Cellulose nanostructures from wood waste with low input consumption

Highlights

• Cellulosic nanostructures were obtained by enzymatic hydrolysis without pretreatment.

• Time of reaction affects only the yield, not the mean size of the structures.

• Needle-like structures with higher crystallinity were obtained without pretreatment.

• Pretreatment surpassing reduced the impacts represented in the LCIA.

Abstract

Currently, cellulose nanostructures (CNS) are obtained using pretreatments of lignocellulosic residues. In this work, different conditions in the enzymatic isolation process were studied to obtain CNS, aiming an alternative eco-friendly method. Two isolation methodologies were used: a) one submitting the residue to an alkaline pretreatment and b) other using the lignocellulosic waste directly without pretreatment. A commercial complex of enzymes (Ctec^®) was used, varying the time to which the substrate was exposed to the enzymes at 12, 24 and 48 h, searching for the better condition to CNS obtaining. The application of alkaline pretreatment removed 73% of hemicellulose and 50% of lignin from wood fiber leading to a more exposed cellulose structure and increased the amount of nanocellulose produced. The obtained nanostructures were needle-shaped of approximately 70 nm wide. However, CNS of similar size and shape were received as well without pretreatment application, allowing their obtainment with lower reagent consumption through a more environmentally efficient route, as presented in the life cycle impact assessment (LCIA) where energy consumption and acidification potential were considered.

Introduction

The growing concern for environmental preservation leads to the development of new materials that combine high efficiency, low prices, and minimum environmental impacts. In this scenario, cellulose nanostructures appear as a remarkable alternative, since it presents interesting mechanical, electrical and optical properties associated with biodegradability and high availability of renewable sources. It presents a high surface area, Young's modulus of around 140 GPa, tensile strength up to 7500 MPa and low density (∼1.5 g cm⁻³). Besides, just as cellulose, nanocellulose is renewable, biodegradable and non-toxic (Abdul Khalil et al., 2016, Abitbol et al., 2016, Grishkewich et al., 2017).

Cellulose is an abundant material found in some bacteria and plants. The development of agriculture generates enormous amounts of waste, agro-residues, which can be applied to obtain nanocellulose (Ditzel et al., 2017, Kallel et al., 2016). The lignocellulose biomass is, generally, major composed of cellulose varying from 30 to 50% wt., hemicellulose with 19–45% wt. and lignin counting with 15–35% wt. (Lee et al., 2014). There are also other components such as fats, carbohydrates, and greases, defined as extractives, and mineral substances (ashes) (El Achaby et al., 2018).

The most common routes for nanocellulose obtaining involve the removal of physical barriers, hemicellulose, and lignin, that shield the structure from the process of cellulose scale down (de Castro et al., 2016, Mariano and Dufresne, 2017, Zhao et al., 2017). The processes, usually denominated pretreatments, consist of lignocellulose structure opening, leading to the removal of fiber constituents, swelling of the biomass, increase of the surface area and a decrease of components molar masses (Cao et al., 2017, Chen et al., 2018, Espinosa et al., 2017). Often, the procedures are repeated several times to increase the results. However, it also increases the energy and reagent consumption required for the process (Henriksson et al., 2007). After the pretreatment, the crystalline regions of cellulose must be released from the amorphous ones and other residual compounds as well (Benini et al., 2018, Chen and Lee, 2018). Those processes influence the surface and bulk properties along with the cellulose source as thermal stability, polymorphism, and dimension of the nanoparticles, which can bring valorization of unexplored biomass since different cellulose sources and isolation methods can provide new structures and different properties (Abdul Khalil et al., 2016, Klemm et al., 2018, Zhao et al., 2017).

It is usual to classify nanocelluloses according to their structure as bacterial nanocellulose (BNC), cellulose nanofiber (CNF) and cellulose nanocrystals (CNCs) (Klemm et al., 2018). Mechanical and enzymatic isolation from pure cellulose usually provide NFC, that is individual or associated nanofibrils, removed from the plant cell wall (Sirviö et al., 2015). The enzymatic isolation method presents great potential. It does not generate or utilize toxic products, being extraordinarily specific and acting in mild conditions of temperature and pressure (Chen et al., 2018).

Enzymes are proteins specialized in catalyzing biological reactions, such as cellulose hydrolysis breakage. In nature, cellulose/hemicellulose is degraded by a group generally named as cellulases, which presents a high degree of synergism during the hydrolysis. The commercial cellulase enzyme preparations are usually a complex mixture of them, what improves their conversion and decreases costs of isolation (Chen et al., 2018, Henriksson et al., 2007, Karim et al., 2017). Recently, the technique of enzymatic hydrolysis has been studied and presents good results. Tang et al. (2015) studied enzymatic hydrolysis followed by ultrasonication after application of alkaline and acid pretreatment in corrugated containers and obtained an increase of 25% in the yield of nanostructures extraction after 36 h of hydrolysis. Cui et al. (2016) produced nanostructures from microcrystalline cellulose pulp, using only the enzymatic method and obtained rod-like particles with a length of 200–400 nm with a yield of 20% conversion after 120 h of hydrolysis. Similarly, Chen et al. (2018) obtained particles of approximately 30 nm with monodispersion using an enzymatic complex to hydrolyze eucalyptus pulp for 5 h (Chen et al., 2018).

Generally, enzymatic hydrolysis tends to result in lower yields than other methods. However, it is an isolation process eco-friendlier than usual acid hydrolysis, as confirmed by Life Cycle Assessment (LCA), within a cradle-to-grave approach (Kargarzadeh et al., 2018). LCA aims the improvement of operations processes while reducing environmental impacts along the product lifecycle. According to ISO 14040:2006 Life Cycle Impact Assessment (LCIA) can be used to evaluate the ecological consequences of the studied phases of a product system. Thus, LCIA appears as a helpful tool for assessing cellulose nanostructures obtaining (Piccinno et al., 2018, Tan et al., 2018). Although LCA has already been used for evaluating nanomaterials application, studies addressing their manufacturing processes are still scarce. Kargarzadeh et al. (2017) pointed out the fact that electricity consumption, raw material efficiency, and water consumption are the main environmental concerns regarding cellulose nanofiber (CNF) production. Abitbol et al. (2016) emphasize that among the isolation methodologies, the enzymatic stands out by its high productivity and low water consumption.

This work proposed the obtaining of nanostructures from Pinus taeda usual wood flour, obtained as industrial waste, utilizing a commercial enzymatic complex. It also evaluates the effects of the enzymatic hydrolysis in this material, which has not yet been clarified by the literature, due to natural recalcitrance from the subtract. The novel method avoids pretreatment application to first cellulose pulp obtaining, as usually reported (Asad et al., 2018, Dubey et al., 2018), and presents environmentally promising results. The morphology and properties of the obtained structures were characterized, searching for a process to increase value to the pinewood waste, while reducing inputs consumption.