Fry, Stephen

Whale, Eric

Donohoe, Christian Luke

2023-06-22T12:15:37Z

2023-06-22T12:15:37Z

2023-06-22

The valorisation of biomass is a key step towards reducing the dependence of chemical production streams on non-renewable sources and for the development of a green economy. Sugar beet pulp (SBP), a secondary biomass product from sugar extraction, can be oxidised with hydrogen peroxide and bleach to produce an industrially valuable, viscous, cellulose-rich material called Curran® which is currently used as a rheology modifier for plastics, paints, and cardboard preparative mixtures. This study investigated the effects these oxidants have on cellulose within the plant cell wall of SBP. The physical and chemical properties of Curran® were influenced by the properties of the cellulose within, and these changes have been measured according to the degree of oxidation and surface accessibility of the cellulose against the monosaccharide composition and viscosity of the oxidised SBP, in comparison with similarly oxidised Whatman paper. This was through methods such as conductometric titrations, novel [3H]oligosaccharide adsorption measurements, and successive ammonium oxalate and sodium hydroxide extractions alongside dynamic viscometric measurements and quantification of cellulase digest products. The viscosity of oxidised SBP suspensions was found to be dependent on the stability of parenchymal cell-ghosts. If these collapsed, then the viscosity was dependent on the properties of cellulose. Rhamnogalacturonan-I was found to be linked to the surface of the cellulose in a manner that was resistant to oxidation. Under the acidic hydrogen peroxide free-radical oxidation and alkaline NaOCl oxidation applied, up to 3.5% of all cellulosic glucose residues in SBP were oxidised. Both oxidation treatments in either order were required to produce carboxylic acid groups preferentially over aldehyde or ketone groups; carboxylic acid groups are preferable for further functionalisation. The (NaOH-soluble) hemicellulose a fraction of the material was found to contain oxidised cellulose, fragmented by the H2O2 oxidation step. The sum of these observations showed that hydrogen peroxide fractured the surface of cellulose during oxidation, solubilising pectin that was non-covalently bound to the cellulose surface and splitting aggregated cells into free-flowing units. This modification of the surface of the microfibrils by hydrogen peroxide would create acidic cellulosic fragments that are still water-insoluble at pH 7, but NaOH-extractable. Bleach oxidation is important for whiteness and viscosity improvement, but this introduces chlorine compounds into the waste stream and its use should be limited. To replace NaOCl oxidation, the esterification of cellulose has been suggested for viscosity improvement of oxidised SBP. Finally, the adsorption of [3H]cellopentaitol and [3H]cellohexaitol onto paper in suspension has been tested as a binding assay to estimate the surface accessibility of the cellulose microfibrils. The binding strength of each probe was measured, and this method successfully showed the accessibility of variously oxidised Whatman paper samples.

https://hdl.handle.net/1842/40712

http://dx.doi.org/10.7488/era/3472

en

The University of Edinburgh

cellulose

biocomposite

sugar beet

Characterisation of cellulose within oxidised sugar beet pulp

The characterisation of cellulose within oxidised sugar beet pulp

Thesis or Dissertation

Doctoral

PhD Doctor of Philosophy