Document Type : Original Research Paper
Department of Environmental Science and Engineering, Guru Jambheshwar University of Science and Technology, Hisar (Haryana), India
Wheat is one of the most common crops among the crops in India and more than 94 million tonnes of wheat are produced per year . The main higher productivity state of wheat grown are Punjab, Haryana, Madhya Pradesh, and Uttar Pradesh. Billion tons of wheat crop residue wheat straw after harvesting have been left in the field. It is an easily available renewable source of lignocellulosic biomass. Wheat straw is an agricultural by-product of the wheat crop and is available in abundant in north India. The higher C-N content of wheat straw leads to lower bio-degradability in contrast to other agricultural remains. A smaller amount of it is utilized for animal feed, the production of biogas and biofuel, and other industrial applications such as the extraction of cellulose and hemicellulose. However, most farmer prefers to burn wheat crop residues directly in the field because it is the cheapest and quiet method to manage crop residues , . It is an important basis for worse air quality . Blaze releases due to forest fire, owing to high carbon content levels . Residues blazing release a higher extent of air pollutants such as N2O, CO2, CH4, CO, SO2, NH4, HC, VOCS and suspended particulate matter in varied ranges depending on the residual composition , . The mentioned pollutants in the air cause hostile effects on human health. They can cause diseases to the skin, cataracts, pulmonary tuberculosis, pulmonary diseases, pneumoconiosis, eye irritation, bronchitis, and blindness. Road accident cases also increase throughout the stubble burning period.
Consequently, it is of major concern to grow economic and scientific methods for recovering cellulose from agricultural wastes. Lots of efforts have been made by various researchers and engineers to produce environmental-friendly composite constituents from agricultural wastes , , , , .
Cellulose shows excellent mechanical properties, such as high strength (2-6 GPA) and toughness  lower density, a high length-to-diameter ratio , , macroscopic quantum tunneling effect, surface effect and smaller size effect, low coefficient of thermal expansion , high Young’s modulus (138 GPa)  biodegradability and biocompatibility, high availability as a renewable material, barrier properties, dimensional constancy. Presently, agricultural residue containing cellulosic materials that are green, maintainable, and environmentally approachable  has been extensively established for nanocellulose extraction , . The exceptionality of nanocellulose elements depends on the characteristics of the material and methodology used in manufacturing . Nanocellulose size generally has a diameter of 1-100 nm or lesser than that in a single dimension. Based on the extents, preparation methods and functions, nanocellulose is classified as cellulose nanofibers (CNF) ,cellulose nanocrystals (CNC) , ,  and biological nanocellulose.
The various methods used for the extraction of nanocellulose by the various researchers worldwide example, described in Table.1 were electrospinning , , , oxidation by reduction of the crystalline fibres in the nanosized units , , high pressure homogenization and refining , , , enzymatic hydrolysis , , cryo-crushing , , micro-fluidization , , , higher intensity ultrasonication , , , steam explosion , . Amongst the various methodologies used in the preparation of nanocellulose, acid hydrolysis is the utmost protruding and widely used method , , ,  that disintegrates amorphous segment and disordered cellulose which provides distinct crystals along with higher notch of crystallinity , , . Extracted nanocellulose has wider applications in the field of environmental science/engineering such as water treatment , energy as fuel cell applications , as an effective solar cell substrate  removal of dyes, heavy metals, fluoride , phytoremediation  and other areas such as drug delivery, food packaging, paint industry, etc.
On a structural and compositional basis, a stiff outer cover that shields the plant cell is called a cell wall. A cell wall is composed of three layers: middle lamella, primary wall, and secondary wall . Middle lamella has a higher quantity of lignin and is mainly accountable for the binding of adjacent cells . The primary wall is around 30–1000 nm thicker and has three main components-pectin, hemicellulose, and cellulose whereas cellulose microfibrils (MFs) are organized cross-wise . The secondary cell wall is composed of three layers: inner (S1), middle (S2), and outer layer (S3) which differs in the angle of microfibrils regarding the axis of fiber . Amongst all, the middle layer is of utmost value that comprises cellulose in higher quantities. Cellulose is connected via β-1,4 glycosidic linkage and most abundant biopolymer on the earth. The cell wall of woody fibers is made up of repetitive crystalline structures which result from the accretion of cellulosic chains, called microfibrils . The secondary wall is composed mostly of cellulose microfibrils, allied parallel and compactly packed in a plane helix . Microfibrils are made up of elementary fibrils that were previously considered the lowest fiber morphological units . The details of the plant cell wall are shown in Fig.1. (a) and (b).
The novelty of the research
Due to the lack of a sustainable management plan for re-utilizing of the wheat straw, if we do stubble burning it leads to air pollution which may cause danger to the biotic and abiotic components of the environment. The present research was done to manage wheat straw by extracting cellulose and conversion of cellulose into nanocellulose further enhanced its utilization due to increased surface area using a facile approach. For large production of nanocellulose chemical method is time-saving, cost-effective, and worldwide accepted. Extracted nanocellulose has a wider range of applications in the field of food packaging, the paint industry, and pollution abatement due to versatile properties like non-toxicity, biodegradability, and renewability.
MATERIALS AND METHODS
Wheat straw samples were collected from the vicinity of rural Hisar city in 2019. The collected samples were washed, dried, cut into small pieces (0.5-1 cm) ground, and sieved with 80-micrometer mesh. The powder samples were oven dried at 500C and stored in an airtight bottle. Nanocellulose was prepared in multi-step treatment processes shown in Fig.2. Alkaline and bleaching treatment were given with sodium hydroxide and sodium chlorite, respectively further acid hydrolysis was carried out with sulphuric acid. After the completion of each step, the material was washed with double deionized water to neutralize the pH of the processed material. Analytical-grade chemicals were used for experimentations.
Estimation of lignocellulosic content
Goering and Van Soest’s method (1970)  was used for the estimation of cellulose, hemicellulose, lignin, and ash content of raw wheat straw.
Extraction of Nanocellulose
Take 500 gm of dried wheat straw, add 4% (w/v) of sodium hydroxide solution to the beaker stir at 80°C for 1 hour. Cool down the mixture solution and then washed it with double deionized water till the pH of the solution becomes neutral. Filter the mixture and dried for further treatment. The lignin and hemicellulose content was removed with NaOH from wheat straw to obtain cellulose during alkaline treatment.
After the alkali treatment remaining mixture of the wheat, and straw powder was bleached with 8% (w/v) sodium chlorite solution at 70°C. The bleaching process was repeated 2-3 times. Bleached fibers were filtered and cleaned in each cycle with double deionized water till the pH becomes neutral. Bleached fibers were dried at 45°C for 12 hrs. in a hot air oven. The majority of lignin was removed during the bleaching process.
The bleached wheat straw fibers were soaked in 20% (w/v) sulphuric acid solution at 40°C on the hot plate for one hour. 500 ml distilled water was added to the solution to stop the response. After the acid hydrolysis, the acid-treated mixture solution was centrifuged at 4000 rpm for 15 minutes and washed the mixture with double deionized water until the pH becomes neutral. The suspension was sonicated for one hour at 45°C to improve the texture of cellulose. and then magnetic stirring was done four to five times and similar treatments were done by .
After acid hydrolysis cellulose was extracted. To further increase the surface area of cellulose, the cryo-crushing process was carried out. The liquid N2 was utilized for the fibers frozen of cellulose. The cell walls were ruptured and finally, nanocellulose is obtained. Dry the prepared nanocellulose at 45°C for 24 hours in an oven and store it in an airtight storage bottle for further use.
Characterization of Nanocellulose
Fourier Transform Infrared Spectroscopy (FTIR)
The FTIR spectrum of the raw wheat straw, alkali treated wheat straw powder, bleached treated wheat straw fibers, acid hydrolyzed wheat straw powder, and nanocellulose were analyzed by Perkin-Elmer Fourier Transform Infrared instrument with the wave ranges between 4000-500 cm-1. The samples were mixed with potassium bromide (KBr) and makes its pellets for FTIR analysis.
X-Ray Diffraction (XRD)
Samples of raw wheat straw, alkali treated wheat straw powder, bleached treated wheat straw fibers, acid hydrolyzed wheat straw powder, and the extracted nanocellulose were X-rayed using an X-ray Diffractometer with monochromatic CuKα energy basis, having 2θ ranges between 10º-50º having phase 0.04 and time of scanning is five min. The samples are placed on the sample holder in fine powdered form and uniformly leveled to obtain proper X-ray exposure.
Field Emission Scanning Electron Microscopy (FESEM)
The morphological structure of the raw wheat straw powder and nanocellulose was analyzed by Field Emission Scanning Electron Microscope instrument (FESEM) Merlin Compact, Carl Zeiss with 30 KV acceleration voltage and 1.6 nanometers at 1 KV. A gold coating layer was applied to the sample using “ion sputter coater” before analysis.
Transmission Electron Microscopy (TEM)
The nanocellulose was observed by TEM using The Tecnai G2 20 (FEI) S-Twin transmission electron microscope with an acceleration voltage of 200 KV. Ion milling of the sample is done to reveal the pristine sample surface for imaging, after that sample was put on the copper-coated grid. Uranyl acetate solution was used to stain the grid and then the grid was allowed to dry at room temperature.
Differential Scanning Calorimetry (DSC)
The nanocellulose was analyzed for DSC using Q-10, TA Instruments Waters. 3mg of sample was placed in a platinum container and heated the sample from 0-4000C at a heating rate of 50C/min provided with the helium atmosphere.
The thermal stability of nanocellulose was characterized using METTLER. Heating of the 6mg sample at 50°C to 850°C having a rate of heating 5°C/min. In the nitrogen atmosphere, the measurements were carried out at a gas flow rate of 10 ml min-1. Thermal stability was obtained as the weight loss rate as a function of time.
Atomic Force Microscopy
The morphological and topographical surface structure of the extracted nanocellulose from wheat straw was determined by a multimode scanning probe microscope (Bruker) AFM. Cantilevers were made of silicon material having a frequency of 230 kHz and images were taken in tapping mode at room temperature. The rate of scanning was 1.5 Hz.
The surface charge of extracted nanocellulose in suspension form was determined by Malvern Zetasizer Nano-ZS90 at 250C with 0.8872 cP viscosity using the dynamic light scattering method. The dispersant used was double deionized water.
Particle Size Distribution
The particle size of the extracted nanocellulose in suspension form was determined by Malvern Zetasizer Nano-ZS90 at 250C with 0.8872 Cp viscosity using the dynamic light scattering method. The dispersant used was double deionized water.
RESULTS AND DISCUSSION
Estimation of Lignocellulosic Content
The lignocellulosic contents of wheat straw were analyzed using Georing and Van Soest method (1970). The observed amount of cellulose (36.1%), hemicellulose (30.3%), lignin (17%), and ash content (9.2%) were found in the raw sample of wheat straw. The results of various researchers are compared with the present study (shown in Table.2). Variation in the composition of lignocellulosic contents may be due to the selection of a different variety of wheat crops and environmental conditions. Further, we extracted cellulose contents from the samples and removed non-lignocellulosic components (hemicellulose and lignin) using chemical treatment. The details of extraction of cellulose from wheat straw using different steps are given in the material and methods.
Characterization of Nanocellulose
The extracted nanocellulose was characterized using several techniques such as Fourier Transform Infrared Spectroscopy (FTIR) to determine organic element composition, X-Ray diffraction (XRD) to analyze the structure of the sample, Field Emission Scanning Electron Microscopy (FESEM) to analyze the morphology of the sample surface, Transmission Electron Microscopy (TEM) to analyze the size, morphology, compositional structure and texture, Thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC) for thermal analysis, Zeta Potential to determine stability and Particle Size Distribution to determine particle size.
Fourier Transform Infrared Spectroscopy (FTIR)
Spectra of FTIR “Raw wheat straw, alkali treated, bleached fibers, acid hydrolysis and nanocellulose”, described in Fig.3. in which, on X-axis (cm-1) denotes wavenumber and on Y-axis transmittance T (%). Peaks at 3421-3433 cm-1 observed stretching of the O-H band, which arises because of the vibrations in the hydroxyl group bonded by hydrogen and similar results have been observed by , . The peaks at 2900-2930 cm-1 show the aliphatic saturated C–H stretching vibration and similar results have been observed by . The band at 1630-1650 cm-1 shows the C=C stretching and bending mode of absorbed water, that the nanocellulose has a strong affinity for water. Peaks at 1195 cm-1 (O=S=O peak of asymmetric stretching vibration), 1033.23 cm-1 (O=S=O peak of symmetric stretching vibration), and 624 cm-1 (C-S stretching vibration peak) these peaks show the presence of SO3H group in acid hydrolysis treatment. Peaks at wavelength 1430.50 cm-1 and 1638.41 cm-1 in extracted nanocellulose show stretching and binding vibrations of O-H and C-O, -CH2 and -CH bonds in cellulose and C=C stretching, water molecules vibrations absorbed in cellulose, respectively and similar results were observed by . Peaks observed at 1430-1450 cm-1 (except in untreated wheat straw) and 1370 and 1316 cm-1 in acid hydrolyzed and nanocellulose observed respectively, represent C–H stretch and C–H or O–H bending and similar results have been observed by . The two peaks at 1050-1060 cm-1 and 897 cm-1 indicate C–O stretching and C–H deformation vibrations of nanocellulose and similar results have been observed by ,  which can be seen in all the spectra.
X-Ray Diffraction (XRD) Analysis
Degree of crystallinity by X-Ray Diffraction (XRD)
XRD patterns of raw wheat straw, alkali treated, bleached fibers, acid hydrolysis, and nanocellulose, shown in Fig.4. XRD peaks about 2theta =16º, 22º, and 35º indicating that extracted nanocellulose crystals adopt a crystalline structural formation and similar results have been observed by . Crystallinity is stated as crystalline region diffraction divided by the sample’s total diffraction and similar results have been observed . The crystallinity index of all the analyzed samples was calculated by using the Segal method  and is illustrated in Table.4. The successively rise in the crystallinity index was due to the elimination of hemicellulose and lignin contents in a certain amount to the amorphous region in alkali process, majority of lignin is eliminated in bleaching process which results in rearrangement of the crystalline spheres. In the acid hydrolysis process, hydronium ions diffuse into cellulosic amorphous regions and assign glycosidic bonds of hydrolytic cleavage, which ultimately issues distinct crystallites and similar results have been observed by . Laterally, the bleaching treatment proficiently removes the enduring amorphous mechanisms. By increasing the crystallinity of fibers of wheat straw, rigidity, toughness, so, the fiber strength also increases. After giving acidic treatment, fibers’ crystallinity increases, and similar results have been observed by , . For raw wheat straw, alkali treated, bleached fibers, acid hydrolyzed wheat straw, and nanocellulose diffraction patterns have been shown in Fig.4.
The formula for determination of the crystallinity index:
where the maximum intensity of diffraction of (002) lattice peak is I002 and the intensity value for amorphous cellulose is Iam. Amorphous region scattered energy is restrained at diffraction angle i.e., (2theta =18º) and diffraction angle around 2theta=22º is located diffraction peak.
The percentage of crystallinity index in raw wheat straw is 33.3% which increases to 68.96% in extracted nanocellulose as shown in Table 4. These results illustrate that the crystallinity is gradually increased. The higher crystallinity index values can be tacit by the elimination of amorphous non-cellulosic complexes, such as hemicellulose and lignin tempted by using alkali, bleaching, and acid hydrolysis in purification processes. It seems that the crystallinity value depends on different kinds of plants, the hydrolysis process, and the extent of fiber purification. More the tensile strength of the fibers results in the crystallinity of the chemically treated fibers and similar results have been observed by , .
Field Emission Scanning Electron Microscopy (FESEM)
FESEM analysis shows the morphology of Raw wheat straw and Nanocellulose shown in Fig.5 (a) and (b). The raw wheat straw has an irregular porous structure. After the chemical treatments, the extracted nanocellulose has a regular shape having straight fibers connected. These straight fibers connected indicate the homogeneity of nanocellulose.
The area, angle, and length of the raw wheat straw and extracted nanocellulose was calculated using ImageJ software (shown in Table 5 and Table 6). The mean area was reduced by 0.038 nm2, the mean angle was reduced by theta=26.58 and the mean length was reduced by 0.629 nm. It shows that parameters were reduced as the process proceeds from raw wheat straw to the extraction of nanocellulose due to a reduction in particle size.
Transmission Electron Microscopy (TEM)
Fig.6. (a) TEM analysis shows the morphology of nanocellulose produced from wheat straw. The extracted nanocellulose has a spherical shape structure connected, showing the regular shape. The spherical structure has numerous properties and applicability. The size and shape of the nanocellulose affect the properties such as rheology, stability, and optical characteristics having a spherical and square structure of nanocellulose mainly regulates the application of nanocellulose and similar results have been observed . Fig.6. (b) The histogram shows the diameter of nanocellulose as 25-32 nm which has a 2% count, 32-40 nm which has a 5% count 40-47 nm which has a 2% count, and 47-55 nm which has a 1% count. A maximum of 5% count occurs at the 32-40 nm diameter range.
Differential Scanning Calorimetry (DSC)
The thermal behavior of the extracted nanocellulose from wheat straw was analyzed by differential scanning calorimetry (DSC) shown in Fig.7. The thermal behavior of the nanocellulose material varied from 0 to 400ºC. The endothermic peak was observed at 148.73ºC having an enthalpy of 9.1167 J/g and another sharp endothermic peak was observed at 193ºC having an enthalpy of 49.276 J/g. The exothermic peak at 336.75ºC with enthalpy 58.190 J/g shows the thermal degradation of nanocellulose. Through the treatment processes, nanocellulose crystallites might reorganize and reorient, which could lead to a more compact crystal structure that might have better thermostability and similar results have been observed .
Thermal stability of extracted nanocellulose from wheat straw has been analyzed by TGA as shown in Fig.8. Because of variation in chemical assemblies amongst lignin, cellulose, and hemicellulose, they decompose in the diverse temperature range. The initially smaller amount of weight loss was observed due to the evaporation of water that is loosely bounded to the nanocellulose surface at a temperature below 120ºC. The first degradation of the processes of cellulose occurs i.e., desiccation, depolymerization, and disintegration in the units of glycosyl, and similar results have been observed . While the nanocellulose disintegration temperature is observed at 360ºC-850ºC and finally decomposition occurs. Alike thermograph patterns show higher residue quantity of wheat straw fibers and similar results has been observed by . Fiber weight prevails next to heating at a temperature of more than 400ºC because of the existence of contents of carbon and similar results have been observed .
Atomic Force Microscopy (AFM)
Nanocellulose extracted from wheat straw has been analyzed by Atomic Force Microscopy (AFM) to determine its morphological and topographical structure. Fig.9. shows a 3D image of nanocellulose surface that has bell shape structure on a smooth surface with a particle height of 3.2 nm because of the homogeneous nature of the extracted nanocellulose. It arises due to the elimination of pectic polysaccharides that forms definite nanocrystal of cellulose and similar results have been observed . The mean roughness of 110.4 nm was calculated using Gwyddion software of the extracted nanocellulose.
Zeta potential determines that extracted nanocellulose has a mean zeta potential value of -22.2 millivolt showing a negative charge on nanocellulose surface as shown in Fig.10. It indicates that nanocellulose is moderately stable. Stability can be defined as (±10-20 mV) low stability, (±10-20 mV) moderate stability, (> ±30 mV) indicates good stability, and (> ±60 mV) very good stability .
Particle Size Distribution
The dynamic light scattering method was used to determine the particle size distribution of the extracted nanocellulose. It has a particle size of 58.77 nm with respect to intensity as shown in Fig.11. Particle size was observed in the nano range. From the obtained particle size it is confirmed that nanocellulose was extracted successfully.
In this study, the nanocellulose was extracted from wheat straw and different characterizations have been carried out. Chemical processes removed undesirable non-cellulosic constituents and mechanical processes aided in the individualization of cellulosic fibers to nano dimensions.
● FTIR showed that the obtained material was nanocellulose which contains traces or a very less amount of hemicellulose and lignin. Peaks at wavelength 1430.50 cm-1 and 1638.41 cm-1 both show that cellulose is obtained. The band at 1630-1650 cm-1 shows the bending mode of absorbed water, that the nanocellulose has a strong affinity for water and the removal of lignin and hemicellulose is observed.
● XRD results showed that the nanocellulose was crystalline having a 68.96% Crystallinity index.
● Morphological analysis was carried out through FESEM and it showed that the untreated wheat straw has an irregular porous structure but the extracted nanocellulose has a regular shape having straight fibers connected.
● TEM analysis showed that the extracted nanocellulose has a spherical shape structure connected, showing the regular shape, the obtained spherical shape regulates the nanocellulose for further applications.
● DSC results show that the exothermic peak at 336.75ºC with enthalpy 58.190 J/g shows the thermal degradation of nanocellulose.
● Thermal analysis, TGA showed that the nanocellulose decomposition temperature was observed around 360ºC.
● AFM determination showed the topographical structure of nanocellulose that has a bell-shaped structure on a smooth surface with a particle height of 3.2 nm and mean roughness was 110.4 nm.
● The Zeta potential of extracted nanocellulose shows a negative surface charge and it is moderately stable.
● Extracted nanocellulose has a particle size of 58.77 nm. The facile approach used in the successful extraction of nanocellulose from wheat straw provides prospects for efficient use of wheat straw which else may become a danger to the environment these days because of burning, mainly in north India. The study proves that nanocellulose can be successfully extracted from wheat straws.
● Extracted nanocellulose has wider applications in the field of environmental science/engineering such as purification of water, pollution abatement, phytoremediation, and other areas such as food packaging, paint industry, and drug delivery.
CONFLICT OF INTEREST
The authors declare no conflict of interest.