Guayule stems fractionation and sugars recovery by different pretreatment technologies

Highlights

• Guayule bagasse has undergone 4 different processes to produce sugars and lignin.

• Dilute acid, organosolv, steam explosion, and alkaline washing were compared.

• Steam explosion associated with alkaline washing produced 133 g kg⁻¹ glucose.

• Monomeric sugars can be obtained up to 317 g kg⁻¹ from the guayule bagasse.

Abstract

The bagasse of guayule, obtained after extraction of natural rubber and resins, can be used to produce 2nd generation fermentable sugars and lignin. For this purpose, four pretreatment technologies were applied to deconstruct the guayule bagasse: diluted acid (DA), steam explosion (SE), organosolv (OS), and the steam explosion followed by alkaline washing (AW). The obtained lignocellulosic pulp underwent enzymatic hydrolysis (EH) to produce glucose. The conditions that gave the highest hydrolysis yield were assessed. A mass balance was elaborated for all processes which were compared on the basis of sugars yield, water and chemical consumption, and energy expenditure footprint. The greatest amount of glucose (133 g kg⁻¹ of dry guayule bagasse, corresponding to 71% enzymatic hydrolysis yield) was obtained using SE followed by AW, whereas SE and OS had lower yields (104 g and 106 g kg⁻¹, respectively). The highest overall monomeric sugars yield, including xylose, was obtained by DA (317 g kg⁻¹) followed by OS (243 g kg⁻¹) pretreatment, although the sugar streams must be purified in both processes by removing the acid and the solvent, respectively. SE is a process with a limited requirement of chemicals and energy input, which are significantly higher for AW and for OS, respectively.

Introduction

Guayule (Parthenium argentatum Gray) is a bushy perennial plant native of Mexico and Southern Texas (US). It is a halophytic shrub that has received special attention in the past years from the rubber industry such as the Bridgestone/Firestone Inc. as an alternative source of natural bulk rubber and latex [[1], [2], [3]]. Guayule plant is considered an industrial crop comparable to rubber tree not only for the quality of the rubber but also because it does not compete with food crops for arable and fertile lands, being adapted to grow in arid-semiarid and salty soils under low-input agronomical condition [4]. From the branches of guayule, it is also possible to extract resins [5] which can be exploited as a marketable by-product in several industrial sectors such as woody composites and wood preservatives, anti-microbial formulations, as well as a fungicidal component in coatings. Conventionally, latex is extracted on freshly harvested bushes with water, while resins and gum are extracted with a mixture of organic solvents like acetone and hexane [3]. However, latex and resins account for no more than 20% of the guayule biomass dry weight. Therefore, the post-extraction residual biomass, named guayule bagasse (GB), is a valuable lignocellulosic waste which, according to an optimized process of circular economy, can be further investigated to recover second-generation sugars and lignin for biorefinery purposes. Despite these recent advancements, little information on further reuse of the residual GB is available in the recent literature [6].

To obtain fermentable monomeric sugars, it is necessary to deconstruct the lignocellulosic matrix into its macro-constituents: cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are the biopolymers from which glucose and C5 sugars (mainly xylose) are obtained, respectively, by hydrolysis with acid or specific enzymes.

The deconstruction of lignocellulosic materials is mainly aimed to obtain easily hydrolysable cellulose which is achieved through a pretreatment technology that requires high temperature (100–200 °C) and high energy inputs. Hydrothermal pretreatment has been found to be an effective and cost-efficient method for a wide range of lignocellulosics from poplar, wheat straw, and many more industrial crops since it converts raw material into highly digestible fibre fractions with up to 60–100% glucan-to-glucose convertibility [[7], [8], [9]]. As alternatives to these hydrothermal approaches, various pretreatments options have been identified, such as mechanical, thermomechanical, chemical, and organosolv technologies, including the most recent green-solvents-based processes [10] as well as the biological pretreatment by white-rot fungi [11]. In general, there is no pretreatment equally suitable for every biomass, but its choice depends on the type of biomass, on its specific recalcitrance, and on the targeted products. Furthermore, many other factors contribute to the choice of pretreatments, such as the technology development level, the cost of the plant, and the market associated with the co-products [12]. Single technological approach on different lignocellulosic have been widely reported, whereas cases of multiple treatments on the same biomass are overlooked in the literature. In this scenario, the pretreatment of GB by multiple technologies seemed to be an opportunity to valorise this residual biomass.

As matter of fact, GB has been studied by using only specific saccharification technologies which are not widely used at industrial scale. For instance, an efficient process based on a high-pressure supercritical CO₂ pretreatment step afforded a final glucose yield of 77% [13,14]. In another study, the ammonia fibre expansion (AFEX) technology was applied to guayule-derived biomass fractions with a final glucose yield of about 50% with respect to the initial glucans content. However, the residual rubber or resin had some negative effects on the fermentability to ethanol [15].

This work proposes to effectively exploit the guayule residue obtainable after the extraction of the rubber. It compares four technologies that are typically used for the pretreatment of lignocellulosic biomasses to explore the potentiality of GB to be recycled through a biorefinery approach for producing 2nd-generation sugars and lignin. Basing on these assumptions, GB was treated with: (i) dilute acid (DA), (ii) steam explosion (SE), (iii) steam explosion followed by alkaline washing (AW), and (iv) organosolv (OS) technologies; then, the glucose produced via enzymatic hydrolysis (EH) was quantified. The efficacy of EH was evaluated by using the glucose yield as a reference parameter to identify the most efficient pretreatment. In order to optimize the whole process, the mass balances were screened to identify the most effective pretreatment in GB fractionation and sugars recovery on the basis of the inputs of water, energy, and chemicals. Finally, a critical comparison among the yielded sugar from GB with other biomass was discussed.