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Article

Zirconium(IV) Metal Organic Frameworks with Highly Selective Sorption for Diclofenac under Batch and Continuous Flow Conditions

by
Anastasia D. Pournara
1,
Evangelos K. Andreou
2,
Gerasimos S. Armatas
2 and
Manolis J. Manos
1,3,*
1
Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece
2
Department of Materials Science and Technology, University of Crete, 70013 Heraklion, Greece
3
Institute of Materials Science and Computing, University Research Center of Ioannina, 45110 Ioannina, Greece
*
Author to whom correspondence should be addressed.
Crystals 2022, 12(3), 424; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst12030424
Submission received: 4 March 2022 / Revised: 15 March 2022 / Accepted: 16 March 2022 / Published: 18 March 2022
(This article belongs to the Special Issue Organic-Inorganic Hybrids: Synthesis, Property and Application)
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Abstract

:
Diclofenac (DCF) is among the most effective non-steroidal anti-inflammatory drugs (NSAIDs) and at the same time one of the most consumed drugs worldwide. Since the ever-increasing use of diclofenac poses serious threats to ecosystems, its substantial removal is crucial. To address this issue, a variety of sorbents have been employed. Herein we present the diclofenac removal properties of two metal organic frameworks, namely [Zr6O4(OH)4(NH2BDC)6]·xH2O (MOR-1) and H16[Zr6O16(H2PATP)4]·xH2O (MOR-2). Batch studies revealed fast sorption kinetics for removal of DCF− from water as well as particularly high selectivity for the drug vs. common competitive species. Moreover, the composite MOR-1-alginic acid material was utilized in a sorption column, displaying remarkable removal efficiency towards DCF− anions. Significantly, this is the first time that column sorption data for removal of NSAIDs using MOF-based materials is reported.

1. Introduction

Access to clean freshwater represents a challenge not only for developing countries but also for the developed world. The rapid urbanization and the improved standards of living along with escalating demands in the prevention and treatment of human diseases has led to an increase in the production and the use of pharmaceutical products [1,2,3]. Pharmaceuticals are emerging pollutants [4,5], which access the water stream via excretion, once they have completed their role in body systems, having aquatic ecosystems as their final destination. These compounds represent a unique category of pollutants as they are not passive but rather, they are bioaccumulated, sustained, and toxic to living organisms [6]. Among these pharmaceuticals, diclofenac (Figure 1) is classified as an emerging micropollutant. Diclofenac is extensively used as a non-steroidal anti-inflammatory drug (NSAID) and is commonly found in the form of carboxylate anions (pKa ~4) in surface and ground water as well as in drinking water [7,8,9,10]. Therefore, the removal of this emerging contaminant from our potable water and aquatic resources is becoming an important issue [11]. So far, various conventional methods, including coagulation–flocculation [12], biodegradation [13,14], photodegradation [15], chlorination [16], and sorption [17,18,19], have been adopted to remove pharmaceuticals from aquatic ecosystems. Among them, sorption is considered as a promising method considering its cost efficiency, ease operation, and low energy consumption [4,20]. The main challenge of the sorption is the development and selection of suitable sorbents. Up to now, carbonaceous materials including activated carbons (ACs) have been used widely for the removal of such pollutants from water, owing to their high surface area and relatively low price [21,22]. Moreover, other sorbents such as MCM-41 [23], SBA-15 [11], amberlite XAD-7 [24], and bentonite [25], have also been studied. Since most traditional sorbents still suffer from considerable limitations in real world applications, such as low sorption capacity, slow kinetics and short life cycles, researchers are looking for novel materials with superior sorption properties.
Among known sorbents, metal organic frameworks (MOFs) have recently attracted significant interest as they combine high porosity, tunable chemical composition, a variety of functional groups, etc. [26,27,28]. The reported studies on the capture of NSAIDs by MOFs have been focused on batch sorption investigations, which in most cases are incomplete [29,30,31,32,33,34,35]. For example, sorption data in the presence of competitive ionic species have not been included for most studied MOFs and, thus, the performance of MOF sorbents under realistic conditions is largely unknown. In addition, practical wastewater remediation requires treatment under continuous flow [36]. Till now, to the best of our knowledge, there are no investigations on MOFs for removal of pharmaceuticals under such conditions.
Our group has reported two MOFs, [Zr6O4(OH)4(NH2BDC)6]∙xH2O (MOR-1) and H16[Zr6O16(H2PATP)4]∙xH2O (MOR-2) (NH2-H2BDC = 2-amino-terephthalic acid; H2PATP = 2-((pyridin-1-ium-2-ylmethyl)ammonio)terephthalate) (Figure 1) [37,38], which have shown excellent sorption properties for inorganic anions and anionic dyes because of their facile anion binding properties.
Here, we report on the diclofenac anion (DCF−) capture properties of MOR-1 and MOR-2. These MOFs have shown quite fast sorption for removal of DCF− from water as well as particularly high selectivity for the drug vs. common competitive anions. The excellent DCF− sorption properties of MOR-1 and MOR-2 are due to the strong interactions of the drug with the framework of the MOFs. In addition, for the first time we present here column sorption data for removal of NSAIDs using MOF-based materials. Specifically, the composite form of MOR-1 with alginic acid (HA) mixed with silica sand was utilized as the stationary phase in the sorption column, which has shown capability for quantitative removal of DCF− from water as well as reusability. These results point toward the practical utilization of MOFs for wastewater treatment applications related to removal of pharmaceuticals.

2. Materials and Methods

2.1. Materials

All reagents and solvents were purchased from Alfa-Aesar or Sigma-Aldrich and used as received. The water used was purified through a Millipore system.

2.2. Syntheses

The syntheses of MOR-1/MOR-1-HA as well as MOR-2 materials were reported previously by us (references [37,38], respectively).

2.3. Physical Measurements

Powder X-ray diffraction measurements were carried out on a Bruker D2-Phaser X-ray diffractometer (CuKa radiation source, wavelength = 1.54184 Å). Unit cell indexing and Le Bail refinement data were performed using TOPAS [39]. ATR-IR spectra were collected in the range of 4000–400 cm−1 using an Agilent Cary 630 FTIR photometer. DCF− determination regarding the sorption experiments was performed by UV-vis spectroscopy with an Agilent Cary 60 UV-vis spectrophotometer from 200 to 800 nm. Scanning Electron Microscope (SEM) images were recorded on a JEOL JSM6390LV SEM. N2 adsorption–desorption isotherms were determined at 77 K on a Quantachrome Nova 3200e sorption analyzer. The activation of the samples involved EtOH-exchange, supercritical CO2 drying, and then degassing at 393 K under vacuum (<10−5 Torr) for 12 h. The specific surface areas were estimated using the Brumauer–Emmett–Teller (BET) method for the isotherm data in the 0.05–0.25 relative pressure (P/Po) range. Zeta potential was evaluated with a Malvern Zetasizer Nano ZS (Malvern Panalytical, Worcestershire, UK).

2.4. Batch DCF− Sorption Studies

The isolation of MOFs loaded with DCF− was performed as following: MOR-1 or MOR-2 (~0.04 mmol) was treated with a solution of diclofenac sodium (C0 = 195 ppm; 0.61 mmol) in water (10 mL, pH ~7), under magnetic stirring for ~20 min. Then, the solids were isolated by filtration, washed several times with water and acetone, and dried in the air. The DCF− uptake from solutions with concentrations in the range 50–5000 ppm was studied by the batch method at V:m ~1000 mL/g, room temperature, and 10 min contact. DCF− analysis was undertaken via UV-Vis and the obtained data were used for the determination of DCF− sorption isotherms. The competitive sorption experiments were conducted with DCF− solutions containing Cl−, NO3−, and SO42− with concentrations from 6.5 to 650 mM. This study was also performed with the batch method at V:m ratio ~1000 mL/g, room temperature, and 10 min contact. The sorption kinetics were determined via DCF− sorption experiments of various reaction times (1–60 min). For each experiment, a 10 mL sample of DCF− solution (0.65 mM; 195 ppm) was treated with 10 mg of MOR-1 or MOR-2 under magnetic stirring for the chosen reaction times. The filtrates from the various reactions were analyzed for their DCF− content with UV-Vis.

2.5. Preparation of the Column

A glass column (0.7 cm ID column) was partially filled with a mixture of 50 mg of MOR-1-HA composite and 5 g of sand (50–70 mesh SiO2). As in previous studies [37,38], the column was treated with ~7 mL HCl (4 M) solution and deionized water.

2.6. Column Sorption Studies

Several samples of the diclofenac solution (0.07 mM) were passed through the column (flow rate 1 mL min−1), collected at the bottom in glass vials and analyzed with UV-Vis. The regeneration of the column-desorption of diclofenac was achieved by its treatment with ~7 mL of HCl acid (4 M) solution. After regeneration, the column was treated with enough water to remove excess acid. The column containing only sand or alginic acid as stationary phase showed no diclofenac sorption capacity.

3. Results and Discussion

Our sorption studies involved detailed batch investigations of MOR-1 and MOR-2 for removal of DCF− including determination of sorption kinetics, sorption isotherms and selectivity vs. common competitive anions. As a second step, the column sorption of DCF− by using MOR-1-HA composite was investigated in detail. All experiments were performed with solutions of pH = 7 to simulate conditions found in most natural waters [40].

3.1. Batch Sorption Studies

3.1.1. Sorption Kinetics

To understand the sorption kinetics, sorption of DCF− by MOR-1 and MOR-2 was investigated through variable time sorption experiments. It is evident from Figure 2 that the sorption process for DCF− was outstandingly rapid, as the equilibrium was achieved within 5–6 min for both materials. Interestingly, MOR-1 removed ~98% of the initial DCF− content (DCF−: C0 = 195 ppm; 0.65 mM, pH ~6) within only 1 min of contact, respectively, while MOR-2 removed 96% of DCF− at the same time.
The kinetics data was fitted with the classic kinetic models, i.e., pseudo-first order and pseudo-second order models [41,42], whose mathematical expressions are given below:
q t = q e 1 − e x p − K L t
q t = k 2 q e 2 t 1 + k 2 q e t
where qt = the amount (mg/g) of ion sorbed by the MOF at different reaction times (t), qe = the amount (mg g−1) of ion absorbed in equilibrium, KL = the Lagergren or first-order rate constant and k2 is the second-order rate constant [g/(mg∙min)] [42]. The fitting parameters of the above are presented in Table 1. According to the correlation factors (R2), the Ho and Mckay’s pseudo-second-order equation could better describe the kinetics data for DCF− sorption by MOR-1 and MOR-2 than the pseudo-first-order model, indicating that chemisorption process occurs for DCF− binding [43]. Furthermore, the rate constants (Table 1) indicate faster sorption of DCF− anions by MOR-1 than MOR-2. Presumably, the diffusion of the relatively bulky DCF− anions is facilitated by the larger pores and channels of MOR-1 (pore sizes for MOR-1 and MOR-2 are 0.83–0.88 and 0.55 nm, respectively).

3.1.2. Sorption Thermodynamics

To better understand the sorption of the pharmaceuticals, we investigated and analyzed the equilibrium data, using the Langmuir, Freundlich, and Langmuir–Freundlich isotherm models. The mathematic expressions of these models are the following [44,45]:
(a)
Langmuir
q = q m b C e 1 + b C e
(b)
Freundlich
q = K F C e 1 n
c)
Langmuir–Freundlich
q = q m b C e 1 n 1 + b C e 1 n
where q (mg/g) is the amount of the ion sorbed at the equilibrium concentration Ce (ppm), qm is the maximum sorption capacity of the sorbent, b (L/mg) is the Langmuir constant related to the free energy of the sorption, KF and 1/n are the Freundlich constants. The parameters of Langmuir, Freundlich, and Langmuir–Freundlich (LF) isotherms, found after the fitting of the isotherm DCF− sorption data (Figure 3) are presented in Table 2.
The isotherm data of DCF− by MOR-1 and MOR-2 can be fitted very well with both Langmuir and LF models, as evidenced by the high R2 values (>93%). However, the sorption of DCF− by MOR-1 and MOR-2 may be better described by the Langmuir model, as the values of 1/n from the fitting with the LF model were found close to 1. When 1/n values tend to be equal to 1, the LF equation coincides with the Langmuir model. The maximum sorption capacity was calculated to be 315 ± 4 mg of DCF− per g of MOR-1 and 254 ± 3 mg of DCF− per g of MOR-2.
The affinity of the sorbents for the examined pharmaceutical can be described by the distribution coefficient Kd which is given by the equation:
K d = V C 0 − C f / C f m
where C0 and Cf are the initial and equilibrium concentration of the ion (ppm), respectively, V is the volume (mL) of the testing solution, and m is the amount of the sorbent (g) used in the experiment. The maximum Kd values for sorption of DCF− by MOR-1 and MOR-2, obtained from the batch equilibrium studies, were found to be 1.8 × 105 and 5.2 × 104 mL/g. These values are particularly high and indicate the exceptional affinity of MOR-1 and MOR-2 materials for the examined pharmaceutical.

3.1.3. Selectivity Studies

The selectivity of MOR-1 and MOR-2 for DCF− anions was investigated by performing sorption experiments in the presence of high concentrations of common competitive anions, such as Cl−, NO3−, and SO42−. The DCF− removal efficiency of MOR-1 and MOR-2 seems not to be influenced significantly in the presence of 100-fold excess of Cl− or NO3− anions, with DCF− removal capacities found more than 85% in any case (Figure 4). Interestingly, MOR-2 removed 92% of the initial DCF− content, even in the presence of 1000-fold excess of Cl−. Moreover, in the presence of competitive SO42− the ability of both materials to remove DCF− from water was notable, especially for MOR-2 which can remove 71% of initial DCF− even in the presence of 1000-fold excess of sulfate in the solution. Furthermore, we conducted experiments with DCF− solutions containing simultaneously Cl−, NO3−, and SO42− anions. Likewise, MOR-1 and MOR-2 can remove DCF− with only a small decrease in their sorption capability (69% and 78% removal percentages for MOR-1 and MOR-2, respectively). Overall, both MOR-1 and MOR-2 materials are proved to be excellent sorbents with significant selectivity for DCF− towards other co-existing anions and, thus, they seem to be very promising candidates for the remediation of real-world wastewater.

3.2. Column Sorption Study

In light of the remarkable sorption capacities of MOR-1 towards DCF−, we extended our studies to the investigation of the sorption properties under continuous flow conditions. However, pristine MOR-1 cannot be utilized as stationary phase in columns, considering that this material is isolated as a fine powder and its use in columns results in column clogging or release of the material to the flowing solution. Thus, we utilized the composite form of MOR-1 with alginic acid (MOR-1-HA) [37]. As in our previous studies [37,38,46], the column is filled with a mixture of the MOF composite material and silica sand (MOF composite:sand mass ratio = 1:100). Column sorption studies were performed with a DCF− solution of initial concentration ~20 ppm (pH ~ 7). These investigations revealed that 10 mL of effluent contain no detectable DCF, whereas relatively high removal capacities (>70%) were observed even after passing 50 mL through the column (Figure 5). The sorbent can be easily regenerated by washing it with HCl solution (1.2–4 M). After the regeneration, the column can be reused showing removal capacities close to those of the first run. Even after a third run, the column largely retains its initial sorption capacity.

3.3. Characterization of the DSF−-Loaded MOFs Mechanism of the DCF− Sorption by MOR-1 and MOR-2

The zeta potential of the MOR-1 and MOR-2 at pH ~ 7 was calculated close to zero (−0.214 and −0.417 mV, respectively), indicating neutral surface charge. EDS data further confirmed this evidence since no Cl− was detected for both materials. These results indicate that electrostatic interactions of MOR-1 and MOR-2 with DCF are not likely. Powder X-ray diffraction (PXRD), unit cell indexing, and Le Bail refinement data for the pharmaceutical-loaded MOFs revealed that the structures of the MOFs were retained after the sorption process (Figure 6).
Scanning Electron Microscopy (SEM) images indicate identical morphological characteristics for the pristine MOFs and loaded materials (Figure 7).
FT-IR data for the DCF-loaded MOFs indicated characteristic features of the organic molecules indicating the successful binding of the DCF− anions (Figure 8). The most evident change is the appearance of the characteristic band at 740 cm−1 in both MOR-1@DCF− and MOR-2@DCF− spectra, corresponding to the C-Cl stretching [47]. Furthermore, the changes observed in the region from 3500 to 3300 cm−1 are attributed to the N-H stretching of the secondary amines. In addition, both spectra display a slight shift of the C=O stretching band from 1568 to 1573 cm−1 (MOR-1) and from 1571 to 1576 cm−1 (MOR-2) after the DCF− sorption process, suggesting the participation of the carboxylic acid groups in DCF− sorption [47].
Based on the above characterization data and sorption results, we conclude that DCF− anions are likely ligated to Zr4+ centers of MOR-1 and MOR-2, presumably via their COO− groups. Thus, the mechanism of sorption may involve replacement of terminal OH/H2O groups (e.g., MOR-2 contains 8 terminal OH/H2O groups per Zr6 cluster) by DCF− anions (Figure 9). MOR-1 has sufficiently large pores (0.83–0.88 nm) to host DCF− having a diameter of about 0.83 nm (Figure 1). The insertion of DCF− into the pores of the MOR-1 is further consistent with the significantly reduced BET surface area of MOR-1@DCF− (557 m2/g) in comparison to that of pristine MOR-1 (1097 m2/g) (Figure 10). In contrast, the small pore dimensions of MOR-2 (0.55 nm) likely restrict the entrance of DCF− into the voids of the MOF and the sorption of DCF− by MOR-2 may occur predominantly on the external surface. Still, the BET surface area of MOR-2@DSF− (214 m2/g) is significantly smaller than that of MOR-2 (354 m2/g) (Figure 10).

3.4. Comparison of MOR-1 and MOR-2 with Other MOF-Based Sorbents

In Table 3, we present selected DCF− sorption characteristics of various MOF-based sorbents in neutral aqueous media and compare them with those of the MOR-1 and MOR-2. Most of these sorbents display maximum sorption capacities much higher than those of our materials. However, in most cases the equilibrium is achieved after long stirring periods, which is a significant drawback for real applications requiring instant removal of pollutants from aqueous media. In contrast, MOR-1 and MOR-2 show the fastest sorption kinetics among MOF-based sorbents with equilibrium times of only 5–6 min. Another important feature that both MOR-1 and MOR-2 display is their capability to remove DCF− satisfactorily under antagonistic conditions commonly encountered in natural waters. Such studies for reported MOF-based sorbents are scarce. However, the critical point of this study is the utilization of MOR-1 in an ion sorption column. To the best of our knowledge, MOR-1 is the first MOF with demonstrated sorption capability for DCF− under flow conditions, offering a great opportunity for further applications in the field of real waste remediation.

4. Conclusions

In conclusion, we reported the detailed DCF− batch sorption studies of MOR-1 and MOR-2. These materials displayed fast sorption kinetics (equilibrium time ~5–6 min) and satisfying removal capacities. Interestingly, the kinetics conducted in this study revealed the fastest DCF− sorption processes by MOFs ever reported. At the same time, MOR-1 and MOR-2 have shown noticeably selective sorption towards DCF− in aqueous samples containing a variety of coexisting anions, which reveals that these materials may be suitable for real world applications. It is worth emphasizing that MOR-1, in its composite form with alginic acid, is the first MOF used in a sorption column for elimination of pharmaceuticals and was found highly efficient for the removal of DCF− from contaminated aqueous solutions. Overall, the present study indicates that MOF-based sorbents are promising for the removal of pharmaceuticals under realistic conditions and for practical water treatment requiring continuous flow. Efforts to develop more effective MOF and MOF-composite sorbents for capture of pharmaceuticals from aqueous media are underway in our laboratory.

Author Contributions

Investigation, A.D.P., E.K.A. and G.S.A.; data curation, A.D.P., E.K.A. and G.S.A.; writing—original draft, A.D.P.; conceptualization, M.J.M.; writing—review and editing, M.J.M.; supervision, M.J.M. All authors have read and agreed to the published version of the manuscript.

Funding

The research work was supported by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the “First Call for H.F.R.I. Research Projects to support Faculty members and Researchers and the procurement of high-cost research equipment grant” (Project Number: 348).

Data Availability Statement

All data in this study are provided in the present manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 12 00424 g001 550
Figure 1. Representation of the structures of (a) diclofenac sodium salt, (b) MOR-1 and (c) MOR-2. Color code: C, grey; Cl, green; O, red; N, blue; Zr, cyan.
Figure 1. Representation of the structures of (a) diclofenac sodium salt, (b) MOR-1 and (c) MOR-2. Color code: C, grey; Cl, green; O, red; N, blue; Zr, cyan.
Crystals 12 00424 g001
Crystals 12 00424 g002 550
Figure 2. Fitting of the kinetics data (solid line) with the Ho–Mckay’s second-order equation for the sorption of DCF− by MOR-1 (a) and MOR-2 (b).
Figure 2. Fitting of the kinetics data (solid line) with the Ho–Mckay’s second-order equation for the sorption of DCF− by MOR-1 (a) and MOR-2 (b).
Crystals 12 00424 g002
Crystals 12 00424 g003 550
Figure 3. Equilibrium DCF− sorption data for (a) MOR-1 and (b) MOR-2 materials (pH ~ 7). The solid line represents the fitting of the data with the Langmuir model.
Figure 3. Equilibrium DCF− sorption data for (a) MOR-1 and (b) MOR-2 materials (pH ~ 7). The solid line represents the fitting of the data with the Langmuir model.
Crystals 12 00424 g003
Crystals 12 00424 g004 550
Figure 4. DCF− sorption data for (a) MOR-1 and (b) MOR-2 in the presence of various competitive anions.
Figure 4. DCF− sorption data for (a) MOR-1 and (b) MOR-2 in the presence of various competitive anions.
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Crystals 12 00424 g005 550
Figure 5. (a) The 1st run, (b) 2nd run, and (c) 3rd run of DCF− sorption under continuous flow conditions for MOR-1-HA (initial DCF− concentration = 21 ppb).
Figure 5. (a) The 1st run, (b) 2nd run, and (c) 3rd run of DCF− sorption under continuous flow conditions for MOR-1-HA (initial DCF− concentration = 21 ppb).
Crystals 12 00424 g005
Crystals 12 00424 g006 550
Figure 6. Le Bail plot of (a) MOR-1, (b) MOR-2, (c) MOR-1@DCF−, and (d) MOR-2@DCF−. Cell indexing data from Le Bail refinement: MOR-1 (space group Fm-3m): a = 20.785(4) Å, V = 8979(6) Å3; MOR-2 (space group I4/m): a = 14.631(5) Å, c = 20.743(9) Å, V = 4441(4) Å3; MOR-1@DCF− (space group Fm-3m): a = 20.771(3) Å, V = 8962(5) Å3; MOR-2@DCF− (space group I4/m): a = 14.59(1) Å, c = 20.64(1) Å, V = 4391(9) Å3. Blue crosses: experimental points; red line, violet crosses: experimental points; red line: calculated pattern; black line: difference pattern (exp-calc); green bars: Bragg positions.
Figure 6. Le Bail plot of (a) MOR-1, (b) MOR-2, (c) MOR-1@DCF−, and (d) MOR-2@DCF−. Cell indexing data from Le Bail refinement: MOR-1 (space group Fm-3m): a = 20.785(4) Å, V = 8979(6) Å3; MOR-2 (space group I4/m): a = 14.631(5) Å, c = 20.743(9) Å, V = 4441(4) Å3; MOR-1@DCF− (space group Fm-3m): a = 20.771(3) Å, V = 8962(5) Å3; MOR-2@DCF− (space group I4/m): a = 14.59(1) Å, c = 20.64(1) Å, V = 4391(9) Å3. Blue crosses: experimental points; red line, violet crosses: experimental points; red line: calculated pattern; black line: difference pattern (exp-calc); green bars: Bragg positions.
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Crystals 12 00424 g007 550
Figure 7. SEM images of (a) MOR-1, (b) MOR-2, (c) MOR-1@DCF−, and (d) MOR-2@DCF−.
Figure 7. SEM images of (a) MOR-1, (b) MOR-2, (c) MOR-1@DCF−, and (d) MOR-2@DCF−.
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Crystals 12 00424 g008 550
Figure 8. ATR-IR spectra of diclofenac sodium salt, (a,b) MOR-1, MOR-1@DCF−, and (c,d) MOR-2, MOR-2@DCF−.
Figure 8. ATR-IR spectra of diclofenac sodium salt, (a,b) MOR-1, MOR-1@DCF−, and (c,d) MOR-2, MOR-2@DCF−.
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Crystals 12 00424 g009 550
Figure 9. Suggested mechanism for the capture of DCF− by MOR-1/MOR-2.
Figure 9. Suggested mechanism for the capture of DCF− by MOR-1/MOR-2.
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Crystals 12 00424 g010 550
Figure 10. N2 adsorption isotherms (77 K) for (a) MOR-1, MOR-1@DCF−, and (b) MOR-2, MOR-2@DCF−.
Figure 10. N2 adsorption isotherms (77 K) for (a) MOR-1, MOR-1@DCF−, and (b) MOR-2, MOR-2@DCF−.
Crystals 12 00424 g010
Table
Table 1. Sorption kinetic parameters of MOR-1 and MOR-2 for DCF− removal.
Table 1. Sorption kinetic parameters of MOR-1 and MOR-2 for DCF− removal.
SorbentPseudo-First Order ModelPseudo-Second Order Model
KL(min −1)qe1 (mg g−1)R2k2 (g mg−1 min−1)qe2 (mg g−1)R2
MOR-14.261920.50.261920.83
MOR-22.191870.280.131880.88
Table
Table 2. Modeling parameters for the DCF− removal by MOR-1 and MOR-2.
Table 2. Modeling parameters for the DCF− removal by MOR-1 and MOR-2.
SorbentLangmuirFreundlichLF
qe
(mg/g)		b
(L/mg)		R2KF
(L/g)		nR2qe
(mg/g)		b
(L/mg)		nR2
MOR-1315 ± 40.18 ± 0.020.99132 ±247.9 ± 1.80.76318 ± 50.17 ± 0.021.12 ± 0.130.99
MOR-2254 ± 30.4 ± 0.050.97157 ± 2314 ± 4.80.57252 ± 30.39 ± 0.030.73 ± 0.080.99
Table
Table 3. Comparison of the sorption characteristics of MOR-1 and MOR-2 with those of other MOF-based sorbents.
Table 3. Comparison of the sorption characteristics of MOR-1 and MOR-2 with those of other MOF-based sorbents.
MOF-Based SorbentCapacity mg/gEquilibrium TimeSelectivity vs.ReusabilityColumn StudyRef.
Fe3O4@MIL-100(Fe)400250 minNANANA[29]
[Cu(BTTA)]n•2DMF650450 minNAReusableNA[30]
Magnetic GO/ZIF-8/g-AlOOH-NC2594.350 minNAReusableNA[31]
Defective UiO-66321120 minVarious pharmaceutical pollutants NANA[32]
MOF-8088335 h NANANA[33]
Fe3O4@MOF-100(Fe)377.3624 hNAReusableNA[34]
MOR-13155 minCl−, NO3−, SO42−Reusable3 runsThis work
MOR-22545 minCl−, NO3−, SO42−ReusableNAThis work
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Pournara, A.D.; Andreou, E.K.; Armatas, G.S.; Manos, M.J. Zirconium(IV) Metal Organic Frameworks with Highly Selective Sorption for Diclofenac under Batch and Continuous Flow Conditions. Crystals 2022, 12, 424. https://0-doi-org.brum.beds.ac.uk/10.3390/cryst12030424

AMA Style

Pournara AD, Andreou EK, Armatas GS, Manos MJ. Zirconium(IV) Metal Organic Frameworks with Highly Selective Sorption for Diclofenac under Batch and Continuous Flow Conditions. Crystals. 2022; 12(3):424. https://0-doi-org.brum.beds.ac.uk/10.3390/cryst12030424

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Pournara, Anastasia D., Evangelos K. Andreou, Gerasimos S. Armatas, and Manolis J. Manos. 2022. "Zirconium(IV) Metal Organic Frameworks with Highly Selective Sorption for Diclofenac under Batch and Continuous Flow Conditions" Crystals 12, no. 3: 424. https://0-doi-org.brum.beds.ac.uk/10.3390/cryst12030424

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