## 2 Materials and Methods
### Hydrothermal Dechlorination  
Figure 1 shows a schematic diagram of the batch-type hydrothermal dechlorination facility utilized in this work. The experimental system consists of an autoclave reactor, a cylindrical electronic heat jacket, a circulation cooling water system, a carrier gas supply cylinder and a gas-sampling system with a water-cooling bath. The PVC was hydrothermally dechlorinated in a batch-type autoclave reactor whose volume is around 500 mL with a vertical size of 190 mm and a diameter of 58 mm.  
Before the HT dechlorination, the PVC pre-mixed with deionized water as the mass ratio (dry basis) of 1:15 (7 g PVC and 105 g deionized water) water was loaded into a glass tube with a volume of 450 mL. A certain amount of sodium hydroxide (0%, 1%, 3% and 5% of PVC amount) was also added and then well mixed. After that, the tube was moved into the reactor which was subsequently sealed. Then the reactor was placed in the cylindrical electronic heat jacket and the argon with a purity of 99.999% supplied from a cylinder was used to replace the atmosphere in the reactor for about 5 min. Finally, the reactor was heated up to the target temperature and the mixer was kept stirring at a speed of 50 rpm to ensure the uniformity of reaction temperature within the samples. During the reaction, the temperature was measured by a thermocouple installed inside the reactor to get a feedback signal for controlling the furnace and maintained within \(\pm\)5 \({}^{\circ}\)C of the target temperature for the desired time (residence time). After a certain time, the electric heater was turned off and the reactor was cooled down by a circulation cooling water system. When the temperature was lower than 90 \({}^{\circ}\)C, the outlet valve was slightly opened to collect the gas as shown in Figure 1. After the pressure and temperature were fell down to the atmospheric and room temperature, the glass tube was taken out to collect the products, which were separated into two parts by filtration. The solid parts noted as hydrochar was well dried in an electric oven at 103-105 \({}^{\circ}\)C for 24 h and stored in a desiccator for further analysis. The amount of Cl in the liquid sample was determined by potentiometric titration and the chlorine content in hydrochar was determined according to GB/T 7139-2002 (eqv ISO 1158: 1998), in which the flask combustion-ion titration method was adopted.  
In the current research, four reaction temperatures 180 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, 220 \({}^{\circ}\)C and 240 \({}^{\circ}\)C were used for the HT dechlorination experiments, respectively. Five residence times 0, 15, 30, 45, and 60 min were utilized for 220 \({}^{\circ}\)C to investigate the effect of the residence time on the HT chlorination of PVC. The residence time for the other temperatures was fixed as 60 min and the NaOH dosage ranged from 0% to 5% of the mass of PVC. Each HT dechlorination experiment and test was repeated three times respectively to reduce the experimental errors.  
Figure 1: Schematic diagram of the equipment for hydrothermal dechlorination of PVC.  
After obtaining the chlorine content in the liquid, the HT dechlorination efficiency (DE) could be derived by the following equation:  
\[\text{HT dechloination efficiency }(\text{DE},\ \%)=\frac{\text{Cl content in liquid}}{\text{Cl content in PVC}}\times 100\% \tag{1}\]