Mechanism And Experimental Study On POP_s Degradation By Mechanochemical Method

The Stockholm Convention (SC) on POPs is a global treaty designed to protect the environment and human health from POPs. As a developing country, China faces a more complex environmental problem than developed countries. The control and reduction of POPs is a significant problem in China's environmental protection strategy. Take PCBs and PCDD/Fs for example:identified dioxin wastes released by key industries must be environmentally sound disposaled by 2015, and all PCBs-containing wastes must complete the environmentally sound disposal by 2028.This dissertation launched a series of fundamental experimental study, with aim to investigate the mechanochemical degradation mechanism of PCBs and PCDD/Fs. The main contents of this dissertation are formed by the five parts as follows.1. The relationship between the dechlorination rate of chlorinated organic and the parameters which affecting the mechanochemical degradation efficiency was experimentally studied. These parameters include:grinding time, revolution speed, materials of the milling media, ball-to-powder ratio(BPR), ball mill fill-factor, the type and ratio of the reducing agent. And organic bromine compounds were also treated by MC means, and the highest debromination rates was 27.6%.2. Mechanochemical treatment of typical dioxin precursor was experimentally studied. PCP was mixed with CaO in a molar ratio of 1:20. And the mixture was co-grinding in a planetary ball mill at the revolution speed of 400 r/min. The effects of the milling time and additives on the dechlorination rate were investigated. The results indicate more than 99% of the PCP concentration was reduced when 6 h grinding is conducted.The yield of Cl ion concentration reached 99% after 8 h grinding. Organic chlorine of PCP is gradually changed into inorganic ion-state chlorine as milling proceeding. The addition of quartz to the grinding mixture facilitated dechlorination. The resulting product was characterized by X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), thermogravimetric analysis (TG) and ion chromatography (IC). On the basis of the experimental results, the decomposition mechanism was derived that free radicals were induced by grinding on CaO surfaces and subsequently the charge transfer occurred on the organic PCP. C-Cl bonding in PCP tended to be cut off to produce some intermediates or products. Cl atom splited off from the C-Cl bond of PCP. At the same time, CaO reacted with the Cl atom to form Ca(OH)Cl intermediate. Cl substituents on PCP have been gradually removed from the structure of a benzene ring. PCP can be dechlorinated following the pathways of dechlorination and oxidation. With a series of complicated reactions induces, a mixture of CaCl2·nH2O and amorphous carbon are formed as the final products.3. The mechanochemical decomposition of the virulent polychlorinated dibenzo-p-dioxins and polychlorinated dibenzof urans (PCDD/Fs) in fly ash was studied by planetary mill. Calcium-based additives such as waste eggshell powder were added as reducing agent. Effects of milling time and rotation speed of the ball mill on degradation efficiency of PCDD/Fs for the total concentration and I-TEQ concentration were investigated. The grinding products were characterized with regard to their structure and size by scanning electron microscopy(SEM). The experimental results indicate that by using eggshell powders, more than 50%of the total concentration and I-TEQ concentration of PCDD/Fs were reduced when 8 h grinding is conducted. The degradation efficiency increases with an increase in rotation speed. It is reasonable to choose 300-400 r/min for large-scale utilization. The destruction and dechlorination reaction occoured simultaneously in mechanochemical treatment of PCDD/Fs.4. The mechanochemical decomposition of polychlorinated biphenyls (PCBs) in soil was studied using a horizontal mill. Grinding a mixture of PCBs and calcium oxide with or without quartz was conducted, and the addition of quartz facilitated dechlorination. Effects of milling time and additives on the degradation efficiency of PCBs were investigated:74%of the total concentration and 78%of the World Health Organization toxic equivalence quantity (WHO-TEQ) of PCBs were reduced after 20 hr of grinding. Formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) during mechanochemical destruction of PCBs was evaluated. The results demonstrate that the problem of PCDFs formation can be overcome.5. Density functional theory calculations were performed to obtain the structures, ionization potentials and electron affinities of 13 polychlorinated biphenyls. A popular three-parameter hybrid density functional, B3LYP, was utilized with the 6-31G(d) and 6-311G(d,p). The structures of non-ortho-chlorinated PCBs have small torsional angles. The optimized structures of the cations and anions of the all the selected PCBs tended to be more planar than the structures of the corresponding neutrals. The anions of non-ortho-chlorinated PCBs have nearly coplanar structures. The addition of an electron increases the C-Cl bond length. Except for MoCBs, the electron affinities of the selected PCBs were positive. The electron affinities and ionization potentials increase with the chlorination level of PCBs. This suggested that the higher chlorinated PCBs are inclined to be reductive dechlorinated, and the lower chlorinated PCBs tend to be oxided.