Facile Sonochemical Synthesis of Flexible Fe-Based Metal-Organic Frameworks and Their Efficient Removal of Organic Contaminants from Aqueous Solutions.

An iron-based metal-organic framework, MIL-53(Fe), was synthesized via the simple sonochemical method, which is a facial and fast strategy, and their adsorption performance for organic contaminants removal from aqueous solutions was studied. The crystal structure and morphology analysis indicate that the sonochemical synthesis of MIL-53(Fe) particles was faster than the solvothermal preparation method, showing high crystallinity with a downsized hexagonal bipyramid shape. Furthermore, the prepared MIL-53(Fe) exhibited enhanced adsorption capability for the organic dyes compared to metal-organic framework prepared via the solvothermal method and showed excellent maximum adsorption capability for the methyl orange removal from aqueous solutions. Based on the superior adsorption properties and facile synthesis, MIL-53(Fe) prepared by ultrasound irradiation has a potential application for an efficient, economic, and ecofriendly wastewater purification process.

Introduction

Recently, the discharge of textile wastewater effluent containing dyes and endocrine-disrupting chemicals (EDCs) has become a social and environmental issue because such chemicals are detrimental to the environment and living organisms.[1] For instance, dyes such as methyl orange (MO) and methylene blue (MB) are challenging to chemically degrade and are potentially carcinogenic in nature. MO is a widely used anionic dye in textiles, printing, papers, and food.[2] MB is mostly used in cationic dyes for wood, silk, and cotton.[3] EDCs may cause developmental and reproductive disorders in humans and wildlife, through their endocrine-disrupting effects.[4,5] Especially, bisphenol A (BPA) is one of the highest production and consumption volume chemicals in the world; however, it is a known EDC that is acutely toxic to the aquatic organisms in the 1000–10,000 mg L–1 range for both fresh water and marine species.[6] Owing to their toxicities, the removal of MO, MB, and BPA from wastewater is considered vital for environmental sustainability in the field of water treatment.

Adsorption methods have been widely used to remove organic contaminants from aqueous environments because of the simplicity of the equipment and their ease of operation.[7−10] Metal–organic frameworks (MOFs) have been investigated as promising adsorbents of hazardous contaminants in water because of the well-defined pore structure of MOFs.[11] MOFs are porous crystalline hybrids consisting of metal ions connected by organic linkers that exhibit the advantages of both inorganic and organic porous solids. They have drawn interest because of the ease of tunability of their morphology, pore structure, and chemical properties via changes in the connectivity of the metal ions and the nature of organic linkers.[12]

Among the various demonstrated MOFs, the MIL-53 family has the advantages of high chemical stability and wide chemical versatility.[13] Specifically, MIL-53(Fe), that is, iron-containing MIL-53, is known to have the advantage of biocompatibility and low cost of production.[14] MIL-53(Fe) has an insufficient surface area because the presence of guest molecules only opens its pores. Fascinatingly, it can be swelled and used to capture guest molecules from aqueous solutions because of its flexible structure.[15] Horcajada et al. reported that MIL-53(Fe) could be swelled and used to deliver drug molecules with the size of ca. 5 nm.[16] There are some issues with the preparation method of MIL-53(Fe). The majority of MIL-53(Fe) has been synthesized via solvothermal methods that involve long reaction times and high reaction temperatures, which may result in low productivity.[17−19] Furthermore, toxic mineralizing agents, such as hydrofluoric acid, are widely used to synthesize MIL-53(Fe) creating issues with environmental pollutant effluents.[15,20] Thus, developing a faster, greener, and cost-effective synthetic method for MIL-53(Fe) with enhanced adsorption performance is required.

To prepare an economical and ecofriendly MIL-53(Fe), sonochemical synthesis may be among the best candidates as a synthesis method. The acoustic cavitation resulting from ultrasound (UTS) irradiation facilitates accelerated reactions and the homogeneous nucleation of MIL-53(Fe), derived from the generation of high temperatures (> 4,700 °C), pressures (> 2000 atm), and rate of temperature change (> 1011 °C s–1) because of the violent collapse of cavitation bubbles.[21] From an economical and environmental perspective, UTS irradiation is also advantageous because it could be utilized to facilely synthesize MOFs without using toxic mineralizing agents.[22] In addition, downsizing of MOFs often provides enhanced physical and chemical properties because of their controllable diffusion kinetics and morphologies.[23] Localized high temperature and pressure by UTS irradiation promote enhanced crystallization kinetics, controlled shape, and phase selectivity.[24] The size of MOFs can be reduced via UTS irradiation with improved nucleation. It is reported that the flexibility of the MOF increased by the reduction of the particle size.[25] We expected that MIL-53(Fe) prepared via UTS irradiation will show enhanced adsorption performance for organic pollutants in an aqueous solution.

This study focused on the sonochemical synthesis of MIL-53(Fe) and its application in adsorption for dyes and EDCs from aqueous solutions. We successfully obtained dodecahedral MIL-53(Fe) by UTS irradiation, facilitating fast production. In addition, the adsorption efficiency of synthesized MIL-53(Fe) was investigated. To the best of our knowledge, sonochemically synthesized MIL-53(Fe) shows superior adsorption performance for the MO removal in an aqueous solution than previously reported adsorbents.

Materials and Methods

Materials

Terephthalic acid (98%), iron(III) chloride hexahydrate (98%), MO (85%), MB (97%), BPA (99%), cetyltrimethylammonium bromide (CTAB, 99%), pluronic F-127 (BioReagent), and dimethylformamide (DMF, 99.8%) were obtained from Sigma Aldrich.

Preparation of MIL-53(Fe)

The MIL-53(Fe)_UTS was prepared following a modified method of a previously described procedure.[12] Terephthalic acid (6 mmol) was dissolved in 30 mL of DMF. Thereafter, an equivalent amount of iron(III) chloride hexahydrate (6 mmol) was dissolved in the terephthalic acid/DMF solution. The solution was then transferred to a reactor (reactor volume: 50 mL) that was equipped with an ultrasonic generator probe (VCX 750, Sonics & Materials, Inc.). This experiment was performed at 33% of maximum power for 2 h without temperature control (final temperature: ca. 70 °C). The product, denoted as MIL-53(Fe)_UTS-1, was filtered and washed three times with DMF and ethanol. Finally, the product was vacuum-dried at 100 °C overnight. When the amount of terephthalic acid was changed to 3 mmol, the prepared sample was referred to as MIL-53(Fe)_UTS-2. To compare the adsorptive properties, the MIL-53(Fe) series was solvothermally synthesized (150 °C, various reaction times) under identical reactant concentrations; in this case, the samples are referred to as MIL-53(Fe)_solvo-1-t and MIL-53(Fe)_solvo-2-t.

Characterization

The X-ray diffraction (XRD) pattern was recorded with a New D8 Advance X-ray diffractometer (Bruker, USA) using Cu Kα radiation. Fourier-transform infrared (FT-IR) spectra were acquired on a Nicolet iS10 IR spectrophotometer (Thermo Fisher Scientific, USA) with the sample in potassium bromide pellets. Thermogravimetric analysis (TGA) was conducted using a Q500 system (TA Instruments, USA) at a scan rate of 5 °C min–1 under an air atmosphere. The nitrogen adsorption–desorption isotherm was obtained using a 3Flex surface characterization analyzer (Micromeritics, USA) at −196 °C. The sample was vacuum-dried at 100 °C overnight prior to analysis. Field-emission scanning electron microscopy (FE-SEM) photographs were obtained on a SUPRA 55VP field-emission scanning electron microscope (Zeiss, Germany). Zeta potential was evaluated using a Z-1000 electrophoretic light scattering spectrophotometer (ELS, Otsuka Electronics, Japan).

Batch Adsorption Experiments

MIL-53(Fe) (5 mg) was added to an MO, MB, and BPA solution (20 mL) at a specific pH (usually 5.6) and then stirred at 30 °C for different durations. The concentrations of MO, MB, and BPA were determined using an ultraviolet–visible spectrophotometer at wavelengths of 464, 665, and 276 nm, respectively. The adsorption capacity was obtained according to the following equation:where qe (mg g–1) is the equilibrium adsorption capacity of MIL-53(Fe), Ci (mg L–1) is the initial concentrations of contaminants, Ce (mg L–1) is the equilibrium concentrations of contaminants, V (mL) is the volume of the solution, and M (mg) is the weight of MIL-53(Fe).

The adsorption kinetic models to explain the adsorption kinetics of MO, MB, and BPA on the MIL-53(Fe)_UTS include the pseudo-first-order (PFO) and pseudo-second-order (PSO) models. These two kinetic equations are as follows:where qt (mg g–1) is the adsorption capacity of MIL-53(Fe) at time t, and k1 (min–1) and k2 (g mg–1 min–1) are the PFO rate constant and the PSO rate constant, respectively.

The adsorption isotherm models to explain the adsorption isotherms of MO, MB, and BPA on the MIL-53(Fe)_UTS include the Freundlich and Langmuir isotherm models. These two isotherm equations are as follows:where KF ((mg g–1) (L mg–1)) is the Freundlich constant, n (unitless) is the heterogeneity factor, KL (L mg–1) is the Langmuir constant, and qL (mg g–1) is the maximum adsorption capacity of MIL-53(Fe)_UTS. The procedure followed to evaluate the adsorption kinetics and isotherm is described in detail in the Supporting Information.

Simultaneous Adsorption Experiments

The selective adsorption performance of MIL-53(Fe) toward target contaminants was evaluated by dispersing 5 mg of the adsorbents into 20 mL of the aqueous solutions at a specific pH (5.6), with a concentration of 25 mg L–1 MO, 25 mg L–1 MB, 25 mg L–1 BPA, 25 mg L–1 F127, and 25 mg L–1 CTAB.

The effect of other contaminants on the adsorption of MO, MB, and BPA on MIL-53(Fe) was determined using eq :where Rq,i (unitless) is the ratio of adsorption capacities for the contaminant "i" at equilibrium; qm,i and qs,i (mg g–1) are the adsorption capacities for the contaminant i in the multicomponent system and the single-component system for the same initial concentration, respectively.

Results and Discussion

Optimal Conditions To Prepare MIL 53(Fe)

As shown in Figure , despite the absence of mineralizing agents in the synthesis process, the pattern of MIL-53(Fe)_UTS exhibits high intensities, indicating the high crystallinity of the sample regardless of the change in the molar ratio. The XRD pattern of MIL-53(Fe)_UTS showed the strong diffraction peaks (200), (110), and (1 1 −1), which was consistent with those of previously reported MIL-53(Fe).[12,21] For MIL-53(Fe)_solvo, all the samples exhibit a crystalline diffraction pattern except for MIL-53(Fe)_solvo-1_16h. Only MIL-53(Fe)_solvo-1_24h showed a similar pattern to MIL-53(Fe)_UTS. The XRD patterns for MIL-53(Fe) can differ depending on pore opening because MIL-53(Fe) exhibits a breathing behavior. We consider that MIL-53(Fe)_UTS and MIL-53(Fe)_solvo-1_24h include an open pore structure; however, other MIL-53(Fe)_solvo samples exhibit a close pore structure.[16] According to the Debye–Scherrer equation, broad and weak peaks in the XRD pattern for MIL-53(Fe)_solvo indicate that the crystallite size of MIL-53(Fe)_solvo samples are smaller than those of MIL-53(Fe)_UTS. These results suggest that UTS irradiation can accelerate the nucleation and growth of MIL-53(Fe) despite a much shorter reaction time compared to the solvothermal method.

Figure 1

XRD patterns for (a) MIL-53(Fe)_UTS, (b) MIL-53(Fe)_solvo-1, and (c) MIL-53(Fe)_solvo-2, and the pink bars were stimulated based on the corresponding CheckCIF file of MIL-53(Fe).[26]

The adsorption capacities of the MIL-53(Fe)_UTS were measured to verify the optimal preparation condition for adsorption ability. The adsorption measurement was conducted for 840 min in a solution with initial MO, MB, and BPA concentrations of 200, 200, and 100 mg L–1, respectively. MIL-53(Fe)_UTS-2 exhibited higher adsorption capacities for MO, MB, and BPA (494.2, 221.9, and 160.1 mg g–1, respectively) than MIL-53(Fe)_UTS-1 (Figure ). Based on these results, MIL-53(Fe)_UTS-2 and MIL-53(Fe)_solvo-1_24h were employed in further analyses to document their structural properties and adsorption behavior.

Figure 2

Adsorption capacities for MIL-53(Fe)_UTS.

Comparison of MIL-53(Fe)_UTS and MIL-53(Fe)_solvo

Figure a shows that all FT-IR spectra contain IR bands that are characteristic of the MIL-53(Fe) structure. The characteristic band at ∼1685 cm–1 corresponds to the C=O stretching vibration of terephthalic acid, and the band at ∼750 cm–1 indicates the C–H bending vibration of the benzene ring. The IR bands at 3450, 2940, 1597, 1393, and 540 cm–1 are associated with the O–H stretching vibration, aromatic C–H stretching vibration, asymmetric and symmetric COO– stretching vibrations of terephthalate, and Fe–O stretching vibration, respectively. These bands suggest that the iron ions were linked to the terephthalate molecules in MIL-53(Fe)_UTS as well as MIL-53(Fe)_solvo.[27]

Figure 3

(a) FT-IR spectra, (b) TGA curves, (c) N2 adsorption–desorption isotherms, (d) HK pore size distributions (for micropores), and (e) BJH pore size distributions (for mesopores and macropores) of MIL-53(Fe)_UTS and MIL-53(Fe)_solvo.

The TGA curves of MIL-53(Fe) indicated that the first weight loss below 250 °C resulted from the evaporation of the solvent from the framework (Figure b). The second weight loss, which occurred between 250 and 500 °C, was associated with the decomposition of MIL-53(Fe), resulting in an iron oxide-carbon composite.[28] When the temperature reached 500 °C, this phase transformed to a pure iron oxide phase, and above 500 °C, the weight of the material was relatively stable.

N2 adsorption–desorption isotherms (Figure c) were measured to obtain specific surface areas and pore size distributions (Table , Figure d,e of MIL-53(Fe). The adsorptive behaviors of all MIL-53(Fe)s might be the combination of Type I and Type II with H3 hysteresis, suggesting the existence of micropores, mesopores, and macropores in their structure.[29]Table indicates that the increase in the pore size might have resulted from the defect generation caused by UTS irradiation, although an inverse relationship is noted for the specific surface area by pore extension.[30,31] The meso- and macropore sizes, acquired using the Barrett–Joyner–Halenda (BJH) method, were significantly different between samples, while the micropore size, determined using the Horvath–Kawazoe (HK) method, was almost similar. The increase in the pore size can be attributed to the absence of an organic linker between each ferric ion.

Table 1

Specific Surface Area and Average Pore Size of MIL-53(Fe)

sample	specific surface area (m2 g–1)	HK pore size (nm)	BJH pore size (nm)
MIL-53(Fe)_UTS	5.01	1.01	32.91
MIL-53(Fe)_solvo	7.81	0.95	5.45

As shown in Figure , MIL-53(Fe)_solvo exhibited a clear hexagonal bipyramid morphology with a larger size (0.6–2.6 μm) because of the slow nucleation and sufficient growth occurring during the long reaction time (24 h). However, MIL-53(Fe)_UTS showed a slightly ambiguous morphology with a smaller size (0.4–1.8 μm) because of faster nucleation and insufficient growth during a shorter reaction time (2 h), which corresponds with previous results.[21]

Figure 4

FESEM photographs and size distribution for MIL-53(Fe)_UTS and MIL-53(Fe)_solvo.

The adsorption capacities of MIL-53(Fe)_UTS and MIL-53(Fe)_solvo were compared through normalization by the specific surface area to identify the intrinsic affinity of MIL-53(Fe) toward organic contaminants. Adsorption was measured for 840 min in a solution with initial MO, MB, and BPA concentrations of 200, 200, and 100 mg L–1, respectively. The MIL-53(Fe)_UTS had much higher MO, MB, and BPA adsorption capacities (98.6, 44.3, and 32.0 mg m–2, respectively) than MIL-53(Fe)_solvo (Figure ). It is hypothesized that the enhanced adsorptive properties of MIL-53(Fe)_UTS appear to be due to the reduced size of particles generated by UTS irradiation with the increased flexibility of frameworks.

Figure 5

Normalized adsorption capacities for MIL-53(Fe)_UTS and MIL-53(Fe)_solvo.

Adsorption Behaviors of MIL-53(Fe)_UTS in the Single-Component System

Figure a displays the relationship between the adsorption of MIL-53(Fe)_UTS for MO, MB, and BPA at C0 (200, 200, and 100 mg L–1, respectively) and a time period from 0 to 840 min. The MO, MB, and BPA adsorption increased rapidly and attained equilibrium after 10 min. The initial rapid adsorption was attributed to an instantaneous adsorption process arising from abundant vacant adsorption surface sites.[32] The adsorption kinetics of MO, MB, and BPA on MIL-53(Fe) _UTS were analyzed with PFO and PSO kinetic models. Table indicates that a PSO kinetic model can fit MO, MB, and BPA adsorption. The adsorption kinetics of MIL-53(Fe) _UTS slowed in order from BPA to MB to MO according to their PSO rate constants and initial adsorption rates (h = k2 × qe2). This could be attributed to properties (e.g., mass transfer rate and diffusivity) resulting from the molecular size and molar mass.[33] The molecular sizes and molar masses of MB (1.41 × 0.60 × 0.38 nm3 and 319.9 g mol–1) and MO (1.48 × 0.60 × 0.45 nm3 and 327.3 g mol–1) are higher than those of BPA (1.24 × 0.54 × 0.44 nm3 and 228.3 g mol–1), which could result in slower adsorption of MB and MO because molecules with a large molecular size and high molar mass seldom diffuse through the boundary layer near the adsorbent.[6,34]

Figure 6

(a) Adsorption kinetics and (b) adsorption isotherms of MIL-53(Fe)_UTS for MO, MB, and BPA.

Table 2

Adsorption Kinetic Parameters for MIL-53(Fe)_UTS

adsorbate	pseudo-first-order kinetic			pseudo-second-order kinetic
	qe	k1	R2	qe	k2	h	R2
MO	483.5	0.0884	0.9951	510.5	0.0003	78.2	0.9957
MB	194.3	0.1906	0.9560	199.7	0.0023	91.7	0.9629
BPA	150.8	0.2743	0.9929	152.7	0.0071	165.6	0.9951

Figure b displays the relationship between the adsorption of MIL-53(Fe)_UTS for MO, MB, and BPA for 840 min and at various C0 (1000, 200, and 200 mg L–1, respectively). The adsorption capacities of MIL-53(Fe)_UTS for MO, MB, and BPA increased rapidly at lower initial concentrations and then continued to increase gradually at higher initial concentrations. This phenomenon was attributed to a limited number of adsorption sites on the adsorbent.[35] The adsorption behavior analysis of MIL-53(Fe)_UTS was conducted by employing the Freundlich and Langmuir isotherm models. The resulting correlation coefficients indicated that the Freundlich model approximated the adsorption of MO on MIL-53(Fe)_UTS and the Langmuir model approximated the adsorption of MB and BPA (Table ). Therefore, the most appropriate fitting for each sample is plotted. The type of adsorption for MO can be predicted according to the Freundlich constant n, which describes whether the adsorption process is favorable (0 < 1/n < 1) or not.[36] The n values for the adsorption of MO by MIL-53(Fe)_UTS showed that the adsorption was favorable. Furthermore, the Freundlich model illustrated that MO was adsorbed onto MIL-53(Fe)_UTS to form a multilayer, suggesting that several adsorption interactions existed between MIL-53(Fe)_UTS and MO.[37] The zeta potential of MIL-53(Fe)_UTS was evaluated to explain why MIL-53(Fe)_UTS exhibited a superior MO adsorption capacity (> 1,200 mg g–1). Figure S1 shows that the MIL-53(Fe)_UTS had a surface charge of approximately 20.75 mV at a pH value of 5.6, which implied that there were electrostatic attractions between positively charged MIL-53(Fe)_UTS and negatively charged MO molecules. This positive surface charge was higher than that of MIL-53(Fe)_solvo (approximately 0 mV at a pH value of 5.6), as shown in Figure S2. Therefore, several MO adsorption mechanisms were proposed: (1) π–π interaction between the MO benzene ring and organic linkers within MIL-53(Fe); (2) electrostatic attraction between MIL-53(Fe) and MO anions.[38,39]

Table 3

Adsorption Isotherm Parameters for MIL-53(Fe)_UTS

adsorbate	Freundlich isotherm			Langmuir isotherm
	KF	n	R2	qL	KL	R2
MO	137.7	3.0	0.9551	1,485.3	0.0064	0.8700
MB	34.1	2.6	0.9592	240.9	0.0522	0.9758
BPA	26.1	2.5	0.8970	222.4	0.0537	0.9928

In the case of MB and BPA, the type of adsorption can be categorized using the equilibrium parameter RL, which is defined as follows:[5]

The adsorption process can be categorized using the RL value as unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0). The RL values showed that MIL-53(Fe)_UTS favorably adsorbed MB (0.1608–0.7540) and BPA (0.0931–0.7487).

The Langmuir model indicated that MB and BPA formed monolayers on MIL-53(Fe)_UTS.[40] The maximum adsorption capacities for MB and BPA were estimated using the Langmuir model to be 240.9 mg g–1 (0.8 mmol g–1) and 222.4 mg g–1 (1.0 mmol g–1), respectively. The different adsorption performances for each molecule could be explained in terms of different interactions between MOF surfaces and adsorbed molecules. BPA molecules are easily adsorbed with π–π interactions onto the surface of MIL-53(Fe)_UTS, while the adsorption of MB was hindered by electrostatic repulsion between MB cations and positively charged MIL-53(Fe)_UTS. Adsorption mechanisms of MIL-53(Fe)_UTS were proposed (Figure ).

Figure 7

Scheme for the adsorption of MIL-53(Fe)_UTS for MO, MB, and BPA.

The maximum adsorption capacities, rate constants, and synthetic conditions of MIL-53(Fe)_UTS were compared with those in previous reports using various adsorbents to confirm the potential of MIL-53(Fe)_UTS as an adsorbent toward dyes and EDCs.[2,41−52] As shown in Table , the maximum adsorption capacity of MIL-53(Fe)_UTS for MO was significantly higher than those of other previously studied adsorbents. Regarding the removal of MB, only MIL-100(Fe) and the alkali-activated multiwalled carbon nanotube (MWCNT) exhibited higher adsorption capacity than MIL-53(Fe)_UTS. However, its adsorption rate and synthetic conditions were slower, toxic, and tedious.[43,52] For the removal of BPA, most adsorbents exhibited higher adsorption capacity than MIL-53(Fe)_UTS. However, the synthetic conditions of these adsorbents included toxic metal ions such as chromium, hazardous agents such as hydrofluoric acid, high reaction temperatures, and long reaction times.[41,42,49−51] MIL-53(Fe)_UTS had the advantages of short reaction times (2 h), mild reaction temperature (< 100 °C), and no usage of toxic and hazardous agents in the synthesis procedure. UTS irradiation can solve these as-forementioned problems of conventional MOF preparation. Considering the concerns involved with DMF, minimizing usage or no usage of DMF as the solvent is required for further research using UTS irradiation because DMF is also a recognized carcinogen, harmful to the environment and easily causes secondary pollution.[53,54]

Table 4

Comparison of the Maximum Adsorption Capacities (mg g–1)/Rate Constants (g mg–1 min–1) and Synthetic Conditions of Various Adsorbents

adsorbent	MO	MB	BPA	synthetic/experimental pH conditions	ref
mesostructured MIL-53(Al)			465/0.0007	F127, DMF, 130 °C for 2 d/pH 6.5	(41)
MIL-101(Cr)			253/0.0139	HF, NH4F, DMF, 220 °C for 8 h/pH 5.6	(42)
MIL-100(Fe)	1045/0.0002	736/0.0001		HF, HNO3, 150 °C for 4 d/pH 5.0	(43)
MOF-235(Fe)	477/0.0009	187/0.0002		DMF, 80 °C for 1 d/pH 5.6	(44)
Ce(III)-doped UiO-66(Zr)	640/0.0184	145/0.0207		DMF, 120 °C for 1 d/pH 7.0	(45)
mesostructured MIL-101(Cr)	208/0.0030	22/0.0490		DMF, CTAB, NaAc, 220 °C for 8 h/n.a.	(2)
PED-MIL-101(Cr)	194/0.0030			HF, NH4F, 220 °C for 8 h, additional steps for functionalization/pH 5.6	(46)
commercial MIL-53(Al)	177/0.0570			n.a./pH 6.0	(47)
HKUST-1/GO		183.5/n.a.		120 °C for 12 h, Hummer method for GO preparation/n.a.	(48)
MIL-53(Cr)			421.0/0.0019	175 °C for 7 h, microwave irradiation/pH 6.3	(49)
MIL-101(Cr)- (OH)3			97/n.a.	HF, NH4F, DMF, 220 °C for 8 h, additional steps for functionalization/pH 7.0	(50)
argan nut shell-microporous-carbon			1408/0.0040	1. heating from RT to 500 °C (5 °C min–1) and maintaining for 1 h	(51)
				2. after cooling, heating from RT to 800 °C (10 °C min–1) and maintaining for 2 h/pH 6.5
alkali-activated MWCNT	149/0.0020	400/0.0007		1. preparing MWCNT by catalytic chemical vapor deposition method	(52)
				2. after mixing with KOH, heating to 750 °C for 1 h/pH 7.0
MIL-53(Fe)_UTS	1485/0.0003	241/0.0023	222/0.0071	DMF, ultrasonic irradiation, 2 h/pH 5.6	this study

To gain a deeper understanding of the structure and the pH effect of the MIL-53(Fe)_UTS in aqueous solutions, the MIL-53(Fe)_UTS was soaked in deionized water (pH 4, 5.6, and 10) for 65 h. All XRD patterns were identical, indicating that the MIL-53(Fe)_UTS was stable in the pH range of 4–10. New Bragg maxima appeared in the XRD pattern compared to the as-synthesized MIL-53(Fe)_UTS, showing a partial structural transformation of the MIL-53(Fe)_UTS in the aqueous solution (Figure S3). In general, as the synthesis of the MIL-53(Fe) proceeds in DMF, and the as-synthesized MIL-53(Fe) retains DMF molecules in its pore structure (MIL-53(Fe)[DMF] form). If sufficient solvent removal or exchange of the as-synthesized MIL-53(Fe) does not occur, the MIL-53(Fe) still retains DMF molecules.[12,21,26] On the contrary, if sufficient solvent exchange occurs (exchange times >12 h), the MIL-53(Fe) can retain H2O molecules instead of DMF molecules (MIL-53(Fe)[H2O] form).[15,55−57] Based on the XRD results and the presence or absence of solvent exchange in the process, we can conclude that the MIL-53(Fe)_UTS retained DMF molecules (MIL-53(Fe)[DMF] form) and that the MIL-53(Fe)_UTS_H2O retained H2O molecules (MIL-53(Fe)[H2O] form) in their pore structures. Several studies have reported on the structure of the MIL-53(Fe)_solvo obtained via XRD analyses. However, to the best of the authors’ knowledge, none of the previous studies have focused on the structure of the MIL-53(Fe)_UTS using XRD analyses. Therefore, we first indexed the XRD results and then extracted the lattice parameters of the MIL-53(Fe)_UTS using TREOR90 (Tables S1 and S2). The lattice parameters of the MIL-53(Fe)_UTS_H2O were estimated to be lower than those of the MIL-53(Fe)_UTS, which was in accordance with the results reported by Horcajada et al.[16] and Loiseau et al.[58] The −OH–O hydrogen bonds with the terminal H2O molecules on the iron octahedrons or the oxygen atoms of the bridging carboxylate groups might be attributed to the lattice contraction of the MIL-53(Fe)_UTS. The subnanometer lattice parameter of the MIL-53(Fe)_UTS-H2O can hinder the diffusion of large-sized organic molecules, imposing size selectivity for subnanometer-sized organic contaminants.

The pH effect evaluation on the adsorption (initial concentration of 100 mg L–1) and surface charge was also carried out. As shown in Figure S4, the adsorption of MO, MB, and BPA depends on the pH value of the aqueous solution. At acidic conditions, a high electrostatic attraction exists between the positively charged surface of the MIL-53(Fe)_UTS and the anionic dye MO. As the pH of the system increases, the number of positively charged sites decreases, lowering the adsorption of anionic dye because of decreased electrostatic attraction. However, the adsorption of the cationic dye showed the opposite trend with increasing pH. The adsorption of neutral EDCs was observed to be almost constant in the pH range of 4–10.

Simultaneous Adsorption of MIL-53(Fe)_UTS in the Multicomponent System

In the textile industry, surfactants are used to improve the binding between dyes and fabric.[59] Therefore, it is required to evaluate the simultaneous adsorption of MIL-53(Fe)_UTS in multicomponent systems. We select CTAB as an ionic model surfactant and F127 as a nonionic model surfactant. Table lists the results of the simultaneous adsorption of MO, MB, and BPA on MIL-53(Fe)_UTS. The types of simultaneous adsorption can be categorized as synergism (Rq > 1), antagonism (Rq < 1), and noninteraction (Rq = 1) according to the literature.[60]

Table 5

Ratios of Adsorption Capacities of MIL-53(Fe)_UTS

adsorbent	Rq,MO	Rq,MB	Rq,BPA
MIL-53(Fe)_UTS	0.95	0.45	0.46

MIL-53(Fe)_UTS exhibits an Rq,MO of 0.95, indicating that the MO adsorption capacity of MIL-53(Fe)_UTS remains nearly unaffected by the presence of MB, BPA, and coexisting organic substances such as F127 and CTAB in multicomponent systems. This phenomenon is in agreement with the adsorption behaviors of MIL-53(Fe)_UTS in a single-component system because the interactions of MIL-53(Fe) with MO were stronger than those with MB and BPA. Moreover, this phenomenon illustrates that coexisting organic substances cannot enter the pore and competitively interact with active sites in MIL-53(Fe)_UTS because F127 and CTAB have high molecular weights of 12,600 and 364.5 g mol–1, respectively.

Considering the sufficient adsorptive property and greener preparation method of MIL-53(Fe)_UTS, it could be regarded as an economical, ecofriendly, and efficient adsorbent for the treatment of wastewater.

Conclusions

In this study, we experimentally confirmed that the sonochemical synthesis of the flexible MIL-53(Fe) MOF is faster than the conventional solvothermal method, and the size of the MIL-53(Fe) particles prepared via UTS irradiation was reduced. Furthermore, the prepared MIL-53(Fe)_UTS particles exhibited enhanced adsorption performance for organic pigments compared to MIL_53(Fe) in aqueous solutions. These results suggested that the particle size of the flexile MOF influences the adsorption capacity toward organic compounds. Anionic MO was more efficiently adsorbed by MIL-53(Fe)_UTS than BPA and MB molecules because of the assistance of π–π interactions and electrostatic attraction. Fortunately, the maximum adsorption capacity of MIL-53(Fe)_UTS for MO was significantly higher than those of other previously reported adsorbents. MIL-53(Fe)_UTS had advantages over other MOF-based adsorbents in that it had high adsorption capacities and could be synthesized using facile and cost-effective methods without the use of toxic chemicals during synthesis. Flexible MIL-53(Fe)_UTS could be considered as an efficient, economical, and ecofriendly adsorbent for the purification of contaminated water on an industrial scale.