# Cellulose Recovery from Agri-Food Residues by Effective Cavitational Treatments
## 3 Biomass Treatments for Cellulose Recovery
### Acoustic Cavitation (AC) as Effective Biomass Pretreatment
The effect of bubbles implosion coupled with the effect of aqueous-ILs appear to decrease the degree of polymerization and crystallization of cellulose, which might contribute to increasing its EH rate. EmimOAc has been frequently considered the most effective IL for cellulose dissolution and pretreatment aiming to EH ([PERSON] et al., 2013) [104]. Conversely, pretreatment with ChOAc enhanced the cellulose saccharification to the comparable degree to EmimOAc,suggesting how the acetate anion has a pivotal role in the reduction of cellulose crystallinity. In general, anions act as hydrogen bond acceptors that interact with the hydroxyl group of polysaccharides, thereby destabilizing their crystalline structure. In contrast, the inhibitory effects of ChOAc on cellulase were much lower than that of EmimOAc, because the cholinium cation was reported to be more biocompatible than the imidazolium one. Hence, ChOAc appear to be a promising option to EmimOAc for the IL based pretreatment and for the in situ saccharification of biomass. Environmentally friendlier alternatives to ILs are deep eutectic solvents (DESs), which consist in a mixture of a hydrogen bond acceptor (HBA), often a quaternary ammonium salt, and a hydrogen bond donor (HBD), such as alcohols, acids, amines or carbohydrates. In literature, the terms DES and IL have been used interchangeably but it is necessary to point out that these are two different types of solvent. [PERSON] et al. (2019) ([PERSON] et al., 2019) demonstrated that sequential ultrasonication and DES pretreatment allowed lignin isolation and Xyl recovery from oil palm fronds (OPF). In detail, the UAT was performed in water (70% of amplitude, 30 min, S/L ratio of 1:10 \(w/v\)) and then pretreated using DES (choline chloride:urea 1:2, S/L ratio of 1:10 \(w/v\), 120 \({}^{\circ}\)C for 4 h). The combined process decreased the lignin content by 36.42% while the DES pretreatment alone reduced the lignin content by 14%. Moreover, FT-IR confirmed the disruption of lignin C-O-C aryl-alkyl ether bonds in the sequential pretreated OPF, while XRD showed that crystallinity index of raw OPF was at 48.92% but the combination of recommended US/DES conditions provided a matrix with a crystallinity index of 59.31%. Figure 11 reports the samples' images obtained through FE-SEM analysis. In particular, the sequential pretreatment of both US and DES showed the highest OPF structure degradation, where lignin fraction was dissolved and large amount of cellulose fraction was exposed to the surface (Figure 11d). Additionally, the amount of monomeric sugars present in the OPF after the sequential protocol was also evaluated by the authors: HPLC analysis performed on the OPF hydrolysate showed that the two-step pretreatment was able to yield a Xyl recovery of 58%. Recently, a new class of DES has been synthetized on the basis of binary deep eutectic solvents and applied as a biomass pretreatment process: TDES (ternary deep eutectic  
Figure 11: FE-SEM image of (**a**) Raw OPF (**b**) ChClurea pretreated only OPF (6) US pretreated only OPF (70%, 30 min) (**d**) US-DES pretreated OPF (70%, 30 min ChClurea). Reprinted from Ultrasonic Sonochemistry, 58, 104598, Copyright (2019), with permission from Elsevier. solvents). [PERSON] et al. (2020) [106] developed a US- and MW-assisted TDES process, able to isolate cellulose and to fractionate lignin from the recalcitrant structure of garlic skin (GS). TDES composed by choline chloride (ChCl), glycerol and aluminium chloride (Al2O3-6H2O) with a 1:2:0.2 mole ratio has been synthetized and used as solvent to evaluate the optimal US frequency for the process. The combined protocol consisted of sequential sonication and MW heating (80\({}^{\circ}\), 2 h) on the mixture of GS and TDES with a 1:10 by weight ratio. The authors firstly investigated the effect of different US function modes of single frequency (20, 28, 35, 40, 50, 60 kHz), dual frequency (20 + 28, 20 + 40, 20 + 60, 35 + 50 and 40 + 60 kHz) and ternary frequency (20 + 28 + 40, 20 + 35 + 50, 20 + 40 + 60 kHz).