Introduction

Brazil is one of the main maize/corn (Zea mays) producers in the world. Brazilian maize harvesting in 2012 was around 71.4 million tons [1]. This means that annually huge amounts of maize wastes are discarded in landfills, leading to the reduction of landfill lifetime. In a maize field in Brazil, depending on several conditions, one hectare can produce about 2–3 tons of grain and about 6–12 tons of byproducts such as straw and stalks [2]. Maize straw is known to have high carbon to nitrogen ratio (C/N) in comparison to wheat straw, rice straw, potato stalks and soybean byproducts [2, 3]. This feature makes maize straw more resistant to degradation by soil microorganisms, meaning that its decay takes a longer time [2]. Moreover it was recently reported that agrowastes with distinct carbon and nitrogen contents can significantly alter soil nitrous oxide flux rates and influence soil–atmospheric emissions of N2O, a long-lasting greenhouse gas which, after carbon dioxide and methane, is also believed to contribute to global warming [3].

In light of this information it is essential to find new usages to agricultural wastes. In the present paper, our research interest is focused on extracting and characterizing cellulose from maize straw with the aim to isolate cellulose nanocrystals as well.

Cellulose extraction from biomass has been subject of intense research because the world needs the application of green chemistry principles in modern life. Each of the five continents shows characteristic biomass sources and due to this diversity, original papers are constantly being published on the extraction of cellulose and its applications. Examples of recently studied biomass sources are wheat straw [4–6], sugarcane straw [7], sugarcane bagasse [8, 9], hemp and flax straws [10] rice straw [11, 12], rice husk [13], soybean hulls [14], jute [15–17], banana stems and pineapple leaf fiber [17], oil palm residue [18] and the grass species Miscanthus × gigantheus [19]. In the search towards global sustainable development, each land must find new applications for the biomass wastes it produces in high quantities, so as to avoid high transport costs of the agrowastes to the local industries as well as environmental impacts. The latter may arise due to open air burning of the wastes that leads to air pollution or their accumulation on the soil, as aforementioned for the case of maize straw.

Many different processes have been used to separate lignocellulosic biomass into its components. These processes can be generally classified as physical (an example is the ball-mill process), chemical (acid or alkaline pulping, solvents, ozone or peroxide bleaching) and physico-chemical (for example, steam explosion) [20]. Also special procedures have been described, for instance the cavitation assisted delignification of wheat straw that was recently reviewed [5]. It was observed that cavitation does not degrade the fibrillar structure of cellulose and helps to remove lignin being a promising approach to reduce energy consumption in paper making industries [5].

In addition, the use of ionic liquids to fractionate lignocellulosic biomass into its various components has been intensively studied as shown by a recent review on this topic [21]. In the case of sugarcane bagasse, it was reported the use of an ionic liquid that completely dissolves it and allows the separation of cellulose [9].

Maize straw typically contains between 28 and 44 % of cellulose, whereas wood has average cellulose content in the range 40–44 % [10, 22, 23]. In spite of a cellulose content comparable to that of wood, maize straw has been rarely studied as a source of cellulose. A recent paper described that maize straw produces cellulose fibers that after acetylation efficiently adsorb diesel oil [23]. These fibers could be used as oil sorbents in oil spill cleanup. Considering maize biomass, the separation of cellulose was reported from cornstalks [24], corncobs [20] and cornhusks [25], i.e. the outer covering of corncobs.

Cellulose is the main constituent of the cell walls of higher plants, and due to its abundance is a cheap and accessible biodegradable material. It is a linear semicrystalline polymer composed of glucopyranose units linked together by β-(1, 4) linkages being enclosed by other two main components: hemicellulose and lignin, which are amorphous. Controlled hydrolysis of cellulose removes the amorphous regions and releases the cellulose nanocrystals which are elongated crystalline nanoparticles in the form of rigid rods. These nanocrystals are commonly called ‘whiskers’. In recent years the number of publications about cellulose whiskers has increased considerably [26]. This interest is mainly due to their high mechanical properties and the abundance of cellulose in biomass. Because of these properties, great improvements in mechanical properties of polymer nanocomposites containing cellulose nanocrystals have been observed [27]. Since these nanocrystals show hydrophilicity, the production of hydrogels containing cellulose whiskers offers an interesting potential for novel biomaterials in tissue engineering and drug delivery. On the other hand, the incorporation of these nanocrystals into electrospun polymeric nanofibers offers the possibility of their use in fields such as optoeletronics, sensor development, catalysis and filtration [28]. Whiskers have been obtained from different cellulose sources and their dimensions have been shown to depend on their source [29]. Recent papers report the isolation of cellulose whiskers from jute [30], Phormium tenax leaf fibers [31], leaves of Pandanus tectorius [32], sesame husk [33], rice straw [11] and rice husk [13]. Up to our knowledge, cellulose whiskers have not been isolated from maize straw.

The aims of this work are: (1) to extract cellulose from maize straw by a chlorine-free process and (2) to isolate and characterize cellulose whiskers from maize straw cellulose. In the present work, as in other studies [13, 34], the techniques of thermogravimetry (TGA) and attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR) were successfully used to characterize crude lignocellulosic biomass and pulped/bleached cellulose samples. The techniques of light scattering (LS) and transmission electron microscopy (TEM) were used to confirm that cellulose whiskers were indeed obtained.

Materials and Methods

Materials

Maize straw was supplied by the agronomic experimental station of Federal University of Rio Grande do Sul (UFRGS). Hexane (Fmaia, Brazil), ethanol (Fmaia, Brazil), sodium hydroxide (Labsynth, Brazil), hydrogen peroxide (CAQ Química, Brazil), nitric acid (Fmaia, Brazil), acetic acid (CAQ Química, Brazil), tetra-acetylethylenediamine (TAED) (Acros Organics, New Jersey, USA) were used as received. All solvents and reagents were of analytical grade. Microcrystalline cellulose (MCC) was supplied by Quimsul (Brazil).

Procedures

Isolation of Cellulose

The removal of maize straw extractives like pectin, suberin, cutin and others was done by means of a Soxhlet extraction, using hexane (2 h), ethyl alcohol (2 h) and deionized water (4 h), in this sequence, according to the increase of polarity. The straw was then dried in oven at 80 °C. The average value for the extractive content of maize straw was 25 %. The extractive-free straw was treated with an aqueous solution of 5 % (w/v) NaOH using a procedure described for soybean hulls [35] but here adapted to maize straw. The straw suspension was treated in an autoclave (model Stermax 20EHD), at 121 °C and 2 atm of pressure, in a 1:100 straw to liquor ratio (g/ml). The time of reaction was tested for 15, 30, 45 and 60 min. The final pulp was filtrated and washed with deionized water until neutral pH. Cellulose bleaching was based in a procedure already described for wheat straw [36]. It consisted in two steps: (1) The cellulose pulp was treated with a 2 % (v/v) H2O2 solution and a 0.2 % (w/v) tetra-acetyl ethylene diamine (TAED) solution, at pH = 12, for 12 h, at 48 °C, under stirring, in a 1:25 straw to liquor ratio (g/ml). The pulp was filtrated and washed with deionized water until neutral pH. (2) Dried cellulose pulp was treated with an 80 % (v/v) acetic acid solution, in a 1:33 straw to liquor ratio (g/ml) and with a 65 % (v/v) nitric acid solution in a 1:4 straw to liquor ratio (g/ml), at 120 °C, under stirring, for 30 min. The solid was filtered and washed with water and ethyl alcohol until neutral pH.

The overall yield of cellulose departing from maize straw was 20 wt%. Figure 1 shows the physical appearance of some samples. It can be noticed that the dark brown color of crude maize straw disappeared after the bleaching procedures. The purified cellulose sample is a white powder.

Fig. 1
figure 1

Sample of crude maize straw CC, CC15 (alkaline treated cellulose pulp after 15 min autoclave), CCBI (pulp after first bleaching step), and CCBII (purified cellulose after second bleaching step)

Full size image

The various designations for maize straw and cellulose samples of this study are listed in Table 1 as follows: commercial microcrystalline cellulose (MCC); crude maize straw (CC); maize straw free from extractives (CCF); cellulose pulp after 15 min in autoclave with NaOH (CC15); cellulose pulp after 30 min in autoclave with NaOH (CC30); cellulose pulp after 45 min in autoclave with NaOH (CC45); cellulose pulp after 60 min in autoclave with NaOH (CC60); cellulose after the first bleaching step (CCBI); purified cellulose after the second bleaching step (CCBII).

Table 1 Abbreviations for the various cellulosic samples
Full size table

Isolation of the Cellulose Whiskers

Purified cellulose (CCBII) was treated with a 64 % (w/w) H2SO4 solution, in a 1:9 cellulose to liquor ratio (g/ml) following the procedure of Dong et al. [37]. The acid hydrolysis was carried out at different times: 15, 30, 60, 90, 120, 150 and 180 min, at 25 °C, under stirring. The reaction was stopped by the addition of cold deionized water. After 1 h of centrifugation at 3,000 rpm, the suspension was ultrasonicated for 15 min in order to get uniform dispersion. The excess of sulfuric acid was removed by putting the suspension in dialysis membranes of regenerated cellulose with cut-off 12,000–14,000 Da (Fisher). The dialysis was conducted in deionized water until neutral pH for a week.

Characterization

Maize Straw and Cellulose

ATR-FTIR spectra were collected with 64 scans and a resolution of 2 cm−1, in a Nicolet 6700 spectrophotometer. Dried maize straw, intermediate samples and purified celluloses were analyzed by ATR-FTIR in order to make clear structural changes along the various steps of cellulose extraction.

TGA measurements were conducted at a heating rate of 10 °C min−1 under N2 (50 ml min−1), using a TA Instruments model TGA Q5000 IR. Sample weight was typically kept at 7 mg. The TGA microbalance had a precision of ±0.1 μg.

The XRD experiment was performed using a Siemens D-500 diffractometer. Purified maize straw cellulose (CCBII) was scanned in the reflection mode using an incident X-ray of CuKα with wavelength of 1.54 Ǻ at a step width of 0.05° min−1 from 2θ = 0–40°. The Segal method was used to calculate the crystallinity of the samples [38]. The Scherrer equation was used to calculate the crystal thickness [39].

Cellulose Whiskers

For TEM images, drops of maize straw whisker aqueous suspensions were deposited on glow-discharged carbon-coated TEM grids and the excess of water was left to evaporate. The specimens were negatively stained with 2 % uranyl acetate and observed using a JEOL JEM 1200FxII electron microscope operating at 80 kV. The whiskers’ dimensions were determined with the aid of the Image Tools software.

Light scattering (LS) experiments were performed with a Brookhaven Spectrogoniometer BI-200 and a 35 mW Coherent laser operating at 633 nm. A Glan-Thomson prism (extinction ratio better than 10−7) was placed at the entrance of the photomultiplier. The correct position of the prism was checked by using the depolarization ratio of toluene (ρV = 0.359) [40].

Results and Discussion

Cellulose and cellulose whiskers were extracted from maize straw using a non-pollutant, chlorine-free methodology. In the literature related to this topic, only a few papers have highlighted the use of totally chlorine-free technologies (TCF) [18, 36]. The acid and alkaline solutions obtained as residues can be easily neutralized, resulting products that are not harmful to the environment.

Characterization of Maize Straw Cellulose

Spectroscopic Characterization

The raw materials and the samples obtained after each step of the extraction procedure were analyzed with respect to their chemical structure by ATR-FTIR spectroscopy. Figure 2 shows the spectra in the range 2,000–800 cm−1 of the different samples (CC, CCF, CC15, CCBI, CCBII). The spectrum of MCC was also included [41].

Fig. 2
figure 2

ATR-FTIR spectra for CC, CC extractive-free, CC15 (alkaline treated cellulose pulp after 15 min autoclave), CCBI (pulp after first bleaching), CCBII (purified cellulose after second bleaching) and commercial microcrystalline cellulose (MCC) in the range from 2,000 to 800 cm−1

Full size image

The main components of maize straw are cellulose, hemicellulose and lignin. These substances usually present different oxygen containing functional groups such as OH, C=O, C–O–C and C–O–(H) as reported in literature [36, 42]. The absorption bands found for each sample were assigned and are shown in Table 2.

Table 2 Assignment of infrared absorption bands of the samples, as designated in Table 1
Full size table

Lignin has characteristic bands in the range of 1,500–1,600 cm −1 related to aromatic ring vibrations. By looking at the spectra of crude maize straw (CC) and maize straw free of extractives (CCF), it can be confirmed that lignin is present in these samples due to the absorptions at 1,514 and 1,604 cm−1 (Fig. 2). The presence of lignin can be also confirmed by the absorptions at 1,244 cm−1 which indicate C–O–C stretching of aromatic ether linkages. According to Viera et al. [42], when these bands are absent in the spectra of cellulosic materials, it means that most of the lignin was removed. So we can conclude that this is the case for the pulped (CC15) and bleached (CCBI and CCBII) samples. As mentioned by Morán et al. [43], an absorption in the region of 1,730 cm −1 is due to the presence of C=O linkage of hemicelluloses. In Fig. 2 an absorption in this region can be seen in CC and CCF spectra but not in the other spectra. According to some authors [36, 42, 44], an absorption at 1,716 cm−1 indicates that some acetylation has occurred. Taking into account that acetic acid was used in the second step of bleaching, this is a valid explanation to the appearance of this band in the spectrum of CCBII sample.

Thermogravimetric Analysis (TGA)

Comparing the DTGA curves of the crude maize straw (CC) and the extractive-free straw (CCF), shown in Fig. 3, it can be noticed that the crude material started its decomposition at lower temperature (~160 °C × ~200 °C). This means that components degrading below 200 °C were mainly composed by soluble compounds [12]. Hemicellulose and lignin started to degrade at 200 °C and this can be seen in the degradation pattern of the extractive-free straw. TGA analyses were used to choose the optimum time of reaction in the autoclave procedure. The cellulose pulps obtained after the alkaline treatment from 15 to 60 min in autoclave showed higher thermal stability, indicating the extensive removal of hemicelluloses and lignin. It can also be observed that all samples obtained after alkaline pulping in autoclave (from CC15 to CC60) showed similar peaks of cellulose decomposition, around 360 °C. Comparing the DTGA curves of the crude straw (CC) and the extractive-free one (CCF), it could be seen that their degradation peaks were much wider than those of the other samples, due to the presence of lignin and hemicelluloses. The degradation peak of the sample pulped for 15 min in autoclave (CC15) is as narrow as those of CC30, CC45 and CC60. This result suggests that shorter and longer times of alkaline treatment show the same efficiency in removing hemicelluloses and lignin. Moreover, if some lignin eventually remains after the pulping step, it is expected to be removed in the next two bleaching steps. So it is not necessary to increase the energy consumption by increasing the duration of the pulping step. Therefore the optimum time of reaction chosen was the shortest one (15 min).

Fig. 3
figure 3

DTGA curves of crude maize straw and of samples obtained after pulping (see Table 1 for sample designation)

Full size image

X-Ray Diffraction (XRD)

The crystallinity of the cellulose fibers was studied by X-ray diffraction. According to the review of Bansal et al. [45] the method of Segal is still the most widely used to calculate the crystallinity index of cellulose being cited in several works [46–48]. It is a simple method that does not need background subtraction and it uses the peak heights. Through Segal’s method, the crystallinity index is calculated by the following equation:

$$ {\text{X}}_{\text{C}} = 100\left( {{\text{I}}_{200} - {\text{I}}_{\text{AM}} } \right)/{\text{I}}_{200} $$
(1)

where I200 is the 200 peak, the highest peak, which represents both crystalline and amorphous material. The lowest height between 200 peak and 110 peak is IAM and represents only the amorphous part.

Figure 4 shows the X-ray diffraction pattern of maize straw cellulose as well as the values of the crystallinity index (calculated by using Eq. 1) and the crystal thickness. The sample has a high and narrow peak at 2θ of about 22° and a lower peak at about 15°.

Fig. 4
figure 4

X-ray diffraction pattern of CCBII cellulose (30 min bleaching) as well as values of crystallinity index (Xc) and crystal thickness (L)

Full size image

The crystal thickness was obtained through the Scherrer equation as follows:

$$ {\text{L}}_{[200]} = {\text k}\uplambda /\left( {\upbeta_{[200]} \cos \theta } \right) $$
(2)

where, L[200] is the crystallite size (or crystal thickness), k is the Scherrer constant which is 0.84 for cellulose [39] and λ is the X-ray wavelength. The variable β[200] corresponds to the lattice [200] peak width at half its intensity.

Crystallinity index (Xc) and crystal thickness (L) values for maize straw cellulose resulted as Xc = (75.5 ± 0.7) % and L = (38 ± 3) Å. The value of the crystallinity index of our maize straw cellulose is very close to that found for commercial microcrystalline cellulose (MCC) which is Xc = (79.1 ± 0.2) % [41, 49]. The crystal thickness of MCC resulted as L = (43 ± 2) Å showing that the crystals of maize straw cellulose and MCC have the same thickness within the experimental error. In addition the values found for the crystal thickness agree well with the data of Abbott et al. [39]. These results mean that the overall multistep procedure employed is adequate to obtain highly crystalline cellulose when departing from maize straw being very efficient to remove lignin and hemicelluloses.

Characterization of Cellulose Whiskers

Light Scattering (LS)

In this study light scattering technique was used to confirm that whiskers were indeed obtained, since it is known that they are able to depolarize light [50]. The ρv parameter is defined as the ratio of depolarized scattered light intensity to the polarized one as given by Eq. 3:

$$ \rho_{\text{v}} = {\text{I}}_{\text{VH}} /{\text{I}}_{\text{VV}} $$
(3)

where ρv: depolarization ratio, IVH: scattered light intensity collected at VH geometry, i.e. the incident beam is vertically polarized and the scattered beam has a direction perpendicular to the incident one, IVV: scattered light intensity collected at VV geometry, i.e. incident and scattered beams are vertically polarized.

It is expected that the intensity depolarized by a scattering center, and consequently the depolarization ratio, increase with its optical anisotropy. In other words, the higher the optical anisotropy of a molecule, the higher should be its depolarization ratio. According to Berne and Pecora [51] the molecular optical anisotropy may be in turn related to the anisotropies of units comprising the chain and to properties that determine the molecular configuration, as for example, barriers to internal rotation around chemical bonds. Therefore the depolarization ratio can be considered as a property intrinsic of anisotropic chain molecules.

The depolarization ratio (ρv) was determined for maize straw cellulose whiskers prepared at various hydrolysis times (15, 30, 60, 90, 120, 150, 180 min) to verify if ρv could indicate an optimum time. However, from the results showed in Table 3 it can be seen that ρv is independent of the hydrolysis time and shows the average value of 0.021.

Table 3 Depolarization ratio of maize straw cellulose whiskers obtained at various hydrolysis times at 25 °C (precision in ρV is ca. 10 %)
Full size table

It is interesting to note that the depolarization ratio value of the maize straw cellulose whiskers is the same as that of cotton whiskers [50]. On the other hand it is higher than for amylose [52]. According to Beck-Candanedo et al. [29], the whisker length is source-dependent. In this work it was found that this is not the case for the depolarization ratio. Such behavior can be explained taking into account that ρv can be related to the molecular optical anisotropy, as aforementioned. As different biomass sources contain cellulose, composed of the same glucopyranose units connected by β-(1,4) linkages, the depolarization ratio should be the same for cellulose whiskers regardless the source from which cellulose originates. In addition, cellulose whiskers and amylose are indeed expected to have different values for the depolarization ratio ρv, since the glucopyranose units in amylose are connected by α-(1,4) linkages and ρv may be considered dependent on the molecular configuration.

Transmission Electron Microscopy (TEM)

TEM images are highly regarded for showing features of individual cellulosic nanocrystals. The whisker sample prepared at 150 min hydrolysis time was chosen for TEM characterization because it gave higher depolarized signals in LS when compared to samples of shorter hydrolysis times. Figure 5 shows a TEM micrograph of this sample, where structures in the form of needles can be seen as well as some aggregates. Similar observations were reported for cellulose whiskers of other sources [53, 54]. The aggregates of these strongly interacting nanostructures are present due to the high density of hydroxyl groups on the surface of cellulose chain molecules favouring the formation of hydrogen bonds [55, 56].

Fig. 5
figure 5

TEM image of maize straw cellulose whiskers prepared at 150 min of hydrolysis time (bar 0.5 μm)

Full size image

The whiskers’ dimensions determined by TEM ranged from 333 to 458 nm in length and 15 to 25 nm in width. The average length value was L = (388 ± 43) nm, while the average thickness was d = (19 ± 2) nm. Therefore the aspect ratio shows an average value near 20. This is a very interesting result considering that the reinforcing effect of cellulose whiskers in polymer nanocomposites has been seen to follow a percolation model [57]. This model states that the mechanical properties of composites are mainly affected by factors that ensure or interfere with the formation of percolated networks, such as the aspect ratio of the fillers [27]. Cellulose whiskers isolated from rice straw show L/d value around 10 [11], from ramie L/d = 12 [58]) and from jute L/d = 6.5 [30]. By comparing these values of aspect ratio, it can be concluded that the percolation threshold of maize straw cellulose whiskers possibly will occur at a lower filler volume than for rice straw, ramie and jute whiskers.

Conclusions

The search for low-cost, sustainable and renewable resources is critical to meet the growing environmental concerns and energy demands of modern society. In this work, the simple and environmental friendly method employed for the extraction of cellulose from lignocellulosic fibers of crude maize straw was well succeeded. It was observed to be efficient in removing hemicellulose and lignin as confirmed by thermogravimetric analysis (TGA), attenuated total reflectance fourier transform infrared spectroscopy (FTIR-ATR) and X-ray diffraction analysis (XRD). The overall multistep procedure is adequate to obtain highly crystalline cellulose.

Light scattering (LS) and transmission electron microscopy (TEM) showed that whiskers were indeed obtained after acid hydrolysis of maize straw cellulose at room temperature. We have found that the depolarization ratio (ρV) does not depend on the cellulose source but depends on the molecular configuration. TEM technique showed the presence of needle-like nanostructures where some of them are aggregated due to hydrogen bonds. The aspect ratio of these maize straw cellulose whiskers was determined as L/d = 20, a value high enough for their potential application as nanofillers in polymer matrices. In comparison to other whisker sources where the acid hydrolysis is usually conducted at 45 °C, maize straw cellulose whiskers show the advantage that they can be obtained at 25 °C with good aspect ratio and relatively well separated.

By using XRD data and the Scherrer equation, we have calculated that the crystal of maize straw cellulose has a thickness of (38 ± 3) Å. This value can be assigned to the framework of cellulose chains in the elementary fibril. By comparing the whisker average thickness (d = 19 nm), as determined by TEM, and the crystal thickness of maize straw cellulose, we can conclude that whiskers are arranged laterally in bundles with average thickness around five times that of crystal thickness.