Metal Organic Framework-74 for Ultra-Trace Arsenic Removal from

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Metal Organic Framework-74 for Ultra-Trace Arsenic Removal from Water: Experimental and Density Functional Theory Studies Belal J. Abu Tarboush, Ali Chouman, Antranik Jonderian, Mohammad Ahmad, Mohamad Hmadeh, and Mazen Al-Ghoul ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00501 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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ACS Applied Nano Materials

Metal Organic Framework-74 for Ultra-Trace Arsenic Removal from Water: Experimental and Density Functional Theory Studies Belal J. Abu Tarboush1, Ali Chouman1, Antranik Jonderian2, Mohammad Ahmad1, Mohamad Hmadeh2,* and Mazen Al-Ghoul2,* 1

2

Department of Chemical and Petroleum Engineering, American University of Beirut, Beirut, Lebanon, PO Box 11-0236. Riyad El-Solh, Beirut, Lebanon

Department of Chemistry, American University of Beirut, Beirut, Lebanon, PO Box 11-0236. Riyad El-Solh, Beirut, Lebanon

*

E-mail: [email protected] * E-mail: [email protected]

Abstract This study investigates and compares Arsenic, As (V), removal from aqueous media using the water-stable zinc-metal organic frameworks (Zn-MOF-74) prepared via room-temperature precipitation (RT-Zn-MOF-74) and solvothermal procedure (HT-Zn-MOF-74). The Zn-MOF-74 crystals possess an average particle size of 66 nm and 144 µm for RT-Zn-MOF-74 and HT-ZnMOF-74, respectively. Moreover, the nano-sized RT-Zn-MOF-74 exhibited superior performance to that of HT-Zn-MOF-74. While the BET surface area of the RT-Zn-MOF-74 was smaller than the HT-Zn-MOF-74, higher adsorption uptake took place onto the room temperature synthesized ones, due to their small particle size and better dispersion. Adsorption isotherm studies showed that Langmuir isotherm was effective for the adsorption of As(V) onto RT-ZnMOF-74 and HT-Zn-MOF-74 with a maximum adsorption uptake (qmax) value of 99.0 mg g−1 and 48.7 mg g−1, respectively. This value exceeds most reported maximum adsorption capacities at neutral pH. The thermodynamics of adsorption revealed a spontaneous endothermic process that is due to the substitution of adsorbed water molecules by arsenate in the pores of the MOF 1 ACS Paragon Plus Environment

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crystal. This was further investigated using plane-wave density functional theory (DFT) calculations. This study constitutes a direct evidence for the importance of tuning the size of MOF crystals to enhance their properties. Keywords: Arsenic removal, metal organic frameworks, adsorption, nanomaterial, water treatment.

1. Introduction High levels of inorganic arsenic in water is a great threat to public health, and individuals can be exposed to arsenic through routine based actions such as drinking water, smoking tobacco, inhaling industrial toxics, eating crops irrigated with contaminated water and consuming food prepared with tainted water.1-2 Arsenic is naturally present as arsenate (As(V)) at relatively high concentrations in the groundwater in many places including India, Bangladesh, Mexico, and the United States of America.1-3 According to the World Health Organization (WHO) and the Environmental Protection Agency (EPA), threshold concentration of arsenic in the drinking water4-5 is 10 µg L−1 and it is thus essential to reduce human intake of arsenic, especially in water, to alleviate the grave impact of its contamination. For that purpose, many methods have been devised ranging from oxidation, coagulation, ion exchange to membrane techniques and phytoremediation.6-15 Nevertheless, the most promising method used for inorganic arsenic removal is adsorption, because of its low cost, high efficiency and ease of operation mostly in fixed beds or packed columns.4, 16 In the literature, there are many adsorbents reported in the literature for the removal of As(V) such as Boehmite and alumina (20.7 mg g-1)17 activated carbon (3.09 mg g-1), UiO-66 (303 mg g-1), Fe(III)-Si (21.1 mg g-1)18-19, hydrated ferric oxide, activated alumina, iron and manganese coated sand, and other natural and synthetic materials.20-

2 ACS Paragon Plus Environment

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22

However, the arsenic adsorption capacity of many of these conventional adsorbents is

unsatisfactory and thereby new adsorbents are being investigated. Particularly, metal-organic frameworks (MOFs)23-24 are some of the most recent and interesting discoveries in material science.25 They are porous solids made-up of inorganic clusters and coordinated to organic ligands via strong bonds.26 Due to their tunable pore size, high internal surface area, good thermal stability, and the exposed active sites, MOFs have received much of attention recently and they are widely used in variety of applications including gas storage, selective separation, catalysis, drug loading and biochemical imaging.27-30 Nevertheless, most of the reported MOF structures are moisture sensitive and consequently they cannot be used for water treatment processes. Among the large numbers of MOF structures to date, only a tiny fraction is found to be chemically stable and have been explored as adsorbents for water purification. UiO-66, HTZn-MOF-74, ZIF-8, AUBM-1 and MIL-53 are some examples of chemically stable MOF structures that have been studied recently for pollutants removal from water.31-42 Therefore, exploring and developing new MOF adsorbents that are chemically stable is particularly interesting. Herein, we investigate the use of the highly stable Zn-MOF-74 [Zn2(DOT); DOT = 2,5-dioxidoterephtalate] (Figure 1) as a new sorbent for removing As(V) from water. This MOF structure is constructed by joining the DOT units with infinite rods of Zn3O3(CO2)3. Each zinc cation is coordinated to three carboxyl, two hydroxyl and a solvent molecule (N,N-dimethyl formamide) to make an extended 3D framework with honeycomb-like pores (Figure 1) that intersect to form one-dimensional helical chains of cis-edge-linkedconnected metal–oxygen coordination octahedra. The channels have an accessible diameter of nearly 1.1 nm size. The most important property of MOF-74 is the presence of metal open sites which endows it with high adsorption capacity. Recently, many investigations have been carried out to understand the



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binding mechanisms of different gases on these open sites by experiments and density functional theory (DFT) simulations.43-45 Besides its chemical stability and high surface area, Zn-MOF-74 is environmental friendly and can be prepared solvothermally and at room temperature.46 Therefore, Zn-MOF-74 is prepared using both routes (Scheme 1) and the effect on the particle size, morphology and the adsorption capacity is explored. Interestingly, changing from microparticles size (prepared at high temperature), to nanoparticles size (prepared at room temperature), enhances the arsenic uptake by almost doubling the maximum adsorption capacity. A series of characterizations is performed on the Zn-MOF-74 including scanning electron microscope (SEM), powder x-ray diffraction (PXRD), Thermo-Gravimetric Analysis (TGA), Brunauer-Emmet-Teller (BET), and Fourier Transform infrared spectroscopy (FTIR). Furthermore, thermodynamic and kinetics parameters of the adsorption process are calculated and an arsenic adsorption mechanism over the RT-Zn-MOF-74 nanoparticles is proposed. We finally perform a DFT investigation to validate the experimental results and provide an explanation for the mechanism of adsorption on the open metal sites, especially through the exchange of adsorbed water molecules by the As(V) entity.

Scheme 1. Synthesis scheme of hydrothermal HT-Zn-MOF-74 and room-temperature RT-ZnMOF-74.


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(A)

(B)

Figure 1. Crystal structure of Zn-MOF-74, view along c axis showing the honeycomb like pores; color code: C : black, Zn : blue, O : red, H : pink (A) coordination mode of zinc atom by DOT forming rod like secondary building units (B).

2. Experimental and Theoretical Methods 2.1. Synthesis and Reagents All chemicals were of analytical grade and used without any purification. Zinc nitrate hexahydrate (98%, SIGMA-ALDRICH) and dihydroxyterephtalic acid (98%, SIGMAALDRICH) were used as the precursor salt and the organic linker, respectively. N,Ndimethylformamide (DMF) and ethanol were used as organic solvents to dissolve the reactants. Sodium arsenate (98%, SIGMA-ALDRICH) was used to prepare arsenic stock solution where specific amount of Na3AsO4 was dissolved in pure water. The stock solution was then thinned in pure water to prepare for the different batches of adsorption experiments.



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Zn-MOF-74 crystals were prepared using two methods; room temperature precipitation and solvothermal procedure. For room-temperature synthesis, 0.081 g of 2,5-dihydroxyterephtalic acid and 0.19 g of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) were dissolved in solution composed of 5 ml of DMF, 600 µL ethanol and 600 µL water. The solution was then left on magnetic stirrer at room temperature and after 1 min, 300 µL of triethylamine (TEA) were added to the solution and kept on magnetic stirrer for another 1 hour after which the precipitates were collected by centrifugation. For the solvothermal procedure, the Zn-MOF-74 was synthesized as per the reported procedure in the literature

47-48

where similar quantities, used for the room-

temperature synthesis, of organic linkers and metal precursors were dissolved in a solution composed of 10 mL of DMF, 1 mL ethanol and 1 mL water, and the mixture was heated to 120 ºC for 24 hours. The collected samples from the room temperature precipitation and solvothermal method were collected and washed with DMF and Dichloromethane (DCM) several time to ensure the purity of the synthesized MOFs.49 RT-Zn-MOF-74 and HT-Zn-MOF74 designate the MOF-74 synthesized at room temperature and at high temperature (120 ºC), respectively (Scheme 1).

2.2. Characterization For powder XRD (PXRD) analysis, the produced MOFs were dried after washing and inserted into a Bruker D8 discover XRD diffractometer which employs a Cu Kα radiation (λ = 1.5406 Å) that operates at 40 kV and 40 mA, with 2θ ranging between 5º and 40º at a scanning rate of 0.3 º min−1 and a total analysis time of 180 min. The confirmation of the MOF structure was done by comparing the recorded PXRD pattern with the simulated single crystal structure of MOF-74 reported in the literature. Thermal stability of the produced MOFs, thermogravimetric analysis


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(TGA), was also performed on a TG 209 F1 Iris (Netzsch, Germany). The procedures included heating a few milligrams of the MOF sample in the presence of nitrogen from 30 ºC to 1000 ºC constant temperature ramp equal to 5 ºC min−1. Nitrogen adsorption/desorption isotherms were measured using Micromeritics ASAP 2420 and Brunauer–Emmet–Teller (BET) equation was used to calculate the surface area. The prepared MOFs were activated by washing them for 3 days with DMF and further 3 days with DCM to ensure the removal of the unreacted starting materials from the MOF crystals. The samples were then activated via degassing at 120 ºC overnight and finally N2 adsorption/desorption isotherms were measured. The morphology of the prepared MOFs was invistigated using Scanning electron microscopy (SEM) and in order to account for possible aggregation resulting from preparation procedures. After washing and drying, a thin layer of gold (5 nm) was used to coat the MOFs sample and then it was inserted into a Tescan Mira SEM, operating at 5 kV with Oxford detector for energy dispersive X-ray (EDX) characterization to provide SEM photographs of the prepared MOFs. 2.3. As(V) Adsorption Experiments Arsenic removal by adsorption using RT-Zn-MOF-74 and HT-Zn-MOF-74 was studied using batch adsorption experiments. All arsenic adsorption experiments were performed at room temperature excluding the experiments that involve studying the effect of temperature. The equilibrium adsorption capacity qe (in mg g−1) was obtained using the following equation:

qe =

(C 0 − C e ) ×V m

(1)



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where C0 represents the initial concentration of As(V) (mg L−1), Ce is the concentration of As(V) at equilibrium (mg L−1), V is the volume of the aqueous solution (L), and m is the mass of MOF74 (g). For the adsorption kinetics, and the effect of pH and temperature experiments, 10 ml volume of the model solution (water and arsenic) was placed in a glass bottle and mixed with 10 mg of the each of the MOFs, separately, at a mixing rate of 200 rpm at room temperature. The initial concentration of arsenic was maintained at 40 mg L−1 (40 ppm). The kinetics experiments were carried out for 48 hours where multiple glass bottles were used to represent different time intervals. Similar procedures were used to study the effect of pH on arsenic removal and the pH of the solution was tuned using 0.1 M HCl and NaOH. For the thermodynamic analysis for the As(V) removal, a temperature range between 25 ºC−45 ºC was used. Adsorption isotherm experiments were performed using the same procedures employed for kinetic experiments except that the concentration range of arsenic was assorted from 5 mg L−1 to 150 mg L−1. For most of the samples, three replicate AA measurements were performed to ensure a standard error of less than 1% for each one. Arsenic concentration was measured using Thermo Labsystems, solar atomic absorption spectrophotometer (AA) with ASX-510 autosampler, and Solaar data acquisition.

2.4. Density Functional Theory Calculations All DFT calculations were completed by the PWscf code of the Quantum Espresso software package50 using the generalized gradient approximation (GGA) for the exchange-correlation energy with the Perdew-Burke-Ernzerhof (PBE) functional51 form. The non-local functional vdW-DF252 was used to include the van der Waals interaction between the MOF-74 and the


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guest molecules. In all calculations the atomic coordinates were relaxed while the lattice parameters were kept fixed. A 1x1x1 supercell was used as a periodic model to represent our system. The Monkhorst-Pack k-point sampling grids were set at 1×1×4. The Ultrasoft pseudopotentials (USPPs)53 were used with the plane-wave kinetic energy and charge cut-offs set to 340 eV and 3,400 eV, respectively. The self-consistent field (SCF) convergence tolerance was set to 5.0 × 10−7 eV. The Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm was employed to optimize nuclear coordinates until all force components converged to within 0.01 eV Å−1 and the total energy converged to within 5.0 × 10−6 eV atom−1 and, respectively. We started with the experimental crystal structure of Zn-MOF-74 obtained from Cambridge Crystallographic Data Centre (CCDC: 265095). Two adsorption sites were investigated: the primary adsorption site located on the zinc atom’s open site, and the secondary adsorption site which is electrostatically bound to the primary adsorbate. The binding energies between the adsorbates and the Zn-MOF74 were calculated using the following equation:

∆E = EMOF−74/ A − (EMOF−74 + EA)

(2)

where EA and EMOF-74 are the energies of the adsorbate and adsorbent, respectively, and EMOF-74/A is the energy of the system after adsorption. In the case of the negatively charged adsorbate, arsenate anion, on the neutral Zn-MOF-74 unit cell, a compensating jellium background was automatically inserted in the unit cell to achieve charge neutrality.54 Moreover, in this case, the arsenate was positioned at the middle of the MOF-74 channel as the initial state to cancel the contribution of the charge of the jellium on the total energy.55

3. Results and Discussions 3.1. Characterization 9 ACS Paragon Plus Environment


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Figure 2A depicts the PXRD patterns of the Zn-MOF-74 samples prepared using solvothermal and room temperature precipitation. It is clear that the synthesized compounds are highly crystalline and the peaks comply with the simulated ones56, which indicate the high purity of the RT-Zn-MOF-74 and HT-Zn-MOF-74 samples. Nevertheless, a broadness in the main peaks of RT-Zn-MOF-74 sample can be noticed, which validates the formation of nanosized crystals as evidenced by the calculation of the particle diameter using the Scherrer’s equation (66 nm for RT-Zn-MOF-74 and 144 µm for HT-Zn-MOF-74). After being exposed to water and with adsorbed arsenic on their surfaces, XRD patterns affirm that the crystallinity of Zn-MOF-74 is well-retained demonstrating the high stability of Zn-MOF-74 in water. Thermogravimetric Analysis (TGA) is employed to study the thermal stability of Zn-MOF-74. The percent weight loss related to the synthesized MOFs is based on control samples that contains only the MOFs. Figure 2B shows the percent weight loss versus temperature. As can be seen in the Figure 2B, considerable weight loss is observed below 200 ºC for the as synthesized RT and HT-Zn-MOF-74 samples. Between 15−20% weight loss is seen in this range and this can be due to the evaporation of water and other solvents. At ca. 100 ºC, while the coordinated water on the metal center remains bonded, there are some weakly adsorbed water molecules inside the cavity that start to desorb. As the heating process continues, the water molecules originally attached to the metal sites remain adsorbed

at ca. 100 ºC

and start to desorb at higher

temperature almost (ca. 200 ºC)47. Another major weight loss is observed between 200 ºC−500 ºC which represents the decomposition of the MOFs and agrees with what has been reported in the literature49. Furthermore, after decomposition at 500 ºC a total weight loss of 30%−40% of the initial weight is witnessed for the virgin MOFs. For the MOFs after the adsorption studies, it is clear from the TGA analysis that MOF-74 remains thermally stable at high temperature after


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the adsorption of As(V), and in the temperature range of 100 ºC−200 ºC the remaining residue from the used Zn-MOF-74 after adsorption is higher than that before adsorption. This is probably due to the presence of fewer guest anions in the MOF structure after adsorption.49

The surface areas for the RT and HT-Zn-MOF-74, evaluated by the BET method following degassing the sample at 120 ºC, are found to be 690 m2 g−1 and 1201 m2 g−1, respectively (Figure 2C). The decrease in the BET surface area of the nanocrystalline RT Zn-MOF-74 is consistent with the reported values of BET surface area of M-MOF-74 (M=Zn, Mn, Mg, Co, Ni) which are found57 to be ranging from 400 m2 g−1 to 1000 m2 g−1. The pore volume for the RT and HT-ZnMOF-74 are found to be 0.826 cm3 g−1 and 0.566 cm3 g−1, respectively (Figure S1). The HT-ZnMOF-74 isotherm is of type I where the maximum quantity adsorbed is reached at low relative pressure while that of the RT-Zn-MOF-74 is of type IV. This can be explained by the use of triethylamine (TEA) as a reagent in the synthesis of RT-Zn-MOF-74, which can probably act as an etching agent for the wall of the pores by coordinating to the zinc cluster and therefore providing the mesoporous behavior to the MOF structure. Similar observations were also found in nano-sized Mg-MOF-74 using TEA, which competes with the terephthalate linkers.58 In addition, the small particle size obtained at room temperature (less than 100 nm) introduces similar behavior in the adsorption of N2 molecules.

SEM study has been performed to understand and clearly see the structural features of the MOF crystals using both synthesis routes.



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Figure 2. XRD Analysis of Zn-MOF-74 before and after adsorption of As(V) (A). TGA analysis of Zn-MOF-74 before and after adsorption of As (V) (B). N2 Isotherms of RT-Zn-MOF-74 and HT-Zn-MOF-74 (C).

The obtained SEM images for RT and HT-Zn-MOF-74 are shown in Figure 3 and clearly exhibit that aggregation of particles resulted in both cases; however, the HT-Zn-MOF-74 particles have exhibited much higher levels of aggregation. The effect of aggregation might severely limit the adsorption capacity due to the reduction in the number of available adsorption sites. Remarkably, the particles obtained at room temperature are small (less than 100 nm) while the particles obtained at high temperature are relatively larger crystals (more than 100 µm). These values are consistent with the calculated ones from PXRDs using the Scherrer equation.


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Figure 3. SEM images of RT-Zn-MOF-74 (A, B). SEM images of HT-Zn-MOF-74 (C, D).

3.2. Arsenate adsorption Arsenate removal data for the HT-Zn-MOF-74 display much lower adsorption capacity, roughly 50% less than that of RT-Zn-MOF-74. This is believed to be due to the severe aggregation problems connected with the formation and use of HT-Zn-MOF-74. The nanosized MOF crystals should have more available adsorption and exposed sites than those in the large particles obtained via solvothermal technique. Indeed, the adsorption of As(V) on the surface of large particles results in an increase in the internal and external mass transfer limitations. Since the adsorption of As(V) is much higher for RT-Zn-MOF-74, the full adsorption study including the effect of pH and temperature, the thermodynamic and kinetic of the arsenic removal is performed on the RT-Zn-MOF-74 sample. 13 ACS Paragon Plus Environment


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3.2.1. Effect of pH The pH of the solution is essential in water treatment since it may affect the adsorption mechanism by altering the interaction of adsorbents and the adsorbates, mainly by controlling the surface charges and the ionization of the adsorbate in the solution.59 Arsenic, for example, exists in different forms in aqueous solutions at different pH values (speciation). It exists in the mono and deprotonated forms in the pH range of 2−10.60 The effect of pH on the percent removal of arsenate using RT-Zn-MOF-74 is conducted in pH range of 2−10 as shown in Figure S2A. The percent removal efficiency of arsenate by RT-Zn-MOF-74, denoted by R(%), is shown to be dependent on the pH of the solution. At an initial arsenate concentration of 40 ppm, and adsorbents dosage of 1 g L−1, around 80% removal can be achieved at pH around 7. The removal of arsenate using RT-Zn-MOF-74 is less effective in very acidic and basic solutions. Moreover, the percent removal efficiency increases from ca. 50% to ca. 80% when the solution pH is increased from 2 to 8. Its important to mention that Zn-MOF-74 is unstable at low pH and therefore no experiment was done at pH lower than 2. As Figure S2A depicts, the maximum removal of arsenate is acheived at pH value between 6 and 8. Two adsorption mechanisms can be investigated to elucidate the mechanism of adsorption: Electrostatic interactions or chemical reaction between the arsenate molecules and the functional groups on the surface of the RT-ZnMOF-7461. We believe that the mechanism of adsorption of arsenate onto RT-Zn-MOF-74 is effectively due not only to chemical interactions that lead to the substitution of water molecules inside the pore channels by the arsenate anions. The FTIR spectra in Figure S3 show medium wide-ranging band at 3400 cm−1 due to υ(OH) which is expected for MOF-74 materials owing to their high hygroscopic character. Interestingly,


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it can be seen that all functional groups originally present on RT and HT-Zn-MOF-74 are intact even after arsenate adsorption and are thus accessible for interaction with the metal ions.

3.2.2. Effect of temperature Studying the effect of temperature on arsenate adsorption is extremely important. For example, polluted water from different industries are released over different temperatures depending on the process that they are generated from. The temperature effect on the adsorption isotherm is presented in Figure S2B. The increase in the temperature from 20 ºC to 40 ºC enhances the percent arsenate removal from 77% (30.8 mg g−1) to 93% (37.2 mg g−1). This reflects the endothermic nature of the arsenate adsorption on the surface of the RT- Zn-MOF-74. The reason behind the increase in the adsorption capacity with the increase in temperature might be related to the presence of strong interaction forces between the arsenate molecules and RT- Zn-MOF-74 particles. In this paper, we chose to work at 30 ºC which still shows a high removal (85%) but closer to ambient temperature than 40 ºC.

3.3. Adsorption kinetics The effect of contact time on the adsorption of As(V) by RT- Zn-MOF-74 is depicted in Figure 4A. The remaining concentration of As(V) in the solution decreased as adsorption time increases, which lowers the static driving force of mass transfer. Thereby, the adsorption is rapid at the beginning when concentration gradient is high, and after 30 minutes, as the system approaches equilibrium, adsorption becomes slower. Equilibrium is established after 60 minutes and the adsorption capacity at equilibrium is calculated to be 36.1 mg g−1.

The adsorption

kinetics are considered rapid in comparison with typical adsorbents62 and this can be explained



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by the fact that the ultra-dispersion of sorbent particles leads to lower external mass transfer limitations.63 The mechanism of the adsorption processes whose limiting step might involve adsorption on a surface, chemical reactions or transport through diffusion is explored by studying the parameters extracted from the following kinetic models.

The most common models used to analyze

adsorption kinetics are Pseudo first-order and pseudo second-order kinetic models. A pseudo first-order kinetic model suggests that the rate of change in the solute concentration with time and the changes in the concentration of the adsorbate and the amount of adsorbent over time are logarithmically proportional.64 On the other hand, a pseudo second-order kinetic model proposes that the number of active sites occupied on the adsorbent is directly proportional to the adsorption capacity. The pseudo first-order49 (Eq. 3) and pseudo second-order38 (Eq. 4) model equations are given as follow:

ln(qe − qt ) = ln qe − k1t

(3)

t t 1 = + 2 qt k 2 qe qe

(4)

where qe (mg g−1) is the capacity at equilibrium calculated experimentally, qt (mg g−1) is the amount adsorbed at time t, k1 (min−1) and k2 (g mmol−1 min−1) are the pseudo-first-order rate and pseudo second-order rate constants respectively. The results of the kinetic adsorption studies are shown in Figure 4B,C. Table 1 provides the obtained kinetic parameters.

Table 1: parameters of As(V) adsorption kinetics by Zn-MOF-74. Kinetic Model

Parameter

Value


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Pseudo-First-Order

Pseudo-Second-Order

(A)

qe (g mg−1) k1 (min−1) R2

21.9 6.1 x 10–2 0.997

qe (g mg−1) k2 (g mg−1 min−1) R2

38.7 4.3 x 10–3 0.993

(B)

(C)

Figure 4. Effects of contact time on the removal of As(V) by the adsorbent Zn-MOF-74. Co = 40 mg L−1, V = 10 mL, m = 10 mg (A). Pseudo first-order kinetics of As(V) removal by Zn-MOF74 (B). Pseudo second-order kinetics of As(V) removal by Zn-MOF-74 (C). According to the correlation coefficient (R2) it can be concluded that the kinetics of arsenate adsorption on Zn-MOF-74 followed the pseudo first-order model. Nonetheless, the calculated capacity by pseudo-second-order (37.03 mg g−1) is closer to the experimental value (36.1 mg g−1) than that of the pseudo first-order (21.92 mg g−1). Therefore, both physisorption and chemisorption mechanisms for the adsorption of As(V) on Zn-MOF-74 should be considered. Both pseudo first-order and pseudo second-order models can successfully express the overall rate of adsorption process; however, the two models cannot predict whether internal or external diffusion is the rate controlling step during the adsorption. As a result, intra-particle diffusion model is used in order to examine the rate controlling step using kinetic data. Intra-particle diffusion model65 at which the adsorbed amount at time t (qt) can be written as:



qt = kid ⋅ t1/2 +θ

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(5)

where kid is defined as the intra-particle diffusion rate constant (mg g−1/2 min−1/2), and θ is a constant linked to the thickness of the boundary layer (mg g−1) and when the value of θ is large, the boundary layer effects becomes greater.66 As shown in Figure S4, the adsorption process is described by three linearity plots, suggesting that three diffusion steps occur in the transport process of arsenate on RT-Zn-MOF-74 nanoparticles67. The first step is the transfer of arsenate molecules from the boundary film to the surface of RT-Zn-MOF-74, after which arsenate molecules transfer from the surface to the intraparticle active site and finally the diffusion of arsenate molecules on the active or binding sites of RT-Zn-MOF-74 until saturation is achieved. Using the intra-particle diffusion model, the rate constants of the three linear steps (the slope of the linear plots) are calculated and found to be 5.75, 1.72, 0.0631 for the first, second and third linear plots, respectively. Therefore, it is concluded that the rate controlling step during the adsorption process is the intraparticle diffusion and the external diffusion is the fastest step.68

3.4. Adsorption isotherm The arsenate adsorption isotherms onto RT-Zn-MOF-74 are studied at pH 7.8 and temperature of 30 °C as it is found to be the optimal conditions. 10.00 mg of adsorbent are added to 10.00 ml aqueous samples (pH = 7.8) of adsorbate with different arsenate concentrations varying between (5 mg L−1−150 mg L−1) at temperature T = 30 ºC. The adsorption capacity of both RT-Zn-MOF74 and HT-Zn-MOF-74 increases as the amount of As(V) in the sample solution increases as shown in Figure S5. This expected trend is due to the fact that a larger concentration gradient (the driving force of mass transfer) leads to the availability of more arsenate molecules in the vicinity of the active sites on the surface of the MOF particles. The experimental maximum


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adsorption capacity represented by the saturation point of both RT and HT-MOF-74.0 are 81.7 mg g−1and 45.1 mg g−1, respectively. Furthermore, the data are evaluated using the following linearized Langmuir (6) and Freundlich (7) isotherms:

Ce Ce 1 = + qe qmax k1qmax

ln q e = ln k1 +

1 log C e n

(6)

(7)

where qe (mg g−1) is the adsorption capacity at equilibrium, Ce (mg L−1) is the equilibrium concentration of adsorbate, qmax is the maximum adsorption capacity, KL (L mg−1) is an equilibrium constant that represents the binding strength; KF (L mg−1) and n are Freundlich constants that specify the intensity and adsorption capacity, respectively. The experimental data for both MOFs are fitted with Langmuir and Freundlich models and plotted in Figure 5, and the parameters of the best fits are summarized in Table 2. Based on the higher correlation coefficients (R2) criterion, the Langmuir isotherm is found to fit the adsorption experiments of both RT-Zn-MOF-74 and HT-Zn-MOF-74 with maximum adsorption capacities, qmax, of 99.0 mg g−1 and 48.7 mg g−1, respectively. A previous study on the adsorption of mercury(II) using HT-Zn-MOF-74 reported a close value of maximum capacity (63 mg g−1).49 Our nanosized MOF removes twice as much arsenic from water under the same experimental conditions. In fact, the adsorption capacity of RT-Zn-MOF-74 is higher than many other adsorbents used and reported in the literature for removing As(V) at neutral pH including zirconium oxide (45 mg g−1)69, Iron(III)-loaded chelating resin (70 mg g−1)70, Iron(III) oxideloaded melted slag (57 mg g−1)71, titanium dioxide (30 mg g−1)72, calcined mesoporous silica (11



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mg g−1)73. These results demonstrate the importance of particle size control for the adsorption processes.

Table 2: The Langmuir and Freundlich parameters and correlation coefficients for arsenate adsorption onto RT-Zn-MOF-74 and HT-Zn-MOF-74. Model

Parameters

Langmuir isotherm

(RT-Zn-MOF-74) @ 30 ºC

(HT-Zn-MOF-74) @ 30 ºC

qmax

99.0

48.7

q bC q e = max e 1 + bC e

b

1.1 x 10–1

3.5 x 10–2

R2

0.953

0.964

Freundlich isotherm

KF

17.3

1.20

qe = K F Ce

1/n

0.41

0.45

R2

0.933

0.959

(A)

1/ n

(B)

Figure 5. Langmuir isotherm for As(V) adsorption onto RT-Zn-MOF-74 and HT-Zn-MOF-74 (A). Freundlich isotherm for As(V) adsorption onto RT-Zn-MOF-74 and HT-Zn-MOF-74 (B).

Removal of ultra-trace As(V). RT-Zn-MOF-74 can also be considered as a promising adsorbent for the removal of ultra-trace (ppb) arsenate from water, especially that the threshold


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concentration of arsenic in drinking water is at 10 ppb.4 To test this promise, similar procedures employed above are used with an initial arsenate concentration of 40 ppb and adsorbent masses varying between 2 mg and 20 mg. The highest removal in the ppb range is obtained for the case of 5 mg as shown in Figure 6. The removal percentage is also compared with an initial arsenate concentration of 40 ppm for the same mass of adsorbent. For example, lowering the mass from 10 mg to 5 mg results in improving the removal percentage from 57% (22.8 µg g−1) to 68% (54.4 µg g−1). This might be attributed to the negative aggregation effect, which becomes more pronounced in the ppb range and that can be diminished by decreasing the mass of the adsorbent to a certain extent.74

40 ppm 40 ppb

100 80 60

R%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 20 0 2 mg

5 mg

10 mg

20 mg

Mass of Adsorbent

Figure 6. Effects of the mass of adsorbent RT-Zn-MOF-74 on the removal efficiency of As(V). pH = 7.8, T = 30 ºC, t = 1 hr.

Moreover, several regeneration experiments for the used Zn-MOF-74 have been performed using a combination of NaCl, HCl, and Ethanol. The best recycling result was achieved by using about 0.1 mM HCl in ethanol, and 40% removal efficiency was obtained after the second adsorption



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cycle for 10.0 mg of used adsorbent and 50 ppm As(V) original solution (Figure S6). The noticeable decrease in the adsorption capacity after reuse needs to be further optimized.

3.5. Thermodynamic Analysis To provide better explanation for the nature and mechanism of the arsenate adsorption onto RTZn-MOF-74 surface, adsorption data at various temperatures, are utilized to find the thermodynamic parameters of adsorption such as change in standard free Gibbs energy (∆G0), change in standard enthalpy (∆H0), and change in standard entropy (∆S0). The thermodynamics of arsenic removal by the adsorbent RT-Zn-MOF-74 is assessed using the following Van’t Hoff equation (Eq. 8) 75, at four different temperatures:  ∆H 0 ln( K ) = −   R

 1 ∆S 0  + R T

(8)

where K is equilibrium constant computed as the ratio K = qe/Ce (L g−1). Figure S7 represents the Van’t Hoff plot and Table 3 contains the values of the aforementioned thermodynamic parameters including the Gibbs free energy at each of the 4 temperatures (∆G0 = ∆H0 − T∆S0). Table 3: Thermodynamic parameters at T = 20 ºC, 30 ºC, 35 ºC and 40 ºC. ∆H0 (kJ mol−1)

46.2

∆S0 (J mol−1 K) 184.7

∆G0 (kJ mol−1) 293 K

298 K

303 K

313 K

‒2.94

‒3.86

‒4.78

‒6.62

The endothermic nature of the adsorption of arsenate onto RT-Zn-MOF-74 was confirmed by the positive value of ∆H0. On the other hand, spontaneity of the adsorption is supported by the negative values of ∆G0. The positive value of 46.2 kJ mol−1 at 30 ºC for ∆H0 is most probably 22 ACS Paragon Plus Environment

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due to chemisorption consisting of the dehydration process of the metal atom and its environment, as discussed in the section on the TGA analysis, and the concomitant substitution with the arsenate. It has been established that there are several water environments in MOF-74, and upon heating the removal of the water molecules usually spreads from the center of the channels where they are weakly adsorbed towards the channel walls, where they are strongly coordinated to the metal.47 In this last case, the metal atom, in its solvated state, is sixcoordinated where one bond belongs to a single water molecule while the coordination sites that remained are filled with oxygen atoms belonging to the organic linker. Such dehydration step, which involves a transition from an octahedral structure into a five-coordinated square pyramidal, is more favored at higher temperatures. Therefore, the reported endothermic process is clearly due to the replacement of the various water molecules, including the one bound to the metal, by the arsenate anion, which becomes more favored as temperature is increased. This will be further investigated in the following section. Moreover, the positive value of ∆S0 proposes an increased randomness at the solid/solution interface with an increase in the degree of freedom of the adsorbed arsenate and could imply that the aforementioned dehydration mainly occurs at the pore center and away from the metal cluster of the MOF crystal, as corroborated by the TGA data and the intra-particle diffusion model.

3.6. Adsorption energies via DFT calculations In order to better understand the experimental thermodynamic data of the adsorption of arsenate on Zn-MOF-74, solid-state DFT energy and geometry calculations are performed. As previously described, Zn-MOF-74 consists of zinc atoms coordinated to the organic linker 2,5dihydroxyterephthalic acid (DOT). Each Zn2+ is coordinated to five oxygen atoms from the



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linker in a configuration close to the square pyramidal one. This configuration leaves an open metal site which can act as Lewis acid endowing MOF-74 with its good adsorption character. Therefore, when MOF-74 is in contact with water, the water molecules act as a Lewis base and coordinate to the Zn2+ open sites forming a primary layer. After saturation of the open metal sites, the excess water molecules form hydrogen bonds with the primary adsorbed water (P) and with the linker’s oxygen atoms as shown in Figure 7. The adsorption energies of primary water molecules on the open sites are calculated and found to be –58 kJ mol−1, while the adsorption energy of the secondary water (S) molecules is found to be –50 kJ mol−1. These values for the water adsorption enthalpies and for the bond distances (Figure 7) coincides with previously reported work.76 On the other hand, in the presence of the adsorbate monodeprotonated arsenate ions (H2AsO4–), the adsorption energy on the open sites is calculated to -91 kJ mol–1 (Figure 7). This value, which is more negative than that of water, indicates that arsenate is a stronger Lewis base than water and this is probably due to the negative charge of the arsenate ions. In other words, the adsorption of the arsenate molecules is thermodynamically more favourable than that of the primary water molecules as shown in the energy diagram in Figure 7. Therefore, in order for the arsenate to reach the open metal site, the water molecules in its vicinity has to be removed first. In the case of only two layers of water, the energy cost of this process is the sum of the two binding energies for primary and secondary water molecules reported above and is equal to −108 kJ mol–1. This value is incompletely compensated by the –91 kJ mol−1 adsorption energy for the arsenate leading to an overall positive adsorption enthalpy of +19 kJ mol−1. Consequently, the more water layers are included, the more positive the enthalpy of adsorption, which is consistent with the value obtained experimentally of 46 kJ mol−1 as reported above. It is noteworthy that the difference between ∆E and ∆H for our system is small because it involves solids and liquids.


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Figure 7. Energy level diagram of the adsorption of water and arsenate on Zn-MOF-74. In the primary adsorbed water (P), the distance between the O and Zn is 2.271 Å. When a second water is added this distance shortens to 2.181 Å. This bond strengthening is due to the hydrogen bonding between the hydrogen of the primary water and the oxygen of the secondary water (S). In addition to the hydrogen bonding between the secondary water and the O of the Zn-MOF-74 framework. On the other hand, the distance is shortest between the O of the arsenate and Zn (1.970 Å).

4. Conclusions In this study, Zn-MOF-74 was investigated for the first time as adsorbent for arsenic removal from water. The effect of particle size on the adsorption properties was evaluated by studying two sets of MOF-74 samples synthesized in a two-size regime (nano and microsized). Interestingly, the nano-sized MOF removed two time more As(V) from water than the conventional MOF-74 (99.0 mg g−1 vs 48.7 mg g−1). This can be explained by the creation of more adsorption sites and less agglomeration at the nanoscale, which enhance the As(V) removal efficiency. Furthermore, the thermodynamic and kinetic parameters were investigated and



Page 26 of 33

showed that the adsorption of As(V) on the MOF nanocrystals was an endothermic process and chemisorption processes were evidenced. DFT calculations for the adsorption energies of primary and secondary waters as well as of arsenate endorses the endothermic nature of the process and confirms that the mechanism of adsorption occurs via the successive substitution of the different layers of water surrounding the open metal site by the arsenate entity. Finally, this work demonstrated the importance of size tuning of the adsorbent and gave a promising assessment for possible application of nano-Zn-MOF-74 in industrial waste water treatment for removing As(V).

Supporting Information Pore size distribution of RT-Zn-MOF-74 obtained by BET. Effects of pH and Temperature on the removal of arsenate As(V). FTIR analysis for the RT and HT Zn-MOF-74 before and after adsorption. Intraparticle diffusion kinetics graph. Effects of initial concentration on the removal of arsenate As(V) onto RT and HT Zn-MOF-74. Recycling adsorption capacity. The Van’t Hof plot of ln(K) versus inverse temperature.

Acknowledgments We gratefully acknowledge the funding provided by the American University of Beirut Research Board and the K. Shair Central Research Science Laboratory. MH acknowledges support from the Masri Institute (#103214) and MG acknowledges support from Lebanese National Council for Scientific Research (LNCSR). The DFT computations were performed on resources provided by the High-Performance Computing Center of Texas A&M University at Qatar. We also thank


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Miss Manal Ammar for the invaluable discussions and help in the experiments and Ms. Mona Kanso for her help in some of the experiments.

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Metal Organic Framework-74 for Ultra-Trace Arsenic Removal from

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