Lignocellulose is the most abundant component in nature since it refers to plant material. Beyond the enormous utilization of lignocellulose by human being, unignorable amount of waste is also formed simultaneously. Agro-industrial lignocellulosic wastes can cause environmental pollutions if not processed before discharged. An innovative approach for lowering the detrimental influences of lignocellulosic wastes is to consider them as a source of useful products rather than a waste to be decontaminated. Beyond the conventional techniques for evaluation of the wastes, new emerging techniques and the use of new solvents have drawn attention recently. Among new generation solvents, deep eutectic solvents (DESs) have been increasingly used in the treatment of lignocellulosics to produce value-added products such as biofuels, chemicals, and solvents and also used for the recovery of bioactive phenolic compounds. DESs are used extensively for fractionation of lignocellulosic wastes, often in combination with enzymatic hydrolysis of the biomass. On the other hand, extraction and recovery of bioactive compounds are also under research using DESs. This mini review summarizes the very recent literature reports on the use of DESs in treating agro-industrial wastes within the concept of valorization of biomass.
- agro-industrial wastes
- bioactive phenolic compounds
- deep eutectic solvents
- lignocellulosic biomass
Along with the increase of the global consumption manner of the humanity, the general waste amount has been increasing significantly. Global municipal solid waste estimated to increase to 2.2 billion tons annually by the third decade of 2000 . The accumulation of this huge amount of waste creates tedious environmental problems such as the generation of greenhouse gases along with the physical appearance. Despite the studies on the recycling and recovery processes, landfill is still commonly used procedure for the waste disposal in many countries .
The main constituent of the municipal solid waste is the lignocellulosic waste having a percentage of 29 . The lignocellulosic waste consists of paper, garden waste, wood, food, and also agricultural wastes. In this chapter, we will focus on the agricultural lignocellulosic waste. A general classification for lignocellulosic waste consists of three subclasses , namely, wood leftovers, farming crops, and secondary biomass. Logging leftovers, wastes from pulp, and paper industry are the subclasses of wood leftovers, whereas grasses, short rotation crops, as well as oil and grain crops belong to farming crops. On the other hand, secondary biomass has also two subclasses, namely, municipal solid wastes and food processing wastes.
Lignocellulose represents the matter of plants in general terms. It is the most abundant sustainable carbon source, and the main constituent is lignin that consists of complex organic polymers. Agricultural lignocellulosic biomass is composed of ~35–50% cellulose, 20–35% hemicellulose, and 10–25% lignin . Lignin forms the plant cell walls providing the mechanical endurance to the plant (Figure 1). They are mainly composed of monolignols that are methoxylated derivatives of benzene.
The carbohydrates found in the lignin structure are cellulose (Figure 2) and hemicellulose (Figure 3) that are covalently and hydrogenically bonded to lignin molecules. As a linear-chain polysaccharide, cellulose is made up of D-glucose monomers that are linked with ß-1-4 glycosidic bonds . Hydrogen bonding interactions are present between linear chains that are found in microfibrils , and cellulose has several types of crystalline structure. This complex structure provides the rigid and recalcitrance to dissolution of cellulose. Hemicellulose is structurally similar to cellulose as it also consists of polysaccharides, but it has a lower chain amount. On the other hand, hemicellulose contains branched heteropolymer consisting of pentoses – mostly D-xylose and D-arabinose; hexoses – mostly D-mannose, D-glucose, and D-galactose; and sugar acids – mostly 4-
Since lignocellulosic biomass is the most abundant natural source in the world, it may be evaluated as an alternative to unsustainable sources in many aspects. On the other hand, the wastes formed by lignocellulosic materials cause environmental problems arising from organic constituents with high COD and BOD degrees. Most of the lignocellulosic wastes contain phenolic compounds that may cause damage to the environment when discharged without any treatment . These wastes may produce odor, soil pollution, and harborage for insects, if not processed further . A promising approach to reduce the pollution problem of the lignocellulosic biomass is to use them as raw materials as a resource of value-added products such as biofuels (bioethanol, biogas, and biohydrogen), chemicals, and solvents  and also to use them for the recovery of bioactive phenolic compounds (flavonoids, phenolic acids, stilbenes, and tannins) . According to the recent literature, the use of lignocellulosic biomass is encouraged as a natural source to be used in biotechnological process that will spontaneously lead to a decrease in the pollution effects of the waste. There are various methods to evaluate the lignocellulosic biomass such as fractionation or recovery of the valuable compounds. Besides conventional methods, the use of green techniques has gained a considerable attention due to environmentally friendly characteristics. In this mini review, the very recent literature on the use of deep eutectic solvents (DESs) in treating agro-industrial wastes, within the concept of valorization of biomass, is summarized.
2. Deep eutectic solvents
For both chemical and pharmaceutical processes, solvents are essential constituents. They are utilized in a broad range of fields including bulk chemicals, medicines, cleaning agents, dyes, and so on. The solvents used in such processes are mainly petroleum-based organic solvents, as well as ammonia and water. However, along with the increasing consciousness related to the environment, the solvents that are regarded as eco-friendly have been the focus for many researchers and are regarded as green solvents. Besides the formerly known green solvents such as biosolvents and supercritical fluids, ionic liquids and lately deep eutectic solvents have been extensively utilized in various areas with an increasing trend. Additionally, green solvents are encouraged in many fields of research to promote sustainable processes .
DESs are one of the most popular green solvents that are mostly known as nontoxic, recyclable, and nonflammable, and they have low vapor pressures [18, 19, 20]. They can be easily prepared in the laboratory using numerous substances in different molar ratios that result in diverse properties of DES such as polar, nonpolar, acidic, and basic. The most common method to prepare a DES is to mix the constituents in a certain molar ratio at a certain temperature until a homogeneous liquid form is obtained [18, 21] (Figure 4). In another method, the constituents (mostly solid) are mixed together with water, and subsequent evaporation of the excess water under vacuum is performed, which is called evaporation method . Similar steps are followed for the freeze-drying method; indeed, water is removed by freeze-drying . In grinding method, a glovebox in nitrogen atmosphere is used to grind the solid in a mortar till clear liquid is obtained . If a novel DES to be formed for the first time, many tests should be performed to prove the “deep eutectic” property of the solvent. Otherwise, published articles’ protocols should be followed exactly to synthesize DES in the correct form.
The very first description of DES was made by Abbot et al. as the liquid formed between a variety of quaternary ammonium salts and carboxylic acid. Later on, DESs were classified into four groups : Type I: Organic salt + Metal salt; Type II: Organic salt + Metal salt hydrate; Type III: Organic salt + hydrogen bond donor (HBD); and Type IV: Metal salt + HBD. Lately, many researchers presented so many different DESs from so many different types of molecules that the definition of DES converged to a simple form: DESs are composed of two or more components, which in minimum two of them have a hydrogen bonding interaction ability: one as a HBD and one as a hydrogen bond acceptor (HBA) . On the other hand, DESs that are formed by natural compounds such as organic acids, sugars, and choline chloride are called natural deep eutectic solvents (NADESs) . NADESs may be classified as sugar based (glucose-fructose-water, glucose fructose-sucrose-water), polyol based, acid based, and so on. Since DESs can be formed by a number of components, physicochemical properties vary from type to type. Therefore, one can tune the physicochemical property by changing the type and the molar ratio of the constituents. Depending on the type of the constituents, viscosity of DESs may be low or high. High viscosity DESs are hard to be handled, but in some cases, they are preferred to be used as a mixture of alcohol and water to decrease the viscosity. DESs generally have low melting points. This is related to the hydrogen bond interaction between the constituents. Some DESs were reported to have a glass transition temperature [22, 24, 27]. Density ranges of 800–1600 kg/m3 are presented in the literature, but in general, they have higher density than water [28, 29, 30]. On the other hand, hydrophobic DESs are reported to have lower density than hydrophilic DESs [31, 32, 33].
The use of DESs in different fields such as biochemistry, electrochemistry, synthesis, nanomaterials, separation, and metal processing [25, 34, 35, 36, 37, 38] has been increasing since 2003, when it was first described. Recently, they were shown to be used as solvents in many types of enzyme-catalyzed reactions such as esterification, transesterification, polymerization, and hydrolysis [39, 40, 41, 42, 43]. On the other hand, their catalytic effects in several different types of reactions have also been reported [39, 40, 41, 44, 45, 46, 47, 48]. In detail, the number of DES-related publications was more than 300 between 2009 and 2013, while it was only 29 until 2008 . In 2017, the number of publications on DESs reached up to almost 750 .
3. Fractionation of agro-industrial wastes with deep eutectic solvents and recovery of lignin
In the studies carried out within the scope of sustainability, agricultural lignocellulosic wastes such as corn straw, rice straw, wheat straw, fruit wastes, and sunflower stalk have been subjected to various treatments prior to conversion processes. The most challenging step in such a process is the resistance of lignocellulosic material to degradation; therefore, a treatment method is required prior to utilization. These methods can be chemical, physical, mechanical, physicochemical, or biological. In some cases, a combination of these methods is also preferred since each one has different advantages and disadvantages . The most commonly used method is the chemical pretreatment; however, it has undesired environmental impacts. On the other hand, physical pretreatments require high energy, whereas biological pretreatments progress relatively slowly. Therefore, in addition to efficiency, cost, environmental impacts, and ease of use should be taken into consideration for the selection of the pretreatment method.
Ionic liquids as green solvents are effective and promising solvents in the pretreatment of lignocellulosic biomass [31, 51]. However, high prices and toxic properties limit their utilization in industrial applications . Recently, DESs that have superiority to ionic liquids due to their low cost, low volatility, biodegradability, easy preparation techniques, and environmental friendliness have been successfully used in the pretreatment of lignocellulosic materials .
Casal et al.  were among the first researchers to report that the solubility of wheat stalk in DES was promising. Later on, Francisco et al.  studied the solubility of alkali lignin, cellulose, and starch in DESs prepared with choline chloride and carboxylic acid. They reported that lignin was soluble in DESs, whereas cellulose was nearly insoluble, which was a promising result. Among the tested eutectic solvents, the best result was obtained with ChCl-lactic acid (LA) (1:9). Afterward, several researchers treated agro-industrial lignocellulosics with DESs and reported satisfactory results. Procentese et al.  pretreated corncob with different choline chloride-based DESs and achieved a total of 41 g fermentable sugars from 100 g corncob after a subsequent enzymatic saccharification. The concentrations of inhibitory agents, that is, acetic acid and furfural were low following the pretreatment with DESs. The authors also reported that the decrease in lignin and hemicellulose contents increased the crystallinity index (CrI) of the pretreated biomass. Zhang et al.  pretreated corncob with DESs consisting of choline chloride as HBA and monocarboxylic acid, dicarboxylic acid, or polyalcohol as HBDs. SEM, XRD, and FTIR analyses of treated corncob showed that pretreatment with DESs disrupted the structure of biomass. Polyalcohol-ChCl was found to be more effective in lignin extraction than others. Kumar et al.  treated rice straw with lactic acid-betaine and lactic acid-ChCl NADESs and could extract high purity of lignin (>90%). They also reported that approximately 60% of lignin could be separated from the lignocellulosic material. Additionally, higher lignin solubility was achieved when lactic acid-ChCl was used in the treatment. The addition of water (5%) during pretreatment caused a further increase (about 22%) in the extracted amount of lignin. The authors also reported that the CrI of biomass decreased after pretreatment and that subtle structural differences were detected in the crystalline and also amorphous zones of the cellulosic portions. Procentese et al.  treated waste lettuce leaves with ChCl-glycerol and used the pretreated biomass sequentially in the enzymatic hydrolysis and acetone-butanol-ethanol fermentation. The authors reported that less energy was consumed with the use of DES than both NaOH and steam explosion pretreatment techniques for the same degree of fragmentation. In the study of rice straw pretreatment using DES, Hou et al.  reported that two-step pretreatment increased the yield of sugar by creating a synergism. The researchers found that the yield of glucose was 90.2% as a result of sequential ChCl-oxalic acid and ChCl-urea pretreatments, and also the addition of water during the process increased the yield. Procentese et al.  investigated the production of fermentable sugars from biomass by pretreating apple residues, potato peels, coffee silverskin, and brewer’s spent grains with ChCl-glycerol and ChCl-ethylene glycol. The highest glucose yield was 0.20 with ChCl-glycerol and 0.19 with ChCl-ethylene glycol. Liu et al.  treated wheat straw with triethylbenzyl ammonium chloride/lactic acid (TEBAC/LA)-based deep eutectic solvents under different conditions. The authors reported that the use of TEBAC/LA (1:9) at 373 K for 10 h provided the highest subsequent enzymatic hydrolyses of cellulose and xylan. About 80% removal of lignin was achieved using TEBAC/LA DES in the pretreatment. New et al.  investigated the effect of water content of ChCl-urea (1:2) on delignification of oil palm fronds and showed that aqueous DES provided more lignin removal than pure DES. The presence of 30% (v/v) water in DES was reported as the best amount for optimal delignification (16.31%). Ong et al.  used two-pot sequential pretreatment for oil palm fronds. They ultrasonicated the palm fronds in water and subsequently pretreated with ChCl-urea. The authors reported that the ultrasound pretreatment facilitated the degradation of lignin matrix by DES. The hydrogen bonding between the halogen component of ChCl and the hydroxyl groups of lignin was proposed to be a facilitation in the cleavage of ether or ester bonds among hemicellulose and lignin. At the optimum conditions (70% amplitude and 30 min), 36.42% of lignin removal and 58% of xylose recovery were achieved. Tan et al.  synthesized several DESs using ChCl and organic carboxylic acids and used them in the pretreatment of oil palm empty fruit bunch. It was reported that the presence of hydroxyl moiety and short alkyl chain enhanced the biomass fractionation and lignin extraction. ChCl-LA (1:15) and ChCl-formic acid (1:2) extracted more than 60 wt% of lignin. Fang et al.  proposed that a hydrothermal pretreatment could reduce the recalcitrance of lignocellulosic biomass if applied before a DES treatment. The hydrothermal pretreatment was performed at 200°C for 10 min with 10% dry matter loading. The results showed a consistency with the initial proposal. Both xylan and lignin removals were successfully enhanced around 25% during the treatment using ChCl-glycerol (1:2). Similar liquid hot water pretreatment was studied by Tian et al.  for the delignification of poplar wood shavings. To provide a mutual agreement for both hemicellulose recovery and solid yield, 170°C was preferred as temperature for the hot water extraction for 40 min. For the subsequent DES treatment step, acidic eutectics were prepared by using ChCl as HBA and formic acid, acetic acid, or lactic acid as HBDs in a molar ratio of 1:2. The hydrothermal processing together with DES treatment increased the lignin selectivity and also the porosity of the resulting cellulose. The ionic properties of the DESs were proposed to provide the selective lignin removal and cellulose deconstruction, thereby increasing cellulose chemical reactivity. A 79.8% of solid yield and 54.4% of hemicellulose removal were reported in the study. Chen et al.  aimed to obtain platform chemicals such as furfural, 2,3-butanediol by the pretreatment of switchgrass with ChCl-ethylene glycol. They reported that neat ChCl-ethylene glycol provided a removal of only 24% of lignin, while acidified form provided 87% removal. They also could enrich cellulose up to 72.6% in pretreated switchgrass with the solid loading levels of between 20 and 27%. At this high level of solid-loading efficient, removal of lignin and xylan was achieved. Lim et al.  synthesized new DESs using potassium carbonate and glycerol in different molar ratios. The most appropriate molar ratio was reported as 1:7 in terms of pH, viscosity, and thermal stability. They tested different parameters such as temperature (110–150°C), reaction time (40–120 min), and solid-to-liquid ratio (1:8–1:12) on the treatment of rice straw. They could achieve 73.8% cellulose under the optimum conditions that were a temperature of 140°C, a reaction time of 100 min, and a solid-to-liquid mass ratio of 1:10. CrI was reported to increase to 60% from 52.8% after the treatment. Wan and Mun  tested the use of different DESs [ChCl-urea (1:2), ChCl-citric acid (1:2), and ChCl-glycerol (1:1)] for the treatment of sago waste. The optimum pretreatment conditions were reported as 110°C and 3 h at 5% solid loading. According to the apparent structural disruption created by ChCl-urea, it was selected as the DES to give the best result. The authors subsequently performed enzymatic hydrolysis to be mentioned in the next part. A distinct study presented the
The main idea of the utilization of DES in the pretreatment is the possibility of the strong intramolecular hydrogen bonds in DES to promote the breakage of the hydrogen bonds in the lignocellulosic structure [12, 80, 81, 82]. There are also some reports on the mechanism of the interaction between DES and lignocellulosic components. In general aspects, to increase the solubility of a hydrophobic compound in an aqueous solvent, the following well-known methods are utilized, that is, cosolvency, hydrotropy, complexation ionization, and the use of surface-active components . Therefore, the mechanism of the enhanced solubility of lignin in DES-water mixtures is investigated in terms of hydrotropic effect. Such a study was conducted by Soarez et al. . The authors reported that syringic acid solubility was increased by decreasing the polarity of the carboxylic acids in DESs. Apart from the hydrogen bond interactions and pi-pi interactions, the main reason for high solubility was reported as the dispersive interactions between organic acid alkyl chain and syringic acid. Furthermore, when urea was used instead of choline chloride, a fourfold increase in the solubility of lignin was reported. Nearly, 50% of DES-water mixtures provided the best solubility of the monomer. This was explained by the hydrotropic mechanism. They obtained the same result when they used organosolv and kraft lignin and proved the mechanism by dynamic light scattering. Xia et al.  searched for the weak fractionation efficiency of ChCl-glycerol using different techniques such as quantum mechanics calculations and solvatochromic parameters. The intramolecular interactions of lignin-carbohydrate complexes were found to be stronger than the interactions with DES and lignin-carbohydrate complexes. Interestingly, chloride ion in DES was reported to be surrounded by mutually anionic hydrogen bonds and cationic hydrogen bonds. This case resulted in a lowered ability of occupied-site anions and insufficient protons, which meant inactive acidic sites. To overcome this, a ternary DES was formed by adding the aluminum chloride into DES. The resulting supramolecular complexes of chlorine ion-metal cation-hydrogen bond acceptor showed deep eutectic characteristics and resulted in a significant enhancement of the efficiency of the lignin extraction. On the other hand, Alvarez-Vasco et al.  found that DESs have the ability to cleavage ether bonds without affecting C-C linkages. Considering the ability of the solvents with high ß and π* to provide solubility of lignin, such as dimethylsulfoxide and pyridine, DESs can be possibly declared as new green candidates in lignin solubility.
Aforementioned studies clearly show that the use of DES is a good alternative for the removal of lignin from agro-industrial wastes to conventional pretreatment methods. Subsequent use of these pretreatment products allows the lignocellulosics to be valorized for several industries (Figure 5).
3.1 Enzymatic hydrolysis of biomass components
Apart from the treatment studies on the lignocellulosic waste by DESs, additional enzymatic hydrolysis is performed in many studies to remove lignin. Some of the above-mentioned literature contains subsequent enzymatic hydrolysis of the treated biomass as summarized. Procentese et al.  performed hydrolysis using Cellic CTec 2 enzymes (Novozyme) after increasing the digestibility of corncob by DES pretreatment. The hydrolysis conditions were 50°C, 180 rpm, and up to 80 h in a rotary shaker. The saccharification rate was found to be the highest at 80°C for the ChCl-imidazole pretreated sample from which 55% lignin was successfully removed. The enzymatic glucose and xylose yields increased with the increasing pretreatment temperature. The highest recovery of the initial carbohydrates was reported as 76%. In their following research, Procentese et al.  studied the enzymatic hydrolysis with the same commercial enzyme after the treatment of waste lettuce leaves with DESs. The completion time for the hydrolysis of the pretreated biomass was reported as 9 h. On the other hand, the higher the pretreatment temperature was the higher fraction of monomers was obtained during enzymatic hydrolysis. The authors also used the enzymatic hydrolysate of the pretreated lettuce in the batch culture of
4. Extraction and recovery of flavonoids from agro-industrial lignocellulosic wastes
Agro-industrial wastes represent sources of phenolic compounds that have beneficial effects to health due to their antioxidant, antimicrobial, anti-inflammatory, and immune-stimulant properties [87, 88, 89]. The prevention of cancer and cardiovascular diseases by phenolic compounds is attributed to their antioxidant and scavenging properties against reactive oxygen species. Apart from their use in biomedical applications, phenolic compounds can also be used in food industry as nutraceuticals. More than 8000 phenolic structures are identified in the structure of plants . The extraction of biophenols from plants is always attractive; moreover, during the last decade, the recovery of phenolic compounds from agro-industrials has gained enormous attention. In spite of their distinct health beneficial properties, the massive phenolic compounds in lignocellulosic wastes have detrimental effects on the environment. The removal/recovery of phenolics from biomass has been conventionally performed with organic solvent extraction; however, recent studies show that DESs can be successfully used in the extraction [91, 92, 93, 94]. On the other hand, the use of DESs in biomass processing is less studied than other applications of DESs and needs to be improved . Below, the very recent studies dedicated to the DES selection and condition development for the extraction of polyphenolic bioactive compounds, especially flavonoids, from most abounded agro-industrial wastes are briefly summarized.
Jeong et al.  tested several DESs for the recovery of anthocyanin from grape skin and reported that ChCl combined with citric acid, D-(+)-maltose, and fructose was the most effective ChCl-based DESs. In addition, a newly designed DES – citric acid-D-(+)-maltose (4:1) – provided considerably high level extraction yield of anthocyanin. Under the optimized conditions identified by the response surface methodology, total anthocyanin content was found to be 63.36 mg g−1 using the new DES. Radosevic et al.  used ChCl-based DESs containing glucose, fructose, xylose, glycerol, and malic acid for the recovery of phenolics from grape skin and tested the biological activity of extracts
The number of studies in open literature on the extraction of bioactive compounds with DESs has been rapidly increasing. Therefore, DESs are easily expected to be used more for the extraction of bioactive phenolic compounds from various sources in the near future. On the other hand, the following issues should be more extensively studied; recovery of phenolics from DESs, stability of phenolics in DESs, reusability of DESs and the scale-up of the extraction processes. Additionally, the determination of biological activities of the phenolic compounds in DES extracts appears to be another field to be focused on in the near feature.
5. Instruments used for the extraction and recovery from lignocellulosic wastes
For the extraction and recovery of lignin or flavonoids from lignocellulosic waste, mostly preferred method is the heating and stirring method [54, 55, 56, 57, 58, 59, 60, 61, 63, 65, 76, 78, 99, 103, 105, 106, 107, 112] in which oil bath is used to achieve relatively high temperatures. Apart from this conventional method, advanced techniques including the use of ultrasound and microwave irradiations are rarely studied for the extraction and recovery from lignocellulosic wastes. The yield of an extraction from a lignocellulosic material is related to the isolation of the target molecule from the matrix . Ultrasound- and microwave-assisted procedures facilitate the isolation of the target molecules. Microwave heating process was reported to be a very efficient method as it promotes the selective bond cleavage and the stretching of certain bonds at a higher level due to the microwave irradiation . Indeed, in the case of microwave, the disruption of the hydrogen bonds occurs by the dipole rotation of the molecules, and subsequently, dissolved ions migrate that lead to an increase in the penetration of the solvents into the matrix. This procedure results in an easier recovery of the target molecules. Beyond this, the pressure formed by the microwave improves the porosity of the matrix, thereby letting better contact of the solvent with the lignocellulosic material . Related to these positive effects, microwave may be used either in the pretreatment of the biomass or in the extraction process directly. Ultrasound-assisted method is also advantageous when compared to classical methods. Similar to microwave-assisted procedure, it lets a better solvent penetration into the matrix. Ultrasound-assisted extraction creates microsteam in the cells that enhances the mass transfer. Ultrasonication creates longitudinal waves that form alternating compressions in the solvent, which leads to cavitation and gas bubbles. In this period, the bubbles expand, and at a point, the gas in the bubble condenses and a considerable amount of energy is released . With the crashing of the condensed molecules, shock waves occur that lead to very high local temperature and pressure values. Regarding the effectiveness of the two nonconventional methods, we believe that researchers will pay attention to use them in the challenging delignification or extraction processes. Both microwave- and ultrasound-assisted extractions reduce working times and also increase the yield.
6. Different aspects of the utilization of deep eutectic solvents
After the use of DES in both fractionation of lignin and recovery of valuable compounds from lignin, DES can be recycled. The solvents that work as an antisolvent such as water, ethanol, or acetone result in the solidified form of DES and can be easily removed from the media. After the evaporation or freeze drying of the antisolvent, DES can be obtained in a pure form as mentioned before by several authors [51, 102, 115]. The removal of water in DES can be performed by adding acetone, thereby precipitating DES. The precipitated DES can be reused again by heating.
Beyond the advantages of the use of DES in the treatment of lignocellulosic waste, some drawbacks of DESs were also reported and reviewed in the literature . At high temperatures, DES was reported to decompose to the hydrogen bond donor and acceptor . Moreover, low boiling point component was reported to evaporate. The decomposition temperatures of some DESs were also reported in the literature . Another disadvantage of DESs was reported as the hygroscopicity; however, since certain amount of water is intendedly added during some treatments, this would not be a real disadvantage for many treatments to our opinion.
Another issue is the toxicity of DESs. As green solvents, they are assumed to be nontoxic, especially when natural components are used to form the eutectic mixtures. The most commonly used HBA in DESs is ChCl. Since cholinium is a component of vitamin B complex and the ever first DES was defined using ChCl, ChCl-based DESs are regarded as safe. However, according to the latest research articles, nontoxicity may not be generalized for all types of DESs, including the ones containing the common quaternary ammonium salt. Herein, some of the reports on the toxicity of different types of DESs are summarized.
Hayyan et al.  were among the first researchers working on the toxicity of DESs. Four different types of DESs [HBA: ChCl HBDs: glycerine, ethylene glycol, triethylene glycol, and urea (molar ratio: 1:3)] were studied for their toxicity toward
Regarding the above-mentioned reports, it can be concluded that the toxicity of a DES varies according to the type and molar ration of the components. The presence of an organic acid increases the overall toxicity of a DES. The eutectic mixture is generally more toxic than its individual components. This may be related to hydrogen bonds between the components. On the other hand, NADESs were shown to exhibit lower toxicity than DESs; moreover, natural based or not, all DESs exhibit lower toxicity than ILs. Therefore, “green solvent” characteristic should be proved, or the proven green solvents should be used especially in large-scale applications. On the other hand, new strategies should be developed to predict the toxicity level of a candidate green solvent in the near future.
Recent trends in the reduction of pollution effects of lignocellulosic waste are to convert them into useful products or to recover the natural components within them. Treatment of wastes provides the utilization of materials or substrates in the production of value-added products. However, most of the pretreatment methods used for lignocellulosics are energy consuming operations since they need to use high temperature and pressure for the removal of lignin. Deep eutectic solvents are new generation solvents that find several applications in chemistry and chemical engineering. Extraction methodology for the recovery of bioactive phenolic compounds from plants can also be used for agro-industrial wastes. Treatment with DESs requires less energy. Additionally, DESs facilitate accessibility to cellulose by dissolving lignin at low temperatures and pressures. On the other hand, the task-specific DES for the target lignocellulosic waste differs according to the type of the waste. Therefore, one should search the literature and start with the promising DES and extend the research to similar kind of DESs to achieve high recovery yields. Apart from that, a new and target-specific DES may be synthesized since enormous number and type of components are candidates for new DESs.
DESs have unique properties such as polarity, conductivity, and viscosity depending on their composition. Therefore, novel DESs to be prepared and treatment conditions to be improved will help to solve environmental problems originated from agro-industrial wastes and also to develop new platforms for the production of valuable products such as chemicals, biofuels, and bioactive phenolic compounds.
Even promising results are published on the use of DESs for the lignocellulosic wastes, the mechanism of the treatment and the changes on the structure of DES still need to be clarified. Additionally, a detailed structural analysis on the extracted and purified biomass components relevant for the purpose should be revisited.
Conflict of interest
The authors declare no conflict of interest.