TiO2掺杂

Colloids and Surfaces A:Physicochem.Eng.Aspects 384 (2011) 519–528Contents lists available at ScienceDirectColloids and Surfaces A:Physicochemical andEngineeringAspectsj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /c o l s u r faPhotocatalytic degradation of mixed azo dyes in aqueous wastewater using mesoporous-assembled TiO 2nanocrystal synthesized by a modified sol–gel processPatladda Wongkalasin a ,Sumaeth Chavadej a ,b ,Thammanoon Sreethawong a ,b ,∗a The Petroleum and Petrochemical College,Chulalongkorn University,Soi Chula 12,Phyathai Road,Pathumwan,Bangkok 10330,Thailand bCenter for Petroleum,Petrochemicals,and Advanced Materials,Chulalongkorn University,Bangkok 10330,Thailanda r t i c l ei n f oArticle history:Received 23November 2010Received in revised form 8April 2011Accepted 8May 2011Available online 13 May 2011Keywords:TiO 2nanocrystalMesoporous assembly Photocatalysis Degradation Azo dye mixturea b s t r a c tIn this work,several operational parameters affecting the photocatalytic degradation of mixtures of two azo dyes –Acid Yellow (AY)monoazo dye and Acid Black (AB)diazo dye –including types of dye,initial dye concentration,photocatalyst dosage,dissolved oxygen,initial solution pH,and water hardness concentration,were investigated by using a mesoporous-assembled TiO 2nanocrystal photocatalyst.The experimental results showed that the initial concentration of both azo dyes in the dye mixture greatly affected the degradation efficiency.It was interestingly found that the degradation efficiency of one azo dye could be improved by the presence of the other with suitable concentrations.At AY and AB concentrations of 2.5and 5mg/l,the optimum conditions for the highest degradation efficiency of both azo dyes were a photocatalyst dosage of 10g/l,a dissolved oxygen of 7.5mg/l,and an initial solution pH of 4.5.Moreover,even though water hardness negatively affected the degradation efficiency,the pH adjustment could be applied for enhancing the degradation of the dyes present in the extremely hard solution.© 2011 Elsevier B.V. All rights reserved.1.IntroductionOrganic dyes belong to the largest groups of pollutants normally found in wastewaters discharged from textile and other relevant industrial processes.The wastewaters containing organic dyes can cause a serious environmental problem since they are extremely resistant to microbial degradation,and thus can be unavoidably converted to toxic or carcinogenic compounds [1–3].Particularly,an abundant class of synthetic coloring organic compounds is azo dyes,such as Methyl Orange,Acid Yellow,and Acid Black,which can be characterized by the presence of one or more azo groups (–N N–)linked between aromatic rings in their molecular structure [4,5].A number of technologies for removal of dye pollu-tants have been investigated,such as adsorption,biodegradation,advanced oxidation,and membrane filtration [6–10].However,the above mentioned processes are unable to completely eliminate the dye pollutants,since they mostly transfer the dye compounds from aqueous to another phase,leading to secondary pollution problems and posing a major drawback of such the treatment processes.In∗Corresponding author at:The Petroleum and Petrochemical College,Chula-longkorn University,Soi Chula 12,Phyathai Road,Pathumwan,Bangkok 10330,Thailand.Tel.:+6622184144;fax:+6622154459.E-mail address:thammanoon.s@chula.ac.th (T.Sreethawong).addition,some cost-intensive regeneration/recycle steps of adsor-bents,as well as sludge post-treatment,are necessary.Among the new oxidation methods called “advanced oxidation processes”(AOPs),heterogeneous photocatalysis using titanium dioxide (TiO 2)as a photocatalyst is highly considered a promis-ing destructive technology for the treatment of polluted air and water because of a number of advantages [4,11–14].Firstly,this process can destroy the polluting compounds by decomposing into ending non-toxic substances with the aid of light irradiation in UV or near-UV region.Secondly,environmentally friendly mate-rials can be employed as a semiconductor photocatalyst,especially the most widely used TiO 2.Thirdly,this process can be carried out under mild conditions,i.e.room temperature and atmospheric pressure.Fourthly,it can bring about the complete degradation of most organic pollutants,without causing the secondary pollution problems.Finally,it is currently receiving an increasing attention because of the use of sunlight as the clean and renewable source of irradiation.The photocatalytic processes originate from the irradiation of light with energy equal to or greater than the band gap energy of the TiO 2photocatalyst (∼3.2eV for the anatase phase TiO 2).When the photocatalyst absorbs the irradiating light with suitable wavelengths,the electrons and holes are produced and transferred along the crystalline lattice to the photocatalyst surface.The elec-trons and holes trapped on the photocatalyst surface can react with0927-7757/$–see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.colsurfa.2011.05.022520P.Wongkalasin et al./Colloids and Surfaces A:Physicochem.Eng.Aspects384 (2011) 519–528both water and dissolved oxygen molecules to generate several oxygen active species,such as OH•,OH2•,O2•−,and H2O2.These active species can further attack organic dye molecules to cause them decomposed.The detailed mechanisms for the photocatalytic dye degradation using the TiO2photocatalyst have already been demonstrated in a number of literatures[4,12–20].In our previous work,the photocatalytic degradation of Methyl Orange(MO)monoazo dye was investigated by compara-tively using a synthesized nanocrystalline mesoporous-assembled TiO2and several non-mesoporous-assembled commercial TiO2 photocatalysts[21].The results showed that the synthesized nanocrystalline mesoporous-assembled TiO2photocatalyst cal-cined at an optimum temperature of500◦C exhibited the highest efficiency in the MO dye degradation.This indicated that the photo-catalyst mesoporous structure with uniform pore size distribution plays a significant role in enhancing the reactant accessibility and the subsequent photocatalytic reactions.Since the photocat-alytic degradation of dye mixtures has been so far rarely reported; therefore,in this present work,by following the previous work’s accomplishment,the experimental investigation was performed on the photocatalytic degradation of mixtures of two azo dyes –Acid Yellow(AY)monoazo dye containing1azo group and Acid Black(AB)diazo dye containing2azo groups–as model mixed contaminants present in textile wastewater,by using the mesoporous-assembled TiO2nanocrystal photocatalyst synthe-sized by a sol–gel process with the aid of a structure-directing surfactant.The influences of various operational parameters affect-ing the photocatalytic degradation of azo dye mixtures,including type of dye,initial dye concentration,photocatalyst dosage, dissolved oxygen,initial solution pH,and water hardness con-centration,were systematically studied with the ultimate goal to obtain,for thefirst time,the optimum conditions for the high-est degradation efficiency of the dye mixtures in extremely hard aqueous wastewater.2.Experimental2.1.MaterialsTetraisopropyl orthotitanate(TIPT,Ti(OCH(CH3)2)4,Merck), laurylamine hydrochloride(LAHC,CH3(CH2)11NH2·HCl,Merck), acetylacetone(ACA,CH3COCH2COCH3,Carlo Erba Reagents),Acid Yellow(AY,C16H9N4Na3O9S2,Nacalai Tesque),Acid Black(AB, C22H14N6Na2O9S2,Nacalai Tesque),hydrochloric acid(37%HCl, Lab Scan Asia),sodium hydroxide(NaOH,Lab Scan Asia),calcium chloride(CaCl2,Ajax Finechem),magnesium chloride(MgCl2,Ajax Finechem),and distilled water were used in this present work.All chemicals were of analytical grade and used without further purifi-cation.The TIPT was used as the titanium precursor for synthesizing the mesoporous-assembled TiO2nanocrystal photocatalyst.The LAHC was employed as a structure-directing surfactant,behaving as a mesopore-forming agent,as well as a gel formation-inducing agent since a gelation cannot occur in the absence of the LAHC. The ACA,as a ligand-modifying agent,was applied to slow down the hydrolysis step of the titanium precursor by forming a less water-sensitive TIPT/ACA complex[22].2.2.Photocatalyst synthesis procedureThe mesoporous-assembled TiO2nanocrystal photocatalyst was synthesized via a sol–gel method with the aid of the structure-directing surfactant[21,23].For a typical synthesis,a2.405g of the ACA was initially mixed with6.821g the TIPT to obtain a TIPT-to-ACA molar ratio of unity.The mixed solution was homogenized by gently shaking.Afterwards,a60ml of0.1M LAHC aqueous solution (1.331g of the LAHC dissolved in60ml of distilled water)with a pH of4.2was added to the ACA-modified TIPT solution until the TIPT-to-LAHC molar ratio of4:1was obtained.The mixture was then continuously stirred at40◦C overnight to obtain transparent yel-low sol.After that,the yellow sol-containing solution was placed into an oven at80◦C for a week in order to achieve the complete gel formation.Subsequently,the gel was dried at80◦C overnight to remove the solvent,which was mainly the distilled water used in the preparation of the LAHC aqueous solution.The dried gel,so-called zero gel,wasfinally calcined at500◦C for4h to eliminate the LAHC surfactant,and the desired mesoporous-assembled TiO2 nanocrystal photocatalyst was consequently obtained.2.3.Photocatalyst characterization techniquesThe N2adsorption–desorption isotherms of the synthe-sized mesoporous-assembled TiO2nanocrystal photocatalyst were obtained by using a nitrogen adsorption–desorption apparatus (Quantachrome,Autosorb-1)at a liquid nitrogen temperature of−196◦C.The Brunauer–Emmett–Teller(BET)approach using adsorption data over the relative pressure(P/P0)ranging from 0.05to0.35was used to determine the specific surface area of the synthesized photocatalyst.The Barrett–Joyner–Halenda(BJH) approach was used to determine the mean pore size and pore size distribution of the photocatalyst sample.The sample was degassed at250◦C for2h to remove any physisorbed gases before the mea-surement.The sample morphology was observed by a scanning elec-tron microscope(SEM,JEOL,6500FE)and a transmission electron microscope(TEM,JEOL,2000CX)operated at15and200kV,respec-tively.The particle size of the synthesized TiO2photocatalyst was determined from both the SEM and TEM images.X-ray diffraction(XRD)was used to identify the crystalline structure of the synthesized TiO2photocatalyst.A Rigaku PMG-A2XRD system generating monochromated Cu K␣radiation with a continuous scanning mode at the scanning rate of2◦/min and operating conditions of40kV and30mA was used to obtain the XRD pattern.Crystallite size(D)of the photocatalyst was calcu-lated from the line broadening of X-ray diffraction peak according to the Sherrer formula[24],as shown in the following equation: D=Kˇcos(Â)where K is the Sherrer constant(normally taken as0.89), is the wavelength of the X-ray radiation(0.15418nm for Cu K␣),ˇis the full width at half maximum(FWHM)of the diffraction peak measured at2Â,andÂis the diffraction angle.The UV–visible spectrum of the synthesized TiO2photocatalyst was obtained by using a UV–visible spectrophotometer(Shimadzu, UV-2550)in the wavelength range of200–600nm at room tem-perature with BaSO4as the reference.Afterwards,the absorbance spectrum was used to estimate the band gap energy(E g,eV)of the photocatalyst.The absorption onset wavelength( g,nm),or band gap wavelength,was the crossing point between the line extrap-olated from the onset of the rising part and x-axis of the plot of absorbance as a function of wavelength( ,nm).The E g was then determined by using the following equation[25]:E g=1240gThe point of zero charge(PZC)of the synthesized TiO2photocat-alyst was examined by suspending the photocatalyst in deionized water with different initial pHs,by using HCl and NaOH as the pH-adjusting agents.The mixture was continuously stirred for1h to achieve the complete adsorption/desorption equilibrium.After that,the solution pH was measured and recorded asfinal pH.TheP.Wongkalasin et al./Colloids and Surfaces A:Physicochem.Eng.Aspects 384 (2011) 519–528521PZC was determined as the point,where the initial pH is equal to the final pH.2.4.Photocatalytic activity testing and evaluationThe photocatalytic degradation experiments of dye mixtures were performed in an open Pyrex glass reactor.Single and mixed AY and AB dye solutions with various concentrations in the range of 0–10mg/l were freshly prepared and used for the photocat-alytic activity testing.A desired amount of the synthesized TiO 2photocatalyst was suspended in the aqueous solutions of both sin-gle and mixed dyes under various reaction conditions by using a magnetic stirrer.Prior to the photocatalytic activity testing,the continuously suspended mixture was left for 30min in a dark envi-ronment to establish the adsorption/desorption equilibrium of the dye molecules on the photocatalyst surface.The photocatalytic reaction was started by exposing the mixture in the reactor to UV light irradiation (4lamps,11W low-pressure Hg lamp,Philips).The suspension was periodically withdrawn from the reactor,and the samples were then centrifuged to remove the photocatalyst pow-der.The supernatant samples were analyzed for the concentration of both dyes by the UV–visible spectrophotometer and for the total organic carbon present in the dye solution by a TOC analyzer (Shi-madzu,TOC-5000A)to follow the dye degradation.The degradation efficiency was calculated by the following equations:UV–visible analysis :Degradation efficiency (%)=C 0−C C 0×100TOC analysis :Degradation efficiency (%)=TOC 0−TOC TOC 0×100where C 0and C denote the concentration of both dyes obtained from the UV–visible analysis at t =0and t =t ,respectively.TOC 0and TOC denote the total organic carbon obtained from the TOC analysis at t =0and t =t ,respectively.Moreover,the pseudo-first-order rate constant (k )of the pho-tocatalytic degradation of dyes was determined by the following equations [4,21,26]:lnC 0C=ktor lnTOC 0TOC=ktThe k value was calculated from the slope of the straight-line plot between ln(C 0/C )or ln(TOC 0/TOC)and time (t ).Both the degra-dation efficiency and the pseudo-first-order rate constant were used as the indicators to assess the effects of various reaction con-ditions on the photocatalytic degradation of both dyes.To study the effect of dissolved oxygen on the photocatalytic degradation efficiency,ultra-high pure N 2,air,and O 2gases were used to vary the dissolved oxygen concentration during the photo-catalytic experiment.A dissolved oxygen (DO)meter (Orion,Model 860)was used to measure the value of the dissolved oxygen in the solution.In addition,it has to be noted that the symbols used to represent the photocatalytic activity results (Figs.8–12are as follows:[“X”for “1st number”:“2nd number”];where “X”can be either “AY”or “AB”from the UV–visible analysis,or “TOC”from the TOC analysis;“1st number”is the AY concentration in the dye mixture;and “2nd number”is the AB concentration in the dye mixture.3.Results and discussion3.1.Characterization results of the synthesized TiO 2photocatalystThe N 2adsorption–desorption analysis is a very reliable technique commonly employed to investigate the mesoporous structure of any powder sample.The N 2adsorption–desorption20406080100Relative pressure, P/P 0A d s o r b e d a m o u n t (c m 3(S T P )g -1)10203040Pore diameter (nm)D v (m m 3n m -1g -1)Fig.1.N 2adsorption–desorption isotherms (a)and pore size distribution (b)of the synthesized mesoporous-assembled TiO 2photocatalyst.isotherms of the synthesized TiO 2photocatalyst shown in Fig.1(a)exhibit typical IUPAC type IV pattern with dominant H2-type hys-teresis loop [27].The hysteresis loop reveals the presence of the mesoporous structure (mesopore size between 2and 50nm)in the synthesized photocatalyst.An abruptly increasing step in N 2adsorption volume was observed in the P /P 0range of 0.5–0.9because of the capillary condensation of N 2into the pores;and this is a predominant characteristic of mesoporous materials.The pore size distribution of the synthesized TiO 2photocatalyst shown in Fig.1(b)is very narrow and monomodal,indicating its exquisite quality.It can be observed that the pore size of the synthesized TiO 2photocatalyst is quite small and uniform.The textural proper-ties of the investigated TiO 2photocatalyst are as follows:a specific BET surface area of 84.3m 2/g,a mean pore diameter of 6.18nm,and a total pore volume of 0.16cm 3/g.The SEM and TEM images (for both low and high magnifica-tions)of the investigated TiO 2photocatalyst are shown in Fig.2.The SEM and low-magnification TEM images demonstrate the forma-tion of TiO 2aggregates comprising three-dimensionally disordered uniform-sized nanoparticles,where the high-magnification TEM image reveals the high crystallinity of the photocatalyst,as clearly evidenced by the presence of lattice fringes.The average parti-cle size of the TiO 2nanoparticles is as small as approximately 10–15nm.The mesopores of the aggregated TiO 2nanoparticles can be clearly observed from the SEM and low-magnification TEM images.Therefore,from the N 2adsorption–desorption,SEM,and TEM results,the mesoporous structure of the synthesized TiO 2pho-tocatalyst originated from the assembled aggregation of the TiO 2nanoparticles.522P.Wongkalasin et al./Colloids and Surfaces A:Physicochem.Eng.Aspects 384 (2011) 519–528Fig.2.SEM image (a)and TEM images with (b)low magnification and (c)high magnification of the synthesized mesoporous-assembled TiO 2photocatalyst.The XRD pattern of the synthesized mesoporous-assembled TiO 2photocatalyst is shown in Fig.3(a).The dominant peaks at 2Âof about 25.2,37.9,47.8,53.8,and 55.0◦,which represent the indices of (101),(004),(200),(105),and (211)planes (JCPDS Card No.21-1272)[28],respectively,well correspond to the crys-talline structure of the pure anatase phase TiO 2,without other phase impurity,e.g.the rutile phase,which can be detrimental for the photocatalytic activities as experimentally observed in our pre-vious works [21,23].The crystallite size of the investigated TiO 2nanocrystal calculated from the line broadening of the anatase (101)diffraction peak by the Sherrer formula is 13.64nm,which is consistent with the particle size estimated from the TEM analysis.Therefore,each nanoparticle observed by the TEM analysis can be possibly considered as a single crystal.In order to investigate the light absorption ability of the synthe-sized mesoporous-assembled TiO 2photocatalyst,the UV–visible spectrophotometry was employed.The UV–visible spectrum of the synthesized TiO 2photocatalyst is shown in Fig.3(b).It can be clearly seen that the absorption band of the TiO 2photocatalyst is approximately in the range of 200–400nm.The dominant absorp-tion band at this low wavelength range elucidated the existence of Ti species as the tetrahedral Ti 4+.This absorption band plausi-bly originates from the electronic excitation of the valence band O2p electron to the conduction band Ti3d level [29].The absorp-tion onset wavelength of the synthesized TiO 2photocatalyst was observed at 387nm,which is exactly conformed to the band gap energy of the anatase phase TiO 2of 3.2eV.The point of zero charge (PZC)is also a very important parameter used to determine the surface charge property of the photocata-lyst under a particular environment.The point of zero charge is defined as the pH,at which the surface charge of the photocata-lyst is neutral,i.e.the number of positive charges on the surface being equal to that of negative charges.Fig.4shows the correla-tion between initial pH and final (equilibrium)pH of the aqueous solutions suspended with the synthesized mesoporous-assembled TiO 2nanocrystal.From the plot,the PZC of the synthesized photo-catalyst is approximately 5.8,which is similar to the reported value of 6.0±0.3for TiO 2in literature [30].3.2.Photocatalytic AY and AB degradation resultsIn the photocatalytic activity testing experiments,the UV–visible spectrophotometry was used to investigate the effects of various reaction parameters on the degradation of single and mixed azo dyes.The UV–visible spectra profiles of AY and AB measured separately are shown comparatively in Fig.5.From the figure,the max values of AY and AB at the absorption maxima were 425and 619nm,respectively.For the mixture of AY and AB,P.Wongkalasin et al./Colloids and Surfaces A:Physicochem.Eng.Aspects 384 (2011) 519–528523(a)(b)6050403020I n t e n s i t y (a .u .)2theta (degree)Anatase (101)Anatase (105)Anatase(004)Anatase (200)Anatase(211)00.010.020.030.04600500400300200A b s o r b a n c e (a .u .)Wavelength (nm)Absorption onset= 387 nm Fig.3.XRD pattern (a)and UV–visible spectrum (b)of the synthesized mesoporous-assembled TiO 2photocatalyst.it is clear that the max values of both dyes did not change upon their mixing at different irradiation times during the course of photocatalytic degradation over the mesoporous-assembled TiO 2nanocrystal photocatalyst,as shown in Fig.6.This indicates that the decreases in absorbance at their max values can be reliably used to evaluate the degradation performance for each dye.Gupta2345678765432F i n a l p HInitial pHFig.4.Correlation between initial pH and final pH at equilibrium of the aqueous solution suspended with the synthesized mesoporous-assembled TiO 2photocata-lyst.00.020.040.06700600500400300λAY,max = 425 nmλAB,max = 619 nmWavelength (nm)A b s o r b a n c e (a .u .)Fig.5.UV–visible spectra of AY and AB measured separately.00.020.040.06Wavelength (nm)A b s o r b a n c e (a .u .)Fig.6.Exemplified UV–visible spectra of a mixture of AY and AB as a function of irradiation time during their photocatalytic degradation using the synthesized mesoporous-assembled TiO 2photocatalyst.et al.[31]also reported that the photocatalytic degradation of a mixture of Crystal Violet and Methyl Red dyes could be investi-gated independently by using their corresponding max values of 590and 430nm,respectively.3.2.1.Effect of initial dye concentrationThe six dye system series prepared by mixing AY and AB at dif-ferent concentrations (see Table 1)were used to investigate the effect of initial dye concentration on the photocatalytic degradation performance in order to find their suitable mixing concentrations by using the synthesized mesoporous-assembled TiO 2nanocrystal photocatalyst.The investigated initial AY and AB concentrations for their mixtures are shown in Table 1.There were 2sets of the photocatalytic experiments:the first set with different initial AY concentrations at 2.5,5,and 10mg/l and various initial AB concen-trations in the range of 0–10mg/l (named as Series I,Series II,and Series III,respectively),and the second set with different initial ABTable 1Various conditions of initial dye concentration of AY and AB used for photocatalytic activity testing.System seriesInitial dye concentration (mg/l)AYABSeries I 2.50,2.5,5,7.5,10Series II 50,2.5,5,7.5,10Series III 100,2.5,5,7.5,10Series IV 0,2.5,5,7.5,10 2.5Series V 0,2.5,5,7.5,105Series VI0,2.5,5,7.5,1010524P.Wongkalasin et al./Colloids and Surfaces A:Physicochem.Eng.Aspects 384 (2011) 519–528(a)(b)0.020.040.060.080.1107.552.50R e a c t i o n r a t e c o n s t a n t , k (h -1)Initial AB concentration (mg/l)AY for Mix I AY for Mix II AY for Mix III AB for Mix I AB for Mix II AB for Mix III0.020.040.060.080.1107.552.50R e a c t i o n r a t e c o n s t a n t , k (h -1)Initial AY concentration (mg/l)AY for Mix IV AY for Mix V AY for Mix VI AB for Mix IV AB for Mix V AB for Mix VIFig.7.Effect of initial dye concentration on degradation rate constant for (a)con-stant AY concentrations at 2.5,5,and 10mg/l and various AB concentrations and (b)constant AB concentrations at 2.5,5,and 10mg/l and various AY concentrations using the synthesized mesoporous-assembled TiO 2photocatalyst (photocatalyst dosage =5g/l,reaction volume =80ml,initial solution pH =4.5,irradiation time =2h,and UV lamp power =44W).concentrations at 2.5,5,and 10mg/l and various initial AY con-centrations in the range of 0–10mg/l (named as Series IV,Series V,and Series VI,respectively).The mixtures with a total volume of 80ml containing 5g/l of the mesoporous-assembled TiO 2photocat-alyst were initially used for the photocatalytic activity testing.The degradation rate constants (k )at various initial dye concentrations are shown in Fig.7(a)for constant initial AY concentrations and in Fig.7(b)for constant initial AB concentrations.It can be observed that at any given initial AY and AB concentrations,the degrada-tion rate constant of AY is always higher than that of AB,indicating that AY could be more easily degraded by the TiO 2photocatalyst.This is possibly because AY is a monoazo dye with less molecular complexity than AB,which is a diazo dye,resulting in an easier attack to AY dye by several photogenerated oxygen active species.When maintaining the initial concentration of AY or AB at 2.5,5,and 10mg/l and increasing the initial concentration of the other,the degradation rate constants of both dyes were found to reach the maximum at the other dye concentration of 2.5mg/l,and then gradually decreased possibly due to too large number of the dye molecules to be attacked by the oxygen active species and also due to the large fraction of useless light absorbed by the dye molecules instead of the TiO 2photocatalyst.It is interestingly found that the degradation rate constants of AY (Series I,Series II,and Series III in Fig.7(a))and of AB (Series IV,Series V,and Series VI in Fig.7(b))are enhanced in the presence of the other dye (AB and AY,respec-tively),especially at the other dye concentration of 2.5mg/l.The higher degradation efficiency of any given dye in the mixture as compared to the single dye is possibly because the other dye,withsuitable concentration,acts as a mediator in reducing the compet-itive interaction/repulsion between the dye molecules themselves and between the dye molecules and the photocatalyst surface,resulting in greater possibilities for the dye molecules to reach the photocatalyst surface and be attacked by the oxygen active species generated by photoinduced mechanism at the photocatalyst sur-face.However,the exact explanation is still unclear at this stage,and further profound experiments are required to perform in the future work.It is also interestingly found that when maintaining initial AB concentration constant and varying initial AY concentration (Fig.7(b)),the degradation rate constants of both dyes tended to be higher than those obtained when maintaining initial AY con-centration constant and varying initial AB concentration (Fig.7(a)),especially at initial AY and AB concentrations of 2.5and 5mg/l,respectively,for the Series V.Since a mixture of 5mg/l AY and 2.5mg/l AB and a mixture of 2.5mg/l AY and 5mg/l AB provided the highest photocatalytic efficiency in each experimental set,these two mixtures were selected for using in further experiments to optimize the reaction parameters.3.2.2.Effect of photocatalyst dosageThe effect of photocatalyst dosage on the photocatalytic degra-dation of the mixtures of AY and AB using the synthesized mesoporous-assembled TiO 2nanocrystal photocatalyst is shown in Fig.8(a)for the mixture of 5mg/l AY and 2.5mg/l AB and in Fig.8(b)for the mixture of 2.5mg/l AY and 5mg/l AB.It is to importantly note that in the case of the photolysis (self-photodegradation of dyes in the absence of the photocatalyst),the degradation efficiency was very comparatively low;therefore,the photocatalyst is essentially required for the photocatalytic reactions.The results shown in Fig.8indicate that significant increases in the photocatalytic degradation efficiency to reach the maxima were observed when the photocat-alyst dosage increased up to 10g/l.However,when it increased beyond 10g/l,the photocatalytic degradation efficiency tended to adversely decrease.The photocatalytic degradation efficiencies at this optimum photocatalyst dosage of 10g/l for the mesoporous-assembled TiO 2nanocrystal were 99.1%of AY and 95.7%of AB for the mixture of 2.5mg/l AY and 5mg/l AB,and 98%of AY and 94.2%of AB for the mixture of 5mg/l AY and 2.5mg/l AB.The TOC results also showed a similar trend of the degradation efficiency to those determined by the UV–visible spectrophotometry.The maximum TOC removal for the mixture of 2.5mg/l AY and 5mg/l AB was 92.5%,which was higher than that for the mixture of 5mg/l AY and 2.5mg/l AB (91.3%).These satisfactorily high TOC removal efficiencies imply the complete degradation of the organic dye molecules,pointing out that the synthesized mesoporous-assembled TiO 2nanocrystal photocatalyst possesses excellent property for use in the wastew-ater treatment.Regarding the above observed optimum photocatalyst dosage,the results can be explained in terms of the active site availabil-ity on the TiO 2photocatalyst surface,the light absorption ability of the photocatalyst,and the light penetration depths into the reac-tion suspension [4].With increasing photocatalyst dosage up to the optimum value of 10g/l,the TiO 2surface active sites and the light absorption ability of the TiO 2photocatalyst correspondingly increase due to a larger amount of the TiO 2nanoparticles available in the photocatalytic system.On the other hand,at higher photo-catalyst dosages than the optimum value,there is only a portion of light-sensitive and active TiO 2nanoparticles in the region near the photocatalytic reactor wall that can totally absorb the irradiating light.The photocatalyst suspended also has a greater probability to be agglomerated,and therefore lesser amounts of the photocatalyst are responsible for absorbing the irradiating light,leading to lower number of photogenerated active species for the subsequent dye。