Environ Sci Pollut Res DOI 10.1007/s11356-017-8577-5

RESEARCH ARTICLE

Zeolitic imidazolate framework-8 for efficient adsorption and removal of Cr(VI) ions from aqueous solution Mahdi Niknam Shahrak 1 & Mahboube Ghahramaninezhad 1 & Mohsen Eydifarash 1

Received: 27 October 2016 / Accepted: 6 February 2017 # Springer-Verlag Berlin Heidelberg 2017

Abstract Heavy metals are emerging toxic pollutants in which the development of advanced materials for their efficient adsorption and separation is thus of great significance in environmental sciences point of view. In this study, one of the zinc-based zeolitic imidazolate framework materials, known as ZIF-8, has been synthesized and used for chromium(VI) contaminant removal from water for the first time. The as-synthesized ZIF-8 adsorbent was characterized with different methodologies such as powder X-ray diffraction (XRD), thermo-gravimetric analysis, FT-IR, nuclear magnetic resonance spectroscopy, and UV–Vis spectra of solid state. Various factors affecting removal percentage (efficiency) are experimentally investigated including pH of solution, adsorbent dosage, contact time and initial concentration of Cr(VI) to achieve the optimal condition. The obtained results indicate that the ZIF-8 shows good performance for the Cr(VI) removal from aqueous solution so that 60 min mixing of 2 g of ZIF-8 adsorbent with the 2.5 ppm of Cr(VI) solution in a neutral environment will result in the highest separation efficiency around 70%. The time needed to reach the equilibrium (maximum separation efficiency) is only 60 min for a concentration of 5 mg L−1. Structure stability in the presence of water is also carefully examined by XRD determination of ZIF-8 under different contact times in aqueous solution, which suggests that the structure is going to be destructed after 60 min immersed in solution. Electrostatic interaction of Cr(VI) anions by positively charged ZIF-8 is responsible for Cr(VI) Responsible editor: Guilherme L. Dotto * Mahdi Niknam Shahrak [email protected]

1

Department of Chemical Engineering, Quchan University of Advanced Technology, Quchan, Iran

adsorption and separation. Moreover, equilibrium adsorption study reveals that the Cr(VI) removal process using ZIF-8 nicely fits the Langmuir and Toth isotherm models which mean the adsorbent has low heterogeneous surface with different distributions of adsorption energies during Cr(VI) adsorption. Equilibrium adsorption capacity is observed around 0.25 for 20 mg L−1 of initial Cr(VI) solutions. Keywords ZIF-8 . Adsorption . Heavy metals . Chromium(VI) ions

Introduction Pollution of groundwater resources by various contaminants such as heavy metals and their separation from aqueous solutions is one of the most important problems in the world (Kano et al. 2014). Among various existing heavy metals in the aqueous solutions, chromium(VI), due to high water solubility and toxicity (Kano et al. 2014), has the highest destructive effect on the environment specially water resources (Dai et al. 2012). The main sources of chromium(VI) ions are industries such as tanning factories, wood preservatives, leather tanning, textiles, rubbers, and artificial fertilizers (Dai et al. 2012; Arenas et al. 2007). Wastewater from these industries can vent Cr(VI) into environment (Dai et al. 2012). According to World Health Organization (WHO), the allowed levels of chromium(VI) in drinking water is only around 0.1 mg L−1(ppm) (Wilbur 2000). The common techniques used for removal of chromium(VI) ion from aqueous solution include membrane filtration, liquid extraction, electrolysis, ion exchange, reverse osmosis, and adsorption. Among them, adsorption technique has attracted significant attention due to relatively low-cost process, easy operation, high removal efficiency, and its applicability for various pollutants (Bakhtiari and Azizian 2015; Ke et al. 2011). So far,

Environ Sci Pollut Res Fig. 1 The 3-D crystalline structure of some important MOFs used in aqueous solutions so far. a MOF-5. b ZIF-8. c MIL101(Cr) (Khan et al. 2013; Rowsell and Yaghi 2004; Li et al. 1999)

(a) various porous adsorbents such as activated carbons, zeolites, and meso-porous materials have been widely studied for adsorptive removal of chromium(VI) compound (Bakhtiari and Azizian 2015; Khan et al. 2013). On the other hand, metal– organic frameworks (MOFs), a subset of advanced porous materials, have emerged many potential application in various processes including adsorption/separation, catalysis, chemical sensing, drug delivery, ion exchange, electrode materials, magnetism, polymerization, imaging, and membranes (Bakhtiari and Azizian 2015; Khan et al. 2013; Ahmed and Jhung 2014) due to some specific properties such as large apparent surface areas, permanent porosity, tunable pore properties, elegant pore size, and selective uptake of small molecules (Bakhtiari and Azizian 2015; Bertke 2012; Rowsell and Yaghi 2004). These materials are generally consist of two parts: a metal ion or a cluster of metal ions (inorganic part) and a linker (organic part) which are connected via one, two, and three dimensions to make a MOF (Khan et al. 2013). Although different types of MOFs have been used for pollutant removal from gaseous

2-MIm (80.5 mmol)

Dissolving in aqueous ammonia

Dissolving in ethanol

(c)

phase, their application in aqueous solutions due to MOF hydrothermal stability problem is relatively rare. Figure 1 demonstrates some of the most used MOFs for contaminant separation from aqueous environments. In 2015, Li and his co-workers employed zeolitic imidazolate framework (ZIF)-67 absorbent for the removal of Cr(VI) (Li et al. 2015a, b). Their results indicated that the required time to reach equilibrium condition is between 20 and 60 min. Furthermore, they reported Bequilibrium adsorption capacities are 5.88, 9.32, and 13.34 mg g−1 for 6, 10, and 15 mg L−1 of initial Cr(VI) solutions, respectively^ (Li et al. 2015a, b). Maleki et al. (2015) synthesized a MOF based on copper–benzen etricarboxylates (Cu–BTC) and used it to remove Cr(VI) (Maleki et al. 2015). They investigated the effect of three operational parameters including pH, adsorbent dosage, and initial concentration of Cr on the process efficiency. Their results demonstrated that the maximum percentages of chromium removal occur at neutral conditions at pH = 7 (Maleki et al. 2015). In this year also, Bhattacharjee et al. prepared ZIF-90 and used it for Hg(II) removal from water (Bhattacharjee et al. 2015). They reported that the adsorbent is able to show 96–98% removal of Hg(II) ions in a low concentration ranging from 0.1 to 10.0 mg L−1 (Bhattacharjee et al. 2015). Li et al. synthesized MOF-808 and employed it in arsenic removal process. They concluded from their comprehensive experimental studies that the adsorption capacity of arsenic is 24.83 mg g−1 (Li et al. 2015a, b). In 2015, Abbasi and his co(011)

Zn(NO3)2. 6H2O (39.0 mmol)

(b)

Drying for 4h at 80⁰Cin a vaccum oven

(134)

(233)

(114)

(222)

(013)

(112) (022)

(002)

The product centrifuged and washed with Ethanol

Intensity (a.u.)

Mixing at 25 ⁰C for 24 h

ZIF-8(as synthesized) ZIF-8(Simulated)

5

10

15

20

25

30

35

40

2-Theta

ZIF-8 Fig. 2 The flow chart for preparation of ZIF-8 by a simple method

Fig. 3 Experimental XRD patterns of as-synthesized ZIF-8 (red, top) in comparison with the simulated XRD patterns (black, bottom) using the single X-ray crystal structure data

Environ Sci Pollut Res

Fig. 4 FT-IR spectra of the as-synthesized ZIF-8

workers have synthesized a new 3D cobalt (II) metal–organic framework nanostructure absorbent to remove several heavy metals such as Pb2+, Cd2+, Al3+, Hg2+, and Fe3+ (Abbasi et al. 2015). They stated that maximum adsorption of metal ions on absorbent is occurred at pH about 6. Moreover, they claimed that the synthetic adsorbent to remove the heavy metals is suitable (Abbasi et al. 2015). Finally, in a recent work, Bakhtiari and Azizian (2015) employed as-synthesized MOF-5 for copper ion separation from an aqueous solution. Only the effect of two parameters, temperature and pH, were characterized by them. However, they concluded that the rate of adsorption is fast, and the adsorption capacity of MOF-5 is 290 mg g−1 for copper ion. Furthermore, their results illustrated that acidic environments are more appropriate for Cu ion removal. In this article, one of the most stable structures among MOF materials in aqueous solutions,1 ZIF-8 (Canivet et al. 2014; Peterson et al. 2008), is synthesized and used for the adsorption of chromium(VI) ion from aqueous solution. Effect of various parameters such as solution pH, adsorbent dosage, initial solution concentration, and absorbent contact time on the removal efficiency is studied too. To the best of our knowledge, adsorption and separation of Cr(VI) ions using zinc-based ZIF-8 material have not been addressed previously.

Experimental Standard solutions and reagents All chemicals were used without purification; all the solutions were prepared in 18 MΩ cm−1 doubly deionized water (DDW). Ammonia solution (25%) and ethanol (98%) were purchased from Merck company; 2-methylimidazole (2-MIm) (purity 99%) and zinc nitrate hexahydrate (purity 99%) were obtained from Sigma-Aldrich and were used for the synthesis of the zeolitic imidazolate framework-8 (ZIF-8). The stock solution of Cr(VI) at a concentration of 1000 mg L−1 was freshly prepared by dissolving the appropriate amounts of K2Cr2O7 (purity >99%, Sigma-Aldrich) in DDW. The working standard solutions were obtained by the appropriate dilution of the stock standard solutions just before use. The pH adjustment was carried out with 1.0 mol L−1 NaOH (Sigma-Aldrich) solution. A solution (0.025%) of 1,5-diphenylcarbazide (DPC, Sigma-Aldrich) was prepared by dissolving 25 mg of DPC in 5 mL of acetone (Merck) and 10 mL of 5 M H2SO4 (Merck). This mixture was diluted to 100 mL with DDM and was used for estimation of Cr(VI) spectrophotometrically. Instruments

1

It should be noted that, although the stability of MOFs in the presence of water is one of the most important challenges to apply MOFs in practical applications, the application of ZIF-8 is increasing in many industries due to its simple synthesis and higher stability in comparison with the other MOFs. MOF stability in aqueous solutions can be found in more details elsewhere (Ayati et al. 2016).

Physicochemical properties of the absorbent (as-synthesized ZIF-8) was characterized by analytical methods such as powder X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (NMR), and UV–Vis

Environ Sci Pollut Res Fig. 5 13C NMR spectrum of the as-synthesized ZIF-8

spectra of solid state. XRD was performed on a Bruker D8-Advance X-ray diffractometer with Cu Kα (1.54178 Å) radiation, 30 kV/15 mA current, and kilobyte filter. A step scan with an increment of 0.02 in 2 h time and scan rate of 1 min−1 was utilized to obtain better patterns. TGA of ZIF powder was performed in nitrogen atmosphere by PerkinElmer, Pyris 1. Sample held in a platinum pan and heating rate was 10 °C min−1 during the measurements. The FT-IR spectra (400–4000 cm−1) of the adsorbent dispersed in KBr pellets were recorded on a NICOLET system. UV–Vis spectra of solid state ZIF-8 were obtained with a Lambda-35 spectrometer (PerkinElmer). NMR spectra were recorded on a Bruker Avance DPX 300 MHz instrument in DCl/D2O (35%). A spectrophotometer (Agilent Technologies Cary 8454) and pH meter (Metrohm 827) were used for concentration of Cr(VI) at a wavelength of 542 nm and pH determination, respectively. Also, N2 adsorption and desorption isotherms at 77 K that determine the textural properties of ZIF-8 including the specific Brunauer–Emmett–Teller (BET) surface area, pore volume, and pore size distribution were measured by using a Micromeritics ASAP 2020 adsorption instrument.

Preparation of ZIF-8 In this work, we report a novel, fast, eco-friendly, and green method for the synthesis of ZIF-8 structure in

NH3 and EtoH as Bgreen solvents.^ This method is experimentally simple, clean, and high yield which is used for the first time for adsorption of Cr(VI). A simple schematic flowchart diagram of the synthesis procedure is shown in Fig. 2. At first, a solution of Zn(NO3)2. 6H2O (39.0 mmol) in aqueous ammonia (25%) was prepared. Then, a solution of 2-methylimidazole in ethanol (80.5 mmol) was added to this solution slowly, as soon as adding white precipitate was generated. The resulting mixture was stirred at room temperature for 24 h to ensure the reaction was completed. Afterwards, the product was centrifuged and washed with ethanol. Finally, ZIF-8 structure with white color was obtained after drying for 4 h at 80 °C in vacuum oven (Fig. 2). General procedure for removal of Cr(VI) Batches of experiments were carried out to investigate the Cr(VI) adsorption performance in ZIF-8 adsorbent. The pH values were adjusted by the addition of NaOH solution.2 After the equilibrium was reached, the adsorbent was separated from the solutions by centrifuging, and the initial and final metal concentrations were determined by a spectrophotometer through the development of a purple-violet color with 1,5-diphenylcarbazide in acidic solution at a wavelength of 542 nm. The uptake of 2

As it would be explained in the BEffect of solution pH^ section, the adopted pH for all experiments is equal to 7 (neutral environment).

Environ Sci Pollut Res 500

1.2

450 1.0

400 350 CC/g

Absorbance

0.8

0.6

300 250 200

0.4

150 100

0.2

50 0

0.0 200 250 300 350 400 450 500 550 600 650 700 750 800

0

0.2

0.4

wavelength(nm)

Fig. 6 UV–Vis absorption spectra of the as-synthesized ZIF-8

Cr(VI) ions by the adsorbent was evaluated by the removal efficiency (R%) according to Eq. (1)   C 0 −C e  100 ð1Þ R% ¼ C0 where Co and Ce are the initial and equilibrium (final) concentrations of Cr(VI) ions, respectively. It is worthwhile to mention that the agitation speed and temperature were fixed at 670 rpm and 298 K, respectively, through all experiments.

Results and discussion Characterization of the as-synthesized adsorbent The phase structure of the ZIF-8 was determined using a X-ray diffractometer. The spectral and analysis data confirmed successful synthesis of the ZIF-8 structure. As it is shown in Fig. 3, a sodalite (SOD) zeolite-type structure with a lattice constant of 16.992 Å was identified with

0.8

1

Fig. 8 Adsorption/desorption isotherms of N2 at 77 K on ZIF-8. Filled symbols, adsorption; open symbols, desorption

XRD. The peaks observed at 2θ = 7.3, 10.38, 12.74, 14.76, 16.48, 18.1, 22.24, 24.6, and 26.76 were assigned to the (011), (002), (112), (022), (013), (222), (114), (233), and (134) reflection planes of (SOD) topology ZIF-8, respectively, which is in agreement with the XRD pattern simulated from the SOD-type single crystal data reported in the literature (Bustamante et al. 2014; Park et al. 2006). The bonding nature and purity of as-synthesized ZIF-8 structure are studied by the FT-IR. Figure 4 shows the FT-IR spectra of the ZIF-8 structure. The FT-IR spectrum of the produced ZIF-8 (Fig. 4) reveals that the presence of the bands at 3135 and 2929 cm−1 is attributed to the aromatic and the aliphatic C–H stretch of the imidazole, respectively. The major band at 1584 cm−1 is attributed to the stretching vibration of C=N group. The peaks at 600– 1500 cm−1 correspond to the entire ring stretching or bending, and the band at 421 cm−1 is ascribed to Zn–N stretch. These bands are the characteristic of ZIF-8 structure (Park et al. 2006; He et al. 2014; Cravillon et al. 2009; Wang et al. 2015). The FT-IR studies are in agreement with the XRD patterns of the ZIF-8 that both confirm the presence of a SOD zeolite-type structure ZIF-8. Figure 5 also shows the 13C NMR spectrum of the synthesized ZIF-8 sample. The peaks at 149.73, 116.65, Table 1

Fig. 7 TGA curve of as-synthesized ZIF-8 sorbent

0.6 P/P0

Textural properties of as-synthesized ZIF-8

H–K Langmuir Sample BET surface surface area median pore diameter (Å) (m2 g−1) area 2 −1 (m g )

Total pore t-method volume* micropore (cc g−1) volume (cc g−1)

ZIF-8

0.68

1281

1875

*Calculated at P/P0 = 0.99

13.6

0.64

Environ Sci Pollut Res 60 50

Removal %

40 30 20 10 0 7

8

9

10

11

12

Solution pH

Fig. 9 Effect of solution pH on adsorption of Cr(VI) (contact time = 60 min, adsorbent dosage = 0.20 g, initial concentration = 5 ppm)

Fig. 10 An electrostatic interaction schematic of competitive adsorption mechanism of Cr(VI) and OH ions on the positively charged surface of ZIF-8 adsorbent in basic range lower than 9

Effect of solution pH 116.75, and 9.42 ppm corresponded to the resonance signals of the C atom of the N–C–N, N–C–C–N, N–C–C–N and –CH3 substituents of 2-MIm, respectively. These results were compared with those previously reported in the literatures (Morris et al. 2012). Further, the formation of the ZIF-8 structure was also confirmed by UV–Vis spectroscopic technique in solid state. Accordingly, UV–Vis spectra show that other two bands at 225 and 307 nm exactly match with the ZIF-8 absorbance bands (Zahmakiran 2012; Biswal et al. 2013). UV–Vis spectra of the as-synthesized ZIF-8 structure are depicted in Fig. 6. Furthermore, to investigate the thermal stability of produced ZIF-8, its TGA curve was measured as depicted in Fig. 7. Furthermore, in order to determine pore textural properties of produced sample, including the specific BET surface area, Langmuir surface area, pore volume, and pore size distribution, N2 adsorption and desorption isotherms at 77 K were measured in an ASAP-2020 adsorption apparatus (Micromeritics). N2 adsorption and desorption isotherms are shown in Fig. 8. As illustrated in Fig. 8, N2 adsorption/desorption shows the type I IUPAC classification of adsorption isotherms indicating characteristic of adsorbent is microporous (Do 1998). This issue can be obviously concluded from the reported data in Table 1 where more than 85% of pores are in microrange. The pore textural properties of ZIF-8 sample are summarized in Table 1. As it has been shown, median pore diameter is found to be 13.6 Å.3 The BET and Langmuir surface area are also reported as 1281 and 1875 m2 g−1, respectively.

The pH of the aqueous solution has most important variable on the removal of heavy metals. This is partly because the solution pH determines the degree of ionization, speciation of the adsorbate, and surface charge of the adsorbent (Qin et al. 2011; El-Ashtoukhy et al. 2008). So, in order to find out the optimum pH for the maximum removal of Cr(VI), the effect of pH on adsorption was examined at 298 K, 60 min4 contact time, and 0.20 g dosage of ZIF-8. For this purpose, 10 mL of 5 mg L−1 Cr(VI) solution was prepared in different pH, ranging from 7.0 to 12.0 using NaOH solution. The reason for choosing the range of 7.0 to 12.0 for adsorption of Cr(VI) is that the ZIF-8 becomes unstable under acidic pH conditions, and high concentration of Zn+2 was released into the solution indicating the dissolution of ZIF-8. In contrast, ZIF-8 is very stable in neutral and basic conditions and no Zn2+ was detected (Jian et al. 2015). The obtained results of the effect of pH on removal efficiency are shown in Fig. 9. As it is clearly shown in Fig. 9, the adsorption of Cr(VI) on ZIF-8 is strongly dependent on the solution pH. The surface of ZIF-8 is positively charged from the neutral condition (pH = 7) to solution pH below 9, while the surface of ZIF-8 becomes negatively charged at solution pH above 9 (Jian et al. 2015). As the results indicate, the best pH of solution for Cr(VI) removal is found to be 7. Although adsorption of Cr(VI) decreases slightly as the solution pH increases from 7.0 to 9.0, however, in higher values pH of 9.0, the removal efficiency will rapidly decline to zero. This may be due to decrease positive charge density on ZIF-8 in effect of increasing solution pH to basic range,

3 This value is close to the theoretical value obtained from crystallographic data which is about 11.6 Å (Park et al. 2006).

4 It should be noted that this time was chosen as a function of the adsorption kinetics and the solid matrix destruction that will be explained in next sections.

Environ Sci Pollut Res 70 60

Removal%

50 40 30 20 10 0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Adsorbent Dosage (g)

Fig. 11 Effect of adsorbent dosage on adsorption of Cr(VI) (contact time = 60 min, initial concentration = 5 ppm, pH = 7)

and therefore, it may lead to negative resulting in electrostatic repulsion between Cr(VI) and ZIF-8 (Jian et al. 2015; Li et al. 2013). Probably, as the pH values increase, leading to the competitive interaction between Cr2O7−2 and OH − for the limited adsorption sites (Chen et al. 2014), the removal efficiency of Cr(VI) declines (Fig. 10). Finally, it is noteworthy that there is no need to adjust pH solution for the adsorption of Cr(VI) on ZIF-8 sample.

Effect of adsorbent dosage Adsorbent dosage is another important parameter affecting adsorption efficiency because it determines the capacity of an adsorbent for a given initial concentration of the adsorbent (Mousavi and Seyedi 2011). The effect of adsorbent dosage has been studied on Cr(VI) removal at 298 K and at 70

50

Removal%

initial Cr(VI) concentration of 5 mg L−1 at pH 7.0 by following a contact time of 60 min. The results are shown in Fig. 11. The results reveal that removal efficiency increased by increasing the adsorbent dosage from 0 to 0.20 g. This may be attributed to the increase of the number of adsorption sites and surface area with the weight of adsorbent increasing to 0.20 g. It should be mentioned that, when the adsorbent dosage is going to be higher than 0.20 g, there is an adsorption reduction onto the adsorbent surface. It is likely that the active sites on the adsorbent surface are not available because of the agglomeration of the adsorbent’s particles. Thus, the amount of Cr(VI) adsorbed onto the adsorbent surface gets reduced with the adsorbent dosage increasing to a higher values (Nadaroglu et al. 2010; Nouri et al. 2007; Amarasinghe and Williams 2007; Jiang et al. 2009). Effect of contact time

60

40 30 20 10 0

Fig. 13 XRD pattern of ZIF-8 at different contact time immersed in water (changing in intensity and location (2θ) of peaks is observed after 90 and 240 min contact time) (adsorbent dosage = 0.20 g, contact time = 60 min, pH = 7)

0

15 30 45 60 75 90 105 120 135 150 165 180 195 210 225 240

Figure 12 shows the effect of contact time on adsorption of Cr(VI) by ZIF-8 adsorbent. For this meaning, initial concentration of Cr(VI) was adjusted on 5 mg L−1 and adsorbent dosage of 0.20 g was used in room temperature at pH equal to 7.0. As it can be observed, the removal of Cr(VI) increases with contact time up to 60 min. Basically, the removal of Cr(VI) is rapid at this time but it gradually decreases with time until it reaches 75 min of contact time which means maximum separation or removal has been reached.5 This indicates that the concentration of Cr(VI) in the solution decreased rapidly within 60 min, and the removal was practically completed up to 60 min

Contact time (min)

Fig. 12 Effect of contact time on adsorption of Cr(VI) (adsorbent dosage = 0.20 g, initial concentration = 5 ppm, pH = 7)

5 Maximum value was determined by the immersion time in the solution in which the destruction process of the adsorbent matrix begins.

Environ Sci Pollut Res 75 70

Table 2 Adsorption isotherm models used for fitting of experimental equilibrium data (Do 1998)

65

Model

Equation

Langmuir Freundlich Toth Sips

q = qmbCe/[1 + bCe] q = KFCe(1/n) q = qmbCe/[1 + (bCe)n]1/n q = qm(bCe)1/n/[1 + (bCe)1/n]

Removal%

60 55 50 45 40 35

by Zhang et al. (2015) where lower than 0.06 wt% ratio of ZIF-8 powder to water dosage was observed.

30 25

0

5

10

15

20

Initial Concentration (ppm)

Effect of initial concentration of Cr(VI)

Fig. 14 Effect of initial concentration of Cr(VI) on adsorption

(Nadaroglu et al. 2010). The rapid removal of the adsorbate has significant practical importance as it will facilitate smaller reactor volumes ensuring efficiency and economy (Nadaroglu et al. 2010; Kalkan 2006; Aksu 2001). However, because of destruction of the framework at the times over than 75 min in presence of aqueous solution, the adsorption is going down when contact time is increased from 75 to 240 min. This issue can be proved by investigating XRD pattern of ZIF-8 structure at different contact times immersed in solution. As it is illustrated in Fig. 13, the XRD pattern of ZIF-8 at 90 and 240 min contact time is different from those at 60 min and before that (fresh adsorbent). Therefore, it could be concluded that despite previous works (Park et al. 2006) that indicated high hydrothermal stability of ZIF materials, any change in crystallinity and porosity of ZIF-8 crystals after being placed in water depends on contact time and ZIF dosage in water. Similar to these behaviors of the ZIF-8, hydrothermal decomposition has been recently reported

Amount Adsorbed (mg/g)

0.25

0.2

0.15

0.1

0.05

0

0

2

4

6

8

10 12 Ce (mg/L)

14

16

18

20

22

Fig. 15 Adsorption isotherm of Cr(VI) on ZIF-8 at 298 K. The circles are experimental data, and the line indicates the predicated values by Toth isotherm equation

The initial concentration of 2.5, 5, 10, and 20 mg L−1 of Cr(VI) at 60 min contact time and 0.20 g dosage in 298 K at pH 7.0 was investigated. As shown in Fig. 14, the removal efficiency of Cr(VI) decreases with increasing of initial Cr(VI) concentration. This may be due to the fact that once a certain number of Cr(VI) is removed by quantitative adsorbent within a given time, no more adsorption occurs afterwards (Chen et al. 2014). Furthermore, to explore describing the interaction between adsorbate and adsorbent and also finding the maximum adsorption capacity of the Cr ions by the as-synthesized ZIF-8 sample, the maximum adsorption capacity data at 298 K are measured. The adsorption isotherm of the Cr(VI) ions is depicted in Fig. 15. Fitting of various adsorption models (Langmuir, Freundlich, Sips,6 and Toth (Do 1998) in Table 2) on experimental isotherm data (Table 3) reveals that Toth model has the highest R2 (correlation coefficient) for Cr(VI) adsorption on ZIF-8 adsorbent. Nevertheless, because of surface heterogeneity, parameter of Toth model is not very far from unity, which could be concluded that the adsorbent has low heterogeneous surface with different distributions of adsorption energies during Cr(VI) adsorption. This may be also achieved by correlation coefficient of Langmuir model which is very close to Toth R2. Moreover, Table 4 shows the comparison of adsorbent activity of ZIF-8 with other adsorbents in the removal of Cr(VI) and other heavy metals from aqueous in terms of adsorbent dosage, adsorption capacity (mg g−1), removal efficiency(R%), and especially conditions used in the reactions. Need to neutral environment and relatively short contact time is superior of commercial available MOF adsorbent, ZIF-8, in comparison with the other investigated adsorbents for Cr(VI) removal. 6

Also known as Langmuir–Freundlich model.

Environ Sci Pollut Res Table 3 Constants of various isotherm models used for the adsorption of Cr(VI) in ZIF-8 at 298 K

qm (mg g−1)

Model

b (L mg−1)

KF (L mg(1 − (1/n)) g−1)

1/n

R2

Langmuir

0.340

0.140

–

–

0.9955

Freundlich Toth

– 0.380

– 0.141

0.063 –

0.465 1.20

0.9813 0.9960

Sips

0.360

0.119

–

0.932

0.9939

Table 4 Comparison of removal efficiency(R%) and adsorption capacity of Cr(VI) and other heavy metals on various adsorbents in their optimum conditions Adsorbent

pH

Adsorbent dosage (mg L−1)

Contact time (min)

Initial concentration (ppm)

Adsorption capacity (mg g−1)

Heavy metal

R%

Ref.

Activated alumina Almond green hull Waste newspaper Zero-valent iron MOF-5 MOF-808 MOF(TATB) MOF(TATB) MOF(TATB) MOF(TATB) MOF(TATB) ZIF-8 ZIF-8 ZIF-8 ZIF-90-SH ZIF-67 MOF-NC Cubic ZIF-8 Leaf-shaped ZIFs (ZIF-L) Dodecahedral ZIF-8 ZIF-8

4 6 3 5 5.2 4 6 6 6 6 6 4 7 7 – 5 5.4 8.5 8.5 8.5 7

10,000 4000 3000 20,000 400 200 50,000 50,000 50,000 50,000 50,000 – 0.0002 0.0002 0.001 1000 – – – – 20

90 30 60 60 240 – 100 100 100 100 80 30 1440 1440 240 90 240 – – – 60

10 10 5 20 300 5 – – – – – 114 20 20 25 15 – – – – 2.5

– 2.04 59.88 0.67 290 24.83 – – – – – 350 60.03 49.49 22.43 13.34 80 122.6 108.1 117.5 0.15

Cr(VI) Cr(VI) Cr(VI) Cr(VI) Cu As(V) Hg Cd Pb Al Fe Cu As(V) As(III) Hg(II) Cr(VI) Cu As(III) AS(III) As(III) Cr(VI)

99 94 64 84 87 60 67 69 73 93 99 97.2 100 95 89.7 88.9 32 – – – 68

Mor et al. (2007) Sahranavard et al. (2011) Dehghani et al. (2016) Fu et al. (2013) Bakhtiari et al. (2015) Li et al. (2015a, b) Abbasi et al. (2015) Abbasi et al. (2015) Abbasi et al. (2015) Abbasi et al. (2015) Abbasi et al. (2015) Zhang et al. (2016) Jian et al. (2015) Jian et al. (2015) Bhattacharjee et al. (2015) Li et al. (2015a, b) Bakhtiari et al. (2015) Liu et al. (2015) Liu et al. (2015) Liu et al. (2015)

a

a

Present work

Desorption and regeneration studies Convenient recovery and reusability of the MOF adsorbent after the heavy metal adsorption and retaining its adsorption 100

Removal Efficiency (%)

90 80 70

Fresh

60

capacity after the regeneration are very important features for the industrial applications. In this work, a solution of HCl (0.1 M) was employed for regeneration and reusability of ZIF-8 after Cr(VI) adsorption. The Cr(VI) adsorption capacity of ZIF-8 structures undergoing 4 cycles is illustrated in Fig. 16. It is noteworthy that capacity of ZIF remained almost constant for the four adsorption–desorption cycles. So, it could be concluded that ZIF-8 adsorbent can be effectively renewed using HCl solution (0.1 M) and can be employed repeatedly as an efficient adsorbents for practical applications.

50 40

Conclusion

30 20 10 0

1

2

3

4

5

Number of Cycles

Fig. 16 Regeneration of ZIF-8 after Cr(VI) adsorption using HCl solution (0.1 M)

In the present work, ZIF-8 adsorbent, as a subset of metal– organic framework materials, was synthesized and employed to removal of Cr(VI) ions from aqua systems. The effect of various practical factors such as pH, adsorbent dosage, initial concentration, and contact time on adsorption process efficiency was investigated and optimum conditions were chosen

Environ Sci Pollut Res

(as shown in Figs. 9, 10, 11, 12, 13 and 14). The neutral environment (pH value equals to 7) is more favorable for the Cr(VI) removal by the ZIF-8 microcrystals. Structure destruction was also observed by XRD determination when the ZIF-8 immersing in water more than 60 min. The Cr(VI) removal process using ZIF-8 follows Toth as well as Langmuir isotherm model indicating homogeneous surface with the same energy distribution. The electrostatic interaction mechanism is suggested for Cr(VI) species adsorption on the ZIF-8, because interaction between Cr(VI) ions (Cr2O7−2) and positively charged surface of ZIF-8 (from the neutral condition (pH = 7) to solution pH below 9) is dominated. In summary, although ZIF-8 does not show very good adsorption capacity as well as other ZIF materials such as ZIF-67 for the Cr(VI) removal from aqueous solution, it is, however, a very facile, green, and low-cost production that could be a good choice to effectively separate toxic Cr(VI) from water.

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