Rubidium ion capture with composite adsorbent [email protected]

Journal of the Taiwan Institute of Chemical Engineers 84 (2018) 222–228

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Rubidium ion capture with composite adsorbent PMA@HKUST-1 Wei Dai a,∗, Yaoyao Fang a, Le Yu a, Guihua Zhao a, Xiaoying Yan b,∗ a b

College of Chemistry and Life Science, Zhejiang Normal University, Zhejiang Province, Jinhua 321004, PR China National Center for Nanoscience and Technology, Beijing 100190, PR China

a r t i c l e

i n f o

Article history: Received 17 October 2017 Revised 25 December 2017 Accepted 17 January 2018 Available online 6 February 2018 Keywords: Phosphomolybdic acid HKUST-1 Rubidium ion Adsorption

a b s t r a c t We first synthesized a series of phosphomolybdic acid (PMA) promoted HKUST-1 composite adsorbents and investigated their selective adsorption features for rubidium ion (Rb+ ) from model salt lakes. The physiochemical properties of these ad-synthesized materials were analyzed using XRD, thermos gravimetric analysis, scanning electron microscopy, transform infrared spectra, and N2 adsorption, which proved that the PMA was immobilized in the structure of HKUST-1. Batch tests showed that the composite material was able to bind Rb+ ions with strong chemical affinity and exhibited high Rb+ uptake capacities, which is superior to those reported in previous literatures. The pseudo-second-order model can make a good description of the adsorption kinetics, while Freundlich model could well express the adsorption isotherms. The Rb+ uptake mechanism could be mainly attributed to Lewis acid–base interaction between the adsorbents and Rb+ molecules. In addition, the Rb+ sorption selectivity was moderately influenced by any co-ion effect (K+ , Na+ and Cs+ ) due to PMA doping. These studies reveal that our novel composite material might be a promising adsorbent for Rb+ adsorption. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Due to its special chemical and physical properties, rare metals such as rubidium has been widely used in medicine, fiber optic telecommunication systems, night-vision equipment, and genetic engineering, etc. [1–3]. However, with the increasing demands, the price of rubidium metal has rocketed (∼$ 80 0 0/kg) in comparison to the other alkali metals [3]. Therefore, research on rubidium collection has been attracting more and more attention. Though well known that many large hinterland salt lakes with abundant rubidium are existing worldwide, the concentration of rubidium in the lakes is genuinely at trace (≤100 mg/L) [4]. There is an urgent demand to develop new materials and methods for the separation and purification of Rb+ from salt lakes. Several methods including chemical precipitation [5], adsorption [6], ion exchange [7], and liquid extraction [8], have been exploited for Rb+ separation and purification. Among these methods, adsorption is regarded as one of the most competitive technologies in view of its low cost and mild technical conditions [5,6]. Adsorbent, the determinant in adsorption technology, is expected to possess a desired porosity/pore size and functionality in high efficient adsorption. Recently, metal–organic frameworks (MOFs) have attracted much attention because of their large porosity and potential applications [7,8]. A typical example in this field ∗

Corresponding authors. E-mail addresses: [email protected] (W. Dai), [email protected] (X. Yan).

is HKUST-1, one particular type of MOFs containing Cu(II)-paddle wheel-type nodes and 1,3,5-benzenetricarboxylate struts. Thanks to its unique structural properties, HKUST-1 has rapidly thrived in the past decade as an important family of crystalline porous materials [9]. When compared to the conventional adsorbents, such outstanding qualities of HKUST-1, as chemically adjustable porosities, good hydrothermal stability and high internal surface make it an advantageous adsorbent in the field of liquid sorptionrelated fields including the deep desulfurization, adsorption of organic dyes and metal ions, etc. [10,11]. For example, it has been used for the adsorptive removal of dyes from aqueous solutions by Hu et al. [10]. T.T. Wang et al. investigated the adsorption desulfurization performance with bimetal HKUST-1 [11]. Selectively adsorption by means of special interactions between heteropoly acids (e.g. phosphomolybdic acid, PMA) with target molecules [12] is one of its feature functions. Comparison with other supporters, further introduction of PMA component to MOFs could enhance its adsorption performance [13]. Nonetheless, PMAdispersed MOFs produced using different methods show rather diverse performance; some even lose their adsorption activities. What is worse, the adsorption capacity of PMA-dispersed MOF hybrids could be significantly reduced by the steric hindrance of excessively grafted PMA. Thus, suitable heteropoly acids as well as proper loading of PMA is critical for the good performance of the hybrids. On the other side, there are other metal cations such as Na+ , K+ and Cs+ coexist with Rb+ in the salt lakes [14,15]. There

https://doi.org/10.1016/j.jtice.2018.01.023 1876-1070/© 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

W. Dai et al. / Journal of the Taiwan Institute of Chemical Engineers 84 (2018) 222–228

350

HKUST-1 PMA(0.25)@HKUST-1 PMA(0.5)@HKUST-1 PMA(1.0)@HKUST-1

300

3

Volume adsorbed (cm /g)

will be a competitive adsorption between Na+ , K+ , Cs+ and Rb+ during the adsorption process onto the adsorbents. Hence, in order to efficiently improve the uptake capacity and selectivity of the rubidium from salt lake, we report the preparation of a novel composite material PMA@HKUST-1 by an impregnation technique of equal volume. Moreover, PMA@HKUST-1 composite material was applied in Rb+ adsorption from model salt lake in the co-exist of Na+ , K+ and Cs+ . For purpose of finding out the possibility of using this MOFs composite material as an effective adsorbent for rubidium capture, a better understanding of the materials’ adsorption mechanism, the adsorption rate and the controlling step of the adsorption is indispensable. Accordingly, kinetics and equilibrium parameters are employed to provide these results.

250 200 150 100 50

2. Materials and methods

0 0.0

0.2

2.2. Preparation of HKUST-1 and PMA@HKUST-1 The HKUST-1 was similarly synthesized according to our previous works (Scheme S.I.1a) [10,11]. Loading of PMA into HKUST1 was done by immersion loading method (Scheme S.I.1b). The recovered solid adsorbent was labeled as PMA(1.0)@HKUST-1. Another two adsorbents, PMA(0.5)@HKUST-1 and PMA(0.25)@HKUST1, were prepared following the Scheme S.I.1b procedure instead a PMA/Cu weight ratio of 0.5/1.0 or 0.25/1.0 was used. 2.3. Characterization Nitrogen adsorption isotherms were recorded at −196 °C with a surface area and porosity analyzer (Micromeritics, ASAP2020) after evacuation of the adsorbents at 150 °C for 12 h. The specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) equation by assuming a section area of nitrogen molecule to be 0.162 nm2 . The total pore volume was estimated to be the liquid N2 volume at a relative pressure of 0.99. The sample phases were determined using X-ray diffraction (XRD, D2 Phaser, Bruker, Cu Kα radiation). The compositions of the samples were examined using field-emission scanning electron microscopy (SEM, Hitachi S-4800). FT-IR spectra were recorded on a Bio-Rad FTS 155 FTIR spectrometer at room temperature in KBr pellets under atmospheric conditions. Thermo-gravimetric analysis was performed using a Netzsch STA 449 C instrument and experiments were conducted with a constant heating rate of 5 °C/min in nitrogen atmosphere. NH3 temperature programed desorption (NH3 -TPD) data were collected by a Thermo Electron TPD/R/O 1100 series catalytic surfaces analyzer equipped with a thermal conductivity detector. 2.4. Batch experiments The model salt lakes of Rb+ /K+ /Na+ /Cs+ was prepared by dissolving RbCl/KCl/NaCl/CsCl in deionized water. Rb+ solutions of different concentrations (0–100 mg/L) were prepared by successive dilutions of the stock solution with deionized water. The Rb+ /K+ /Na+ /Cs+ concentrations were determined using an atomic absorption spectrometer (TAS-990, Beijing Puxi Co., Ltd.). Before adsorption, the HKUST-1 and PMA@HKUST-1 were dried overnight under vacuum at 100 °C and were kept in a desiccator. The adsorbents were added to Rb+ /K+ /Na+ /Cs+ solutions with fixed ions concentrations. After that, at a constant temperature of 25 °C,

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

2.1. Chemicals and reagents All the chemicals (Table S.I.1) used in this study were analytical grade and used without further purification.

223

Fig. 1. N2 adsorption isotherms of HKUST-1, PMA(0.25)@HKUST-1, PMA(0.5)@HKUST-1 and PMA(1.0)@HKUST-1 at 77 K, respectively.

mix well the Rb+ /K+ /Na+ /Cs+ solutions which contain the adsorbents using magnetic stirring for 100 min and filter out the adsorbent with a syringe filter (polytetrafluoroethylene, hydrophobic, 0.5 μm), and the Rb+ /K+ /Na+ /Cs+ concentration was measured using atomic absorption spectrometer. The results revealed that under the concentration range used in this work, the calibration curves satisfied linear rule and were very reproducible (R2 = 0.9999). All the uptake capacities (mg/g) were calculated from the difference between the final and initial concentrations of the adsorbate using the following equation:

qe =

(Co − Ce ) · V M

(1)

where Co and Ce (mg/L) are the initial and equilibrium concentrations of Rb+ /K+ /Na+ /Cs+ solution, respectively; V (L) is the volume of solution, and M (g) is the mass of MOFs materials used. 3. Results and discussion 3.1. Materials characterization 3.1.1. N2 adsorption Fig. 1 and Table S.I.2 plotted the nitrogen adsorption isotherms and summarized textural properties of all the synthesized samples. The isotherm obtained for HKUST-1 belongs to type I with a small hysteresis loop, according to IUPAC classification [16,17], which is typical for microporous materials as reported in the literature [11,17]. The isotherm shapes of the PMA@HKUST-1 sample are between type I and IV. In detail, the steep rise at the low pressure reflects that these materials harbor micropores. And the H4 hysteresis loop emerges at relative pressures of ∼0.4 for all samples except HKUST-1 suggests mesopores are obtained as well, which is credited to the crystallization process. In comparison with the blank HKUST-1 sample, the PMA@HKUST-1 sample displayed inferior total pore volume, specific surface area, and average pore size. Besides, the surface area decreased with the increasing of PMA loading amount while the pore volumes of PMA@HKUST-1 decreased accordingly. Furthermore, when compared with HKUST1, the average pore diameter was reduced in the composites and that was more seriously reduced in PMA(1.0)@HKUST-1, which indicates that the high level loading of PMA may cause pore blocking. In sum, during the production of MOFs, the incorporation of PMA decreased the under-utilized space and thus created more micropores while at the same time shrank the pore diameter. It could

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Fig. 2. SEM images of HKUST-1 and PMA(0.5)@HKUST-1. (A): HKUST-1; (B): PMA(0.5)@HKUST-1.

3.1.3. XRD analysis The X-ray diffraction (XRD) patterns of the HKUST-1 before and after doping PMA are shown in Fig. 3. We know that pure HKUST-1 shows a typical amorphous structure. From Fig. 3, more information could be attained. First of all, the diffraction peaks of HKUST-1 are consistent with those mentioned in the literature, [10]. In addition, the positions peaks of the PMA@HKUST-1 are analogous with the parent HKUST-1, demonstrating the formation of the crystal structure was not inhibited by the impregnation of PMA. Furthermore, it is worthy to mention that the peak intensities displayed for PMA@HKUST-1 were notably stronger than those shown for HKUST-1, implying that by adding a certain amount

PMA(0.5)@HKUST-1

(440)

(422)

(422) (333)

(400)

(222)

PMA(0.25)@HKUST-1 (220)

3.1.2. SEM micrographs The surface morphologies of the parent material and composites can be observed by SEM images. As seen in Fig. 2, being consistent with the previous reports, HKUST-1 exhibits a clear crystalline structure whose shape is pyramidal [10,11]. The PMA@HKUST-1 materials have a smooth surface whose morphology is alike to that of HKUST-1, signifying a minor amount of PMA does not disrupt the formation of crystal structure. The crystal sample of HKUST-1 and PMA(0.5)@HKUST-1, in the SEM image had a double-sided pyramidal shape with about 5–20 μm width. This result illustrates that loading an appropriate amount of PMA was required to maintain the octahedral shape of the crystal structure.

PMA(1.0)@HKUST-1

Intensity (a.u)

be assumed that more PMA was implanted into the inner pores or thin layers were formed on the internal surface of the pores of the MOFs. This, however, could be regarded as an advantage if PMA could strongly interact with the support and preserve on the surface because it would eventually result in a more Rb+ uptake capacity.

HKUST-1

PMA 6

8

10

12

14

16

18

20

22

24

26

28

30

32

2-Theta (degree) Fig. 3. XRD curves of HKUST-1, PMA(0.25)@HKUST-1, PMA(0.5)@HKUST-1 and PMA(1.0)@HKUST-1, respectively.

of PMA, a more ordered structure in the hybrid materials was produced. It was though that HKUST-1 could serve as nucleation sites and help the growth of crystal. Similar conclusions were also observed in the literatures [3,10]. 3.1.4. Thermal analysis The TG/DTG curves acquired for the ignited samples of HKUST1 and PMA@HKUST-1 as well as the gas products releasing process during thermal decomposition in N2 atmosphere were presented in Fig. 4. As can be seen from the graph, PMA@HKUST-1

W. Dai et al. / Journal of the Taiwan Institute of Chemical Engineers 84 (2018) 222–228

110

A

100

100

90 80

qe (mg/g)

Weight percent (wt %)

90 80 70

PMA(1.0)@HKUST-1

60

HKUST-1 PMA(0.25)@HKUST-1 PMA(0.5)@HKUST-1 PMA(1.0)@HKUST-1

50

20 10

PMA(0.25)@HKUST-1

0

HKUST-1 0

100

200

300

400

500

600

700

800

o

Temperature ( C)

B 2 0 -2 -4

HKUST-1 PMA(0.25)@HKUST-1 PMA(1.0)@HKUST-1 PMA(1.0)@HKUST-1

-6 -8 -10 -12 -14 -16 -18 0

100

200

300

400

500

600

700

800

o

Temperature ( C) Fig. 4. Thermal analysis curves TG (A) and DTG (B) for HKUST-1, PMA(0.25)@HKUST-1, PMA(0.5)@HKUST-1 and PMA(1.0)@HKUST-1, respectively.

110 100 90 80 70 60 50 40

HKUST-1 PMA(0.25)@HKUST-1 PMA(0.5)@HKUST-1 PMA(1.0)@HKUST-1

30 20 10 0

10

20

30

0

10

20

30

40

50

60

70

80

90

100

110

t /min

30

Mass difference (%)

60

30

PMA(0.5)@HKUST-1

40

0

70

40

50

qe (mg/g)

225

40

50

60

70

Fig. 6. Effect of contact time on the adsorption capacities of Rb+ on HKUST1, PMA(0.25)@HKUST-1, PMA(0.5)@HKUST-1 and PMA(1.0)@HKUST-1, respectively. (pH = 7, dose = 0.4 g/L, T = 298 K).

and its parent material HKUST-1 display not only extremely comparable weight loss curves but also the same released gas products when the temperature rise from 20 to 800 °C. This further substantiated that the introducing of PMA does not change the structure in HKUST-1, which supported the studies discussed beforehand in SEM and XRD. Two weight-loss steps are demonstrated in the TG/DTG profiles, where the first step, in the range of 50–120 °C and 20 0–30 0 °C, is related to the removal of the guest water and solvent molecules inside the pores [18,19]. ∼10% weight loss is observed for all the prepared materials. The decomposition of organic moieties (e.g. BTC ligand) and complete collapse of the framework can be represented in the ∼300–360 °C range. The ∼53% ∼50%, ∼45%, and ∼40% weight losses in HKUST-1, PMA(0.25)@HKUST-1, PMA(0.5)@HKUST-1 and PMA(1.0)@HKUST-1, respectively. Generally, the outcomes show a reasonable thermal stability for PMA@HKUST-1 framework structure up to 300 °C. The TG and DTG curves of the composites looked rather similar to parent HKUST-1. It implied that the composites PMA@HKUST-1 had similar thermal stability with HKUST-1. In generalization, the outcomes suggest that the framework structure of PMA@HKUST-1 has a sound stability up to 300 °C. 3.1.5. IR spectra Existence of functional groups can be confirmed by IR spectra and the results are plotted in Fig. S.I.1. The spectrum of HKUST-1 matches well with that reported in the literature [18,19]. The bands in the range of 60 0–130 0/cm region can be accredited to the outof-plane vibrations of BTC, and the bands at 1644 and 1568/cm as well as those at 1444 and 1374/cm originate from the asymmetric and symmetric stretching vibrations of the carboxylate groups in BTC, correspondingly. Additionally, the bands of weaker intensity at around 3440/cm approve the existence of a prominent amount of water molecules in the framework. Moreover, the weak and narrow bands at ∼750/cm can be individually owing to δ (C–H) and γ (C–H) vibrations of aromatic rings [20,21]. For PMA@HKUST-1, the band at 965/cm can be ascribed to the PMA vibration, which are stronger than HKUST-1. Therefore, the FTIR spectrum settles the existence of the PMA on PMA@HKUST-1.

Ce (mg/L) Fig. 5. Adsorption isotherms of Rb+ on HKUST-1, PMA(0.25)@HKUST-1, PMA(0.5)@HKUST-1 and PMA(1.0)@HKUST-1, respectively. (pH = 7, dose = 0.4 g/L, T = 298 K).

3.1.6. NH3 -TPD analysis As a well-known method of probing the surface acidity, the NH3 -TPD was conducted over the HKUST-1, PMA(0.25)@HKUST-1, PMA(0.5)@HKUST-1 and PMA(1.0)@HKUST-1. NH3 -TPD profiles of

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a

b

100

85

HKUST-1 PMA(0.5)@HKUST-1

95 90

HKUST-1 PMA(0.5)@HKUST-1

80 75 70

80

qe (mg/g)

qe (mg/g)

85

75 2

y=-0.12x+95.88, R =0.9751

70 65

65 2

y=-0.096x+83.08, R =0.9900

60 55

60

50 55 2

y=-0.13x+72.74, R =0.9642

50

45 40

45 40

2

y=-0.10x+62.18, R =0.9871

0

50

100

150

35

200

0

50

CNa (mg/L)

100

150

200

CK (mg/L)

+

+

c 95

HKUST-1 PMA(0.5)@HKUST-1

90 85

qe(mg/g)

80 75

2

y=-0.081x+84.45, R =0.9333 70 65 60 2

55

y=-0.097x+72.36, R =0.9634

50 45

0

50

100

150

200

CCs+(mg/L) Fig. 7. Effect of Na+ , K+ and Cs+ initial concentration on Rb+ uptake capacity of HKUST-1 and PMA(0.5)@HKUST-1. (a): Na+ ; (b): K+ ; (c): Cs+ .

the adsorbents are listed in Table S.I.3. The total acidity of assynthesized samples of PMA(0.25)@HKUST-1, PMA(0.5)@HKUST-1 and PMA(1.0)@HKUST-1 was about 0.56, 0.63, and 0.81 mmol/g, respectively. It indicated that, the PMA could be loaded into HKUST-1 by immersion loading method. The total acidity of the as synthesized sample was increased with the increase of PMA contents in the PMA@HKUST-1 sample. 3.2. Rb+ adsorption isotherms Rb+ adsorption isotherms were depicted graphically by plotting the adsorption quantity against the liquid-phase concentration at equilibrium. Fig. 5 shows Rb+ adsorption isotherms in solution at equilibrium onto HKUST-1, PMA(0.25)@HKUST-1, PMA(0.5)@HKUST-1 and PMA(1.0)@HKUST-1, respectively. The equilibrium adsorption quantity increases as the equilibrium concentrations increase and then reach adsorption saturation. The equilibrium adsorption data were obtained at 25 °C and the most widely-used two models, Langmuir and Freundlich isotherm, were utilized to probe the adsorption isotherms. The linear forms of the two isotherms are expressed by Eqs. (2) and (3) in Table S.I.4. The corresponding isotherm parameters obtained by linear regression

are displayed in Table S.I.5. The fitted straight lines (Fig. S.I.2) graphically reveal that linearized Freundlich isotherms exhibited a better fit for the equilibrium adsorption data than Langmuir adsorption isotherms (higher R2 , correlation coefficient). Freundlich adsorption model describes the non-ideal adsorption process and refers to multilayer adsorption. It deems the adsorption surface to be heterogeneous, and the adsorption sites are non-identical to occupy due to the differences in adsorption energy. The Kf value gives an idea of the relative sorption affinity, and the n value indicates the intensity of the sorption process. In our case, the n value of less than 1 manifests the heterogeneity of the surfaces of the composite material [22,23]. Moreover, the uptake capacities of Rb+ followed the next order HKUST-1 < PMA(0.25)@HKUST1 < PMA(1.0)@HKUST-1 < PMA(0.5)@HKUST-1. The maximum Rb+ uptake capacity onto PMA(0.5)@ HKUST-1 was 99 mg/g, which is higher than those previously reported adsorbents (Table S.I.6) [3,24–29]. It was found that after doping PMA, the Rb+ uptake capacity of PMA@HKUST-1 was significantly increased in comparison with the HKUST-1, which could probably be explained by the Lewis acid–base interaction mechanism (Scheme S.I.2). Rb+ shows Lewis acid performance because it can accept electrons containing metal cations. PMA has lone pair electron, which exhibit Lewis

W. Dai et al. / Journal of the Taiwan Institute of Chemical Engineers 84 (2018) 222–228

model. With higher correlation coefficient (R2 ) and smaller variances between the calculated and experimental equilibrium data, the pseudo-second-order model provides a better fit, indicating it is more suitable for the adsorption process. As shown in Fig. S.I.4, several multiple-linear curves instead of one are presented, clarifying multiple stages were included in the adsorption process. Therefore, the Rb+ adsorption onto PMA@HKUST-1 could be regarded as more than one process, suggesting the intra-particle transport is not the rate-limiting step. Such result is in parallel with that found in previous works [3,26].

100

PMA(0.5)@HKUST-1

80 Cs

+

qeRb (mg/g)

90 Na

70

Na/Cs

K

Na/K Na

60

HKUST-1

50

Cs/K

Cs K

40

Na/Cs Na/Cs/K

Cs/K 0

1

3.4. The effect of competing ions

Na/K

Na/Cs/K 30

227

2

3

Amounts of interference ions Fig. 8. Effect of interfering ions amounts on Rb+ uptake capacities of HKUST-1 and PMA(0.5)@HKUST-1.

base feature. Thus, the strong chemical bonding between PMA and rubidium ions could lead to a specific adsorption and increase the adsorption capacity. When the doping level of PMA is further increased, the amount of the PMA sites were consequently increased, which arouses the better adsorption capacity of PMA(0.5)@HKUST-1 than PMA(0.25)@HKUST-1. However, it may block the pore and leads to the growth of the diffusion resistance during the mass transfer reactions with more PMA doping, which actually can decline the amount of accessible active PMA sites, and thus the sorbent with higher PMA doping (PMA(1.0)@HKUST-1) shows a lower Rb+ uptake capacity than that with higher PMA loading (PMA(0.5)@HKUST-1). 3.3. Rb+ adsorption kinetics The kinetic curves for Rb+ adsorption of the HKUST-1 samples before and after loading PMA are presented in Fig. 6. The four curves all reveal that the initial Rb+ uptake rate was quite fast during the first 10 min and gradually slowed down after that, with adsorption equilibria reached within 30 min. Under the same experimental conditions, the as-synthesized PMA@HKUST-1 exhibited much higher adsorption capacity than the HKUST-1. The experimental results revealed that the uptake capacity is initially fast, becoming slower later on and ultimately reaches saturation. The fast initial uptake can be described as a region in which the rate of reaction tends to be independent of concentration. Initially the metal ions preferentially occupy many of the active sites of the adsorbent in a random manner, as a result of which the rate of adsorption is faster. The adsorption achieves equilibration in ∼1 h, which indicates that chemical adsorption complexation rather than physical adsorption is the main adsorption mechanism (The Rb+ uptake mechanism could be mainly attributed to Lewis acid–base interaction between the adsorbents and Rb+ molecules). Generally, physical adsorption is weak and slow as compared with chemical adsorption. Further with time the rate of uptake becomes slower and reaches a constant value (the plateau region) when the surface becomes saturated. For further investigation on the mechanisms of Rb+ adsorbed onto the samples, the data were fitted to two kinetics models (that is, pseudo-first-order and pseudo-second-order) and the linear form of both models are displayed in Table S.I.7. Fig. S.I.3 reveals that almost all the experimental data are distributed on the fitted straight lines for the pseudo-second-order model. As well, the calculated equilibrium adsorption quantity approaches to the experimental data of the samples for the pseudo-second-order

It is known that the presence of other ions in the solutions which can compete for the binding sites with the target species can affect the adsorption process. In this study, K+ , Na+ and Cs+ cations were regarded as the competitive cations because these cations are present in most of salt lakes and their ionic radii is close to Rb+ [1,2]. A series of experiments were carried out in the presence of K+ , Na+ and Cs+ whose concentration ranged from 20 to 100 mg/L while the Rb+ concentration was constant at 100 mg/L (Figs. 7 and 8). As known from Figs. 7 and 8, the data clarified that the Rb+ sorption was greatly affected by the amounts and concentrations of competent cations. The adsorption capacity of Rb+ decreases linearly with the increase of Na+ , K+ or Cs+ concentration. The Rb+ uptake capacities of HKUST-1 and PMA(0.5)@HKUST1 decreased from 79 to 37 mg/g, and 99 to 52 mg/g with the increase ions species from one to three. This is probably owing to the high similarity in the ionic radii between K+ , Na+ , Cs+ and Rb+ . Therefore, three ions could compete more with Rb+ ion during the sorption process by the adsorbent. This is in good agreement with the results of the previous studies. In addition, percentage reduction of Rb+ uptake capacities onto HKUST-1 (54%) is higher than that of PMA(0.5)@HKUST-1 (48%) in presence of K+ , Na+ and Cs+ ions, which indicates that PMA doping into HKUST-1 material can diminish the influence of competing ions. The mechanism of Rb+ ions adsorption is possible to achieve by various factors like physical and/or chemical properties of adsorbents, mass transfer process, etc. [3,4,27–29]. For example, Lewis acid–base force might be the mainly effect on the K+ , Na+ and Rb+ adsorption process in this work. According to Pearson’s hard and soft acid–base concept, bases can be classified into two categories. One type of bases is polarizable and the other type is non-polarizable. These two groups are denoted as “soft” and “hard” bases, respectively. Similarly, acids can be classified based on their preferential interactions with hard or soft bases. That is, acids that form strong interactions with hard and soft bases are called hard and soft acids, correspondingly. According to this concept, for the same family elements of Na, K, and Rb, with the increase of atomic number, the electron layer increases, and the radius of the atom increases. The attraction ability of the nuclear electrons to the outermost electrons decreases, and the electron losing ability of the atoms increases. Rb+ exhibiting the characteristic of Lewis acid, are relatively verge on soft than those of Na+ and K+ . Thus, relative soft Rb+ might be strongly attracted to soft Lewis bases, such as phosphomolybdic root ion. Thus, the uptake capacity of Rb+ is higher than those of Na+ and K+ . On the other side, the sieve effect could be the most important for the Rb+ and Cs+ adsorption onto PMA@HKUST-1. For the same pore size, comparison with Ce+ , there would be more Rb+ inside pore structure at a unit time due to smaller ionic radius (the ionic radius of Rb+ and Ce+ are 148 and 169 pm) [12,13]. 4. Conclusions In summary, a novel composite adsorbent (PMA@HKUST-1) was successfully obtained by a simple solvothermal reaction with addi-

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tion of PMA. Characterization of this composite material was confirmed by XRD, TGA, SEM, and N2 adsorption. The pseudo-secondorder model can well express the adsorption kinetics data, while the Freundlich model tallies the adsorption isotherm even better. The maximum adsorption quantity of Rb+ approach to 99 mg/g, which are competitive or even superior to other recently reported adsorbents. The main adsorption mechanism is the Lewis acid– base interaction between the synthesized MOFs and Rb+ molecule, which contribute to the excellent adsorption performance of the PMA@HKUST-1. In addition, the sorption process was kinetically fast with maximum sorption attained in 30 min. The fast adsorption of Rb+ makes industrial operation possible for studied adsorbents, which is extremely essential for Rb+ purification and separation from salt lakes. What is more, the adsorbent appeared to be highly selective toward Rb+ in the presence of K+ , Na+ and Cs+ . And these adsorbents could be considered as a promising adsorbent in the adsorption of rubidium ion from salt lakes if further researches including the optimization of the strategies of sorbent regeneration are successfully studied. Acknowledgments This research is financially supported by Zhejiang Provincial Natural Science Foundation of China under grant no. LY16B060 0 02 and Open Research Fund of Key Laboratory of the Ministry of Education for Advanced Catalysis Materials and Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Zhejiang Normal University. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2018.01.023. References [1] Vdovic S, Sarkisyan D, Pichler G. Absorption spectrum of rubidium and cesium dimers by compact computer operated spectrometer. Opt Commun 2006;268:58–63. [2] Naeimi S, Faghihian H. Performance of novel adsorbent prepared by magnetic metal-organic framework (MOF) modified by potassium nickel hexacyanoferrate for removal of Cs+ from aqueous solution. Sep Purif Technol 2017;175:255–65. [3] Fang YY, Zhao GH, Dai W, Ma LY, Ma N. Enhanced adsorption of rubidium ion by a phenol@MIL-101(Cr) composite material. Microporous Mesoporous Mater 2017;251:51–7. [4] Naidu G, Nur T, Loganathan P, Kandasamy J, Vigneswaran S. Selective sorption of rubidium by potassium cobalt hexacyanoferrate. Sep Purif Technol 2016;163:238–46. [5] Naeimi S, Faghihian H. Application of novel metal organic framework, MIL-53(Fe) and its magnetic hybrid: for removal of pharmaceutical pollutant, doxycycline from aqueous solutions. Environ Toxicol Pharmacol 2017;53:121–32. [6] Ye XS, Zhang SY, Li HJ, Li W, Wu ZJ. Determination rubidium and cesium in chloride type oilfield water by flame atomic absorption spectrometry. Spectrosc Spectraol Anal 2009;29:1–4. [7] Zhao GH, Fang YY, Dai W, Ma N. Copper-containing porous carbon derived from MOF-199 for dibenzothiophene adsorption. RSC Adv 2017;7:21649–54.

[8] Dai W, Tian N, Liu CM, Yu L, Liu Q, Ma N, Zhao YX. (Zn, Ni, Cu)-BTC functionalized with phosphotungstic acid for adsorptive desulfurization in the presence of benzene and ketone. Energy Fuels 2017;31:13502–8. [9] Hu J, Dai W, Yan XY. Comparison study on the adsorption performance of methylene blue and congo red on Cu-BTC. Desalin Water Treat 2016;57:4081–9. [10] Hu J, Yu HJ, Dai W, Yan XY, Hu X, He H. Enhanced adsorptive removal of hazardous anionic dye “congo red” by a Ni/Cu mixed-component metal-organic porous material. RSC Adv 2014;4:35124–30. [11] Wang TT, Li XX, Dai W, Fang YY, He H. Enhanced adsorption of dibenzothiophene with zinc/copper-based metal-organic frameworks. J Mater Chem A 2015;3:21044–50. [12] Wang B, Zhang J, Zou X, Dong HG, Yao PJ. Selective oxidation of styrene to 1,2-epoxyethylbenzene by hydrogen peroxide over heterogeneous phosphomolybdic acid supported on ionic liquid modified MCM-41. Chem Eng J 2015;260:172–7. [13] Zhang L, Ding SD, Yang T, Zheng GC. Adsorption behavior of rare earth elements using polyethyleneglycol (phosphomolybdate and tungstate) heteropolyacid sorbents in nitric solution. Hydrometallurgy 2009;99:109–14. [14] Baghkumeh AM, Faghihian H, Mokhtari S. Functionalized nano magnetic Fe3 O4 –SiO2 core–shell as efficient adsorbent for removal of Pb2+ from aqueous solutions. Desalin Water Treat 2017;78:166–71. [15] Akkaya R. Synthesis and characterization of a new low-cost composite for the adsorption of rare earth ions from aqueous solutions. Chem Eng J 2012;200–202:186–91. [16] Wang WN, Zhang F, Zhang CL, Guo YC, Dai W, Qian HS. Fabrication of zinc oxide composite microfibers for near-infrared-light-mediated photocatalysis. ChemCatChem 2017;9:3611–17. [17] Sun CY, Zhang F, Wang X, Cheng FQ. Facile preparation of ammonium molybdophosphate/Al-MCM-41 composite material from natural clay and its use in cesium ion adsorption. Eur J Inorg Chem 2015;2015:2125–31. [18] Tan P, Xie XY, Liu XQ, Pan T, Gu C, Chen PF, et al. Fabrication of magnetically responsive HKUST-1/Fe3 O4 composites by dry gel conversion for deep desulfurization and denitrogenation. J Hazard Mater 2017;321:344–52. [19] Wee LH, Lohe MR, Janssens N, Kaskel S, Martens JA. Fine tuning of the metal-organic framework Cu3 (BTC)2 HKUST-1 crystal size in the 100 nm to 5 μ range. J Mater Chem 2012;22:13742–6. [20] Zu DD, Lu L, Liu XQ, Zhang DY, Sun LB. Improving hydrothermal stability and catalytic activity of metal-organic frameworks by graphite oxide incorporation. J Phys Chem C 2014;118:19910–17. [21] Deng YH, Cai Y, Sun ZK, Liu J, Liu C, Wei J, Li W, Liu C, Wang Y, et al. Multifunctional mesoporous composite microspheres with well-designed nanostructure: a highly integrated catalyst system. J Am Chem Soc 2010;132:8466–73. [22] Gong R, Ye JJ, Dai W, Yan XY, Hu J, Hu X, Li S, He H. Adsorptive removal of methyl orange and methylene blue from aqueous solution with fingercitron-residue-based activated carbon. Ind Eng Chem Res 2013;52:14297–303. [23] Yu HJ, Wang TT, Yu L, Dai W, Ma N, Hu X, Wang Y. Remarkable adsorption capacity of Ni-doped magnolia-leaf-derived bioadsorbent for congo red. J Taiwan Inst Chem Eng 2016;64:279–84. [24] Huang XJ, An LY, Zhao XY, Chen XQ. Preparation of ammonium tungstophosphate-calcium alginate composite adsorbent and its adsorption properties of rubidium. Adv Mater Res 2013;652–254:2524–8. [25] Guerra DL, Batista AC, Viana RR, Airoldi C. Adsorption of rubidium on raw and MTZ-and MBI-imogolite hybrid surfaces: an evidence of the chelate effect. Desalination 2011;275:107–17. [26] Naidu G, Loganathan P, Jeong S, Johir MAH, To VHP, Kandasamy J, et al. Rubidium extraction using an organic polymer encapsulated potassium copper hexacyanoferrate sorbent. Chem Eng J 2016;306:31–42. [27] Ye XS, Wu ZJ, Li W, Liu H, Li Q, Qing B, et al. Rubidium and cesium ion adsorption by an ammonium molybdophosphate-calcium alginate composite adsorbent. Colloid Surf A 2009;342:76–83. [28] Krys P, Testa F, Trochimczuk A, Pin C, Taulemesse JM, Vincent T, et al. Encapsulation of ammonium molybdophosphate and zirconium phosphate in alginate matrix for the sorption of rubidium(I). J Colloid Interface Sci 2013;409:141–50. [29] Zhu YF, Wang WB, Zhang HF, Ye XS, Wu ZJ, Wang AQ. Fast and high-capacity adsorption of Rb+ and Cs+ onto recyclable magnetic porous spheres. Chem Eng J 2017;327:982–91.