Journal of Water Resource and Protection, 2013, 5, 1227-1234 Published Online December 2013 (http://www.scirp.org/journal/jwarp) http://dx.doi.org/10.4236/jwarp.2013.512131 OpenAccessJWARP TiO2 PC500 Coated on Non Woven Paper with SiO2 as a Binder-Assisted Photocatalytic Degradation of Reactive Black 5 in Aqueous Solution Tomkouani Kodom, Akpn Dougna, Ibrahim Tchakala, Massama-Eya D. T. Gnazou, Gbandi Djaneye-Boundjou, Moctar L. Bawa Laboratory of Water Chemistry, Faculty of Sciences, University of Lom, Lom, Togo Email: tchakodotom@yahoo.fr Received September 27, 2013; revised October 28, 2013; accepted November 25, 2013 Copyright 2013 Tomkouani Kodom et al. This is an open access article distributed under the Creative Commons Attribution Li-cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT Photocatalytic discoloration kinetics of Reactive Black 5 (RB5), a vinylsulfone dye, has been studied spectrophotomet- ricallybyfollowingthedecreaseindyeconcentrationwithtimeatambientconditionsusingaflowloopreactor.UV lump, Black Light Blue (BLB) emitting at maximum wavelength of 365 nm and Ahlstrom Research Service paper con-sistentofTiO2P500coatedonnonwovenpaperwasusedrespectivelyassourceofUVlightandphotocatalyst.At naturalpH,theresultshowsthatphotolysis ofRB5anditsadsorptionin thepresence of photocatalystwasnegligible while the photocatalytic oxidation (PCO) permits 30.8% of RB5 degradation. The degradation of dye was studied under a variety of conditions such as volumetric flow rate, initial pH, photocatalyst reuse, and in the presence of electron ac- ceptorsuchassodiumpersulphate ((Na)2S2O8). The degradationrates werefoundtobestronglyinfluenced byallthe above parameters.Thecirculationflow rateof 108L/hwasthebest. TherateconstantcalculatedwhentheinitialpH was varied shows that pH 3 was more favorable for RB5 removal. Peroxydisulphate ions have the strong effect on RB5 discoloration even in dark without and with photocatalyst. When UV light was used in the presence of photocatalyst, 50 min was enough for quasi-total removal of RB5 with 28 2S O (0.2 M). Keywords: Photocatalytic Degradation; Non Woven Paper; TiO2 PC500; Reactive Black 5 1. Introduction Textileindustriescontinuetobeattractednowadaysdue tothedresssenseincreasinglygrowingofhumanbeing overtheworld.InWestAfricacountries,tanneryfacto- riesandloinclothcraftindustriesaremuch developedto produce textile named basin very valued by many peo- ple. Organics dyes are one of raw materials used and they constitute one of the larger groups of pollutants in waste- water released from these processes. Except the carcino- geniceffectofazo-dyesmuchused,textilewastewaters powderedintotheecosystemwithoutanytreatmentin- volves environmental problems such as esthetic pollution andperturbationofaquaticlife[1].Themorecomplex environmentalproblemsassociatedwiththetextilein- dustry are that organic dyes are resistant to microbial de- gradationandtheycanbeconvertedtothetoxicorcar- cinogeniccompounds.Biologicaltreatmentbecomesso inefficientfordyeremovalinwater[2].Therefore,ad- vanced oxidation methods, such as photocatalysis, have be- cameanattractivewayfortextilewastewaterstreatment [3,4]. Titanium dioxide is the most popular photocatalyst with good activity under UV light or sunlight [5]. Despitetheeffectivenessoftheheterogeneousphoto- catalysisprocess,theoperatingcostforthetotalminer- alizationofhazardousorganiceffluentsremainedhigh due to the obligation to separate the small TiO2 particles from the suspension after treatment [6], limitation of light penetrationwhentheconcentrationofTiO2increases (slurrysystem),reactorconfigurationwhichcanmaxi- mizephotocatalysisefficiency,weakactivityundersun- lightetc.Toavoidtheformersituations,immobilization of catalyst has gained much attention using different me- thodsanddifferentinertsupports.Supportcatalysthas advantage to be reused several times. Theaimofthisstudyistoanalyzethefeasibilityof discolorationofReactiveBlack5(RB5)dyeusinga T. KODOMETAL.1228 TiO2PC500supportednon-wovenfibersbyAhlstrom Firm (France). Some factors that influence the dye photo- catalytic removal were investigated. 2. Material and Methods 2.1. Catalyst Theimmobilizedphotocatalystusedinthisstudywasa commercialtitaniumdioxidephotocatalystfromAhl- stromfirm(France).ItconsistsinPC500TiO2byMil- lenniuminorganicchemicals(anatase:>99%,specific surface area 350 - 400 m2g1, crystallites mean size = 5 - 10nm).TitaniumdioxidePC500wascoatedonnon- wovenfibers(254mofthickness)usinganaqueous dispersionofcolloidalSiO2(EP1069950B1European patent)asbinder.Aspecificsurfaceareaextender(zeo- liteUOP,2000m2g1)wasusedtoincreaseadsorption properties of the photocatalyst. Table 1 shows some phy- sical and chemical characteristic of the whole paper [6]. Before the first photocatalytic test, the paper (7.8 cm 39.8cm)is weighed,washed severaltimesthen driedin anoventoremovealladsorbedwater.Thisoperationis necessary to remove non bound particles which will cre- ateturbidityinsolution.Afterdrying,thelostinmass wasca.0.136g.Beforeeachexperiment,thepaperis wash several times by distillated water. As resulting, there is a lengthening of the paper varying from 0.3 to 0.7 cm in length and in width. The pH of zero charge of photocatalyst was monitored byintroducing0.2gofphotocatalystin40mLofNaCl (0.1M)solutionatdifferentpH0.ThentheresultedpH was measured after one day, second day and finally third day. The pzc is identified as the pH at which pH (pHf pH0) is zero when plotting pH versus the initial pH, pH0 [7]. Table1.PhysicalandchemicalpropertiesofAhlstrompa- per [6]. Composition TiO2 PC50018 gm2 SiO220 gm2 Zeolite UOP2002 gm2 Physical properties Mass par unit area73.45 gm2 Thickness254 m Air permeability2,570 Lm-2s1 Tensile strength (MD)1,100 Nm1 Elongation (MD)3% Water drop2 s 2.2. Dye AsamodeldyecompoundReactiveBlack5(RB5,pu- rity55%)purchasedfromSigmaAldrichhasbeencho- sen.Itisawater-solubleazo-dyewithamolecularfor- mulapresentedinFigure1.Themaximumabsorption for RB5 is 599 nm, that wavelength was used for quanti- tativeanalysis.Solutionswerepreparedbydissolving requisitequantityofthedyeindistilledwater(3-4 Scm1).ThepHwasadjustedtoagivenvalueinthe range 1 - 9 by addition of concentrated H2SO4 or NaOH (analyticalgrades)andwasmeasuredusingaWTWpH 330i pH-meter. The natural pH of 10 mg/L of the dye varies from 5.4 to 5.8 at our laboratory room temperature. 2.3. Photocatalytic Reactor Photocatalysisexperimentswereperformedinaflow loopreactoropentoair,providedbya310.44cm2sur- face ofthinphotocatalystlayer,volumeoftreatment0.9 L solution. A UV-A Black light Blue Lamp, (15 W, and 365 nm) wasusedaslightsourceundercontinuousflowcondi- tions.Theopticalpaththroughthesolutionwas1cm, and the temperature was kept at ca. 29 - 31C. A Master- flexI/PModel77601-00pumpwasusedforsolution circulationduringalltheexperiment.Theschemeofthe reactor is shown in Figure 2. 2.4. Photodegradation Procedure The disappearance of the MO was measured using a Per- kinElmerUV/VisspectrometerLambda2.Allexperi- mentswereperformedatroomtemperatureandatmos- phericpressure.Thephotocatalyticdegradationexperi- mentswerecarriedoutbyloading900mLofdyesolu- tioninthereactor.Inatypicalexperiment,thenonwo- venpapercoated(ca.39.8cm7.8cm=310.44cm2) with TiO2 was fixed on the inner wall of the photoreactor. After adsorption equilibrium was carried out in the dark, thelightwasturnedontoirradiatethesolutionandthe first sample was taken (t = 0). Residual concentrations of thedyeinsolutionateachtimeweredeterminedfrom the calibration lines established. 3. Results and Discussion 3.1. Photocatalysis, Photolysis, Adsorption of Reactive Black 5 on Non Woven Paper Coated with TiO2 PC500 The decrease in concentration during a photocatalytic ex- perimentwouldcomefromphotolysis,adsorptionand thenphotodegradation.Itisimportanttoevaluatethe contributionofeachphenomenon.Figure 3 presentsthe curvesofphotolysisofRB5underUVlight(365nm), OpenAccessJWARP T. KODOMETAL.1229 Figure 1. The molecular formula of Reactive Black 5 (max = 599 nm, molecular weight = 991.8 gmol1). Water at room temperature Figure2.Schematicdiagramofphotocatalyticflowloop reactor (a), Photography of real reactor (b), Initial solution ofRB5(10mg/L)andtheresultingsolutionafterphotode- gradation(t=300min,Experiment8)usingUV+Photo- catalyst. Figure3.PhotodegradationofRB5(10mg/L)versuspho- tolysis and adsorption using TiO2 PC500 coated on non wo- ven paper (Ahlstrom firm), V = 900 mL, pH0 5.8, solution flow 108 L/h. adsorptionindarknessonnonwovenpapercoatedTiO2 PC500 and the photodegradation on the same paper. After210min,only1.3%ofRB5undergoesdecom- position (photolysis) when only UV light was used. This resultissimilartoresultsofKodometal.[4]andD. Chatterjee et al. [8] indicating clearly that RB5 cannot be remove efficiently by photolysis. However, removal per- centage about 13.87% was reported by Zielinska et al. in the case when UV-Visible light was used probably due to dye sensitization in visible [9]. In the presence of photo- catalystnoneilluminated,themaximumamountofRB5 adsorptionwasabout2.5%at30minwhileat210min, the amount adsorbed was 1.3% probably because this big molecule can be desorbed as the solution continues flow- ing.Kineticscurvesofthephotocatalyticdegradationof thedyeshowsclearlytheabilityofphotocatalysisof RB5removalincomparisontophotolysisandadsorp- tion. These curves followed an apparent first-order kine- tics according to Langmuir-Hinshelwood model, confirm- ed by the linear transforms ln(C/C0) = f(t) not shown here. Therateconstantofdisappearanceinmin1correspond- ingtotheslopeofthelatercureforRB5photodegrada- tion after 300 min was 12.4 104 min. 3.2. Effect of Solution Flow Speed on the Rate of RB5 Removal Oneoftheproblemsintheimmobilizedphotocatalysis process is the mass transport limitation because the cata- lyst is immobilized on a solid carrier. This limitation was overcome by complete mixing by creating sufficient tur- bulence in batch system [10] or by solution circulation in flow loop reactor. In the last case, the flow rate can be a determinativefactorforefficientlypollutantremoval.In thisstudy,thedrivingforceofpumpusedpermitsdif- ferentflowratethroughmodulationofaknob.Itisalso importanttonotethatthispumpworksbypressureon thepipecreatingavibrationoftheentirereactor.The effect of flow rate on RB5 photodegradation is illustrated byFigure4.Thisexperimentwasdonewithphotocata- lyst previously used three times. The four speeds used correspond to 52.65 L/h, 108 L/h, 152.4L/hand186L/hrespectively.Theapparentfirst orders kinetic, kapp, calculated for the four flow rates are 5.6103min1,12.7103min1,10.8103min1 and 10.7 103 min1 respectively. The flow rate of 108 L/h is more favorable for RB5 removal. It was found that the constant kapp calculated for the flow rate of 52.65 L/h wasabouttwicelowerthanthatfortheotherthreeflow rates, i.e. 108, 152.4 and 186 L/h. Duringphotocatalyticoxidation,organicsubstrates followthestepsofdiffusion,adsorption,andreaction [11].Forthehighestflowratesapplied,itcanbesup- posedthatmasstransporttowardscatalystsurfacewas beneficial to mass diffusion; therefore the rate constant k will be less affected by the flow rate. On the other hand, whenthevolumetricflowrateof52.65L/hwasused, mass diffusion was a limiting factor of the examined pro- cess.However,weobservedthatRB5degradationrate decreaseforthelasthighestflowrates(152.4L/hand 186L/h)incomparisontotheflowrateof108L/h. Theseflowratescorrespondingtosolutionspeedinthe reactor on supported photocatalyst of 14.3 ms/s and 17.2 m/s respectively combined with the strong turbulence in- ducedbythepump,arelessfavorableforpollutantdif- fusion-adsorption-reactionthan108L/h.Ifweconsider theremovalefficiencyat186,152.4,and180L/h,these OpenAccessJWARP T. KODOMETAL.1230 Figure4.ChangesofRB5concentrationintimefordif- ferentsolutionflowrates;initialdyeconcentration:10 mg/L, V = 900 mL, pH0 = natural pH. resultsareinagreementwiththoseofM.A.Behnajady andN.Modirshahlawhoreportedthatwithdecreasing volumetricflowratefrom120to60mlmi1,removal efficiency of Acid Orange 7 in a tubular continuous-flow photoreactor is increased [12]. However, their volumetric flowratesarelesshighthanthevaluesobtainedinthis study.Takingaccounttheresidenttimeofthereactant, one can clearly say that, the volumetric flow rate of 108 L/h(solutionspeed:8.6m/soverphotocatalyst)wasfa- vorable for RB5 removal. So for the rest experiment, we have used as flow rate of solution, 108 L/h. 3.3. Determination of Point of Zero Charge (PZC or pHpZC) The oxide surfaces of TiO2 and SiO2 in water are ampho- teric and could be positively charged, negatively charged or neutral depending on the pH of the solution. For below thepHofzerocharge(pHpzc),theoxidenetsurface charge is positive and above pHpzc the net surface charge is negative. Indeed, when a metal oxide is put in contact with an aqueous medium, it develops an electrical charge due to the amphoteric behavior of its surface sites, i.e., its hydroxyls groups. So it is important to know the isoelec- tricalpointespeciallyinheterogeneouscatalysisallow- ing to control or explain some phenomenon bound to the variationofpH.ThepHofzerochargewasdetermined using the method described by M. Mullet et al. [7]. Fig- ure 5 shows the variation in pH allowing the determina- tion of pHpzc. The variations in pH are explained by the proton equi- libriathatoccuratthesurfaceofphotocatalyst.Asex- pected,atpH0valuesbelowthepzcofmaterial,anin- crease in pH (pH > 0) is observed. In these range of pH, the hydroxyls groupsatthesurface ofthe paper become protonated and so positively charged. On the contrary, at pH0 values above the pzc, a decrease in pH (pH < 0) is observed.InthisrangeofpH,thehydroxylsgroupsbe- pHf pH0 Figure 5. Plot of the variation in pH = pHf pH0 in 0.1 M NaCl solution, 2 g of photocatalyst. comedeprotonatedandsonegativelycharged[7].The pzc is identified as the pH at which pH is zero. It occurs at a pH of about 5.4. This value results from four materi- als(nonwovenpaper,TiO2PC500,SiO2andZeolite). ThesmallvariationatpH2isduetotheinfluenceof SiO2 which pzc is 2.5 while for TiO2 the pzc is reported to be 6.2 [1]. 3.4. Effect of Initial pH of the Solution on the Rate of RB5 Photooxidation According to the previous result on point of zero charge, thebehaviorofthematerialwillchangewhenpHwill vary.ItiswellknownthatpHisadeterminativefactor that needs to be controlled in order to enhance the rate of degradation [2]. Six different pH included natural or free pH of RB5 were chosen to determined the effect of initial pHofsolutionontherateofRBphotooxidation.The kineticsofRB5removalobtainedatthesepHsare presentedinFigure6.NaturalpHofRB5measuredat 10mg/Lvariesfrom5.4to5.8probablyduetothein- fluence of laboratory atmosphere. Figure 6 shows clearly the pH dependence of RB5 photodegradation in the range of1to9.Thephotodegradationratewasbetterinacid medium. However, decolorization of RB5 was the fastest at pH 3 than pH 2 and pH 1. Nevertheless, the adsorption ratewashigherinthelatterpHsshowingthatsilica (pHpzc=2.5[1])asabinderperturbsactivityinacidic medium. pHmayaffectphotocatalyticoxidation(PCO)ina number of ways [13]. First,thechargeofthedyemoleculeswithionizable functional groups and the surface of the TiO2 catalyst are bothpHdependent.pHchangescanthusinfluencethe adsorption of dye molecules onto the catalyst surfaces, an importantstepforPCOtotakeplace.Thephenomenon explains well the increase in rate of RB5 removal in acid mediumsinceRB5isanacid(anionic)dyewhichad- sorption is strong. Indeed, in acid media, TiO2 surface is positivelychargedandthuselectrostaticattractionbe- OpenAccessJWARP T. KODOMETAL.1231 Figure 6. Effect of initial pH on the rate of RB5 degradation withnonwovenpapercoatedwithTiO2PC500,RB5(10 mg/L, Vsolution = 900 mL). tween sulfonic moiety of the azo dye and positive charg- es of the oxide is increased. Forthesecondview,HOcanbeformedbythere- actionbetweenhydroxideionandpositiveholeinva-lence band (h+VB) according to Equation 3. AnalkalineconditionwouldthusfavorHOforma- tion and enhance degradation. However, we observe that therateofRB5degradationwasnotpositivelyaffected inalkalinemediaandthepeuso-firstorderconstantde- creaseswhenpHincreases(Figure7).InAlkalinecon- dition, there is repulsion between RB5 and TiO2 avoiding adsorption step. Third,theTiO2particlestendtoagglomerateunder acidicconditionandthesurfaceareaavailablefordye adsorption and photon absorption would be reduced [14]. Thisbehaviorwouldexplainthedecreaseinratewhen pH varies from 2 to 1. Moreover, the adsorption at these pHswasimportantprobablybecauseofthepresenceof SiO2,asbinderwithapointofzerochargearound2.5. Thisstudyshowsthatthephotocatalyticprocessoccurs mainlyonthephotocatalystsurface(directphotocataly- sis), and not in the bulk solution (indirect photocatalysis), thusadsorptionofsubstratesisessentialfortheirphoto- catalytic degradation [15]. 3.5. Reuse of the Photocatalyst during RB5 Photodegradation Thepossibilityofreusingthephotocatalystwasexam- inedtodeterminethecosteffectivenessofthemethod. Successive tests of the photocatalytic degradation of Re- activeBlack5usingthesamephotocatalystwereper- formed. The regeneration of the catalyst can be done in a very simple way. After the degradation, the photocatalyst wasthenthoroughlywashedwithdistilledwaterand reusedfordegradationwithafreshlotofdyesolution. Theexperimentalresultsofelevensuccessiveexperi- ences interrupted by photocatalyst washing are shown in Figure 8. Figure7.EffectofinitialpHonthepseudo-first-order rate constantforthephotocatalyticoxidation(PCO)ofRB5 (normalized curve). Figure8.Successivetestsofthephotocatalyticdegradation of Reactive Black 5 using the same photocatalyst. An increase of photocatalytic efficiency was observed from the first experiment until eighth (8th) experiment and thenaslightlossofefficientwasobserved.Theevolu- tion oftheapparentfirstorderconstantisrepresentedin Figure 9. One can clearly see that a strong and gradual increase ofefficiencyoccursformbeginningtoeighthtest.The apparentfirstorderconstantwasincreased6.6timesat 8th test in comparison to the first test. The monitoring of adsorptionduring30minindarknessindicatedthatthe amountadsorbedatsuccessiveexperimentincreasesin thesamedirectionasphotocatalyticoxidationrate(Ta- ble 2). Indeed, when the photocatalytic was wetted with water, there was an elongation of the paper about 3.6% in length and2.5%inwidthexplainingtheincreaseofadsorption and then photoactivity. On the other hand, the amorphous silica used as binder wouldhaveanimportantinfluenceonthephotoactivity of TiO2 P500. In fact, F. Thevenetet al. [16] noticed, in their study done in gas phase, that the inhibitor effect of SiO2isnottobeneglectedtakingcareaboutefficiency. In liquid phase as in present work, repetitive use coupled with intermediate washing could cause silica detachment. OpenAccessJWARP T. KODOMETAL.1232 Table2.Adsorptionamountatsuccessiveexperimentsof RB5 oxidation with the same paper. Experiment numberAmount of dye adsorption during 30 min21.2% 32% 43% 54.3% 66.4% 86.9% 96.8% 115.2% Figure9.Evolutionofapparentfirstorderconstant,kapp, according to successive tests of RB5 removal with the same paper coated with TiO2 PC500. TiO2 particles are then denuded leading to an increase of photoactivity. However, there is a limitation consistent of layer saturation with byproducts which could be removed by washing the paper in acid solution. This result shows thatthephotocatalystcanbeusedseveraltimes.How- ever,itisnecessarytotakecareofthemanipulationof the paper to avoid all liberation of the powder. 3.6. Effect of Addition Peroxydisulphate Ions Upon irradiation of TiO2, electrons on the surface of the semiconductor are produced at the conduction band, and positive holes areformedin thevalanceband.Theelec- tronsandholescaneitherrecombineandproducether- malenergy,orinteractwithothermolecules.Theholes may react either with electron donors in the electrolyte or withhydroxideiontoproducepowerfuloxidizingspe- ciessuchashydroxylradicals,whichoxidizeorganic compoundsonthesurface[17].Accordingtothis,the relevantreactionsatthesemiconductorsurfacecausing the degradation of dyes can be expressed as follows: 2 2TiO h TiO h e 2 VB SH O h H OH (2) VB SHO h HO 2(3) 2 CBe O O (4) degradation productsSDye HO (5) degradation productsVBDye h (6) To avoid recombination, so increase photocatalytic ef- ficiency,additionalofagoodelectronscavengeristhe suitableway.Hydrogenperoxide(H2O2)andperoxydi- sulphateions(28 2S O ),strongoxidants,havenbeenpro- posedtoplaythisrole.Wehavefocusedthisstudyon effectofperoxydisulphateontherateofRB5photode- gradation. Kinetics of RB5 disappearance in the presence of 28 2S O are presented in Figure 10. Figure10showsthatperoxydisulphatehasastrong effectontherateofRB5oxidation.Weremarkthatthe solutionfadedevenindarkandwithoutTiO2showing thattheseionsareabletooxidizeRB5themselves.V. Augugliaroetal.havereportedthisbehavioronMethyl Orange photodegradation using TiO2 P25 in slurry form. However,itwasfoundthattherewasnomineralization in these conditions when total organic carbon was moni- tored. Due to this behavior, experiments were conducted inthepresenceof 28 2S O butwithoutTiO2andwithout irradiation,inthepresenceofwithTiO2and withoutirradiationandfinallyinthepresenceof 28 2S O 28 2S O withTiO2underirradiation.Thepercentagesofcolor abatement at 20 min are reported in Table 3. Astheinitialconcentrationsofincreases,the effectontherateofRB5abatementincreasesinevery condition.Theseresultcomparedtothecaseofabsence of 28 2S O 28 2S O showthatRB5disappearancewasveryfast, althoughisshowedtobemoresignificantinrespectto thatofcorrespondingheterogeneoussystems: 28 2S O + TiO2 and 28 2S O + TiO2 + h. In presence of 28 2S O the solutionofRB5fadedquicklyandturnstorosebefore total discoloration. Without possibility to monitor TOC measurement due tothelakeofTOCanalyzer,wehavemeasuredquickly the absorbance at different wavelength when the solution was rose at 20 min in two cases (Figure 11). These absorbencies lower than the absorbance of RB5 at initial concentration of 10 mg/L (0.395 - 0.4 a.u), show clearlythedisappearanceofReactiveBlack5.Theby products absorb very weakly in the range of wave lengths studied(Visible).Thisshowsthatthesebyproducts,cer- tainlystillorganicin nature,arederived fromthecleav- ageofazobonds,becausethecleavageof-N=N-bonds leadstothedecolorizationofdyes[18].Indarkness 28 2S O ionsareabletooxidizedirectlyRB5molecules. Inthepresenceofphotocatalyst,theincreaseofRB5 (1) OpenAccessJWARP T. KODOMETAL.1233Table 3. RB5 (10 mg/L) abatement at 20 min in presence of peroxydisuphate ions. Experiment conditions Color abatement at 20 min (%) 228S O (0.05 mol/L) + UV31.1 228S O (0.05 mol/L) + TiO2 PC500 + UV61.6 228S O (0.1 mol/L) + Dark46.5 228S O (0.1 mol/L) + UV73.6 228S O (0.1 mol/L) + TiO2 PC500 + Dark54.4 228S O (0.1 mol/L) + TiO2 PC500 + UV84.1 228S O (0.2 mol/L) + Dark71.1 228S O (0.2 mol/L) + TiO2 PC500 + UV94.3 TiO2 PC500 + UV, first experiment0.4 TiO2 PC500 + UV, height experiment13.1 Figure 10.effect on PCO of RB5, 10 mg/L, V = 900 mL,concentrations (0, 0.1, and 0.2 mol/L). 22 8S O 8 22S O Figure11.AbsorbanceevolutionofsolutionsofRB5+ (0.05 M) after 20 min of contact. 22 8S O disappearanceisduetoadditionaleffect1)decreasein pH (0.05 M of pH0 = 3.2; 0.1 M of 28 2S O 28 2S O pH0 = 2.7 and 0.2 M of pH0 = 2.5) involving the increase of RB5 adsorption even in dark and under il- lumination,2)inhibitionoftheelectron-holerecombi- nationatthesemiconductorsurfaceilluminatedaccord- ing to the following equations [19,20]: 28 2S O 2SOCB22 8 4 4S O SO e 2SO(7) 4 2 4SO H O HO H 28 2S O(8) Theadditionalproductionofhydroxylradicalsunder illumination would lead to the best activity in these con- ditions. 4. Conclusion The results of our study showed that TiO2 PC500 coated on nonwovenpaperilluminated withUV lightcouldbe efficientlyusedtodegradetheReactiveBlack5(RB5). Photodegradationefficiencyofdyewasneglectedwhen photolysiswascarriedoutintheabsenceofTiO2and smallintheabsence oftheUVlight.Thekineticcurves ofthephotocatalyticdegradationfollowapseudo-first kineticsorderwithrespecttodyeconcentration.There- sults indicated that the degree of degradation of RB5 was obviouslyaffectedbyvolumetricflowrate,stepofthe reuse, pH, and addition ofas electron scavenger. Thehigh rate constant,kapp, wasobtainedwith volumet- ricflowrateof108L/h.Thisflowpermitsthebestad- sorption-diffusion-reactionprocessinflowloopreactor usedinthiswork.SuccessiveexperimentsofRB5re- moval at natural pH with the same paper show the gain in photocatalytic efficiency until height use follows by loss ofefficiencyprobablyduetosaturationofthepaperby byproducts.However,theuseofTiO2coatedonAhl- strom non-woven fibers could be a promising method as it can avoid the tedious final filtration of Titania in slur- riesandpermitthephotocatalystrecycling.Thezero pointchargedeterminedwasabout5.4.Hence,atmore acidic pH values, the particle surface is positively charg- ed, while at pH values above 5.4, it is negatively charged. In this study, it has been shown that the degradation rate forthemodelcompoundunderinvestigationisstrongly influencedbythereactionpH,wheretheefficiencyof degradationrateforthedecompositionofthedyewas betteratpH3,whereasitwaslowerinalkalinemedia. Thepersulphateionscanefficientlyoxidizethedyein theabsenceofTiO2andligh.Underillumination,the efficiencywasmaximizedduetoadditionalproduction of hydroxyl radicals further to peroxydisulphate reaction with photogenerated electron in conducting band of TiO2. Theseresultssuggestthatphotocatalysismaybeenvis- agedasamethodforthetreatmentofdilutedcolored wastewaters,inparticular,intextileindustriesorloin- cloth factories in West-Africa countries. OpenAccessJWARP T. KODOMETAL. OpenAccessJWARP 1234 5. Acknowledgements TheauthorsthanktheInternationalFoundationforSci- ence(IFS)andtheWorldAcademyofScienceforAd- vancementinDevelopingCountries(TWAS)fortheir precious financial support for this research. The authors also wish to thank Ahlstrom Research and Servicesforsupplyingthephotocatalyticnonwovenpa- perviaDr.Ing.BertrandGombertofUniverstyofPoit- iers. 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