## 1 Introduction  
Poly (vinyl chloride) is one of the most widely applied chlorinated plastics, and it has been utilized in many areas such as packaging, wrappings, bottles and containers because of its low price and good performance [1]. Accordingly, large amounts of PVC-containing wastes are being generated and needed to be disposed of. Basically, the recycling of polymeric solid wastes can be carried out by re-extrusion, mechanical, chemical and energetic recovery [2]. For PVC-containing wastes, several methods have been proposed for recycling. Among them, thermal recycling (pyrolysis, gasification, hydrogenation, etc.) and incineration have been the main technologies widely applied. However, PVC is rather resistant to incineration because of its high chlorine content (56.8%). As for the thermal degradation, it suffers from toxic emissions and generate a lot of hydrochloric acid (HCl), which can cause corrosion in the furnace and lead to the release of trace amounts of further harmful gases such as organohalogen compounds and substantial amounts of chlorinated dioxins and furans, resulting in severe environmental and health problems. For landfilling, phthalate and heavy metals (derived from plasticizer and stabilizer) such as lead, cadmium, and tin can leach out, which might contaminate the soil and ground water. In particular, phthalate can be an endocrine disruptor. Recently, it has been suggested that phthalate leakage from PVC products, such as blood bags, baby pacifiers, and toys, can cause health problems [3]. Therefore, dechlorination is an optimal option for the safe disposal of chlorinated organic wastes. Many studies have investigated PVC degradation by means of both thermal and wet treatments. [PERSON] et al. reviewed the thermal degradation of PVC with emphasis on the recent progress in PVC pyrolysis [4]. The dehydrochlorination of PVC mainly happened at a temperature range of 250-320 \({}^{\circ}\)C, depending on the material, the stabilizers and also the additives [5]. According to the thermogravimetry-mass spectrometry (TG-MS) analysis, the degradation could be divided into three stages [6], proceeding from an apparent zero-order reaction to an apparent first-order reaction [7]. To improve the chlorine removal efficiency, many other methods based on pyrolysis have been developed recently. [PERSON] et al. [8] utilized a gas-liquid fluidized bed reactor to investigate the chlorine behavior and reported that the dechlorination efficiency (DE) was dependent on the reaction temperature and 99.5% of Cl could be removed at a reaction temperature of 300 \({}^{\circ}\)C in 1 min. [PERSON] et al. [9] studied the co-pyrolysis behavior of PE (poly ethylene), PVC and PS (poly styrene) and stated that the dechlorination of PVC occurred simultaneously with the rearrangement and cyclization of conjugated polyene, leading to the formation of dioxins. Parts of methyl (-CH\({}_{3}\)) and methylene (-CH\({}_{2}\)-) groups disappeared during the co-pyrolysis of PVC with PS/PE. Pyrolysis is efficient for removing chlorine from PVC, whereas a high temperature was required and the problems of secondary pollution such as emission of HCl, dioxins and corrosion of facilities still existed. Hydrothermal treatment (HT) is another efficient technology which has the advantage of presenting simple and fast reaction rates, and of being a homogeneous reaction without the limitations of mass transfer [4]. It has been widely applied in the field of wastes to energy (WtE) [1; 3; 10; 11; 12; 13; 14; 15; 16]. In this system, supercritical or subcritical water works as a solvent and reagent for reactions of organic compounds [17]. [PERSON] and [PERSON] [18] reported that 100% of chlorine could be removed at 19.3 MPa and 300 \({}^{\circ}\)C. To dechlorinate PVC under mild conditions and also to improve the DE, some additives were utilized. Among them, NaOH was one of the most popular additives.