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Adsorptive Removal of Artificial Sweeteners from Water Using Metal-Organic Frameworks Functionalized with Urea or Melamine Pill Won Seo, Nazmul Abedin Khan, Zubair Hasan, and Sung Hwa Jhung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11115 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 15, 2016

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ACS Applied Materials & Interfaces

Adsorptive Removal of Artificial Sweeteners from Water Using Metal-Organic Frameworks Functionalized with Urea or Melamine

Pill Won Seo, Nazmul Abedin Khan, Zubair Hasan, Sung Hwa Jhung* Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Daegu 702-701, Republic of Korea.

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ABSTRACT: A highly porous metal-organic framework (MOF), MIL-101, was modified to introduce urea or melamine via grafting on open metal sites of the MOF. Adsorptive removal of three artificial sweeteners (ASWs) was studied using the MOFs, with or without modifications (including nitration), and activated carbon (AC). The adsorbed quantities (based on the weight of the adsorbent) of saccharin (SAC) under various conditions decreased in the order urea-MIL-101 > melamine-MIL-101 > MIL-101 > AC > O2N-MIL-101; however, the quantities based on unit surface area are in the order melamine-MIL-101 > urea-MIL-101 > MIL-101 > O2N-MIL-101. Similar ASWs [acesulfame (ACE) and cyclamate (CYC)] showed the same tendency. The mechanism for very favorable adsorption of SAC, ACE, and CYC over urea- and melamineMIL-101 could be explained by H-bonding on the basis of the contents of –NH2 groups on the MOFs and the adsorption results under a wide range of pH values. Moreover, the direction of Hbonding could be clearly defined (H acceptor: ASWs; H donor: MOFs). Urea-MIL-101 and melamine-MIL-101 could be suggested as competitive adsorbents for organic contaminants (such as ASWs) with electronegative atoms, considering their high adsorption capacity (for example, urea-MIL-101 had 2.3 times the SAC adsorption of AC) and ready regeneration.

KEYWORDS: adsorption; artificial sweeteners; melamine; metal-organic frameworks; urea


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1. INTRODUCTION Emerging contaminants,1–5 especially organic ones6 such as pharmaceutical and personal care products (PPCPs)7–12 and artificial sweeteners (ASWs),13,

14

have attracted much attention

recently because they are produced and used in huge amounts and can be dumped into the environment inadvertently. Although the negative impact of such chemicals has not been defined explicitly, various adverse effects on plants, animals, and human beings have been reported.1–14 In particular, ASWs13–20 have attracted much attention recently because (i) removal of ASWs in wastewater treatment (WWT) is not sufficient; (ii) some ASWs are found in water resources; (iii) many ASWs are excreted (through feces and urine) into the environment without change or digestion; and (iv) production and consumption of ASWs are increasing steadily. New technologies16, 19 have been investigated for removing ASWs from water. Removal of ASWs has been attempted by various methods such as ozonation, advanced oxidation processes (AOPs), biofiltration, coagulation–flocculation, chlorination, and photocatalytic decomposition; however, there have been some drawbacks. For example, AOPs require high energy, and ozonation causes the formation of residual byproducts.21,

22

Therefore, further research is

essential considering the inefficiency of conventional or existing technologies. Adsorptive removal of organic chemicals from water is a promising technology23–27 considering its low energy consumption, mild operation conditions, little production of secondary products, and easy operation. Carbonaceous materials including graphene24 and carbon nanotubes25 were actively studied as potential adsorbents. Low-cost adsorbents including waste materials have also been investigated for removal of organic pollutants from water.28 However, only a few results18, 29 have been reported on removal of ASWs from water via adsorption.



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Remarkable progress has been achieved recently in the field of porous materials because of developments in mesoporous materials30–31 and metal-organic frameworks (MOFs).32–37 In particular, MOFs have been studied for removal of harmful materials from water, fuel, or air.38–42 Hasan et al. applied MOFs, with or without modifications, to adsorb a few organics such as naproxen, clofibric acid, and diclofenac-Na.43–45 Considering their huge and designable porosity, MOFs will be very effective in removing hazardous organics such as ASWs from water, especially when modified suitably to offer an adequate adsorption mechanism.

Table 1. Properties of saccharin, acesulfame and cyclamate.13, 17 ASWs

Molar mass (g/mol)

Water solubility (g/L)

pKa

Saccharin

183.18

4 13

~1.5

Acesulfame

163.15

270 13

2.0 13

Cyclamate

179.24

133 13

1.9 13


17

or 2.2 13

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O C

MIL-101

NH2 H2N

O

O

O

Cr O O

Cr

Cr O O

O

O O

O

O

O Cr

O O

O

Urea or Melamine, Toluene

Cr O

reflux 12h

O

Cr

O O

O

O

O

O

H2N

O O

N N

NH2 N

NH2

Scheme 1. Scheme presentation to show the grafting methods to introduce urea or melamine on MIL-101.

In this work, we utilized a MOF, MIL-101,46 considering its high porosity, stability, and ready modification via grafting on its open metal sites (OMSs).47 As summarized in Scheme 1, we introduced melamine or urea on MIL-101 by coordination of amino groups (of melamine or urea) onto the OMSs of MIL-101 under reflux conditions considering the relatively low basicity of melamine and urea. To the best of our knowledge, there is no report on grafting of urea or melamine on MOFs with OMSs, probably because of the relatively low basicity of the grafting materials. We selected saccharin (SAC), acesulfame (ACE), and cyclamate (CYC) as representative ASWs because these ASWs are often detected in aquatic environments.13–20 SAC is one of the most well-known ASWs;15,

16

it changes very little in the human body and is

excreted into the environment.15 It has also been reported that SAC can be produced as a soil metabolite from certain herbicides.17 Moreover, SAC is used as a brightener in nickel plating plants;18 therefore, removal of SAC from the used solution is essential for recycling of plating 5 ACS Paragon Plus Environment


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solution. Additionally, SAC has been found not only in the wastewater of Germany and Switzerland19 but also in the surface water of Germany.48 Adsorptive removal of SAC has been done using activated carbons (ACs).18 ACE and CYC are also interesting because they can be used as a marker for wastewater contributions.49 Removal of ACE and CYC has been attempted using various techniques such as photocatalysis,50 biodegradation,28,

51

and an AOP.52–55 The

molar mass, water solubility, and pKa values of SAC, ACE, and CYC are presented in Table 1,13, 17

which shows that the three adsorbates are quite acidic and readily soluble in water, hindering

their ready removal from water. Even though adsorptive removal of ASWs is not very common,18,

29

adsorption may be a possible solution for removing ASWs. In this study,

adsorptive removal of the ASWs SAC, ACE, and CYC was first performed using highly porous MOFs with or without modification in order to estimate possible application of MOFs in adsorptive removal of ASWs and to understand the relevant mechanism of adsorption.


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2. EXPERIMENTAL 2.1. Materials

Chromium nitrate nonahydrate [Cr(NO3)3·9H2O, 99%] and terephthalic acid (TPA, 99%) were purchased from Samchun and Junsei Chemicals, respectively. Urea (99%) and melamine (99%) were obtained from Daejung Chemicals. Ethanol (99.5%), nitric acid (60%), sulfuric acid (98%), and toluene (99.5%) were procured from OCI Chemicals. Ammonium fluoride (NH4F, 97%) was purchased from Junsei Chemicals. SAC (H-form, C7H5NO3S, 98%), ACE (K-form, C4H4KNO4S, 99%), and CYC (Na-form, C6H12NNaO3S, 98%) were obtained from Alfa Aesar. AC (granular, 2–3 mm, practical grade) was purchased from Duksan Pure Chemical Co., Ltd. Other chemicals, including solvents, were obtained from commercial vendors. All of the chemicals were used without further purification. 2.2. Synthesis and modification of MOFs MIL-101 was synthesized hydrothermally according to a reported method56 with a small modification. Cr(NO3)3·9H2O (2.22 g) and TPA (0.922 g) were mixed in water (30 mL) and put in an electric oven at 210 °C. After this temperature was maintained for 8 h, the autoclave was cooled to room temperature, and green solid products were recovered by double filtration, washing (with deionized water), and drying at 100 °C for 12 h. The obtained MIL-101 was further purified using an ammonium fluoride solution.57 Urea or melamine was introduced on MIL-101 using grafting47, 57 on the OMSs of the MOF. To ensure successful grafting, the reaction was run under reflux (in toluene solvent) for 12 h. O2N-MIL-101 was obtained by nitration of MIL-101 according to a reported method.58 After 7 ACS Paragon Plus Environment


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modification, the obtained MOFs were separated by filtration, washed with ample water, dried at 100 °C for 12 h with an electric oven, and stored for use.

2.3. Characterization

The crystal phases of the MOFs were confirmed using an X-ray diffractometer (D2 Phaser, Bruker, using CuKα radiation). The porosities of the MOFs were measured at liquid nitrogen temperature using a surface area and porosity analyzer (Micromeritics, Tristar II 3020) after evacuation of the samples at 150 °C for 12 h. The Brunauer–Emmett–Teller (BET) equation was applied to evaluate the surface areas. Successful modification (grafting and nitration) of the MOFs was confirmed using a Fourier transform infrared (FTIR) spectroscope (Jasco FTIR-4100) equipped with an attenuated total reflectance (resolution: 2.0 cm−1) accessory. The nitrogen contents of the modified MOFs were obtained using chemical analysis (Elemental Analyzer, ThermoFisher Flash 2000). 2.4. Adsorption of ASWs from water using MOFs ASW solutions of the desired concentrations were prepared by dissolution of SAC, ACE, or CYC in deionized water and successive dilutions. In this study, the proton form of SAC was used; whereas, the salts of ACE and CYC were applied. Before adsorption, the MIL-101s and AC were dried overnight under vacuum at 100 °C and stored in a desiccator. An exact amount of the adsorbents (5.0 mg) was put in an ASW solution (20 mL) with a fixed concentration. The solutions containing an adsorbent and adsorbates were mixed well for a fixed time (up to 12 h) at 25 °C using a magnetic stirrer. After adsorption for a predetermined time, the solution was separated from the adsorbents with a syringe filter (polytetrafluoroethylene, hydrophobic, 0.5 µm). The ASW concentrations were determined by measuring the absorbance of the solutions 8 ACS Paragon Plus Environment

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using a spectrophotometer (Shimadzu UV spectrophotometer, UV-1800). The absorbances at 267, 226, and 314 nm were used to calculate the concentrations of SAC, ACE, and CYC, respectively. If needed, a UV measurement was conducted after the ASW solutions were diluted. The pH values of the virgin solutions (10 ppm) of SAC, ACE, and CYC were 4.4, 6.5, and 6.6, respectively. To measure the adsorbed amount of SAC under various pH conditions, the pH of the SAC solution was adjusted with 0.1 M aqueous solutions of HCl or NaOH. Considering the pH of rain or stream water, the pH of the ACE and CYC solutions was maintained at 7.0 by adding a few drops of NaOH (0.1 M) solution to the ASW solutions. The adsorption isotherms for SAC over the MIL-101s were obtained by adsorption for 12 h (a sufficiently long time for equilibrium) using SAC solutions with various initial concentrations. Calculations including the adsorbed quantity (qt) and maximum adsorption capacity (Q0) were done according to a reported method.43, 59



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3. RESULTS AND DISCUSSION

3.1. Synthesis, modification, and characterization of MOFs

Table 2. BET surface areas and nitrogen contents of MIL-101s and the maximum adsorption capacities for SAC over the MIL-101s. Adsorbent

SABET (m2/g)

N content (wt %)

Q0 (mg/g)

Q0 (100*mg/m2)

MIL-101

3030

0

53.4

1.76

urea-MIL-101

1970

3.17

86.4

4.39

melamine-MIL-101

1350

70.1

5.21

O2N-MIL-101

1620

18.7

1.16

12.1 2.58


(b)

1000

MIL-101

3

Simulated MIL-101 MIL-101 urea-MIL-101 melamine-MIL-101 O2N-MIL-101

(a)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Quantity adsorbed (STP-cm /g)

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800

urea-MIL-101 600

O2N-MIL-101 melamine-MIL-101

400

AC 200

5

10

15

20

25

30

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/Po)

2 theta (deg)

Figure 1. (a) XRD patterns of MIL-101s and (b) nitrogen adsorption isotherms of MIL-101s and activated carbon.

Successful synthesis and modification of the MOFs were confirmed by the X-ray diffraction (XRD) and porosity analyses. As shown in Figure 1a, the XRD patterns of the obtained MIL101s were very similar to the calculated one, confirming the successful synthesis and integrity of the crystal structure even after modification via nitration or grafting with urea and melamine. Even though the adsorbed amounts of nitrogen (Figure 1b) and BET surface areas (Table 2) were decreased by the modifications, all of the MOFs were highly porous (>1300 m2/g). The decrease in the porosity of MOFs upon modification is quite well-known and is explained by the contribution of an additional volume of functional groups and/or partial destruction of the MOF structure via modifications in harsh conditions.



MIL-101 urea-MIL-101 melamine-MIL-101

Transmittance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1608 1265

1211

707

2500

2000

1500

1000 -1

Wavenumber (cm )

Figure 2. FTIR spectra of pristine MIL-101, urea-MIL-101, and melamine-MIL-101. Inset shows the enlarged part of the spectra from 750 to 700 cm-1.

The obtained MOFs, with or without grafting, were analyzed using FTIR spectroscopy to confirm successful grafting with urea and melamine. As illustrated in Figure 2, several bands at 1608, 1265, 1211, and 707 cm−1 appeared after grafting with urea and melamine. The FTIR results confirmed successful grafting because the bands at 1608, 1265/1211, and 707 cm−1 are the result of N–H bending, C–N stretching, and N–H wagging, respectively. Chemical analysis results (shown in Table 2) also confirmed the introduction of N after modification of the MOFs by grafting (with urea or melamine) and nitration. Therefore, it can be concluded that highly porous MIL-101s with various functional groups such as –NH2 and –NO2 were obtained, in accordance with Scheme 1 (for urea and melamine grafting). This is the first report on grafting of urea or melamine on MOFs with OMSs, although there are several results for the introduction of 12 ACS Paragon Plus Environment

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organic species via grafting typical amines.57, 60, 61 The introduction of urea and melamine onto MOFs is meaningful because of the introduction of an amide group (with low basicity) and of a large N content (and with two –NH2 groups), respectively.

3.2. Adsorptive removal of SAC using MOFs 5

(a)

(b) 2

Amount adsorbed (100*mg/m )

75

Amount adsorbed (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Activated carbon MIL-101 urea-MIL-101 melamine-MIL-101 O2N-MIL-101

50

25

0

4

MIL-101 urea-MIL-101 melamine-MIL-101 O2N-MIL-101

3

2

1

0 0

2

4

6

8

10

12

14

0

2

4

Time (h)

6

8

10

12

14

Time (h)

Figure 3. Effect of adsorption times on the adsorbed amounts of saccharin over MIL-101s and activated carbon. Figures (a) and (b) show the adsorbed amounts of saccharine based on the unit weight and surface area, respectively, of adsorbents. The initial concentration of saccharine was 10 ppm. The solid lines are guides to the eye.

In this study, MOFs were first applied in the adsorption of SAC from water. As shown in Figure 3a, adsorption was completed in 4 h at 25 °C irrespective of the adsorbents used. However, the adsorbed quantities (qt) depended strongly on the applied MOFs; further, qt decreased in the order urea-MIL-101 > melamine-MIL-101 > MIL-101 > AC > O2N-MIL-101, showing the competitiveness of some of the MIL-101s in the adsorption of SAC. In particular, urea-MIL-101 had 2.3 times the q12 h value of AC. To further understand the adsorption of SAC, qt was calculated on the basis of the unit surface area (rather than the unit weight, as shown in Figure 3a) of the MOFs, because the surface area is one of the most important parameters for 13 ACS Paragon Plus Environment


controlling adsorption when there is no special interaction mechanism.62 As presented in Figure 3b, the qt per unit surface area was in the order melamine-MIL-101 > urea-MIL-101 > MIL-101 > O2N-MIL-101. Therefore, it can be summarized that MIL-101s grafted with urea and melamine were very effective for adsorbing SAC according to the adsorbed amounts based on unit weight and surface area, respectively.

6

(a)

(b) 2


Amount adsorbed (mg/g)

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60


30

4


2

0

0 0

2

4

6

8

10

0

2

4

6

8

10

Ce (ppm)

Ce (ppm)

Figure 4. Adsorption isotherms of saccharin over MIL-101s at 25 oC. Figures (a) and (b) show the isotherms of saccharine based on the unit weight and surface area, respectively, of adsorbents (MIL-101s). The solid lines are guides to the eye.

To obtain the maximum adsorption capacity (Q0) of various MIL-101s, adsorption was performed using SAC solutions with a wide range of concentrations. Even though the adsorption was practically completed in 4 h (Figure 3a), the adsorption time was extended up to 12 h to ensure adsorption equilibrium. Figures 4a and 4b show the adsorption isotherms for SAC over various MIL-101s based on the unit weight and surface area of the MOFs, respectively. The general tendencies of Figure 4 agree perfectly with the results of Figure 3. The isotherms were converted into Langmuir plots, as shown in Figure S1, and the obtained Q0 values derived from 14 ACS Paragon Plus Environment

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the linear plots are summarized in Table 2. The exact Q0 values (based on the unit weight of MIL-101) decreased in the order urea-MIL-101 > melamine-MIL-101 > MIL-101 > O2N-MIL101, which agrees exactly with the results of Figures 3a and 4a. Similarly, the Q0 values per unit surface area were in the order melamine-MIL-101 > urea-MIL-101 > MIL-101 > O2N-MIL-101, which also agrees nicely with the results of Figures 3b and 4b. Therefore, it can be concluded that MIL-101s grafted with urea and melamine are the most effective adsorbents of SAC according to the adsorbed amounts based on unit weight and surface area, respectively. 3.3. Mechanism of SAC adsorption The favorable contributions of grafted melamine and urea on MIL-101 to adsorption of SAC is remarkable. In contrast, O2N-MIL-101 was very inefficient in adsorption of SAC according to the amounts adsorbed based on both the unit weight and surface area. Considering the very favorable contribution of urea or melamine and the negative effect of the –NO2 group on MIL101, we assumed a favorable interaction between the –NH2 (of MOFs) and SAC. On the other hand, there might be a repulsion between SAC and the –NO2 group of the adsorbent.



90

80

(a)

(b)

MIL-101 urea-MIL-101

60

Zeta potential (mV)

60

q12 h (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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30

MIL-101 urea-MIL-101

40

20

0

-20

-40

0 2

4

6

8

10

2

3

4

5

6

7

8

9

10

11

pH

pH

Figure 5. Effect of pH on (a) the adsorbed amounts of saccharine over MIL-101 and urea-MIL101; (b) the zeta potentials of the two MIL-101s. The solid lines are guides to the eye.

Recently, various adsorption mechanisms such as electrostatic, π–π, hydrophobic, and Hbond interactions have been suggested to explain the adsorption of various organics over MOFs from both aqueous and non-aqueous phases.39, 40, 63 To understand the adsorption mechanism of SAC over the MOFs, the effects of the solution pH on the adsorbed amounts and zeta potentials were investigated. Urea-MIL-101 and MIL-101 were selected as representative MOFs considering that the former yielded the best performance in SAC adsorption (based on unit weight) and the latter is the pristine (standard) MOF in this study. As shown in Figure 5a, the qt values were quite stable at the investigated pH values for both of the MIL-101s, even though there was a slight decrease at pH > 7 or pH < 3 over the urea-MIL-101. As presented in Figure 5b, the surface charges decreased steadily with increasing pH, and isoelectric points (IEPs) were observed at pH ~ 7.5 over the two MOFs.


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CH3 O

O

O

H

O

S

H

O

O

H

N O

N C O

S

H O

O

H

H

H

H

OH

N

O

N H

N

S

C

N

N N

N

C

O

MIL-101 surface

Scheme 2. Plausible mechanism for adsorption of saccharin, acesulfame and cyclamate (from left to right) over urea-MIL-101. Dotted lines show H-bond between adsorbates and the MOF.

Considering the pKa of SAC (~2.0),13, 17 most of the SAC will be in the deprotonated form with negative charge under the investigated conditions. If a frequently applied electrostatic interaction is considered as the potential mechanism, the appreciable amount of adsorbed SAC at pH 8–10 cannot be explained because of a repulsive interaction between SAC anions and the negative surface of urea-MIL-101. On the basis of the most favorable contribution of –NH2 in both urea- and melamine-MIL-101 (based on the unit weight and surface area, respectively) to SAC adsorption (or, in contrast, the negative contribution of the –NO2 group), H-bonding can be 17 ACS Paragon Plus Environment


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suggested as a plausible adsorption mechanism. Because of the O− on SAC (at pH > 2.0) and the –NH2 on the grafted MIL-101s, the adsorbate SAC (especially after deprotonation) and the MOFs can act as the hydrogen acceptor and donor, respectively. The relatively stable qt values in a wide pH range (3–7) can also be explained by H-bonding because the SAC anion and –NH2 group on urea-MIL-101 will be very stable at those pH values. Therefore, there is little decrease in qt with increasing pH even though the surface charge of urea-MIL-101 decreases steadily. When the pH exceeds 7.5, the surface charge of the MOF will be negative, and therefore, qt will decrease with increasing pH because of electrostatic repulsion. Even though qt decreased slightly with increasing pH from 7.5, qt was quite stable, probably because of the very favorable contribution of H-bonding to SAC adsorption. The reason for a very slight decrease in qt with decreasing pH from 3 to 2 might be that the SAC is partially neutral (therefore, the efficiency of H-bonding will be decreased). Moreover, a small decrease in electrostatic attraction (because of the partially neutral SAC and positive surface of urea-MIL-101) might be another reason for the lower qt at pH 2 than at pH 3. On the basis of the above discussion, H-bonding can be presented as a plausible adsorption mechanism for SAC over urea-MIL-101, as shown in Scheme 2. The contribution of H-bonding will be considerable, similar to that in adsorption of p-nitrophenol,64 because hexagonal structures are generally very stable. The very remarkable adsorption (based on surface area) of SAC over melamine-MIL-101 can also be explained by the H-bonding mechanism, considering the high density of –NH2 on the grafted MOF, melamine-MIL-101 (even with a decreased surface area). The adsorption mechanism of H-bonding has been applied recently to explain adsorptive purification of both water64-66 and fuel.58, 67–70 The effect of pH on the qt value of SAC over pristine MIL-101 (Figure 5a) can be easily explained by a simple electrostatic interaction. Alternatively, qt should decrease steadily with 18 ACS Paragon Plus Environment

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increasing pH because of the stable negative SAC anion and decreasing surface charge (from positive to negative with an IEP at pH 7.5). The small qt even at the highest pH might be the result of a contribution from a simple van der Waals interaction or pore-filling effect.62 3.4. Adsorptive removal of ACE and CYC using MOFs

(a) 2

2

(b)

14


14


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


12 10


8 6 4 2

12 10


8 6 4 2 0

0 0

2

4

6

8

10

12

14

0

2

4

6

8

10

12

14

Time (h)

Time (h)

Figure 6. Effect of adsorption times on the adsorbed amounts of (a) acesulfame and (b) cyclamate over MIL-101s. The adsorbed amounts are calculated based on the surface area of MIL-101s. The initial concentration of the two adsorbates was 10 ppm. The pH of the solutions was 7.0. The solid lines are guides to the eye.

Inspired by the very favorable contribution of –NH2 in the MOFs to adsorption of SAC, the similar ASWs ACE and CYC were adsorbed over the modified or unmodified MOFs. As shown in Figure 6, the qt value (based on the surface area of the MIL-101s) for both ACE and CYC decreased in the order melamine-MIL-101 > urea-MIL-101 > MIL-101 ~ O2N-MIL-101, which is the same as the order for SAC adsorption (Figures 3b and 4b). As illustrated in Figure S2, the qt values (based on unit weight) for both the ASWs were in the order urea-MIL-101 > melamineMIL-101 > MIL-101 > O2N-MIL-101, which is also very similar to the results for SAC in Figures 3a and 4a. Moreover, the qt values in Figure S2 show the competitiveness of the MOFs



(especially urea-MIL-101 and melamine-MIL-101) with conventional AC in adsorptive removal of ASWs. Therefore, the adsorption results for ACE and CYC are very similar to that for SAC, suggesting the contribution of the same adsorption mechanism. Considering the O− on the –SO2 of ACE and CYC (after deprotonation because of high acidity, as shown in Table 1), the Hbonding can be explained by interaction between the –NH2 (of the grafted MIL-101s) and O− (of the –SO2 of ACE and CYC) as the H donor and H acceptor, respectively. The plausible Hbonding adsorption mechanism is also displayed in Scheme 2. 3.5. Reusability of MOFs

80

(b)

(a)

Transmittance (a.u.)

urea-MIL-101 (purified after saccharin adsorption)

60

q12 h (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40

20

urea-MIL-101 (fresh)

urea-MIL-101 (after saccharin adsorption)

Pure saccharin

0 urea-MIL-101

1st reuse

2nd reuse

3rd reuse

2000

1800

1600

1400

1200

1000

800

-1

Wavenumber (cm )

Figure 7. (a) Reusability of urea-MIL-101 for the adsorption of saccharine from water. The lower and upper horizontal lines show the performances of activated carbon and pristine MIL-101, respectively. (b) FTIR spectra of saccharine and MIL-101s in various stages.

The reusability of an adsorbent is very important for viable applications in industry. The reusability of urea-MIL-101 for SAC adsorptive removal was evaluated by repeated adsorptions after acetone washing. As shown in Figure 7a, the qt value for SAC was quite stable during reuse, 20 ACS Paragon Plus Environment

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even though qt exhibited a slight steady decrease. Importantly, the qt value for SAC over ureaMIL-101 is very competitive against not only AC but also the pristine MIL-101. FTIR spectroscopy was also applied to understand the reusability of urea-MIL-101 for SAC adsorption. As presented in Figure 7b, the FTIR bands corresponding to SAC appeared on ureaMIL-101 after adsorption of SAC, showing the successful adsorptive removal of SAC from water. More importantly, the FTIR spectrum of urea-MIL-101 after adsorption of SAC was very similar to that of fresh urea-MIL-101 when the used MOF was washed with acetone. Therefore, the FTIR results confirm not only adsorption of SAC but also ready purification of the used MOF (which is essential for adequate reusability), in agreement with the results of Figure 7a.

4. CONCLUSION Urea or melamine was introduced for the first time on highly porous MIL-101 via grafting on OMSs of the MOF and applied for adsorptive removal of the ASWs SAC, ACE, and CYC from water. The following conclusions could be drawn from this study. First, urea and melamine, even with low basicity, can be grafted on the MOF under reflux. Second, urea-MIL-101 and melamine-MIL-101 showed the highest uptake for the three ASWs based on the unit weight and surface area of the MOF, respectively. Third, the very favorable adsorption over urea-MIL-101 and melamine-MIL-101 could be explained by H-bonding, and the direction of the H-bonding (H acceptor: adsorbates; H donor: MOFs) could be clearly defined. Fourth, on the basis of its adsorption capacity and reusability, urea-MIL-101 could be suggested as a competitive adsorbent for ASWs, especially those with electronegative atoms or –SO2 groups.



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Funding Sources This research (for all authors) was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning (grant number: 2015R1A2A1A15055291). ASSOCIATED CONTENT Supporting information Chemical structures of adsorbates (ASWs), Langmuir plots for saccharin adsorption over MIL101s, and the adsorbed amounts of acesulfame and cyclamate over MIL-101s and activated carbon. This information is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Phone: 82-53-950-5341; Fax: 82-53-950-6330; E-mail: [email protected] ABBREVIATIONS ACE, acesulfame; ACs, activated carbons; AOP, advanced oxidation processes; ASWs, artificial sweeteners; BET, Brunanur-Emmett-Teller; CYC, cyclamate; FTIR, Fourier transform infrared spectroscope; IEPs, isoelectric points; MOFs, metal-organic frameworks; OMSs, open metal sites; PPCPs, pharmaceuticals and personal care products; SAC, saccharin; UV, ultraviolet; WWT, waste water treatment.


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