3.1.1. Acid treatment
Pretreatment by acids seems to be the most widely applied method for the transformation of lignocellulose biomass [
73]. Pretreatment and delignification of oat husks by diluted solutions of sulfuric [
73,
74,
75,
76,
77], nitric [
11,
39,
78,
79,
80,
81,
82], hydrochloric [
73], peracetic [
40] acid is described.
The authors of paper [
74] optimized pretreatment of oat husks with diluted sulfuric acid by using of Response Surface Methodology (RSM). The optimal acid treatment condition values were 130 °C, solid-to-liquid ratio (S:L) of 1:8 (w/v), concentration of H
2SO
4 1% (v/v) and pretreatment time 19 min. However, the authors did not discuss the content of residual lignin in the product. In another study, Soleimani and Tabil [
76] showed that the optimal conditions of oat husk pretreatment were 130 °C, S:L of 10% (w/v), time of delignification 40 min and 0.05 M of H
2SO
4.
Budaeva et al. systematically investigated the oat husk acidic delignification in the presence of nitric acid solutions (nitric method) [
11,
39,
78,
82,
83,
84,
85,
86,
87,
88,
89,
90]. Temperature, reaction time and concentration of acid were varied in the range of 90-96 °C, 2-4 h and 2-6%, respectively. Optimum conditions revealed were S:L of 1:20 (w/v) at 94-96 °C, 4 h. This method allows one to reduce the lignin and hemicellulose content in husk by the factors of 1.5-2.4 and 4-5, respectively.
It should be emphasized that an application of pure acidic treatment does usually not let one to carry out the oat husk processing sufficiently for any materials to be produced. To increase a delignification degree, it is proposed to combine acidic treatment with other methods. One of the most common ways is a combination of acidic and alkali techniques. The biomass processing technique called nitric-alkaline is carried out via two subsequent stages (nitric acid first, alkali second) [
44,
86,
91] and favors decreasing residual lignin and hemicelluloses [
87,
92]. According to the proposed process scheme, lignin undergoes nitration but remains insoluble. Modified biopolymer is extracted by diluted solution of any alkali at a temperatures less than 100 °С. Thus, in the patent [
79] is shown that sequential two-stage treatment of oat husks with diluted nitric acid (2% (w/w), S:L of 1:15 (w/v), 90-95 °C, 6-12 h) and sodium hydroxide solution (2% (w/w), S:L of 1:10 (w/v), 80-95 °C, 5-6 h) enables to produce cellulose sample with residual lignin of 0.6-2.5%. The authors of [
44,
93] decreased an amount of residual lignin to 0.3% in cellulose derived from oat husks. Interestingly, the reaction product contained 93.3-94.9% of α-cellulose. Polymerization degree was 1050-1420. The cellulose sample was fibers , the size of which was around 30-50 μm in width and 25-850 μm in length.
Sulfuric acid may substitute nitric one in two-stage acid-base oat husk processing. Thus, Soleimani and Tabil [
94] spent oat husk in H
2SO
4 solution followed by treatment in NaOH. Therein, the hemicellulose and lignin contents were reduced to 0% and 7%, respectively, cellulose extracted represented fibres. In the manuscript [
95] two-stage acid-base oat husk delignification included bleaching by 1.4% acidified sodium chlorite solution, with pH adjusted to 3.0-4.0 by glacial acetic acid (70 °C, 5 h) to remove lignin. The bleached fibers were washed several times in distilled water and treated with a 5 wt.% potassium hydroxide aqueous solution (24 h, room temperature, and then at 90 °C for 2 h) to leach hemicelluloses. Cellulose yield was 34%. The purity of the cellulose sample was confirmed by IR spectroscopy. The bands at 1247 cm
-1, 1516 cm
-1 and 1729 cm
-1 which correspond to vibrations of the C–O aryl group, the C=C bond in the aromatic ring of lignin, and C=O of carbonyl and acetyl groups in the xylan were absent in the spectra. The particle size and crystallinity index of the resulting cellulose were 10-45 µm and 52.7%, respectively.
The authors of [
40,
42,
96,
97] proposed to isolate cellulose of oat husks via treatment with peracetic acid generated in-situ. Delignification was carried out in the presence of 50% acetic acid, 38% hydrogen peroxide at 60 °С, S:L of 1:12.5 (w/v). Reaction time was 24 h. The resulting product yield was 30-33%. Amount of residual lignin was 3%. The removal of non-cellulose components was confirmed by IR and NMR spectroscopy. CI of the product of 35% was higher compared to one of pristine oat husk due to deletion of amorphous domains of cellulose in biomass. A microscopy of the product confirms the formation of long separate fibers of the polysaccharide. Cellulose was of I type.
To increase the purity of the cellulose some authors propose a multi-step treatment of oat husks. The technique includes subsequent application of acids, bases and/or hydrogen peroxide. The treatment of intermediate cellulose by bases and/or H2O2 can be carried out several times to improve the product quality.
Kashcheyeva et al. [
39] separated cellulose from oat husks by multi-step process which consisted of preliminary hydrolysis of the substrate ([HNO
3] – 0.2-0.4 wt.%, 90-95 °C, 1 h, S:L(w/v) = 1:15), a treatment by 3-6 wt.% solution of HNO
3 (90-95 °С, 10-12 h, S:L(w/v) = 1:15), subsequent bleaching by 3-6 wt.% and 0.5-1.0 wt.% NaOH (90-95 °С, 2-4 h, S:L(w/v) = 1:15 and 90-95 °С, 1-2 h, S:L(w/v) = 1:10), and, finally, souring with 0.5-1.0 wt.% HNO
3 (40-60 °C, 15-30 min, S:L(w/v) = 1:15). The yield of cellulose produced by such a multi-step experiment reached 23% and 1 and 2.5 wt.% of lignin and hemicelluloses were revealed in the product. Another approach applied by the authors differed from the previous one by sequence of stages. After hydrolysis ([HNO
3] – 0.2-0.4 wt.%, 90-95 °C, 1 h, S:L(w/v) = 1:15), treatment by 3-6 wt.% NaOH (90-95 °С, 6-8 h, S:L(w/v) = 1:15) preceded the reaction with a 3-6 wt% HNO
3 (90–95 °C, 4-6 h, S:L(w/v) = 1:10). While the inversion of the process steps increased cellulose yield to 34%, the amounts of residual lignin and hemicelluloses achieved 2.1 and 6.9 wt.%, respectively. α-cellulose content was 92.5% in both cases.
There are some other results reported which describes multi-step approach for isolation of cellulose of oat husks. The authors of [
80,
83] investigated delignification of oat husks as follows: 1) preliminary hydrolysis by 0.1 % dilute HNO
3 (91-93 °С, 1h), 2) treatment of the pulp in solutions of 4% (w/w) NaOH and 4% (w/w) nitric acid under 92 °C.
The obtained sample contained 1.7% of lignin. In order to remove residual lignin the produced wet pulp was additionally pretreated with 2% (w/w) sodium hydroxide solution at 60 °C and washed with 0.2% (w/w) sodium hydroxide solution and water. After this procedure the sample was followed by cleaning with a water-ethanol mixture. The final cellulose contained less than 0.5% of residual lignin. Cellulose consisted of coiled plane fibers and had an advanced surface (thickness of 2–5 μm, wideness of 20–30 μm and a length of 300–800 μm). The authors believed that an application of ethanol predicted resorption of lignin solved on a cellulose surface.
Sjöstedt carried out multi-stage acid delignification of oat husk followed by alkali or peroxide treatment [
73]. The first stage was a prehydrolysis of biomass under following conditions: 0.1% HCl or 0.0137-0.0274 M H
2SO
4, 160 °C, S:L of 6:1, 15 rpm, 0.5-1h. In the second stage, alkali delignification (soda pulping) was carried out at 170 °C, 4% NaOH, S:L of 8.3:1, 4h. Finally, the third stage was a two-step bleaching of obtained pulp via treatment with hydrogen peroxide with the addition of EDTA and peroxide treatment with the addition of sodium hydroxide. After alkaline delignification pulp contains 87.7 - 98.1 % cellulose and 0.1 - 0.05 % lignin. Pretreatment with sulfuric acid results in better removal of lignin. However, the degree of cellulose polymerization was 238-279, while after hydrochloric acid treatment the degree of polymerization varied from 405 to 437. According to the data of solid-state NMR analysis, the obtained cellulose samples belong to the Cellulose I type (a peak was observed in the region of 64.9ppm). Mass average molar mass of pulp was 89431, which is 4.8 times lower than in the feedstock.
Shahi et al. [
98] treated of black oat as following: 1) neutral detergent solution and acid detergent solution (100 °C, 1 h) using a fiber analyze, 2) alpha-amylase, and sodium sulfite, 3) alkaline hydrogen peroxide (1 N NaOH, 3% hydrogen peroxide). In the sample obtained cellulose contain lignin was less 2%. Cellulose derived from black oat exhibited of high thermal decomposition temperature (~ 370 °C), which was even higher than commercial cellulose.
3.1.2. Alkali treatment
According to literature data, the possibility of separation cellulose by alkali delignification is also discussed [
39,
48,
59,
73,
99,
100,
101,
102]. The advantages of alkaline treatment are the low cost of the reagent, low process temperatures, atmospheric pressure, the possibility of reusing the residual alkaline solution, and the less aggressive effect of the reagent on equipment compared to acid treatment.
The idea of alkaline delignification is to remove lignin from the lignocellulosic matrix of a plant. The mechanism of alkaline hydrolysis is that the uronic ester bond that functions as cross-link is broken by saponification, and the crosslinks between the xylan chain and other polymer units can be broken. Finally, lignin is removed and cellulose and hemicellulose are retained [
103]. The destruction of the bonds between lignin and hemicellulose in lignin-carbohydrate complexes also leads to an increase in the hydrophilicity of lignin and promotes its dissolution [
104,
105]. Alkali treatment influences cellulose via decreasing polymerization degree of the polysaccharide. It may one to obtain cellulose being suitable for chemical trasformations into bioethanol, for example. Also the cellulose derived via technique under consideration can be processed to furfural derivatives by catalysis.
In work [
59] alkali pretreatment was carried out by dilute solution NaOH at 94-96 °C during 4h. This treatment leads to an increase in the cellulose content of the obtained product compared to the feedstock and to a decrease in the amount of lignin from 18.1% to 5.4%. Nowadays, scale production of alkali-treated cellulose of oat husks is also under consideration. Skiba et al. carried out pretreatment of oat hulls with 2% (w/w) alkaline solution at 80 °C during 1 h and at 2100 rpm in pilot installation comprised of a rotary-pulsating apparatus and a 100 L stainless steel reactor [
102]. Loading of oat hulls were 1.5 kg. The obtained pulp was washed with 0.5% (by weight) sodium hydroxide solution, then with water. In the pulp from the oat hulls reducing the content of pentosans by a factor of 5, acid-insoluble lignin by a factor of 3.4 were observed, whereas the cellulose content increased by 1.9 times. Kashcheyeva et al. [
39] increased scale of oat husk treatment in a pilot installation (S:L (w/v) =1:15, 10 kg of substrate, [NaOH] – 3-6 wt.%, 90–95 °С, 6–8 h). After the delignification, 10-13 wt.% of lignin и 6.1-7.7 wt.% of hemicelluloses were found in the cellulose product, its yield was 41%.
Similar to acidic approach, for the total removal of non-cellulose components of oat husks, additional treatment is required. Acids, hydrogen peroxide and bases are used for bleaching the cellulose semi product. Purification can be repeated several times for increasing quality of the target material.
Previously, cellulose separated from the oat hulls by alkaline delignification method with sodium hydroxide was shown to be additionally treated by nitric acid. Cellulose has a degree of polymerization of 1139 and an index of crystallinity equal to 68% [
106]. This method showed a high degree of reproducibility [
87,
107].
Valdebenito et al. [
49] proposed tree-step way of oat husk transformation. It consisted of the following subsequent treatments 1) 0.1 M NaOH, 30 °C, 18 h, 2) 0,1 М HNO
3, 85 °С, 1 h, 3) 3% H
2O
2 at 70 °С, 1 h. Residual content of lignin and hemicelluloses were less 1 and 26%, respectively, cellulose yield achieved 63%.
Oliveira et al. [
48] obtained cellulose fibers from oat husks by alkali pretreatment with 4% (w/v) NaOH solution at 80 °C, 4 h. Cellulose semi product was subsequently bleached by the mixture of NaOH (0.017 g/L), acetic acid (0.075 g/L) and sodium chlorite (0.017 g/L). S:L ratio was 1:1, 90 °C. Reaction time was 4 h. Obtained sample of cellulose fibers contain 94% cellulose, 1.4% hemicelluloses and 1% lignin. The cellulose fibers had a structure similar to commercial cellulose fiber samples. The index of crystallinity was 63.5% (cellulose I type). During the transformation, from 100 g of the oat husks the researchers obtained 18.4 g of cellulose.
Along with acid and alkali treatments some alternative agents are also investigated for oat husk transformation to cellulose. Dehkhoda et al. [
108] studied oat husk delignification by ammonium persulfate (0.15 М (NH
4)
2S
2O
8, 900 rpm, 60-70 °С, 20 h). As reported by the authors, the advantage of this method of cellulose isolation is the simplicity of the method, the one-pot organization of the process. The technique does not require additional processing, including energy-consuming mechanical methods. The removal of non-cellulose components occurs with high efficiency. Such an extraction let one to derive nanocellulose which has high degree of crystallinity [
109,
110,
111]. In the process of heating a solution of ammonium persulfate, the formation of free radicals and particles, SO
4- and H
2O
2, occurs. The active species penetrate into the amorphous regions of cellulose, destroy non-crystalline domains of the polysaccharide and lignin. The process let one to form nanocrystalline cellulose. Using XRD, the resulting nanocellulose was shown to be of I type, the crystal lattice was monoclinic. It was also confirmed that the cellulose extraction method used did not lead to the transformation of the cellulose I into cellulose II. The CI of the prepared sample estimated was 57%. The average size of the resulting nanocellulose particles was 30 nm. The absence of lignin impurities in the nanocellulose sample was confirmed by FT-IR.
3.1.3. Combination of chemical and physical treatment approaches
In case of cellulose isolation of oat husks, interesting results can be obtained via combination of chemical and physical treatment techniques. For the processing oat husks ultrasonication, steam explosion, microwave heating are discussing.
Winuprasith et al. [
112] recovered cellulose from oat bran to compare 1) chemical treatment carried out by 5% w/w solution of KOH (90 °C, 2 h) and 1 %w/w HCl (80 °C, 2h), 2) microwave treatment (800 W, 5 min, 5 cycles) and 3) hydrothermal treatment (120 °С, 2h). All the recovery techniques were followed by bleaching with hydrogen peroxide (30% v/v, 90 °C, 3 h). Unfortunately, the authors of [
112] did not discuss the yield, CI and purity of the target product. Lawford et al. pretreated the feedstock by a combination of steam explosion (1 min) and 0.5-1% (w/w) sulfuric acid treatment [
75]. This procedure destroyed the fiber structure of the raw material. Budaeva et al. used hydro-thermobaric processing of the substrate in a high-pressure reactor [
81,
113,
114] under 1-2 MPa Time of exposure varied from 0 to 1200 s and temperature was chosen in the range of 179-230 °C. After the pretreatment, the resulting fibrous product was additionally treated with 2% (w/w) sodium hydroxide solution under 55 °C during 30 min. H
2O
2 was added into the suspension. Mass fraction of lignin in obtained cellulose were varied from 0.8 to 13.5%.
Machado et al. [
100] compared several approaches to oat husk delignification. The authors investigated the following agents:
- -
alkali pretreatment with sodium hydroxide solutions (0.5%, 1%, 2%, 3% or 4% (w/v)) spent at S:L of 1:10 (w/v), 121 °C, 0.5 h;
- -
alkali pretreatment with H2O2 solution (7.5% (v/v), pH = 11.5, S:L of 1:25 (w/v), 25 °C, 1 h);
- -
alkali treatment using ultrasonication (1% (w/v) NaOH solution, S:L of 1:7 (w/v), power 90 W, frequency 20 kHz, 20 min);
- -
sequential alkaline (1% NaOH solution (w/v), S:L of 1:10 (w/v), 121 °C, 0.5 h) and ultrasonic pretreatment (water, S:L of 1:7 (w/v), power 90 W, frequency 20 kHz, 20 min).
It is shown that an increase in the concentration of an alkali solution leads to a decrease in the content of lignin in a cellulose-containing product from 13.5 to 2.2%. Pretreatment of oat husk with H2O2 reduces the residual lignin content to 7.3% in the pulp product. Single ultrasonic pretreatment demonstrated low efficiency in terms of delignification. Only 1.3% of lignin was removed. At the same time sequential alkali and ultrasonic treatment of the biomass showed the results similar to NaOH treatment only. Thus, the authors demonstrated that ultrasonication seems to be ineffective for the production of cellulose from oat husks.
An influence of a combination of chemical acid or alkali treatments and steam treatments on the delignification of oat husks was also reported in [
115]. The substrate was pretreated in a steam in an autoclave (1.5 atm, 121 °C, 0.5 h, S:L of 1:9.1 (w/v)) or undergoes ultrasonication (80 °C water, S:L of 1:9.1 (w/v), 30 min). Then, biomass was dispersed in 1% sulfuric acid solution or in 10% sodium hydroxide solution or water. The content of residual lignin in the samples of the cellulose product ranged from 12.6 to 7.2%. The index of cellulose crystallinity was in the range of 47-77%. Analysis of the obtained data showed that steam treatment is more effective compared to ultrasonic. The lowest content of residual lignin and highest crystallinity index was in the sample subjected to steam treatment combined with H
2SO
4.
The authors of [
50,
71,
116] applied reactive extrusion combined with alkali-peroxide treatment of oat husks [
117]. To carry out the reaction, a sample of oat husk was placed in a sealed container with sodium hydroxide solution (pH = 11.5), the moisture content of the husk was 32%, and kept for 24 h at 7-10 °C. Then a solution of hydrogen peroxide (7 g H
2O
2 per 100 g of hulls) was added to the resulting mixture. The material was extruded in a single-screw extruder with four heating zones, where the temperature was maintained at 110 °C in all zones. After extrusion, the samples were dried (60 °C, 24 h) and crushed. It was shown that the removal of lignin resulted in the production of cellulose samples with a more open morphology and greater surface area and porosity. Reactive extrusion may be an alternative fiber modification method that has several advantages, such as short processing time and no wastewater generation. However, the authors did not discuss the residual lignin amount.
Reactive extrusion can be also combined with acid-alkali treatment. Debiagi at al. proposed two ways of the biomass delignification: 1) three-step reactive extrusion (firstly NaOH (10 wt.%), secondly double treatment with H
2SO
4 (2 wt.%)) combined with ultrasonication (15 min, 20 kHz), 2) subsequent pretreatment by peracetic acid (50% acetic acid, 38% hydrogen peroxide and 12% distilled water at 60 °C for 48 h, S:L of 1:16.6 (w/v)), extrusion with H
2SO
4 (2 wt.%) and ultrasound (15 min 20 kHz) to produce cellulose I type [
53]. It was found that the first three-step approach let one to produce cellulose which contains 17% of hemicelluloses and 9.5% of lignin. CI was 68%. Pretreatment by peracetic acid followed by activation with H
2SO
4 and ultrasonication reduces hemicelluloses and lignin to 3 and 7.7%. CI of the target material was 80%.
In conclusion, the following main points may be emphasized. One-step acid or alkaline treatments of oat husks at low temperatures allow one to reduce the lignin content in the composition of the cellulose product to 5-13% only. The advantage of this process is the use of low temperatures (usually up to 100 °C). However, due to the presence of lignin impurities, the use of such cellulose without additional purification is limited.
The combination of acid and alkaline treatment results in a reduction in residual non-cellulosic components in the final product. Such cellulose can be used for the synthesis of ethanol or for the production of composites based on it. Reiteration of treatment (acid-base and/or peroxide-base) combined with additional bleaching by bases or hydrogen peroxide propose a way for obtaining cellulose which has high purity and suitable for the synthesis of nanocellulose, including bacterial cellulose, composites and hydrogels, cellulose esters. The degree of polymerization and crystallinity index in this process vary in the ranges of 800-1400 and 60-70%, respectively. Disadvantage of the method is a quite low yield of the target polysaccharide. It may vary from 18 to 35%. Purified cellulose is also suitable for catalytic transformations of the polysaccharide to alcohols (sorbitol, ethylene and propylene glycols), organic acids (formic, levulinic, glycolic) and furan derivatives
A combination of chemical and physical (steam explosion, hydro-thermobaric processing, ultrasonic pretreatment, microwave treatment, reactive extrusion) processes is also under consideration. An analysis of the literature data showed that the most promising method that can be implemented on an industrial scale is the use of a combination of chemical processing and reactive extrusion. Steam treatment is also perspective. This method is most actively used by research groups in Brazil. The advantages of the method include simplicity of the organization, small reaction time, low quantities of solvents involved.