Introduction
Lignocellulosic biomass (LCB), the cheapest, bio-renewable resource that amounts to a global yield of about 1 billion 3 million tons every year (Balan, 2014). The bulk of LCB is composed of biological polymers such as hemicellulose, cellulose, and lignin, which are strongly associated with each other by hydrogen and covalent bonds thus forming a highly recalcitrant structure (Akhtar et al., 2016). Lignocellulosic biomass involves majorly, organic materials that include crops and agro-wastes, woods, grass, algae, etc. all of which are available renewable resources.
The LCB hydrolysis is displayed in the release of many reducing sugars that are highly required in the production of biofuels such as bioethanol, biogas, phenols, aldehydes and organic acids. In the last two decades, LCB such as wheat straw, sugarcane bagasse, timothy grass, rice straw, barley, and woody raw materials, softwoods, forest residues, and paper pulps have been extensively researched for biofuel production (Low et al., 2015 and Akhtar et al., 2016).
Green/Novel pretreatment methods of LCB
In recent years, the concept of green chemistry” has gained increasing interest as a possible approach to the challenge of developing available biorefinery concepts (Clark et al., 2009). In achieving this goal, it is central to develop technology that uses raw materials more efficiently to eliminate waste and avoid the use of hazardous and toxic materials. Selected green methods currently being pursued pretreatment of lignocellulosic biomass are summarized below.
Biological: Enzymatic pretreatment
Biological pretreatment is getting great relevance because it is an effective, safe, and environmentally friendly approach (Sindhu et al., 2016) unlike the conventional pretreatment method for lignin degradation which requires a large input of energy and also causes pollution. Biological pretreatment instead, employs the use of microorganisms, such as fungi bacteria, and actinomycetes, which synthesize cellulolytic, hemicellulolytic, and ligninolytic enzymes to degrade cellulose, hemicellulose, and lignin respectively. There are two families of ligninolytic enzymes which play an important role in enzymatic degradation namely; phenoloxidase (laccases) (Lac) and peroxidases (lignin peroxidase (LiP), versatile peroxidase (VP), and manganese peroxidase (MnP) (Zamocky et al., 2014). These are heme-containing glycoproteins that require hydrogen peroxide (H2O2) as an oxidant.
The biological pretreatment method can be considered a “green” method. It is cost-effective and easy to operate, it requires low energy inputs, and no chemical additives, resulting in few inhibitors generation and it does not cause environmental pollution. However, this method is affected by many disadvantages; in general, the main drawbacks are the low hydrolysis rate obtained, the necessity of a large sterile area which should be maintained during all processes, the slow rate of growth of the fungi that limits on farm-scale application and protracted process time and the need for monitoring this microorganism growth.
Ionic liquid pretreatment
Ionic liquids (ILS) are a special group of organic salts that can exist in liquid form at relatively low temperatures (<100 ºC) and some can even exist in liquids at room temperature (Zhu et al., 2006). They are usually molten salts or oxides. Ionic liquids are special solvents that can dissolve chemical compounds that are otherwise considered insoluble in the conventional solvent. Typically in ionic liquids, the cations are organic while the anions may be inorganic or organic (Marsh et al., 2018). Some common cations and anions from ionic liquids used for biomass dissolution are 1-ally1-3-methyl imidazolium, Zolium chloride, Acetate (CH3COOH), Glycine (C2H5NO2) (Marsh et al., 2018). Chemical compounds extracted or dissolved in ionic liquids may be recovered by using an antisolvent such as water, ethanol, acetone, or supercritical CO2. Many factors impact ionic liquid pretreatment of biomass. They include ionic liquid type, ionic liquid to biomass ratio, pretreatment time and temperature, and water content. There are several possible basic strategies for ionic liquid pretreatment of lignocellulosic biomass. These are; (i) dissolution of cellulose in processed cellulose material such as pulp cellulose in an ionic liquid, (ii) removal of lignin and hemicellulose by dissolving the lignocellulose biomass in an ionic liquid while leaving cellulose behind, and (iii) dissolution of all the biomass components and then selectively recover the needed components (Wang et al., 2018).
Most of the ILS are recoverable and reusable. They possess the striking advantages of negligible vapor pressure, non-volatility, non-toxicity, larger thermal and chemical stability, and most importantly the adjustable nature of their cations and anions on which the properties of the ILS depend (Chen et al., 2017). These are the reasons why ILS have often been described as green solvents. Regardless of their distinctive chemical properties, ILs present the major shortcomings of being expensive and toxic to microorganisms and enzymes (Brandt-Talbot et al., 2017). Nevertheless, further studies on these aspects with low-cost recovery technology and its toxicity to enzymes are still needed for economically viable large-scale applications.
Deep eutectic solvents
Deep eutectic solvents (DESs) are a new generation of ionic fluids composed of two or three cheap and safe components which are capable of associating with each other, through hydrogen bond interactions, to form a eutectic mixture (Zhang et al., 2012). They are usually liquids at temperatures lower than 100 °C. DESs and ILs are much alike in terms of their physicochemical properties but their low-cost synthetic technology and biodegradability make them versatile alternatives to ILs (Zdanowicz et al., 2018). DESs are mostly derived by mixing a quaternary ammonium salt with a metal salt or hydrogen bond donor (HBD) which is capable of forming a complex with the halide ion of the quaternary ammonium salt e.g. choline chloride with urea (reline), choline chloride with ethylene glycol (ethylene), choline chloride with citric acid, etc. (Smith et al., 2014).
Pretreatment of LCB using DESs has attracted much interest in recent years. DESs have demonstrated potential as novel solvents targeting the conundrum encountered with the conventional pretreatment methods of lignocellulosic biomass and have been proven to be multifunctional insolubilization, extraction, and subsequent production of value-added products from lignocellulosic material (Hansen et al., 2020).
In conclusion, pretreatment is an extremely important step for bioconversion of LCB to sugars for further processing into industrially important bioproducts such as biofuels, chemicals, and enzymes. Although the novel/green pretreatment methods are not entirely flawless, their environmental friendliness has necessitated the need for further exploration to improve their overall effectiveness and economic viability.
References
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Dr. Haliru Musa is a Lecturer II in the School of Science and Information Technology, Department of Biological Sciences. He acquired his PhD in Bioprocess Engineering from Universiti Malaysia Perlis, Malaysia.
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