## 3 Results
### Effect of HT Dechlorination on Functional Group Existing in PVC  
The functional groups of the PVC and the hydrochar were analyzed by FTIR spectrometry and the spectra are shown in Figure 4. The spectra were interpreted according to previously published data [9; 21; 23; 26; 27]. Figure 4a shows the spectrum of the original PVC. The strong absorption bands at around 2914 cm\({}^{-1}\) and 2851 cm\({}^{-1}\) were attributed to -CH\({}_{2}\) asymmetric and symmetric stretching [27], certifying the existence of the methylene (-CH\({}_{2}\)-) group. The absorption band at 2817 cm\({}^{-1}\) existing in PVC, which was assigned to C-H stretching in the phase, diminished in the hydrothermally dechorinated products. The C-H stretching band on neighboring CHCl groups at 2969 cm\({}^{-1}\) disappeared after the hydrothermal process [27]. The absorption bands at around 1427 cm\({}^{-1}\) and 1332 cm\({}^{-1}\) were ascribed to the C-H wagging and bending vibration [12]. The bands at 1255 cm\({}^{-1}\) corresponded to the skeletal vibration (Cl-C-H). The bands at 1097 cm\({}^{-1}\) and 964 cm\({}^{-1}\) were ascribed to the C-C stretching and -CH\({}_{2}\) wagging, respectively. Besides, the bands between 600 cm\({}^{-1}\) and 700 cm\({}^{-1}\), 834 cm\({}^{-1}\) could be attributed to C-Cl stretching vibrations. The existence of plasticizer could be verified by the absorption bands of PVC at 487 cm\({}^{-1}\) and 428 cm\({}^{-1}\). After the HT dechlorination, the bands at about 2920 cm\({}^{-1}\), 2850 cm\({}^{-1}\), 1427 cm\({}^{-1}\) and 960 cm\({}^{-1}\) were obviously weakened. The peak at 2914 cm\({}^{-1}\) of pure PVC assigned to -CH\({}_{2}\) stretching was shifted to 2922 cm\({}^{-1}\) (as shown in Figure 4b) and 2921 cm\({}^{-1}\) (as indicated in Figure 4c) in the hydrochar,  
Figure 3: Effect of residence time on HT dechlorination of PVC. while the skeletal vibration at 1255 cm\({}^{-1}\) for pure PVC shifted to 1249 cm\({}^{-1}\) and 1250 cm\({}^{-1}\), and the trans -CH\({}_{2}\) wagging of PVC presenting at 964 cm\({}^{-1}\) shifted to 969 cm\({}^{-1}\) and 968 cm\({}^{-1}\). The peaks at around 1330 cm\({}^{-1}\) and 1250 cm\({}^{-1}\) (corresponding to the C-H bending vibration and the skeletal vibration Cl-C-H) almost disappeared, implying that the skeletal chain was destroyed and part of the chlorine was removed by the hydrothermal reaction. The nucleophilic substitution, cyclization, hydrolysis, oxidation, and thermal degradation intricately proceeded and overlapped to generate aromatic compounds and O-functionalized low-molecular-weight compounds [1,3]. The possible pathway could be summarized as shown in Figure 5, in which the dechlorination proceeded in two ways as dehydrochlorination and substitution (depicted in the dashed box). As for the substitution, one possible and direct way is to replace the -Cl with -OH, a given two-carbon segment. The other way is to replace both the -H and -Cl with two -OH, a given two-carbon segment. After that, the intramolecular and intermolecular dehydration would happen to generate H\({}_{2}\)O and HCl. In that case, the functional groups including -OH and C=C-OH would be detected in the hydrochar. In addition, if the dehydrochlorination played a main role, the C=CH would appear. As shown in Figure 4, the bands at about 3017 cm\({}^{-1}\) assigned to C=CH appeared and the intensity of C=C (corresponding to 1600 cm\({}^{-1}\)) was increased, indicating the elimination of HCl during the HT dechlorination of PVC. The peak at 750 cm\({}^{-1}\) ascribed to C-Cl disappeared, indicating the deformation of the functional group and also the skeletal chain. The HT dechlorination of PVC was evidenced by the decreasing intensity of the C-Cl stretching vibration or even the disappearance of chlorine-containing functional groups. The increasing intensity of the C=CH and C=C stretching vibrations in the hydrochar indicated that the dehydrochlorination (elimination reaction) was a main route for chlorine removal during HT dechlorination.