Zeolitic imidazolate frameworks: Experimental and molecular simulation studies for efficient capture of pesticides from wastewater

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Zeolitic imidazolate frameworks: Experimental and molecular simulation studies for efficient capture of pesticides from wastewater Reda M. Abdelhameeda,*, Mohamed Tahab,*, Hassan Abdel-Gawada, Fathia Mahdya, Bahira Hegazia a b

Applied Organic Chemistry Department, Chemical Industries Research Division, National Research Centre, 33 EL Buhouth St., Dokki, Giza 12622, Egypt Materials Science and Nanotechnology Department, Faculty of Postgraduate Studies for Advanced Sciences, Beni-Suef University, Beni-Suef, Egypt

A R T I C LE I N FO

A B S T R A C T

Keywords: ZIF-8/67 Ethion insecticide Prothiofos insecticide Removal Molecular simulation

The agricultural drainage water represents the largest part of the contaminated water with pesticide, it was necessary to remove all pesticide from it. In this paper, two different zeolite imidazole frameworks (ZIF-67 and ZIF-8) based in two different metal ions (cobalt and zinc) was investigated. The preceding two nanoparticles adsorbents were used for the elimination of two common pesticides, prothiofos and ethion. Surprising results lie in the selectivity of pesticide toward different metal ions. The maximum adsorption capacities of prothiofos onto ZIF-8 and ZIF-67 were 366.7 and 261.1 mg g−1, respectively. Whereas in ethion; the maximum adsorption capacities were 279.3 and 210.8 mg g-1 for ZIF-8 and ZIF-67, respectively. Both pesticide adsorption followed pseudo-second-order kinetics and best fit the Langmuir adsorption model. The adsorption of ethion and prothiofos insecticides on to the surface of ZIFs were confirmed by using Fourier transform infrared (FTIR) spectroscopy and energy-dispersive X-ray spectroscopy (EDS). The adsorption of the pesticide molecules onto the ZIF surfaces was investigated in dry system using Grand canonical Monte Carlo (GCMC) simulation. The GCMC simulation provides information on the adsorption energies and maximum adsorption capacities. Molecular dynamics simulation was also revealed the effect of water solvent on the adsorption mechanism. This combination of simulations provides a complete picture of the adsorption mechanism which is needed for understanding the adsorption process of ZIF materials and the practical applications uses.

1. Introduction

textile industry [8]. Recently, MOFs as excellent adsorbent material were used for the elimination of inorganic pollutants from wastewater including heavy metals (As5+, Cd2+, Cr6+, and Hg2+) [9–13], excessive halide ions [14], and radioactive substances (133Ba, 99Tc, 129I, 232 Th, 235U and 238U) [15]. Not only MOFs were used for removal of inorganic pollutants but also extends to organic contaminants like dyes (e.g., MO, MB and RhB) [16,17], pharmaceuticals (e.g., diclofenac sodium) [18] and pesticides (e.g., insecticides, herbicides and fungicides). Here as we are focusing on removal of pesticide from wastewater we will describe in more details the recent research of using MOFs in order to eliminate pesticide from wastewater. So far, removal of pesticides from water using MOFs were relatively rare. The 2,4-D and MCPP pesticides were removed from water using MIL-53 and UiO-66, respectively [19,20], and the results proved that pesticide can interact with MOFs structure via π–π stacking or electrostatic interactions. The mechanism of adsorption is very important and depending on its origin the anionic framework (NKU-101) can adsorb the cationic herbicides diquat (DQ) and (methyl viologen (MV) [21]. NKU-101 can adsorb

The scientist divided the pollutants in wastewater onto two main category, the first is inorganic pollutants such as heavy metal ions and radioactive substances, and the second is organic pollutants include pesticides, polyaromatic hydrocarbons, dyes, and pharmaceuticals [1]. Because these kinds of contaminants have serious problem for both human and nature, therefore, its removal represent great research interest. A lot of techniques were used for wastewater treatment for example filtration, precipitation and coagulation. However, these techniques are complex and need high maintenance costs [2]. Therefore, the researchers are mainly focus on removing water pollutants by adsorption [3,4]. Efforts were done to produce some porous adsorbents like activated carbon, clays and aluminophosphates but unfortunately they were not satisfactory because it’s shortage of functional groups and low surface areas. Metal organic frameworks (MOFs) are a class of new hybrid material used in photocatalysis [5], separation [6], purification [7] and

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Corresponding authors. E-mail addresses: [email protected] (R.M. Abdelhameed), [email protected] (M. Taha).

https://doi.org/10.1016/j.jece.2019.103499 Received 20 August 2019; Received in revised form 20 October 2019; Accepted 21 October 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Reda M. Abdelhameed, et al., Journal of Environmental Chemical Engineering, https://doi.org/10.1016/j.jece.2019.103499

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methyl viologen with the highest adsorption capacity 160 mg g−1, while NKU-101 can adsorb diquat with 200 mg g−1 capacity. Iron based metal-organic frameworks (MIL-(Fe)-100) were used to uptake phosphate from water samples and gave 93.6 mg g−1 maximum uptake capacity [22]. UiO-66 is used to eliminate dimethyl-4-nitrophenyl phosphate (methyl-paraoxon) from wastewater [23]. Copper-based metal–organic framework (Cu-BTC) was applied for adsorbing 14Cethion insecticide from aqueous solution; the capacity of sorbent is about 122 mg g−1. In addition to, Cu-BTC MOF can be used again after six regenerations cycles. The mechanism of ethion adsorption goes under coordinates of phosphoryl (PeO) group with copper (II) atoms through the pore of Cu-BTC [24]. ZIF-8 was incorporated with magnetic multi-walled carbon nanotubes producing new composite (M-M-ZIF-8) which used for the organophosphorus pesticides removal from water and soil. The mechanism of adsorption depend on sharing of electrons between the vacant active sites of M-M-ZIF-8 and the organophosphorus pesticide molecules [25]. More active research has been done for the adsorptive elimination of glyphosate in water using NU-1000 and UiO-67. The importance of NU-1000 and UiO-67 is possessing Lewis acid nodes which push it to interact with the Lewis base phosphate group of the glyphosate. NU-1000 shows high efficiency and better reusability performance compared to UiO-67. This activity occurs as the result of the presence of larger pore diameters in the NU-1000. The calculations showed that the interaction energy of glyphosate with the zirconium metal ion in NU-1000 is higher (-37.63 KJ mol-1) in comparison with UiO-67 (-17.37 KJ mol-1), which reflect the higher efficiency of NU-1000 [26]. Atrazine insecticide was removed from water using ZIF-8, UiO-66, and UiO-67. The percent removal of atrazine insecticide from water was reached to 98% for both MOFs (UiO-67 and ZIF-8), whereas UiO-66 is ineffective. UiO-67 show higher efficiency after regeneration proving its efficiency to eliminate atrazine in water [27]. MIL-101(Cr) modified with amino (NH2–) or urea (UR2–) groups were used to remove Glyphosate from water. The higher adsorbing capacity of glyphosate was 64.25 mg g-1 by using NH2-MIL-101(Cr), whereas UR2-MIL-101(Cr) did not reach the best adsorption performance because of the steric hindrance [28]. Not only purification of contaminated water is the target of pesticide adsorption but also determination of pesticide with high selectivity and sensitivity are from the other goals. Very recently, structured magnetic framework composites (Fe3O4@TMU-21) were used to remove pyrethroid from fruit juice samples [29]. Porous carbons (PCs) doped with Pt nanoparticles@ zirconium oxide (ZrO2) were prepared by using zirconium based metal organic framework (UiO-66 MOFs) as a template. They show strong convergence to the phosphate group and the presence of nitro group on the methyl parathion (MP) insecticide leads to producing highly electrocatalytic activity [30]. In the present work, two different ZIF MOFs (ZIF-67 and ZIF-8) were chosen for studying the pesticides adsorption (prothiofos and ethion) from contaminated water, the best conditions for adsorption were investigated as well as molecular simulation was studied.

2.2. Zeolitic imidazolate frameworks-8 (ZIF-8) synthesis ZIF-8 was synthesized according to Shi et al [33] with slightly modification. 6.56 g of 2-methylimidazole in methanol (120 mL) was added to 2.95 g of Zn (NO3)2·6H2O in 60 mL of methanol. The reaction mixture was stirred for 20 min, filtered off, washed with methanol several times. The resulting white precipitate was dried in an oven at 80 °C until used. 2.3. Zeolitic imidazolate frameworks-67 (ZIF-67) synthesis ZIF-67 was prepared according to Shi et al [33] with some modification. ZIF-67 was synthesized as follow: 2.93 g cobalt nitrate hexahydrate was reacted with 6.49 g 2-methylimidazole (Hmim) in 200 mL methanol. The mixture was stirred for 8 h at room temperature. Purple precipitate was obtained, filtered, washed with methanol, and dried to 80 °C until used. 2.4. Characterization PXRD was obtained using an X-ray diffractometer (X’Pert MPD Philips) with a scan range of 2θ recorded from 3.5 to 50 degrees at 0.02 degrees per second of a scan speed. Current and voltage were set at a 40 mA and 40 mV, respectively. Surface morphology and particles size of ZIFs before and after reaction were observed by scanning electron microscopy (SEM, Hitachi S-4800, Japan) with an acceleration voltage of 5 kV. X-ray energy dispersive spectrometer (EDS) was used for the analysis of specific elemental species and concentrations. FT-IR spectrum of solid samples before and after reaction were attained using a FTIR spectrometer (Mattson 5000 FTIR spectrometer) using potassium bromide disc and measured at full scan from 400 to 4000 cm−1. 2.5. Batch experiments The experiments of adsorption were done in 15 mL glass centrifuge tubes under varying experimental conditions which included initial concentration and reaction time. During batch experiments 0.01 g ZIF-8 or ZIF-67 and 50 mg L−1 pesticide concentration was stirred at 250 x g and 303 K for up to 8 h. During the experiment sample vials were periodically removed from the reaction shaking table at specific time intervals and centrifuged and the aqueous solution was extracted three times by 10 mL chloroform and dried under anhydrous sodium sulphate. Three parallel experiments were conducted. The absorbance of ethion and prothiofos was measured by UV spectrophotometer (JASCO) at λ =270 nm and the concentration of the mixed contaminants before and after the reaction were measured by the peak area. The quantity of prothiofos and ethion insecticide adsorbed onto ZIFs under different conditions was calculated according to Eq. (1):

Qt =

(Co − Ct ) V m

(1)

Where Qt is the adsorption concentration quantity of ZIFs, t is the time of ethion adsorption measured by mg g−1. C0 is the initial concentration of ethion insecticide and Ctis the residual concentration of ethion insecticide at time t (mg L−1), V, m are the liquid volume (L) and mass of ZIFs (g); respectively. The removal efficiency (R) of prothiofos and ethion insecticide are calculated in accordance to a Eq. (2):

2. Material and method 2.1. Materials and chemicals Zinc nitrate hexahydrate (Zn (NO3)2·6H2O, ≥99.0%) was get from Sigma Aldrich. 2-Methylimidazole (C4H6N2, > 98%) was obtained from Sigma Aldrich. Methanol (CH3OH, ≥99.5%) was obtained from Alfa. Chromatography grade acetonitrile (C2H3N, ≥99.9%) was get from Sigma Aldrich. Distilled water was taken from the laboratory via a laboratory water purification system. Ethion and prothiofos were synthesized according to previous published procedure [31,32]. The structure of prothiofos and ethion is shown in Fig. 1.

%R =

(Co − Ct ) 100 Co

(2)

2.6. Computational details The structures of ZIF-8 and ZIF-67 crystals were taken from previously reported cif files [34,35]. All calculations in this investigation were done using the Studio of Materials. The unit structure of ZIF-8 or 2

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Fig. 1. Structure of prothiofos and ethion.

ZIF-67 is optimized by density functional theory (DFT) with a DNP basis set. The generalized gradient approximation (GGA) with the PerdewBurke-Ernzerhof (RPBE) functional were applied [36]. Prothiofos and ethion were also optimized at the same level of theory. The effective core potential was used to treat the core electrons ZIF-8 or ZIF-67, while all electrons were considered for the pesticide molecules. The DFT calculations were carried out by DMol3 code [37,38]. The obtained optimized structures and atomic charges were used for the Grand Canonical Monte-Carlo simulation and molecular dynamics simulations. The absorption and adsorption calculations (MC simulation) were computed by Absorption and Adsorption Modules. The MD simulations were computed by FORCITE Module. In the MC and MD simulation calculations, the pesticide and water molecules were optimized by the COMPASSII force field [39]. The forces of electrostatic and van der Waals were treated with the Ewald method and the atom based summation method, respectively. The overall quality for all calculations was set to fine. The absorption of two prothiofos or two ethion molecules onto ZIF-8 and ZIF-67 were performed by MC simulation. In the adsorption calculations, a larger surface is built consisting of (2 × 2 × 2 Å) unit cell of each crystal structure. A vacuum slab with 35 Å thicknesses was built on the ZIF-8 and ZIF-67 surfaces and the lattice parameters were (34.1 Å × 34.1 Å × 68.1 Å). The top surface atoms were chosen for the interaction with the prothiofos and ethion insecticide compound. The highest loading of prothiofos and ethion on the ZIF-8 and ZIF-67 surfaces were computed within distance of 7 Å above the surface. The lowest-energy structures obtained from the adsorption calculations were filled with water molecules and the simulated using MD method. The cubic boxes were simulated with NVT ensemble at 298.0 K for 30 ns, and the time step was set to 1 fs. The Nose thermostat was used for controlling the temperature.

Fig. 2. PXRD of ZIF-8, ethion@ZIF-8 and prothiofos@ZIF-8.

3. Result and discussion 3.1. Characterization of MOFs 3.1.1. XRD analysis For good understand of the possible mechanisms of ZIFs removal of pesticide, the PXRD patterns of ZIFs before and after batch reactions were analyzed. Fig. 2 show the PXRD spectra of ZIF-8, the spectrum show peaks at 7.3, 10.4, 12.7, 14.7, 16.4, 18.0, 24.5 and 26.6° which fit well with XRD of previously reported ZIF-8 [40]. After reaction with ethion and prothiofos, the most diffraction peaks placement showed no movement in 2 theta positions. ZIF-67 particles were synthesized in very pure phase as we concluded from PXRD diffraction in Fig. 3. As we having a distinctive peaks related to ZIF-67 after adsorption of pesticide meaning that the framework of ZIF-67 was no change in its structure. The average crystallite size of ZIF-8, ethion@ZIF-8 and prothiofos@ZIF8 are calculated from the Scherrer equation [41], It was found that the average crystallite sizes were 75.3, 84.1 and 87.1 nm, respectively. Interestingly, the crystallite sizes of pesticide adsorbed samples have bigger than the pure ZIF-8. On the other hand, the average crystallite size as calculated from the Scherrer equation of ZIF-67, ethion@ZIF-67 and prothiofos@ZIF-67 were 169.1, 196.8 and 201.1 nm, respectively.

Fig. 3. PXRD of ZIF-67, ethion@ZIF-67 and prothiofos@ZIF-67.

3.1.2. SEM-EDS analysis The texture of ZIFs before and after adsorption of ethion and prothiofos was tested by SEM (Fig. 4,5). The SEM images of pure ZIFs exhibited rhombic dodecahedron and these finding are in parallel to the previously reported [40]. The particle sizes of ZIF-8 and pesticide@ZIF8 are about 120 nm, while the particle sizes of ZIF-67 and pesticide@ ZIF-67 are about 250 nm as proved by SEM images. As concluded from SEM images, the textures of ZIFs were not change greatly after adsorption of pesticide. However, the SEM images appeared that the surface of ZIFs became coarse after the adsorption due to contact with pesticide (Fig. 4b, c, 5b, c). Moreover, the element analysis of ZIF-8 before adsorption showed the presence of carbon (C), nitrogen (N), oxygen (O) and zinc (Zn) (Fig. 4). After adsorption the major change was appearance in sulfur (S), phosphorous (P) and chlorine (Cl), this 3

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Fig. 4. Scanning electron microscope image and EDX of [a] ZIF-8; [b] Prothiofos@ZIF-8 and [c] Ethion@ZIF-8.

Fig. 5. Scanning electron microscope image and EDX of [a] ZIF-67; [b] Prothiofos@ZIF-67 and [c] Ethion@ZIF-67.

change was related to adsorption of prothiofos. While in ethion adsorption, ZIF8-8 consistent was changed with only S and P elements due to ethion have only S, O, C, and P in its structure. Moreover, the elemental analysis of ZIF-67 before reaction with pesticide is composed from C, N, O and cobalt (Co) (Fig. 5), after pesticide adsorption it is completely changed and S, P elements were appeared 3.1.3. FTIR analysis FTIR of ZIFs before and after reaction with pesticide were shown in Fig. 6 and 7. ZIFs before reaction with pesticide show bands at 3135 and 2929 cm−1 attributed to the aromatic and the aliphatic CeH stretch of the imidazole, respectively. The peak at 1584 cm−1related to the C]N stretch, the bands at 1350–1500 cm−1 are attributed to the aromatic ring stretching. The bands at 900-1350 cm−1 are corresponding to in-plane bending of the ring, bands below 800 cm−1due to out-of-plane ring bending. After reaction with pesticide, beside the old bands of ZIFs, new bands at 2980, 2936 and 2901 cm−1 ascribed to aliphatic CeHs. Sharp peak at 1016 cm-1 is assigned to POeEt and a band at 960 cm-1 to POCeC. Bands at 816/796, 651and 505 cm-1 are related to PeOEt, P]S and PeS, respectively. The stretching mode of CH aromatic ring of prothiofos was appeared at 2867 cm−1.

Fig. 6. FTIR of ZIF-8, ethion@ZIF-8 and prothiofos@ZIF-8.

4

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Fig. 8. effect of pesticide concentration on ZIFs adsorption process and Langmuir model fitting.

Fig. 7. FTIR of ZIF-67, ethion@ZIF-67 and prothiofos@ZIF-67.

3.2. Adsorption isotherms

This meaning that Langmuir model is perfect fitting with data of ZIF-67 (Fig. 8). Table 1 showed that adsorption of pesticide by ZIFs was more consistent with the Langmuir adsorption model. These finding prove that the adsorption mechanism is monolayer adsorption of both insecticide (prothiofos and ethion) on to the surface of ZIFs. Using ZIF-8, the maximum adsorption capacity of prothiofos was greater than ethion. To determine the best-fit isotherm models, Chi-square statistic (χ2) was used to evaluate the data and it was calculated for the both isotherm models [42–44]. The values of χ2 show that Langmuir isotherm model was the best model for explanation of pesticide adsorption onto MOFs because the values of χ2 were much lowers than the values when using Freundlich model (Table 1).

The drawing of insecticide equilibrium concentration with the quantity of insecticide adsorbed at constant temperature gives us sorption isotherms. In this experiment 30 °C was selected as the ideal environmentally temperature. The importance of adsorption isotherm comes from its best information on the adsorbent capacity and the reaction mechanism. The widely used adsorption models are the Freundlich and Langmuir models. The Langmuir model proved that ethion and prothiofos are uptake as a monolayer and all adsorption centres are the same. The Langmuir model has been written as non linear equation (Eq. 3):

Qe =

Qm kL Ce 1 + kL Ce

3.3. Kinetics analysis

(3)

Where Ce represents the equilibrium concentration of ethion and prothiofos, Qe is the equilibrium adsorption capacity, Qm is the highest adsorption capacity of ZIFs and KL is the Langmuir constant. In comparison, the Freundlich experimental model (Eq.) does not make any assumptions about surface adsorption and adsorption is considered heterogeneous. 1

Qe = kF Cen

The adsorption capacity of both ZIF-8 and ZIF-67 were investigated over time to determine the kinetic parameters. The data of time dependent were suitable for both a pseudo-first-order and a pseudosecond-order kinetic model (Table 2) by using the following equations: The non-linear equation of pseudo-first-order model is written as follow:

Qt = Qe (1 − exp−k1 t )

(4)

(5)

The non-linear equation of pseudo-second-order model is written as follow:

Where Qe is the equilibrium adsorption capacity, Ce is the ethion and prothiofos equilibrium concentration; KF is Freundlich adsorption equilibrium constant, n is an empirical parameter. The 1/n has value between 0 and 1 and it shows the effect of concentration on adsorption strength. At 30 °C, R2 (coefficient of determination) values for prothiofos was respectively 0.921 and 0.980 for the Langmuir and Freundlich equations using of ZIF-8 and 0.968 and 0.997 in the case of ZIF-67 (Table 1).

Qt =

k2 Qe2 t 1 + k2 tQe

(6)

Where qe and qtof ZIFs are the adsorption capacity at equilibrium at time t, respectively; k1 and k2 are the rate constant of the pseudo-firstorder, pseudo-second-order kinetic model, respectively. The kinetics

Table 1 Parameters of isotherm for adsorption of ethion and prothiofos onto ZIFs. Samples

Prothiofos@ZIF-8 Prothiofos@ZIF-67 Ethion@ZIF-8 Ethion@ZIF-67

Freundlich parameters

Langmuir parameters

n

KF

R2

χ2

Qm (mg g−1)

KL

R2

χ2

7.5 ± 2.1 5.3 ± 0.9 7.7 ± 1.7 4.9 ± 1.1

164.6 ± 31.2 78.3 ± 1.5.3 120.5 ± 21.2 52.71 ± 14.6

0.921

532

36.9

287

0.997

22.1

0.960

434

0.999

9.5

0.961

204

0.1518 ± 0.028 0.0375 ± 0.002 0.0814 ± 0.003 0.0171 ± 0.002

0.980

0.968

366.7 ± 10.8 261.1 ± 3.1 279.3 ± 1.6 210.8 ± 5.4

0.991

46.3

5

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Table 2 Parameters of adsorption of ethion and prothiofos onto ZIFs. Samples

Qe (mg g−1) experimental

pseudo-first-order parameters Qe (mg g

Prothiofos@ZIF-8 Prothiofos@ZIF-67 Ethion@ZIF-8 Ethion@ZIF-67

329 ± 5.8 237 ± 6.7 254 ± 4.3 186 ± 3.4

269 ± 26.7 223 ± 4.33 264 ± 10.35 179 ± 1.52

−1

)

pseudo-second-order parameters χ2

Qe (mg g−1)

K2

R2

χ2

0.698

428

21

232

0.993

27

0.942

182

0.986

37

0.954

109

0.00572 ± 0.00142 0.00221 ± 0.00032 0.00091 ± 0.00024 0.00502 ± 0.00077

0.976

0.961

333 ± 14.7 241 ± 9.9 260 ± 28.7 192 ± 6.88

0.991

25

K1

R

0.119 ± 0.0112 0.609 ± 0.0379 0.391 ± 0.0405 0.884 ± 0.0271

results for both prothiofos and ethion show the best fit to pseudosecond-order kinetic model with R2about 0.999 (Table 2). The bestfitting of kinetic models can be determined by using Chi-square statistic (χ2) [42]. It well known that if the data from the model are similar to the experimental data means χ2 has small number and vice versa. As presented in Table 2, the values of Chi-square statistic of Pseudosecond order model are lower than corresponding values for First-order model. This meaning that the Pseudo-Second Order kinetic model is more favorable to describe the sorption of pesticide onto MOFs (Fig. 9).

2

nanocage of ZIF-8 are presented in Fig. 10. Both Fig. 10 (a and b) show that the two (−P = S) groups of the dithiophosphate moieties of ethion molecule was able to form a number of intermolecular hydrogen bonds with the imidazolium’s hydrogen of two neighbors of six-membered ring opening channels, in which each (−P = S) group formed six hydrogen bonds with three imidazolium rings (Fig. 10b). In case of prothiofos absorption, the (−P = S) and a halogen (Cl) groups were also able to form six intermolecular hydrogen bonds with two neighbors of six-membered ring windows (Fig. 10d). Similar results were obtained for the absorption of both Pesticides inside ZIF-67 (structures not shown). The isosteric heats of ethion and prothiofos in ZIF-8 were, respectively, -36 kcal·mol−1 and 38 kcal mol-1, whereas their values in ZIF-67 were, respectively, 40 kcal mol−1 and 41 kcal mol-1; and this explains the high adsorption capacity of prothiofos as compared to ethion. On the other hand, both pesticides show higher affinity to the ZIF-67 than ZIF-8, which contradict the experimental observation, suggesting that these compounds interact with the frameworks metal ion. Generally it is known that Zn ion exhibit stable complexes with ligands as compared with Co ion [45], and thus offers a possibility for explaining the observed contradiction. We, therefore, examined the adsorption of ethion/prothiofos on the ZIF-8/67 surfaces to determine whether these molecules coordinate with the metal ions. The maximum loading of prothiofos and ethion on (2 × 2 × 2) unit cell of ZIF-8 were 14 and 11 molecules, and on (2 × 2×2) unit cell of ZIF-67 were 10 and 9 molecules, respectively. This result agrees with the trend observed in the highest adsorption capacity (Qm). Fig. 11 shows the main interactions of ethion and prothiofos with metal ions of ZIF-8. Prothiofos was mainly interacted with Zn through (−P = S······Zn) and a (Cl······Zn) interaction with distances 2.232 Å and 2.931 Å, respectively (Fig. 11a). The chlorine atom was also formed hydrogen bond with an imidazolium’s hydrogen. Fig. 11b displays that prothiofos was also interacted with ZIF-8 via (−P − S······Zn) and (−P = S······Zn). Their bond distances were 2.240 Å and 3.385 Å, respectively. This interaction was stabilized by three intermolecular hydrogen bonds between a chlorine atom and imdidazolium’s hydrogens, as well as a π–π stacking between the prothiofos benzene ring and an imidazolium ring. On the other hand, both (−P = S······Zn) groups of ethion molecule was found to form two (−P = S······Zn) interactions with bond distance 2.187 Å and 2.181 Å. Similar results were observed for the absorption of ethion and prothiofos on the ZIF-67 surface (structures not shown). To estimate the effect of the water molecules on the adsorption of prothiofos and ethion on ZIF-8/67 surfaces, the lowest-energy configurations of the adsorbed molecules obtained from the MC simulation were solvated with water and simulated using molecular dynamics method. By visual inspection of the adopted molecules, it was found that the adsorbate molecules are stably adsorbed parallel to the ZIF-8/67 surfaces. As an example, Fig. 12 displays snapshots of as adsorption of 10 prothiofos molecules on ZIF-67 surface in dry and aqueous systems. To get more insights on the interactions between the adsorbate

3.4. Adsorption mechanism ZIF-8 or ZIF-67 structure is comprised of large central cavities (nanocage, ∼ 11.6 Å of diameter) and interconnected by narrow channels (∼ 3.40 Å of diameter). The nanocage is formed by self-assembly of eight six-membered ring Zn/CoMeIM opening windows of ca. 3.4 Å in diameter, and the narrow channel is assembled of six fourmembered ring Zn/CoMeIM opening windows of ca. 0.8 Å. The nanocage and small channels lie through the body diagonals and faces of the unit cell, respectively. In case of no structural defects for ZIF-8 and ZIF-67, the access of the sorbates into the pore cavities depends on the relation between the kinetic diameter of the sorbate molecule and the 3.4 Å window size. The adsorption isotherms for prothiofos and ethion in ZIF-8 and ZIF67 frameworks from 1 atm to 10 atm were performed using MC simulation. Results show that the maximum loading of each unit cell (at 10 atm) was two prothiofos or two ethion molecules, where one is localized at the center of the nanocage cavity whereas the other one is placed at two of the eight corners of the nanocage. Thus, the adsorption site for both pesticides are inside the main cavity ‘nanocage’. The lowest-energy configurations of ethion and prothiofos inside the

Fig. 9. Effect of time on the adsorption of pesticide on to ZIFs and pseudosecond-order kinetic model fitting. 6

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Fig. 10. The lowest-energy configuration of ethion (a and b) and prothiofos (c and d) molecules loaded into the central nanocage of ZIF8 in dry system, as obtained from the MC simulation. For clarity purpose, all four and sixmembered ring windows were removed except the two windows hydrogen bound to ethion (b) and prothiofos (d). Carbon, Grey; Hydrogen, white; Oxygen, red; Nitrogen, blue; Sulfur, yellow; Phosphorus, magenta; Chlorine, cyan. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

(> S····Zn) RDF appear at ∼ 1.9 Å and 2.4 Å, respectively. The chlorine atoms have low affinity toward the Zn, the corresponding (−Cl····Zn) RDF showed broad peak ∼ 4.8 Å. The (− > P = S····Himid) RDF and (−Cl····Himid) RDF are related to the hydrogen bonds formation, in which they showed peak at ∼3.1 Å and the former peak is greater than the latter. A peak with very low intensity corresponding to the (−Cl····Zn) RDF is observed at 2.4 Å. It is clear form Fig. 13 (b) that the interactions between prothiofos with ZIF-67 are weaker than those with ZIF-8. The (− > P = S····Co), (> S····Co), and (> O····Co) RDFs appear at ∼ 3.0 Å, 3.5 Å, and 3.4 Å, respectively. The (− > P = S····Himid) and (−Cl····Himid) RDFs appear, respectively, at 2.8 Å and 2.9 Å, which they are closer than those in prothiofos + ZIF-67 system. A peak corresponding to the (−Cl····Co) RDF is observed at 3.4 Å. The interactions of ethion with ZIF-8 and ZIF-67 are mainly due to the (− > P = S····Zn/Co) and (− > P = S···· Himid) types, as it can be seen from Fig. 13 (c and d). The (− > P = S····Zn) and (− > P = S····Co)

molecule and ZIF-8/67 surfaces in aqueous solution, the radial distribution functions (RDFs) of some selected atoms of prothiofos/ethion with the metal ions (Zn/Co) or imidazolium’s hydrogen (Himid) of ZIF8/67 surfaces were determined from the MD simulation as shown in Fig. 13. The RDF is an important tool for investigating the molecular interactions, which defines as the probability of appearing a particle “A” within the range of “r + dr” around a particle “B", and expressed as gAB (r). In Fig. 13 (a), the highest intense peaks appears at distance ∼2.17 Å, which attributes the interaction between the (− > P = S) of prothiofos and Zn ion of ZIF-8. This distance is less than the total of van der Walls radii (VDW) of S and Zn atoms, suggesting that the bond (− > P = S····Zn) has covalent character. The interactions of the oxygen atoms (linked to 2,4-dichlorophenyl and ethyl moieties, > O) and sulfur atom (linked to propyl chain, −S) of the prothiofos with Zn ion of ZIF-8 surface have also covalent character, which (> O····Zn) RDF and

Fig. 11. The interactions of prothiofos (a and b) and ethion (c) molecules with ZIF-8 surface in dry system, as obtained from the MC simulation. For clarity purpose, all four and six-membered ring windows those not bound to prothiofos or ethion were removed. Carbon, Grey; Hydrogen, white; Oxygen, red; Nitrogen, blue; Sulfur, yellow; Phosphorus, magenta; Chlorine, cyan. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 7

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Fig. 12. Snapshots of as adsorption of 10 prothiofos molecules on ZIF-67 surface in dry (a) and aqueous systems at 30 ns (b). The zeolitic imidazolate frameworks, stick style (green color); prothiofos molecules ball and stick style (yellow color); water, line style. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

Fig. 13. RDFs for the interactions of prothiofos + ZIF-8(a), prothiofos + ZIF-67 (b), ethion + ZIF-8(c), and ethion + ZIF-67 (d), obtained from MD simulations.

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Fig. 14. The recycle use of ZIF-8 and ZIF-67 for [a]prothiofos and [b]ethion removal. Test conditions: adsorbent (10 mg), temperature (298 K), pH = 7, reaction time (150 min), solution (10 mL), and the concentration of 50 ppm.

RDFs appear at 2.1 Å and 3.0 Å, respectively. This indicates that the ethion is interacting strongly with ZIF-8 compared with ZIF-67.

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3.5. Regeneration and recycle use We found that the adsorbed pesticide can be effectively desorbed by acetonitrile solvent (20 mL), giving elution efficiency about 99.5% (the washing solvent was measured after each washing step for about three times). ZIF-8 and ZIF-67 show excellent recycle use. The first recycle use of ZIF-8 show the adsorption capacity of 298.7 mg g−1 and 240.9 mg g-1, in the case of prothiofos and ethion, respectively, comparable with the initial value of 331.9 mg g−1 and 264.8 mg g−1 for prothiofos and ethion, respectively, (Fig. 14). In addition, the recycle use of ZIF-67 is satisfied; because the first recycle use generate the adsorption capacity of 218.8 mg g−1 and 165.9.3 mg g−1 for prothiofos and ethion, respectively, less than the initial value of 237.9 mg g−1 and 186.5 mg g−1 for prothiofos and ethion, respectively. 4. Conclusions In conclusion, successful preparations of ZIFs (ZIF-8 and ZIF-67) were characterized by PXRD, FTIR and SEM-EDS. Kinetic studies revealed that removal of both ethion and prothiofos by ZIFs was best fit to pseudo-second-order model. Adsorption isotherms studies proved that the process of adsorption best fit the Langmuir model indicating that pesticide adsorption occurred on the ZIFs surface as monolayer adsorbent. Based on MC and MD simulations, the adsorption mechanism was mainly as the result of the interaction of prothiofos and ethion insecticides with the metal ions (Zn/Co) of ZIFs surface in dry and aqueous solution. This interaction was also stabilized with hydrogen bonds formation with imidazolium’s hydrogen of ZIF-8/67. This combined method supplies a mechanistic pathway for understanding structural, dynamics and the changes of energy through adsorption on ZIF surfaces. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by financial assistance received from National Research Centre, Egypt, project number 11070102. 9

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