Research On The Influence Factors And Destruction Mechanism On Base Catalyzed Decomposition Of Hexachlorobezene

The POPs waste is one of the primary sources of the POPs in the regionalenvironment, and endangers the surrounding ecological system. There are stillnumerous POPs waste in stockpile needs further destruction in China. Base catalyzeddecomposition (BCD) is a non-incineration technology based on chemical reductiondechlorination reaction with high efficiency, mobility and flexibility. It has been provedto be a cost-efficiency technology for the treatment of many kinds of POPs wastes.Hexachlorobenzene (HCB) was used as a model compound in this study. The main aimsof this study were to select optimal reagent combinations, to investigate the influence ofreaction parameter on dechlorination efficiency, and to illustrate the dechlorinationpathway and mechanism. The main conclusions are as follows:An orthogonal experiment was performed to investigate the effects of differenthydrogen donor, catalyst and alkali on the HCB dechlorination efficiency. The mostsignificant factor is the alkali followed by the hydrogen donor and the catalyst. Theexperiment results indicate that there are four combinations of reagents are suitable forthe treatment of POPs: polyethylene glycol (PEG)/hydroxide, octadecane/KOH,glycerin/hydroxide, and glycerin/carbonate/Ni. The HCB dechlorination efficiency canreach~100%after3h reaction at250oC with the presence of PEG200#and KOH.The influence of temperature, and different types and proportions of alkali andPEG on the HCB removal and dechlorination efficiency was investigated withsingle-factor experiments. The HCB dechlorination rate increases with the increasing ofreaction temperature. The pseudo first order reaction kinetics rate constant increases byone order of magnitude when the reaction temperature increases from150oC to250oC.The HCB dechlorination efficiency is~100%after1.5h reaction at250oC with themolar ratio of HCB/NaOH/PEG200#=1/18/9. The HCB dechlorination rate increaseswith the increasing of alkali basicity and dosage. There is a linear relationship betweenthe square root of the ionic potential of the alkali metal cation and the logarithm of theHCB dechlorination rate constant. And the HCB dechlorination rate constant increaseslinearly with the increasing of initial alkali mass concentration. The type and dosage ofPEG has certain effects on the HCB dechlorination efficiency, but there is notmonotonous relationship between the HCB dechlorination rate and PEG molecular weight or PEG concentration. The molar ratio of PEG200#and HCB should be greaterthan3:1in order to reach a high HCB dechlorination efficiency.The high concentration HCB in activated carbon can be removed with the presenceof PEG200#and NaOH. After3h reaction at200oC, the HCB concentration decreasesfrom6705ppm to0.986ppm with a dechlorination efficiency of61.24%.Chromatography and spectrometry were used to identify the ultimate products andinvestigate the reaction mechanism. There are two different pathways in the HCBdechlorination with the presence of PEG200#and hydroxide. The primary pathway isthat the HCB reacts with PEG200#and hydroxide to form water-soluble aromaticcompounds which can be further destructed. The minor pathway is thehydrodechlorination of HCB with the hydrogen produced in the reaction to generate lowchlorinated benzenes.