Porous structured MIL-101 synthesized with different mineralizers for adsorptive removal of oxytetracycline from aqueous solution

Abstract

In this work, highly porous MIL-101 materials using hydrochloric acid (HCl) or hydrofluoric acid (HF) as a mineralizer were synthesized. The products were characterized with X-ray diffraction, scanning electron microscopy, thermal gravimetric analysis, and N₂ sorption analysis. The results proved that the mineralizers had an obvious effect on the microstructures and morphologies of MIL-101, and MIL-101(HCl) showed remarkable superiority in a higher specific surface area, smaller particle size and higher crystallinity to the HF counterpart. These materials were then used to adsorb and remove the oxytetracycline (OTC) antibiotics from a water solution and an effective fast adsorption of OTC in aqueous solution was observed in the adsorptive kinetics. Compared with MIL-101(HF), a higher adsorption capacity on the MIL-101(HCl) was obtained, which was probably attributed to the higher specific surface area for the MIL-101(HCl). The adsorption mechanism might be greatly ascribed to π–π interaction, and acid–alkaline interaction between the oxytetracycline molecule and the adsorbent.

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1. Introduction

The heavy use of veterinary antibiotics in intensive animal husbandry and aquiculture for prevention of infectious diseases, especially as animal feed additives, has resulted in serious environmental problems.¹ Long term exposure to even low-level antibiotics has led to the emergence of bacterial resistance among pathogenic microbes, and even triggered the formation of cross- and multiple-resistances in organisms.² It is urgent to develop advanced strategies for eliminating residual antibiotics in wastewater to deal with the antibiotic-related crisis. Adsorption is one of widely used technologies for the effective removal of low-concentration antibiotics,³ and a considerable number of adsorptive materials, such as: activated carbon,⁴ soil clays,⁵ humic substances,⁶ sewage sludge- and waste oil sludge-derived materials,⁷ are currently under active investigation to find an advanced candidate. Nevertheless, there is an increasing demand for the development of adsorbent material to deal with the fact that antibiotics are still present in environmental matrices worldwide.

Metal–organic frameworks (MOFs), an intriguing class of crystalline solids assembled by the connection of metal ions or clusters through molecular bridges,⁸ have attracted significant interest in the last years mainly due to the favorable attributes of extremely high apparent surface areas, chemical modification for targeting desired properties, and optical or magnetic responses to the inclusion of guests. The metrics and surface properties of pores are finely tuned by choosing structural components, such as: organic ligands, metal cations, and counter anions.⁹ To date, MOFs have been shown promising applications in the field of gas storage¹⁰ and separation,¹¹ catalysis,¹² drug delivery carriers for nanomaterials,^13,14 luminescence,¹⁵ and adsorption of organic molecules.¹⁶ Particularly, several studies indicated that MOFs have excellent performance in the adsorptive removal of hazardous organic substances from water.^17,20 However, to the best of our knowledge, very scarce investigations on antibiotics capture using MOF-based adsorbents were reported up to now.^21,22

As an excellent absorbent for removing antibiotics from aqueous solution, two significantly important features should be particularly considered to facilitate the intentional selection of materials with an effective adsorption performance: primarily, everlasting structural stability of adsorptive materials in aqueous systems must be equipped from an application perspective. Additionally, it should be emphasized that many antibiotics are bulky molecules,²³ and the fairly enough pore size adequate to antibiotic giant molecules is precondition for high uptake loadings to allow maximum working capacity. Unfortunately, most of the reported MOFs to date are restricted to microporous regime, and only a small fraction of MOFs with mesoporous structures are reported.^24,25 The small size of their cavities has an extremely detrimental impact on diffusion and mass transfer for the large-size molecules. Notably, MIL-101 (MIL = Materials of Institut Lavoisier), a classic MOF material developed by Férey et al.,²⁶ have received vast attention over the past decade due to its durable water stability and giant pore size. The framework structure exhibits a large pore volume of 1.0 cm³ g⁻¹ and unprecedented surface area (3100–5900 m² g⁻¹) without any loss of crystallinity after water evacuation. Importantly, their crystalline structures are kept intact and porous after exposure to boiling water at 100 °C for 7 days, giving reproducible proof of their hydrothermal stability.²⁷ All these endow the framework with potential application in antibiotic treatment.

Mineralizer is wildly used in many hydrothermal synthesis processes, and it plays the key role in the promotion of dissolution, crystal nucleation and phase growth.²⁸ A great deal of MOFs were synthesized with different mineralizers, such as: HF,²⁶ graphene oxide,²⁹ TEOS and H₂O₂.³⁰ MIL-101(Cr) is originally synthesized with a HF mineralizer.²⁶ However, HF is a highly corrosive and toxic acid and the F⁻ anion in water has the health risks including chronic toxic effects on teeth, food intake, bones and soft tissues.²⁸ Using an acid or a base as the mineralizing agent, the basic structure of the MOFs remains the same, but the charge of the framework may be altered,³¹ which may affect the antibiotics adsorption properties on the MOFs. By the former reasons, same chemical compounds, such as: HNO₃,³² graphene oxide,²⁹ anhydrous sodium acetate,³¹ and tetramethylammonium hydroxide (TMAOH),³³ have been tested to synthesis MIL-101 trying to avoid using the corrosive HF mineralizer and improve the performance of MIL-101 in previous works. Besides, Akiyama et al.⁹ have ever reported that MIL-100 can be developed from HCl mineralizer. However, few researches about preparing the MIL-101 with HCl as mineralizer for absorbing oxytetracycline (OTC) were reported until now.

In the present work, the MIL-101 is prepared with different mineralizers (HCl and HF) as the OTC adsorbent in water solution. Influences of two different mineralizers on microstructure and absorption properties are investigated. The adsorption kinetics and isotherms of MIL-101(HCl) for the removal of OTC from aqueous solution are studied. Our attention focuses on the effect of mineralizer on synthesizing MIL-101 and adsorption mechanism for OTC in aqueous phase with the promising adsorbents. The obtained results can provide useful information for the use of MOFs in capturing other families of antibiotics.

2. Experimental

2.1. Chemicals

Oxytetracycline dihydrate standard compound (>95%, molecular formula: C₂₂H₂₄N₂O₉·2H₂O, molecular weight: 496.46), chromic nitrate nonahydrate (99%), 1,4-benzenedicarboxylic acid (99%), hydrofluoric acid (40 wt% in H₂O) and hydrochloric acid (38 wt% in H₂O) obtained from Aladdin Chemistry Co. Ltd. (Shanghai, China). Absolute ethanol (AR grade) were purchased from Nanjing Chemical factory (Nanjing, China). All solvents and reactants were obtained commercially and used without further purification.

2.2. Preparation of MIL-101s

A routine solvothermal recipe of MIL-101(HCl) involves in following steps: Cr(NO₃)₃·9H₂O (4.0 g, 9.96 mmol) was dissolved in de-ionized water (48 mL, 2650 mmol) and mixed with H₂BDC (1.64 g, 9.87 mmol) and HCl (10 mmol). The reaction solution was then transferred to a closed vessel and heated in a preheated oven at 493 K for 8 h. In contrast, the MIL-101(HF) was prepared by the reported method.²⁶

After natural cooling, a green fine powder was obtained. However, significant amounts of needle-shaped colorless crystals were observed due to the reason that unreacted terephthalic acid was recrystallized as previous report.²⁶ To achieve the higher porosity, a following purification treatment was required to eliminate carboxylic acid which was present both outside and within the pores of material.³¹ Firstly, the crystalline MIL-101 products in solution were filtered doubly filtered using two glass filters with pore sizes between 40 and 100 μm. Secondly, a solvothermal treatment was performed using absolute ethanol at 393 K for 15 h for further purification. Finally, the mixture was cooled, filtered, and washed several times with water, and the resulted powder was dried overnight at 353 K in air.

2.3. Materials characterization

Powder X-ray diffraction data were performed on a D/Max 2200 X-ray diffractometer using monochromatic Cu-Kα radiation under 40 kV and 36 mA with a scan speed of 10° min⁻¹. The Brunauer–Emmett–Teller (BET) surface areas and pore volumes were estimated by N₂ physical adsorption–desorption at 77 K using a surface area analyzer (Quantachrome Corporation). Meanwhile, the Density Functional Theory (DFT) method was used to evaluate the pore size distribution. The morphologies of the samples were analyzed by scanning electron microscopy (SEM Hitachi S4800). A TGA (Thermal Gravimetric Analyzer S-1000) work was conducted to study the stability of the MOFs; ca. 5–10 mg samples was heated to 973 K at 10 K min⁻¹ under an air flow (50 mL min⁻¹). UV-Vis spectra were collected on a UV-Vis spectrophotometer (Shimadzu UV-1800) in a range from 200 nm to 600 nm in ambient conditions.

2.4. Liquid-phase adsorptive experiments

The aqueous stock solution of OTC (120 mg L⁻¹) was prepared by dissolving OTC in deionized water, and different OTC concentrations (20–120 mg L⁻¹) for the adsorption experiments were prepared by successive dilution of the stock solution with water. Prior to the adsorption, the samples were dried by heating in vacuum at elevated temperature until the weight was constant (overnight). In the process of adsorption, an exact amount of the adsorbents was placed in the OTC aqueous solutions with various fixed initial concentrations, and the OTC solutions containing the adsorbents were mixed well with magnetic stirring. After adsorption for a pre-determined time, the adsorbents were rapidly filtered-off from the suspensions with a syringe filter (PTFE, hydrophobic, 0.45 μm), and the OTC concentration was calculated with absorbance (at 276 nm) of mother solution by direct UV-Vis spectrophotometry. The amount of OTC adsorbed on the MOFs at equilibrium was evaluated through the following equation:where q_t (mg g⁻¹) is the adsorbed amount at the adsorption time t (min); C₀ and C_t (mg L⁻¹) are the initial concentration and concentration at the adsorption time t (min), respectively; V (L) is the initial volume of OTC solution, and M (mg) is the mass of the adsorbing material.

2.5. Adsorption kinetics

In order to quantitatively evaluate the equilibrium capacity of adsorbents for removal of OTC and obtain the equilibrium time, the experiments of adsorption kinetics were conducted at pre-determined time intervals ranging from 0 to 300 min. 0.08 g MOF-based adsorbent materials was weighed and added into 100 mL 80 mg L⁻¹ as-preparation OTC solution, the solution was separated from the adsorbents at specified time points and the residual concentration was determined by UV-Vis.

2.6. Adsorption isotherms

To study the removal mechanism, the adsorption equilibrium experiments were conducted at various fixed initial concentrations (20–120 mg L⁻¹) in a series of batch processes. For single batch experiment, a given amount of adsorbent (0.02 g) was placed in 25 mL as-preparation OTC solution at fixed initial concentration.

3. Results and discussion

3.1. Materials characterization

The X-ray diffractograms of the purified MIL-101s shown in Fig. 1 are basically identical, and all peaks match well with the reported literature,²⁶ indicating that the as-prepared samples are MIL-101s and the resulting basic frameworks remain invariant with different mineralizers in the process.³¹ Furthermore, the diffractogram from MIL-101(HCl) has a higher intensity of diffraction peaks than MIL-101(HF) especially at peaks of 2theta around 10°, implying that highly crystalline MIL-101 is easier to obtain using HCl mineralizer in the above synthesis conditions.

Fig. 1 Powder X-ray diffraction patterns of MIL-101(HCl) and MIL-101(HF).

The thermal stability of pure prepared MIL-101s was examined by TGA. As shown in Fig. 2, two weight losses within the 25–100 °C and 350–550 °C are observed in both MIL-101s. The first weight loss can be due to the departure of free water molecules, and the second one may be assigned to the departure of solvent molecules and decomposition of MIL-101 framework. A steady weight (ranging from 100 to 350 °C) indicates that the MOFs adsorbents are thermally stable up to 350 °C, which may be ascribed to the high thermal stability of the carboxylate-metal fragment.³⁴

Fig. 2 TG curves of MIL-101(HCl) and MIL-101(HF).

The porosity properties of the MIL-101s obtained with different mineralizers were characterized by N₂ sorption–desorption isotherms at 77 K, and the results were recorded in Fig. 3. All the MOF samples exhibited a type I N₂ adsorption isotherm with remarkable H4 hysteresis loops below relative pressure at 0.42 indicating the existence of micropore in MIL-101(HCl) and MIL-101(HF). The hysteresis loops may be ascribed to the slits in MIL-101 particles.

Fig. 3 N₂ adsorption–desorption isotherm of MIL-101(HCl) and MIL-101(HF).

The relevant parameters involved in specific surface areas and pore volumes of the used samples are summarized in Table 1, the result shows that MIL-101(HCl) has the higher specific surface area and pore volume than the MIL-101(HF).

Table 1 The specific surface area and pore volume of two MIL-101s

Sample	Specific surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)
MIL-101(HCl)	3090	1.64
MIL-101(HF)	2533	1.28

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Fig. 4 presents the pore size distributions in these synthesized materials. It can be observed that the micro and mesoporous structures are present in the two MIL-101s. The MIL-101(HF) has a pore size distribution centered at 1.09, 1.48, 2.12 and 3.10 nm, indicating the presence of the micro-windows at about 1.09–1.48 nm, which is close to those estimated from crystal structure (≈1.2 nm and 1.6 nm × 1.45 nm), and the meso-cages at about 2.12–3.10 nm, which are moderately smaller than those estimated from crystal structure (≈2.9 nm and 3.4 nm).²⁶ The MIL-101(HCl) has a nearly similar distribution with MIL-101(HF) ranging from 0.9 to 3.3 nm except that an extra pore size of 1.36 nm is observed in the MIL-101(HCl). Importantly, compared with MIL-101(HF), MIL-101(HCl) shows a larger differential pore volume, which may help to interpret the higher specific surface area for MIL-101(HCl).

Fig. 4 Pore size distribution of MlL-101(HCl) and MlL-101(HF).

The high crystallinity of the MIL-101s are evident from the typical octahedral shape and is in accordance with the high quality of the morphologies (Fig. 5a–d). Consistent with the previous report,³⁵ MIL-101(HF) crystals appears with a mean particle size ranging from 0.5 to 1.5 μm. However, MIL-101(HCl) shows the smaller mean particle size ranging from 0.2 to 1.2 μm which may give a further interpretation for the higher specific surface area than MIL-101(HF).

Fig. 5 SEM images of the MIL-101(HCl) with different magnifications (a and b); SEM images of the MIL-101(HF) with different magnifications (c and d).

3.2. Adsorption kinetics

Adsorption kinetics is used to investigate diffusion mechanism, adsorbed control step and influencing factors of adsorbed velocity.³⁶ Herein, the experiments were carried out at the initial concentration of 80 mg L⁻¹, and the amount of OTC adsorbed over MIL-101s as a function of time is displayed in Fig. 6. The results demonstrate that the adsorption is quite fast during the initial period (from 0 to 10 min), which may be attributed to the presence of a large number of vacant surface sites on the exterior surface. With further increasing the contact time, the adsorption rate decreases gradually due to the fact that remaining vacant surface sites are increasingly difficult to be occupied because of the strong repulsive forces between the solute molecules on the solid and bulk phases.³⁷ Subsequently, the diffusion of OTC molecules in the pores encounters great resistance and the adsorption rate of OTC starts to slow down. Similar trend is observed in the adsorption of dye on MIL-101.³¹ After approximately 60 min, evidently, no detectable concentration changes occurr, indicating that the removal equilibrium is reached. The adsorptions are practically completed in 60 min, showing the rapid adsorption of OTC over the MIL-101s. Precisely, adsorption equilibrium for OTC on MIL-101(HCl) is reached earlier than that on MIL-101(HF), due to the much smaller particles and larger exterior surface. The equilibrium quantity of OTC adsorbed on MIL-101(HCl) nearly doubles that on MIL-101(HF). The trend of the adsorption amount is consistent with the surface areas of adsorbents, revealing that the adsorbent textural parameters play a critical role in the adsorption process as reported previously in dye adsorption on MOFs materials.³⁸

Fig. 6 Adsorption equilibrium curves of OTC on MIL-101(HCl) and MIL-101(HF).

To further study quantitatively the reaction kinetics, the changes of adsorption amount with time are analyzed with two common kinetics models which has been widely applied to the adsorption of pollutants from aqueous solutions: the pseudo first-order equation as eqn (2)³⁹ and the pseudo second-order equation as eqn (3).³¹

ln(q_e − q_t) = ln q_e − k₁t (2)

where q_t and q_e are the uptake capacity (mg g⁻¹) at the adsorption time t (min) and the equilibrium time, respectively; k₁ (min⁻¹) represents the rate constant for the pseudo first order equation, and k₂ (g mg⁻¹ min⁻¹) is the symbol of the rate constant for the pseudo second-order equation.

The regression fitting of the kinetic data is fitted by the pseudo first-order and pseudo second-order model. As shown in Table 2, the results modeled by the pseudo second-order kinetic model presents a highly linear relation because R² values are all larger than 0.995, and the values from fitting results are approximately close to adsorption amounts of obtained from experimental results for two MIL-101s. While the pseudo first-order kinetics model does not give a good fit to the experimental data, because the calculated values of q_e deviate largely from the results of the experiment and a poor linear relation for two MIL-101s. Therefore, MIL-101(HCl) is a more effective adsorbent for OTC removal in the viewpoint of adsorption amount.

Table 2 Parameters of kinetics for oxytetracycline adsorption on MIL-101(HCl) and MIL-101(HF)

Samples	qe,exp	Pseudo first-order model			Pseudo second-order model
		qe	k1	R^2	qe	k2	R^2
MIL-101(HCl)	39.88	72.56	0.1526	0.9408	43.10	0.0034	0.9956
MIL-101(HF)	22.40	10.99	0.0518	0.8307	23.09	0.0117	0.9993

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3.3. Adsorption isotherms

The adsorption isotherm experiments were conducted at different temperatures ranging from 5 to 35 °C in various initial OTC concentration shown in Fig. 7. The adsorption quantity of the OTC on MIL-101s drastically increase with the increasing concentration in the experimental initial concentration range. This phenomenon is attributed to the limited adsorption sites on the specified adsorbent as aforementioned. Moreover, it can be seen that the process of enhancing the temperature resulted in a lower amount of adsorbed OTC confirming that the adsorption of OTC antibiotic on MIL-101 adsorbents is an exothermic reaction and lowering the temperature will benefit the adsorption of OTC.

Fig. 7 (A) Adsorption isotherms of OTC on MIL-101(HCl) at 15, 25, 35 and 45 °C; (B) adsorption isotherms of OTC on MIL-101(HF) at 15, 25, 35 and 45 °C.

In order to understand the relationship in the adsorption processes, two typical adsorption isotherms models, Langmuir adsorption isotherm and Freundlich adsorption isotherm, are employed to evaluate the equilibrium adsorption capacity.

The Langmuir adsorption isotherm is recognized as one of the most employed methods to quantify and compare the performance of various adsorbents with predication of maximal solute uptake which assumes that the surface of the adsorbent is homogeneous and has only one type of binding site.⁴⁰ The Langmuir isotherm equation can be expressed as follows:

where q_∞ and k_L are the Langmuir constants representing maximal adsorption capacity and bonding energy constant, respectively, and C_e is the equilibrium concentration of OTC. q_e (mg g⁻¹) is the amount of the OTC adsorbed at the equilibrium.

The Freundlich isotherm, on the other hand, represents an empirical expression assuming that the surface of the adsorbent is heterogeneous and the adsorption capacity depends upon the concentration of the adsorbate.⁴¹ The Freundlich isotherm can be described as follows:

where k_F and n are the Freundlich constants related to the adsorption capacity (mg g⁻¹) and intensity respectively.

The applicability of the isotherm models to adsorption behavior is determined by the correlation coefficient (R²) summarized in Table 3.

Table 3 Langmuir and Freundlich model constants

Samples	T/°C	Langmuir			Freundlich
		q∞	kL	R^2	n	kF	R^2
MIL-101(HCl)	5	115.34030	0.005555	0.98774	0.7722	1.38676	0.99396
	15	54.64481	0.014014	0.97114	0.73432	1.10390	0.99042
	25	44.76275	0.012778	0.97384	0.72198	1.00371	0.99004
	35	31.02699	0.015895	0.98432	0.51291	1.19448	0.99337
MIL-101(HF)	5	91.57510	0.009080	0.98980	0.80498	1.08928	0.99648
	15	58.54801	0.011181	0.98776	0.74424	1.04825	0.99472
	25	21.58895	0.022972	0.97417	0.56438	1.08725	0.99488
	35	20.563438	0.026689	0.97930	0.51291	1.19448	0.99337

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As shown in Table 3, within a temperature range from 5 to 35 °C, the Langmuir and Freundlich models all fit the data well. For Langmuir model, the MIL-101(HCl) shows a higher amount of the OTC adsorbed than MIL-101(HF) in the tested temperature which is consistent with the kinetics results. The calculated adsorption capacity of OTC on MIL-101(HCl) can reach a maximum of 115.34 mg g⁻¹, which is higher than some reported adsorbents, such as: activated sludge (90.9 mg g⁻¹)⁴² and ZIF-8 (28.3 mg g⁻¹).⁴³ The factors leading such a removal capacity may be greatly attributed to benzene rings from H₂BDC and unsaturated metal centers embedded in the MIL-101 framework.⁴⁴ The benzene rings in MIL-101 can attract hydrophobic aromatic OTC due to a π–π interaction between the aromatic rings and the benzene rings in the organic part of the MOFs, which is reported in the previously study.⁴⁵ In addition, the OTC structure exists predominantly as a zwitterion with a positive charge on the tertiary amine functionality and a negative charge on the deprotonated hydroxyl group.⁴⁶ The tertiary amine group is weak alkalinity due to the electronegativity of the nitrogen atom, while the coordinatively unsaturated Cr³⁺ sites endow MIL-101 with Lewis acid character.⁴⁷ An acid–alkaline interaction between Lewis open metal sites and the tertiary amine group may also help explain the adsorption mechanism.

4. Conclusion

In summary, by using the HCl mineralizer, a higher crystalline, smaller particle size and larger specific surface area MIL-101 framework is prepared in comparison with the conventional HF mineralizer. The kinetics and thermodynamics experiments indicate that the MIL-101 (HCl and HF) exhibit remarkably fast OTC adsorption kinetics in water solution. The high specific surface area for MIL-101(HCl) improves adsorption kinetics and capacity in comparison with MIL-101(HF). The mechanism is likely to be ascribed to π–π interaction, acid–alkaline interaction. Such outstanding adsorption performance makes MIL-101(HCl) a promising adsorbent candidate to adsorb and remove OTC from wastewater in the viewpoint of adsorption amount and rate.

Acknowledgements

Financial supports for this work by National Nature Science Foundation of China (No. 2014FB129, 51104075, 51364023 and 31160146).

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