Physicochemical Pretreatment

Steam Explosion

Steam explosion is one of the traditional and widely used physicochemical pretreatments for biomass hydrolysis. Steam explosion is a mild, chemical-free (typically) and cost-effective approach to pretreat lignocellulosic biomass. Water is involved to heat the biomass under pressure followed by a sudden decompression of the reaction vessel to explode the cellulosic fibers (Pielhop et al. 2016). Temperature involved in this process varies from 160 to 260°C and pressure varies from 0.69 to 4.83 MPa (Yu et al. 2012).

The disruption of lignocellulosic biomass because of the steam explosion leaves behind the sugar polymers accessible to enzymatic saccharification. Hemicellulose degradation and transformation of lignin to pseudo-lignin are also the outcomes of the steam explosion (Vivekanand et al. 2013). Some factors such as residence time.

temperature, biomass particle size and physicochemical properties can affect the efficiency of the steam explosion. As compared to other harsh pretreatment approaches, the steam explosion is a less reactive and less corrosive approach.

Liquid Hot Water

The pretreatment method involving liquid hot water requires water at higher temperatures (160-240°C) and pressures to retain its liquid state (Ruiz et al. 2013). This approach helps structural modification of lignocellulosic biomass along with the separation of hemicelluloses and amorphous cellulose, extraction of water-soluble components, although lignin remains unaffected (Nanda et al. 2018c). The quality of sugar generated is governed by the temperature employed while its quantity depends on the reaction time (Yu et al. 2010).

Ammonia Fiber Explosion

Ammonia fiber/freeze explosion (AFEX) method is a physicochemical process, which allows substantial delignification of biomass along with negligible degradation of sugars. AFEX is a cost-effective and less energy-intensive pretreatment process operating at moderate temperatures (60-90°C) and pressures (4-5 MPa) involving 1-2 kg of ammonia per kg of biomass (Peral 2016). Lower moisture content in the biomass and moderate conditions favor AFEX in an efficient pretreatment with lesser production of sugar degradation (or inhibitory) products. This process is somewhat similar to the steam explosion with the difference that liquid anhydrous ammonia is employed at elevated pressures and moderate temperatures followed by rapid de-pressurization (Chundawat et al. 2007).

Ammonia Recycle Percolation

In ammonia recycle percolation (ARP) strategy, nearly 5-15% of aqueous ammonia is permissible to percolate over a packed bed reactor retaining the biomass at a rate of about 5 mL/min (de Jong and Gosselink 2014). ARP is an improved approach over AFEX, due to its potential to remove 75-85% of lignin content and dissolve 50-60% of hemicellulose (Kim and Lee 2005). In this process, ammonia can be recycled. The pretreatment of corn stover and switchgrass with ARP resulted in the removal of 60-80% and 65-85% lignin, respectively (Iyer et al. 1996). Jonathan et al. (2017) researched dilute ammonia pretreatment of corn stover and reported the release of hemicelluloses from carbohydrate-lignin complex.

Hydrodynamic Cavitation

The approach of hydrodynamic cavitation accelerates the chemical reactions to pretreat a broad range of biomass with variable contents of cellulose, hemicellulose and lignin (Gogate 2016). The efficient removal of lignin along with the enhanced porosity of biomass is an outcome of this pretreatment method, which leads to effective saccharification of carbohydrates during enzyme hydrolysis. Other benefits associated with hydrodynamic cavitation are lower energy requirements and lower chemical catalyst loadings (Gogate and Pandit 2000).

The processing time of hydrodynamic cavitation is relatively shorter, which makes it an attractive option for a potential alternative to other pretreatments. Hydrodynamic cavitation has less complexity in its operating system, which can also be modified into a continuous process. Microbubbles are produced in the operating system when the fluid pressure declines and the fluid moves through a compression device such as a venture tube or an orifice plate (Patil et al. 2016). When microbubbles are generated, grown and collapsed, it gives rise to the cavitation phenomenon (Gogate and Pandit 2000). Inside the bubble, there exist drastic conditions such as high temperatures and pressures, which release a high amount of energy (Saharan et al. 2013). Along with these reactions, water molecules are detached in the cavities, thus ensuring the formation of potential oxidative radicals. The potential oxidative radicals are also liberated in the medium when the bubbles collapse (Badve et al. 2014). Due to the presence of these oxidative radicals, oxidation and degradation of lignin take place, thus resulting in its efficient elimination.

Supercritical Fluids

Supercritical fluids are a promising option for the pretreatment, fractionation, component extraction, structural modification, oxidation, liquefaction and gasification of waste organic biomass (Nanda et al. 2018c). Supercritical fluids behave likes gases and liquids, which open up their many applications. The critical temperature (T_c) and critical pressure (P_c) of water are 371°C and 22.1 MPa, respectively (Reddy et al. 2014). Water with its temperature and pressure lower or near to its critical points is called subcritical water, whereas that exceeding the critical points is called supercritical water. Subcritical and supercritical water are considered green solvents due to the innocuous nature of water and its abundance (Okolie et al. 2020a). Moreover, high-moisture containing biomass such as microalgae, sewage sludge and food waste can be effectively used in such pretreatments because no biomass drying is required since the reaction medium is water. This approach has the feasibility of application at large-scale industrial operations. Though this technology has been assessed in the pioneer stage, the last few years have witnessed it as a promising and green technology for pretreating, oxidizing, gasifying and incinerating different waste biomasses (Reddy et al. 2015; Reddy et al. 2017; Reddy et al. 2019).

Supercritical CO, is also used for biomass pretreatment, fractionation and biofuel upgrading (Reddy et al. 2018). The critical temperature and critical pressure of C0₂ is 31°C and 7.4 MPa. respectively, exceeding which it transforms into supercritical C0₂. The disruption of the crystalline structure of biomass occurs synchronously with an effective lignin elimination when the pretreatment process employs supercritical fluids. Subcritical C0₂ and subcritical water ameliorate the conversion of cellulose having the eco-friendly option as the usage of C0₂ curtails the impact of the greenhouse gas on the atmosphere (Liang et al. 2017). During this method, the moisture content of biomass plays a crucial role as C0₂ combines with water, thereby producing carbonic acid, which leads to hemicellulose hydrolysis.

Biological Pretreatment

Enzymatic hydrolysis involving a cocktail of enzymes such as (3-glycosidases, cel- lulases, hemicellulases, lignin-modifying enzymes and lignin-degrading enzymes plays an important role in delignifying lignocellulosic biomass and depolymerizing cellulose and hemicellulose sugars for fermentation to fuels and chemicals (Alvarez et al. 2016; Kumar et al. 2016). Microorganisms producing cellulase differ from each other in possessing different components of cellulase, as there are multiple enzyme components present in a cellulose enzyme system. Based on the biocatalytic activity, cellulases are classified into endocellulases, exocellulases, cellobiases, oxidative cel- lulases and cellulose phosphorylases. The reducing and non-reducing ends are generated when endoglucanase works upon linear cellulose molecules. These ends are targeted by exoglucanase, which highlights more inner sites for endoglucanase binding. The products generated from the activity of exocellulases are further catalyzed by cellobiases or (3-glycosidases. Radical reactions are catalyzed by oxidative cellulases, whereas phosphates are used to depolymerize cellulose in the biocatalysis aided by cellulose phosphorylase. Pseudomonas aeruginosa is an important microbial source of exoglycosidases (Kumar and Kumar 2017).

Many microorganisms have the potential to degrade cellulose, hemicellulose and lignin into their constitutive sugars. Prominent among them are bacteria, actino- mycetes and fungi. These methods are a cost-effective and eco-friendly approach to obtaining the goals of pretreatment, thereby circumventing various challenges faced by other physical and chemical pretreatment methods. Through biological pretreatment, several constituents of biomass like lignin, cellulose, hemicellulose and polyphenols can be hydrolyzed or degraded (Sindhu et al. 2016). Glucose, arabinose, xylose, etc. are the mono sugars that are generated because of bacterial and fungal hydrolysis of cellulose and hemicellulose.

Selective degradation of lignin and hemicellulose occurs by the action of brown, white and soft-rot fungi, among which white-rot fungi are highly efficient. Biological pretreatments also occur in ambient temperatures and pressures, which significantly differs from other pretreatment methods in being environmental friendly. Furthermore, biological pretreatments do not require acid, alkali or other reactive and corrosive pretreating agents. Biological pretreatment methods are advantageous in being cost-effective and less energy-intensive, although they can be time-intensive. This limitation is associated w'ith biological pretreatments because of the low rate of hydrolysis dependent on bacterial or fungal metabolism and enzymatic saccharification. Many researchers have explored a w'ide variety of microorganisms to explore lignocellulosic enzymes.

Conclusions

Bioconversion technologies towards efficient alcohol-based biofuel production involve novel pretreatment methods. Cost-effective, energy-efficient and sustainable pretreatment technologies are required for the industrial-scale conversion of waste biomass to recover fermentable sugars. The emergence of the new process technologies can lead to seeking sustainable alternatives for high-efficiency pretreatment of a wide variety of biomass. Novel and efficient pretreatment methods can be exploited for the production of biofuel that could meet the mounting energy demands. To get high-value biofuels and platform chemicals, more research on pretreatment technologies is required that can efficiently delignify lignocellulosic biomass, degrade holocellulose and prevent the formation of inhibitory and undesired degradation byproducts. Moreover, such pretreatment technologies should also have the flexibility for continuous-scale operations and feature short reaction time, less energy input, capital cost, less maintenance, safe operation. recycling of waste products and ability to be integrated into the bioconversion processes.