## 3 Results and Discussion
### Cascade Protocol for Grape Stalk Valorization  
US-assisted delignification process afforded some valorization pathways for GS, as represented in Figure 2.  
Acid-insoluble lignin from GS was precipitated and isolated from the obtained liquid fraction after acidification at pH = 2. The analysis of its physical properties could be interesting in the frame of a complete further valorization; for instance, lignin contains a high number of phenolic groups, with remarkable antioxidant activity, useful in biomedical applications as anticancer, antiviral, or for diabetes treatment [37], among others; furthermore, by liquefaction or pyrolysis, a liquid bio-oil with 100 different kinds of phenolics can be obtained, useful for bioenergy purposes [38; 39]. Finally, the oxidation of lignin affords bioaromatics (mainly vanillin and syringaldehyde [3]), endowed with many industrial applications in foods, drugs, or cosmetics, as well as long-chain fatty acids as biofuel precursors, as recently reported for GS as well [21]. Figure 2: Cascade protocol by applying enabling technologies (UAE and MW-assisted radiation) for the full valorization of grape stalks (GS). In this work, a high amount of lignin has been removed by US-alkaline-assisted treatment, in agreement to other reported works [40]. After NaDES treatment, no solid was recovered upon delignification with ChLAGly, while a low amount of lignin was isolated when using both ChLA (30 mg) and ChLeVA (10 mg). This finding could be explained by the formation of depolymerization products and acid soluble lignins due the synergy between cavitation effect and NaDESs. Moreover, the precipitation could be hindered by the successful interaction of NaDESs with the exposed phenol groups in lignin that could help keep lignin in the liquid phase by the formation of H-bonds. The characterization by FT-IR and SEM (only for alkali lignin) of the isolated lignins after GS-delignification is shown in Figure 3.  
Alkaline treatment with NaOH lead to a less cross-linked structure, as it is possible to observe in the FT-IR spectrum (Figure 3a, black line). Indeed, the bands related to C=C and C-O stretching for aromatic rings--as well as the C-O and C-H vibration of aliphatic CH\({}_{2}\)- or phenol OH bonds (1600-1000 cm\({}^{-1}\)) [2; 41; 42]--were found to be less intense compared to NaDES lignins. This fact is in accordance with the literature, where [PERSON] et al. reported that a high structural modification of the starting lignin occurs after alkaline treatment [43]. Moreover, both carbonyl and C-O bands (1728 and 1025 cm\({}^{-1}\), respectively) are less intense in alkali lignin compared again to NaDES lignins, suggesting that the amount of ether linkages is considerably lower. By contrast, NaDESs delignification was useful to obtain a less-modified lignin with a cross-linked structure (Figure 3a, red and blue lines). This fact  
Figure 3: FT-IR spectra for NaDES and alkali lignin (**a**); and SEM images at 500\(\times\), 1.00 K\(\times\), and 2.00 K\(\times\) magnification for alkali lignin (**b**). can be observed by both aromatic region (1600-1000 cm\({}^{-1}\)) and the sharp bands related to C=O and C-O linkages. As mentioned, these solvents establish hydrogen bonds with lignin monomers, thus avoiding deep structural modifications. Furthermore, as a higher amount of lignin was recovered by US-alkaline-assisted treatment in contrast to NaDESs, it was possible to perform the SEM analysis for this solid (Figure 3b). The morphology showed plenty of irregular and hollow spheres according to the literature [44,45], which conglomerated and aggregated through intermolecular forces and adsorption during lignin precipitation [44]. All these spherical structures observed by SEM suggest an aromatic moiety, in agreement to previous work [2], which may be related to the monolignols involved in lignin structure. In order to fully valorize each recovered fraction after US-assisted GS pretreatment, the liquid fraction recovered after lignin precipitation was considered.