Subcritical Water-Assisted Fractionation of Lupin Hull for Production of Cellulose Nanofiber Hydrogels and Aerogels

• Author / Creator

  Ciftci, Deniz

• In recent years, there has been a growing interest in transitioning to a bio-based economy by replacing petroleum-based products due to multiple drivers, such as growing environmental concerns, consumer demand, and the need for minimization of hazardous waste. Lignocellulosic biomass, composed of mainly cellulose, hemicellulose and lignin, has a major role in this transition, owing to its renewability, biodegradability and abundancy to obtain high-value bio-based products, such as cellulose nanofibers, hydrogels and aerogels. Subcritical water technology is a promising alternative to be employed for biomass processing to minimize the use of hazardous chemicals and waste production. The objective of this thesis was to employ subcritical water technology in the enrichment of cellulose from lupin hull, a legume byproduct of the agri-food industry, and then convert the obtained cellulose to nanofiber, and form cellulose nanofiber hydrogels and aerogels by supercritical CO2 (SCCO2) drying. Non-cellulosic fractions of the lupin hull, namely, hemicellulose and lignin, were removed with subcritical water treatment, and the effects of process parameters (pressure: 50-200 bar, temperature: 160-220 ºC, flow rate: 2-10 mL/min, and pH: 2-12) on lupin hull fractionation were investigated. At the optimized conditions (180 °C, 50 bar, 5 mL/min, and pH 6.2 within 40 min) for maximum hemicellulose sugar yield (85.5%) removed in the extract, a cellulose enriched residue (~80% cellulose) with increased crystallinity and thermal stability was obtained. To compare with the traditional methods used, a combination of sodium hydroxide (NaOH) treatment (5-20%, 25-75 °C and 2-10 h) followed by acidified sodium chlorite (ASC) treatment (1.7%, 75 °C for 2-6 h) was investigated for the fractionation of lupin hull, and the treatment efficiencies were compared with those of subcritical water for the isolation of cellulose fibers. The effect of lignin content on the treatment efficiency was also examined by processing canola straw, a high-lignin biomass, in addition to lupin hull. The amount of non-cellulosics removal was higher for lupin hull (~90%) than that of canola straw (~80%) at the conditions of 15% NaOH/99 °C/6 h followed by 6 h ASC treatment, indicating that low lignin content favors the biomass fractionation. The subcritical water treatment was as efficient as the optimized NaOH treatment (15% NaOH/99 °C/4 h) that yielded a cellulose-enriched lupin hull residue of ~80% cellulose. Therefore, NaOH treatment was replaced with subcritical water treatment in the subsequent process to obtain cellulose nanofibers to reduce the use of chemicals. Subcritical water-treated cellulose-enriched residue obtained at the optimized conditions was further purified with an ASC treatment, and then the resultant purified cellulose was fibrillated with ultrasonic treatments at varying amplitudes (20-80%) for 15-35 min to obtain cellulose nanofibers. Increasing ultrasonication amplitude and time resulted in enhanced fibrillation, with the smallest average nanofiber diameter of 15 nm at 80% amplitude and 35 min ultrasonication time. Rheological characterization of the aqueous suspensions of cellulose nanofibers obtained by ultrasonication was conducted in the concentration range of 0.1-1.9 wt.%. All suspensions, except at 0.1 wt.% concentration, formed hydrogels. Regardless of the concentration, all samples showed a typical shear-thinning behavior. Increasing concentration of the suspension resulted in hydrogels with an increase in the dynamic moduli, forming a stronger gel network due to highly entangled structures. Finally, highly porous (96.6-99.4% porosity) and lightweight (0.009-0.05 g/cm3 density) cellulose nanofiber aerogels were formed from hydrogels with 1-2 wt.% concentration using SCCO2 drying and freeze drying methods. The resulting cellulose nanofiber aerogels with the highest specific surface area of 115 m2/g, the highest porosity of 99.4% and the lowest density of 0.009 g/cm3 were obtained by SCCO2 drying of 1 wt.% hydrogel, which had a three-dimensional open nanoporous (~8 nm) network structure. The results suggest that subcritical water technology is a promising method to enrich cellulose from lignocellulosic biomass, while reducing the use of chemicals. Also, lupin hull is a good cellulose source to obtain nanofibers to form hydrogels and aerogels without using chemical crosslinkers. The obtained cellulose nanofiber hydrogels and aerogels could be considered as potential candidates for many applications in food packaging, nanocomposites, paper reinforcement, coating additives, tissue engineering scaffolds, filtration media, thickening agents, rheology modifiers, and adsorbents.

• Subjects / Keywords

  • Cellulose nanofiber
  • Subcritical water
  • Lupin hull

• Graduation date

  Spring 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R39G5GS0G

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.