# Cellulose Recovery from Agri-Food Residues by Effective Cavitational Treatments
## 3 Biomass Treatments for Cellulose Recovery
### Acoustic Cavitation (AC) as Effective Biomass Pretreatment
Compared with the no-ultrasonic pretreatment, the highest lignin removal (73%) and the highest cellulose increase (166%) have been obtained exploiting the 20 + 28 + 40 kHz triple frequency mode. According to the authors, the multi-frequency modes allow one to avoid the formation of ultrasonic barrier (shorter ultrasonic transmission distance and weaker cavitation effect) caused by the increase of cavitation bubbles' number at high frequencies. In addition, two different metal chlorides (FeCl3 6H2O and CrCl3 6H2O) and two different hydrogen bond donors (oxalic acid and urea) were used to synthetize TDES, but the best results in terms of lignin removal rate were obtained with the ChCl-Gly-Al DES (Figure 12). Moreover, [PERSON] et al. investigated the effect of thermal treatment comparing MW heating and oil bath (OB) heating for each of synthetized DES, after the US treatment (20 + 28 + 40 kHz, 30 min) or alone. Higher lignin removal has been achieved again with the ChCl-Gly-Al DES. The sequential US + MW protocol (80 \({}^{\circ}\)C for 2 h) allowed one to obtain a higher lignin removal (90.14%) compared to the ones obtained with the US followed by OB heating at 110 \({}^{\circ}\)C for 4 h (87.36%), OB and MW alone (76.10% and 83.71%, respectively). The pretreated GS has also been characterized by FT-IR, XRD and SEM analysis. While FT-IR confirmed the disruption of chemical bonds between cellulose and lignin (C-O-C aryl-alkyl ether bond), XRD confirmed the decreasing of GS crystallinity may be due to the cellulose hydrogen bonds disruption. SEM images obtained on the pretreated GS revealed that US + MW + TDES pretreatment is more effective than both OB and MW alone and US + OB + TDES (Figure 12 b-d, respectively) in terms of surface morphology alteration and inner fibre exposure. As stated by the authors, the TDES combined with US and MW represents a low-cost cellulose isolation process that could improve later stage in biorefinery, especially for the conversion of lignocellulosic biomass into bio-based platform chemicals. Figure 12: SEM images of GS under different pretreatment methods in TDES (ChCl-Gly-Al). Reprinted from _Ind. Crops and Prod._, 149, 112357, Copyright (2020), with permission from Elsevier. In recent years, especially in the green chemistry area, some special DES that were produced using natural products have been studied and generally called natural deep eutectic solvents (NADES). These new classes of DES and GVL were tested as suitable solvents, coupled to US and MW, in the biomass delignification process by [PERSON] et al. (2018) [107]. In detail, WS (wheat straw) has been treated with both GVL and different NADESs coupled to US and MW technologies for delignification purposes. NADES1 was composed by lactic acid/choline chloride (10:1) while NADES2 was composed by lactic acid/glycerol/choline chloride (1:1:1). The MW-assisted pretreatment was performed in a multimode reactor with a S/L ratio of 1:50 and a variety of GVL/H2O ratios at 90-120 \({}^{\circ}\)C (1500 W-30 min). The UAT has been performed in a high-power ultrasonic bath (40 kHz and 200 W) with a WS/liquid ratio of 1:50 and different GVL/H2O ratios at 35 and 50 \({}^{\circ}\)C for 60 and 120 min. The lignin content of unpretreated and pretreated samples has been determined by Py-GC/MS/FID (analytical pyrolysis coupled with GC/MS/FID). Despite best delignification (45%) being achieved with NaDES2 (lactic acid/glycerol/choline chloride) under MW irradiation at 120 \({}^{\circ}\)C in 30 min, all the pretreatment experiments enabled good cellulose recovery. Moreover, the untreated and pretreated samples of WS have been characterized by nitrogen adsorption-desorption isotherm method to better understand the WS porosity modification. In general, UAT in presence of biobased solvents increased WS porosity both in the GVL and NADES samples. In comparison with conventional extraction, MW and US treatment with green solvents presented significant advantages in terms of sustainability.