High-efficiency promotion on dechlorination of polyvinyl chloride in subcritical water treatment by introducing waste concrete

Abstract

Polyvinyl chloride (PVC) is one of the most widely used plastics in the world. Due to the generation of corrosive HCl and chlorine-containing organic pollutants during the recycling process, waste PVC can cause great environmental risks. At the same time, a large number of waste concrete in the construction industry have not been effectively utilized. Subcritical water (SubCW) has showed good prospect for treatment of PVC due to that dechlorination can be carried out with lower energy consumption. In this study, a high-efficiency and low-temperature SubCW treatment for dechlorination of PVC using waste concrete as an enhancer was proposed. The effects of temperature, residence time, solid-to-liquid ratio and PVC-to-Concrete ratio on dechlorination efficiency, chlorine distribution and weight loss ratio were studied. The PVC dechlorination efficiency could be enhanced to as high as 95.04% by introducing waste concrete. Significant dechlorination occurred under the conditions of 220 ℃, 60 min, solid-to-liquid ratio of 1:30 g/mL, and PVC-to-Concrete ratio of 4:1. Under the optimal conditions, all the removed chlorine from PVC was transferred into the liquid phase in the form of inorganic chlorine. Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscope (SEM), and elemental analysis were used to analyze the PVC dechlorination mechanism. The dechlorination pathway of PVC was found to include both direct dehydrochlorination and hydroxyl substitution. The enhancement mechanism of the waste concrete on the PVC dechlorination could be attributed to the rapid capture ability of CaCO3 for HCl. The SubCW-Concrete process proposed in this study was beneficial to both the low-temperature dechlorination of PVC and the resource utilization of waste concrete.