A novel and universal metal-organic frameworks sensing platform for selective detection and efficient removal of heavy metal ions

Chemical Engineering Journal 375 (2019) 122111

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A novel and universal metal-organic frameworks sensing platform for selective detection and efficient removal of heavy metal ions

T

Shi-Wen Lva,b,1, Jing-Min Liua,b,1, Chun-Yang Lia,b, Ning Zhaoa,b, Zhi-Hao Wanga,b, ⁎ Shuo Wanga,b, a b

Tianjin Key Laboratory of Food Science and Health, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, China School of Medicine, Nankai University, Tianjin 300350, China

H I GH L IG H T

G R A P H I C A L A B S T R A C T

universal sensing platform for both • Aanalyzing and removing heavy metal was proposed.

amino-group functionalization • Simple would light the MOF materials. MOF showed • Amino-functionalized great adsorption ability for Fe , 3+

Cu

2+

2+

and Pb

.

mechanisms of detection and • Possible adsorption were fully investigated. study opens up new potential of • The MOF application via proper surface modification.

A R T I C LE I N FO

A B S T R A C T

Keywords: Metal-organic frameworks Heavy metals Fluorescent detection Adsorptive removal

Pollution generated by heavy metals has become a global environmental issue with much public concern. For the first time, the current research presented series of amino-decorated MOFs as a universal sensing platform that demonstrated great potential for detecting and removing heavy metals with remarkable specificity and capability. Concretely, the amino-functionalized MIL-101(Fe) synthesized via a simple one-step method possessed satisfactory fluorescence due to the linker emission, and the excellent sensing performance of MIL-101-NH2 in detecting Fe3+, Cu2+ and Pb2+ with a low LOD and a broad liner range could be achieved. Furthermore, MIL101-NH2 exhibited great adsorption ability and reusability in removing Fe3+, Cu2+ and Pb2+ from aqueous solution, and the saturated adsorption capacities reached up to 3.5, 0.9 and 1.1 mM/g. More importantly, other series MOFs constructed from organic linkers with amino groups decoration including MIL-101-NH2(Cr), MIL53-NH2(Al), UiO-66-NH2(Zr) and MOF-5-NH2(Zn) also showed similar performance in the detection and removal of metal ions. Possible mechanisms for the fluorescence quenching and adsorption were investigated. All in all, the research results not only met the requirements of sensitively detecting and efficiently removing heavy metals, but also provided novel inspirations for future application of MOFs in the field related to environmental protection via specific surface functionalization.

⁎

Corresponding author at: No. 94 Weijin Road, Tianjin 300350, China. E-mail address: [email protected] (S. Wang). 1 These authors contribute equally to this article. https://doi.org/10.1016/j.cej.2019.122111 Received 13 May 2019; Received in revised form 11 June 2019; Accepted 29 June 2019 Available online 29 June 2019 1385-8947/ © 2019 Published by Elsevier B.V.

Chemical Engineering Journal 375 (2019) 122111

S.-W. Lv, et al.

1. Introduction

China). The aqueous solutions of metal ions were prepared using there chlorate salts. All other chemicals used in this study were obtained as analytical grade or higher.

Nowadays, environmental pollution is still one of the most sensitive issues facing mankind, which seriously threaten the ecosystem and human health [1]. Heavy metals, as the most common pollutants, have aroused great concern worldwide because of their adverse health effects [2,3]. For instance, Pb2+ ions released into the body can impair the kidneys, reproductive system, neuronal system, and the function of brain cells [4]; the toxicity of Cu2+ is second to mercury in drinking water, and excessive intake of Cu2+ will damage kidney and liver [5]. Thus, it has become particularly important and necessary to develop effective methods for the trace detection and efficient removal of heavy metals. Metal-organic frameworks (MOFs) are a kind of well-known inorganic-organic hybrid materials in which the main components are metal units and organic linkers [6,7]. Thanks to their fascinating surface properties such as high specific surface areas and tunable porosities, MOFs have been widely explored in various fields, including gas storage [8], adsorption and separation [9], drugs delivery [10], catalysis [11], and sensors [12]. Recently, the interests in the application of MOFs for the detection and removal of heavy metals are growing rapidly. Many researches have demonstrated that MOF-based sensors possessed great potential of detecting heavy metals in aqueous solution, and the mechanism of which mainly involve a host-guest electron transfer between analyte and sensor [13]. Du et al reported amine functionalized UMCM-4 decorated with ethoxycarbonyl isothiocyanate as fluorescent sensor for Fe3+ detection, and as-obtained sensor exhibited high sensitivity and selectivity for detecting Fe3+ in aqueous solution [14]. Samanta et al synthesized butyne functionalized MOFs by ligand modulation, and as-prepared MOF-based probe showed great sensitivity with a detection limit of 10.9 nM for Hg2+ [15]. Noticeably, the common design principle for construction of luminescent MOFs was the introduction of fluorescein or quantum dot, but most of these methods were usually complicated. More recently, MOF-based material as a promising adsorbent for efficient removal of heavy metals from aqueous solution has been studied widely [6]. For instance, Huang et al prepared two zeolite-imidazolate frameworks (namely ZIF-8 and ZIF67) as adsorbents for adsorbing Pb2+ and Cu2+ from wastewater, and results indicated that more than 99.4% of Pb2+ and 97.4% of Cu2+ could be removed [16]. Additionally, a thiol-functionalized Cu-based MOF was reported by Ke et al, who used this MOFs material to remove Hg2+ and a saturated adsorption capacity of 714.29 mg/g could be achieved [17]. However, most of methods developed could only conduct either the detection or removal task separately, which limited their practical applications to some extent. Based on above all, the current work proposed a universal sensing platform based on amino-functionalized MOFs via a simple one-step method, which could achieve the requirements for both sensitively detecting and efficiently removing heavy metals. Taking MIL-101NH2(Fe) as example, the as-obtained chemosensor not only had excellent performance for detecting Fe3+, Cu2+ and Pb2+, but also exhibited a great ability to remove Fe3+, Cu2+ and Pb2+ from aqueous solution. The schematic diagram was displayed in Fig. 1, and the possible mechanisms for the fluorescence quenching and adsorption were studied. Additionally, the application of other series MOFs constructed from organic linkers with amino groups decoration for both detecting and removing heavy metals were also investigated.

2.2. Synthesis of materials MIL-101. The preparation of MIL-101 (Materials of Institute Lavoisier, MIL) was conducted according to a modified method as reported [18]. Briefly, 5 mM of FeCl3 6H2O and 2.5 mM of H2BDC were dispersed into 30 mL of DMF in a Teflon-lined autoclave, and the reaction was carried out at 110 °C for 20 h. And the resulting products were collected and washed with absolute ethanol, then recovered after drying. MIL-101-NH2. The fabrication of MIL-101-NH2 was performed using a similar solvothermal method as above mentioned. The only thing that changed was that the same molar mass of NH2-BDC as organic ligand was used instead of H2BDC in the synthesis process of MIL-101-NH2. Other series MOFs including MIL-101-NH2(Cr), MIL-53-NH2(Al), UiO-66-NH2(Zr; University of Oslo, UiO) and MOF-5-NH2(Zn) were constructed using the organic linkers with amino groups decoration (namely NH2-BDC), and the detailed content was shown in Supplementary Material.

2.3. Characterization The morphology was observed by field-emission scanning electron microscopy (FESEM, JSM-7800F) and transmission electron microscopy (TEM, JEM-2800), respectively. The X-ray diffraction (XRD) patterns were obtained by using an Ultima IV diffractometer with a scan rate of 8° min−1. Infrared absorption spectra were recorded on a Thermo IS50 Fourier transform infrared (FT-IR) spectrometer. A surface area analyzer (ASAP 2460) was used to determine special surface area at −196 °C after evacuation at 150 °C for 12 h. The zeta potential was analyzed through the Zeta-sizer Nano-ZS (Malvern, UK). X-ray photoelectron spectra (XPS) were obtained by a Thermo Scientific Escalab 250Xi spectrometer.

2.4. Luminescent measurements In a typically assay, a certain amount of MIL-101-NH2 was fully dispersed in deionized water to give a desired concentration by ultrasonic treatment. After the pH was adjusted to specific value, different metal ions were then added into mixture. Fluorescence emission spectra were immediately determined by exciting at 332 nm using an F-7100 spectrofluorometer (Hitachi, Japan). The widths of excitation slit and emission slit were all 5 nm.

2.5. Adsorption studies To investigate the adsorption capacities of MIL-101-NH2 for metal ions, 20 mg of MIL-101-NH2 was added into 50 mL of metal ions solutions with the desired concentrations. The adsorption experiments were then conducted using a shaking table at 180 rpm and constant temperature for a fixed time. The metal ion concentrations in solution were measured using the fluorescence detection developed by current study. The adsorbed amounts were calculated as follow:

2. Materials and methods

qt = 2.1. Chemical reagents

(C0 − Ct ) × 50 20

(1)

where qt (mM/g) was adsorbed amount at time t; C0 and Ct (mM) were metal ions concentrations at initial and time t, respectively; 50 and 20 were the solution volume (mL) and mass of the adsorbent (mg), respectively.

Terephthalic acid (H2BDC, 99%), 2-aminoterephthalic acid (NH2BDC, > 98%), N, N-dimethylformamide (DMF, > 99.9%), and all metal chlorides were purchased from Aladdin Chemistry Co. Ltd. (Shanghai, 2

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-NH2

Fe3+

Pb2+

Cu2+ 427nm

332nm

DMF

FeCl3·6H2O

Adsorption removal

Fluorescence quenching

110 oC 20 h

MIL-101-NH2 Fig. 1. Schematic representation for the detection and removal of heavy metals by MIL-101-NH2.

The FT-IR analysis could provide some information to illustrate the structure of samples. The FT-IR spectra of MIL-101-NH2 and MIL-101 were displayed in Fig. 2E. The adsorption bands detected at 623 and 704 cm−1 were assigned to Fe-O bonds [23]. Strong band at 1381 cm−1 correspond to eCOO− vibration was observed, affirming the presence of dicarboxylate linkers in the MIL-101-NH2 and MIL-101 [24]. The peaks located at about 768 and 1256 cm−1 were ascribed to NeH wagging band and CeN stretching band, respectively, which demonstrated the amino groups were successfully grafted onto MIL-101-NH2 [25]. The N2 adsorption isotherms of MIL-101-NH2 and MIL-101 were shown in Fig. 2F. Compared to pure MIL-101, the BET surface area of MIL-101-NH2 reduced from 587.4 to 454.6 m2/g, which was mainly due to the introduction of amino groups [26]. In general, the amino groups of MIL-101-NH2 might protrude into the space of the pores of its surface, and thereby causing a reduction in the BET surface area [27].

2.6. Adsorption isotherms and adsorption kinetics In order to better understand the adsorption process of metal ions over MIL-101-NH2, Langmuir isotherm model and Freundlich isotherm model were employed to fit the adsorption isotherms data according to previous reports [19,20], respectively, by using the following equation:

Ce C 1 = e + qe qmax qmax ·b

(2)

1 lnCe n

(3)

ln q e =lnKf +

where b and Kf were Langmuir constant and Freundlich constant, respectively; n was dimensionless exponent of Freundlich equation; Ce (mM) was metal ions concentration at equilibrium; qe and qmax (mM/g) were equilibrium adsorption amount and maximum adsorption amount, respectively. In addition, the pseudo-first order kinetic model and pseudo-second order kinetic model were applied to describe adsorption kinetics, and the adsorption data was analyzed according to the following equation:

ln(qe − q t ) =lnq e − t

(4)

t 1 t = + qt qe k2·qe2

(5)

3.2. Fluorescence properties of MIL-101-NH2 Fluorescence properties of MIL-101-NH2 and MIL-101 were determined, and the results were shown in Fig. 3A. It was found that MIL101-NH2 showed excellent fluorescence emission at 427 nm under excitation at 332 nm, while MIL-101 had no fluorescence emission under the same conditions. Compared to virgin organic linker, interestingly, the organic linker modified with amino groups (namely NH2-BDC) had similar fluorescence emission with MIL-101-NH2 (Fig. S1). Thus, the fluorescence property of MIL-101-NH2 could be attributed to the linker emission [13], and the fluorescence property was not destroyed during the synthesis process of MIL-101-NH2. Generally, the luminescent MOFs concentration and pH exert important effects on its fluorescence properties [28]. As indicated in Fig. 3B, the fluorescence intensity increased gradually with the reduction of the MIL-101-NH2 concentration. When the concentration was 0.05 mg/mL, the fluorescence intensity came to achieve maximum. Subsequently, a decreased trend appeared in the fluorescence intensity as the concentration continuing to decrease, which could be attributed to the self-quenching behavior of MIL-101-NH2. So the MIL-101-NH2 concentration used in the follow-up experiments was set at 0.05 mg/ mL. It could be observed from Fig. 3C that the fluorescence intensity of MIL-101-NH2 was stronger in alkaline condition than in acid condition, revealing the protonation of amino groups had an adverse influence on fluorescent signal generation. Conclusively, the optimal pH value of MIL-101-NH2 solution should be 7. To investigate the possible application of MIL-101-NH2 as a luminescent sensor for metal ions detection, the fluorescence response of MIL-101-NH2 toward various metal ions was then studied in the presence of different metal ions. The I0/I was used to denote the

where k1 and k2 were the first-order kinetic constant and second-order kinetic constant, respectively. 3. Results and discussion 3.1. Characterization The morphology and microstructure of MIL-101-NH2 were studied by SEM and TEM. As shown in Fig. 2A and B, MIL-101-NH2 presented spindle shaped crystals. Fig. 2B and C exhibited the TEM images of MIL101-NH2 and MIL-101. Obviously, MIL-101-NH2 kept the morphology unchanged after the introduction of amino groups, indicating the decoration with amino groups had no effect on crystal morphology. The composition and crystal form of samples were characterized by XRD, and the results were displayed in Fig. 2D. The XRD patterns of MIL-101NH2 and MIL-101 exhibited distinct characteristic peaks at 8.9°, 10.0° and 16.4°, which were well matched with the previous reports, suggesting the successful construction of MIL-101-NH2 and MIL-101 [21–23]. Furthermore, the main diffraction peaks (2θ = 8.9°, 10.0°, 16.4°) of MIL-101-NH2 were similar with that of MIL-101, which revealed the crystal phase structure was well preserved after amino groups introduced into MIL-101. 3

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Fig. 2. (A) SEM image of MIL-101-NH2; TEM images of MIL-101-NH2 (B) and MIL-101 (C); (D) XRD pattern of MIL-101-NH2 and MIL-101; (E) FT-IR spectra of MIL101-NH2 and MIL-101; (F) N2 adsorption isotherms of MIL-101-NH2 and MIL-101.

Fig. 3. (A) Fluorescence emission spectra of MIL-101-NH2 and MIL-101; Effects of MIL-101-NH2 concentrations (B) and pH (C) on fluorescence intensity; (D) Extent of the fluorescence response of MIL-101-NH2 toward various metal ions (metal ion concentration = 5 mM). 4

Chemical Engineering Journal 375 (2019) 122111

S.-W. Lv, et al.

A 4.0

fluorescence quenching intensity ratio. Remarkably, the test results in Fig. 3D indicated that various metal ions showed different effects on the fluorescence intensity of MIL-101-NH2, such as the interaction with Fe3+, Cu2+, or Pb2+ could drastically quench the fluorescence. The above results illustrated that MIL-101-NH2 might be a good candidate to monitor Fe3+, Cu2+, or Pb2+ with high selectivity and sensitivity in aqueous media.

Fe

3.5

y=15.54x+1.0309 R2=0.9934

I0/I

3.0

0 mM 0.01 mM 0.02 mM 0.04 mM 0.08 mM 0.1 mM 0.2 mM 0.4 mM 0.8 mM 1 mM 5 mM 10 mM

8000

Intensity (a.u.)

2.5 2.0

6000

4000

2000

1.5

3.3. Detection of heavy metals The fluorescence responses of MIL-101-NH2 toward Fe3+, Cu2+, or Pb2+ with different concentrations from 0 to 10 mM were further investigated, and the results were displayed in Fig. 4 and Table 1. The linear relationships between I0/I and metal ions concentration were established, and their linear correlation coefficients were all more than 0.99. The limit of detection (LOD) for Fe3+ was calculated to be 0.0018 mM (S/N = 3). And this result was slightly better than a previous research reported by Chand et al, who used azo-functionalized MOFs as a luminescent probe to detect Fe3+ and the LOD of 0.0024 mM was ultimately obtained [29]. Furthermore, the proposed method for the detection of Cu2+ had a wide linear range from 0.01 to 10 mM and a low LOD of 0.0016 mM, and its performance was better than the previous study [30]. As for the detection of Pb2+, a linear range from 0.01 to 1 mM and a LOD of 0.0052 mM were observed. To assess the practicality of the developed method, the obtained MIL-101-NH2 as the fluorescent sensor was employed to measure the concentrations of Fe3+, Cu2+ and Pb2+ in simulated wastewater via the standard addition method. It was observed from Table 2, the recovery range for the spiked samples was from 94.2% to 101%, confirming the proposed MIL-101-NH2 fluorescent sensor method had great feasibility for the detection of Fe3+, Cu2+ and Pb2+.

0 400

0.04

B

0.08

440

480

Wavelength (nm)

0.12

520

0.16

560

0.20

C (mM) 160

Cu

120

y=15.55x+2.0648 R2=0.9933

I0/I

80

10000

0 mM 0.01 mM 0.02 mM 0.04 mM 0.08 mM 0.1 mM 0.2 mM 0.4 mM 0.8 mM 1 mM 5 mM 10 mM

Intensity (a.u.)

8000

40

6000 4000 2000 0

0

400

0

4

480

Wavelength (nm)

6

520

8

560

10

3.4. Fluorescence properties of other amino-functionalized MOFs

C (mM)

C 3.0

The current study results showed that the fluorescence property of MIL-101-NH2 was mainly contributed by organic linker modified with amino groups, which provided selectivity as well as high sensitivity in detecting Fe3+, Cu2+ and Pb2+. And this fabrication method of the MOF-based fluorescent sensor was simple and feasible. In order to prove whether this fluorescence system arisen from organic linker with amino groups decoration had general applicability, thus, other aminofunctionalized MOFs including UiO-66-NH2(Zr) [26], MIL-53-NH2(Al) [12], MOF-5-NH2(Zn) [31] and MIL-101-NH2(Cr) [32] were prepared to investigate the fluorescence properties. The XRD patterns of as-obtained four amino-functionalized MOFs were shown in Fig. S2. Interestingly, it could be seen from Fig. 5 that all four amino-functionalized MOFs showed excellent fluorescence emission under specific conditions, affirming this simple fluorescence way was applicable for other MOFs. Additionally, the fluorescence response of the four amino-functionalized MOFs toward various metal ions was also determined and the results were displayed in Fig. S3. It was observed that various metal ions exhibited diverse quenching effects for the fluorescence of different amino-functionalized MOFs, indicating the four amino-functionalized fluorescence MOFs had an excellent potential as sensors in the detection of metal ions with high selectivity and sensitivity. Compared to the common preparation method of luminescent MOFs, the developed method in this study was simple and effective, and it also had general

Pb y=2.2714x+0.9535 R2=0.9993

2.5

2.0

0 mM 0.01 mM 0.02 mM 0.04 mM 0.08 mM 0.1 mM 0.2 mM 0.4 mM 0.8 mM 1 mM 5 mM 10 mM

8000

Intensity (a.u.)

I0/I

2

440

1.5

1.0

6000

4000

2000

0 400

0.0

0.2

0.4

440

480

Wavelength (nm)

0.6

0.8

520

560

1.0

C (mM) Fig. 4. Linear relationships of emission intensity of MIL-101-NH2 quenched by Fe3+ (A), Cu2+ (B) and Pb2+ (C).

Table 1 Analytical performance of the proposed method. Analyte

Liner range (mM)

R2

LOD (mM)

Spiked (mM)

Determined (mM)

Recovery (%)

Fe Cu Pb

0.01–0.2 0.01–10 0.01–1

0.9934 0.9933 0.9993

0.0018 0.0016 0.0052

0.05 0.05 0.05

0.0506 ± 0.0105 0.0471 ± 0.0119 0.0500 ± 0.0011

101 94.2 100

5

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S.-W. Lv, et al.

MIL-101-NH2, and the resulted electrostatic interaction between metal ions and adsorbents would afford higher uptake capacity. Significantly, the undesired precipitate of metal ions would appear as the increase of pH [34,35]. Therefore, through comprehensive consideration, the subsequent adsorption tests would be carried out at pH 3 (for Fe3+) or pH 5 (for Cu2+ and Pb2+). The adsorption isotherms of MIL-101-NH2 for Fe3+, Cu2+ and Pb2+ were studied, and the results were shown in Fig. 7A, D and G. Obviously, all adsorption behaviors presented a similar change tendency at varied temperatures. The equilibrium adsorption amounts increased along with increasing the initial concentration of metal ions, and the maximum adsorption capacities of MIL-101-NH2 for Fe3+, Cu2+ and Pb2+ reached up to 3.5, 0.9 and 1.1 mM/g, respectively at 25 °C. Remarkably, an increase in temperature could improve the uptake of metal ions, indicating the adsorption process of MIL-101-NH2 was endothermic and spontaneous [26]. Langmuir model and Freundlich model were used for analyzing isotherm data to help understand adsorption process. Generally, Langmuir model presumes the adsorption behavior occurs by a monolayer manner on a homogenous surface where all the adsorption sites show identical affinities [36], while Freundlich model is wildly used in describing the multilayer adsorption occurred on a heterogeneous adsorption surface [37]. The fitting results were displayed in Fig. 7 and Table 2. Through comparing the correlation coefficients, it was concluded that the adsorption process of MIL-101-NH2 for Fe3+, Cu2+ and Pb2+ could be well described by Langmuir model, which implied the adsorption of Fe3+, Cu2+ and Pb2+ onto MIL-101-NH2 might be a monolayer adsorption process. The results of adsorption kinetics were displayed in Fig. S4A. It was observed that the adsorption capacities of Fe3+, Cu2+ and Pb2+ on

Table 2 Fitting results of Langmuir model and Freundlich model. T (°C)

Freundlich model qmax,

Fe

Cu

Pb

25 35 45 25 35 45 25 35 45

Freundlich

(mM/g)

3.81 4.74 5.37 0.75 0.85 0.99 1.09 1.15 1.22

Langmuir model R

2

0.8511 0.8035 0.7389 0.8208 0.8258 0.8391 0.7700 0.8524 0.8534

qmax,

Langmuir

(mM/g)

3.91 4.41 4.69 1.02 1.14 1.35 1.22 1.29 1.36

R2 0.9953 0.9950 0.9951 0.9954 0.9921 0.9903 0.9991 0.9998 0.9997

applicability for many MOFs. This method only needed to simply decorate organic linker with amino groups, and thereby to obtain great fluorescence emission, and no extra fluorescein was required. 3.5. Removal of heavy metals It has been well known that the MOF-based adsorbents held a great deal of potential applications in the adsorptive removal of metal ions [6,9,33]. Hence, the adsorption capacities of MIL-101-NH2 for Fe3+, Cu2+ and Pb2+ were also investigated. As indicated in Fig. 6A, the removal efficiencies of Fe3+, Cu2+ and Pb2+ were all gradually increased with the increase of pH from 1 to 7. Clearly, Fig. 6B showed the surface charge of MIL-101-NH2 was positive because of the protonation reaction under acidic conditions, which would result in the electrostatic repulsion occurred between metal ions and adsorbents. However, with pH increasing, the negative charges gradually appeared in the surface of

A

B

UiO-66-NH2 (Zr)

MOF-5-NH2 (Zn) MOF-5 (Zn)

Ȝex =328 nm Ȝem=425 nm 400

450

500

Intensity (a.u.)

Intensity (a.u.)

UiO-66 (Zr)

550

Ȝex =317 nm Ȝem=425 nm 400

450

Wavelength (nm)

C

500

D

MIL-53-NH2 (Al)

MIL-101-NH2 (Cr) MIL-101 (Cr)

Intensity (a.u.)

Intensity (a.u.)

MIL-53 (Al)

Ȝex =328 nm Ȝem=425 nm 400

450

500

Wavelength (nm)

550

Wavelength (nm)

550

Ȝex =358 nm Ȝem=450 nm 400

450

500

550

600

Wavelength (nm)

Fig. 5. Fluorescence emission spectra of UiO-66-NH2 (Zr; concentration = 0.05 mg/L and pH = 7) (A), MOF-5-NH2 (Zn; concentration = 0.05 mg/L and pH = 7) (B), MIL-53-NH2 (Al; concentration = 0.05 mg/L and pH = 7) (C), and MIL-101-NH2 (Cr; concentration = 0.5 mg/L and pH = 3) (D). 6

Chemical Engineering Journal 375 (2019) 122111

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A 2.00

B 25

Fe Cu Pb

1.75

Zeta potential (mv)

1.50

20

Ce (mM)

1.25 1.00 0.75 0.50 0.25

15 10 5 0 -5

-10

0.00

-15 1

2

3

4

5

pH

6

1

7

2

3

4

5

pH

6

7

Fig. 6. (A) Effect of initial pH on the removal of metal ions by MIL-101-NH2 (Adsorption conditions: C0 = 2 mM; T = 25 °C; contact time = 180 min); (B) Zeta potentials of MIL-101-NH2 at varied pH values.

A

B 4

0.3

1.2

0.8

0.2

y = 0.2263x + 0.0201 R2 = 0.995

o

25 C o 35 C o 45 C

1

0.4

0.6

0.8

Ce (mM)

1.0

1.2

1.4

1.6

y = 0.2132x + 0.0142 R2 = 0.9951

0.0

E

1.2

3

1.0 0.8

0.4

0.8

F

o

T ( C) 25 35 45

-3

1.6

y = 0.9743x + 0.2843 R2 = 0.9954

0.0

0.2

0.5

1.0

1.5

Ce (mM)

2.0

0.5

1.0

1.5

1.00

Ce (mM)

2.5

T ( C) 25 35 45

Ce/qe

qe (mM/g)

y = 0.7732x +0.0667 R2 = 0.9998

0.5

0.0

0.5

1.0

1.5

Ce (mM)

2.0

2.5

0.5

1.0

1.5

Ce (mM)

0.5

1.0

y = 0.165x +0.0869 R2 = 0.77

T ( C) 25 35 45

y = 0.1522x +0.1464 R2 = 0.8524

0.0

-0.1 -0.2

y = 0.7346x +0.0575 R2 = 0.9997

0.00

0.0

lnCe

0.2

o

25 C o 35 C o 45 C

0.25

-0.5

y = 0.1508x +0.2066 R2 = 0.8534

0.3

0.1

1.0

0.50

-1.0

I

y = 0.8175x +0.0738 R2 = 0.9991

1.5

0.75

o

T ( C) 25 35 45

y = 0.2976x - 0.2765 R2 = 0.8208

-0.8

2.0

o

2.0

y = 0.3076x - 0.151 R2 = 0.8258

-0.6 y = 0.739x + 0.2238 R2 = 0.9903

H

1.25

0

-0.4

1

2.5

-1

lnqe

y = 0.87x + 0.2394 R2 = 0.9921

0.0 0.0

lnCe

-0.2

o

25 C o 35 C o 45 C

-2

y = 0.3358x - 0.0091 R2 = 0.8391

0.2

Ce/qe

qe (mM/g)

0.4

G

1.2

Ce (mM)

2

0.6

o

T ( C) 25 35 45

lnqe

D

0.2

y = 0.3604x + 1.3378 R2 = 0.8511

0.4

0.1

0 0.0

y = 0.366x + 1.5568 R2 = 0.8035

lnqe

2

y = 0.3492x + 1.6816 R2 = 0.7389

y = 0.2553x + 0.0301 R2 = 0.9953

Ce/qe

qe (mM/g)

3

C 1.6

o

T ( C) 25 35 45

0.4

o

-0.3 2.0

2.5

-1.5

-1.0

-0.5

lnCe

0.0

0.5

1.0

Fig. 7. Adsorption isotherms of Fe3+ (A), Cu2+ (D) and Pb2+ (G) on MIL-101-NH2 at varied temperatures (Adsorption conditions: pH = 3 or 5; contact time = 180 min); Fitting the adsorption isotherm curves: (B) Langmuir model for Fe3+; (C) Freundlich model for Fe3+; (E) Langmuir model for Cu2+; (F) Freundlich model for Cu2+; (H) Langmuir model for Pb2+; (I) Freundlich model for Pb2+.

MIL-101-NH2 increased rapidly in the first 60 min, and then continued to increase but at a slower rate until the adsorption equilibrium. It was well known that the pseudo-first-order model was often used to refer to physisorption process, while the pseudo-second-order model was usually applied to describe chemisorption behavior [38]. To better understand the adsorption process, the pseudo-first-order model and

pseudo-second-order model were employed to simulate the kinetic experimental data. The fitting results from Fig. S4 and Table 3 showed that the adsorption of Fe3+, Cu2+ and Pb2+ onto MIL-101-NH2 followed pseudo-second-order model better than pseudo-first-order model based on the comparison of correlation coefficients, indicating the chemisorption might be the rate limiting-step of adsorption [39]. 7

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Additionally, other four MOFs also had different adsorption capacities for metal ions and results were shown in Fig. S5, which suggested other amino-functionalized MOFs could be an alternative candidate for the removal of heavy metals.

Table 3 Fitting results of pseudo-first-order model and pseudo-second-order model. Pseudo-first-order model qe, g) Fe Cu Pb

cal

5.81 0.82 1.21

(mM/

Pseudo-second-order model 2

k1

R

qe, g)

0.0387 0.0288 0.0058

0.9777 0.9488 0.8566

cal

(mM/

4.52 1.09 1.48

k2 (mM/(g min))

R2

5.85 × 10−3 3.06 × 10−2 2.38 × 10−2

0.9902 0.9918 0.9911

3.6. Reusability Generally, the reusability is an important indicator of adsorbent to evaluate the feasibility of practical application. When the adsorption behavior was finished, the MIL-101-NH2 was collected and regenerated by using 10 mM citric acid as a desorption agent [35]. Then the recycle experiment was performed under the same conditions. As indicated in

A After Pb(II) adsorption

O 1s

Pb 4f

Fe 2p

C 1s N 1s

Intensity (a.u.)

After Cu(II) adsorption

O 1s

Cu 2p3

Fe 2p

C 1s N 1s

After Fe(III) adsorption O 1s

Fe 2p

C 1s N 1s

MIL-101-NH2

O 1s

Fe 2p

C 1s N 1s

1200

1000

800

600

400

200

Binding energy (eV)

B

After Fe(III) adsorption

1

402

401

398.5

2

400

399

Intensity (a.u.)

Intensity (a.u.)

399.5

398

Binding energy (eV)

397

402

396

1

402

401

400

399

398

397

Binding energy (eV)

400

399

398

397

Binding energy (eV)

399.0

Intensity (a.u.)

Intensity (a.u.)

399.3

2

401

398.7

1 2

396

After Pb(II) adsorption

After Cu(II) adsorption

399.4

399.1

396

1

402

401

400

398.7

2

399

398

397

Binding energy (eV)

396

Fig. 8. (A) XPS spectra of MIL-101-NH2 before and after adsorption; (B) XPS high-resolution spectra of N 1s for MIL-101-NH2 before and after adsorption. 8

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Transmittance (%)

After Pb(II) adsorption

After Cu(II) adsorption

After Fe(III) adsorption MIL-101-NH2

500

1000

1500

2000

2500

3000

3500

-1

Wavenumber (cm ) -O-C=O asymmetric stretching C-N stretching

1256

1577

1600

-NH stretching

3461 1575

1550

1280

1260

1240

1220

3500

3450

3400

3350

3300

Fig. 9. FT-IR spectra of MIL-101-NH2 before and after adsorption.

Fig. S6, the adsorption capacities of MIL-101-NH2 for Fe3+, Cu2+ and Pb2+ could still achieve 88.1%, 78.8% and 76.9% of first use after six regeneration and reuse cycles, suggesting MIL-101-NH2 presented excellent reusability. Thus, it was believed that MIL-101-NH2 would be a promising candidate for removing heavy metal ions from wastewater.

obtained by decreasing MIL-101-NH2 concentration in the assay system. Meanwhile, many chelating binding sites afforded by amine groups on the surface of MIL-101-NH2 were favorable for the adsorption of Fe3+, Cu2+ and Pb2+. The analysis of FT-IR spectra after adsorption was conducted to further explore the adsorption mechanism. As indicated in Fig. 9, some changes (such as shifting and intensity weakened) were occurred in the peaks assigned to C-N stretching and eNH stretching after Fe3+, Cu2+ and Pb2+ adsorption, demonstrating the chemical interactions happened between amine groups and metal ions. Additionally, a change in the peak ascribed to eOeC]O asymmetric stretching was observed, which suggested that metal ions might have a coordination with the C]O unit [39,41]. The above results showed consistence with the adsorption isotherm analysis that the adsorption of metal ions by MIL-101-NH2 followed a monolayer adsorption process (Langmuir model) of the chemical chelation of target metal ions with the surface amine groups.

3.7. Interaction between MIL-101-NH2 and heavy metals The possible mechanisms for the fluorescence quenching and heavy metals adsorption were explored by testing possible interactions between metal ions and MIL-101-NH2. The full range XPS spectra of MIL101-NH2 before and after adsorption were shown in Fig. 8A, it was found that a peak correspond to Fe exhibited an increase in intensity after Fe3+ adsorption, which might be because Fe3+ was loaded onto MIL-101-NH2. Obviously, two new peaks of Cu 2p3 and Pb 4f were successfully detected after Cu2+ and Pb2+ adsorption, suggesting Cu2+ and Pb2+ have been adsorbed on the MIL-101-NH2. The XPS high-resolution spectra of N 1s before and after adsorption were further analyzed and the results were displayed in Fig. 8B. The N1s spectrum of MIL-101-NH2 could be dissected into two peaks at the binding energy of 399.5 and 398.5 eV, assigning to the nitrogen in the amide (1: eNeC) and amine (2: eNH2), respectively [40]. After Fe3+, Cu2+ and Pb2+ adsorption, a remarkable shift at two fitting peaks (namely 1 and 2) was observed, which could be attributed to the chelation between amine groups and metal ions [4]. The above results (Fig. 3C) have showed the change of amino groups caused by pH possessed an important influence for fluorescence properties of MIL-101NH2, revealing the amino groups played a significant role in fluorescence turn-on or turn-off. Thus, it could be concluded, the chelation between amine groups and metal ions could induce host-guest electron transfer, resulting in the fluorescence quenching. Based on the chelation occurred between amine groups and metal ions, it was found that the lower LOD for the detection of Fe3+, Cu2+ and Pb2+ could be

4. Conclusion Herein, a scalable and cost-effective fluorescence sensor named MIL-101-NH2 was successfully synthesized via a simple one-step method. The results showed that the fluorescent MIL-101-NH2 as a sensing platform showed excellent performance in detecting Fe3+, Cu2+ and Pb2+, and the LOD for Fe3+, Cu2+ and Pb2+ was 0.0018, 0.0016 and 0.0052 mM, respectively. Importantly, other four MOFs functionalized by the same modification method also exhibited satisfactory fluorescence properties, indicating this simple construction method of luminescent MOFs was general applicability. This method only needed to simply decorate organic linker with amino groups to obtain great fluorescence emission, and no extra fluorescein was required. Furthermore, the as-prepared MIL-101-NH2 had great adsorption ability for Fe3+, Cu2+ and Pb2+, and the saturated adsorption 9

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capacities of Fe3+, Cu2+ and Pb2+ reached up to 3.5, 0.9 and 1.1 mM/ g. And other four amino-functionalized MOFs also exhibited good adsorption ability for heavy metals. The analysis of XPS spectra and FT-IR spectra suggested that the chelation between amine groups and metal ions played a significant role in detecting and removing Fe3+, Cu2+ and Pb2+ by MIL-101-NH2. In summary, the proposed method had great potential for practical application in the simultaneous detection and removal of metal ions.

[17]

[18]

[19]

Acknowledgements

[20]

This work was financially supported by National Key R&D Program of China (No. 2018YFC1602401), National Natural Science Foundation of China (No. 21806083), and the Fundamental Research Funds for the Central Universities, Nankai University (No. 63191429, No. ZB19500227).

[21]

[22]

[23]

Declaration of Competing Interest

[24]

The authors declare no conflict of interest. [25]

Appendix A. Supplementary data [26]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122111.

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