Extraction and Characterization of Cellulose Nanocrystals from Corn Husk Waste and Its Application in Bioplastic Preparation

Maize is the most cultivated cereal in the world. In fact, according to data of the Food and Agricultural Organization of the United Nations, global production reached up to 1.16 billion tons in 2020. In Peru, according to data from the Minister of Agriculture (MIDAGRI, 2021), more than 414 thousand tons of corn were harvested in 2020, mainly in the coastal valleys of Lima and Ica regions.

In this context, cereals production has also given rise to considerable amounts of waste, which are the residues generated by the harvesting and processing of the feedstock. In particular maize wastes include stalks, chaffs, steams, silks, and husks; out of these components’ stalks constitute more than half of the stover biomass while the husk accounts for 10% of the total mass. Furthermore, these wastes barely degrade on the soil surface, making their disposal still a challenge, especially in developing countries where the lack of efficient and environmentally friendly techniques ultimately generates environmental threats.

In this sense, open air burning is one of the main disposal strategies for corn waste in Peru, however, this practice has been widely questioned by the scientific community as it constitutes one major source of gaseous and particulate emissions to the atmosphere. In fact, both greenhouse gasses (carbon dioxide (CO2), carbon monoxide (CO), ie.) and volatile organic compounds (ammonia (NH3), sulfur dioxide (SO2), ie.) would get released to the surroundings.

On the other hand, scientists have shown a particular interest in the valorization of different lignocellulosic biomass over the last decade, as it constitutes the largest source of renewable organic material on Earth. In particular, cellulose has received remarkable attention considering its outstanding mechanical properties (tensile strength between 5 and 7 GPa along the chain direction) and its versatility in the synthesis of added-value derivates.

Moreover, micro and nano forms of cellulose are quite relevant because of their unique properties conferred at such a small scale. Cellulose nanocrystals (NCC) are rod-like particles with typical widths and lengths in the range of 5–70 nm and several micrometers, respectively. NCC exhibits low density (~1.46 g/cm3), an outstanding mechanical performance (bending strength estimated at 10 GPa and elastic modulus ranging from 10 and 100 to 200 Pa, respectively), and a high surface area/aspect ratio of 10 up to 70.

Under this context, this research focuses on the valorization of Peruvian corn wastes into cellulose nanocrystals using both mechanical and chemical treatments combined.

The feedstock was collected from local markets in Lima, Peru, and after washing it several times, the sample was dried, milled, and sieved under 850 µm. Afterward, dried corn husk powder was treated with 4% (w/v) sodium hydroxide twice. Then, peracetic acid bleaching was conducted according to a previous methodology. This cellulose sample was characterized by instrumental techniques and gravimetric measurements. Finally, acid hydrolysis was conducted with concentrated sulfuric acid at 45°C followed by centrifugation, dialysis and low-frequency ultrasonication. The nanocrystalline cellulose suspension obtained was used to prepare films by simple casting method.

The cellulose was extracted with a 79.4% yield while its purity was estimated at 70.3%. On the other hand, in order to identify characteristic functional groups of spectral bands, characterization by FT-IR spectroscopy was carried out, and characteristic bands of cellulose were found at 1426 cm-1, 1325 cm-1, 1163 cm-1, and 1110 cm-1. Likewise, the decrease in signals at 1251 cm-1 and 1736 cm-1 indicates that hemicellulose and lignin were removed after the chemical treatments.

At the same time, x-ray diffractograms showed peaks at 22.5° and 34°, characteristics of the crystal polymorphs of cellulose, while the crystallinity index was estimated in 54 by the Segal equation. SEM images showed a change on the structure of the material from irregular and rough to a highly fibrillated network arrangement.

The average hydrodynamic diameter of the isolated CNC was determined by dynamic light scattering (DLS). The nanocrystals dimension was found to be 268 nm, however, its important to highlight that the hydrodynamic diameter represents the dimension of an equivalent sphere for different types of particles (individual size, aggregates, and agglomerates) and thus may not represent the actual size of the particle, in fact, CNC is known for having a high length: width ratio (aspect ratio). Therefore, aiming to obtain a better understanding of the morphology of the crystals, atomic force microscopy was performed into diluted suspensions, it was found that the nanocrystals had a rod-like shape, with an average aspect ratio of 35.

Furthermore, for comparison, commercial nano cellulose was also prepared and the dimension was found to be 218 nm.

It is also important to mention that an interesting effect caused by low-frequency ultrasonication was noticed. As the sonication time increased both the viscosity and the particle size of the nanocrystalline suspension decreased. This effect might be a consequence of the rearranging of the particles caused by cavitation. Previous studies have found that ultrasound induces a decrease in the aspect ratio of suspension while promoting the individualization of particles. Thus, further research studying these effects is still needed.

CNCs films were cast by pouring the suspension into a petri dish and drying overnight. These films were highly transparent and brittle. Their resistance to both alkaline and acidic medium was evaluated by measuring the weight loss over the days and comparing it to polyethylene films.

Regarding the water absorption capacity, the films were able to absorb their weight up to 3 times, approximately. It was noted that the bioplastics obtained had good resistance to acids (sulfuric acid) and bases (sodium hydroxide) up to concentrations of 25% and 40%, respectively. After 5 days there was no loss in the mass of the CNCs films. When these films are compared with those commercially obtained (polyethylene), it was observed that the latter lose 20% of their mass in the presence of 40% alkali, while the former was intact.

This evidence could respond to the high stability of the crystalline structure and the strong bonding between molecules, making it suitable for future applications in the medical and food industries, where these properties are highly appreciated.