# Cellulose Recovery from Agri-Food Residues by Effective Cavitational Treatments
## 3 Biomass Treatments for Cellulose Recovery
### Acoustic Cavitation (AC) as Effective Biomass Pretreatment
The results of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) on pretreated and untreated wood samples showed that cavitation is indeed effective in altering the physicochemical structure of Eucalyptus wood; the applied protocol reduced the amount of alkali metals (e.g., potassium, magnesium and calcium) in the residual biomass. Compared to the control biomass, the residual char content of NaOH pretreated samples increased by 10%- 20%, while the reaction temperature decreased from 361 \({}^{\circ}\)C to 341 \({}^{\circ}\)C. Conversely, in UAT samples treated in presence of acetic acid or distilled water, the residual char content diminished by 12%-21% and there were no significant discrepancies in reactivity apart from a slight increase in reaction rate. Further results showed that UAT has strong influence on the structure texture of the Eucalyptus wood samples. The acoustic cavitation successfully removed cellulose, hemicellulose and lignin from the substrate, increasing the average crystallinity up to 35.5%; the process also enhanced the exposure of the biomass material to the treatment solutions and broke down sample structure generating pits and microchannels. In addition, US removed and prevented the formation of aggregates and biomass clusters. When rapeseed straw (RapS) was pretreated by [PERSON] et al. (2016) [80] either by US, MW or high voltage electrical discharges (HVED) the extraction of reducing sugars and soluble lignin, as well as the delignification, were improved. In terms of energy requirements, HVED induced a significant delignification yield (42.3%) with the lowest energy consumption (in order to achieve the same efficiency, MW and US consumed 9 and 4.5 times more, respectively). Meanwhile, the best EH yields (41.6%) were reached when the RapS were pretreated under US (12 kHz, 400 W, 10-40 min) and HVED in alkaline conditions (at 90 \({}^{\circ}\)C). Figure 4 shows the SEM images of RapS pretreated by US and HVED which were related to the greatest delignification yields. Figure 4A-F shows the shredded morphology of the sample, due to the structural damage caused by UAT and HVED. Moreover, there were more rifts and extra fibres appearing in the pulps after US pretreatment than that recorded in the raw material (Figure 4G), where a slick and uniform surface is revealed. Consequently, the lignin recovery (40%) was achieved thanks to an intensified breakdown of polysaccharides-lignin connections, providing better accessibility to enzymes. Finally, the reported US and HVED were effectively combined with chemical treatments, i.e., NaOH sol. or organosolv (ethanol/water 65% \(v/v\), +1% H\({}_{2}\)SO\({}_{4}\) 20%) for RapS delignification ([PERSON] et al., 2017) [81]. Best results were noticed for HVED-NaOH and US-organosolv approaches. Both technologies enabled a marginally affected lignin with a higher polymerisation degree (in case of UAT) and damaged ferulates bridges (in case of HVED). The lignin fractions recovered from HVED-organosolv treatment included a lower amount of ferulic acid and \(p\)-coumaric acid as compared to those extracted by US-organosolv, while the latter showed higher syringl/guiaacyl (S/G) ratios (1.75) compared to the first (1.31). In addition, pre-treatment with HVED and US-under alkali condition induced positive effect on subsequent cellulose recovery. [PERSON] et al. also described the successful combination of physical and chemical treatments for the rapeseed hulls extraction of phytomelanins ([PERSON] et al., 2016) [82]. Among the reported pretreatments, US and HVED have proven their better activity especially under alkali conditions. The above-mentioned investigation revealed that physical treatment promoted the recovery of the phytomelanin layer and increased the EH of resulting pulps. The results highlighted in particular the effectiveness of US working at 12 kHz with the intensity of 400 W (Figure 5).