biobutanol, by fermentation processes. 

Sample Hemicel- luloses Cellu- Lignin Extraclose lives C H O... 

Table I. Growth of Trichoderma reesei QM9414 (Tr) and T. harzianum IMI275950 (Th) on wheat straw lignocellulose (lignocell.) or derived cellulosic materials from straw (cell., cellulose hemicell-A, hemicellulose-A holocell., holocellulose)...

Substrate cell. hemicell-A holocell. lignocell. lignocell. 

Thaysen, A. C. Bunker, H. J. The Microbiology of Cellulose, Hemicel-lulose, Pectin and Gums Oxford Univ. Press Oxford, 1927. 

Many studies have indicated that covalent linkages must exist between lignin and wood polysaccharides. Separation and analysis of lignin -carbohydrate complexes (LCC) have led to the conclusion that the hemicel-lulose components (xylan and galactoglucomannans in softwood) are bound to lignin mainly through arabinose, xylose, and galactose moieties as shown in Fig. 4-10. 

Under proper conditions, biomass deconstruction will generate streams of monomeric or oligomeric carbohydrates, with glucose and xylose (from cellulose/starch or hemicel-... 

Tenkanen, M., Action of Trichoderma reesei and Aspergillus oryzae esterases in the deacetylation of hemicel-luloses. Biotechnol Appl Biochem 1998,27, 19-24. 

Miller, R. S. and Bellan, J., A generalized biomass pyrolysis model based on superimposed cellulose, hemicel-lulose and lignin kinetics. Combustion Sci Technol 1997, 126 (1-6), 97-137. 

Flash pyrolysis in FFR is a usefnl means to remove snlphnr from coal [19, 21]. As shown by Li et al. [22], it can also be ntilized to remove heteroatom molecules from biomass. Both the yield and the composition of the resnltant gas depend on the biomass composition. The gas ontpnt is richer in hydrogen in the case of cellulose and hemicel-Inlose than in the case of lignin. Smaller biomass particle sizes and higher fast pyrolysis temperatnres also boost hydrogen content. The total of carbon monoxide and hydrogen content is reported to be 65.4% for legnme straw and 55.7% for apricot stone. 

There is limited information regarding potential differences in the nature and amount of juvenile wood and mature wood hemicel-luloses (2). What data are available indicate that there can be limited changes in the relative amounts of some simple sugars, but there is apparently little or no significant difference for either softwoods or hardwoods. 

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