Research On The Preparation Of Microwave Electrodeless UV Lamps And Their Applications On Photo-degradation Of Organic Wastes In Water

The paper reported the study of a new-type ultraviolet(UV) light source called microwave electrodeless UV lamp(MEUVL),which works on gas discharge excited by microwave energy without any help of electrodes.Four kinds of MEUVLs filled with different binary gas mixtures of Ar-Hg,Kr-I₂,Kr-Br₂ and Ar-Se respectively were prepared and their UV emission spectra were measured.The Ar-Hg lamp(185 nm) and Kr-I₂ lamp(206.2 nm) were used to degrade rhodamine B(RhB) dye and dimethyl phthalate(DMP) without addition of oxidants.The results showed that direct photolysis of pollutants by 185 and 206.2 nm radiations supplied a new alternative way to remove organic pollutants from water.The 185 nm vacuum-UV(VUV) intensity of the Ar-Hg lamp amounted to 12% of the total UV output,about 1.7 times as high as that of the conventional low-pressure mercury lamp.The Kr-Br₂ and Ar-Se lamp provided intense radiation at 291 nm and 254 nm respectively.The 207 nm radiation of Kr-Br₂ lamp and the 204/207 nm radiations were all very weak and unable to be used for the photo-degradation of organic pollutants in water in this paper.The Kr-I₂ lamp provided strong emission of atomic iodine line at 206.2 nm and much weaker VUV radiations at 178.3/180.1/183/184.4/187.6 nm of other atomic iodine lines.The 206.2 nm radiation intensity depended much on the Kr gas pressure, filling mass of I₂ and the microwave feeding power as well.With Kr gas pressure 1 Torr and 1₂ filling mass 0.5 mg,both at particularly low levels,the 206.2 nm radiation intensity reached a maximum value and would ascend when the microwave feeding power increased.It was found that by placing a piece of Mo-foil embedded in quartz to the lamp bulb,the 206.2 um radiation intensity could be effectively enhanced by almost 1.2 times and meanwhile the starting time of the Kr-I₂ lamp was shortened from 15 s to less than 5 s.The photodegradation of RhB dye in aqueous solution was achieved by 185 nm radiation emitted from the Ar-Hg lamp without addition of oxidants.The dye degradation followed pseudo-first order kinetics.A strong pH dependence of RhB degradation was observed and acidic condition was preferred.Precipitates were found to be generated during RhB degradation when the initial concentration of RhB solution was above 60mg·L^-1.For 100mg·L^-1 RhB at pH 4.0,the mass of precipitates produced was nearly half of the initial amount of RhB in the solution after irradiation for 75 min.The corresponding degradation ratio of RhB reached up to 99.4%and COD_Cr reduced 77.4%.The precipitates generated during degradation was attributed to the collision reactions between the radical fragments produced by the direct photolysis of RhB by 185 nm.The impact of HO·radicals produced by water absorbing 185 nm radiation was another important pathway for RhB degradation.The photodegradation of RhB dye was also achieved by 206.2 nm radiation emitted from the Kr-I₂ lamp without addition of oxidants.The degradation behavior of RhB by 206.2 nm radiation appeared very similar to the case with 185 nm radiation. Precipitates were also found to be generated during 50 mg·L^-1 RhB degradation.After irradiation for 150 min,the degradation ratio of RhB reached up to 99.5%and COD_Cr reduced 58.9%.The precipitates generated during degradation was also attributed to the direct photolysis of RhB by 206.2 nm,which should be the only pathway to trigger the subsequent degradation reactions.The photodegradation of DMP in aqueous solution achieved by 206.2 nm radiation emitted from the Kr-I₂ lamp without addition of oxidants also indicated the impact of direct photolysis of DMP by 206.2 nm was effective.For DMP with initial concentration of 60 mg·L^-1,the degradation process followed the pseudo-first order kinetics.After 50 min,the degradation ratio of DMP reached up to 92.5%with its orginal carbonyl and phenyl structure well destructed.