Research On The Low-temperature Thermal Dechlorination Pathways Of Chlorinated POPs Contained In Solid Wastes

At present,China had a large amount of solid wastes contains chlorinated aromatics persistent organic pollutants?POPs?which were the historical accumulation.Moreover,in the course of producing of pesticide,combustion and heat treatment process could be generating unintentionally POPs.Therefore,POPs widely exists in various erath environment.Chlorinated POPs have three kinds of characteristics which will lead to cancer,teratogenic and the mutation.And most of them are well-known for their persistence,bioaccumulation,long-range transport potential and toxicity.Chlorinated POPs will damage human lives and environment,and the disposal of POPs is a serious environmental problem.Low-temperature catalytic degradation using metal oxides is a safe,high yield and economy productive alternative technology that is thought of as the most promising method for POPs treatment owing to its non-incineration and mild treatment condition.It is of great applied value and science significance to study the thermal degradation mechanism for site remediation,solid waste treatment and improvement of the ecological environment of the city.Using hexachlorobenzene?HCB?as a model chlorinated aromatics POPs compound,the dechlorination potential of eight kinds of metal oxides were investigated.Then,the metal oxide possess high catalytic activity was compounded with transition metal oxide to form homogeneous solid solutions,the effects of three kinds of two-component metal oxides on the degradation of HCB at low temperature.The aim of this study was to speculate the synergic degradation pathway of HCB under the experimental condition and to discuss the security and universality of the technology,based on the generation and concentration variation of the intermediate products.The conclusions were summarized as follows:?1?Different metal oxides such as ?-Al2O3,?-Al2O3,CaO,MnO2,?-Fe₂O₃,?-Fe₂O₃,Ni2O3 and CuO were used for HCB degradation in sealed systems at temperature of 350 ?under air atmosphere for 1 h.The results indicated that the dechlorination activity was related to the type and dosage of metal oxides.HCB dechlorination rate increased significantly with dosage.When mass ratio of metal oxide to HCB was 100:1,CaO,?-Fe₂O₃,Ni2O3 and CuO were much more effective than other metal oxides with HCB degradation of 65.5%,100.0%,100.0% and 52.4%,respectively,while the dechlorination rate reached 54.4%,81.9%,77.5% and 46.1%,respectively.?2?The major ingredient of two-component metal oxides catalyst was CaO,the second composition was transition metal oxide such as ?-Fe₂O₃,Ni2O3 and CuO,which possess high dechlorination/ hydrogenation catalytic activity.The results showed that dechlorination capacity increased when CaO and ?-Fe₂O₃,Ni2O3 or CuO.HCB dechlorination rate increased significantly with transition metal oxide dosage,and CaO/?-Fe₂O₃ has the best catalytic effect.When mass ratio was mCaO: m?-Fe₂O₃: mHCB= 100:5:1,dechlorination efficiency of HCB was dramatically enhanced.During treatment at 350 °C for 20 minutes,the HCB degradation rate reached 100.0%.And during treatment for 50 minutes,the dechlorination rate?D2?reached about 100.0%.?3?According to the quantitative analysis of residual HCB and intermediates,lower chlorinated chlorobenzenes were detected.By the catalysis of ?-Fe₂O₃ and Ni2O3,DCB-,TrCB-and TeCB-were all detected,when reacted with other six kinds of metal oxides,dechlorinated products barely existed,and by the catalysis of three kinds of twocomponent mixed metal oxides,dechlorinated products were more.The results indicating that a dechlorination/hydrogenation pathway of HCB existed obviously.?4?All treatments showed a certain degree of discrepancy in chlorine balance after low-temperature thermal dechlorination.Raman spectroscopy was used to detect the HCB reaction products with ?-Fe₂O₃,CaO or CaO/?-Fe₂O₃,and the concentration of HCB was 20 mg·g-1?20% w/w?.There was no significant changes in peak of ?-Fe₂O₃ before and after treatment occurred.After reacting with Ca O or CaO/?-Fe₂O₃,HCB sample's color would deepen.And the typical amorphous carbon Raman peaks were present,these findings indicate that HCB generated polymerized products after low-temperature thermal dechlorination,amorphous carbon was one of the end product of HCB.This explaining the phenomenon of discrepancy in chlorine balance and indicating the presence of dechlorination/ polymerization pathway.?5?It is worthy to be mentioned that the dechlorination/ hydrogenation process of chlorobenzene compounds is relate to thermodynamics,based on the effect of reaction time on the distribution of products,as well as HCB was easy to generate stabilize product.The species of the ultimate products of the dechlorination reaction between CaO and HCB were identified by Raman spectroscopy.The ultimate products were CaCO3,amorphous carbon,and CaCl2.At the same time,these products covered in catalyst surface,which result in the deactivation of catalyst.