PHOTOCATALYTIC DEGRADATION OF WASTE WATER ON. THIN FILMS OF TiO2

82Journal of Natural Gas ChemistryVol.10 No.1 2001PHOTOCATALYTIC DEGRADATION OF WASTE WATERON THIN FILMS OF TiO2Wu ZhenghuangDepartment of Applied Chemistry, Taijyuan University of TechnologyTaiyuan, Shanxi 030024Abstract The degradation of organic phosphorous pesticide waste waterusing thin films of TiO2。 which was prepared in an atmospheric verticalchemical vapor deposition system, was studied. The results show that thewafer material for coating TiO2, the photocatalytic time, the TiO2 crystalphase, the pH value and the concentration of pesticides in wastc waterinflucnce the degradation rate. These facts indicate some potential forphotocatalytic treatment of waste water by uliling sunlight.Keywords TiO2 thin films, photocatalytic degradationIntroductionIn recent years, the photocatalytic degradation of inorganic and organic polutants byusing TiO2 powder have been studied!1!. But it is evident that in any waste purificationproccss the filtration and resuspension of semiconductor powders should be avoidcd, ifpossible. In this paper, we will discuss the feasibility of photocatalytic degradation oforganic phosphorous pesticide by mean of TiO2 thin flms and seek better technologicalconditions to improve the efficiency of the photocatalytic process.ExperimentalTiO2 thin flms by CVDThe n-type TiO2 on wafer was prepared in the :中国煤化工MYHCNMHGReceived March 8, 2000;revised version received July 4, 2000Wu Z H etal. Photocatalytic Degradation of Waste Water on Thin Films ofTiO233晶四11123舟13空14目3、文15Fig.1 Flow chart of preparing TiO2 thin flm1.0xygen 2.nitrogen 3. pressure reducing valve 4.quality flowmeter 5. pressurereducing valve 6.manometer 7.vacuum valve 8.TTIP vapor room 9.water vapor room .10. flowmeter 11.deposition room 12.vacuum meter 13.pressure reducing valve14.filter 15.vacuum pump 16. electromagnetic valveThe preparation of TiO2 thin film by CVD mainly consists of three parts. The first isthat TTIP was heated in a quartz tubc and maintained at 60°C, then flowed intodeposition room with nitrogen as the carrier gas. The second is that oxygen tube wasinserted into deposition room. The third is that water vapor was carried into depositionroom by nitrogen. In the deposition room the nozles for coating substrate were placed.The distance betwecn the nozzles and the film is about 2.5 cm. The flm growing timewas about 0.5 h~2 h. The thickncss of the film was mecasured by wcighting and thecrystalline structures of the flm were characterized by XRD using Cu Ka on a standard 0~ 20 diffractormeter. For a film prepared by TTIP there were six peaks with higherintensities at 20 values of 47.70, 55.40, 61.66, 69.23, 75.30 and 76.37 corresponding toplanes of (200), (211), (213), (220), (215) and (301), respectively. The results indicatedthat the deposited film are polyerystallite anatase.Photocatalytic experimentPut 0.01 L solution of organic phosphorous pesticide (3.75x 10+4 mol/L) and TiO2 thinfilm in a quartz tube, tum on ultraviolet light (h =中国煤化工n air bubbleflow into the quartz tube, then let it flow out ofYHC N M H Golution. Thedistance between the ultraviolet light and the TiO2 thin flm is 8 cm. The timc oflightening was 0.5 h~3 h. Fig.2 shows the flow chart of ultraviolet light decgradation of84Journal of Natural Gas ChemistryVo1.10 No.1 2001waste water. The concentration of organic phosphorous pesticide after degradation wasanalyzed on a 721 UV equipment.43Fig.2 Flow chart of photocatalytic degradation of waste water1. ultraviolet light 2. quartz tube 3. TiO2 thin f!m 4. air bubbleResults and discussionEffect of ilumination timeThe initial concentration of organic phosphorous pesticide in solution was 3.75x 10*4mol/L. The copper wire as wafer was coated with TiO2 thin flm. The relationshipbetwcen the photodegradation efficiency of pesticide and illumination time is shown inTable 1.Table 1 Rclations of conversion and ilumination timeIllumination time I min204060Conversion/ %6.0845,988.0It is clcar that the photodegradation efficiency increased with illumination timeincreasing. Becausc the longer ilumination time on the thin film surfacc, the energyadsorbed by TiO2 surface is more than the band-gap energy (3.2 eV). The energy excitedelectron jumped from valence-band to conduct-gap, hence the hole-electron pairs in theTiO2 thin film surface formed. The hole-electron pairs formed will react with 02 and H20,producing .0H and吃, which are necessary for '中国煤化ion of pesticide.As long as thc illumination time is long enougMYHC N M H Gvill completelyphotodegrade into PO2 .Wu Z H et al. Photocatalytic Degradation of Waste Water on Thin Films of TiO,85Effect of solution pHThe effect of pesticide solution pH on the photodegradation efficiency is showm inTable 2.Table 2 Effect of pesticide solution pH on conversionpH1~23~46~79Conversion / %91.263.5.45.93.3Table 2 shows that when solution pH is 1~2 the conversion of pesticide to PO2 is upto 91.2%. The convcrsion decreased with the solution pH increasing. The reactions of02 and H+ on the TiO2 thin fiIm during photodegradation arc as follows;202 +2H*→H2O2 +O2H2O2+c-→.0H + OH~It is clear that higher H+ concentration is in favor of the production of .OH, so the .photodegradation efficiency was higher.Effect of coated wafer materialUnder the conditions of pcsticide concentration of 3.75x 10-4 molL, ilumination timeof 40 min, the effect of coatcd wafer material on the photodegradation efficiency isshown in Table 3.Table 3 Effect of coated wafer material on convcrsionCoatcd waferSiTiCuConversion/ %1.7610.78It can been seen from Table 3 that different coated wafers have different conversions.The photodegradation efficiency decrcased as follows: Cu wire> Ti net> Si wafer, AsCu wire coated with TiO2 film can form a battery on the TiO2 surface, the recombinationof produced hole-electron pairs can bc prevented by oxidation-reduction occuring on theCu wire surface. Hencc coaling Cu wire中国煤化工can cnhancephotodegradation cfficiency.:YHCNMHGEffect of crystal phase86Journal of Natural Gas Chemistry Vol.10 No.1 2001The pesticide concentration was 3.75x 10-4 mol/L. The crystal phase of TiO2 thin filmon Ti net is amorphous, anatase and rutile. The illumination time was 40 min, the effectof crystal phase on photodegradation efficiency is shown in Table 4Table 4 Effect of crystal phase on conversionCrystal structureAmorphousAnataseRutileConversion/ %2.5510.200.7It can be seen from Table 4 that the anatase photodegradation efficiency was higherthan that of amorphous and rutile.Effect of initial concentration of pesticideThe effcct of initial pesticide concentration on photodegradation efficiency is shownin Table 5.Table 5Effcct of initial pesticide concentration on conversionInitial concentration/ mol:L-I2.50x 10-33.75x 10-41.12x104Conversion / %10.245.960.3Table 5 shows that as the initial concentration of the pesticide incrcased, theconversion of pesticide decreased. This is because that as the initial concentrationincreascs, more and more pesticide substancc are adsorbed on the TiO2 surface, but theilluminate time and intensity of light are constant and the 02 and .0H formed on TiO2surface ate therefore constant, so the relative number of 02 and .0H attacking thepesticide compounds decrease. The amount of formed phosphate decreased and thcphotodegradation efficicncy decreased2), on the other hand, according to the reaction ofphotodegradation:H2O一→0H~+ H+when pesticide concentration is lower the H2O is higher, which means more .OH can beproducd, so the photogradation efficiency of trea中国煤化工:d.CNMHGWu Z H et al. Photocatalytic Degradation of Waste Water on Thin Films of TiO287ConclusionsFrom the resultsabove, it can be seen that TiO2 can degrade pesticidephotocatalytically. The best conditions are as follows: the ilumination time is longer, thepH of pesticide solution is 2; the copper wire is used as the coated wafer; the crystalphase of TiO2 is anatase and the initial concentration of pesticide is lower. It is clear thatusing sunlight may convert harmful substrate into harmless one.References1. Control Water Polution, Wastewater Standard Superior. Zhongguo Gongye Jianzhu(Construct Industry of China), 1985, 3852. Zhao Mengyue, Chen Shifu, Tao Yaowu. Photocatalytic Degradation of OrganophosphorousPesticides Using Thin Film of TiO2. Huaxue Keji Shengwu Jishu (Chem. Tech. Biotechnol,),1985, 64: 339~344中国煤化工MYHCNMHG