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Degradation of Recalcitrant Organic Micropollutants by Vacuum-ultraviolet in Water

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Tutor: MaJun
School: Harbin Institute of Technology
Course: Municipal Engineering
Keywords: Vacuum-UV (VUV),advanced oxidation process,micropollutant,hydroxyl radical,kinet
CLC: TU991.2
Type: Master's thesis
Year:  2012
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Water pollution by micropollutants is a current challenge all over the world.Recalcitrant Micropollutants, e.g., endocrine disrupting chemicals (EDCs),polycyclic aromatic hydrocarbon, pharmaceuticals and personal care products(PPCPs), phenolic compound, are of great concern for their high toxicity,bioaccumulation, and persistence in the environment. Traditional processes areinefficient on recalcitrant organics removal, especially for the low concentration ofpollutants.Ultraviolet wave (UV) is an established disinfection method, and the UVenhanced technologies, eg., UV based advanced oxidation technologies, areattractive for high efficiency, environmental friendly characteristics, and theaffordable cost.This study was focused on the efficient removal of recalcitrant organics, andproposed a Vacuum-UV (VUV) technology. The effects of main influential factorson the degradation of benzoic acid (BA) by VUV, which was chosen as a probehereinafter, and kinetic analysis was carried out. The degradation mechanism of BAwas also investigated and proposed. Finally, typical micropollutants bisphenol A(BPA), dimethyl phthalate (DMP), and nitrobenzene (NB) were selected, and theirremoval by VUV was also investigated in real surface water matrix.The results shows that the degradation efficiency of BA increased along withthe increasing temperature and UV dosage. pH had little impact on BA degradationin the range of5-9. Water matrix showed various impacts on the degradationefficiency of BA. The presence of natural organic matter (NOM), bicarbonate, andferrous ion inhibited the degradation, while the presence of chlorine ion had slightlypositive promotation. The degradation of BA in the VUV system agreed well withpseudo-first order kinetics and the apparent activation energy was calculated to beEaapp=(32.50¡À3.05) kJ mol-1.The contribution of some reactive radicals to BA degradation in the VUVsystem was studied. The inhibition of BA degradation by tert-butanol stronglysupports that hydroxyl radical formed by water photolysis was responsible for themain degradation of BA in VUV system. The minor influence of dissolved oxygenon the BA degradation indicated reductive species, hydrated electron and hydrogenatom, made little contribution. Determination and quantification of intermediateswere also carried to probe the mechanism. The formation of ortho-hydroxybenzoicacid, para-hydroxybenzoic acid and meta-hydroxybenzoic acid were quantifiedduring the degradation process of BA. Further study showed that there was an obvious accumulation of3hydroxybenzoic acids followed by subsequent depletionduring the course of BA degradation. Further study on the degradation mechanism inVUV system is needed. Furthermore, the presence of dissolved oxygen indeedimproved the elimination of TOC in the processes, suggesting the enhancement ofmineralization of organics in the VUV systems.VUV could also degrade low level of bisphenol A (BPA), dimethyl phthalate(DMP) and nitrobenzene (NB) efficiently. Even though under low temperature, thedegradation efficiency by VUV was attractive. Though inhibited by water matrix,the VUV system still showed high efficiency in removing low level of BPA, DMP,BA, and NB. As a new advanced oxidation process, VUV process possesses highefficiency, and can be potentially applied. More importantly, the lack of requirementof any oxidant make it an environmental friendly technology in water treatment, andis a promising alternative technology in drinking water treatment.
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