The adsorptive properties of UiO-66 towards organic dyes:A record adsorption capacity for the anionic dye Alizarin Red S.

Marwa S.Embaby ,Saber D.Elwany ,Widiastuti Setyaningsih ,Mohamed R.Saber *

1 Central laboratories,Fayoum Drinking Water and Sanitation Company(FDWaSC),Fayoum 63754,Egypt

2 Food and Agricultural Product Technology Department,Faculty of Agricultural Technology,Gadjah Mada University,Jalan Flora,Bulaksumur,Yogyakarta,55281,Indonesia

3 Chemistry Department,Faculty of Science,Fayoum University,Fayoum 63514,Egypt

1.Introduction

The amounts of wastewater containing dyes and organic pollutants have been significantly increasing due to the various sources from severalindustries including textile,paper,leather,plastic,printing,dyestuffand so on[1].Organic dyes are toxic[2],mutagenic and teratogenic and they may cause serious effects on flora and fauna[3].Dyes,even in low concentration,are visually detected and affect the photosynthetic activity in aquatic life by reducing light penetration[4].

Several physical,chemical,and biological methods are used to remove dyes from the wastewater including coagulation[3],adsorptive oxidation[5],ultra- filtration[6]and biological treatment[7].However,physical methods are typically preferred over chemical and biological methods due to their low cost,simplicity and,more importantly,low level of byproducts left in water bodies[8].In this vein,adsorption techniques appear to be exceptionally efficient in removing color from the industrial effluents[9].

Metal–organic frameworks(MOFs)are porous materials composed of inorganic metal nodes and organic linkers[10].Different types of MOFs have been typically synthesized using conventionalhydrothermal and solvo-thermal methods as well as slow diffusion techniques[11].Recently,the MW-assisted hydrothermal method was successfully applied to decrease MOFs syntheses reaction time up to a few minutes with high yield and high phase purity due to rapid/homogeneous heating and fast kinetics[12].The simple synthesis,large specific surface areas,and high porosity[13]of MOFs render them as efficient adsorbents for diverse applications[14–16],including;gas storage[17],heterogeneous catalysis[18],sensing[19]and for therapeutics and diagnostics in biomedicine[20].

In water treatment,MOFs have shown high capacity to remove different pollutants.Many studies have been reported for removing metals[21,22],anions[23,24],dyes[25–27]and other organics[28,29].Among these studies,the adsorptive/degradation properties of UiO-66 have been thoroughly investigated due to its exceptional water stability and high adsorption capacity[30–33].It was found that UiO-66 has high affinity towards anions;Fluoride[2],Selenate and Selenite[34],Arsenate[35]and Phosphate[36].In contrast,the adsorption of metals on UiO-66 re flects no or negligible efficiency with less than 20%removal of Cd,Cr,Hg and Pb[37].Chen and co-workers,however,have reported high selectivity of UiO66 towards the cationic dyes(Methylene Blue,Rhodamine B and Neutral Red)as compared to the anionic dyes(Methyl Orange and Acid Chrome Black K)[38,39].Chen's observation is clearly contradicting all the previously mentioned reports indicating UiO66 affinity towards anionic species[2,34–37].

In order to resolve these con flicting reports of UiO-66 adsorption selectivity towards anionic species[2,34–36]versus cationic species[38,39],the adsorptive removal of the anionic dyes(Alizarin Red S.(ARS),Eosin(E),Fuchsin Acid(FA)and Methyl Orange(MO))and the cationic dyes(Neutral Red(NR),Fuchsin Basic(FB),Methylene Blue(MB),and Safranine T(ST))on UiO-66 has been investigated.The adsorption process of ARS as an example has been thoroughly investigated using the central composite design(CCD).The kinetics of the adsorption process have been thoroughly evaluated.

2.Experimental

2.1.Chemicals and reagents

Zirconium(IV)Oxide Chloride Octahydrate(ZrOCl2·8H2O)(Merck,Germany),Terephthalic acid(H2BDC)(Loba Chemie,India),Acetic acid(BDH,England),N,N-dimethylformamide(DMF)(Loba Chemie,India),Acetone(Panreac,Spain),Alizarin Red S.(Fluka,United Kingdom),Fuchsin Basic(BDH,England),Methyl Orange(Panreac,Spain),Sodium Hydroxide(NaOH)(Merck,Germany),Hydrochloric acid(HCl)37 wt%(BDH,England),Deionized water were obtained from Barnstead Water Puri fication System(Thermo Scientific,USA).

2.2.Synthesis of UiO-66

UiO-66 was synthesized according to Ref.[40]with slight modi fications.ZrOCl2·8H2O(8.055 g,25 mmol),Terephthalic acid(4.150 g,25 mmol),45 ml Acetic acid and 2.7 ml Deionized water were mixed with 200 ml DMF and stirred for 30 min,then equally divided into four 75 ml PTFE vessels.An Anton Paar microwave oven(MV 3000)was used to irradiate the mixture for 30 min at 120°C then cooled.The productwas centrifuged,washed severaltimes by DMF and acetone then air-dried.Yield=78%.FTIR(KBr):3391 cm-1(H2O),1658 cm-1,1255,1102 cm-1.

2.3.Adsorption experiments

The adsorption experiments were carried out at room temperature in a batch process using an aqueous solution of different dyes;Alizarin R S(ARS),Eosin(E),Fuchsin Acid(FA),Methyl Orange(MO),Neutral Red(NR),Fuchsin Basic(FB),Methylene Blue(MB),and Safranine T(ST).An aqueous stock solution of these dyes(1000 μg·g-1)was prepared by dissolving 1 g of these dyes in a liter of DI water.Uio-66 was added to 100 ml working solutions of dyes(20 mg.L-1)and shaking with 200 r·min-1for 120 min as reported by Chen et al.[38].

Among these dyes,Alizarin Red S.was studied in details to describe the adsorption mechanism.The effects of different parameters on the rate of adsorption process were investigated by varying initial pH,amount of adsorbent,initial dye concentration and contact time,whereas,the solution volume(V)was kept constant,50 ml.

In each experiment a specific amount of UiO-66 was added into 100 ml stoppard conical flask containing 50 ml of the dye,the initial pH was adjusted by HCl and NaOH to be in range 2–10 using a pH meter UltraBasic UB-5,Denver,USA.Then,the mixture was agitated on a mechanical shaker Max Q 2000 from Thermo Scientific(USA)for a different time intervals.The adsorbent was separated from the solution by centrifugation using(Rotina 380,Hettich Zentrifugen,Germany).A Lambda 25 UV–vis Spectrophotometer,PerkinElmer,USA was used for measuring the absorbance of the supernatant at λmax=423 nm to determine the residual dye concentration.

The kineticsofadsorption was investigated by analyzing dye adsorptive uptake from aqueous solution at different time increments using this equation.

where R is removal percentage,c0and ctare the initial and final dye concentrations in mg·L-1,respectively.

Adsorption capacities for Alizarin dye(qi,mg·g-1)were calculated by a mass balance:

where c0and cfare respectively the initial and final concentrations(mg·L-1)of dye,V is the solution volume(L)and m indicates the adsorbent amount(g).

2.4.Central composite design(CCD)

The efficiency of the dye removal process depends on several variables,i.e.initial concentration of Alizarin Red S.dye(x1),pH(x2),adsorbent amount(x3),and shaking time(x4).Therefore,chemometric approaches based on a central composite design(CCD)were used to assess the variables that affect the dye removal process[41,42].

In this study,a CCD was developed based on a five-level experimental design with six replicates at the center point in order to investigate the effects of four independent variables on the dye removal process.The independent variables were coded at five levels(-2,-1,0,1,and 2)and each level was selected based on preliminary trials.The chosen levels for the variables are listed in Table 1 and the CCD consisted of 30 experimental points are provided in Table 2.

Table 1 Selected factors and their levels

The data was fit to a polynomial model to obtain the surface response and evaluate the independent and mutual effects of each factor.The RSM for all factors can be expressed as follows:

where y is the response of the system and xiare the factors.

Assuming that the independent factors are continuous and controllable during the experiments,in order to maximize the response y,it was necessary to find the best estimation for the correlation between independent factors and the response surface using a second-order model:

where x1,x2,…,xkare the factors that in fluence the response y;β0,βii(i=1,2,…,k),βij(i=1,2,…,k;j=1,2,…,k)are unknown parameters andεis a randomerror.Theβcoefficients are obtained by the least square method.

STATGRAPHICS Centurion XVI(Statpoint Technologies,Inc.,USA,trial)was used to obtain the design of experiment(DOE)matrix.Gnumeric 1.12.17.Analysis of Variance(ANOVA)was used to analyzethe experimental results in single factor experiments and the signi ficance of differences between the means was determined using the Least Significant Difference(LSD)test.

Table 2 CCD for four factors with their observed responses

3.Results and Discussion

3.1.Synthesis and characterization

The microwave route to the synthesis of UiO-66 based on the precursor;ZrOCl2·8H2O has been used.This approach proved quite efficient with large scales and less reproducibility issues due to the less hygroscopic character of ZrOCl2·8H2O[43].

The isolation of the target MOF has been confirmed by the identical powder XRD pattern(Fig.1)as compared to literature reports[40].The presence of terephthalate bands is indicated by the appearance of its fingerprint IR frequencies including νas(COO)and νs(COO)stretching frequencies at 1607 and 1408 cm-1respectively.The high frequency region(3800–3100 cm-1)is dominated by broad band centered at 3391 cm-1which is assigned to the hydrogen-bonded adsorbed water and ν(C--H)stretching modes of DMF molecules.While in the 3100–2850 cm-1region,weak bands due to aromatic and aliphatic ν(C--H)modes of benzene ring and DMF are seen.The low frequency region is dominated by intense band centered at 1658 cm-1and two narrow bands at 1255 and 1102 cm-1.These bands could be assigned to the δ(OH2),δ(CH3)and δ(C--O)vibration of the physisorbed water and DMF molecules respectively.

3.2.Adsorption mechanism and selectivity

The affinity of UiO-66 towards anionic vs cationic dyes has been assessed to decisively resolve the con flict in this issue.Different dyes,anionic(Alizarin Red S.(ARS),Eosin(E),Fuchsin Acid(FA)and Methyl Orange(MO))and cationic(Neutral Red(NR),Fuchsin Basic(FB),Methylene Blue(MB),and Safranine T(ST))have been employed on UiO-66.The selected conditions were 100 ml of 20 mg.L-1dye solution,120 min shaking time and 20 mg sorbent as reported by Chen et al.in 2015.The results shown in Fig.2 and summarized in Table 3 clearly show a general affinity towards anionic dyes with the highest removal for Alizarin Red S.and Eosin(about 80%)followed by the other two anionic dyes Fuchsine Acid and Methyl Orange(45.6%and 31%respectively).On the other hand,the adsorption of the cationic dyes is almost negligible(less than 5%).These results are in good agreement with previous reports of UiO-66 affinity towards anionic species[2,34–36].Also,the weak adsorptive removal of cationic dyes agrees with the negligible removalofmetals(Cd,Cr,Hg and Pb)on UiO-66[37].

The adsorption mechanism of similar oxo-anions onto UiO-66 has been proposed to go via anion exchange of the adsorbate with the hyrdoxide bridges in the Zr6O4(OH)4nodes of the MOF[34,35].Similarly,the adsorption mechanism of the present set of sulfonated anionic dyes could proceed via anion exchange ofthe hydroxide bridges with the sulfonate groups of the dyes(Fig.3 top).In neutral media,the negatively charged dye anions are attracted to the MOF nodes probably due to hydrogen bonding between--OH groups on the nodes and--SO3-groups on the dyes.The zeta potential measurements of UiO-66 indicated a point of zero charge at pH=3.9 which means that the MOF has positive surface charges below this pH value,most likely due to the successive protonation of Zr6O4(OH)4nodes into[Zr6(OH)8]4+[35].The positively charged protonated surface offers more--OH interaction sites as well as stronger electrostatic attraction with the zwitterionic--SO3-groups on the dye molecules(Fig.3 bottom)leading to increased adsorption efficiency and capacity.

3.3.Central composite design for ARS adsorption

Based on the previous results,the Alizarin Red S.adsorption process has been thoroughly investigated using the central composite design as a representative anionic dye.The results are successively presented in the following sections.

Fig.1.(a)The nano cavities in UiO-66(back and frontBDC spacers shown in wire modelforclarity.(b)Enlarged picture showing Zr6O4(OH)4 nodes.Zr(violet)and O(red).c)XRDpattern of the pristine UiO-66 sample.Inset:IR spectrum.

Fig.2.Absorbance of anionic dyes a)ARS,b)Eosin,c)AF,d)MO and cationic dyes,e)NR,f)MB,g)FB,h)Safranine T before(blue)and after(red)adsorption on UiO66.

Table 3 Removal of anionic vs.cationic dyes on UiO-66

3.3.1.Effects of the studied parameters

A CCD with 30 runs that included six center points was constructed to evaluate the effect of 4 factors related to dye removal process.The factors measured were the initial concentration of Alizarin Red S.(x1;10–90 mg.L-1),pH(x2;2–10),adsorbent amount(x3;0.005–0.025),and shaking time(x4;30–150 min).The principal response was the level of dye removal(%).Based on the half-fractional CCD,an experimental design was developed and 30 dye removal processes were completed to remove Alizarin Red S.from aqueous medium(Table 2)and the estimated effects of different factors on the dye removal(%)were analyzed based on the following equation:

Fig.3.Zr6O4(OH)4 nodes interacting with anionic dyes via coordinating with sulfonic acid groups(top).Possible interaction through hydrogen bonding/electrostatic attraction between OHgroups of the MOF and--groups of the dye molecules(bottom).The proposed mechanism may be enhanced in acidic medium via MOF successive protonation into[Zr6(OH)8]4+which offers more interaction sites.

Fig.4.Pareto chart for the standardized effects on dye removal.

where nsrefers to the number of points collected at each level and ysrefers to the associated responses.The absolute values of the investigated effects divided by their standard value are plotted in the Pareto chart(Fig.4).The bars crossing the vertical line represent the factor or combination of factors that affect the response significantly.As shown in the figure,two factors have p-values below 0.05,indicating that they are different from zero at the 95.0%confidence level.The factors that gave rise to the main effects,i.e.,pH(x2)and adsorbent amount(x3),had a significant in fluence on the dye removal process.The pH showed a negative effect in that a higher percentage of dye removal was achieved on decreasing the pH.In contrast,the amount of absorbent had a positive effect as the percentage of dye removal increased with increasing sorbent amount.

3.3.2.Prediction capability of the regression model

A predictive equation for dye removal process based on the previously identified significant variables has been developed from the regression model.The smallest possible number of variables for an optimized design has been used in order to avoid a large degree of variability,thus,the equation for the fitted model is:

where y is the percentage of dye removal,x2is the pH and x3is the amount of adsorbent.

The R2value of the fitted model indicates that it explains 81.89%of the variability in the percentage of dye removal.The standard error of the predicted value shows that the standard deviation of the residuals is 8.4786.The predictive capability ofthe regression modelwas evaluated using a plot of the measured vs.predicted values(Fig.5).The model equations were found to be suitable to describe the experimentaldesign as indicated by the high slope values in this plot.The average error for the prediction was 13%,although only four experiments gave very high error values(＞25%)and most of these were for processes that led to low percentage of dye removal.

Fig.5.The prediction capability of the regression model.

Prediction errors for higher recoveries extractions were much lower,with an error of 9%.

3.3.3.Response optimization

Three-dimensionalsurface plots were constructed based on the model in order to predictthe relationships between independentfactors and the response.A plot of the surface response versus pH(x2)and absorbent amount(x3)was constructed based on CCD results(Fig.6).

Fig.6.Response surface plots showing effects of variables(x2,pH;x3,adsorbent amount)on the percentage of dye removal.

As can be observed,a high point was found at which the optimum dye removal was obtained at coordinates for a pH of-2.0 and an adsorbent amount of 1.95221.Based on RSM,the optimized dye removal process was achieved upon applying an initial concentration of 11.82 μg·g-1,adsorbent amount 0.0248 g and shaking time of 36 min,with the lowest pH in the studied range.The pH value was in the corner of the investigated range,however,DeCoste and coworkers have reported that the stability of UiO-66 in strong acidic medium,0.1 mol·L-1HCl,may be affected as indicated by slight changes in both the XRD pattern and FTIR spectrum[44].For this reason,we decided not to go below pH=2 to avoid sorbent decomposition.The structure of the Alizarin R S loaded UiO-66 has been studied by XRD.The collected data reveals a stable structure within the studied range of pH values(2–10).Under these optimum conditions,the adsorption experiment was conducted in three replicates;the maximum removal percentage is 98.4%±1.2%(Fig.7).

Fig.7.The maximum removal of Alizarin Red S.on UiO-66.

3.4.Isotherms

Langmuir,Freundlich,Tempkin and Dubinin–Radushkevich isotherm models were consulted to describe the adsorption process.The equation parameters of these models supply useful knowledge in terms of type of adsorption layer of the adsorbate and distribution of the active sites on the sorbent surface.

Langmuir isotherm is expressed as

where Q0and KLare Langmuir constants which can be calculated from slope and intercept of an Ce/qeversus Ceplot.The correlation coefficient R2is 0.999 which indicates that the adsorption process fits Langmuir model which is typically reported for the adsorption behavior on UiO-66[2,34,36].The maximum adsorption capacity(Q0)of Alizarin Red S.on the surface of UiO-66 is 400 mg·g-1(Table 4).

Table 4 Parameters of different isotherms for adsorption of Alizarin R S on UiO-66

To study the heterogeneity ofthe UiO-66 surface,the linearized form of Freundlich isotherm model was used:

where KFis adsorption capacity at unit concentration and 1/n is adsorption intensity.Plot of lg qeversus lg Ceenables determination of the values of Kfand 1/n as shown in Table 4.1/n value gives insights into the type of isotherm whether it is irreversible(1/n=0),favorable(0＜1/n＜1),or unfavorable(1/n＞1).

Heat of adsorption and the adsorbent–adsorbate interaction on adsorption isotherms were studied by Tempkin using the equation:

where KTthe equilibrium binding constant(L·mg-1)and B1is related to the heat of adsorption.T is the absolute temperature in K,R is the universal gas constant,8.314 J·mol-1·K-1.The experimental data was acceptably fitted to Tempkin isotherm(Table 4).High correlation coefficient implies that the heat of adsorption of all the dye molecules decreases with increasing coverage[45].

Dubinin–Radushkevich isotherm model describes the sorbent characteristics and free energy transported from the molecules to the surface.For this regard,the following equation was used:

K is a constant related to the adsorption energy,Qsis the theoretical saturation capacity and ? is the Polanyi potential.The applicability of this model was evaluated by plotting ln qeversus ?2where B and Qswere calculated.The correlation coefficient of Dubinin–Radushkevich isotherm was found to be 0.7189,indicating a poor correlation to the model(Table 4).

The collective analysis of the results indicates that the adsorption process is highly correlated to two models,Langmuir and Tempkin isotherms which suggest that different sorption mechanisms are involved in this adsorption process[46].

3.5.Kinetics

The adsorption mechanism and rate limiting steps such as diffusion control,chemical reaction and mass transport processes have been investigated using different kinetic models including pseudo first and second order,intra particle diffusion and Elovich model.The pseudofirst order equation is given as:

where qe(mg·g-1)and qt(mg·g-1)are the adsorption capacities at equilibrium and at time t(min),respectively;K1is the pseudo firstorder rate constant(min-1).Under the assumption that the adsorption capacity depends on surface active sites,the pseudo-second order equation is:

where K2is the pseudo-second order rate constant(g·mg-1·min-1).UiO-66 is a porous material so the rate of intra particle diffusion should be investigated using this equation:

where Kpis the rate constantofintra particle diffusion(mg·g-1·min-1/2)and C shows the boundary layer thickness.

Elovich model equation is expressed as

The adsorption kinetic parameters of all the models are summarized in Table 5 which indicates that the theoretical qevalue calculated from first order model is not close to the experimental value whereas the pseudo second order model gives capacity close to the experimental one.Also,the correlation coefficient R2of pseudo-second order is more than 0.99 while 0.89 in the case of first order.Hence,the adsorption process may fit pseudo-second order kinetics.These collective results indicate that the adsorption process may depend on the surface active sites[47]in agreement with other adsorption studies based on UiO66[2,34,36,38].

Table 5 Parameters of different kinetic models for adsorption of Alizarin R S on UiO-66

3.5.1.Record adsorption capacity of UiO-66 for ARS

The maximum adsorption capacity of UiO-66 for alizarin R S has been assessed based on Langmuir model(Q0=400 mg·g-1)(Table 4).This Q0value is significantly higher than the conventional sorbents reported so far for ARS[48–50].Table 6,summarizes the adsorption capacities ofARS on differentsorbents.Itobviously observed that UiO-66 has a capacity about 3 times higher than the best nano sorbent,(Multiwalled Carbon Nanotubes,161 mg·g-1).On the otherhand,the adsorption capacity of UiO-66 towards different adsorbates indicates a significantly higher affinity towards ARS as compared to other pollutants.This significantly high adsorption capacity towards ARS agrees well with the previous reports indicating high affinity of UiO-66 towards different anionic species including,Fluoride[2],Selenate and Selenite[34]and Phosphate[36].However,Chen and coworkers have recently reported the selective adsorption of cationic dyes(Methylene Blue,Rhodamine B and Neutral Red)on UiO-66[38],results that are not reproducible for MB and might be attributed to the experimental error of using filtering media in their set-up.Several types of filtering media have filtering/adsorptive capabilities towards organic dyes[51,52].

Table 6 Comparison of adsorption capacities of different pollutants on UiO-66

4.Conclusions

In summary,this study reveals exceptional adsorptive capabilities of the nano-porous microwave-synthesized MOF UiO-66 towards anionic dyes as compared to cationic dyes.The adsorptive removal of Alizarin Red S.from aqueous solutions has a record adsorption capacity of 400 mg·g-1in acidic conditions.The kinetics of adsorption can be well described by the pseudo second-order model with R20.999.The results of equilibrium studies data showed that the equilibrium follows the Langmuir isotherm with R2=0.999.Optimum conditions for the removal of Alizarin Red S.with UiO-66 as optimized by CCD are 0.0248 g of adsorbent,dye concentration of 11.82 mg.L-1and pH=2 at shaking time of 36 min.This study adds further evidence of the anticipated potential of MOFs in the field of organic pollutants removal from water.

Acknowledgements

The authors would like to thank the FDWaSC for supporting this work.Also,we thank Dr.Ahmed M.Ahmed and Dr.Abdullah A.Khamees for their assistance.

References

[1]G.Crini,Non-conventional low-costadsorbents for dye removal:a review,Bioresour.Technol.97(2006)1061–1085.

[2]X.Zhao,D.Liu,H.Huang,W.Zhang,Q.Yang,C.Zhong,The stability and de fluoridation performance of MOFs in fluoride solutions,Microporous Mesoporous Mater.185(2014)72–78.

[3]I.Uzun,Kinetics of the adsorption of reactive dyes by chitosan,Dyes Pigments 70(2006)76–83.

[4]V.S.Mane,I.D.Mall,V.C.Srivastava,Use of bagasse fly ash as an adsorbent for the removal of brilliant green dye from aqueous solution,Dyes Pigments 73(2007)269–278.

[5]S.J.Allen,G.McKay,K.Y.H.Khader,The adsorption of acid dye onto peat from aqueous solution–solid diffusion model,J.Colloid Interface Sci.126(1988)517–524.

[6]N.K.Amin,Removal of reactive dye from aqueous solutions by adsorption onto activated carbons prepared from sugarcane bagasse pith,Desalination 223(2008)152–161.

[7]I.C.Gon?alves,A.C.Gomes,R.Brás,M.I.A.Ferra,Biological treatment of effluent containing textile dyes,J.Soc.Dye.Colour.116(2000)393–397.

[8]S.Elwany,M.F.Elshhat,N.Burham,Green puri fication of different natural water samples in Fayoum governorate by using a novel and safe modi fied rice husk,Int.J.Adv.Res.2(2014)502–522.

[9]D.Couillard,The use of peat in wastewater treatment,Water Res.28(1994)1261–1274.

[10]H.Furukawa,K.E.Cordova,M.O'Keeffe,O.M.Yaghi,The chemistry and applications of metal–organic frameworks,Science 341(2013)1230444.

[11]G.Ferey,Hybrid porous solids:past,present,future,Chem.Soc.Rev.37(2008)191–214.

[12]J.Klinowski,F.A.Paz,P.Silva,J.Rocha,Microwave-assisted synthesis of metal–organic frameworks,Dalton Trans.40(2011)321–330.

[13]N.Stock,S.Biswas,Synthesis of metal–organic frameworks(MOFs):routes to various MOF topologies,morphologies,and composites,Chem.Rev.112(2012)933–969.

[14]M.Eddaoudi,J.Kim,N.Rosi,D.Vodak,J.Wachter,M.O'Keeffe,O.M.Yaghi,Systematic design of pore size and functionality in isoreticular MOFs and their application in methane storage,Science 295(2002)469–472.

[15]M.R.Saber,A.V.Prosvirin,B.F.Abrahams,R.W.Elliott,R.Robson,K.R.Dunbar,Magnetic coupling between metal spins through the 7,7,8,8-tetracyanoquinodimethane(TCNQ)dianion,Chemistry 20(2014)7593–7597.

[16]X.Zhang,M.R.Saber,A.P.Prosvirin,J.H.Reibenspies,L.Sun,M.Ballesteros-Rivas,H.Zhao,K.R.Dunbar,Magnetic ordering in TCNQ-based metal–organic frameworks with host–guest interactions,Inorg.Chem.Front.2(2015)904–911.

[17]J.R.Li,R.J.Kuppler,H.C.Zhou,Selective gas adsorption and separation in metal–organic frameworks,Chem.Soc.Rev.38(2009)1477–1504.

[18]J.Lee,O.K.Farha,J.Roberts,K.A.Scheidt,S.T.Nguyen,J.T.Hupp,Metal–organic framework materials as catalysts,Chem.Soc.Rev.38(2009)1450–1459.

[19]L.E.Kreno,K.Leong,O.K.Farha,M.Allendorf,R.P.Van Duyne,J.T.Hupp,Metal–organic framework materials as chemicalsensors,Chem.Rev.112(2012)1105–1125.

[20]A.C.McKinlay,R.E.Morris,P.Horcajada,G.Ferey,R.Gref,P.Couvreur,C.Serre,BioMOFs:metal–organic frameworks for biological and medical applications,Angew.Chem.49(2010)6260–6266.

[21]A.Maleki,B.Hayati,M.Naghizadeh,S.W.Joo,Adsorption of hexavalent chromium by metal organic frameworks from aqueous solution,J.Ind.Eng.Chem.28(2015)211–216.

[22]N.Bakhtiari,S.Azizian,Adsorption of copper ion from aqueous solution by nanoporous MOF-5:a kinetic and equilibrium study,J.Mol.Liq.206(2015)114–118.

[23]J.Li,Y.N.Wu,Z.Li,M.Zhu,F.Li,Characteristics of arsenate removal from water by metal–organic frameworks(MOFs),Water Sci.Technol.70(2014)1391–1397.

[24]Q.Zhang,J.C.Yu,J.F.Cai,L.Zhang,Y.J.Cui,Y.Yang,B.L.Chen,G.D.Qian,A porous Zrcluster-based cationic metal–organic framework for highly efficient Cr2O72-removal from water,Chem.Commun.51(2015)14732–14734.

[25]E.García,R.Medina,M.Lozano,I.Hernández Pérez,M.Valero,A.Franco,Adsorption of azo-dye orange II from aqueous solutions using a metal–organic framework material:iron-benzenetricarboxylate,Materials 7(2014)8037–8057.

[26]E.Haque,J.W.Jun,S.H.Jhung,Adsorptive removal of methyl orange and methylene blue from aqueous solution with a metal–organic framework material,iron terephthalate(MOF-235),J.Hazard.Mater.185(2011)507–511.

[27]E.Haque,J.E.Lee,I.T.Jang,Y.K.Hwang,J.S.Chang,J.Jegal,S.H.Jhung,Adsorptive removal of methyl orange from aqueous solution with metal–organic frameworks,porous chromium-benzenedicarboxylates,J.Hazard.Mater.181(2010)535–542.

[28]Y.S.Seo,N.A.Khan,S.H.Jhung,Adsorptive removal of methylchlorophenoxypropionic acid from water with a metal–organic framework,Chem.Eng.J.270(2015)22–27.

[29]Z.Hasan,J.Jeon,S.H.Jhung,Adsorptive removal of naproxen and clo fibric acid from water using metal–organic frameworks,J.Hazard.Mater.209-210(2012)151–157.

[30]S.Pu,L.Xu,L.Sun,H.Du,Tuning the optical properties of the zirconium–UiO-66 metal–organic framework for photocatalytic degradation of methyl orange,Inorg.Chem.Commun.52(2015)50–52.

[31]Z.Sha,J.S.Wu,Enhanced visible-light photocatalytic performance of BiOBr/UiO-66(Zr)composite for dye degradation with the assistance of UiO-66,RSC Adv.5(2015)39592–39600.

[32]Z.Sha,J.L.Sun,H.S.O.Chan,S.Jaenicke,J.S.Wu,Bismuth tungstate incorporated zirconium metalorganic framework composite with enhanced visible-light photocatalytic performance,RSC Adv.4(2014)64977–64984.

[33]Z.Sha,H.S.Chan,J.Wu,Ag2CO3/UiO-66(Zr)composite with enhanced visible-light promoted photocatalytic activity for dye degradation,J.Hazard.Mater.299(2015)132–140.

[34]A.J.Howarth,M.J.Katz,T.C.Wang,A.E.Platero-Prats,K.W.Chapman,J.T.Hupp,O.K.Farha,High efficiency adsorption and removal of selenate and selenite from water using metal–organic frameworks,J.Am.Chem.Soc.137(2015)7488–7494.

[35]C.Wang,X.Liu,J.P.Chen,K.Li,Superior removal of arsenic from water with zirconium metal–organic framework UiO-66,Sci.Rep.5(2015)16613.

[36]K.-Y.A.Lin,S.-Y.Chen,A.P.Jochems,Zirconium-based metal organic frameworks:highly selective adsorbents for removal of phosphate from water and urine,Mater.Chem.Phys.160(2015)168–176.

[37]H.Saleem,U.Ra fique,R.P.Davies,Investigations on post-synthetically modi fied UiO-66-NH2for the adsorptive removal of heavy metal ions from aqueous solution,Microporous Mesoporous Mater.221(2016)238–244.

[38]Q.Chen,Q.He,M.Lv,Y.Xu,H.Yang,X.Liu,F.Wei,Selective adsorption of cationic dyes by UiO-66-NH2,Appl.Surf.Sci.327(2015)77–85.

[39]Q.He,Q.Chen,M.Lü,X.Liu,Adsorption behavior of rhodamine B on UiO-66,Chin.J.Chem.Eng.22(2014)1285–1290.

[40]M.Taddei,P.V.Dau,S.M.Cohen,M.Ranocchiari,J.A.van Bokhoven,F.Costantino,S.Sabatini,R.Vivani,Ef ficient microwave assisted synthesis of metal–organic framework UiO-66:optimization and scale up,Dalton Trans.44(2015)14019–14026.

[41]J.K.Im,I.H.Cho,S.K.Kim,K.D.Zoh,Optimization of carbamazepine removal in O3/UV/H2O2system using a response surface methodology with central composite design,Desalination 285(2012)306–314.

[42]R.Tabaraki,A.Nateghi,Optimization of ultrasonic-assisted extraction of natural antioxidants from rice bran using response surface methodology,Ultrason.Sonochem.18(2011)1279–1286.

[43]F.Ragon,P.Horcajada,H.Chevreau,Y.K.Hwang,U.H.Lee,S.R.Miller,T.Devic,J.S.Chang,C.Serre,In situ energy-dispersive X-ray diffraction for the synthesis optimization and scale-up of the porous zirconium terephthalate UiO-66,Inorg.Chem.53(2014)2491–2500.

[44]J.B.DeCoste,G.W.Peterson,H.Jasuja,T.G.Glover,Y.G.Huang,K.S.Walton,Stability and degradation mechanisms of metal–organic frameworks containing the Zr6O4(OH)(4)secondary building unit,J.Mater.Chem.A 1(2013)5642–5650.

[45]G.Akkaya,A.?zer,Biosorption of Acid Red 274(AR 274)on Dicranella varia:determination of equilibrium and kinetic model parameters,Process Biochem.40(2005)3559–3568.

[46]A.ElNemr,A.El-Sikaily,A.Khaled,O.Abdelwahab,Removal of toxic chromium from aqueous solution,wastewater and saline water by marine red alga Pterocladia capillacea and its activated carbon,Arab.J.Chem.8(2015)105–117.

[47]Y.S.Ho,G.McKay,Kinetic models for the sorption of dye from aqueous solution by wood,Process.Saf.Environ.Prot.76(1998)183–191.

[48]P.B.Wagh,A.Shrivastava,Removal of alizarin red-S dye from aqueous solution by sorption on coconut shell activated carbon,J.Sci.Res.Rep.3(2014)2197–2215.

[49]J.Samusolomon,A.Devaprasath,Removal of alizarin red S(dye)from aqueous media by using Cynodon dactylon as an adsorbent,J.Chem.Pharm.Res.3(2011)478–490.

[50]F.Fu,Z.Gao,L.Gao,D.Li,Effective adsorption of anionic dye,alizarin red S,from aqueous solutions on activated clay modi fied by iron oxide,Ind.Eng.Chem.Res.50(2011)9712–9717.

[51]S.Homaeigohar,A.U.Zillohu,R.Abdelaziz,M.K.Hedayati,M.Elbahri,A novel nanohybrid nano fibrous adsorbent for water puri fication from dye pollutants,Materials 9(2016)848.

[52]R.Lin,B.Chen,G.Chen,J.Wu,H.Chiu,S.Suen,Preparation of porous PMMA/Na+montmorillonite cation-exchange membranes for cationic dye adsorption,J.Membr.Sci.326(2009)117–129.

[53]H.Roopaei,A.R.Zohdi,Z.Abbasi,M.Bazrafkan,Preparation of new photocatalyst for removal of alizarin red-S from aqueous solution,Indian J.Sci.Technol.7(2014)1882–1887.

[54]K.M.Joshi,S.V.Shrivastava,Degradation of alizarine red-S(a textiles dye)by photocatalysis using ZnO and TiO2as photocatalyst,Int.J.Environ.Sci.2(2011)8.

[55]H.V.Jadhav,S.M.Khetre,S.R.Bamane,Removal of alizarin red-S from aqueous solution by adsorption on nanocrystalline Cu0.5Zn0.5Ce3O5,Pelagia Res.Libr.2(2011)68–75.

[56]M.Ghaedi,A.Hassanzadeh,S.N.Kokhdan,Multiwalled carbon nanotubes as adsorbents for the kinetic and equilibrium study of the removal of alizarin red S and morin,J.Chem.Eng.Data 56(2011)2511–2520.

[57]Z.Hasan,N.A.Khan,S.H.Jhung,Adsorptive removalof diclofenac sodium from water with Zr-based metal–organic frameworks,Chem.Eng.J.284(2016)1406–1413.

[58]H.B.Shang,C.X.Yang,X.P.Yan,Metal–organic framework UiO-66 coated stainless steel fiber for solid-phase microextraction of phenols in water samples,J.Chromatogr.A 1357(2014)165–171.