Synthesis of Iron Doped Zeolite Imidazolate Framework-8 ...downloads.hindawi.com/journals/jchem/2017/5045973.pdf · ResearchArticle Synthesis of Iron Doped Zeolite Imidazolate Framework-8

Research ArticleSynthesis of Iron Doped Zeolite Imidazolate Framework-8 andIts Remazol Deep Black RGB Dye Adsorption Ability

Mai Thi Thanh12 Tran Vinh Thien3 Vo Thi Thanh Chau4 PhamDinh Du5

Nguyen Phi Hung6 and Dinh Quang Khieu1

1College of Science Hue University Hue City 530000 Vietnam2Faculty of Physics-Chemistry-Biology Quang Nam University Tam Ky 560000 Vietnam3Faculty of Natural Science Phu Yen University Phu Yen 620000 Vietnam4Faculty of Technology Industrial University of Ho Chi Minh City Quang Ngai Campus Quang Ngai City 570000 Vietnam5Faculty of Natural Science Thu Dau Mot University Thu Dau Mot City 820000 Vietnam6Department of Chemistry Quy Nhon University Quy Nhon City 590000 Vietnam

Correspondence should be addressed to Dinh Quang Khieu dqkhieuhueunieduvn

Received 8 February 2017 Revised 20 March 2017 Accepted 28 March 2017 Published 4 May 2017

Academic Editor Wenshan Guo

Copyright copy 2017 Mai ThiThanh et alThis is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Zeolite imidazole framework-8 (ZIF-8) and the iron doped ZIF-8 (Fe-ZIF-8) were synthesized by the hydrothermal process Theobtained materials were characteristic of X-ray diffraction (XRD) X-ray photoelectron spectroscopy (XPS) scanning electronmicroscope (SEM) nitrogen adsorptiondesorption isotherms and atomic absorption spectroscopy (AAS) The results showedthat the obtained Fe-ZIF-8 possessed the ZIF-8 structure with a large specific area ZIF-8 and Fe-ZIF-8 were used for the removalof Remazol Deep Black (RDB) RGB dye from aqueous solutions The various factors affecting adsorption such as pH initialconcentration contact time and temperaturewere investigatedThe results showed that the introduction of iron intoZIF-8 provideda much larger adsorption capacity and faster adsorption kinetics than ZIF-8 without iron The electrostatic interaction and 120587-120587interaction between the aromatic rings of the RDB dye and the aromatic imidazolate rings of the adsorbent were responsible forthe RDB adsorption Moreover the coordination of the nitrogen atoms and oxygen in carboxyl group in RDB molecules with theFe2+ ions in the ZIF-8 framework played a vital role for the effective removal of RDB from aqueous solution

1 Introduction

It is well-known that textile industries pulp mills anddyestuff manufacturing discharge a considerable amount ofcolored wastewaters which has provoked serious environ-mental concerns all over the world [1] Its removal is thereforeof prime importance Owing to their complicated chemicalstructures dyes are difficult to treat with municipal wastetreatment operations There are many treatment processessuch as chemical precipitation membrane filtration andalum coagulation of which adsorption is considered themost effective method widely employed to treat wastewatercontaining different classes of dyes

Nearly 60 of the dyes used in textile plants are azo dyeswhich are classified as mono- di- tri- and tetra-azo dyes

with the azo groups mainly bound to the benzene andnaphthalene rings Remazol Deep Black (denoted as RDB)RGB is a common diazo reactive dye in aqueous solutionsand widely used in textile industries [2] It is stable andhardly biologically degradable due to the presence of aromaticrings Thus much research attention has been paid to howto eliminate RDB from aqueous solutions Various processesincluding adsorption electrochemistry and biosorption forRDB treatment have been reported Soloman et al [2] stud-ied the electrochemical degradation of hydrolyzed RemazolBlack Performance of the batch recirculation system wascomparatively better than the other rector configurationsstudied with respect to capacity utilization and energy con-sumption Cardoso et al [3] used Brazilian pine-fruit shells(Araucaria angustifolia) in natural form and as adsorbents for

HindawiJournal of ChemistryVolume 2017 Article ID 5045973 18 pageshttpsdoiorg10115520175045973

2 Journal of Chemistry

Remazol Deep Black RGB

N=NN=NNaO3SOCH2CH2O2S

OH NH2

NaO3S SO3Na

SO2CH2CH2OSO3Na

Scheme 1 The structure of RDB molecule

the removal of RDB textile dye from aqueous effluents witha maximum sorption capacity of 746mg gminus1 Biosorptionof the azo dye by growing fungi (Aspergillus flavus) wasinvestigated in batch reactors The nearly complete removalof dye was found at initial concentration up to 250mgL andat pH 45 which was used as working pH value for removal ofdye in all the batch studies The removal of chemical oxygendemand (COD) was found to be 90 at 100mgL initialconcentration of dye [4]

Metal-organic frameworks (MOFs) are hybrid materi-als with ordered three-dimensional frameworks via strongmetal-ligand bonds between metal cations and organiclinkers [5] Since their discoveries MOFs have receivedsignificant attention in their potential applications in gasstorage [6ndash13] separation [14 15] and heterogeneous catal-ysis [16ndash19] The zeolitic imidazolate framework (ZIF) mate-rials have zeolite like topologies and belong to an impor-tant class of MOFs materials with interesting adsorptionseparation and catalytic properties [20ndash23] Among themZIF-8 [Zn(2-methylimidazole)2sdot2H2O] constructed from 2-methylimidazole ligands and Zn(II) center ions exhibitshigher thermal and chemical stability than other MOFs [24]Many studies show that ZIF-8 exhibited efficient removalof arsenic and organic pollutants from aqueous solutionsJiang et al [25] reported that ZIF-8 as a novel adsorbentfor fast removal of 1H-benzotriazole and 5-tolyltriazole withregard to adsorption isotherms kinetics thermodynamicsdesorption and adsorbent regeneration Lin and Chang [26]reported that the adsorption capacity of ZIF-8 was muchhigher than that of fly ash activated carbon zeolites andso forth showing its promising potential for the removal ofhumic acid P-Arsanilic acid which is widely used as feedingadditive in the poultry and pork industries to enhance thefeeding efficiency was efficiently removed by using ZIF-8 asan adsorbent [27] ZIF-8 exhibited the high arsenatearseniteadsorption (up to 50mgsdotgminus1 for As(II) and 60mgsdotgminus1 forAs(V)) [28] Zheng et al [29] reported a free solvent synthesisof core-shell Fe3O4zeolitic imidazolate frameworks-8 (ZIF-8) via two stepsThe introduction ofmagnetic iron oxide intoZIF-8 facilitated the separation of adsorbents by themagneticfield Fe3O4ZIF-8 showed good adsorption properties formethylene blue with a maximum adsorption capacity of202mg gminus1 According to these studies ZIF-8 not only ishighly stable in water but also exhibits promisingly highadsorption capacities Owing to these features ZIF-8 shouldbe promising and feasible adsorbents to organic pollutantsfrom aqueous solutions On the other hand in addition

to being a catalyst iron species can act as adsorptive sitesfor adsorption processes The combination of iron and ZIF-8 (denoted as Fe-ZIF-8) is expected to provide a noveladsorbent due to large accessible surface area and abundantactive surface sites

In this study the introduction of iron into ZIF-8 by aone-step process was performed with the aim of improvingits adsorption ability Fe-ZIF-8 was used as an adsorbent forremoving RDB dye In addition the dye adsorption overZIF-8 was performed for comparison The iron oxide incor-porated into the ZIF-8 framework significantly enhanced aRDB adsorption capacity compared to bare ZIF-8 A possibleadsorption mechanism was suggested based on adsorptionat various pH values and surface charges on Fe-ZIF-8

2 Experimental

21 Materials Zinc nitrate hexahydrate (Zn(NO3)2sdot6H2ODaejung Korea ge99) iron(II) sulfate heptahydrate(FeSO4sdot7H2O Merck Germany gt 99) methanol (CH3OHMerck Germany) and 2-methylimidazole (C4H6N2 Sigma-Aldrich USA 99) were utilized in this paper Remazolblack B RGB (C26H21N5Na4O19S6 molecular weight =99182) was obtained from Thuy Duong Textile CompanyVietnam The structure of RDB is shown in Scheme 1

22 Preparation of ZIF-8 and Iron Doped ZIF-8 (Fe-ZIF-8)ZIF-8 and Fe-ZIF-8 were synthesized as in [30 31] Briefly28mmol of zinc(II) and iron(II) (molar ratio of FeZn = 010or 19) were dissolved in 14mmol of methanol A solutionconsisting of 644mmol of 2-methylimidazole and 14molof methanol was added to the Zn-Fe based solution andvigorously stirred for 24 hs at ambient temperature Nitrogenwas bubbled through the solution to minimize the oxidationreaction of Fe(II) to Fe(III) species Finally this solutionwas centrifuged at 300 rpm and washed thoroughly withmethanol This washing procedure was repeated 3 times Theresultant crystals were dried overnight at 120∘CThe obtainedsamples with the molar ratio of Fe(II)(Zn(II) + Fe(II)) being010 and 110 were named ZIF-8 and Fe-ZIF-8 respectivelyIn the obtained samples ZIF-8 was white and Fe-ZIF-8 waslight brown

23 Determination of the Point of Zero Charge The pH atthe potential of zero charge (pHPZC) of ZIF-8 and Fe-ZIF-8 was measured by the pH drift method [32] To a seriesof 100mL flasks 5mL of 01M NaCl solution and 40mL of

Journal of Chemistry 3

distilled water were added The initial pH value (pHi) of thesolution was adjusted from 23 to 120 by adding either 01MNaOH or 01M HCl The total volume of solution in eachflask wasmade exactly as 50mL by adding distilled waterThe001MNaCl solutionswith different pHvalueswere obtainedNitrogen was bubbled through the solution to eliminate thedissolved CO2 Then 002 grams of the sample was added toeach flask and mixtures were sealed and shaken for 24 hoursthen the final pH (pH119891) of solution was recorded The plotof ΔpH = pH119894 minus pH119891 versus initial pH119894 was conducted Thepoint of intersection of curve with abscissa at which ΔpH =0 provided pHPZC

24 Adsorption Kinetics Study Experiments were conductedin a batch process The 3 L plastic beaker was equippedwith a stainless steel flat blade impeller using an electricmotor to stir the dye solution Samples (ZIF-8 or Fe-ZIF-8) (04 g) were vigorously mixed with 1000mL of RDBsolution in the beaker at a fixed temperature Ten millilitresof solution was drawn at preset intervals of time throughtap and the solid was removed by centrifuging process Theresidual dye concentrations were determined using UV-Visspectrophotometry The experiments were conducted withvarious RDB concentrations ranging from 30 to 50 ppmTheamount of the dye adsorbed by the adsorbent was calculatedby the following equation

119902119905 = 119881 (119862119900 minus 119862119905)119898 (1)

where 119902119905 is the amount of dye adsorbed per unit of adsorbentamount (mg gminus1) at 119905 time 119862119900 the initial dye concentration(mg Lminus1) 119862119905 the dye concentration (mg Lminus1) after the batchadsorption procedure 119881 the volume of dye solution (L) and119898 the mass (g) of the adsorbent It is reasonably assumed thatthe adsorption of dye from solution follows reversible first-order kinetics The heterogeneous equilibrium between dyein solution and solid adsorbent is illustrated as follows [32]

dye (solution) 1198961997888rarrlarr9978881198962

dye (adsorbent) (2)

where 1198961 and 1198962 are the forward and backward rate constantsrespectively

The equilibrium constant 1198700 defined as 11989611198962 could beexpressed [26]

1198700 = 11989611198962 =

(1198620 minus 119862119890)119862119890 (3)

where 1198620 and 119862119890 are dye concentrations (mg Lminus1) at ini-tial and equilibrium time respectively and the others aredescribed above

Pseudo-first-order kinetic of adsorption was investigatedby Natarajan-Khalaf equation [33]

ln119862119900119862119905 = 119896ads119905 (4)

where 119896ads is the rate constant of adsorption process

The slope of the linear plot of ln119862119900119862119905 versus 119905 willprovide the value of 119896ads

Based on the relaxation method [34 35] 119896ads could beexpressed as

119896ads = 1198961 + 1198962 (5)

Then the forward and backward rate constants could bederived from (3) and (5)

The kinetics of diffusion was studied by Webberrsquos intra-particle-diffusion model Webberrsquos intraparticle-diffusionmodel is described in the following equation [36 37]

119902119905 = 119896119901 sdot 11990512 + 119868 (6)

where 119896119901 is intraparticle-diffusion rate constant(mg gminus1minminus05) and 119868 the intercept which reflects thelayer boundary effect

The analysis of the multilinearity in pore and film-diffusion plot using Webberrsquos plot was conducted by usingpiecewise linear regression proposed by Malash and El-Khaiary [38]

In this method the experimental data could be fixed forone two or three linear segmentsrsquo line by Webberrsquos model

One linear segmentrsquos line 119884 = 119861 + 119860119883 (twoparameters)Two linear segmentsrsquo line 119884 = 119861 + 119860119883 + 119862(119883 minus 119863) lowastsign(119883 minus 119863) (four parameters)Three linear segmentsrsquo line 119884 = 119861+119860119883+119862(119883minus119863)lowastsign(119883minus119863)+119864(119883minus119865)lowastsign(119883minus119865) (six parameters)

where the values of 119860 119861 119862 119863 119864 and 119865 are estimated bynonlinear regression 119863 and 119865 called breakpoints are theboundaries between the segmentsTheMicrosoftExcel ldquosignrdquofunction is defined as follows

sign (119883 minus 119886) =

1 if 119909 gt 1198860 if 119909 = 119886minus1 if 119909 lt 119886

(7)

The example for the two linear segmentsrsquo equation wasexpressed as follows

119884 =

119861 minus 119862119863 + 119883 (119860 + 119862) if 119883 gt 119863119861 + 119860119863 if 119909 = 119863119861 + 119862119863 + 119883 (119860 minus 119862) if 119909 lt 119863

(8)

Then the linear equation of the first segment is 119910 = 1198861119909 + 1198871where 1198871 = 119861 + 119862119863 and 1198861 = 119860 minus 119862

Then the linear equation of the second segment is 119910 =1198872119909 + 1198862 where 1198872 = 119861 minus 119862119863 and 1198862 = 119860 + 119862

Nonlinear regression determines the modelrsquos parametersby the least squares methodThis is calculated by minimizingthe sum of squared deviations SSE119878 by numerical optimiza-tion techniques using Solver function inMicrosoft ExcelThefunction for minimization is

SSE119878 =119873

sum1

(119910exp minus 119910est)2 (9)

4 Journal of Chemistry

where 119910exp is experimental datum and 119910est is the valueestimated by model

The determination coefficient 1198772 is obtained by the ex-pression

1198772 = 1 minus SSE119878SSE119879

(10)

where SSE119879 is the total sum of squares equal to sum1198731 (119910exp minus119910mean)2 (119910mean is the mean value of 119910)The comparison of models was based on Akaikersquos Infor-

mation Criterion (AIC) [38ndash40] The AICc determines howwell the data support each model The value of AIC can bepositive or negative The model with the lowest AICs scoreis most likely correct The AICc (for a small size sample) iscalculated for each model from the following equations

AICc = 119873 ln(SSE119879119873 ) + 2119873119901 + 2119873119901 (119873119901 + 1)119873 minus 119873119901 minus 1 (11)

where119873 is the number of experimental points and119873119901 is theparameter sum of model

25 Thermodynamic and Isothermal Studies Experimentalprocedure was conducted as an adsorption kinetics studyHowever the temperature of the process was fixed at 298308 and 318 K The activation energy 119864119886 was determined byArrhenius equation [35]

119896 = 119860119890minus119864119886119877119879 (12)

where 119896 is the rate constant equal to the rate constant 119896ads inNatarajan and Khalaf equation 119860 the frequency factor 119877 gasconstant (8315 Jmolminus1 Kminus1) and 119879 absolute temperature inKelvin

Taking the natural logarithm of both sides of (12) oneobtains

ln 119896 = minus 119864119886119877119879 + ln119860 (13)

By linear plotting ln 119896 versus 1119879 the 119864119886 could be obtainedfrom slope (minus119864119886119877)

Thermodynamic parameters of activation can informwhether or not the adsorption process follows an activatedcomplex or is prior to the final adsorption Thermody-namic parameters of activation including the enthalpy (Δ119867)entropy Δ119864 and free energy Gibbs Δ119866 of activation forRBB adsorption kinetics were obtained by applying Eyringequation [41 42]

The Eyring equation in its thermodynamic version is asfollows

119896 = (119896119887119879ℎ ) 119890minusΔ119866119877119879 = (119896119887119879ℎ ) 119890Δ119878119877 sdot 119890minusΔ119867119877119879 (14)

where 119896 is the rate constant equal to the rate constant 119896adsin Natarajan-Khalaf equation the 119896119887 (13807 times 10minus23 J Kminus1)the Boltzmann constant and ℎ (6621 times 10minus34 J s) the Planckconstant

Taking the natural logarithm of both sides of (14) Eyringequation in linear form was obtained

ln( 119896119879) = ln(119896119887ℎ ) + Δ119878119877 minus Δ119867

119877119879 (15)

By linear plotting ln(119896119879) versus 1119879 Δ119878 and Δ119867 wereobtained from the slope (Δ119867119879) and 119910-intercept [ln(119896119887ℎ)+(Δ119878119877)]

The Gibbs free energy of activation can be obtained by

Δ119866 = Δ119867 minus 119879Δ119878 (16)

In order to assess if the adsorption process is spontaneousor not the thermodynamic parameters of adsorption areneeded The standard Gibbs free energy of adsorption (Δ1198660)is given by the expression [35 43]

Δ1198660 = Δ1198670 minus 119879Δ1198780 (17)

where Δ1198660Δ1198670 andΔ1198780 are the standard Gibbs free energyenthalpy and entropy respectively

Δ1198660 is given by vanrsquot Hoff rsquos equation

Δ1198660 = minus119877119879 ln119870119889 (18)

where 119870119889 is the distribution coefficient of the solute ionsand equals (119902119890119862119890) [27 44 45] and the others are describedabove

By replacing (18) with (17) one obtains

ln119870119889 = minusΔ1198670119877119879 + Δ1198780119877 (19)

The value ofΔ1198670 andΔ1198780 was determined from the slope andintercept of the linear plot of ln119870119889 versus 1119879

The adsorption isotherms were developed at 25∘C Theadsorption time was conducted for 24 hours to confirmsaturation Thereafter supernatant liquid was collected bycentrifugation and the final dye concentrations were deter-mined using UV-Vis spectrophotometry

The experimental data were analyzed according to theFreundlich and Langmuir models

Langmuir Isotherm The Langmuir equation is valid formonolayer sorption onto the surface It could be expressedas follows [37 46]

119902119890 = 119870119871 sdot 119902mom sdot 1198621198901 + 119870119871 sdot 119862119890 (20)

where 119902mom is the maximum monolayer capacity amount(mg gminus1)119870119871 is Langmuir equilibrium constant (Lmgminus1) andthe others are described above

The essential characteristics of the Langmuir isothermcan be expressed in terms of a dimensionless constantseparation factor 119877119871 which is performed as

119877119871 = 1(1 + 119862119900 sdot 119870119871) (21)

Journal of Chemistry 5

where the value of 119877119871 indicates the type of isotherm unfa-vorable (119877119871 gt 1) linear (119877119871 = 1) favorable (0 lt 119877119871 lt 1) orirreversible (119877119871 = 0) [47]Freundlich Isotherm Freundlich equation is an empiricalrelation based on the adsorption of adsorbates onto theheterogeneous surface It is represented as follows [48 49]

119902119890 = 119870119865 sdot 1198621119899119890 (22)

where 119870119865 is the Freundlich constant which is a measure ofadsorption capacity and 119899 an empirical parameter related tothe nature and strength of the adsorption process A largevalue of 119899means that the surface is heterogeneous For valuesin the range 1 lt 119899 lt 10 adsorption is favorable Valuesof 119899 between 2 and 10 represent good adsorption processeswhereas 1 lt 119899 lt 2 indicates that adsorption capacity is onlyslightly suppressed at lower equilibrium concentrations [50]

The parameters of models ((20) and (22)) were estimatedby nonlinear regression method using Solver function inMicrosoft Excel To quantitatively compare the applicabilityof each model apart from the regression coefficient (1198772) theChi-square test (1205942) was calculated as follows [51]

1205942 = sum (119902119890exp minus 119902119890est)2119902119890est (23)

where 119902119890exp and 119902119890est are the adsorption capacity at theequilibrium experimental condition and adsorption capacityestimated by model respectively

A small value of 1205942 indicates that the data obtained fromthe model is consistent with the experimental value

26 Characterization of Materials Thepowder X-ray diffrac-tion (XRD) patterns were recorded by a D8 AdvanceBruker (Germany) with CuK120572 radiation (120582 = 15406 A)The morphology of the obtained sample was determinedby scanning electron microscope (SEM) using SEM JMS-5300LV (Japan) The specific surface area of the sample wasdetermined by nitrogen adsorptiondesorption isothermsusing a Micromeritics 2020 volumetric adsorption analyzersystem (USA) Thermal behaviors of the obtained materialswere analyzed by means of thermal analysis (TG-DTA)using Labsys TG Setaram (France) The element analysis wasconducted by atomic absorption spectrometry (AAS) usingAA6800 Shimazu (Japan) Visible spectrophotometry wasmeasured by Lambda 25 Spectrophotometer PerkinElmer(Singapore) at 120582max of RDB dye (600 nm)

3 Results and Discussion

31 Characterization of ZIF-8 and Fe-ZIF-8 Figure 1 showsXRD patterns of ZIF-8 and Fe- ZIF-8 The XRD pattern ofZIF-8 in this work agreed well with patterns from [30 31]There was a well-defined diffraction (011) at two theta = 716∘in the XRD pattern of ZIF-8 indicating that the crystallinityof ZIF-8 in this work was relatively highTheXRD patterns ofFe-ZIF-8 also exhibited characteristic peaks of ZIF-8 and no

(334

)(2

33)

(114

)

(134

)

(222

)(0

13)

(022

)(002

)(0

11)

(112

)

Fe-ZIF-8

ZIF-8

Inte

nsity

(arb

)500

Cps

10 20 30 40 50 6002 theta (degree)

Figure 1 XRD patterns of ZIF-8 and Fe-ZIF-8

characteristic peaks of iron oxides were observed Howeverintensity of these diffractions decreases as a certain amountof iron was doped into the ZIF-8 framework

SEM images of ZIF-8 and Fe-ZIF-8 are presented inFigure 2Morphology of ZIF-8 consisted of spherical particleswith a diameter around 100 nm while the irregular shapes ofFe-ZIF-8 with sizes around 01ndash100 nm were observed Theintroduction of iron into ZIF-8 caused a significant change inmorphology of ZIF-8

The zinc and iron compositions were analyzed by AASThe results are presented in Table 1 Iron percentage in thefinal product (0116) was higher than the original (0100)ThepH of the synthesized gel was around 45 Then iron(II) waslikely to incorporate completely into ZIF-8 however possiblyZn(II) was partly dissolved in the solutionThis is reason whythere was an increase in the percentage of iron in the finalproduct

The XPS spectra indicated a chemical state of elementthat is iron (Fe2p) and zinc (Zn2p) The peak of Zn2p12 (1044 eV) and Zn2p32 (102096 eV) observed for bothsamples confirmed the existence of Zn(II) (Figure 3) ForZIF-8 the peak of Fe 2p32 was inconspicuous indicatingthat iron was a very minor component (in fact it could notbe detected) Only peak Fe2p32 for Fe(II) at 70998 eV wasdetected implying the main iron in Fe-ZIF-8 was Fe(II) Thepercentage of oxidation state of iron calculated from peakareas was listed in Table 1 It was worth noting that the initialiron source of Fe(III) was also tested to incorporate into ZIF-8 but the solid product was not obtainedThis means that thepresence of Fe(III) in the initial synthesized mixture was notfavorable for the formation of ZIF-8 structure

Based on ZIF-8 with space group of 11986843119898 [52] the cellparameter of ZIF-8 and Fe-ZIF-8 was expressed by

11198892 =

(ℎ2 + 1198962 + 1198972)1198862 (24)

where 119889 is spacing distance 119886 is cell parameter and ℎ 119896 119897 areMiller indexes of diffraction planes

6 Journal of Chemistry

ZIF-8

(a)

Fe-ZIF-8

(b)

Figure 2 SEM observations of ZIF-8 and Fe-ZIF-8

Table 1 Chemical composition of ZIF-8 and Fe-ZIF-8 analyzed by AAS and XPS

AdsorbentAAS XPS

Zn(molg)

Fe(molg)

Molar ratio(Fe(Zn + Fe))

Initial molar ratio(Fe(Zn + Fe))

Fe(II)()

Fe(III)()

ZIF-8 0043 mdash 0 mdash mdash mdashFe-ZIF-8 0038 0005 0116 0100 100 0000

Table 2 Textural properties of ZIF-8 andFe-ZIF-8

Adsorbent 1119878BET (m2g) 2119878Langmuir (m2g) 3119863pore (nm) 4119881pore (cm

3g)ZIF-8 1383 1909 334 116Fe-ZIF-8 1243 1599 206 0641119878BET specific surface area calculated by BETmodel 2119878Langmuir specific surface area calculated by Langmuir model 3119863pore pore diameter calculated by BJHmodel 4119881pore pore volume

The cell parameter of ZIF-8 (16800 A) and Fe-ZIF-8(16977 A) seems to be unchangeable Since the charge andradii of Zn2+ (075 A) and Fe2+ (074 A) are similar it is likelythat Fe(II) can substitute Zn(II) in ZIF-8 or disperse highly inferrous forms

Figure 4 shows the nitrogen adsorptiondesorptionisotherms of ZIF-8 and Fe-ZIF-8 All samples exhibited typeIV with H4 which is characteristic of mesoporous materialsFe-ZIF-8 possessed a shape which is different from ZIF-8 at high relative pressure This result suggests that theporous structure was distorted due to the incorporation ofiron oxides ZIF-8 exhibited a high specific surface area of1380m2sdotgminus1 (calculated by BET model) which was similar tothat found in the previous literature [30 31 53]

The introduction of iron oxide into ZIF-8 lowered thespecific surface area the pore diameter and the pore volumeThe specific surface areas are 1380 and 1243m2sdotg1 for ZIF-8 and Fe-ZIF-8 respectively (see Table 2) This also gaveevidence of the encapsulation of iron oxides within the poresof framework which brought about the lowering of accessiblevoid space for N2 gas molecules

The thermal stability of materials was tested by TG-DTAZIF-8 and Fe-ZIF-8 were found to be highly stable up to220∘C and 250∘C Beyond this temperature the frameworkslowly started to decompose and a flat valley was obtained till

700∘C (Figure 5) The incorporation of iron in ZIF-8 seemsto make the materials more stable This behavior was alsoobserved as TiO2 is doped in ZIF-8 [54]

The stability of ZIF-8 in water with different pHs was alsostudied Figure 6 presents XRD patterns of ZIF-8 which weresoaked in water with pH from 2 to 12 The pH of the solutionwas adjusted byNaOH001MorHCl 001MThe crystallinityof ZIF-8 nanoparticles was retained well at pH gt 3ndash12 whichproved that ZIF-8 was stable in aqueous solutions with pHsin the range of 3ndash12

32 A Study on RDB Adsorption onto ZIF-8 and Fe-ZIF-8

321 Effect of Initial RDB Concentration The experi-mental results for adsorption at various concentrations(30ndash50mgsdotLminus1) with contact time are shown in Figure 7 Asseen from Figure 7 the adsorption capacity of adsorbentincreases as initial dye concentrations go up The RDBadsorption of Fe-ZIF-8 was higher than that of ZIF-8 in thesame initial concentration The adsorption capacity of RDBonto ZIF-8 increased from 3020 to 4214mg gminus1 and thatonto Fe-ZIF-8 increased from 5036 to 7679mg gminus1 as theinitial concentrations increased from 30 to 50mg gminus1 Theinitial dye concentration provided a critical driving force toovercome all the mass transfer resistance of RDB between

Journal of Chemistry 7

ZIF-8 ZIF-8- Zn2p

1050 1045 1040 1035 1030 1025 1020 1015 10101055Binding energy (eV)

10

12

14

16

18

20

Inte

nsity

720 716 712 708 704 700724Binding energy (eV)

760

770

780

790

800

810

Inte

nsity

times102

Experiment lineFe2p32Fe(II)

Baseline

(a)

Fe - ZIF-8- Fe2p Fe-ZIF-8-Zn2p

720 716 712 708 704 700724Binding energy (eV)

1050 1045 1040 1035 1030 1025 1020 1015 10101055Binding energy (eV)

8

10

12

14

16

18

20

22

24

26

Inte

nsity

68

70

72

74

76

78

80

Inte

nsity

times101 times102

(b)

Figure 3 XPS Fe2p and Zn2p core level spectra of ZIF-8 (a) and Fe-ZIF-8 (b)

Fe-ZIF-8

ZIF-8

02 04 06 08 1000Relative presure (PP0)

150200250300350400450500550600650700750800

Adso

rbed

(cm

3 middotgminus

1ST

P)

Figure 4 Nitrogen adsorptiondesorption isotherms of ZIF-8 andFe-ZIF-8

the aqueous solution and the adsorbent surface [55] As aresult high initial RDB concentration might promote theadsorption process Figure 7 indicates that the adsorptionof RDB was fast in the earlier stage (0ndash100 minutes) and

gradually reached the equilibrium It is found that the timerequired to obtain the adsorption equilibrium was around250 minutes

Due to the porosity of ZIF-8 and Fe-ZIF-8 intraparticlediffusion was expected in the adsorption process This wasexamined by plotting RDB uptake 119902119905 against the squareroot of time 11990512 as (6) If intraparticle diffusion is therate-limiting step this plot will give a straight line and anintercept equal to zero However previous studies by variousresearchers showed that these plots represent multilinearity[56] This application often suffers uncertainties caused bythe multilinear nature of their plotTherefore the confidenceintervals for parameters are determined to estimate theiruncertainties If the 95 confidence interval of 119910-interceptdoes not contain zero or it varies from this negativepositivevalue to another 119910-intercept is significantly different fromzero It is concluded that the line does not pass through theorigin or vice versa (in this study the SPSS-version 21was usedto calculate the confidence interval)

8 Journal of Chemistry

DTA

TGA

ZIF-8

TGA

()

TGA

()

minus6282minus24735

minus43552

TGA

DTA

Fe-ZIF-8

minus49985

minus1355

43327∘C

554∘C 19439∘C 47682∘C

52465∘C

minus20

0

20

40

60

80

100

200 400 600 8000Temperature (∘C)

200 400 600 8000Temperature (∘C)

minus20

0

20

40

60

80

100

0

10

20

30

40

minus20

minus15

minus10

minus5

051015202530

minus0447mg

minus1649mg

minus4444mg

minus2524mgminus0641mg

DTA

(휇V

mg)

DTA

(휇V

mg)

Figure 5 TGA-DTA of ZIF-8 and Fe-ZIF-8

Table 3 Comparison of piecewise linear regression for one two and three linear segments by AIC

Adsorbent 119862RDBmgsdotLminus1

One linear segmentrsquos regression Two linear segmentsrsquo regression Three linear segmentsrsquoregression

SSE 1198772 AIC SSE 1198772 AIC SSE 1198772 AIC

ZIF-830 24707 0905 6272 8293 0968 3930 8293 0968 453440 73684 0858 9113 18935 0964 6077 18935 0964 667850 61932 0884 8947 3391 0994 1605 3391 0995 2246

Fe-ZIF-830 83506 0650 9438 5413 0977 2851 5414 0977 342340 207079 0973 11799 31780 0967 7276 31780 0967 802450 98431 0846 9865 2373 0996 677 2133 0997 1001

ZIF-8

Inte

nsity

(arb

)

10 15 20 25 3052 theta (degree)

pH = 2pH = 3

pH = 6

pH = 10

pH = 12

1000

Cps

Figure 6 XRD diffraction patterns of the ZIF-8 soaked in waterwith different pHs (119898ZIFminus8119881solution = 01 g 30ml pH = 2ndash12 sockingtime 24 hours)

Piecewise linear regression proposed by Malash andEl-Khaiary [38] was applied to analyze multilinearity ofWebberrsquos plot Since increasing the number of linear segmentsleads to an increase in the number of regression parametersa decrease in SSE119879 naturally follows For this reason SSE119879solely cannot be used to assess the goodness of fit for modelsThe well-known statistical method for model comparison isAkaikersquos Information Criterion (AIC) [38 57] This methodadvises which model is more likely to be correct The smaller

AIC value indicates a more compatible model For exampleFigure 8 illustrates experimental data and piecewise linearregression lines with initial concentration 50mg Lminus1 for ZIF-8 and Fe-ZIF-8 The experimental points seem to be closeto regression lines for two or three linear segment linesWe could not estimate visually which one is more likelycorrect The values of AICc for RDB adsorption onto ZIF-8 are 8947 and 1605 and 2246 for one-segment two-segment and three-segment models respectively Those forRDB adsorption onto Fe-ZIF-8 are 9865 and 667 and 1001for one-segment two-segment and three-segment modelsrespectively It is decided to accept the model with two linearsegments as the correct model because of the lowest value ofAICc in this model

A comparison of models based on AICc is presentedin Table 3 Table 3 shows that Webberrsquos model analyzedtwo segments linear regression provided the lowest AICccompared to one- or three-segment models In conclusionthe experimental data best fit with the two segmentsrsquo linearregression model Results of piecewise two linear segmentsrsquoregression for different initial concentrations are shown inTable 4 For illustration for 50mg Lminus1 concentration of ZIF-8in Figure 8 the intercepts of the first line in the Webber plotwere minus1019 with 95 confidence interval (minus1228 to minus810)This value of the intercept was significantly different fromzero It means the line did not pass through the origin Thesimilar behaviors were observed for all the other cases Theseresults indicate that the adsorption of RDB dye onto ZIF-8

Journal of Chemistry 9

ZIF-8 Fe-ZIF-830mg

50mg40mg 30mg

50mg40mg

50 100 150 200 2500Time (minute)

50 100 150 200 2500Time (minute)

05

10152025303540455055606570758085

qe(m

gmiddotgminus

1)

0

5

10

15

20

25

30

35

40

45qe(m

gmiddotgminus

1)

Figure 7 Effect of contact time on the adsorption of RDB by ZIF-8 and Fe-ZIF-8 (119862RDB = 30ndash50mgsdotLminus1 adsorbent = 02 g and initial pH =82 119881RDB = 500ml temperature = 30∘C and stirring rate = 500 rpm)

Table 4 Results of piecewise regression for the two linear segments for ZIF-8 and Fe-ZIF-8 (the values in parentheses are at a 95 confidencelevel)

Adsorbent Concentration(mgsdotLminus1)

Break point(minute05)

The first linear segment The second linear segmentIntercept 1 Slope 1 Intercept 2 Slope 2

ZIF-8

30 592 minus283(minus511 minus055) 468 1916

(1758 2074) 075

40 707 minus1029(minus1710 minus347) 483 2706

(2529 2883) 067

50 894 minus1019(minus1228 minus810) 558 3485

(3221 3749) 052

Fe-ZIF-8

30 949 2258(2016 2500) 385 7807

(7500 8115) minus19740 1000 813

(264 1363)682 9879

(9399 10359) minus226

50 894 2538(2359 2716) 601 8015

(7858 8172) minus015

or Fe-ZIF-8 in the first stage was controlled by film diffusion(eg surface adsorption and liquid film diffusion) whilethe second was assigned to intraparticle diffusion [57ndash59]The breakpoints (see Table 4) reflect the adsorption processmoving from one mechanism to another The times of phasetransition calculated by the square of breakpoints variedfrom 36 to 100 minutes The adsorption could be dividedinto two distinct phases by the time of phase transition (i)main adsorption of RDB molecules within 0ndash100 minutes ofthe contact times this process was rather slow compared toother adsorbents [40 57] in which the fast and instantaneousadsorption was observed since the pore sizes of Fe-ZIF-8or ZIF-8 are around 1 nm [45] the adsorption of RDB withfour benzene rings in which each has a critical dimension ofaround 0662 nm [60] on ZIF-8 limited the diffusion of theRDBmolecule to the inner pore structure however abundantactive adsorption sites could be obtained at its external

surface which is helpful to the adsorption of dye molecules(ii) a gradual attainment of the equilibrium where only about5ndash10 of the adsorption was encountered This is due to theutilization of the all active sites on the adsorbent surfaceThe first linear segment indicated a boundary layer effectwhile the second linear segment was assigned to intraparticlediffusion [58 59]

The rate parameter 119896119901 increases with an increase ininitial concentrations However the effect of initial RDB onrate parameters was irregular for RDB adsorption onto Fe-ZIF-8 The irregularity of 119896119901 could be related to that factthat iron incorporated caused the heterogeneity of ZIF-8structure

The experimental data usually exhibit the multilinearplots two or more stages influencing the adsorption processIn the statistical method the piecewise linear regression isrelevant to analyzing the data with multilinearity instead of

10 Journal of Chemistry

ZIF-8One segment

Two segments

Three segments

Fe-ZIF-8

One segment

Two segments

Three segments

2 4 6 8 10 12 14 160t12 (minute12)

2 4 6 8 10 12 14 160t12 (minute12)

qe(m

gmiddotgminus

1)

10mgmiddotgminus

1

qe(m

gmiddotgminus

1)

10mgmiddotgminus

1

Figure 8 Plot of piecewise linear regression for one two and three segmentsrsquo based Webberrsquos model (119862RDB = 50mgsdotLminus1119898adsorbent = 02 g V= 500mL temperature = 30∘C stirring rate = 500 rpm)

ZIF-8

298 K

308 K

318 K

50 100 150 200 2500Time (minute)

05

1015202530354045505560

qe(m

gmiddotgminus

1)

(a)

Fe-ZIF-8

298 K

308 K

318 K

50 100 150 200 2500Time (minute)

0102030405060708090

100110

qe(m

gmiddotgminus

1)

(b)

Figure 9 Effect of temperature on adsorption of RDB dye onto ZIF-8 (a) and Fe-ZIF-8 (b) (119862RDB = 30ndash50mgsdotLminus1 119898ZIFminus8 = 02 g 119881RDB =500ml and stirring rate 500 ppm)

the conventional graphical analysis [61] that might misiden-tify objective break points or numbers of segments

As seen from Figure 9 a possible desorption mightoccur where the RDB uptake appears to fluctuate or evendrop a little This behavior could be attributed to either areversible adsorption or a back diffusion controlling mech-anism [62] The pseudo-first- or second-order kinetic modelsof Lagergren [33] or Ho and McKay [63] respectivelyhave been widely used to investigate the formal kinetics ofadsorption processes [12 27 40 64] However the presentexperimental data could not apply to these models dueto reversible adsorption In the present study the pseudo-first-order kinetics model of Natarajan-Khalaf was used toanalyze the experimental data The rate constant 119896ads forthe adsorption of dye was determined from the slope ofNatarajan-Khalaf rsquos plots The results are listed in Table 5Thehigh coefficient of determination 1198772 (0973ndash0998) implies

that this model was compatible with the experimental dataThe rate constant of adsorption is separated into the rate offorward and reverse adsorption The rate constants for theforward and reverse process are also presented in Table 5It is clear that the adsorption kinetics can be significantlyimproved by the introduction of iron into the ZIF-8 Asshown in Table 5 the rate constants for adsorption could beincreased approximately 5 times by Fe-ZIF-8 and the kineticswith Fe-ZIF-8 were faster than that with ZIF-8

Adsorption thermodynamics was conducted by varyingthe temperature from 298K to 318 K as shown in Figure 9

The thermodynamic parameters including activationenergy 119870eq 1198961 and 1198962 are presented in Table 6 The resultsshowed that equilibrium adsorption capacity 119902eq of bothadsorbents increased with an increase in temperature whichindicated that the process was endothermic The equilibriumadsorption capacity of Fe-ZIF-8 is higher than that of ZIF-8

Journal of Chemistry 11

Table 5 Rate constants for the adsorption and the rate constants for the forward and reverse process and equilibrium constants at differentconcentrations for ZIF-8 and Fe-ZIF-8

Adsorbent 119862RDB(mgL) 119896ads 1198961 1198962 119870119900 1198772

ZIF-830 00023 00009 00014 06429 099540 00025 00009 00016 05625 099850 00046 00015 00031 04839 0990

Fe-ZIF-830 00115 00077 00038 20263 099140 00122 00081 00041 19756 099150 00184 00113 00071 15915 0980

Table 6 Activation energy equilibrium and rate constants for RDB dye adsorption and rate constants for forward and reverse process ofRDB adsorption onto ZIF-8 and Fe-ZIF-8

Absorbent Temp(K) 119870eq

119902eq(mgsdotgminus1)

1198961(times103)(minminus1)

k2(times103)(minminus1)

119896ads(times103)(minminus1)

1198772 119864119886(kJsdotmolminus1) 1198772

ZIF-8298 1272 2889 0890 1800 2700 0990 48270 0991308 1570 4394 1914 2692 4606 0988318 1842 5302 3908 5304 9212 0973

Fe-ZIF-8298 3401 6292 11361 7063 18424 0988 12507 0972308 4079 7750 12851 7876 20727 0994318 5864 8764 17761 7572 25333 0991

Table 7 Activation parameters for RDB dye adsorption onto ZIF-8 and Fe-ZIF-8

Adsorbent Temperature(K)

Δ119878(Jsdotmolminus1sdotKminus1)

Δ119867

(kJsdotmolminus1)Δ119866

(kJsdotmolminus1) 1198772

ZIF-8298 minus2944 5203 6080 0999308 6110318 6139

Fe-ZIF-8298 minus15340 994 5566 0960308 5719318 5873

for each corresponding temperature The increasing rateconstant with an increase in temperature suggests that tem-perature favors the adsorption process for the fast rate toproceed The equilibrium constant 119870eq for Fe-ZIF-8 andZIF-8 changed in the range of 13ndash18 and 34ndash57 respectivelyas temperature varied from 298 to 318 K It is worth notingthat 119870eq in the case of Fe-ZIF-8 is higher and increasesmuch faster than that in the case of ZIF-8 The activationenergy refers to the minimum amount of energy that mustbe overcome to proceed with the adsorption 119864119886 for ZIF-8and Fe-ZIF-8 was 4827 kJ and 1251 kJ respectively Lower 119864119886for Fe-ZIF-8 indicated that Fe-ZIF-8 was more favorable forRDB adsorption than ZIF-8 Low activation energy (below42 kJmolminus1) implies diffusion controlled process because thetemperature dependence of pore diffusivity is relatively weakand the diffusion process refers to the movement of thesolute to an external surface and not diffusivity of materialalong microspore surfaces in a particle [65] 119864119886 with ZIF-8 is slightly higher than 42 kJmolminus1 However a reversibleadsorption was observed as shown in Figure 9 indicating

that the rate-limiting step in this case involved a physical-chemical mechanism and not purely physical or chemicalone Therefore the RDB adsorption onto ZIF-8 was alsoconsidered a diffusion controlled process and so is the RDBadsorption onto Fe-ZIF-8

The activation parameters calculated using Eyring equa-tion are presented in Table 7 This would express whetherthe adsorption process follows an active complex prior tothe final sorption The coefficient of determination washigh for both ZIF-8 and Fe-ZIF-8 although the Eyringequation is not strictly linear with respect to 1119879 [66] Thenegative value of Δ119878lowast (minus2944 J Kminus1molminus1 for ZIF-8 andminus15340 J Kminus1sdotmolminus1 for Fe-ZIF-8) suggests a possibility ofan associative chemisorption through the formation of anactivated complex between RDB molecule and adsorbent[47] [63] Also the negative value of Δ119878 normally reflectsthat no significant change occurs in the internal structureof the adsorbent during the adsorption process [41 66] Thevalues forΔ119867 (52 03 kJmolminus1 for ZIF-8 and 994 for Fe-ZIF-8) suggest that these reactions are endothermic The large

12 Journal of Chemistry

Table 8 Thermodynamic parameters for the adsorption of RDB dye onto ZIF-8 and Fe-ZIF-8

Absorbent Δ1198660 (kJ) Δ1198670 (kJ) Δ1198780 (J) 1198772298K 308K 318K

ZIF-8 minus0599 minus1359 minus1615 16033 55795 0983Fe-ZIF-8 minus1092 minus3600 minus4677 51639 170000 0980

ZIF-8Fe-ZIF-8

4 6 8 10 122pH

0

20

40

60

80

100

120

qe(m

gmiddotgminus

1)

ZIF-8

Fe-ZIF-8

minus3

minus2

minus1

01234

ΔpH

minus3

minus2

minus1

0

1

2

3

4

5

ΔpH

4 6 8 10 12 142pH

3 4 5 6 7 8 9 10 11 12 132pH

pHZPC = 92

pHZPC = 98

Figure 10 Effect of pH on the adsorption of RDB by ZIF-8 and Fe-ZIF-8 (119862RDB = 50mgsdotLminus1 119898Adsorbent = 002 g V = 50mL shaking time =24 h Temp = 30∘C)

positive Δ119866 in both RDB adsorptions onto ZIF-8 and Fe-ZIF-8 imply that these reactions require energy to convertreactants to the product and as the energy requirement issatisfied the reaction proceeds Typically Δ119866 value relatesto the adsorption rate The rate increases as Δ119866 decreases[41 42] This is seen when comparing the data from Tables 6and 7 In Table 6 the rate constant 119896ads of Fe-ZIF-8 is higherthan that of ZIF-8 Table 7 describes the trend for Δ119866 inwhich the Fe-ZIF-8 has the lower Δ119867 value than ZIF-8

The thermodynamic parameters Δ1198670 Δ1198780 and Δ1198660of system were determined using vanrsquot Hoff equation toassess the spontaneity of adsorption process In Table 8 theresults show that the adsorption process using ZIF-8 andFe-ZIF-8 was endothermic as indicated by the positive signof the Δ1198670 value The positive value of Δ1198780 indicates theincreasing randomness at the solid-liquid interface duringthe adsorption of RDB molecules on the adsorbent [43] Thenegative values of Δ1198660 suggest the spontaneous RDB adsorp-tion of RDB on ZIF-8 or Fe-ZIF-8 The more negative valueat higher temperatures implies that the spontaneity increaseswith a temperature increase As the Gibbs free energy changeis negative and accompanied by the positive standard entropy

change the adsorption reaction is spontaneous with highaffinity Consistent with the findings in the kinetics morenegative values of Δ1198660 with Fe-ZIF-8 were obtained com-pared to that with ZIF-8 Again this confirmed the importantrole of iron in ZIF-8 in enhancing the RDB capacity

322 Effect of pH One of the important parameters control-ling the adsorption process is pH Figure 10 shows the effectof pH on the removal of RBB dye from aqueous solutionsThebehaviors of pH effect for RDB adsorption onto ZIF-8 and Fe-ZIF-8 were similar However the RDB adsorption capacity ofFe-ZIF-8 was higher than that of ZIF-8 In general the RDBadsorption capacity of adsorbents was observed to increasesignificantly with an increase in pH from 22 to 6 followedby a slight increase in pH from 6 to 10 and it decreasedsignificantly in further pH increase The values of pHZPC forZIF-8 and Fe-ZIF-8 determined by pH drift method werearound 92 and 98 respectively and are in accordance withprevious results [25 27 67] (the inset in Figure 10) Thevalue of pHZPC indicates that the surface of the adsorbent ispositively charged when pH of the solution is below pHZPCwhile the surface of adsorbent becomes negatively charged atpH of the solution above pHZPC

Journal of Chemistry 13

R

NH2

minusOSO3

SO3minus

SO4minus

(RDB)

+

++++++++

++ +++++++

++ +++++++

++ ++++++++

R

NH2

minusOSO3

SO3minus

SO4minus

(RDB)

RminusOSO3

SO3minus

SO4minus

Fe-ZIF-8Fe2+

Fe2+

2-MelmNH

N

NH2

R㰀R

휋-휋 interaction between thearomatic rings and the aromaticimidazole rings

atoms or oxygen in RDB

the ZIF-8 framework

Coordination of the nitrogen

Electrostatic interaction

molecules to the Fe2+ ions in

-e hydrophobic and 휋 휋

Figure 11 The proposed mechanism of RDB adsorption onto ZIF-8 or Fe-ZIF-8 at pH lt pHZPC 2-Melm 2-methylimidazole

(25)RR

(RDB)

NH3+

HOSO3

SO3H

SO4HOHminus

H+

NH2

minusOSO3

SO3minus

SO4minus

Scheme 2 Equilibrium of RDB in water

As seen in Scheme 1 molecular structure of RDB consistsof two groups of ndashSO3

minus and ndashSO4minus a group of NH2

Commonly equilibrium of RDB in water is expressed as inScheme 2

Therefore the higher pH is the more the equilibriumof (22) shifts to the right With regard to the correlationof the electronic charges of the adsorbentadsorbate andsolution pH values it can be presumed that there might be anelectrostatic interaction between the positively charged ZIF-8surface and the negatively charged site of RDB increase as pHof solution increases up to pHZPC The adsorption capacityof ZIF-8 and Fe-ZIF-8 started to decrease significantly atpH gt sim102 which was probably due to the electrostaticrepulsion of negatively charged RDB and negative ZIF-8In addition the low stability of ZIF-8 framework in pHlower than 3 also contributes to the low RDB adsorptioncapacity (see Figure 6) This electrostatic interaction mech-anism is similar to the adsorption mechanism for phthalicacid [67] and 119901-arsanilic acid on ZIF-8 [27] In additionthe hydrophobic and 120587-120587 interaction between the aromaticrings of the RDB and the aromatic imidazole rings of theZIF-8 framework is also thought to contribute to the RDBadsorption capacity

The increasing RDB uptake of Fe-ZIF-8 is probably dueto the adsorption through other mechanisms addition tothe electrostatic interactionThe enhanced adsorbed amountwith Fe-ZIF-8 probably resulted from an increased number ofadsorption iron sites It is likely that the coordination of thenitrogen atoms and oxygen in RDBmolecules to the Fe2+ ionsin the ZIF-8 framework is responsible for the more efficientadsorption compared with bare ZIF-8 From this discussionthe possible mechanisms of RDB adsorption onto ZIF-8 orFe-ZIF-8 were illustrated in Figure 11

323 Isotherm Adsorption Studies The study of adsorptionisotherms is helpful in determining the adsorption capacitiesfor removal at certain dyes at fixed temperature In the presentwork the equilibrium experiments were operated as if theinitial concentration (1198620 = 50mgsdotLminus1) was kept constantand the absorbent weight varied between 0005 0007 001012 0015 0017 and 0020 g The experimental data wereanalyzed according to the nonlinear form of Langmuir andFreundlichmodel Figure 12 presents the experimental pointsand nonlinear regression curves of Langmuir and Freundlichmodels The parameters of models are listed in Table 9However the maximum adsorption capacity is not obtainedin Freundlich equation Halsey [68] supposed that the maxi-mum adsorption capacity 119902119898 by Freundlich equation couldbe expressed

119902119898 = lim119862119890rarr1198620

1198701198651198621119899119890 (25)

119902119898 calculated based on Freundlich equation is also shown inTable 9

14 Journal of Chemistry

ZIF-8 Fe-ZIF-8

ExperimentalLangmuirFreundlich

ExperimentalLangmuirFreundlich

5 10 15 20 25 30 350Ce (mgmiddotLminus1)

5 10 15 20 25 300Ce (mgmiddotLminus1)

80

100

120

140

160

180

200

qe(m

gmiddotgminus

1)

90

95

100

105

110

115

120

125

130qe(m

gmiddotgminus

1)

Figure 12 Langmuir and Freundlich isotherm models of adsorption RDB dye onto ZIF-8 and Fe-ZIF-8 (119862RDB = 50mgsdotLminus1 V = 40mL119898adsorbent = 0005ndash0020 g pH = 82 shaking time = 24 hours)

Table 9 The parameters of Langmuir and Freundlich models

Adsorbent

Langmuir model Freundlich model

119870119871(Lsdotmgminus1)

119902mom(mgsdotgminus1) 1198772 1205942 119873

119870119865(mgsdot

gminus1sdotmgsdotLminus1)119899119902119898

(mgsdotgminus1) 1198772 1205942

ZIF-8 0594 13376 0974 0254 7800 82344 12735 0878 1209Fe-ZIF-8 0568 19356 0958 2340 4434 92015 22233 0961 2399

The determination coefficient (1198772) and the Chi-squaretest (1205942) for assessing the compatibility of experimentaldata with the isothermal models are listed in Table 9 Thehigh value of 1198772 and low 1205942 suggest that the isothermaldata of ZIF-8 could be well represented by the Langmuirmodel This implies a monolayer adsorption for ZIF-8 ForFe-ZIF-8 both models exhibited similar values of 1198772 and1205942 Moreover favorable characteristic parameters of 119877119871 for

Langmuir isotherm and 119899 for Freundlich isotherm were 0 lt119877119871 = 0034 lt 1 and 2 lt 119899 = 443 lt 10 which indicated thatboth isotherms were favorable These results confirmed thatthe equilibrium data of RDB adsorption onto Fe-ZIF-8 couldbe well fitted by the two adsorption isotherm models Thisresult indicated a monolayer adsorption and the existence ofheterogeneous surface in Fe-ZIF-8 It is worth noting thatmaximummonolayer adsorption capacity 119902mom for ZIF-8 ismuch higher than that for ZIF-8 It was concluded that theintroduction of iron intoZIF-8 enhanced theRDBadsorptiononto Fe-ZIF-8 in terms of kinetics and isotherm adsorption

119870119871 in Langmuir model is the equilibrium constant anddescribes the relation between kinetics and thermodynamicsThen 119870119871 in Langmuir model is thought to be equivalent to119870119900 However the relation of 119870119871 and 119870119900 (1198700 = 11989611198962) (seeTable 5) was not clear in our study It is explained by that factthat in the solid-liquid adsorption system119870119871 in the Langmuir

model no longer reflects the equilibrium constant as itsoriginal meaning In our previous study [40] as isothermaladsorption experiments were operated with constant initialconcentration1198620 and variable weights of adsorbent we foundthat the obtained parameters of Langmuir and Freundlichmodels also increase as the initial concentration increaseswhich should be constant at certain temperature It is thoughtthat these parameters turn out to be empirical coefficientsrather than fixed parameters at a certain temperature

324 Reusability of ZIF-8 and Fe-ZIF-8 To estimate thereusability of ZIF-8 and Fe-ZIF-8 for the removal RDBthe used adsorbents were regenerated by sonication assistedwashing with 10minus3M NaOH solution for 6 h and drying for24 hours at 100∘C and then exploited to adsorb RDB Theadsorption capacity of the regenerated ZIF-8 and Fe-ZIF-8are presented in Figure 13 Although the RDB adsorptiondecreased gradually with an increase in desorption cyclesthe regenerated adsorbents still exhibited good performancefor their adsorption After the desorption for three cyclesthe RDB adsorption capacity of by ZIF-8 and Fe-ZIF-8reached 90 and 95of the adsorption of the initialmaterialrespectively The XRD patterns of adsorbents after the threecycles seem unchangeable (see Figure 14) indicating that theadsorbents were stable in this condition

Journal of Chemistry 15

ZIF-8 Fe-ZIF-8

The second cycle The third cycleThe first cycleThe second cycle The third cycleThe first cycle0

10

20

30

40

50

qe(m

gmiddotgminus

1)

0

5

10

15

20

qe(m

gmiddotgminus

1)

Figure 13 Effect of regeneration cycles of ZIF-8 and Fe-ZIF-8 adsorbents on the adsorption of RDB (119881RDB = 125mL 119862RDB = 50mg Lminus1adsorbent119881RDB = 005 g125mL shaking time = 10 hours)

The third cycle

The second cycle

The first cycle

Fe-ZIF-8

Fe-ZIF-8

Inte

nsity

(arb

)

The third cycle

The second cycle

The first cycle

ZIF-8

ZIF-8

Inte

nsity

(arb

)500

cps

1000

cps

10 15 20 25 30 3552 theta (degree)

10 15 20 25 30 3552 theta (degree)

Figure 14 The XRD patterns of ZIF-8 and Fe-ZIF-8 adsorbents after the three cycles

4 Conclusion

The iron doped zeolite imidazolate framework-8 was syn-thesized by hydrothermal process Ferrous ion as an ironsource could be directly introduced into ZIF-8 to form Fe-ZIF-8 (molar mole of Fe(Fe + Zn) = 0116) The ferrous ionscould replace partially Zn(II) in ZIF-8 structure or exist inamorphous speciesTheZIF-8was stable in aqueous solutionswith pH in ranging from 3 to 12 for 24 hours Both ZIF-8 andFe-ZIF-8 were used to study the RDB adsorptionThe resultsshow that the introduction of iron into ZIF-8 significantlyenhanced the RDB adsorption capacity compared to bareZIF-8 The study based on Webberrsquos intraparticle diffusionshows that the adsorption process with ZIF-8 and Fe-ZIF-8 tended to follow two stages in which the first stage wasfilm-diffusion and the second was an intraparticle-diffusionprocess The kinetic parameters based on Arrhenius andEyring equation proved that the introduction of iron intoZIF-8 provided a much larger adsorption capacity and fasteradsorption kinetics than ZIF-8 In addition to the electro-static interaction mechanism and the hydrophobic and 120587-120587

interaction between the aromatic rings of the RBB dyeand the aromatic imidazole rings of the adsorbent for ZIF-8 the coordination of the nitrogen atoms and oxygen incarboxyl group in RDB molecules to the Fe2+ ions in theZIF-8 framework might explain why Fe-ZIF-8 has a higherRBD adsorption capacity than ZIF-8 The experimental datafor ZIF-8 were well correlated by Langmuir model whilethose for Fe-ZIF-8 were well fitted to both Langmuir andFreundlich models The maximum monolayer adsorptioncapacity for Fe-ZIF-8 (19356mgminus1sdotgminus1) was approximately 14times higher than that for ZIF-8 (13376mgminus1sdotgminus1)

Conflicts of Interest

The authors declare that they have no conflicts of interest

Acknowledgments

Thisworkwas funded byMinistry of Education andTrainingVietnam under the Project B2016-DHH-20

16 Journal of Chemistry

References

[1] F P Van Der Zee and S Villaverde ldquoCombined anaerobic-aerobic treatment of azo dyesmdasha short review of bioreactorstudiesrdquoWater Research vol 39 no 8 pp 1425ndash1440 2005

[2] P A Soloman C A Basha M Velan V RamamurthiK Koteeswaran and N Balasubramanian ldquoElectrochemicaldegradation of Remazol Black B Dye effluentrdquo CleanmdashSoil AirWater vol 37 no 11 pp 889ndash900 2009

[3] N F Cardoso R B Pinto E C Lima et al ldquoRemoval of remazolblack B textile dye from aqueous solution by adsorptionrdquoDesalination vol 269 no 1ndash3 pp 92ndash103 2011

[4] V R Ranjusha R Pundir K Kumar M G Dastidar and T RSreekrishnan ldquoBiosorption of Remazol Black B dye (Azo dye)by the growing Aspergillus flavusrdquo Journal of Environ Sci HealthA ToxicHazardous Substances and Environmental Engineeringvol 45 no 10 pp 1256ndash1263 2010

[5] C Janiak and J K Vieth ldquoMOFs MILs and more conceptsproperties and applications for porous coordination networks(PCNs)rdquo New Journal of Chemistry vol 34 no 11 pp 2366ndash2388 2010

[6] O M Yaghi M OrsquoKeeffe N W Ockwig H K Chae MEddaoudi and J Kim ldquoReticular synthesis and the design ofnew materialsrdquo Nature vol 423 no 6941 pp 705ndash714 2003

[7] P Chowdhury C Bikkina and S Gumma ldquoGas adsorptionproperties of the chromium-based metal organic frameworkMIL-101rdquo Journal of Physical Chemistry C vol 113 no 16 pp6616ndash6621 2009

[8] L Hamon C Serre T Devic et al ldquoComparative study ofhydrogen sulfide adsorption in the MIL-53(Al Cr Fe) MIL-47(V) MIL-100(Cr) and MIL-101(Cr) metal-organic frame-works at room temperaturerdquo Journal of the American ChemicalSociety vol 131 no 25 pp 8775ndash8777 2009

[9] Y Li and R T Yang ldquoHydrogen storage in metal-organic andcovalent-organic frameworks by spilloverrdquo AIChE Journal vol54 no 1 pp 269ndash279 2008

[10] P L Llewellyn S Bourrelly C Serre et al ldquoHigh uptakes of CO2and CH4 in mesoporous metal-organic frameworks MIL-100and MIL-101rdquo Langmuir vol 24 no 14 pp 7245ndash7250 2008

[11] J Yang Q Zhao J Li and J Dong ldquoSynthesis of metal-organicframework MIL-101 in TMAOH-Cr(NO3)3-H2BDC-H2O andits hydrogen-storage behaviorrdquo Microporous and MesoporousMaterials vol 130 no 1ndash3 pp 174ndash179 2010

[12] K Yang Q Sun F Xue and D Lin ldquoAdsorption of volatileorganic compounds by metal-organic frameworks MIL-101influence of molecular size and shaperdquo Journal of HazardousMaterials vol 195 pp 124ndash131 2011

[13] Z Zhang S Huang S Xian H Xi and Z Li ldquoAdsorptionequilibrium and kinetics of CO2 on chromium terephthalateMIL-101rdquo Energy and Fuels vol 25 no 2 pp 835ndash842 2011

[14] R Kitaura K Seki G Akiyam and S Kitagawa ldquoPorouscoordination-polymer crystals with gated channels specific forsupercritical gasesrdquo Angewandte ChemiemdashInternational Edi-tion vol 42 no 4 pp 428ndash431 2003

[15] S Ma D Sun X-S Wang and H-C Zhou ldquoAmesh-adjustablemolecular sieve for general use in gas separationrdquo AngewandteChemie-International Edition vol 46 no 14 pp 2458ndash24622007

[16] D-Y Hong Y K Hwang C Serre G Ferey and J-S ChangldquoPorous chromium terephthalate MIL-101 with coordinativelyunsaturated sites surface functionalization encapsulation

sorption and catalysisrdquo Advanced Functional Materials vol 19no 10 pp 1537ndash1552 2009

[17] Y K Hwang D-Y Hong J-S Chang et al ldquoSelective sul-foxidation of aryl sulfides by coordinatively unsaturated metalcenters in chromium carboxylate MIL-101rdquoApplied Catalysis AGeneral vol 358 no 2 pp 249ndash253 2009

[18] N V Maksimchuk M N Timofeev M S Melgunov et alldquoHeterogeneous selective oxidation catalysts based on coor-dination polymer MIL-101 and transition metal-substitutedpolyoxometalatesrdquo Journal of Catalysis vol 257 no 2 pp 315ndash323 2008

[19] Z Saedi S TangestaninejadMMoghadam VMirkhani and IMohammadpoor-Baltork ldquoMIL-101 metal-organic frameworka highly efficient heterogeneous catalyst for oxidative cleavageof alkenes with H2O2rdquo Catalysis Communications vol 17 pp18ndash22 2012

[20] A Phan C J Doonan F J Uribe-Romo C B Knobler MOrsquoKeeffe and O M Yaghi ldquoSynthesis structure and carbondioxide capture properties of zeolitic imidazolate frameworksrdquoAccounts of Chemical Research vol 43 no 1 pp 58ndash67 2010

[21] J R Long and O M Yaghi ldquoThe pervasive chemistry of metal-organic frameworksrdquo Chemical Society Reviews vol 38 no 5pp 1213ndash1214 2009

[22] H Wu W Zhou and T Yildirim ldquoHydrogen storage in aprototypical zeolitic imidazolate framework-8rdquo Journal of theAmerican Chemical Society vol 129 no 17 pp 5314ndash5315 2007

[23] H Bux A Feldhoff J Cravillon M Wiebcke Y-S Li and JCaro ldquoOriented zeolitic imidazolate framework-8 membranewith sharp H2C3H8 molecular sieve separationrdquo Chemistry ofMaterials vol 23 no 8 pp 2262ndash2269 2011

[24] J Cravillon S Munzer S-J Lohmeier A Feldhoff K Huberand M Wiebcke ldquoRapid room-temperature synthesis andcharacterization of nanocrystals of a prototypical zeolitic imi-dazolate frameworkrdquo Chemistry of Materials vol 21 no 8 pp1410ndash1412 2009

[25] J-Q Jiang C-X Yang and X-P Yan ldquoZeolitic imidazolateframework-8 for fast adsorption and removal of benzotriazolesfrom aqueous solutionrdquo ACS Applied Materials and Interfacesvol 5 no 19 pp 9837ndash9842 2013

[26] K-Y A Lin and H-A Chang ldquoEfficient adsorptive removal ofhumic acid from water using zeolitic imidazole framework-8(ZIF-8)rdquoWater Air and Soil Pollution vol 226 article 10 2015

[27] B K Jung J W Jun Z Hasan and S H Jhung ldquoAdsorptiveremoval of p-arsanilic acid from water using mesoporouszeolitic imidazolate framework-8rdquo Chemical Engineering Jour-nal vol 267 pp 9ndash15 2015

[28] M Jian B Liu G Zhang R Liu and X Zhang ldquoAdsorptiveremoval of arsenic from aqueous solution by zeolitic imidazo-late framework-8 (ZIF-8) nanoparticlesrdquo Colloids and SurfacesA Physicochemical and Engineering Aspects vol 465 pp 67ndash762015

[29] J Zheng C Cheng W-J Fang et al ldquoSurfactant-free synthesisof a Fe3O4ZIF-8 core-shell heterostructure for adsorption ofmethylene bluerdquo CrystEngComm vol 16 no 19 pp 3960ndash39642014

[30] S Eslava L Zhang S Esconjauregui et al ldquoMetal-organicframework ZIF-8 films as low-120581 dielectrics inmicroelectronicsrdquoChemistry of Materials vol 25 no 1 pp 27ndash33 2013

[31] M Zhu S R Venna J B Jasinski and M A CarreonldquoRoom-temperature synthesis of ZIF-8 the coexistence of ZnOnanoneedlesrdquo Chemistry of Materials vol 23 no 16 pp 3590ndash3592 2011

Journal of Chemistry 17

[32] A Kumar B Prasad and I M Mishra ldquoAdsorptive removalof acrylonitrile by commercial grade activated carbon kineticsequilibrium and thermodynamicsrdquo Journal of Hazardous Mate-rials vol 152 no 2 pp 589ndash600 2008

[33] N Kannan andMMeenakshisundaram ldquoAdsorption of CongoRed on various activated carbons AComparative StudyrdquoWaterAir and Soil Pollution vol 138 no 1ndash4 pp 289ndash305 2002

[34] A J Ahamed V Balakrishman and S Arivoli ldquoKinetic andequilibrium studies of Rhodamine B adsorption by low costactivated carbonrdquo Archives of Applied Science Research vol 3pp 154ndash166 2011

[35] P Atkins and J D Paula Physical Chemistry Oxford UniversityPress New York NY USA 2010

[36] J Crank The Mathematics of Diffusion Clarendon Press Lon-don UK 1975

[37] W J Weber and J C Morris ldquoKinetics of adsorption on carbonfrom solutionrdquo Journal of the Sanitary Engineering DivisionProceed American society of civil Engineers vol 89 no 2 pp 31ndash60 1963

[38] G F Malash and M I El-Khaiary ldquoPiecewise linear regressiona statistical method for the analysis of experimental adsorptiondata by the intraparticle-diffusion modelsrdquo Chemical Engineer-ing Journal vol 163 no 3 pp 256ndash263 2010

[39] H Motulsky and A Christopoulos Fitting Models to BiologicalData Using Linear and Non-Linear Regression GraphPad Soft-ware San Diego Calif USA 2003

[40] B H Dang Son V Quang Mai D Xuan Du N Hai Phongand D Quang Khieu ldquoA study on astrazon black AFDL dyeadsorption onto Vietnamese diatomiterdquo Journal of Chemistryvol 2016 Article ID 8685437 11 pages 2016

[41] T S Anirudhan and P G Radhakrishnan ldquoThermodynamicsand kinetics of adsorption of Cu(II) from aqueous solutionsonto a new cation exchanger derived from tamarind fruit shellrdquoThe Journal of Chemical Thermodynamics vol 40 no 4 pp702ndash709 2008

[42] K G Scheckel and D L Sparks ldquoTemperature effects on nickelsorption kinetics at the mineral-water interfacerdquo Soil ScienceSociety of America Journal vol 65 no 3 pp 719ndash728 2001

[43] E I Unuabonah K O Adebowale and B I Olu-OwolabildquoKinetic and thermodynamic studies of the adsorption of lead(II) ions onto phosphate-modified kaolinite clayrdquo Journal ofHazardous Materials vol 144 no 1-2 pp 386ndash395 2007

[44] Y Liu ldquoIs the free energy change of adsorption correctlycalculatedrdquo Journal of Chemical and Engineering Data vol 54no 7 pp 1981ndash1985 2009

[45] Q Song S K Nataraj M V Roussenova et al ldquoZeoliticimidazolate framework (ZIF-8) based polymer nanocompositemembranes for gas separationrdquo Energy and EnvironmentalScience vol 5 no 8 pp 8359ndash8369 2012

[46] I Langmuir ldquoThe constitution and fundamental properties ofsolids and liquids Part I Solidsrdquo The Journal of the AmericanChemical Society vol 38 no 2 pp 2221ndash2295 1916

[47] T W Weber and R K Chakravorti ldquoPore and solid diffusionmodels for fixed-bed adsorbersrdquo AIChE Journal vol 20 no 2pp 228ndash238 1974

[48] H M F Freundlich ldquoOver the adsorption in solutionrdquo Journalof Physical Chemistry vol 57 pp 385ndash471 1906

[49] I Tosun ldquoAmmonium removal from aqueous solutions byclinoptilolite determination of isotherm and thermodynamicparameters and comparison of kinetics by the double expo-nential model and conventional kinetic modelsrdquo International

Journal of Environmental Research and Public Health vol 9 no3 pp 970ndash984 2012

[50] I A W Tan A L Ahmad and B H Hameed ldquoAdsorptionof basic dye on high-surface-area activated carbon preparedfrom coconut husk equilibrium kinetic and thermodynamicstudiesrdquo Journal of Hazardous Materials vol 154 no 1ndash3 pp337ndash346 2008

[51] A Asfaram M Ghaedi and G R Ghezelbash ldquoBiosorption ofZn2+ Ni2+ and Co2+ from water samples onto Yarrowia lipoly-tica ISF7 using a response surface methodology and analyzedby inductively coupled plasma optical emission spectrometry(ICP-OES)rdquoRSCAdvances vol 6 no 28 pp 23599ndash23610 2016

[52] O Karagiaridi M B Lalonde W Bury A A Sarjeant OK Farha and J T Hupp ldquoOpening ZIF-8 a catalyticallyactive zeolitic imidazolate framework of sodalite topologywith unsubstituted linkersrdquo Journal of the American ChemicalSociety vol 134 no 45 pp 18790ndash18796 2012

[53] Y Du R Z Chen J F Yao and H T Wang ldquoFacile fabricationof porous ZnO by thermal treatment of zeolitic imidazolateframework-8 and its photocatalytic activityrdquo Journal of Alloysand Compounds vol 551 no 25 pp 125ndash130 2013

[54] X Zeng L Huang C Wang J Wang J Li and X LuoldquoSonocrystallization of ZIF-8 on electrostatic spinning TiO2nanofibers surface with enhanced photocatalysis propertythrough synergistic effectrdquo ACS Applied Materials and Inter-faces vol 8 no 31 pp 20274ndash20282 2016

[55] V Vadivelan and K Vasanth Kumar ldquoEquilibrium kineticsmechanism and process design for the sorption of methyleneblue onto rice huskrdquo Journal of Colloid and Interface Science vol286 no 1 pp 90ndash100 2005

[56] G A Seber and C J Wild Nonlinear Regression Wiley Seriesin Probability and Mathematical Statistics Probability andMathematical Statistics John Wiley amp Sons New York NYUSA 1989

[57] M A Al-Ghouti M A M Khraisheh M N M Ahmad and SAllen ldquoAdsorption behaviour of methylene blue onto Jordaniandiatomite a kinetic studyrdquo Journal of Hazardous Materials vol165 no 1ndash3 pp 589ndash598 2009

[58] A J Ahamed and B J Suganthana ldquoAdsorption of Cr-VIon commercially available ash powdermdasha kinetic approachrdquoJournal of the Indian Chemical Society vol 83 pp 792ndash7952006

[59] B Y M Bueno M L Torem F Molina and L M S deMesquita ldquoBiosorption of lead(II) chromium(III) and cop-per(II) by R opacus equilibrium and kinetic studiesrdquoMineralsEngineering vol 21 no 1 pp 65ndash75 2008

[60] C E Webster R S Drago and M C Zerner ldquoMoleculardimensions for adsorptivesrdquo Journal of the American ChemicalSociety vol 120 no 22 pp 5509ndash5516 1998

[61] R R Sheha and A A El-Zahhar ldquoSynthesis of some ferromag-netic composite resins and their metal removal characteristicsin aqueous solutionsrdquo Journal of Hazardous Materials vol 150no 3 pp 795ndash803 2008

[62] N K Lazaridis T D Karapantsios and D Georgantas ldquoKineticanalysis for the removal of a reactive dye from aqueous solutiononto hydrotalcite by adsorptionrdquoWater Research vol 37 no 12pp 3023ndash3033 2003

[63] Y S Ho and G McKay ldquoPseudo-second order model forsorption processesrdquo Process Biochemistry vol 34 no 5 pp 451ndash465 1999

18 Journal of Chemistry

[64] J X Lin S L Zhan M H Fang and X Q Qian ldquoTheadsorption of dyes from aqueous solution using diatomiterdquoJournal of Porous Materials vol 14 no 4 pp 449ndash455 2007

[65] K G Scheckel and D L Sparks ldquoDissolution kinetics of nickelsurface precipitates on clay mineral and oxide surfacesrdquo SoilScience Society of America Journal vol 66 pp 689ndash694 2001

[66] C-C Kan M C Aganon C M Futalan and M L P DalidaldquoAdsorption of Mn2+ from aqueous solution using fe and mnoxide-coated sandrdquo Journal of Environmental Sciences vol 25no 7 pp 1483ndash1491 2013

[67] N A Khan B K Jung Z Hasan and S H Jhung ldquoAdsorptionand removal of phthalic acid and diethyl phthalate fromwater with zeolitic imidazolate andmetal-organic frameworksrdquoJournal of Hazardous Materials vol 282 pp 194ndash200 2015

[68] G D Halsey ldquoThe role of surface heterogeneity in adsorptionrdquoAdvances in Catalysis vol 4 pp 259ndash269 1952

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 201

International Journal ofInternational Journal ofPhotoenergy

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Carbohydrate Chemistry

International Journal ofInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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