Study On The Degradation Of Polyvinyl Chloride By Three-dimensional Electrochemical Reactor

With the rapid development of the plastics industry,a large amount of plastic waste is discarded into the environment,causing serious damage to the ecological environment.Polyvinyl chloride（PVC）is widely used because of its low cost,wide application range and good stability.At present,the degradation methods of PVC mainly include pyrolysis,biodegradation and photodegradation.Pyrolysis usually requires high temperature,high pressure and dechlorination process,so it has the disadvantages of high energy consumption and high cost.In comparison,the condition of biodegradation is mild and the cost is low,but its degradation ability is limited and requires a long degradation time.Photocatalytic degradation is promising,but it suffers from environmental limitations and the high cost of photocatalysts.In this study,PVC was degraded by handmade three-dimensional electrochemical reactor for the first time.Optimal reaction conditions were determined by single factor experiments to efficiently degrade PVC.In addition,the changes of morphology and chemical properties of PVC,as well as the formation and degradation of microplastics in the reactor were introduced in detail.Furthermore,the degradation mechanism of PVC in three-dimensional electrochemical reactor was revealed by analyzing the role of free radicals and degradation intermediates.The main conclusions are as follows:（1）Voltage,PVC loading and reaction temperature are vital factors affecting the degradation efficiency of PVC.The optimal reaction conditions were determined by single factor experiments with 15 V,4.0 mg PVC/g tourmaline loading and 90℃reaction temperature.Under the reaction conditions,after 12 hours of degradation,the concentration of Cl^-was 343.7 mg/L,the concentration of TC was 250.9 mg/L,and the degradation rate of PVC was 51.9%.（2）During the degradation process,PVC gradually changed from transparent to milky white,and finally turned yellow,from smooth and flat to rough,and finally broken and powdered.After 12 hours of degradation,the contents of carbon and chloride in PVC decreased,the oxygen content increased,and oxygen containing groups such as C=O and O-H were introduced.Carbonyl index（CI）increased from0.51 to 1.29.The Mw and Mn decreased by 21.6%and 28.9%,respectively.The results showed that PVC underwent the chemical bonds C-C and C-Cl breaking and oxidation under the action of electric field.Meanwhile,PVC were gradually degraded from high molecular weight polymers to low molecular weight oligomers.（3）In the three-dimensional electrochemical reactor,PVC was degraded and broken into microplastic fragments,which fall off the surface of the particle electrode into the solution.The shape of microplastics were mainly fibers and fragments.By indirect oxidation,these microplastics gradually broke from large size to small size and even degraded into dissolved organic matter.The amount of microplastics decreased with the increase of reaction time.It was indicated that three-dimensional electrochemical reactor can also degrade microplastics.（4）Hydroxyl radical（^·OH）played an important role in the degradation of PVC.Through the analysis of intermediate products,the degradation pathway was obtained.After PVC was dechlorinated to form conjugated polyene,it might be oxidized under the action of^·OH to form oxygenated compounds such as alcohols,carboxylic acids and aldehydes,or cyclized to form benzene series such as ethylbenzene.Then,these organics were degraded into short chain alkane compounds,and finally mineralized into CO₂and H₂O.