Comparative analysis of methods for producing nanocellulose from wheat straw and bran, with co-extraction of valuable products

Ceaser, Regan (2019-12)

Thesis (MEng)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Nanocellulose production has recently attracted much attention from most researchers due to its variable applications in fields such as food, packaging, and medicine. Nanocellulose production from agricultural resides requires cellulose-rich pulp as the precursor which is mainly obtained by bleaching the biomass after pretreatment. Bleaching of the agricultural residues results in the loss and disruption of other cell wall components such as hemicellulose, hydroxycinnamic acid and lignin. However, these cell wall components are valuable products that when extracted can improve the treatment process. It is therefore necessary to develop a method to obtain a cellulose-rich for nanocellulose production while co-extracting these other cell wall components as value-added products. This study focused on developing and optimising a method to produce cellulose-rich pulp to further produce nanocellulose while co-extracting hemicellulose, lignin, ferulic and p-coumaric acid from wheat bran and wheat straw, which are agricultural residues from the wheat production industry. A two-stage alkaline treatment was the selected method for the extraction of hemicellulose, lignin, ferulic and p-coumaric acid while producing a cellulose-rich pulp. The first alkaline treatment stage was termed asa mild alkaline treatment stage due to the mild process conditions (1.5-2.5 wt. % NaOH conc., 20-40°C at 16 hours) used. The mild alkaline treatment was optimised by a central composite design with a response surface methodology targeted at extracting high yields of hemicelluloses and ferulic acid (for wheat bran) or p-coumaric acid (for wheat straw) with the extraction of lignin as a by-product of the treatment. The second stage alkaline treatment termed as the alkaline delignification step was conducted and optimised at treatment conditions of 6-10 wt. % NaOH for wheat bran and 8-12 wt. % NaOH for wheat straw for 30-90 min at 121⁰C. The alkaline delignification optimisation was focused on obtaining a cellulose-rich pulp with minimal lignin content for nanocellulose production whereas extracting as much lignin as possible with hemicelluloses as by-product. The hemicelluloses, lignin and cellulose-rich pulp obtained were analysed by compositional analysis, Fourier Transform Infrared Spectroscopy (FTIR) and a thermogravimetric analysis (TGA) whereas the extracted hydroxycinnamic acids (ferulic and p-coumaric acid) were analysed by compositional analysis and their antioxidant activity determined. The total yield of hemicelluloses and lignin obtained after the two-stage alkaline treatment from wheat straw was 77% and 68%, respectively whereas the p-coumaric yield was 85% with an antioxidant activity of 45%. In addition, the wheat straw cellulose-rich pulp obtained after the two-stage alkaline treatment had a cellulose recovery, cellulose content, hemicelluloses content, lignin content and crystallinity of 97%, 76%, 3%, 5% and 55%, respectively which were within the range for application in nanocellulose production. The results indicated that two-stage alkaline treatment was beneficial for producing cellulose-rich wheat straw with a high crystallinity for nanocellulose production. Yields of 52% and 94% were obtained for the two-stage alkaline treatment of hemicelluloses and lignin, respectively for wheat bran whereas a ferulic acid yield of 65% with a 15% antioxidant activity was obtained. Furthermore, the wheat bran cellulose-pulp obtained had cellulose recovery, cellulose content, hemicellulose content, lignin content and crystallinity of 83%, 48%, 31%, 6% and 41%, respectively. The cellulose recovery indicated cellulose loss during the delignification step whereas the hemicellulose content was high enough to inhibit enzymatic hydrolysis to produce nanocellulose. The alkaline delignified wheat bran cellulose produced required further treatment to be applied in nanocellulose production and was therefore not used in the nanocellulose production stage of this study. Results from nanocellulose production from delignified wheat straw using sulfuric acid, hydrochloric acid and enzymatic treatment indicated that although sulfuric acid produced cellulose nanoparticles presented the highest yield (34%) and crystallinity (75%), it resulted in the lowest maximum thermal decomposition temperature (309⁰C) as compared to both hydrochloric acid and enzymatic treatment. In addition, the yield, zeta potential and maximum thermal decomposition temperature for nanoparticles produced by enzymatic treatment (17.16 ± 2.30%, -15.2 ± 0.6 mV and 378°C, respectively) were similar to that of hydrochloric acid treatment (20.31 ± 1.24%, -16.3 ± 1.50 mV and 380°C, respectively). It was interesting to note that, the yield, zeta potential and maximum thermal degradation temperature were obtained at a shorter time (4.64 h) for enzymatic treatment than the hydrochloric acid treatment (7.41 h), resolving the issue of extended enzymatic treatment times for nanocellulose production. Furthermore, the crystallinity obtained for hydrochloric acid produced nanoparticles (70%) was closer to that of sulfuric acid (75%) produced nanoparticles and higher than enzymatic hydrolysis produced nanoparticles (48%). Hydrochloric acid produced cellulose nanoparticles resulted in improved polydispersity index (0.53 ± 0.20) and fiber morphology (514 ± 50 length) as compared to enzymatic produce nanoparticles (0.92 ± 0.13 PdI and >1 μm length). It can be concluded that between enzymatic and hydrochloric acid treatments, the latter resulted in nanoparticles with improved properties than the former. KEYWORDS Wheat straw; Wheat bran; Alkaline treatment; Hemicellulose; Lignin; p-Coumaric acid; Ferulic acid; Nanocellulose

AFRIKAANSE OPSOMMING: Die produksie van nanocellulose het die afgelope tyd baie aandag gekry van die meeste navorsers as gevolg van die wisselvallige toepassings op gebiede soos voedsel, verpakking en medisyne. Die produksie van nanocellulose uit landboukoshuise benodig sellulose-ryke pulp as die voorloper wat hoofsaaklik verkry word deur die biomassa na behandeling met blykmiddels. Behandeling van die landboureste lei tot die verlies van ander selwandkomponente soos hemicellulose, hidroksisinnamiensuur en lignien. Hierdie selwandkomponente is egter waardevolle produkte wat die behandelingsproses kan verbeter wanneer dit ontgin word. Dit is dus nodig om 'n metode te ontwikkel om 'n sellulose-ryke eindproduk van nanocellulose-produksie te verkry, terwyl hierdie ander selwandkomponente as waardetoevoegingsprodukte saamgewerk kan word. Hierdie studie het gefokus op die ontwikkeling en optimalisering van 'n metode om sellulose-ryke pulp te vervaardig waarvan nanocellulose verder geproduseer word, terwyl dit hemisellulose, lignien, feruline en p-coumaric acid van koring semels en koringstrooi, wat landbou-residu's uit die koringproduksiebedryf is, saamwerk. 'N Alkaliese behandeling met twee fases was die geselekteerde metode vir die ekstraksie van hemisellulose, lignien, feruline en p-coumaric acid, terwyl 'n sellulose-ryke pulp geproduseer word. Die eerste alkaliese behandelingsfase word as ligte alkaliese behandelingsfase gedoen as gevolg van die ligte prosesstoestande (1,5-2,5 wt.% Konsentrasie NaOH, 20-40 °C op 16 uur). Die ligte alkaliese behandeling is geoptimaliseer deur 'n sentrale saamgestelde ontwerp met 'n responsoppervlak-metodologie wat daarop gemik is om hoë opbrengste van hemiselluloses en feruliensuur (vir koring semels) of p-coumaric acid (vir koringstrooi) te onttrek met die ekstraksie van lignien as 'n by- produk van die behandeling. Die tweede fase alkaliese behandeling, wat as die alkaliese delignifikasiestap bekend staan, is uitgevoer en geoptimaliseer tydens behandelingstoestande van 6-10 wt.% NaOH vir koring semels en 8-12 wt.% NaOH vir koringstrooi vir 30-90 min by 121°C. Die optimalisering van die alkaliese delignifikasie was daarop gefokus om 'n sellulose-ryke pulp met 'n minimale lignieninhoud vir nanocellulose-produksie te verkry, terwyl soveel as moontlik lignien met hemicelluloses as neweproduk verkry word. Die hemiselluloses, lignien en sellulose-ryke pulp wat verkry is, is met behulp van samestellingsanalise, Fourier Transform Infrared Spectroscopy (FTIR) en 'n termogravimetriese analise (TGA) geanaliseer, terwyl die onttrekte hidroksikannamiese sure (feruline en p-coumaric acid) deur middel van komposisie-analise en hul antioksidant aktiwiteit bepaal. Die totale opbrengs van hemicelluloses en lignien wat na die tweestap-alkaliese behandeling van koringstrooi verkry is, was onderskeidelik 77% en 68%, terwyl die p-coumaric acid opbrengs 85% was met 'n antioksidantaktiwiteit van 45%. Boonop het die koringstrooi sellulose-ryke pulp wat na die tweestap-alkaliese behandeling verkry is, 'n sellulose-herstel, sellulose-inhoud, hemicellulose-inhoud, lignieninhoud en kristaliniteit van onderskeidelik 97%, 76%, 3%, 5% en 55% gehad. wat binne die reeks vir toediening in nanocellulose-produksie was. Die resultate het aangedui dat tweestadige alkaliese behandeling voordelig was vir die produksie van sellulose-ryk koringstrooi met 'n hoë kristaliniteit vir nanocellulose-produksie. Opbrengste van 52% en 94% is behaal vir die tweestap-alkaliese behandeling van hemicelluloses en lignien, onderskeidelik vir koring semels, terwyl 'n feruline suuropbrengs van 65% met 'n 15% antioksidant aktiwiteit verkry is. Verder het die koring semelselle sellulose-pulp sellulose herstel, sellulose-inhoud, hemicellulose-inhoud, lignieninhoud en kristaliniteit van onderskeidelik 83%, 48%, 31%, 6% en 41% gehad. Die sellulose-herstel het sellulose-verlies tydens die delignifikasiestap aangedui, terwyl die hemicellulose-inhoud hoog genoeg was om ensiematiese hidrolise te inhibeer om nanocellulose te produseer. Die alkaliese delignifiseerde koring semelsellulose wat geproduseer is, het verdere behandeling toegepas in die produksie van nanocellulose en is daarom nie in die nanocellulose-produksiestadium van hierdie studie gebruik nie. Resultate uit die produksie van nanocellulose uit delignifiseerde koringstrooi met behulp van swawelsuur, soutsuur en ensiembehandeling het aangedui dat hoewel swaelsuur geproduseerde sellulose-nanodeeltjies die hoogste opbrengs (34%) en kristaliniteit (75%) lewer, dit tot die laagste maksimum temperatuur vir ontbinding van die temperatuur gelei het ( 309°C) in vergelyking met beide soutsuur en ensiematiese behandeling. Daarbenewens was die opbrengs, zeta-potensiaal en maksimum termiese ontbindingstemperatuur vir nanodeeltjies wat geproduseer word deur ensimatiese behandeling (onderskeidelik 17,16 ± 2,30%, -15,2 ± 0,6 mV en 378°C) soortgelyk aan dié van soutsuurbehandeling (20,31 ± 1,24% , -16,3 ± 1,50 mV en 380°C, onderskeidelik). Dit was interessant om daarop te let dat die opbrengs, die zeta-potensiaal en die maksimum termiese afbrekingstemperatuur op 'n korter tyd (4,64 uur) verkry is vir ensimatiese behandeling as die soutsuurbehandeling (7,41 uur), wat die kwessie van verlengde ensimatiese behandelingstye vir nanocellulose opgelos het. Verder was die kristaliniteit wat verkry is vir soutsuur geproduseerde nanopartikels (70%) nader aan dié van swaelsuur (75%) geproduseer nanodeeltjies en hoër as enzymatiese hidrolise geproduseer nanodeeltjies (48%). Soutsuur geproduseerde sellulose-nanodeeltjies het gelei tot 'n verbeterde polydispersiteitsindeks (0,53 ± 0,20) en veselmorfologie (514 ± 50 lengte) in vergelyking met die enzymatiese produkte van nanopartikels (0,92 ± 0,13 PdI en> 1 μm lengte). Die gevolgtrekking kan gemaak word dat laasgenoemde tussen ensiematiese en soutsuurbehandelings nanodeeltjies tot gevolg gehad het met beter eienskappe as eersgenoemde.

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