Marwa M.Ibrahim

Department of Chemistry,Faculty of Education,Ain Shams University,Roxy 11711,Cairo,Egypt

Keywords:CeO2-ZrO2-Al2O3 Surfactants Ultrasonic co-precipitation method Optical properties Wastewater Adsorption

ABSTRACT To preserve the environment for civilization,we should remove the pollutants like toxic dyes by friendly and cost efficacious method.In this study,the effect of surfactants or mixed surfactants on physicochemical,optical and adsorption properties of ternary mixed oxide CeO2-ZrO2-Al2O3 (CZA) are investigated.The ternary mixed oxide CZA was prepared by surfactants or mixed surfactants assisted ultrasonic coprecipitation method.The physicochemical and optical properties are estimated by different techniques like XRD,TEM,EDX,FTIR,SBET and UV-Vis/DR.The CZAT and CZAC have hybrid shapes and high surface area.The adsorption properties of ternary mixed oxides adsorbents were characterized by sono-removing anionic dyes such as Congo red(CR)and Remazol red RB-133(RR).The different factors like contact time,different dye concentrations and temperatures also studied.The kinetics and isotherms applications showed that,the adsorption process was followed pseudo second order kinetics and the Freundlich isotherm model.Also,the adsorption is spontaneous and endothermic process through the thermodynamic study.Finally,the results showed that the ternary mixed oxide nano-adsorbent (CeO2-ZrO2-Al2O3) is promising and functional materials for anionic dye sweep from wastewater.

1.Introduction

In last decade,mixed metal oxides which called also heterometal oxide attracted attention and works in variety of fields such as physics,chemistry and material science [1,2].Mixed metal oxides are produced from combination of two metals or more in oxide matrix.The variety of these fields is due to the advantages of mixed metal oxides and these advantages are the sum of advantages of all individual metal in mixed metal oxide [2].CeO2-ZrO2-Al2O3considers one example of ternary mixed oxide and has the advantages of individual oxide which formed.CeO2is rare-earth oxide material considered as oxygen supply because it has the capacity to store/discharge oxygen(redox shift between Ce4+and Ce3+).It has chemical and biological stability,corrosion prevention,absorbs only UV light and it has the ability to adsorb the pollutants [3,4].Zirconia(ZrO2) is n-type semiconductor transition metal oxide used in different applications because has chemical and physical properties like high transparent in visible and near-IR region,large band gap,low optical loss,high refractive index,good mechanical properties and fracture toughness and biocompatibility [5,6].In addition to,Al2O3,metal oxide with high surface area,large pore volume,unique optical and biomedical properties,has different shapes (nanoplates,nano sheets and flowers),it has thermal and chemical stability and low cost.So,it used as support of another metal oxide because make dispersion of active species on the surface [4,7].

Various physical and chemical methods used to prepare the mixed metal oxides like vapor condensation,spray pyrolysis,thermochemical/flame decomposition of metal-organic precursors,thermal decomposition,combustion synthesis,co-precipitation,hydrothermal,microwave-assisted hydrothermal,sol-gel,lowtemperature wet chemical,reverse microemulsion/micelle and sono-chemical method [2,4,8].Recently,the scientists begin to use additive substances like surfactants during the preparation methods due to control the physicochemical properties of mixed metal oxide such as crystal size,surface area,porosity,morphology and hydrophobic or hydrophilic properties [9,10].Surfactants are called surface active agent and it consist from two parts,a hydrophilic polar head(water loving)and a hydrophobic tail(water hating).Four categories of surfactants are presents due to the charge on polar head:cationic,anionic,amphoteric and non-ionic surfactants.The presence of surfactants leads to decrease the surface tension of solution consequently increased the surface area.The increase in surface area is due to the formation of micelle.In case of low concentration of surfactants(critical micelle concentration (CMC)),the molecules self-organize itself and form micelle.This micelle decreases the aggregation of precursor particles and control of crystallization process so increased the surface area[11,12].And then,the choice of suitable kind of surfactants is necessary step for formation of efficient nanoparticles for determined applications.

Adsorption is one of the most important applications which mixed metal oxide used in it as adsorbent.Adsorption considered way to remove pollutants from wastewater and has advantageous rather than different methods of removal such as simple design and operation,high efficiency,adsorbents easily available,can be reused and eco-friendly [13-15].Although CeO2-ZrO2-Al2O3ternary mixed oxide prepared previously and used as catalyst in different reactions like dehydrogenation of ethylbenzene,CO oxidation,reduction of nitrogen oxides,methane steam forming reaction,partial oxidation of methane and motorcycle [7,16-20],The adsorption studies on CeO2-ZrO2-Al2O3which treated by surfactants and mixed surfactants during preparation is notably uninvestigated.

The focus of this study was a research for effect of surfactants and mixed surfactants on physicochemical,optical and adsorption properties of ternary mixed oxide CeO2-ZrO2-Al2O3.So,ternary mixed oxides which have different surface area and morphology prepared by surfactants and mixed surfactants assisted ultrasonic precipitation method.Also,the adsorption properties are studied for removal of anionic dyes like CR and RR.Thermodynamic and different models of kinetics and isotherms have been calculated.

2.Experimental

2.1.Synthesis of CeO2-ZrO2-Al2O3 mixed oxide nanoparticles

CeO2-ZrO2-Al2O3ternary mixed oxide nanoparticles were prepared by modified co-precipitation method using surfactant and ultrasonic radiation.Different types of surfactants and mixed surfactant with same molar ratio used for this preparation like Cetyltrimethylammonium bromide(CTAB,cationic),Sodium dodecyl sulfate (SDS,anionic),Triton X-100 (nonionic),SDS:CTAB,and SDS:Triton X-100.Cerium nitrate,zirconium oxynitrate and aluminum nitrate were dissolved in bi-distilled water and mixed in the same molar ratio under vigorous stirring.Then the appropriate amounts of surfactants or mixed surfactants were added to the previous solution and stirred for 1 h at room temperature.After that ammonia solution was added drop wise to the above mixture solution to attain pH=9 under sonication using UP50H sonication probe (50 W,30 kHz).After addition,the solution was again exposed to ultrasonic radiation for further 15 min to perform the complete reaction of mixed solution with ammonia.The temperature of the reaction mixture was maintained constant at room temperature using water bath.After 6 h of aging at 333 K,the formed yellowish white precipitate was centrifuged and washed with bidistilled water several times to ensure the complete removed of surfactant,impurities and pH reached to 6-7.The resultant precipitate was dried at 373 K to obtain mixed hydroxide then calcined at 873 K for 3 h.For comparison the CeO2-ZrO2-Al2O3ternary mixed oxide nanoparticles were prepared by modified co-precipitationultrasonic method without any surfactant.The following nominations are used throughout the text CZA,CZAC,CZAS,CZAT,CZASCand CZASTto refer to CeO2-ZrO2-Al2O3nanoparticles prepared without surfactant,using CTAB,SDS,Triton X-100,SDS:CTAB,and SDS:Triton X-100,respectively.All reagents were obtained from commercial sources of analytical grade and used without further purification.

2.2.Instrumental analyses

The phase structure,morphology,surface and optical analysis of nanosized ternary mixed oxides was measured using different techniques like X-ray diffraction(XRD) with a Philips diffractometer equipped with a CuKα1 radiation as X-ray source,Transmittance electron microscope (TEM) with energy dispersive spectroscopic(EDS)using JEM-2100CX(JEOL),The surface properties(SBET)with Quantachrome TouchWinTMversion 1.11,Fourier-transform infrared spectroscopy (FTIR) by Jasco IR 4100 spectrometer (Japan)and UV-Visible absorbance and diffuse reflectance spectroscopy(UV-Vis/DR) by JASCO V-550 spectrometer (Japan) supply with unit for diffuse reflectance spectra.

2.3.Adsorption behavior of different anionic dyes using ternary mixed oxide CZA

To study the effect of different surfactants and dual surfactants on adsorption properties of ternary mixed oxides,Congo red (CR)and Remazol Red RB-133 (Reactive Red RB133,RR) as examples for anionic dyes were chosen for this study.The structures and characteristics of these dyes are presented in Table 1.The adsorption experiments were carried out by 0.1 g of catalysts are immersed in 100 ml of dye solution has concentration 5 × 10-5mol·L-1in the presence of ultrasound and at room temperature using a water bath to keep the temperature constant.The solution samples were bringing out at time intervals of sonication and centrifuged to remove the catalyst.The concentration of residual dye in the solution was measured at λmaxof dyes using UV-Vis spectroscopy until reaching balance.The calibrations curves and Beer’s law used to determine the initial and equilibrium dyes concentrations.The following Eqs.(1) and (2) are used to calculate the adsorption capacity of dye (qt) and amount of dye adsorbed at equilibrium (qe)

whereC0,CtandCe(mg·L-1)are the initial,at timetand equilibrium liquid dye concentrations,respectively,Vis the volume of dye solution (L) andMis the mass of adsorbent used (g) [4].In addition to,several factors are studied like initial dye concentrations,contact time and different temperatures.Also,different modules of kinetics,isotherms of adsorption and thermodynamic study are investigated.Finally,the highly active CZATadsorbent are used for adsorption of mixture of two dyes (CR and RR)

3.Results and Discussion

3.1.Characterization of CeO2-ZrO2-Al2O3 nanoparticles

3.1.1.XRD

Fig.1 shows the XRD images of CeO2-ZrO2-Al2O3nanoparticles which prepared by modified ultrasonic co-precipitation method using different surfactants and mixed surfactants.Table 2 displays the crystal properties like 2θ,lattice parameter,full width at half maximum (FWHM) and crystal size which calculated using the Scherrer equation.It is obvious from Fig.1 and Table 2 that,all nanoparticles samples have four master peaks (2θ ≈ 28.79°,33.8°,48.3°,and 57.6°) for a fluorite like cubic CeO2phase and no distinguished peaks of ZrO2and Al2O3are appeared due to highdispersion and the formation of solid solution [21].Also,no peculiar peaks of surfactants like CTAB,SDS and Triton X-100 which signal to high purity of ternary mixed oxides.The reason of formation of solid solution is the ionic radii of Zr+4and Al+3are smaller than Ce+4thus it entered in lattice of Ce4+leading to the shrink-age of lattice (Ce+4(0.101 nm),Zr+4(0.084 nm) and Al+3(0.067 nm) for coordination number 6) [20,22].As well,the second notice which indicating the formation of solid solution is the shift in the value of 2θ and lattice parameter(Fig.1 and Table 2).The lattice parameter of CeO2(cubic)is estimated bya=d√h2+l2+k2[(h l k)parameters for the plane (1 1 1) anddisd-spacing].It is clear from analysis data that,the lattice parameters of our samples are less than pure CeO2(a=0.5410 nm) and this confirm the formation of solid solution and lattice contraction[4].All ternary mixed oxides have small crystal sizes and this appear from broad peaks and large value of FWHM.The crystallite size is rated and it is found in the range 2.6-6.4 nm.Likewise,the peak intensity of CeO2-ZrO2-Al2O3which prepared by Triton X-100 and CTAB surfactants are smaller than other prepared samples and this compatible with the value of FWHM.So,the sample prepared with these surfactants have the highest value of FWHM(Table 2 and Fig.1)indicating the formation of small crystal than the others[4].The presence of surfactants affected on crystallization process of samples.In case of CTAB (ionic surfactant;cationic),molecules of CTAB dissolved in aqueous solution forming CTAB+types as head with long hydrophobic tail.These types formed micelles in which the positive part offers to water molecules.The main units to form oxide are metal hydroxide which has negative charge.So,the chemical interaction (electrostatic interaction) between the seed of oxide(negative charge) and CTAB+ions (positive charge) takes place formed complex which suppresses the growth of metal oxides(small crystal size).Also,the presence of long hydrocarbon chain as hydrophobic part of CTAB acts as capping agent which slow down the crystal growth[11,23].While SDS(ionic surfactant;anionic) dissolved in water forming SDS-types as head and hydrocarbon chain as tail which forming micelles in which a negative part offers to water molecules.After that,the complex is formed between SDS-and the positive charge of metal oxide seeds M+due to electrostatic interaction.The outer surface of the hydrophilic end is occupied by numerous M+ions.These micelles restrict the access of metal ions to metal oxide seeds which leads a formation of chemically activated lattice defeats on the surface.These lattice defects will be nucleation and growing center in the lattice[11,24].The triton X-100(non-ionic surfactant)formed from polyethylene oxide as head and hydrophobic aromatic hydrocarbon group as tail.This surfactant is assembled formed micelles then the complex is formed between polyethylene oxide and positive charge of metal oxide seeds M+through oxygen bond.The presence of bulky group in tail makes steric hindrance for nucleus growing(small crystal size)[11,25].Actually,the presence of mixed surfactant in ultrasonic co-precipitation method effects on crystal growth.The dual surfactants contain a mixture of anioniccationic (SDS-CTAB) and anionic-non-ionic (SDS-triton X-100)decreases the crystal size than anionic surfactant only(SDS).This is due to the addition of SDS leads to formation of nucleation center which is capping by another surfactant(CTAB or triton X-100)and inhibit crystal growth [10].Fig.2 shows the mechanism of formation of metal oxide in the presence of surfactants and dual surfactants.

Table 1 Structure and characteristics of the anionic dyes

Fig.1. XRD patterns of ternary mixed oxide CeO2-ZrO2-Al2O3.

Table 2 Physicochemical and optical properties of ternary mixed oxide CZA

Fig.2. The mechanism of formation of mixed metal oxide in the presence of surfactants and dual surfactants.

3.1.2.TEM &EDX

The morphology of particles prepared by surfactants assisted ultrasonic co-precipitation method is shown in Fig.3.Fig.3(a)show high agglomeration of nanoparticles as cluster for CZA prepared without surfactant.By using surfactants like CTAB,SDS and Triton X-100,the coalescence decreased due to the surfactant decrease surface tension of the precursor as shown in Fig.3(b)-(d),respectively [3].In case of CTAB and Triton X-100,the hybrid shapes of well dispersed nanoparticles like sphere,cubic with mean particle size 20 nm and small sheets are observed in Fig.3(b) and (c) which lead to high surface area.On the other hand,by using SDS as surfactant,large spherical nanoparticles with mean particle size 37 nm are observed in Fig.3(d).In the absence of surfactants,the first stage of the precipitation reaction produced primarily Ce(OH)3,whose nuclei are spherical,but the ultimate shape of the particles is rod like.With the pH value below 10,the original spherical nuclei of Ce(OH)3were rapidly oxidized to Ce(OH)4or cerium (IV) hydroxide/oxide with high aggregation of spherical nanoparticles as cluster [26].The presence of surfactants can promote the self-assembly of CeO2and formation of different morphologies nanomaterials like nanospheres,nanorod and nanosheets.The nanorod is formed through the surfactants interact preferentially with the(1 1 1)plane to(1 0 0).The exposed surfaces tend to combine together to minimize the surface energy to form a cubic plane structure.After that,the CeO2nanoplates converted to nanorod [27].The nanosheets were found to be formed through two-dimensional self-organization of initially formed small ceria nanocrystals,followed by an in-situ recrystallization process [28].

Fig.3. TEM and EDX images of (a) CZA,(b) CZAC,(c) CZAS,(d) CZAT and (e) EDX of CZAT.

The EDX spectra of ternary mixed oxide CeO2-ZrO2-Al2O3are presented in Fig.3(e).The peaks corresponding to Ce,Zr,Al and O are clearly observed in the EDX spectrum and the results are clearly indicating the composition of mixed oxide.

3.1.3.Surface area measurements

Fig.4 shows adsorption-desorption isotherms and pore size distribution for ternary mixed oxides CeO2-ZrO2-Al2O3prepared by ultrasonic co-precipitation method using different surfactants or mixed surfactants.All isotherms can be ascribed as Type II,which is associated to either mixed pores or nonporous system according to the IUPAC classification[Fig.4(a)][29].Table 2 shows the surface characterization of the samples like surface area,volume of monolayer,total pore volume and average pore radius which nominated asSBET,Vm,Vpandr′,respectively.It is obvious from Table 2 that,the surface area of samples prepared by surfactant or mixed surfactant assisted precipitation method is bigger than one prepared without surfactant except SDS.The reason of increasing surface area in samples is surfactants inserted into particles layer then removed during calcinations and removed many pores on the surface which leading to higher surface area [30].The other reason for increasing surface area is decreasing the crystal size and hybrid shapes.So,samples treated by Triton X-100 and CTAB have the highest surface area and sample treated by SDS has the lowest surface area due to crystal size(XRD and TEM measurements) as shown in Table 2.The value ofr′of all ternary mixed oxides is approximately 4 nm.So,the pores formed from very small cavities of size near to frontier of meso and micropores.The value ofVmandVpare consent with the sequence ofSBET.

Fig.4. Nitrogen adsorption-desorption isotherms of ternary mixed oxide CZA (a),Pore size distribution of ternary mixed oxide (b).

The pore size distributions of samples are presented in Fig.4(b)and the value of mean pore diameter are calculated and shown in Table 2.From Fig.4(b) and Table 2,the samples CZA,CZASand CZASChave distribution in micropores region besides small amount in mesopores region with mean pore diameter 3.8 nm.While the CZAC,CZATand CZASThave distribution in mesopores region(5.14-7.44 nm).Thence,modification of ultrasonic co-precipitation method by using different surfactants or mixed surfactants changes the mean pore diameter.So,the cationic,non-ionic and mixed surfactant (anionic-nonionic) lead to form pores in mesopores region.The mixed surfactant (anioniccationic) and anionic surfactant makes pores in micro and mesopores region like unmodified sample.

Fig.5. FTIR of ternary mixed oxide CeO2-ZrO2-Al2O3.

Fig.6. UV-vis/DR spectra of CeO2-ZrO2-Al2O3 ternary mixed oxide.Inset:plot of(αhυ)2 versus photonenergy (hυ).

Fig.7. Factors effecting on adsorption capacity of CR&RR dyes using CZAT(a)contact time,(b)dye removal versus time,(c)initial dyes concentration,and(d)temperature.

3.1.4.FTIR

To recognize the functional group of ternary mixed oxide which prepared by surfactant or mixed surfactant assisted ultrasonic coprecipitation method,FTIR is studied in Fig.5.From the spectra,all samples have the peaks at ≈3426 and 1634 cm-1due to the vibrations of physically adsorbed-OH and bending mode of H2O functional groups,respectively [31].The presence of hydrated ternary metal oxide on the surface is appeared at 2900 and 2850 cm-1[4].Appearance of band at 1450 cm-1is due to non-bridging OH group [32].The peak at 1100 cm-1is attributed to environment water absorbed on ceria surface [33].In addition to,the bands at 869,523 and 453 cm-1are assigned to stretching Ce—O and some interaction among Ce (IV),Zr (IV) and Al (III) through oxygen or hydroxide bridge formed ternary mixed oxide[4,21].Furthermore,no special peaks of surfactant or mixed surfactants are appeared in spectra indicating the purity of ternary mixed oxide and these data is compatible with XRD measurements.

3.1.5.UV-Vis/DR

The optical properties of prepared ternary mixed oxides were investigated by UV-Vis/DR spectroscopy.Fig.6 shows the UVVis/DR spectra of the mixed oxide which untreated or treated by surfactants during preparation method.It is obvious from this figure that the sample CZA which prepared without treating by surfactant has a broad absorption edge at 330 nm to lower wavelength.This band is due to the superposition of absorption band of CeO2and ZrO2[7].By using surfactants or mixed surfactants,blue shift of absorption edge and tailing take place except in case of SDS (red shift and tailing).The optical band gap of samples is calculated and presented in Table 2 and Fig.6 inset using the equation [αhν=A(hν -Eg)1/2].In this equation the symbolshν express the photon energy,Aconsidered a constant for a direct transition and α=2.303×ln(Io/I)/t(absorbance is ln(Io/I)and thickness expressed byt) [5].It is obvious from Table 2 and Fig.6 inset that the band gap increases by using surfactants or mixed surfactants in preparation methods.But in case of samples modified by SDS,this observation is failed because the optical band gap decreases than unmodified one.The reason of decreasing band gap in case of SDS is the largest crystal size[34].The values of band gaps of all samples equal 1.96-2.61 eV and this suggested that the ternary mixed oxides are potential materials for absorption of UVlights.

3.2.Adsorption etude on ternary mixed oxide CZA

3.2.1.Factors affecting on adsorption process I.The influence of contact time.

The contact time considered one of the important factors which influence on the success of adsorption process.The excellent adsorbent adsorbed high concentration of the dyes in short times and reached to equilibrium.Fig.7(a)shows the effect of contact time on the adsorption of CR and RR dyes(5×10-5mol·L-1)on CZATas representative example.It is obvious from this figure that the rapid adsorption of two dyes takes place at 20 min and reached to 23.12 and 47.92 mg·g-1of CR and RR,respectively.After 20 min,the adsorption of dyes slows gradually until to reach equilibrium.The rapid adsorption in first 20 min attributed to the high active sites on the surface of adsorbent.After that,the free adsorption sites are decreased due to the dyes molecules is agglomerated at vacant sites[35].The adsorption capacityqeand the time removal of dyes on different nanoadsorbent is shown in Table 3.The CZAThas the highest adsorption capacity and the lowest time of removal of dyes (35.21 mg·g-1,120 min and 61.05 mg·g-1,135 min for CR and RR,respectively) due to mixed shape and high surface area of nanoparticles as shown in TEM and surface area measurements.Fig.7(b) shows the dye removal as function of time for different dyes CR and RR using CZAT.From this figure,the removal of RR is higher than CR until to equal at 75 min this is attributed to RR is monoazo while the CR is diazo and the color removal in case of monoazo is faster than diazo dyes [36].

II.The influence of initial dyes concentration.The study of impact of initial dye concentration on the adsorption process is substantial driving force to beat the impedance of mass transfer of dyes between solid and aqueous stage[4].Fig.7(c)presented the effect of initial CR &RR dyes concentration (2.5 × 10-5-7.5 × 10-5mol·L-1) at 303 K on adsorption capacity (qt) using CZATas representative example.The figure shows that,by increasing the initial dye concentration,the adsorption capacity increased.At higher concentration of dyes adsorbed on catalyst,higher interaction between dyes and catalyst in addition to concentration gradient is present.This concentration gradient leads to a mass transfer of dyes from bulk of solution to surface of catalyst and increase the adsorption capacity [4,37].Also,the mass transfer is increased by magnetic stirring.

III.The influence of temperature.Temperature has dynamic factors in adsorption process of dyes.So,the effect of temperature determines the adsorption process is endo or exothermic process.If the adsorption capacity increases by increasing temperature,the adsorption process is endothermic.When the adsorption capacity decreases by increasing temperature,the adsorption process is exothermic.Fig.7(d) displays the effect of different temperatures(288 K,303 K and 323 K)on the adsorption capacity of CZATusing CR&RR dyes(5×10-5mol·L-1).It is obvious from this figure that,the adsorption capacity increased by elevation the temperature from 288 K to 323 K (endothermic process).The increasing of adsorption capacity is attributed to enhancing the mobility of dyes molecules by increasing the temperature and induction of new active sites available for adsorption [35].

3.2.2.Application of adsorption kinetics models

Different kinetics models are used to determine the suitable kinetic model for adsorption batch systems and the suitable one has the linear plot.Also,from these models,kinetics rate is calculated which is important in commercial field.The adsorbent has high kinetics rate for removal of dye is the best one.The four kinetics models (Pseudo first order,Pseudo second order,Intraparticle diffusion and Elovich) were used in this study and the details for these models are shown in Table 4 such as equation,relation in graph,data from graph and description of models.Fig.8(a)-(d)shows the plot of four kinetics models using different dyes (CR and RR) and CZATas adsorbent (representative example) (Pseudo first order (a),pseudo second order (b),intra particle diffusion (c)and Elovich model (d)).Also,Table 3 shows the results obtained from these figures.Fig.8 and Table 3 elucidate that,adsorption process is pseudo second order reaction because the value of correlation coefficient (R2) has the highest value and the value of adsorption capacity calculated (qecal.) is near to adsorption capacity experimental(qeexp.).Although the value ofqecal.is compatible withqeexp.in case of pseudo first order but theR2is lower than pseudo second order.So,the adsorption process is pseudo second order which means chemisorption mechanism being rate determining step.In addition to,the linear plot in case of intraparticle diffusion do not pass through the origin and have intercept.So,other factors which specify on rate determing step rather than the intraparticle diffusion.Also,the figure in this model has two grades and the second grade has low adsorption rate compared to the first grade.This is attributed to the amount of dyes remained in the solution is small.In Elovich model,the value ofR2is small with compared to pseudo second order reaction.Finally,the pseudo second order reaction is the best model.The highest adsorption rate is found in CZATsample (1.75 × 10-3,CR and 1.94 × 10-3,RR)

Fig.8. Adsorption kinetics models for CR &RR dyes using CZAT (a) pseudo first order,(b) pseudo second order,(c) intraparticle diffusion,and (d) Elovich.

3.2.3.Application of adsorption isotherms

Four types of isotherms are used to show the mechanism of adsorption in this study,the four isotherms are Langmuir,Freundlich,Tempkin and Dubinin-Radushkevich(D-R)and the details of these isotherms are shown in Table 3.The graphs of these isotherms are presented in Fig.9 and the data obtained from these graphs are shown in Table 5.These isotherms display the connection between the dye concentration on the catalyst and on the solution at constant temperature and equilibrium [38,39].Fig.9(a)-(d) shows the plot of four isotherms using different dyes (CR and RR)and CZATas representative example[(a)Langmuir,(b)Freundlich,(c) Tempkin and (d) Dubinin-Radushkevich (D-R)].From Fig.9 and Table 5,the adsorption of different dyes follows Freundlich isotherm because the high value ofR2(0.99) and the adsorption process is favorable because thenvalue higher than 1.In case of Langmuir and Tempkin,theR2value is less than Freundlich isotherm.Also,Dubinin-Radushkevich isotherm has correlation coefficient lower than Freundlich isotherm and theEvalue is lower than 8 kJ·mol-1.So,the adsorption process is physical in nature.

The structures of dyes and adsorbents effect on the adsorption process.The heterogenous surface of CZA adsorbent contains hydroxyl groups which make hydrogen bond with S,N and O atoms in dye molecule.The highernvalue which obtained from Freundlich isotherm refers to the bond energy between dye and adsorbent.So,the chemisorption is taken place rather than physisorption.In addition to,the multilayer of dye is takes place through the adsorption process [40].

3.2.4.Application of thermodynamic studies

The following equations are used to calculate some thermodynamic parameters like ΔG:Gibbs free energy change(kJ·mol-1),ΔH:enthalpy change (kJ·mol-1) and ΔS:entropy change (kJ·mol-1·K-1).Keare the thermodynamic equilibrium constant (L·g-1).

Table 4 Kinetics and isotherms models information

Fig.10 presented the relationship between lnKeand 1/Tfor adsorption of different dyes (CR and RR) on CZATat different temperatures.The value of ΔSand ΔHwere estimated from the intercept and the slope.These calculated thermodynamic parameters are listed in Table 6.The higher positive values of ΔHrefer to the adsorption process is endothermic and chemisorption.In addition to,the positive value of ΔSpoint out high ordering of adsorption reaction.Also,the negative value of ΔGindicated the adsorption process is spontaneous and favored at high temperature.

3.2.5.Adsorption of mixed dyes (CR and RR)

UV-visible spectra of a mixture of CR and RR as a function of adsorption time using the highly active adsorbent CZATis presented in Fig.11.It is clear from this figure that,the λmaxof two dyes are changed after mixing and the mixture of dyes has a new λmaxat 512 nm at time 0.Also,the color of mixed dyes decreases(absorbance decrease) and still constant at 180 min.The λmaxshifted to 522 nm at time 180 min.This shift in λmaxindicating that the adsorption of CR is takes place initially then the adsorption of RR because the λmaxof RR is 520 nm (still remaining in solution).The decolorization efficiency of mixed dye is equal to 77.1% and this value is smaller than the individual dye (CR=99.5% and RR=99.3%).The value of the decolorization efficiency of mixed dyes using the CZATmakes it a promising material in purification of wastewater by adsorption.

3.2.6.Comparison of CZA adsorbent efficiency with other adsorbents

Fig.9. Adsorption isotherms for CR &RR dyes using CZAT (a) Langmuir,(b) Freundlich,(c) Tempkin,and (d) Dubinin-Radushkevich (D-R).

Table 5 Isotherms parameters of the adsorption of different dyes on ternary mixed oxide

Table 6 Thermodynamic parameters of the adsorption of different dyes on ternary mixed oxide

Fig.10. Plot of ln Ke against 1/T for CR &RR dyes using CZAT.

Fig.11. UV-visible spectra of a mixture of CR and RR as a function of time during the adsorption using CZAT.

Table 7 Comparison of experimental adsorption capacities of CR and RR dyes for different adsorbent

Table 7 compared between experimental adsorption capacities for adsorption of CR and RR dyes on CZA mixed oxide with other different adsorbents on the literature.It is obvious,from this table,that the CZATadsorbent has the highest experimental adsorption capacity of CR and RR dyes compared to other adsorbents.So,the ternary mixed oxide has highest potential for water purification application.

4.Conclusions

In conclusion,this study demonstrated a preparation of ternary mixed oxide CeO2-ZrO2-Al2O3(CZA) by surfactant/mixed surfactant assisted ultrasonic co-precipitation method.The type of surfactants or mixed surfactants effects on morphology,surface and optical properties of CZA.The CZAThas the highest surface area,different shapes and highest band gap.Also,these mixed oxides are applied as adsorbents for anionic dyes (CR and RR).The CZAThas the highest removal efficiency for both dyes.The adsorption process is spontaneous,endothermic and it follows the pseudo second order reaction and Freundlich isotherm.At the end,the ternary mixed oxide has highest potential for water purification application.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.