Synthesis of a bifunctional metal-organic framework membrane for the removal of anionic and cationic heavy metals from wastewater

doi:10.21203/rs.3.rs-2008163/v1

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Synthesis of a bifunctional metal-organic framework membrane for the removal of anionic and cationic heavy metals from wastewater

https://doi.org/10.21203/rs.3.rs-2008163/v1

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A novel adsorption composite composed of metal-organic frameworks with bifunctional groups of -NH₂ and -COOH [UIO-66-NH₂(-COOH)] loaded onto the fiberglass membrane labelled as F-MOFs-M was fabricated by a two-step method. This surface-modified composite could simultaneously capture both anionic and cationic heavy metal ions from wastewater, and the adsorption priority followed an order of SbO₃⁻, CrO₄²⁻, AsO₄³⁻, Pb²⁺, and Ni²⁺. The adsorption capacity for AsO₄³⁻, CrO₄²⁻, SbO₃⁻, Ni²⁺, and Pb²⁺ was 55 mg·g^− 1, 189 mg·g^− 1, 342 mg·g^− 1, 16 mg·g^− 1, and 133 mg·g^− 1, respectively. It could be used for the quick removal of these toxic ions by withdrawing the membrane after adsorption. Additionally, the prepared F-MOFs-M provided an excellent stability in 7 days of continuous work, and could be easily regenerated and reused without significant decrease in removal performance even after 5 filtration cycles. The adsorption performances and their corresponding mechanisms were exploited by a series of advanced techniques. The MOF-derived materials loaded on membrane provide a new application scenario in the remediation of polluted water bodies.

Wastewater

MOFs-derived material

adsorption mechanism

heavy metals

sequential adsorption

Water pollution is seriously threatening the survival of human being, plants, and animals (Han et al. 2022). The widespread water pollution by heavy metals is one of the most grievous water environmental problems. The wastewaters containing various heavy metals from natural and anthropogenic sources are responsible for the pollution of water bodies (Zhou et al. 2020). Among the toxic elements AsO₄³⁻, CrO₄²⁻, SbO₃⁻, Ni²⁺, and Pb²⁺ are considered to be the most hazardous and are listed as priority pollutants in waters by EPA (Kaur et al. 2022, Wu et al. 2021). Therefore, it is extremely urgent to dispose heavy metal contaminated waters for relieving their environmental threats.

Various techniques have been applied for the removal of inorganic metal ions from wastewaters, including ion exchange (Sharma et al. 2021), adsorption (Huo et al. 2019), and photocatalytic degradation (Jayakrishnan et al. 2021), among which the adsorption method received broad attention due to its simplicity, high effectiveness, fast and safe, as well as the feasibility for the simultaneous removal of multiple pollutants (Abuzalat et al. 2022). However, the adsorption method requires complex high-speed centrifugation or high-pressure filtration for the separation of the adsorbents after adsorption. Membranes loaded with different adsorbents have been certified as an effective application for the removal of microorganisms, organic substances, heavy metals, and dissolved solids (Liu et al. 2017, Zhu et al. 2021a, Zhu et al. 2021b). Recently, numerous researchers were devoted to develop functional membrane materials to remove various pollutants from wastewaters (Lee et al. 2021, Reddy et al. 2022, Yan et al. 2021). For example, Li et.al (Li et al. 2020) fabricated a dual-functional MIL-53 based membrane, which could be used for the separation of drifting oil and immobilization of soluble dye. Therefore, the adsorbent-based composite membrane material has been expected to have high potential implication in the simultaneous removal of water pollutants.

Metal-organic frameworks (MOFs), composed of various organic ligands and metal centers, have been attracting considerable interest due to their high surface areas, adjustable pore sizes and functional groups, chemical properties, and great potential as adsorbent, drug carrier, and plant sensing (Dissegna et al. 2018, Jomova et al. 2011, Liang et al. 2020, Wang et al. 2020, Yuan et al. 2021). Among the reported adsorbents, MOFs have been regarded as one of the most feasible materials for the removal of heavy metals. However, quite a few reports showed that MOFs had poor stability in water, for example, about 80%-100% ZIF-8 was dissolved in water within 24h contacting time (Zhang et al. 2019a). Moreover, most of the previous studies only delt with the removal of a single type of cation or anion e.g. Cd²⁺, Cu²⁺, Pb²⁺, and CrO₄²⁻, UO₂²⁺, etc (Abuzalat et al. 2022, Bui et al. 2022, Chen et al. 2021, Xiong et al. 2020, Zhang et al. 2021). The introduction of various functional groups such as -SH, -COOH, and -NH₂ into MOF-based materials has shown a dramatic impacts on the their adsorption performance (Collins et al. 2016). For instance, Leili Esrafili et al. (Esrafili et al. 2021) fabricated a dual-functionalized MOFs (TMU-81) by the introduction of the sulfonyl (-SH) and amide (C = O) groups for removing the Cd, Cu, and Cr. However, the adsorption capacity and stability of TMU-81 was weakened at neutral pH, since the nucleophilic oxygen-containing C = O group was restrained due to the lack of internal synergistic effect between –SH and –C = O. How to avoid the structure collapse of MOF materials during adsorption process and increase their performance, especially for the simultaneous removals of multiple toxic pollutants at a wide pH range, has become a bottleneck. Additionally, the adsorption mechanisms of the MOF with multi-functional groups for the simultaneous removal of anions and cations were complicated and unclear. Hence, the establishment of a simple and easy-operating method for synthesizing high-efficient and mu1ti-functioning MOF materials and elucidating its effecting mechanisms are urgently needed in order to overcome above bottleneck.

As such, a two-step fabrication approach was established, by which a new bifunctional MOF (UIO-66-NH₂(-COOH)) was synthesized and successfully loaded on the fiberglass membrane, thus a novel composite named as fiberglass@ UIO-66-NH₂(-COOH) with abbreviation of F-MOFs-M was obtained. The F-MOFs-M was proved to have strong absorbability and high stability in the quick removal of cationic and anionic heavy metal ions including AsO₄³⁻, CrO₄²⁻, SbO₃⁻, Ni²⁺, and Pb²⁺ from wastewater. Moreover, it was easy to separate from the treated wastewater by simple withdrawing the membrane after adsorption of the target toxic ions. The adsorption mechanisms of the fabricated membranes were elucidated by series techniques including SEM, FTIR, XPS, and ESI-HR-MS.

Chemicals and Material

Hydrochloric acid (HCl), nitric acid (HNO₃), and methyl alcohol (Me-OH) were of guaranteed grade, purchased from Sinopharm Group (Shanghai, China). The original fiberglass membrane was from Bositai Shop (Chongqing, China). 2-aminoterephthalic acid, ZrCl₄, benzene-1, 3, 5-tricarboxylic acid, and benzoic acid were of guaranteed grades purchased from Sigma (Shanghai, China). The Ultrapure water prepared using a Milli-Q water system (18.25 MΩ·cm^− 1) was used throughout all the experiments.

The standard stock solutions of 1000 mg·L^− 1 of AsO₄³⁻, Na⁺, Mg²⁺, K⁺, Ca²⁺, Sc³⁺, MnO₄²⁻, Cr³⁺, CrO₄²⁻, Fe²⁺, Cu²⁺, Cl⁻, NO₃⁻, SbO₃⁻, PO₄³⁻, Ni²⁺ and Pb²⁺ were all purchased from the National Center for Analysis and Testing of Steel Materials, China, and the calibration solutions with specific concentrations were diluted stepwise accordingly by weighing method.

Synthesis of fiberglass @ UIO-66-NH2(-COOH) modified membrane

The UIO-66-NH₂ was used as the starting MOF material and post modification of UIO-66-NH₂ was conducted by the introduction of –COOH group for obtaining the bifunctional MOF material, in which a single 2-aminoterephthalic acid was replaced by a benzene-1, 3, 5-tricarboxylic acid during the fabricated ligand exchange (Huang et al. 2016). The final absorbent F-MOFs-M was then prepared by a two-step hydrothermal growth method as illustrated in a schematic diagram (Fig. 1a).

A piece of original fiberglass membrane (aperture of 0.5 µm, thickness of 25 µm to 50µm) immersed in 5 mL N, N-dimethyl formamide (DMF) at room temperature for 24 hours. The soaked fiberglass membrane and 0.23 g of the ZrCl₄ were added in 10mL DMF with ultrasonic vibration at 60 ℃ for 20 min (solution A); Meantime, 0.36 g of the 2-aminoterephthalic acid and 1.23 g of the benzene-1,3,5-tricarboxylic acid were dissolved in 25 mL DMF separately in Teflon-lined bomb (solution B). Solution A, solution B, and 1.22 g of benzoic acid and 0.64 g HCl (50%) were mixed and followed by ultrasonic vibration for 5 min, respectively. The mixture solution was transferred into Teflon-lined bombs and maintained in an oven at 130 ℃ for 24 h. After cooling to room temperature, the pink fresh F-MOFs-M was successfully prepared, which was washed with DMF and methyl alcohol three times, then immersed in methyl alcohol (50 mL), and the supernatant was exchanged with methyl alcohol every 24 h for 3 days and then dried overnight at 130℃ under vacuum.

Characterization

The surface morphology of the F-MOFs-M was observed by scanning electron microscopy (SU3500, Tokyo, Japan), its structure was inspected by X-ray diffraction (Bruker, Germany) analyses at the conditions of Cu Kα1 radiation (γ = 1.5418A) and the 2-theta range from 5° to 80°. The surface functional groups and chemical compositions of membranes were determined using Fourier transform infrared spectra (FTIR, Nicolet Summit spectrometer, Thermo scientific USA) and the X-ray photoelectron spectrometer (Thermo 250xi, USA), respectively. High-performance liquid chromatography (HPLC, UltiMate 3000, Dionex) combined with mass spectrometry (MS, Q Exactive Hybrid Quadrupole-Orbitrap, Thermo Scientific, Bremen, Germany) was used to analyze ion fragments for a mass range of 50-1000 m/z using a solution MOFs (UIO-66-NH₂(-COOH)). The UIO-66-NH₂(-COOH) and 5% DMF were diluted with 30% Me-OH before ESI-HR-MS. A robust High Resolution Inductively Coupled Plasma mass spectrometry (HR-ICP-MS, Thermo scientific USA) with a concentric model nebulizer and a cyclone spray chamber was applied for the determination of the target ions.

Adsorption studies

The equilibrium isotherms (50-1000 mg·L^− 1) were investigated by adding 0.1 g the F-MOFs-M to a 50 mL solution in a thermostat oscillator and vibrated for a certain time. After then, the F-MOFs-M was taken out those solutions, the concentration of target elements in the collected solutions were diluted by deionized water, which were determined by HR-ICP-MS. The adsorption kinetics were studied by adding to 0.05 g the F-MOFs-M to a 20 mL solution containing the target ions (containing 100 mg/L of CrO₄²⁻ and SbO₃⁻ and 50 mg/L of AsO₄³⁻, Ni²⁺, and Pb²⁺) at constant temperature of 25℃. At reaction time intervals were 5, 10, 20, 40, 60min, and longer time. The concentrations of the target ions in the original solution and remaining solution were diluted by deionized water, which were determined by HR-ICP-MS. All the analyses were conducted in triplicate.

A series of 20 mL mixture solutions containing each of 10 mg/L of AsO₄³⁻, CrO₄²⁻, SbO₃⁻, Ni²⁺, and Pb²⁺ were adjusted to pH of 1, 3, 5, 7, 9, and 10, separately, using 1% HNO₃ and 1% NaOH. Then, 0.05 g the F-MOFs-M was added to these mixture solutions, respectively. These solutions were shaken at 25℃ for 3 h. Subsequently, the mixture solution was centrifuged at 10,000 r/min for 5 min. Then, the F-MOFs-M was separated from the mixture solution by a tweezer. The concentration of the target ions and Zr in the original solution and post-reaction solution were determined by HR-ICP-MS.

To exam the selectivity of F-MOFs-M on the target ions, each of 5 mg·L^− 1 Na⁺, Mg²⁺, K⁺, Ca²⁺, Sc³⁺, MnO₄²⁻, Cr³⁺, Fe²⁺, Cu²⁺, Cl⁻, NO₃⁻, PO₄³⁻, Ni²⁺, Pb²⁺, and 10% humic substance, which might coexist with the target ions in wastewater, as well as the same concentrations of the target ions were mixed together in a 20 mL adsorption vessel, 0.05 g F-MOFs-M as adsorbent then added for adsorption experiment, following similar procedure as described above.

Description and analysis of the F-MOFs-M

The SEM micrographs of the sample are shown in Fig. 1b. The original fiberglass membrane has a glazed surface, excellent randomly oriented fiber, which is the construction of the MOFs vector. To grow MOFs on the fiberglass membrane, the ZrCl₄ was doped onto the fiberglass membrane as a precursor of Zr-O bonds. The main reason to employ the fiberglass membrane was that the surface of the fiberglass membrane possessed many hydroxide radicals (Li et al. 2020). The precursor could not only act as a bridge to assist UIO-66-NH₂(-COOH) to grow on the surface of the fiberglass membrane but also provide excellent mechanical stability for the final composite membrane F-MOFs-M. The UIO-66-NH₂(-COOH) growing on the membrane surface seems like a “chocolate bar” (Fig. 1c). Herein, for the first time, we produced a straightforward two-step hydrothermal synthesis to create a composite membrane loaded with UIO-66-NH₂(-COOH). The interior structure of the F-MOFs-M possessed abundant porosities, which favorited the adsorption of heavy metals from the wastewater. Additionally, physical synthesis had also been applied to the preparation of composite materials (Hakimifar &Morsali 2019). Huo et al (Huo &Yan 2012) reported a fast magnetization of MOFs MIL-101 for rapid magnetic solid-phase extraction of polycyclic aromatic hydrocarbons (PAHs) environmental water samples by physical synthesis. The F-MOFs-M with dispersive skeletons and tightly hierarchical structures are superior to the composite membrane prepared by the physical synthesis (Fig. 1d), in terms of providing a large contact area between the water and adsorbent while also enhancing membrane flux.

The XRD patterns of the UIO-66-NH₂ and the F-MOFs-M are shown in Fig. 2a. The diffraction peaks of UIO-66-NH₂(-COOH) on the surface of the membrane were similar to those for UIO-66-NH₂ (Browe et al. 2017), indicating that the modification of UIO-66-NH₂ with -COOH groups did not impact the original crystal structure (Katz et al. 2013). The chemical composition of UIO-66-NH₂ and the F-MOFs-M are characterized by FT-IR spectra presented in Fig. 2b. The stretching vibration of the C-H, C-C, benzene ring, O-H, and C-N bond corresponded to the peaks at 1244 cm^− 1, 1386 cm^− 1, 1502 cm^− 1, 2996 cm^− 1, and 3475 cm^− 1, respectively. The peaks at 1572 cm^− 1(the C-O bond) and 1442 cm^− 1 (the C-C vibration mode ) (Zhao et al. 2019) came from -COOH and C = O bond in the infrared spectra of UIO-66-NH₂(-COOH). Two notable peaks at 3475 cm^− 1and 3462 cm^− 1 were ascribed to the amino groups, which were due to the different stretching modes of the N − H bond (El-Mehalmey et al. 2018). Besides, the vibration of the C = N and C = O bonds caused two significant peaks in the infrared spectra of UIO-66-NH₂(-COOH) at 1576 cm^− 1 and 1683 cm^− 1. These peaks revealed that the –NH₂ on UIO-66-NH₂(-COOH) had been successfully converted from C − N to N-H using 2-Aminoterephthalic Acid and 1, 3-Benzenedicarboxylic acid.

X-ray photoelectron spectrometry is used to validate the chemical compositions of the F-MOFs-M (Fig. 3). As represented in Fig. 3a, the deconvolution of the Zr3d3/2 and Zr3d5/2 peaks at around 185.26 eV and 182.83 eV are attributed to zirconium in the Zr⁴⁺ state, indicating that Zr ions existed as ZrO₂ in the tetragonal phase (Zhao et al. 2018). The O1s XPS spectrum is shown in Fig. 3b. The peak at 529.73 eV was attributed to lattice oxygen due to Zr-O bonds. The peak at approximately 531.68 eV was attributed to oxygen components of the carboxylate groups O-C = O due to the unreactive ligand. As shown in Fig. 3c, the binding energy of the N1s2/1 peaks at around 401.52 eV and 399.16 eV were attributed to -NH₂/NH₃⁺ and N-H in the 2-aminoterephthalic acid, respectively (Zhao et al. 2019). In a word, the surface of F-MOFs-M had a large number of –NH₂ and –COOH, which contributed to improving the hydrophilicity and coordination.

The smallest unit cell of UIO-66-NH₂(-COOH) consisted of a cubic structure of cationic Zr₆O₄(OH)₄ nodes, which was indicated by ESI-HR-MS. As shown in Fig. 4, the characteristic peaks at m/z = 675.12, m/z = 681.41, m/z = 687.47, and m/z = 699.72 were ascribed to ⁹⁰Zr₆O₈H₇⁻, ⁹¹Zr₆O₈H₇^{− 92}Zr₆O₈H₇⁻ and ⁹⁴Zr₆O₈H₇⁻, respectively. The isotopic abundance ratios of ⁹⁰Zr, ⁹¹Zr, ⁹²Zr, and ⁹⁴Zr were consistent with the natural isotopic composition of Zr. These data combined with Zr isotopic fingerprints allow identification of the Zr-MOF. Many researchers studied MOFs with hexazirconium oxo hydroxo (Zr₆O₄(OH)₄) cluster nodes (Audu et al. 2016, Deria et al. 2015, Katz et al. 2013, Mondloch et al. 2014). For example, UIO-66 consisted of a cubic framework of cationic Zr₆O₄(OH)₄ nodes (formed in situ via hydrolysis of ZrCl₄) and 1,4-benzene dicarboxylate linkers (BDC). Unfortunately, the best operating condition of the instrument was a mass range of 50 to 1000 m/z using the measurement for negative mode. The theoretical mass of UIO-66-NH₂(-COOH) overstepped the range of m/z = 1000. Therefore, the molecular weight of the complete MOFs could not be obtained. In this context, the characteristic peaks at m/z = 675.12, m/z = 681.41, m/z = 687.47, and m/z = 699.72 were ascribed to the smallest unit cell of UIO-66-NH₂(-COOH). It could be deduced from the results that the UIO-66-NH₂(-COOH) had at least two missing-linker sites per node (Fig. 4).

All the aforementioned investigations demonstrated that the F-MOFs-M had super hydrophilicity, sheet structure, and alive-linker site making it promising for the removal of harmful ions in wastewater.

Adsorption of performances

Effect of initial pH

The pH value should be optimized to improve the adsorption efficiency and reduce interference from the matrix. As shown in Fig. 5, the removal efficiencies of CrO₄²⁻ by F-MOFs-M were higher than 75% at the pH ranging from 1 to 9, The high removal rate occurred at pH > 2; The removal rates of AsO₄³⁻ were over 80% within the pH range of 1–10, and the highest removal efficiency occurred at pH 9. Insignificant influences of pH on SbO₃⁻, Ni²⁺, and Pb²⁺ adsorptions were observed in the pH range of 1 to 8, and the optimal pH for relatively higher removal efficiencies for these metals was about 7. Cationic metals could be precipitated and negative charges on the surface of MOFs might reject anion groups at the pH values higher than 8. All the results indicated that the F-MOFs-M had an excellent capacity for capturing all the target metal ions in a wide pH range.

Adsorption isotherms and kinetics

Adsorption isotherms: the adsorption isotherms of all the tested ions on the F-MOFs-M are shown Fig. 6. It showed that the adsorption of AsO₄³⁻, CrO₄²⁻, SbO₃⁻, Pb²⁺, and Ni²⁺ increased rapidly at lower equilibrium concentrations, then a plateau appeared with the further increase of equilibrium concentration. Two often-used empirical equations, the Langmuir and Freundlich isotherm models, were used to describe the characteristics of the adsorption isotherms, which could be expressed as nonlinear and linear forms (Equ. i, ii):

$qe=qmax*k1\frac{Ce}{1+k1*Ce} \left(\text{i}\right)$ $\frac{Ce}{qe}=\frac{1}{qm*k1}+\frac{Ce}{qm} \left(\text{i}\text{i}\right)$

where qe (mg·g^− 1) was the adsorption amount of the target ions at equilibrium; q_max (mg·g^− 1) was the maximum adsorption capacity; $qm$ was the mono-layer adsorption capacity (mg·g^− 1); the $k1$ was the Langmuir constant, and $Ce$ (mg·L^− 1) was the equilibrium concentration.

The Freundlich models also had two forms expressed as:

$qe=k2*{Ce}^{(1/n)}$ (iii)

$$log\left(qe\right)=log\left(k2\right)+\frac{1}{n}logCe \left(vi\right)$$

Where qe (mg·g^− 1) was the adsorption amount at equilibrium concentration $Ce$ (mg·L^− 1) n and $k2$ were the simulating equation constants relevant to adsorption characteristics.

The simulating results are listed in Tables 2 and 3. All the adsorption isotherms are well described by the two equations, with Langmuir equation being better for all the tested ions except for AsO₄³⁻ as indicated by the higher determination coefficients (R²). The adsorption of AsO₄³⁻ by F-MOFs-M might prefer multilayer adsorption, in consistent with the previous reports (Zhang et al. 2019b).

The maximum adsorption capacity (q_max) of AsO₄³⁻, CrO₄²⁻, SbO₃⁻, Pb²⁺, and Ni²⁺ by F-MOFs-M obtained from Langmuir equation were 54.96 mg·g^− 1,188.9 mg·g^− 1, 324.0 mg·g^− 1, 132.5 mg·g^− 1, and 15.87 mg·g^− 1, respectively, which were accordingly lower than those of the synthesized bifunctional MOFs [UIO-66-NH₂(-COOH)] material due to the partly occupations of the active sites used to connect with the membrane (Su et al. 2020). However, the maximum adsorption capacities of most target ions by F-MOFs-M were still higher than many reported MOFs (Table 3). Besides, MOFs loaded on fiber glass (F-MOFs-M) helped to increase the stability of MOFs, and supplied great advantage for the later handling of the absorbent after absorption by simply picking it up.

Adsorption kinetics: the absorption kinetics of AsO₄³⁻, CrO₄²⁻, SbO₃⁻, Pb²⁺, and Ni²⁺ on F-MOFs-M are shown in Fig. 7. It was clear that the concentration of target elements were decreased by rising time. In the first 30 min, CrO₄²⁻ and SbO₃⁻ contents were rapidly decreased, while AsO₄³⁻, Pb²⁺, and Ni²⁺ contents were tardily reduced and after that, the adsorption rate of those target ions was remarkably decreased by raising the reaction time. After 120min of reaction, the adsorbed amounts of target elements on F-MOFs-M reached equilibrium, suggesting that the F-MOFs-M had fast adsorption dynamics for the removal of multifarious ions from water. The F-MOFs-M hydrolysis time is about 7 days, much longer than the equilibrium time of target elements, which guaranteed the removal of target elements in wastewater.

The experimental results were fitted with the pseudo-second-order kinetic model using the equation (v) below:

$$\frac{t}{Qt}=\frac{t}{Qe}+\frac{1}{k3*{Qe}^{2 }} \left(v\right)$$

Where k₃was characteristic constants of the pseudo-second-order adsorption (g·mg^− 1 min^− 1), Q_t was the amount of analyst adsorbed at the time $t$ (mg·g^− 1), and Qe (mg·g^− 1) was the adsorption capacity unit of adsorbent at equilibrium (mg·g^− 1).

The high correlation coefficients (R²) of all elements were revealed near 0.995 between time (t) and t/Qt (Fig. 7). Meantime, the adsorption rate coefficient (k₃) of AsO₄³⁻, CrO₄²⁻, SbO₃⁻, Pb²⁺, and Ni²⁺ were calculated to be 1.28×10^− 3 g·mg^− 1 min^− 1, 9.02×10^− 2 g·mg^− 1 min^− 1, 1.78×10^− 2 g·mg^− 1 min^− 1, 8.01×10^− 3 g·mg^− 1 min^− 1, and 3.12×10^− 3 g·mg^− 1 min^− 1, respectively, which were higher than those adsorbed materials under similar conditions (Table S1). The results indicated that the F-MOFs-M possessed a faster adsorption capacity. Moreover, the experimental data was certified to the calculated Qe value, indicating the adsorption process of AsO₄³⁻, CrO₄²⁻, SbO₃⁻, Pb²⁺, and Ni²⁺ onto the composite membranes fitted pseudo-second-order kinetic model.

Simultaneous capture of cations and anions

To study the simultaneous capture of cations and anions, the removal performances of F-MOFs-M on single solutions of AsO₄³⁻, CrO₄²⁻, SbO₃⁻, Pb²⁺, and Ni²⁺ and those mixed solutions were researched (Fig. 8). The adsorption efficiency for each ion from the mixed solution is shown in Fig. 8a. It shows that oxyanions could be captured more preferentially than cations. The adsorption efficiencies were between 75% and 85% for oxyanions, and 20%-50% for cations. When the single cations separately appeared, each removal efficiency of Pb²⁺ and Ni²⁺ are 82% and 66%, and Cr³⁺, K⁺, Ca²⁺, and Mg²⁺ were just only 12%, 11%, and 9%, respectively (Fig. 8b). Therefore, with the presence of the oxyanions inhibited the capture of cations, and the more remarkable inhibiting effect was shown for Pb²⁺.

When multiple ions are competitive for adsorption, the adsorption partition-coefficient (K_d) is a relative metric of the selectivity adsorption (Cosgrove et al. 2019, Xu et al. 2020), and it was calculated by the equation below:

K_d5=Q_e/C_e

Where Q_e is the adsorption capacity after equilibration (mg/g); C_e is the equilibrium concentration (mg/L); K_d5 is the adsorption partition coefficient at the initial concentration of 5 mg/L.

As shown in Table 4, by comparing the K_d5 values the results show a selective adsorption order of SbO₃⁻ > CrO₄²⁻ > AsO₄³⁻> Pb²⁺ > Ni²⁺. Generally, the electronegativity directly determined the adsorption selectivity of ions, the metal ions with stronger electronegativity easily interacted with the adsorbent (Pan et al. 2022). However, the adsorption capacity of Pb²⁺ was much lower than that of SbO₃⁻ (Liu et al. 2019). To explore this, the results of the Zeta potential of the F-MOFs-M show that the F-MOFs-M carries a positive charge on the surface during the reaction (Fig. 9). A positive surface charge favored the preferential adsorption of anion groups. Therefore, the adsorption selectivity depended on the surface charge of the F-MOFs-M and the self-characteristics of the adsorbate.

As observed, the F-MOFs-M had a better affinity to oxyanions in the presence of cations. The UIO-66-NH₂(-COOH), which has two missing-linker sites per node at least (Fig. 4), captured oxyanions significantly faster and in greater quantities than cations. According to the Hofmeister effect (Hua et al. 2018, Nezamzadeh-Ejhieh &Afshari 2012), and many of the studies reported (Xiong et al. 2020, Xu et al. 2020) that the chemisorption ability of some metals obeyed the sequence of Pb²⁺༞Ni²⁺, Pb²⁺༞Cd²⁺, Ni²⁺༞Cd²⁺. Herein, the F-MOFs-M obtained a preferential adsorption order of oxyanions(SbO₃⁻ > CrO₄²⁻ > AsO₄³⁻)༞cations(Pb²⁺༞Ni²⁺).

Recycling and stability of the F-MOFs-M

The reproducibility of the resulting membrane is crucial for its practical application (Glomstad et al. 2017). And therefore, the elution and reusage of the F-MOFs-M were further studied. The SbO₃⁻, CrO₄²⁻, AsO₄³⁻, Pb²⁺, and Ni²⁺ loaded membranes were stirred continuously in HNO₃ (0.5 M) at 25℃ for 360min. the results showed that the cations-loaded (Pb²⁺, and Ni²⁺) F-MOFs-M were easily eluted, achieving almost 98.5% and 97.6% regeneration rates. Meanwhile, the anions (SbO₃⁻, CrO₄²⁻, and AsO₄³⁻)-loaded F-MOFs-M were also easily eluted with over 98% elution rates. To obtain the recycling of the F-MOFs-M, the regenerated membranes were then used for the 5 times regenerated cycles. It is certain that the F-MOFs-M still possesses over 90% of the original removal rates even after 5 times reuse (Fig. 10), which indicated that the F-MOFs-M could be regenerated and recycled without losing its removal efficiency.

The bottleneck of MOFs in practical application as absorbents is their structural instability in solution. Therefore, the hydrolysis process of the fabricated F-MOFs-M in an aqueous solution was investigated using HR-ICP-MS (Table 1) within 3 h to 60 days. The results showed that the UIO-66-NH₂(-COOH) could stably exist in an aqueous solution within at least 7 days. Obviously, it possessed an excellent chemical stability, which was closely related to the electrostatic configuration in water.

Adsorption Mechanisms

The previous investigations showed the F-MOFs-M exhibited superior adsorption performance for the tested harmful ions, which might attribute to the abundant functional groups and the exposed active sites in the UIO-66-NH₂(-COOH) structure. As shown in Fig. 2a, after-adsorption treatment, the sharp diffraction modes corresponding to the extruded UIO-66-NH₂(-COOH) arise three broad peaks at around 14.68°, 19.33°, and 51.70° following the (400), (420), and (137) diffraction peaks of the metal peaks, indicating the successful adsorption of heavy metals. The second evidence comes from Fig. 2b by the FT-IR spectra, the infrared spectrum of UIO-66-NH₂(-COOH), the peak of the O-H disappeared and the metal-O peak appeared. A characteristic peak at about 875 cm^− 1 proved the existence of a metal-O bond after adsorption, and three peaks at 564 cm^− 1, 584 cm^− 1and 568 cm^− 1 were closely related to the symmetric tensile vibration of Zr-O (Audu et al. 2016, Huo et al. 2019). For instance, the XPS spectra of As3d (Fig. 11) is shown two visible shifts at 46.72 eV to 45.68 eV after AsO₄³⁻ loading, suggesting that the chemical environment of AsO₄³⁻ had changed. Furthermore, the peaks resulting from the Zr isotope ratio were used to calculate the smallest unit cell of UIO-66-NH₂(-COOH) by ESI-HR-MS. The results show that the Zr atoms contained unsaturated binding sites, and target oxyanions are coordinated to the Zirconium oxide octahedron nodes (Fig. 4) (Katz et al. 2013). The active binding sites on the Zirconium oxide octahedron nodes were excellent binders for the AsO₄³⁻.

Additionally, as displayed in N1s and O1s spectrum after-absorption (Fig. 3c and b), the characteristic peaks at 401.52 eV and 399.16 eV are ascribed to -N-H, N⁺, and C-N bonds, respectively. The -NH and N⁺ own a higher peak after absorption, suggesting that the cations are being promoted to obtain inert nitrogen. Compared with the O1s XPS spectrum of before-absorption (Fig. 3b), the peak for chemisorbed oxygen is more prominent. The peak at 533.72 eV could consider being –COO⁻, decreased its binding energy, which might be formed the hydrogen bonds with cations or promoted the transformation of C = O oxygen species as active oxygen. According to reports (Liu et al. 2020, Tang et al. 2021, Zhao et al. 2019), the C = O bond could increase the chemical adsorption capacities of ions such as Hg²⁺, Pb²⁺, and Ni²⁺. As mentioned above, the lone pair of electrons functional group went through coordination and covalence with target cations.

Herein, a novel composite membrane of metal-organic frameworks (F-MOFs-M) was fabricated by two steps hydrothermal methods. This membrane was attractive for its ability to fast, efficient, and ordinal removal SbO₃⁻ > CrO₄²⁻ > AsO₄³⁻> Pb²⁺ > Ni²⁺ from wastewater. And the F-MOFs-M was sufficiently reusable and over 90% of target ions removal efficiency were retained even completing five cycles. The harmful anions and cations adsorption mechanism may identify means to fabricate more distinct MOFs@composite membrane for multi-application. Moreover, we provided a new evidence-based MOFs structure (Zr₆O₄(OH)₄) with two available missing-linker sites per node by ESI-HR-MS. The results would provide a promising candidate for constructing a multifunctional composite membrane for the removal of harmful ions in wastewater.

Fundings

This work was supported by the National Nature Science Foundation of China (42177012) and the Open Fund of Key Laboratory of Testing and Tracing of Rare Earth Products for State Market Regulation (TTREP2022YB01).

We thank Dr. Bi-Yue Ding (Southwest University) for providing valuable technical assistance in taking photos using the Scanning Electron Microscope.

Author Contributions

Shiqing Wei provided ideas and designed research. Hao Cui, Yi Song, Jinjiang Pan, and Zhenmao Jiang performed sample pretreatment and data analysis. Hao Cui, Yi Song, Jinjiang Pan, Zhenmao Jiang, and Lei Li wrote the manuscript. All authors discussed the results and approved the final manuscript.

Ethics approval and consent to participate: Not applicable.

Consent for publication: Not applicable.

Competing interests: Not applicable.

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Table 1

The composite membrane of dissolution efficiency% of Zr⁴⁺ at 3h to 60 days

Time	UIO-66 dissolution efficiency%	UIO-66-NH₂(-COOH) dissolution efficiency%
3h	0.15	0.19
24h	0.27	0.26
2d	0.53	0.77
7d	3.53	6.68
15d	7.64	12.44
30d	15.43	21.43
60d	35.66	44.42

Table 2

The comparison of the maximum adsorption capacity between the synthesized composite membrane and those reported in literature

Target Anions	MOFs	Isotherm models	q_max mg·g^-1	pH	Ref
CrO₄^2-	Cu-MOF	Langmuir	107	4	(Zhang et al. 2018)
CrO₄^2-	ZJU-101(Zr)	Langmuir	245	/	(Zhang et al. 2015)
CrO₄^2-	NU-1000	Langmuir	37	4	(Lin et al. 2017)
CrO₄^2-	Zn-MOF	Langmuir	43.8	7	(Fu et al. 2018)
Pb²⁺	ZIF-8	Langmuir	390.5	5.5	(Wang et al. 2019)
Pb²⁺	MIL-53	Langmuir	90.8	5.8	(Alqadami et al. 2018)
Ni²⁺	Chitosan-UiO-66	Langmuir	60	5	(Wang et al. 2016)
AsO₄^3-	UiO-66-NH₂	Langmuir	76.9	7	(Chang et al. 2019)
AsO₄^3-	Fe₃O₄@UiO-66	Freundlich	73.2	7	(Huo et al. 2019)
AsO₄^3-	Cubic ZIF-8	Langmuir	122.6	8.5	(Liu et al. 2015)
AsO₄^3-	HP-UiO-66	Freundlich	248.8	6	(Xu et al. 2020)
SbO₃^-	UiO-66-NH₂	Langmuir	105.4	1.5-12	(He et al. 2017)
SbO₃^-	UIO-66	Langmuir	99.5	1.5-12	(He et al. 2017)
SbO₃^-	NU-1000	Langmuir	2-500	2.3-9.5	(Li et al. 2017)
CrO₄^2-	the F-MOFs-M	Langmuir	188.92	2	this work
AsO₄^3-	the F-MOFs-M	Freundlich	54.96	9	this work
SbO₃^-	the F-MOFs-M	Langmuir	324.06	7	this work
Ni²⁺	the F-MOFs-M	Langmuir	15.87	7	this work
Pb²⁺	the F-MOFs-M	Langmuir	132.5	7	this work

Table 3

Isotherm parameters of the target oxyanions sorption using the F-MOFs-M

Element	Langmuir isotherm models			Freundlich isotherm models
	q_max mg·g^-1	k1	R²	n	k2	R²
AsO₄^3-	54.96	0.01	0.912	3.42	7.23	0.998
CrO₄^2-	188.9	0.02	0.994	4.89	7.59	0.989
SbO₃^-	342.0	0.02	0.995	5.62	8.64	0.954
Pb²⁺	132.5	0.02	0.993	5.66	7.77	0.912
Ni²⁺	15.87	0.01	0.959	3.75	7.45	0.902

Table 4

Characteristic electronegativity and K_d5 values of heavy metals

ions	K_d5	Electronegativity
AsO₄^3-	69.05	2.18
CrO₄^2-	82.30	1.66
SbO₃^-	83.90	2.05
Pb²⁺	21.56	2.33
Ni²⁺	17.84	1.91

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Synthesis of a bifunctional metal-organic framework membrane for the removal of anionic and cationic heavy metals from wastewater

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Introduction

Materials And Methods

Chemicals and Material

Synthesis of fiberglass @ UIO-66-NH2(-COOH) modified membrane

Characterization

Adsorption studies

Results And Discussions

Description and analysis of the F-MOFs-M

Adsorption of performances

Simultaneous capture of cations and anions

Recycling and stability of the F-MOFs-M

Adsorption Mechanisms

Conclusions

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References

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