Experimental Study On Hydrothermal Dechlorination Of PVC

Because of the chlorine,organic pollutants from plastics are of environmental pollution,bioaccumulation,and high toxicity,which seriously endanger the surrounding ecological environment and human health.The PVC(Polyvinyl Chloride)was chosen as parent material to conduct the experiments in this work.It intends to explore the effect of hydrothermal dechlorination(HTD)on the chlorine removal behaviors and optimize the operating parameters of HTD.To improve the HTD of chlorinate waste plastics and energy efficiency of this process,the nickel and alkaline additives were also added in the HTD to investigate their effect on the chlorine behaviors and the corresponding mechanism.The main findings are summarized as follows:For HTD experiment,three PVC samples(KL-10,S-65 and HB-65)were treated to study the effect of PVC structure,hydrothermal temperature and reaction time on the PVC dechlorination efficiency.The results showed that the worst dechlorination efficiency was achieved in S-65 due to the largest molecular weight and the most stable structure.The HTD of PVC was improved greatly with the increase of hydrothermal temperature.At a reaction time of 5h,the dechlorination efficiency of three PVC samples(KL-10,S-65 and HB-65)increased from 57.03% to 90.49%,68.63% to 90.18% and 75.60% to 90.97% when the temperature ranged from 210 to 230,increased by 58.67%,31.40% and 20.33%,respectively.Furthermore,the dechlorination efficiency was also increased by reaction time.At a temperature of 220oC,the dechlorination efficiency of PVC increased by an order of magnitude and achieved 88.26%,87.31%,and 84.75% when the reaction time ranged from 1 h to 5 h.The breakage of C-Cl bonds can be promoted by nickel during the HTD of PVC,and thus the hydrothermal temperature and reaction time could be reduced when achieving the same dechlorination efficiency.When the dosage of nickel was 0.05 M,the dechlorination efficiency of PVC(KL-10,S-65 and HB-65)was 98.08%,93.46%,and 92.73%,respectively,at a temperature of 220oC for 3h.According to the results from FTIR and SEM,the microscopic surface of hydrochar generated from HTD of PVC with nickel was more bulky and the pores were denser.Besides,dechlorination pathways among which dehydrochlorination(elimination reaction)and hydroxyl substitution are the main ones in the normal system,became diversifier in the nickel-involved reaction system.Among these pathways,the dehydrochlorination that is-Cl and –H in the PVC was directly removed from the structure in the form of HCl plays the most important role.For the hydroxyl substitution,the polyene double bonds would be further generated resulting from the chemical dehydration in the hydrothermal reaction system.According to the previous results,some alkaline additives were chosen and added into the HTD of PVC in order to improve the dechlorination efficiency.The effect of additives type and dosage on the HTD of PVC were investigated,and it was indicated that the poisoning effect of hydrochloric acid on the nickel could be reduced by alkaline additives.When 0.3M Na OH was added,the dechlorination efficiency of PVC increased from 14.89%,84.94% and 79.52% to 96.24%,91.66% and 95.45%,respectively.,Moreover,dechlorination efficiency of PVC increased firstly and then dropped with additive dosage increasing under the same hydrothermal conditions(220oC,3h).In order to investigate the mechanism of the HTD with additives,the SEM were performed to investigate the microsurface of hydrochars dechlorinated from PVC.According to the SEM,it was found that some white precipitates appeared on the surface of products,which may result in the coverage of active sites and the prohibition of HTD of PVC.This presumably accounted for the decreased dechlorination efficiency while the dosage was increased.This study provides scientific idea for clean and efficient utilization of chlorinated organic wastes.