## 3 Results
### DES Treatment
#### 3.2.3. DES Recycling Assessment  
The recycling of DES is crucial in the reduction in operating cost and meets the requirements of green chemistry. Herein, recycling experiments were carried out under the optimal DES treatment conditions mentioned above. Figure 6a shows the recycling performance of ChCl-LA on the fractionation of green bamboo. Contrary to CR yield and cellulose retention, lignin removal exhibited a decreasing trend with increasing run number, which is probably related to the decrease in acidity and the increase in viscosity and impurity of recycled DES [55; 56; 57]. Although it requires in-depth understanding of the recycling performance of recycled DES, the recycling assessment clearly demonstrates that  
Figure 5: FTIR analysis of CR prepared under **(a)** ChCl:LA (1:3), 90–160 \({}^{\circ}\)C, 3 h, 1:25 g/mL without water addition, **(b)** ChCl:LA (1:3), 140 \({}^{\circ}\)C, 2 h, 1:10 g/mL, and water addition of 0–60% (v%); XRD analysis of CR prepared under **(c)** ChCl:LA (1:3), 90–160 \({}^{\circ}\)C, 3 h, 1:25 g/mL without water addition, **(d)** ChCl:LA (1:3), 140 \({}^{\circ}\)C, 2 h, 1:10 g/mL, and water addition of 0–60% (v%); **(e)** FTIR and **(f)** XRD analysis of \(\alpha\)-cellulose and cotton before and after DES treatment.  
ChCl-LA can be recycled and reused, exhibiting the potential to reduce the operating cost for industrial consideration.  
The weak FTIR signals corresponding to lignin (Figure 6b) and the similar _CrI_ of CR (Figure 6c) indicate that ChCl-LA can be recycled five times without a significant decrease in delignification efficiency. Additionally, [PERSON] et al. [58] pointed out that when the residual lignin was less than 36.0%, the adverse effect of lignin on enzymatic hydrolysis efficiency via non-productive enzyme adsorption and steric hindrance was not remarkable. Therefore, achieving complete removal of lignin from bamboo may not be essential and cost-effective for the following saccharification process of CR.  
It is generally believed that HSQC-NMR is a powerful technique for lignin structural characterization. The side-chain (\(\delta_{\text{C}}\)/\(\delta_{\text{H}}\) 50-95/2.5-6.0 ppm) and aromatic (\(\delta_{\text{C}}\)/\(\delta_{\text{H}}\) 95-160/5.5-8.5 ppm) regions of HSQC spectra of DEL after the 1 st and 5 th run of DES recycling are presented in Figure 7. The main cross-signals are assigned according to previous publications [47; 59; 60; 61; 62; 63], as listed in Table S2. A correlation for the \(\gamma\)-position cross-signal of the \(\beta\)-O-4' (A) substructure is observed at \(\delta_{\text{C}}\)/\(\delta_{\text{H}}\) 59.8/3.73 ppm, while a  
Figure 6: (**a**) Investigation of DES recycling on solid yield, lignin removal, and cellulose retention, (**b**) FTIR spectra of HTB and CR treated with recovered DES, (**c**) XRD spectra of HTB and CR treated with recovered DES.  
C\({}_{\gamma}\)-H\({}_{\gamma}\) correlation of \(\gamma\)-acylated A can be seen at \(\delta_{\rm C}/\delta_{\rm H}\) 63.0/4.30 ppm, and acylation probably occurs between the hydroxyl group at the \(\gamma\)-position of lignin and the carboxyl group in lactic acid [62]. It is believed that ferulate (FA) is responsible for cross-linking in the cell wall of grasses [59], and a C\({}_{2}\)-H\({}_{2}\) correlation of FA can be observed at \(\delta_{\rm C}/\delta_{\rm H}\) 110.2/7.30 ppm, implying that a slight amount of FA is still associated with lignin [64]. Furthermore, the contents of guiaacyl- (G), syringl- (S), and \(p\)-hydroxyphenyl (H)-type lignin substructures, as well as the S/G ratio are inset in Figure 7. Since the value of S/G decreases dramatically from 11.6 to 3.4, S-type lignin substructures were probably demoxylized and partially converted to G-type lignin substructures [16].