hf-free synthesis of nanoscale metal–organic framework nmil

HF-Free Synthesis of Nanoscale Metal−Organic Framework NMIL-100(Fe) as an Efficient Dye AdsorbentShengxia Duan,†,‡ Jiaxing Li,*,†,∥,⊥ Xia Liu,†,‡ Yanan Wang,†,‡ Suyuan Zeng,*,§ Dadong Shao,†

and Tasawar Hayat∥

†Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei230031, People’s Republic of China‡University of Science and Technology of China, Hefei 230026, People’s Republic of China§School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, People’s Republic of China∥NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia⊥Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, and School for Radiological andinterdisciplinary Sciences (RAD-X), Soochow University, Suzhou 215123, People’s Republic of China

*S Supporting Information

ABSTRACT: A hydrofluoric acid (HF)-free solvothermal method was used tosynthesize nanoscale metal−organic framework NMIL-100(Fe), which exhibitedcomparable physicochemical properties as those prepared by traditional methods,but with a mild and environmentally benign synthesis condition. XRD, TGA, N2adsorption, FT-IR, SEM, and TEM were employed to characterize the as-prepared NMIL-100(Fe), which was further applied as an effective adsorbent fordye adsorption, including two cationic dyes, rhodamine 6G (R6G) andrhodamine B (RB), and an anionic reactive red 120 (RR 120) with highadsorption efficiencies and capacities. The adsorption process can be welldescribed by pseudo-second-order kinetic model and Langmuir isotherm model.Hydrogen bonding and electrostatic interaction were revealed for the adsorptionof the two cationic dyes and one anionic dye onto NMIL-100(Fe), respectively,as investigated by mechanism studies. Thermodynamic analyses indicate thatadsorption processes for cationic and anionic dyes are entropy-driven endothermic and enthalpy-driven exothermic processes,respectively. This environmental-benign synthetic strategy for NMIL-100(Fe), as well as its high adsorption efficiency andcapacity, might be used for the fabrication of other nanoscale metal−organic frameworks, and the potential applications ofNMIL-100(Fe) in real wastewater treatment.

KEYWORDS: HF-free, Metal−organic framework, NMIL-100(Fe), Adsorption, Dyes

■ INTRODUCTIONNowadays, organic dyes have been widely used in variousindustries, such as textile, tannery, leather, rubber and paint.1

However, water contaminated by the toxic organic dyes ispretty threating to the environment and human health, andtheir sustainability in the entire food chain might even make thesituation worse.2,3 Thus, it is very important to solve theproblems as mentioned above, and it will be better if thesynthesis of raw materials can be achieved simultaneously. Sofar, various physicochemical technologies have been compre-hensively investigated for separation of organic dyes fromaqueous solutions including adsorption,4,5 oxidation,6 mem-brane filtration,7 chemical coagulation,8 photocatalysis,9,10 andso on. Among these methods, adsorption is considered as themost attractive technology because the process of adsorptioncan be carried out with high efficiency, economical feasibility,and ease of operation.11,12

With great potential of metal−organic frameworks (MOFs)in adsorption, MOFs constructed from coordinated ligands and

metal ions have been used for the removal of dyes,13

alkylaromatics and phenols,14−17 pharmaceuticals (furosemideand sulfasalazine),18 and sulfur compounds.19−22 Generally,MOFs are highly crystallized materials with bulk phases, whichare not suitable in the application of adsorption. Nevertheless,the shape and size of the material are very significant to tune itsproperties. Therefore, it is necessary to scale MOFs down toform nanoscale metal−organic frameworks (NMOFs),23,24

often exhibiting conspicuous physical and chemical properties,which are dependent on size and shape but unobservable intheir bulk analogues.25 Meanwhile, the yield of most of NMOFsmaterials is very low, further limiting their applications in realoperation. Moreover, the studies on NMOFs for rapid dyeremoval are still rare and will be of great interest in enrichingthe varieties of NMOFs materials. Hence, it is a crucial task to

Received: March 1, 2016Revised: April 25, 2016Published: May 3, 2016

Research Article

pubs.acs.org/journal/ascecg

© 2016 American Chemical Society 3368 DOI: 10.1021/acssuschemeng.6b00434ACS Sustainable Chem. Eng. 2016, 4, 3368−3378

pubs.acs.org/journal/ascecg

http://dx.doi.org/10.1021/acssuschemeng.6b00434

investigate NMOFs and to ensure that the NMOFs woulddemonstrate great adsorption ability due to their uniquestructure.Typically, MIL-100(Fe), owing a rigid porous crystal

structure, has received great attention in preceding literatureswith respect to its catalytic,26 separation,27 heat trans-formation,28 dye adsorption,29 and gas storage properties.30

In the above literatures, MIL-100(Fe) with bulk shape hasalways been synthesized by hydrothermal methods withhazardous reagents including nitric and hydrofluoric acids(HF). However, HF is an industrially weak acid in water, whichis very erosive and stubborn, indicating that using HF willprevent MOFs from mass production at higher temperatureand pressure.31,32 Moreover, the subsequent purificationprocess of the synthesized MIL-100(Fe) in the previousliteratures is very cumbersome,26−30 making the yield of MIL-100(Fe) much low and inhibiting its large scale synthesis. Thus,it is essential to establish a facial synthetic method to obtainMIL-100(Fe), especially a more environmental-friendly meth-od.In this work, we have synthesized a new kind of nanoscale

material MIL-100(Fe) (NMIL-100(Fe)) with high yield in theabsence of nitric acid (HNO3) or HF by a solvothermalmethod using ferric chloride (FeCl3) and 1,3,5-benzenetricar-boxylic acid (1,3,5-H3BTC) (Figure 1). The as-prepared

NMIL-100(Fe) was applied in wastewater treatment byabsorbing two cationic dyes, rhodamine 6G (R6G) andrhodamine B (RB), and one anionic reactive red 120 (RR120) (Table S1). The effects of reacting time, pH, andtemperature on the adsorption performances of NMIL-100(Fe)were comprehensively investigated. Possible adsorption mech-anisms were also proposed. This work might provide apotential exploration of NMIL-100(Fe) in the adsorption field.

■ EXPERIMENTAL SECTIONSynthesis of NMIL-100(Fe). 1,3,5-H3BTC (Chimi Vo, 98%, 4.0

mmol) was dissolved in a 1:1 (vol./vol.) mixture solution of water andethanol (40 mL), and stirred magnetically until the solution becametransparent. FeCl3 (Chimi Vo, 98%, 4.0 mmol) was added into thesolution with continuous stirring until they became colloidal. Theconsequent solution was then held in a Teflon-lined autoclave andkept reacting at 140 °C for 12 h. After being cooled to roomtemperature, the reaction product was obtained by centrifuging andthe crude product was scoured with distilled water and absolute

ethanol alternately for several times to remove any unreacted reactants,and then dried under vacuum at 60 °C for 12 h.

Characterizations. All chemicals and solvents were obtainedcommercially and used as received. The crystallinity of the NMIL-100(Fe) was investigated with XRD using a Philips X’Pert Pro Superdiffractometer with Cu Kα radiation (λ = 1.541 78 Å). The surfacefunctional groups in the NMIL-100(Fe) materials were determinedwith Fourier transform infrared spectroscopy (FT-IR) using samplesprepared as pellets with KBr in the 4000−400 cm−1 region.Thermogravimetric analysis (TGA) was performed in a TGA-50/50H analyzer (Shimadzu, Kyoto, Japan), for which the heating rate was10 °C·min−1, under nitrogen atmosphere. Textural properties ofNMIL-100(Fe) synthesized above were investigated with N2adsorption−desorption measurements at 77 K using a nitrogenadsorption method (Quantachrom Autosorb IQ-C). The morpholo-gies of the samples were studied with the help of a Hitachi S-4800 fieldemission scanning electron microscopy (FESEM) instrument. Thetransmission electron microscopy (TEM) was obtained on a JEOL2010 with 200 kV of accelerating voltage. The ζ-potential was testedby Zeatsizer Nano ZS (Malvern, England). UV−vis absorption spectrawere gained with spectrophotometer (Shimadzu UV-1800) usingquartz cells (10 × 4 mm light path).

Batch Adsorption Studies. The adsorption properties of theNMIL-100(Fe) material were then evaluated by applying it to removaldye molecules from aqueous solutions. This was carried out by mixingthe NMIL-100(Fe) materials with aqueous solutions of the dyes (R6G,RB, and RR 120), whose chemical structures are shown in Table S1.For adsorption studies, adsorption experiments were carried out inbatch mode with a constant solution volume of 9 mL and a fixedadsorbent amount of 30 mg. Stock solutions of three dyes wereprepared at an initial concentration of 0.01 mol·L−1, which werefurther diluted to various concentrations for different adsorptionstudies. Three temperatures, 298.15, 308.15, and 318.15K, wereselected for isothermal studies with different initial dye concentrations.Equation 1 was used to calculate the adsorption capacities of theNMIL-100(Fe) for different dyes.

=−

qC C V

m( )

e0 e

(1)

where C0 (mol·L−1) and Ce (mol·L

−1) are the initial and equilibriumconcentrations of dyes in the solution, V (L) is the total volume of thesuspension, and m (g) is the mass of adsorbent.

For adsorption kinetic studies, the volume of dye solution and theconcentration of NMIL-100(Fe) were both set the same as those inthe experiments described above. Moreover, the initial concentrationsof the three dyes were set as 1 × 10−4 mol·L−1, and the contact timeswere varied between 1 and 30 min. The amount of the three dyesadsorbed on NMIL-100(Fe) at time t was then calculated using eq 2:

=−

qC C V

m( )

tt0

(2)

where C0 (mol·L−1) and Ct (mol·L

−1) are the initial concentration ofthe three dyes and the liquid-phase concentrations of three dyes attime t, respectively, V (L) is the total volume of the suspension, and m(g) is the mass of adsorbent. The concentrations of dye remaining inthe solution were obtained by measuring their correspondingabsorbance maxima and calculated from standard calibration curves.The variables concerned in this study included contact time,temperature and the solution pH. The solution pH was regulated byadding 0.1 mol·L−1 NaOH or 0.1 mol·L−1 HCl. All of the experimentswere performed in duplicates for data consistency.

■ RESULTS AND DISCUSSIONCharacterization of NMIL-100(Fe) Materials. NMIL-

100(Fe) was manufactured by a simple one-step solvothermalmethod. Compared with the previous synthetic methods forMIL-100(Fe) by using iron powder and 1,3,5-H3BTC as themetal source and the ligand respectively, with the aid of HNO3

Figure 1. Synthetic procedure of NMIL-100(Fe) and the simulatedstructure of NMIL-100(Fe).

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DOI: 10.1021/acssuschemeng.6b00434ACS Sustainable Chem. Eng. 2016, 4, 3368−3378

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and HF in ultrapure water,26−30 the iron powder wassubstituted by FeCl3 and a mixture of EtOH/H2O solutionwas used as the solvent without any HNO3 or HF, making thesubsequent purification procedure much more easily. In thisstudy, the ligand 1,3,5-H3BTC was first dissolved in a mixtureof EtOH/H2O solution by decreasing the polarity of thesolvent, making the deprotonation of the ligand easier. Uponthe addition of FeCl3 into the solution, the solution became ayellowish red colloid, indicating the successful self-assembly ofFe3+ with 1,3,5-H3BTC. The high reaction temperature at 140°C enhanced the crystallinity of NMIL-100(Fe), which mayreplace the role of HF. Comparing with the previous reportedXRD patterns of NMIL-100(Fe),26−30 the experimental XRDpattern of the as-synthesized NMIL-100(Fe) was in accordgreatly, indicating good phase purity of NMIL-100(Fe) material(Figure 2a). Meanwhile, the diffraction peaks centered at10.75°, 18.25°, 20.17°, 24.15°, and 27.40° can be indexed to the(4 2 8), (9 5 1 1), (1 2 8 8), (1 6 8 8), and (10 4 20) planes ofcubic NMIL-100(Fe).33,34 As shown in Figure 2b, NMIL-

100(Fe) displays two distinct weight losses in the range of 30−470 °C. The first weight loss (12.78%) in the range from 30 to150 °C result from the loss of the three inside and twocoordinated water molecules. The other loss (57.92%) in therange from 325 to 465 °C is concerned with the collapsingframework. The overall weight loss confirmed the formula ofFe3O(H2O)2(OH)[C6H3(CO2)3]2·nH2O (n ≈ 3.0)],32 indicat-ing that this HF-free synthetic route can produce comparablethermal stable NMIL-100(Fe) materials as traditional HF-containing synthesis. However, the detailed weight loss ofNMIL-100(Fe) material is different from other samples. Forinstance, the MIL-100(Fe)26 synthesized by Serre et al. has thestructure formula of FeIII3O(H2O)2F·{C6H3(CO2)3}2·nH2O (n≈ 14.5), which shows three weight losses between 0 and 575°C. The first weight loss (∼9%) of MIL-100(Fe),20 from insideand coordinated water molecules, is in a range from 0 to 175°C, whereas that of NMIL-100(Fe) synthesized in this study isin the range from 30 to 150 °C. The final weight loss (35.3%)at 575 °C is related to the combustion of 1,3,5-H3BTC,

Figure 2. Characterization of the as-synthesized NMIL-100(Fe): (a) XRD patterns; (b) TGA curve; (c) FT-IR spectrum; (d) N2 adsorption−desorption isotherms and the pore size distribution (inset) (e) SEM image; and (f) TEM image.



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whereas that of NMIL-100(Fe) (57.92%) is in a the range from325 to 465 °C. All the three weight losses of MIL-100(Fe) aredifferent from the thermal analysis of NMIL-100(Fe)synthesized in this work. Moreover, compared with the thermalanalysis of NMIL-100(Fe) synthesized in this work with MIL-100(Fe) synthesized by Yan et al., there still exists greatdifference. The MIL-100(Fe) synthesized by Fu et al.27 displaysthree weight losses in the range of 25−700 °C. The weight loss(∼7.0%) from 25 to 250 °C resulted from the departure of thefree water molecules in the pores and the water moleculesinterrelated with the iron trimers. The last weight loss (∼55%)at higher temperature (>320 °C) resulted from the collapsingframework. As mentioned above, there exists great difference ofdetailed weight loss between NMIL-100(Fe) material and MIL-100(Fe), indicating that different synthetic methods mayinfluence the structure of the samples.26−30

The FT-IR spectrum of NMIL-100(Fe) clearly confirms thesuccessful self-assembly of FeCl3 and 1,3,5-H3BTC (Figure 2c).Moreover, it indicates that the method for purifying NMIL-100(Fe) in this work is remarkably effective in removingresidual 1,3,5-H3BTC, as observed from the decreased intensityof distinct CO stretching (1718 cm−1) from 1,3,5-H3BTC,after the final wash by deionized water and ethanol. As shownin Figure 2d, the textural properties of NMIL-100(Fe) wereinvestigated by N2 adsorption and desorption. And the poresize distribution curve is also exhibited in Figure 2d. It can beclearly seen that NMIL-100(Fe) synthesized in this workshowed typical IV adsorption profiles and H1-type hysteresisloops, which manifest the orderly mesoporous channels. N2adsorption-based surface area analysis gave 1.24 m2·g−1 of BETsurface areas for NMIL-100(Fe), with pore size distributioncentered at about 3.94 nm. Although there exist greatdifferences in BET surface areas between NMIL-100(Fe) andthe previous reported MIL-100(Fe), there was little differencebetween their porosities. For instance, BET surface area ofMIL-100(Fe) reported by Fu et al.27 is 1598 m2·g−1 whereasthe pore diameter centered at 3.7 nm, which is similar to thepore diameter of as-synthesized NMIL-100(Fe). Moreover,MIL-100(Fe) reported by Huo et al.29 gave the BET surfacearea of 1626 m2·g−1 and a pore diameter at 3.2 nm, respectively,which is also similar to that of as-synthesized NMIL-100(Fe).The differences in BET surface areas between NMIL-100(Fe)and the previous reported MIL-100(Fe) can be explained asfollows: (i) there is no F− in this framework, which does nottake the role of supporting the framework, leading to thedecrease of the BET surface areas, (ii) this low BET surfaceareas may also be ascribed to the N2 isotherm obtained in thedifferent p/p0 ranges for the calculation of BET surface areas. Inother words, the additional N2 uptake at higher relativepressures is probably related to adsorption within theinterparticular voids, which supports the presence of smallnanoparticles. This phenomenon may hinder the N2 into thepores of NMIL-100(Fe). (iii) The different synthetic methodmay also lead to the decrease of the BET surface area. TheSEM and TEM images of as-synthesized NMIL-100(Fe) arepresented in Figure 2e,f, respectively, clearly displaying ananoscale size rather than a bulk size. Moreover, amorphousmorphology was also observed for the as-synthesized NMIL-100(Fe), which is different from the previous reportedcrystallized structures of MIL-100(Fe). For instance, MIL-100(Fe) materials synthesized by Fu et al. are crystallinephased, showing a cubic morphology with a wide range of sizebetween 0.5 and 50 μm.27 MIL-100(Fe) materials synthesized

by Huo et al. also display bulk phase with the size in the rangeof 0.5−5 μm.29 Because the crystallized bulk MIL-100(Fe) arenot suitable in the application of adsorption process, the easilyobtained amorphous NMIL-100(Fe) materials provide a newperspective to apply the MOFs to the environment treatment.Moreover, this synthetic route is HF-free. As compared withthe hazardous and detrimental HF-containing synthetic routes,the current method is more likely to be scaled up from theprospect of application. The characteristic surface charge ofNMIL-100(Fe) was investigated by the measurement of ζ-potential in the pH range of 2.0−12.0, shown in Figure S1. Andthe point of zero charge, pHpzc is 4.55.

Adsorption Kinetics. The adsorption kinetic study plays animportant role in the adsorption process analysis, and candepict the adsorption rate that controls the residual time of theadsorption process at the interface between solid and solution.To reveal the dye capture behavior of NMIL-100(Fe), the dyeadsorption kinetics were performed with the sample concen-tration of 3.33 g·L−1 at 298.15 K, as shown in Figure 3.

Adsorption equilibria can be achieved within 15−20 min in alladsorption processes of the three dyes, indicating very fastadsorption kinetics of the dyes on NMIL-100(Fe) surfaces,which is ascribed to the abundance of active sites in the surface.This fast adsorption efficiency is crucial for the possiblepractical application in removing organic dyes by NMIL-100(Fe) from wastewater.To understand further the adsorption behavior of the three

dyes onto NMIL-100(Fe), four kinetic models were used toanalyze the mechanism of the adsorption process, pseudo-first-order model,35 pseudo-second-order model,36 intraparticlediffusion,37 and the Boyd model.38 Equation 3 expresses theLagergren’s pseudo-first-order kinetic model:

− = −q q qk

tlog( ) log2.303te e

1(3)

where k1 (min−1) is the pseudo-first-order rate constant, and qe

(mmol·g−1) and qt (mmol·g−1) are the adsorption amount of

dye molecules onto NMIL-100(Fe) at equilibrium and atdesired time t, respectively. The values of k1 and qe wereevaluated from Figure 4a, and the results are shown in Table 1.The correlation coefficients (R2) obtained are relatively lower(0.9582, 0.9731, and 0.9861 for R6G, RB, and RR 120,respectively), suggesting that the pseudo-first-order model is

Figure 3. Effect of contact time on the adsorption of the three dyes,R6G, RB, and RR 120, respectively, on the NMIL-100(Fe), Cdye (initial)= 1 × 10−4 mol·L−1, m/V = 3.3 g·L−1, T = 298.15 K, pH = 6.0 ± 0.1.



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inappropriate for characterizing the adsorption process in thisstudy.The pseudo-second-order kinetic model was further applied

to simulate the adsorption behavior, as shown in eq 4:

= +tq k q q

t1

21

t 2 e2

e (4)

where k2 (g·mmol·min−1) is the rate constant of pseudo-second-order adsorption. By plotting t/qt vs t (Figure 4b), thevalues of k2 and qe (mmol·g−1) were obtained, and they werealso shown in Table 1. The correlation coefficients calculatedby pseudo-second-order model are all above 0.999, implyingthat the adsorption process follows the pseudo-second-orderkinetic model.The adsorption of three dyes were further simulated with

intraparticle diffusion and the Boyd models, focusing on theadsorbate transportation from external surface to pores and therate-controlling adsorption process, respectively. The intra-particle diffusion model could be expressed as eq 5:

= +q k t Ct i1/2

(5)

where ki is the diffusion rate constant and C is a constant. Asshown in Figure 4c, the plot of qt against t

0.5 is of multilinearportions, suggesting that there are over one process, whichcould affect the adsorption process. The first linear linesindicate the dye diffusion from solutions to the surfaces ofNMIL-100(Fe), followed with slow diffusions into pores in thesecond stage. The last stage manifests the final equilibriumstatus.31 However, no stage passes through the origin,manifesting that the intraparticle diffusion is not the only ratecontrolling step in the adsorption process. Moreover, decreasedslopes were observed with increasing adsorption time,indicating decreased diffusion rates, which might be attributedthe slowly blocking pores by absorbed dyes.39,40

Equation 6 presents the Boyd kinetics equation:

π= − −F Bt1

6exp( )2 (6)

where F is the fraction of solute and parameter B is amathematical function of F, given by eq 7:

=Fq

qt

e (7)

Figure 4. Four different kinetic models for adsorption of dyes onto NMIL-100(Fe) (a) pseudo-first-order, (b) pseudo-second-order, (c) intraparticlediffusion, (d) Boyd, T = 298 K, Cdye (initial) = 1 × 10−4 mol·L−1, m/V = 3.3 g·L−1.

Table 1. Parameters for Pseudo-First-Order and Pseudo-Second-Order Models for the Adsorption of R6G, RB, and RR 120

adsorbate pseudo-first-order pseudo-second-order

R6G k1 (min−1) 2.48 k2 (g·mmol·min−1) 150.08

qe (mmol·g−1) 0.0486 qe (mmol·g−1) 0.0492

R2 0.9582 R2 0.9999RB k1 (min

−1) 2.91 k2 (g·mmol·min−1) 249.34qe (mmol·g

−1) 0.0465 qe (mmol·g−1) 0.0469R2 0.9731 R2 0.9999

RR 120 k1 (min−1) 2.51 k2 (g·mmol·min−1) 180.12

qe (mmol·g−1) 0.0437 qe (mmol·g−1) 0.0442

R2 0.9861 R2 0.9999



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with the substitution of eq 7 into eq 6, the kinetic expressioncan be expressed as eq 8:

= − − −Bt F0.4977 ln(1 ) (8)

The plot of Bt vs t, named Boyd plot, is used to differentiate theexternal transport vs intraparticle diffusion by observing theorigin of the plotted straight line. The intraparticle diffusion isfavored by passing through the origin and vice versa. Figure 4dclearly indicates the dominant film diffusion adsorptionprocedure as the rate-limiting process for three dyes, which isin well agreement with the observation from intraparticlediffusion model.Adsorption Mechanism. Although these three dyes have

similar adsorption behaviors, there still exists difference in theadsorption mechanism during the adsorption process for theremoval of the three dyes on NMIL-100(Fe). To characterizethe possible adsorption sites on NMIL-100(Fe), FT-IR studieshave been conducted for different dyes. The FT-IR spectra forNMIL-100(Fe), R6G, and R6G adsorbed NMIL-100(Fe) arecollected in Figure 5 and discussed as an example due to similar

structures of R6G and RB exhibiting similar FT-IR spectra inFigure S2. The hydroxyl peak at 3411 cm−1 is observed forNMIL-100(Fe), which blue-shifted toward higher wavelength(3426 cm−1), similarly with the CO stretching of CO from1718 to 1724 cm−1 after adsorption of the R6G. Meanwhile, theCO stretching vibration of R6G red-shifted from 1026 to1020 cm−1. These shifts clearly indicated the formation ofhydrogen bondings between CO and NH functionalgroups of the R6G dye and the CO and −OH functionalgroups of NMIL-100(Fe).Surface complexation via electrostatic interactions has been

demonstrated to be the predominant mechanism regardingadsorption of dyes on various adsorbents. To verify whetherelectrostatic interaction is also dominant in the removal of R6Gby NMIL-100(Fe), the adsorption of R6G by NMIL-100(Fe)under different pH values (3−12) are shown in Figure 6. Itshows that the pH values could hardly affect the adsorptioncapacity of the NMIL-100(Fe) toward R6G, suggesting thatelectrostatic interaction is not the mechanism for the removalof R6G by NMIL-100(Fe). On the other hand, the CCvibration of NMIL-100(Fe) at 1630 cm−1 had no shift after theadsorption of R6G, indicating the absence of π−π stackinginteractions between the phenyl rings of NMIL-100(Fe) andphenyl rings of R6G. Thus, the mechanism of dyes removal, by

adsorption onto NMIL-100(Fe), can only be attributed to theforming of hydrogen bondings between R6G molecules andNMIL-100(Fe).Similar adsorption experiments have been performed on RR

120 for NMIL-100(Fe). Although similar adsorption mecha-nisms are observed for R6G and RB, the adsorption mechanismof RR 120 is measured to be electrostatic interaction confirmedby the effect of the pH values (Figure 6), because FT-IR studiesindicated that there were no evident changes of vibrations onNMIL-100(Fe) after adsorption of RR 120 and (Figure S3).The adsorption process of R6G and RB is independent of pHwhereas that of RR 120 is strongly pH dependent. A highadsorption capacity of RR 120 is observed at pH < 5.02, andthen undergoes a rapid decrease when pH values increase from5.02 to 12.05. The RR 120 solution pH relative to the pHpzc ofNMIL-100(Fe) can qualitatively account for the effect of pH onthe adsorption capacity of NMIL-100(Fe). When the solutionpH value is under 4.55 (Figure S1), a surface positively chargedwill arise in the NMIL-100(Fe), which will have a strongelectrostatic attraction to the negatively charged dye molecules.Because of the significantly low hydrolysis constant value of thesulfonate groups of the dye molecules (2.1), this functionalgroup could easily dissociate and the dye molecule and thus hasnet negative charges in the experimental conditions. Thedecreasing adsorption capacity at higher pH values may resultfrom the abundant OH− ions in solution leading to the strongelectrostatic repulsion between the anionic dye molecules andthe negatively charged surface. Moreover, there are neitherextra exchangeable anions on the exterior surface of the NMIL-100(Fe) with higher pH values, resulting in the decrease of theadsorption capacity. Additionally, inferior adsorption of RR 120in basic conditions is also related to the excess OH− ions thatwould compete with anionic dyes to occupy the adsorptionsites. With decreasing pH values, more protons will be providedand adsorbed on the surface of NMIL-100(Fe), increasing thepositive charges on surface, as well as the increased amount ofadsorbed anionic dyes. Thus, it is apparent that the interactionbetween NMIL-100(Fe) and RR 120 is electrostatic interaction.Moreover, on the basis of the kinetic data analysis and FT-IRstudies, it can be concluded that the adsorption for R6G andRB can be attributed to abundant binding sites (hydrogenbonding interactions) between chemical groups in R6Gmolecules and the CO and −OH functional groups ofNMIL-100(Fe). Scheme 1 gives an explanatory illustration

Figure 5. FT-IR spectra of (a) NMIL-100(Fe); (b) R6G + NMIL-100(Fe); (c) R6G.

Figure 6. Effect of pH on the adsorption of the three dyes ontoNMIL-100(Fe), T = 298.15 K, Cdyes (initial) = 3.5 × 10−4 mol·L−1, m/V= 3.33 g·L−1.



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about the adsorption mechanism of the three dyes onto NMIL-100(Fe).Adsorption Isotherm. The adsorption isotherms are very

meaningful to investigate the adsorption abilities of dyes ontoNMIL-100(Fe). The Langmuir and Freundlich isothermmodels41,42 are used to analyze the adsorption isotherms, andthe former one is often applied for the monolayer adsorptionprocess:

=+

qQ bC

bC1emax e

e (9)

where Ce (mol·L−1) is the concentration of dyes in solution atequilibrium status, qe (mmol·g−1) is the adsorption capacity atequilibrium status, Qmax (mmol·g−1) is the maximumadsorption capacity, and b (L·mol−1) is the Langmuir constantconcerning the adsorption energy. The affinity between thethree dyes and NMIL-100(Fe) can be estimated with theLangmuir dimensionless separation factor RL, which can bederived from eq 10:43,44

=+

RbC

11L

0 (10)

where C0 (mol·L−1) is the initial concentration of dyes and b(L·mol−1) is the Langmuir constant. The RL values can be usedto confirm whether the adsorption process is irreversible (RL =0), favorable (0 < RL < 1), linear (RL = 1), or unfavorable (RL >1). And in this work, all RL values obtained are in the range of0−1, indicating the adsorption process here is favorable.The Freundlich isotherm model is expressed as eq 11:

=q K C ne f e

1/(11)

where Ce (mol·L−1) is the concentration of dyes in solution atequilibrium status, qe (mmol·g−1) is also the adsorptioncapacity at equilibrium status, Kf (mmol·g−1) and n correspond

to Freundlich constants concerned with the adsorption capacityand heterogeneity factor.Figure 7 depicts the adsorption isotherms of three different

dyes on NMIL-100(Fe) at 298.15 K, and the adsorption

isotherms at 308.15 and 318.15 K are plotted in Figure S4. Theparameters calculated based on Langmuir and Freundlichmodel are shown in Table 2 and Table S2, respectively. Theadsorption isotherm data fit better with the Langmuirisothermal with R2 values higher than 0.99, demonstratingmonolayer adsorption process. Moreover, the thermodynamicperformance suggests the uniform binding energy on thesurface of the adsorbent, i.e., there is no interaction orcompetition between the adsorbed dyes molecules/anions dueto the identical adsorption activity of the whole surface, and thisis favorable for the formation of the almost completemonolayer coverage on the adsorbent surfaces. For R6G andRB, the values of Q0 calculated by the Langmuir model, havethe highest value at T = 318.15 K and the lowest value at T =

Scheme 1. Schematic Illustration on the Adsorption Mechanism of Three Dyes onto NMIL-100(Fe)

Figure 7. Langmuir (solid line) and Freundlich (dash line) isothermsfor the dyes adsorption onto NMIL-100(Fe) at 298.15 K.



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298.15 K, which indicates that the adsorption is an endothermalprocess, confirming the better adsorption performance underhigher temperature. On the contrary, an opposite phenomenonwas observed for RR 120 with the maximum at T = 298.15 Kand the minimum at T = 318.15 K, demonstrating anexothermal adsorption process. Table 3 presents the maximum

monolayer adsorptions values of three dyes onto differentadsorbents in the solutions, indicating higher adsorptioncapacities of our present adsorbent, NMIL-100(Fe).Generally speaking, large surface area is desirable for better

adsorption capacity in the adsorption process for dyeremoval.46 However, the surface area has taken no dominantrole in improving the adsorption capacities of the adsorbents,whereas the type and the content of functional groups on theadsorbents surface could also influence the adsorptioncapacities of the adsorbents. Because NMIL-100(Fe) is aconstructed from 1,3,5-H3BTC and Fe3+, there are largeamount of CO and −OH functional groups willing to

combine with dye molecules. As confirmed by FT-IR studiesand the influence of pH values, the adsorption for R6G and RBcan be attributed to abundant binding sites (hydrogen bondinginteractions) between chemical groups in R6G molecules andthe CO and −OH functional groups of NMIL-100(Fe), andthe adsorption for RR 120 can be attributed electrostaticinteraction. Thus, it can be suggested that NMIL-100(Fe) isstill of relatively high adsorption capacity for both cationic dyes,R6G, RB, and the anionic RR 120 removal even with lowsurface area due to its large amount of CO and −OHfunctional groups.

Thermodynamic Studies. Temperature-dependent ad-sorption isotherms are plotted to calculate the thermodynamicparameters, including ΔHo, ΔSo, and ΔGo, of which ΔHo andΔSo are derived from the slope and intercept of the ln Kd vs 1/T curve (Figure 8) according to following equation:62

=Kq

Cde

e (12)

= Δ − ΔK

SR

HRT

ln d

o o

(13)

where R (8.314 J·mol−1·K−1) is ideal gas constant, and T (K) istemperature in Kelvin. The ΔGo for specific adsorption couldbe obtained from the fundamentally thermodynamic equation:

Δ = Δ − ΔG H T So o o (14)

The values obtained from eq 13 and eq 14 are listed in Table 4.The analysis about thermodynamic parameters is beneficial forthe insightful study of the mechanism of dyes adsorption ontoNMIL-100(Fe).Generally, the adsorption process could be suggested as

physi-sorption when the ΔGo value is in the range of −20 to 0kJ·mol−1, whereas a chemisorption process with ΔGo valuebetween −400 and −80 kJ·mol−1.63,64 The adsorptions of thethree dyes onto NMIL-100(Fe) owns all negative changes infree energy, when initial concentration of adsorbent is 3.33 g/L.Additionally, the enthalpy value of adsorption process is alsoavailable in distinguishing chemical and physical adsorption.The ΔHo ranging from 83 to 830 kJ·mol−1 indicates a chemicaladsorption, whereas that ranging from 8 to 25 kJ·mol−1

indicates a physical adsorption. The low values of ΔHo forthe three dyes give clear evidence that the process can be takenas a physi-sorption process. For these three dyes, the negativevalues of ΔGo suggest the spontaneous adsorption of dyes onto

Table 2. Parameters for the Langmuir Isotherm Model ofThree Different Dyes

adsorbate parameters 298.15 K 308.15 K 318.15 K

R6G Q0 (mmol·g−1) 0.1679 0.1749 0.1850KL (L·mol−1) 3.82 × 105 4.24 × 105 4.37 × 105

R2 0.9930 0.9992 0.9989RB Q0 (mmol·g−1) 0.1434 0.1528 0.1601

KL (L·mol−1) 4.28 × 105 4.41 × 105 4.53 × 105

R2 0.9979 0.9936 0.9987RR 120 Q0 (mmol·g−1) 0.1299 0.1108 0.0645

KL (L·mol−1) 1.07 × 105 9.02 × 104 7.35 × 105

R2 0.9991 0.9996 0.9991

Table 3. Maximum Dye Removal Capacity of VariousAdsorbents

adsorbent sample adsorbate adsorption capacity (mg·g−1) ref.

graphene oxide R6G 23.3 45FWH-250 R6G 51.06 46palm shell powder R6G 19.65 47almond shell(Prunus dulcis)

R6G 32.6 48

hexadecylfunctionalizedmagnetic silicananoparticles

R6G 35.6 49

H-0.2 R6G 36.6 50NMIL-100(Fe) R6G 88.62 present

studyAC RB 88.0 51Fe-Ben RB 98.62 52A-TRB RB 212.77 53unmodified biomass RB 25.2 54CK-30 RB 270 55NMIL-100(Fe) RB 76.69 present

studyCP-bentonite RR 120 81.97 56BTA RR 120 84.5 57Fe3O4/MgAl-LDH RR 120 101.0 58activated carbon RR 120 267.2 59IL-Fe3O4 RR 120 166.67 60Chara contraria RR 120 112.83 61NMIL-100(Fe) RR 120 190.95 present

study

Figure 8. Liner plot of ln Kd versus 1/T of the three dyes adsorptiononto NMIL-100(Fe), m/V = 3.33 g/L.



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NMIL-100(Fe). For dyes R6G and RB, the positive values ofΔHo manifest that the adsorption process is endothermicnaturally and also entropy-driven, whereas for RR 120, thevalue of ΔHo is negative and the adsorption process is enthalpy-driven. And all of which are in good agreement with the studyabout the adsorption equilibrium isotherms.

■ CONCLUSIONSA HF-free solvothermal method was used to synthesizenanoscale metal−organic framework NMIL-100(Fe), whichexhibited comparable physicochemical properties as thoseprepared by traditional methods, but with a mild andenvironmentally benign synthesis condition. The presentsynthetic route will lead to a facile way for the synthesis ofNMIL-100(Fe) aiming at practical applications. NMIL-100(Fe)was further utilized as an efficient adsorbent for both cationicand anionic dyes. Fast adsorption efficiencies and highadsorption capacities were observed for all dyes adsorptions,making NMIL-100(Fe) be a viable and new water treatmentmaterial. Besides, from a fundamental standpoint, the resultsobtained may be helpful for the in-depth research to broadenthe application fields of nanoscale metal−organic frameworks.

■ ASSOCIATED CONTENT*S Supporting InformationInformation as mentioned in text. The Supporting Informationis available free of charge on the ACS Publications website atDOI: 10.1021/acssuschemeng.6b00434.

Molecular structures of three dyes R6G, RB, and RR 120;effect of pH on ζ-potential of NMIL-100(Fe); FT-IRspectra; Langmuir and Freundlich isotherms; parametersfor the Freundlich isotherm model of three different dyes(PDF).

■ AUTHOR INFORMATIONCorresponding Authors*J. Li. Tel/Fax: +86-551-65596617. Email: [email protected].*S. Zeng. Email: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors acknowledge the financial support provided byNational Natural Science Foundation of China (1U1530131,21272236, 21373106), special scientific research fund of publicwelfare profession of China (201509074) and JiangsuProvincial Key Laboratory of Radiation Medicine andProtection and the Priority Academic Program Developmentof Jiangsu Higher Education Institutions.

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Table 4. Thermodynamic Parameters for R6G, RB, and RR 120 Adsorption on NMIL-100(Fe)

dyes temperature (K) ΔGo (kJ·mol−1) ΔHo (kJ·mol−1) ΔSo (J·mol−1·K−1)

R6G 298.15 −2.69 13.71 136.10308.15 −2.82318.15 −2.96

RB 298.15 −2.89 5.63 115.87308.15 −3.01318.15 −3.12

RR 120 298.15 −2.41 −27.87 −56.93308.15 −2.36318.15 −2.30



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