Study On Degradation Of Hexachlorobenzene In Soil By Mehcanochemical Method And Its Reaction Mechanism

Persistent organic pollutants（POPs）do great harm to the ecology and human beings.The historical legacy,leakage and contaminated sites of POPs need to be disposed and repaired.Mechanochemical technology has great advantages in the field of POPs pollution control due to its thorough reaction,no solvent consumption,simple operation and no secondary pollution.In this study,hexachlorobenzene（HCB）contaminated soil was selected as the object.The economic and efficient combination of mechanochemical treatment additives was selected,with determined parameters of the ball milling process.And the mechanochemical degradation mechanism of HCB in soil was explored to support the application.The main conclusions are as follows:The effects of alkaline materials,neutral materials,natural minerals and some solid wastes on the degradation effect of hexachlorobenzene in soil were studied.Alkaline materials performed the best,and the degradation rate of hexachlorobenzene increased with alkalinity.The materials such as Fe₃O₄,albite,calcined dolomite and red mud also presented great effect.The combination of albite and Fe₃O₄ displayed the highest degradation efficiency of HCB in soil,and the degradation rate reached 73.5%in 2h.Though,little effect was found on the destruction of HCB with calcium magnesium phosphate fertilizer on the basis of albite as the main grinding agent,it can be used as a fertilizing means to complete the supplement of nutrients during the remediation of contaminated soil.Single factor experiments were carried out to study the effects of material ratio,ball size ratio,ball milling time,rotating speed and additive amount on the degradation of HCB.It was found that the degradation rate of hexachlorobenzene increased with the increase of ball to material ratio,reaction time,rotating speed and additive amount.The effects of soil texture type,soil organic matter content,soil moisture content and compound polluted soil were also explored on the mechanochemical degradation of HCB.The degradation rate of HCB in different soil types was in the order of Sand>loam>clay.The content of soil organic matter was closely related to the degradation effect of HCB.Due to the application of a large amount of exogenous humic acid,it was closely combined with HCB,resulting in enrichment and improving the degradation effect.With the increasement of moisture content in soil,degradation effect of HCB got worse.For Pb²⁺,Cd²⁺and HCB contaminated soil,the removal of HCB and fixation of Pb²⁺and Cd²⁺can be achieved simultaneously by mechanochemical method,and the adverse effects of different kinds of pollutants on each other’s can be ignored.The degradation mechanism of HCB in soil was studied by means of chemical analysis and characterization analysis.The results of spectrum show that the main products are low chlorobenzene,such as C₆H₅Cl,C₆H₂Cl₄,C₆HCl₅,etc.,which proves the degradation path of HCB.The results of the analysis of the milling mixtures show that the continuous action of high density mechanical force makes the mixtures fine,with the surface structure destroyed and the surface defects produced,which leads to lattice failure and the decrease of crystallinity.During this period,Si-O and Al-O fracture,which increased the content of reactive oxygen in the reaction system,and produced phenoxy on the surface of the material,and increased the activity of the reaction.At the same time,Fe²⁺is transformed into Fe³⁺,which provides free electrons for the mechanical and chemical reactions of materials.Under the joint attack of free radicals and free electrons,C-Cl was broken down in HCB molecules,then dechlorination and mechanochemical destruction of HCB were achieved.