Experimental and molecular dynamics study on dye removal from water by a graphene oxide-copper-metal organic framework nanocomposite

Journal of Water Process Engineering 34 (2020) 101180

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Experimental and molecular dynamics study on dye removal from water by a graphene oxide-copper-metal organic framework nanocomposite

T

Mostafa Dadashi Firouzjaeia, Farhad Akbari Afkhamib, Milad Rabbani Esfahania,*, C. Heath Turnera, Siamak Nejatic a

Department of Chemical and Biological Engineering, University of Alabama, Tuscaloosa, United States Department of Chemistry and Biochemistry, University of Alabama, Tuscaloosa, United States c Department of Chemical and Biomolecular Engineering, University of Nebraska−Lincoln, 12 Lincoln, Nebraska 68588-8286, United States b

A R T I C LE I N FO

A B S T R A C T

Keywords: Water treatment Wastewater treatment Dye adsorption Metal-organic framework Graphene-oxide Langmuir Freundlich Molecular dynamics

In this study, a novel copper-based metal-organic framework (Cu-MOF), immobilized on graphene oxide (GO), was fabricated via ultrasonication method. The synthesized GO-Cu-MOF was used as an adsorbent, and the kinetics data for the removal of dye molecules investigated along with the molecular dynamics simulations. Various parameters such as solution temperatures and pH, dye, and adsorbent concentrations were studied to evaluate the performance of the adsorbent in removing a model contaminant based on the real-world water treatment conditions. The synthesized adsorbent was characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Ultraviolet-visible spectroscopy (UV–vis), Brunauer–Emmett–Teller (BET) analysis, and Zeta Potential. The characterization results showed full exfoliation of GO in Cu-MOF. The adsorption kinetic results followed the rapid adsorption process with a pseudo-second-order characteristic. The GO-Cu-MOF exhibited higher adsorption capacity of 173, 251, and 262 mg/g at 25 °C, 45 °C and 65 °C compared to 106, 117 and 142 mg/g adsorption capacity of Cu-MOF at the same temperature. The dye removal experiments suggest that the acidic condition and the higher temperature (65 °C) favors the adsorption of Methylene blue (MB) on GO-Cu-MOF compound. The molecular dynamics simulation performed to calculate the adsorption energy for Cu-MOF and GO-Cu-MOF. The calculated adsorption energy of -323 (kCal/mol), and -119 (kCal/mol) for GO-Cu-MOF and Cu-MOF was in agreement with the experimental data.

1. Introduction Dyes are known as the major water pollutants [1,2]. Using adsorbents is one the most effective ways for removing dyes from water and wastewaters [3–8]. Different carbon-based materials such as activated carbon have been used as effective adsorbent. Although this category of material is efficient in removing dyes still their production from wood or coal is not sustainable and economical [9–11]. Therefore, many studies have been carried out to introduce alternative materials as the replacement for activated carbon [3,9,12–16]. Metal-organic frameworks (MOFs), consist of the organic ligands and metal ions, possess a unique structure with high porosity and surface area [17,18]. The possibility to generate a new engineered structure by changing the metal ion and organic ligand makes MOFs appropriate candidates for various applications such as gas separation

⁎

[19], drug delivery [20], and gas adsorption [21,22]. The most common ligands for MOFs synthesis are of 1H-1,2,4-triazole,benzene1,3,5-tricarboxylic acid (BTC). Copper, zinc, iron, and manganese ions are the common metallic ions for the synthesis of MOFs [17]. The organic part of the MOF with various functional groups and numerous active sites provides more opportunities to act as a bridge for reaching to the other compounds [23–30]. MOFs are efficient adsorbents for phenols [31–33], sulfur compounds [34,35], pharmaceuticals [33], and dyes [36]. The bulk shape of the MOFs is a setback for their adsorption proficiency. Therefore, it is necessary to modify MOF morphology towards a more hospitable environment for dye absorbency [37]. Copper as a low-cost metal ion showed acceptable dye adsorption [38,39]. Asfaram et al. [40] investigated the adsorption of Auramine-O (AO), Erythrosine (Er), and MB into copper-doped zinc sulfide nanoparticles loaded on activated

Corresponding author. E-mail address: [email protected] (M. Rabbani Esfahani).

https://doi.org/10.1016/j.jwpe.2020.101180 Received 13 November 2019; Received in revised form 29 January 2020; Accepted 6 February 2020 2214-7144/ © 2020 Elsevier Ltd. All rights reserved.

Journal of Water Process Engineering 34 (2020) 101180

M. Dadashi Firouzjaei, et al.

the 4000-400 cm−1 range on a Bruker Tensor 27 FT-IR spectrometer (Bruker Scientific LLC, Billerica, MA, USA) using KBr discs.

carbon and reported adsorption capacity of 123 mg/g for AO, 123.5 mg/g for MB, and 84.5 mg/g for Er. Cheng et al. [41] reported the complete decomposition of acid orange 7 dye using the copper(II) complex of dithiocarbamate-modified starch. Graphene-Oxide (GO) is one of the pioneers in adsorption applicability and productivity [42]. The low-cost and high flexibility towards modification, candidate GO as the effective dye (such as methyl violet, methyl green, and neutral red) adsorbent [43]. The high reactivity of the GO is due to the different functional groups such as epoxide and hydroxyl groups on the basal planes and carboxyl groups on the edges [44]. The high number of active sites, negative electrical surface charge, and multiple oxygen-containing functional groups provide a facile reaction environment with other compounds for GO [45–47]. In this study, the novel hybrid adsorbent of Cu-MOFs embellished with GO was synthesized through the rapid, facile, and eco-friendly fabrication method and was characterized comprehensively. Finally, the mechanisms and effective factors on the kinetics of the dye removal process were examined at different operational conditions along with the molecular dynamics simulation of the adsorption process.

2.4. Adsorption procedure Adsorption experiments were conducted in 40 mL flasks. The experiments were carried out at different initial dye solutions (0−150 mg/L), different temperature (25, 45 and 60 °C) and different pH (3–12) (with constant dye solution of 70 mg/L, and 0.01 g adsorbent). All the flasks were mixed up in aqueous condition on the reciprocating shaker (rpm = 150) for 24 h at the controlled temperature (T = 25 C°). The pH of the solutions was adjusted using 0.1 M HCl and 0.1 M NaOH. The concentration at equilibrium state (Ce) was fitted to the Freundlich [48], Langmuir [49] and Langmuir−Freundlich (L−F) [50] models. The pseudo-first-order and pseudo-second-order equations were used to investigate the effect of contact time (kinetics) on dye adsorption. Aliquots were taken from the supernatant during the adsorption process and were analyzed by the UV–vis spectroscopy. 2.5. Molecular dynamics (MD) simulations

2. Materials and methods All MD simulations were performed using Materials Studio 6.0 (MS) software (Accelrys Inc., San Diego) [51]. The Cu-MOF and GO-Cu-MOF structures, methylene blue and water molecule, were built using ‘built module’ of MS software. Methylene Blue was located on the adsorbent using ‘Adsorption Locator Module. All initial molecular structures were geometry-energy minimized by the ‘Forcite Module’ of the MS software. The adsorbents were packed with the water (1 g/cm3) using ‘Amorphous Cell Module’ of the MS software. The annealing process carried out using ‘Universal’ [52] forcefield in 100 annealing cycles with an initial temperature of 300 °C and mid-cycle temperature of 500 °C. After full relaxation of the molecular structures, the molecular dynamics simulation of all the systems were performed using ‘COMPASS’ [53] forcefield in room temperature with time steps of 1 fs for a period of 500 ps. The van der Waals and electrostatic interactions were calculated with a cut-off distance of 18.5 Å, and the smooth particle mesh Ewald sum (SPME) [54] method was implemented to calculate longrange electrostatic interactions, with a Fourier spacing of 0.5 Å and Ewald accuracy of 10−5. The Nose-Hoover thermostat [55] was used to maintain the temperature, with a temperature difference of 10 °C, decay constant of 0.1 ps, Q ratio of 0.1, and collision ratio of 1. To avoid undesirable interactions of molecules with the walls of the simulation cell, the periodic boundary conditions (PBC) [56] method was implemented. The lattice parameters of the simulation were 27.9:34.4:65.2 (a:b:c) (Å), and 90°:114.748°:90° (α:β:γ).

2.1. Materials Copper nitrate hemipentahydrate (Cu(NO3)2.2.5H2O), ethanol (Purity, 99 %) and 1, 3, 5-benzentricarboxylic acid (BTC) were purchased from Merck, Germany. Graphene-oxide (GO) nanopowders (US1022) was purchased from US Research Nanomaterial, Inc. (Houston, USA). Methylene blue (MB) (C16H18ClN3S.H20, λ = 665 nm) was purchased from Merck, Germany. 2.2. Synthesis of GO-Cu-MOF adsorbent GO-Cu-MOF was synthesized by ultrasonication method [47]. Briefly, 20 mL solution of BTC (0.5 g) and copper nitrate hemipentahydrate (0.5 g) in water stirred for 30 min. in room temperature. To this solution, 10 ml solution of GO (50 g/L) was added and the final mixture was sonicated using a sonicator probe (Cole-Parmer, 750 W Ultrasonic Processor, USA) set at 20 kHz frequency and 100 W output power, with a 0.6-second pulse. Finally, the mixture was dried at the 40 °C in an oven (Thermo Fisher, Gravity convection) for 24 h. The same procedure was employed for the fabrication of Cu-MOF except addition of GO. Fig. 1 schematically illustrates the fabrication process along with chemical structure of GO-Cu-MOF. 2.3. Characterization

3. Result and discussion The crystalline patterns of fabricated MOF nanoparticles were analyzed using a diffractometer (Bruker D8, Germany) with a Cu Kα radiation in 2θ mode from 0° to 60°. X-ray photoelectron spectroscopy was performed on a Kratos spectrometer (Axis 165 XPS/ Auger, Shimadzu, Japan) equipped with a 100 μm monochromatic Al Kα Xray. The Brunauer–Emmett–Teller (BET) analysis was carried out for assessment of the surface area and pore volume of the fabricated MOFs using Autosorb iQ (Quantachrome, USA). The Zeta potential of the nanoparticles (aqueous 0.5 M) was measured by the Nano ZS Zetasizer (Malvern, UK). The UV–vis spectra were collected using a Genesys 10 s (Thermo Fisher, USA) spectrometer. The MB concentrations calibration curve was created at 665 nm wavelength. The transmission electron microscopy was performed on a FEI Tecnai (F-20, Thermo Fisher, USA). The surface morphology of nanoparticles was determined using a scanning electron microscope (JEOL FE 7000, JEOL, USA). The samples were coated with 5 nm gold layer using a sputter coater (Leica EM ACE600) Energy dispersive spectroscopy (EDX) (JEOL 7000, JEOL, USA) was employed for elemental analysis of the MOF nanoparticles. The FT-IR spectra were measured in

3.1. Characterization analysis The XRD patterns of the Cu-MOF and GO-Cu-MOF was depicted in Fig. 2a. The multiple characteristic peaks were labeled indicating the presence of Cu-BTC as reported in the literature [57]. The peaks at 2θ = 6.88°, 9.35°, 13.3°, 16.4°, 17.16°, 18.78°, 21.45°, 23.54°, 26.21°, 28.63°, and 29.0° represent (200), (220), (4000, (331), (422), (511), (440), (620), (444), (731), (822), and (751) indices [58,59]. A peak at 2θ = 42.3° confirmed the presence of GO in a GO-Cu-MOF pattern [60]. The crystal size average (D (nm)) was calculated using the Debye − Scherrer equation (Eq. 1) [61]

D=

Ks λ B cos θ

(1)

Where Ks is dimensionless shape factor equal to 0.9 for Cu Kα radiation, λ (nm)= wavelength (0.15405 nm for Cu Kα radiation), θ is the diffraction angle, and B is the peak width at half-maximum (rad). The crystalline size of the Cu-MOF particles is about 32. 2 nm using the peak 2

Journal of Water Process Engineering 34 (2020) 101180

M. Dadashi Firouzjaei, et al.

Fig. 1. The fabrication process of the nanoparticles. (a) Stirring solution, (b) Ultrasonication, (c) MOF powders (d) high magnification of ultrasonication, and (f) chemical structure of GO-Cu-MOF.

at 1600 cm-1 which was assigned to the sp2 C]C bonds and the phenyl CC ring. []68] In addition, COCee epoxy group vibration at 1100 cm1 and OHe deformation vibration around 1400 cm-1, and COeH stretching vibration at 1253 cm-1 also appeared in the spectrum of GO.2 Moreover, the broadband between 2700 and 3500 cm−1 corresponds to the OeH groups (COeH stretching mode) [69]. The two bands of BTC ring at 1112, and 938 cm-1 correspond to the OeH in-and out-of-plane bending modes while 793 and 729 cm-1 peaks correspond to the CeC ring of BTC out of plane mode. The major peaks at the 1390 cm-1 and 1440, and 1705 cm-1 attributed to the CeO and, asymmetric and symmetric types of CO] of the carboxylic groups, respectively and the band at 1605 cm-1 is attributed to the presence of aromatic C]C stretching mode [67,70]. As illustrated in Fig. 2b, the FT-IR spectra of the Cu-MOF and GO-Cu-MOF represents similarities. The strong absorption peaks related to the stretching mode of CeO and CO] were assigned at 1384, 1435 and 1699 cm−1, respectively. The shift of these three peaks in comparison with the free BTC might be due to the interaction of the COOH group of BTC with Cu ions during the formation of Cu-BTC coordination compound. Fig. 3 (a, b, and c) exhibits the SEM images of the typical sheet-like structure of GO. Fig. 3 (d, e, and f) shows the crystalline structure of Cu-MOF, as discussed in previous sections. The similarity between SEM images of the GO-Cu-MOF and Cu-MOF revealed, as concluded based on the XRD information, that GO completely exfoliated, and Cu-MOF covered most of the surface of the nanoparticle. Figure S3 and S4 show the EDX mapping and spectrum of GO-Cu-MOF, respectively. Table S2 summarized the elemental composition of NPs extracted from the EDX analysis. The EDX data is in perfect agreement within the XPS data. The uniform distribution of copper element in the GO-Cu-MOF EDX map revealed that there was not any agglomeration in GO-Cu-MOF along

at 2θ = 9.35°. The XRD result of Cu-MOF and GO-Cu-MOF showed a similar pattern (Fig. 2a). The GO exfoliation can be explained by the two possible interactions between GO and Cu-MOF: (i) the π-π stacking interaction between aromatic rings of GO and Cu-MOF, (ii) possible hydrogen bonding between BTC carboxylic group of Cu-MOF and hydroxyl and carbonyl GO. The UV–vis spectrum (Figure S1) of GO-Cu-MOF revealed three peaks around 260, 280, and 290 nm, corresponding to the electronic transition of aromatic rings, π→π* and n→π* transitions of C]O bonds, respectively [62,63]. Figure S2a shows the XPS spectra of the GO, GO-Cu-MOF, and CuMOF. The characteristic peaks for Cu (2p) around 954 and 934 eV revealed the existence of the copper nanoparticles (especially in GO-CuMOF) [64]. Table S1 shows the atomic concentrations of the GO, GOCu-MOF, and Cu-MOF. The lower carbon concentration and the higher copper concentration from GO to Cu-MOF were in agreement with the experimental fabrication procedure. The GO C1 s spectrum showed four carbon bands including (CeC, CC) ∼284.7 eV, (COH and COC]eee) at ∼285.5 eV and ∼286.8 eV at the ∼288.3 eV for carboxylate carbon [47,65]. The C1s spectra of the Cu-MOF and GO-Cu-MOF (Figure S2h and Figure S2i) indicated three peaks for CeC, CC at 284.7 eV, CO at 285.2 eV, and CO]e] at 288.7 eV [66]. In the GO-Cu-MOF sample around 286.9 eV, the epoxy group peak of GO emerged which verified the presence of the GO in the GO-Cu-MOF structure [47]. The highresolution spectra of the Cu-MOF and GO-Cu-MOF revealed the presence of Cu (2p1/2) and Cu (2p3/2) at around 934 and 953 eV for both samples that confirmed Cu-BTC formation in both samples [67]. Fig. 2b represents the FTIR spectrums of materials. The FT-IR spectrum of GO showed an intense peak at 1725 cm−1 corresponds to fingerprint C]O stretch from carboxyl groups and a nearly broadband 3

Journal of Water Process Engineering 34 (2020) 101180

M. Dadashi Firouzjaei, et al.

Fig. 2. (a) The XRD pattern of GO-Cu-MOF and Cu-MOF NPs, (b) the FTIR spectrum of BTC, GO, Cu-MOF and GO-Cu-MOF.

shift of the FTIR spectrum of Cu-MOF and GO-Cu-MOF after dye adsorption. The XRD patterns of GO-Cu-MOF and loaded GO-Cu-MOF was shown in Fig. 4b. The sharp peak of MB around 2θ = 16o exist at both the MB and loaded GO-Cu-MOF. These two peaks were overlapped each other completely and confirmed the dye presence in GO-Cu-MOF particles. Both the loaded GO-Cu-MOF and the GO-Cu-MOF patterns approximately followed the same shape, but with the difference in peak angle and intensity. For instance, a peak at 2θ = 9.35o represented (220) indices of Cu-BTC shifted to 10.6° for the loaded sample. The same inclination was observed for other characteristic peaks. This change in peak location and intensity along with peak shifts in FTIR spectrum of the loaded-GO-Cu-MOF (Fig. 4a) all justify that the major adsorption mechanism was the surface adsorption due to the bondage between active groups of adsorbents and MB [72]. The TEM images of loadedGO-Cu-MOF, clearly showed the dye particles in the structure of GO-CuMOF (Fig. 5). The MB visual size distribution and clear boundaries between dye particles demonstrated no dye agglomeration, which decreasesd the chance of dye physical adsorption as one of the main adsorption mechanisms.

with TEM images of GO-Cu-MOF. Table S3 presents the porosity and surface area of the fabricated nanoparticles. The mesoporous volume (Vm) of GO-Cu-MOF (2.1 cm3 g−1) was around three times greater than the mesoporous volume of Cu-MOF (0.74 cm3 g−1). The total pore volume of GO-Cu-MOF was 2.2 times greater than the pore volume of Cu-MOF. Also, surface area (as) of the GO-Cu-MOF was around four times greater than Cu-MOF. The adsorption-desorption results (Figure S3c) show that GO-Cu-MOF had hysteresis in its curve (Type3), resulting from higher mesoporous volume share [71]. The mesoporous regions in the structure directly affect the surface area and increase the active sites. Figure S3d, and S3e show the Zeta potential of fabricated MOFs (adsorbents) and dye (MB) at different pH. Due to the cationic and anionic nature of MB and adsorbents, respectively, by increasing the pH from acidic to basic, the adsorbents negative Zeta potential and MB solution positive Zeta potential were increased. 3.2. Adsorption characterization Fig. 4c and Figure S5 show the EDX map and spectrum of GO-CuMOF nanoparticles after adsorption of MB 50 mg/L solution (loaded GO-Cu-MOF). Since only one sulfur atom existed in the MB molecular formula, and due to the sulfur mapping (Fig. 4c) and number of sulfur atoms, it can be concluded that MB covered the surface of the material. This proof is in agreement with the FTIR spectrum of loaded GO-CuMOF (Fig. 4a). The characteristic peaks of GO-Cu-MOF and Cu-MOF were shifted to lower or higher wavenumbers and showed the surface bonding between MB and adsorbents. Table S4 summarizes the peak

3.3. Adsorption evaluation Series of dye adsorption tests were conducted for systematic evaluation of the adsorption process of MB on the fabricated NPs at different conditions as listed in section 2.

4

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Fig. 3. SEM images of the NPs at three different resolutions, (a, b, and c) GO, (d, e, and f) Cu-MOF, and (g, h, and i) GO-Cu-MOF.

3.3.2. Kinetics of adsorption process Fig. 7a and b show the dye concentration pattern with adsorption contact time. The same trend was reported in the literature [75,76]. Both materials were shown fast dye uptake in the first 2 h of the process with a slight reduction in 8 h. The equilibrium stage was reached after twelve hours towards the end of the adsorption process for 24 h. The adsorption behavior can be interpreted based on two different mechanisms of chemical interaction and physical entrapment. The fast adsorption at the first 30 min of contact time implied the external surface diffusion [77]. The numerous active surface sites of organic ligands in Cu-MOF and various functional groups on GO provided appropriate physicochemical environments for chemical interaction (surface adsorption) with dyes molecules to be captured. After the saturation of these sites, the probability of chemical adsorption decreased since there were fewer places available for interacting with the dyes molecules, then the physical adsorption (intraparticle adsorption) became dominant. Finally, the electrostatic dye repulsion phenomena happened until reaching the equilibrium stage. Table S5 represents the curve fitting of experimental data with the pseudo-first-order and pseudo-second-order equations and, based on the analyzed data; the pseudo-second-order reaction is the valid expression of the adsorption

3.3.1. Effect of pH on adsorption Fig. 6 illustrates the effect of pH on the adsorption of MB on the fabricated MOFs compounds. For both adsorbents, the adsorption versus pH curves followed the same trend. The adsorption of MB on the MOFs decreased by increasing pH. The electrostatic interactions affect the reaction of cationic groups of methylene blue and anionic groups of Cu-MOF and GO-Cu-MOF at lower pH. This interaction can occur through two mechanisms including (i) methyl groups of MB and carboxylate group of Cu-BTC (in both Cu-MOF and GO-Cu-MOF), (ii) carbonyl and epoxide groups of GO-Cu-MOF [72]. Increasing the pH weakened the electrostatic interaction between the dye and adsorbents, since the dye solutions zeta potential and adsorbents change in the same direction (Fig. 6) and then at the alkaline pH, π-π interactions and van der Waals bonds are the dominates. GO-Cu-MOF at lower pH showed around 20 % more adsorption than the Cu-MOF. This behavior can be justified based on the fact that the presence of the extra functional groups such as the carbonyl and epoxide in the GO-Cu-MOF, as a result of GO exfoliation, provided more adsorption active sites and hospitable environment for dye capture [73]. Also, there is a possible interaction between benzene rings of GO and MB might [74].

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Fig. 4. (a)The FTIR spectra of pristine Cu-MOF, loaded Cu-MOF, GO-Cu-MOF, and – loaded GO-Cu-MOF, (b) The XRD patterns of loaded GO-Cu-MOF with MB at a solution with 10 mg/L concentration, MB, and pristine GO-Cu-MOF, and (c) The EDX mapping of loaded GO-Cu-MOF with MB dye (after process of adsorption with 50 (mg/L) solution).

made monolayer coverage. There are more available capture sites in nanoparticles for dyes in lower initial dye concentration in comparison with higher initial dye concentration. Increasing the temperature from 25 to 65 °C for both adsorbents enhanced the Qmax. The Qmax for CuMOF increased from 106 to 142 mg/g and for GO-Cu-MOF increased from 173 to 262 mg/g for. This change might be the result of adsorption active site reinforcement at higher temperature [78,79]. The active sites reinforcement happen due to the breakage on internal bonds of the adsorbents (GO-Cu-MOF) at a higher temperature and results in the generation of more active sites [78,79]. Also, dye molecules can penetrate more easily into the material network at higher temperature due to the increment in their diffusion potential which is higher at

reaction. 3.3.3. Thermodynamics analysis of the Isotherms adsorption process Fig. 8c and d demonstrates the isotherm curves of Cu-MOF and GOCu-MOF. Also, Table 1 represents isothermal parameters obtained from the Langmuir, Freundlich, and Langmuir-Freundlich models. The Qmax value for Cu-MOF at 25 C° was 110 mg/g, and for GO-Cu-MOF it was 155 mg/g that implied 40 % increment in adsorption. As can be seen in Fig. 7c and d, at lower concentrations of dye reaching the equilibrium occurs rapidly and intense. This intensity shows the probability of adsorption in different regions. This is in agreement with the hypothesis that dye molecules were adsorbed on the outer surface of materials and

Fig. 5. (a) TEM image of loaded-GO-Cu-MOF particles (100 mg/L), (b) High magnification image of loaded GO-Cu-MOF with Methylene Blue molecular structure. 6

Journal of Water Process Engineering 34 (2020) 101180

M. Dadashi Firouzjaei, et al.

Fig. 6. Effect of pH on adsorption of MB on GO-Cu-MOF and Cu-MOF at the constant temperature of 25 °C. MB concentration of 70 mg/L and adsorbents concentration 1 g/L.

Fig. 8. The molecular dynamics simulation system (Red Color = Oxygen atom, White Color=Hydrogen atom, Gray Color = Carbon atom, Blue Color = Nitrogen atom, Yellow Color = Sulfur atom, and Mustard Color = Copper atom.).

higher temperature [3]. adsorption process. The adsorption energy was calculated by the implementation of four different systems including; (i) adsorbent-waterdye, (ii) adsorbent-water, (iii) water-dye, and (iv) water in Eq. (2). All the systems were individually annealed, and then MD simulation was

3.3.4. Molecular dynamics simulation of adsorption The molecular dynamic (MD) simulation of the adsorption process was performed to clarify the intermolecular interaction of the

Fig. 7. The effect of contact time on adsorption of MB onto (a) GO-Cu-MOF and (b) Cu-MOF, MB initial concentration effect on (c) GO-Cu-MOF and (d) Cu-MOF at pH = 3 and different temperatures 25, 45 and 65 °C. 7

Journal of Water Process Engineering 34 (2020) 101180

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Table 1 Equilibrium parameters T (oC)

for adsorption of MB onto Cu-MOF and GO-Cu-MOF.

Langmuir Equation

Qe =

Cu-MOF 25 45 65 GO-Cu-MOF 25 45 65

a

Qmax KL Ce 1 + KL Ce

Freundlich Equation

Langmuir- Freundlich Equation

Qe = KF Ce1/ n

Qe =

Qmax (KLF Ce )b 1 + (KLF Ce )b

Qmax (mg/g)

KL (L/mg)

R2

KF (mg(1−1/n)L1/n g-1)

N

R2

Qmax (mg/g)

KLF [(L/mg)1/b]

b

R2

110 157 164

0.007 0.011 0.006

0.98 0.99 0.99

2.38 2.51 4.58

1.53 1.43 1.60

0.98 0.99 0.99

106 117 142

0.023 0.035 0.034

1.61 1.51 1.36

0.99 0.99 0.99

155 166 174

0.010 0.018 0.023

0.98 0.98 0.99

3.40 6.69 12.38

1.45 1.71 2.044

0.95 0.94 0.93

173 251 262

0.009 0.00148 0.0027

0.939 0.754 0.853

0.99 0.99 0.99

a

Qe is dye concentration at equilibrium in solid phase, Qmax (mg/g) is the maximum amount of dye adsorption, KL(L/mg) is Langmuir equilibrium constant, KF (mg(1−1/n) L1/n g-1) is Freundlich adsorption capacity constant, n is adsorption intensity, KLF [(L/mg)1/b] is Langmuir-Freundlich constant, b is Langmuir-Freudlich heterogeneity constant. Table 2 The molecular dynamics simulation results. The energy of the Systems (kCal/mol)

H2O

H2O + MB

H2O + Adsorbent

H2O + MB + Adsorbent

Adsorption Energy (kCal/mol)

Cu-MOF GO-Cu-MOF

−1609.5 −1609.5

−802.5 −802.5

−17164.9 −18748.8

−16477.5 −18265.1

−119.7 −323.3

Fig. 9. The influential parameters affecting the adsorption properties.

towards enhanced adsorption capability. The increased number of functional groups and surface area are the two main contributions of GO to Cu-MOF regarding the dye adsorption process. These modifications resulted in better electrochemical adsorption chemistry for capturing dye for GO-Cu-MOF. Also having more surface area increased the probability of dye adsorption for GO-Cu-MOF. Pore size incrimination also increases the chance of physical adsorption for dyes.

carried out. Fig. 8 shows the schematic of the adsorption system and the total energy of all the systems [80].

ΔEAbsorption = EAbsorbent + H2 O + MB − EMB + H2 O + E H2 O − EAbsorbent + H2 O

(2)

In here, EAdsorbent+H2O+MB is the total energy of the system containing adsorbent, water, and MB after the simulation. The same interpolation is true for the other three systems. Figure S6 depicted the snapshots of the systems after simulation. The calculated adsorption energies were given in Table 2. The higher negative adsorption energy results in the stable adsorption process. The GO-Cu-MOF showed approximately three times more negative adsorption energy than Cu-MOF (Table 2). These data agreed with the experimental resulted explained in Section 3.2 and 3.3. As elaborated in previous sections, the more functional groups and active sites provided by GO increased the possibility and feasibility of the MB adsorption and facilitated the interactions like π-π interactions and hydrogen bonding between the adsorbent and MB. Fig. 9 compared and summarized the influential parameters affecting the adsorption behavior of Cu-MOF and the novel fabricated GO-Cu-MOF compound. Based on the obtained data, all the physicochemical parameters are in favor of GO-Cu-MOF. The presence of GO in the structure of the Cu-MOF changed the surface chemistry of NPs

4. Conclusion The novel GO-Cu-MOF was synthesized as an adsorbent for dye removal in water treatment. The various analysis showed the exfoliation of GO in Cu-MOF structure that enhanced the adsorption ability of the GO-Cu-MOF compared to the Cu-MOF. The GO exfoliation was characterized through the electronic transition of aromatic rings, π→π* and n→π* transitions of C]O bonds. The GO-Cu-MOF showed enhanced surface area, pore size, negative charge, and more active groups resulted in 20 % higher dye removal in comparison to Cu-MOF nanoparticles. The GO-Cu-MOF exhibited fast (30 min) dye adsorption using surface adsorption (chemical interaction) followed by the intraparticle adsorption (physical interaction) mechanism. The maximum adsorption 8

Journal of Water Process Engineering 34 (2020) 101180

M. Dadashi Firouzjaei, et al.

capacity of Cu-MOF increased from 106, 117 and 142 mg/g at 25, 45, and 65 °C to 173, 251, and 262 mg/g for GO-Cu-MOF at 25, 45, and 65 °C, respectively. The adsorption evaluation tests demonstrated around 90 % dye adsorption into GO-Cu-MOF at acidic (pH 3) and high temperature (65 °C). The kinetics of dye adsorption into the GO-Cu-MOF was modeled by the Langmuir-Freundlich and followed the pseudosecond-order model. The calculated adsorption energy (-323 (kCal/ mol), and -119 (kCal/mol) for GO-Cu-MOF and Cu-MOF) based on the MD simulation confirmed the interpretation of the experimental results regarding the enhanced adsorption ability of the GO-Cu-MOF for dye removal in the presence of water.

[17] [18]

[19]

[20]

[21]

Declaration of Competing Interest [22]

None. Acknowledgment

[23]

The authors gratefully acknowledge use of the resources of the Alabama Water Institute and the Department of Chemical and Biological Engineering at The University of Alabama.

[24]

[25]

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jwpe.2020.101180.

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