Eco-friendly sorbent of bacterial cellulose/metal–organic framework composite membrane for effective bisphenol a removal

Abstract

Environmental hormones such as bisphenol A have attracted enormous attention due to their potential threat to humans and environment. Herein, we synthesized a BC/MIL-53 (Al)-F127 composite membrane by the vacuum filtration method with a mesostructure metal–organic framework MIL-53 (Al)-F127 and bacterial cellulose as the substrate. The BC/MIL-53 (Al)-F127 composite membrane exhibited efficient adsorption of bisphenol A in aqueous solution. The results exhibited that 94% of bisphenol A could be adsorbed in approximately 120 min, and the equilibrium sorption amounts of bisphenol A reached approximately 8.3 mg/g. Meanwhile, the mechanism of adsorption was explored. The optimum pH and temperature for the adsorption of bisphenol A were 6 and 40°C, respectively. The removal efficiency of bisphenol A was maintained at 90% after five repeated cycles, indicating the advantage to separate freely without a complex filtration system. The results of adsorption indicated that the BC/MIL-53 (Al)-F127 membrane has great potential in the aspect of the sewage treatment as a prospective sorbent.

Keywords

BC/MIL-53 (Al)-F127 bisphenol A adsorption metal–organic framework bacterial cellulose nanometer material composite membrane

Introduction

Endocrine disrupting chemicals (EDCs) are released into the aqueous system through human production and life, and seriously affect the normal metabolism, hormone synthesis, and endocrine of organisms.¹ This chemical has been frequently detected in aquatic environment throughout the world.^2,3 Among these EDCs which have various structures and properties, bisphenol A (BPA) (2, 2-bis (4-hydroxyphenyl) propane), with wide usage in plastics,⁴ and poor degradation ability,⁵ was detected at different concentrations in drinking water, groundwater, and surface water.⁶ Some researches have reported that some cell functions could change due to BPA at concentrations ranging from 1 pmol/L to 1 nmol/L. Therefore, the US Environmental Protection Agency has regarded BPA as a chemical which could disrupt the endocrine. Besides, they had stated that the removal of BPA became a global and social issue.⁷

At present, various approaches have been investigated to remove BPA from wastewater, such as photocatalytic,⁸ oxidation,⁹ adsorption,¹⁰ biodegradation,¹¹ and electrochemical.¹² And the adsorption technologies have been regarded as the most common process to effectively remove BPA from wastewater owing to simple operation, low energy consumption, producing fewer secondary products, and regeneration. Fortunately, various adsorbents, such as activated carbon,¹³ graphene oxides,¹⁴ zeolites,¹⁵ bentonites,¹⁶ carbon nanotubes,¹⁷ graphene,¹⁸ and chitosan,¹⁹ have been studied and synthesized to separate organic pollution for water purification. However, these adsorbents are limited by the disadvantages of low adsorption capacity and a short life cycle. Therefore, it is quite significant to explore new adsorption materials for sewage disposal.

As a new type of nano-porous materials with a shifted network structure, the structure of metal–organic frameworks (MOFs) are constructed by combining organic ligands with metal ions.²⁰ MOFs exhibit more superior absorption properties than other conventional porous materials due to the greater porosity, higher specific surface area, modifiable surface, and adjustable pore size.^21,22 So far, many researchers have studied and synthesized various types of MOFs which have different framework structures in the application of gas storage,²³ chemical sensing,²⁴ catalysis,²⁵ photoelectromagnetic materials,²⁶ adsorption, and biomedicine. Up to now, many research works have been devoted to the field of adsorption; nevertheless, heavy metal ions contained in MOFs that are harmful to the environment and the micropore structure (less than 2 nm) limit the application in wastewater treatment, which could easily cause secondary pollution to the environment and prevent large molecules especially in the water phase from entering internal surface, respectively. Therefore, the research works about the application of MOFs in removing environmental pollutants have become more urgent and meaningful due to the emerging global environmental pollution crisis. MOFs with mesoporous structure were carried out using the linker-expanding and supramolecule-templating methods to resolve such a barrier. The surfactant-templating method was applied to prepare traditional mesoporous MOFs, which can prevent the skeleton instability and interpenetration of MOFs with stretched ligands.^27–29 In recent years, MOFs, such as MOF-199 (Cu)³⁰ and MOF-5 (Zn),³¹ have attracted much more attention in the application of wastewater treatment due to the excellent adsorption performance.³² Nevertheless, these MOFs probably cause potential pollution risks due to the toxic heavy metal ions contained in some MOFs. Therefore, it is still a huge challenge to investigate environmentally friendly and biocompatible MOFs materials to effectively separate organic pollutants from aqueous solution. However, in the other hand, the powder form of MOFs limits the application in the purification of water from organic pollution, such as the difficulties in separation, serious quality loss, and recycling, which can easily cause secondary pollution for the environment and also a challenge for other solid adsorbents. In order to overcome this shortcoming, it is necessary to immobilize MOFs on some substrates, which have good chemical and physical stability, and multiple active sites. The composite material with excellent recyclability will offer a great possibility to remove organic pollution from aqueous solution.

Nanofibers have the advantages of low cost, fine structure, high mechanical strength, and low thermal expansion coefficient, so nanofibrous membrane has become ideal candidate among these materials on account of its strength, large surface area, and high porosity. Bacterial cellulose (BC) is a type of natural biological material synthesized by some bacteria,³³ which has a complex 3D porous structure formed by cellulose nanofibers. It has a large specific surface area with aqueous stability^34,35 so that it can provide abundant active sites to combine with MOFs. Besides, it has many unique physical and chemical and mechanical properties, such as hydrophilicity, and nontoxicity,³⁶ and good biocompatibility and degradability, which are much friendlier to the environment. BC nanofibrous membranes are ideal substrates to host adsorbents to fabricate composite materials for the absorption of BPA in aqueous solution.

Herein, we synthesized an environmentally friendly material of mesostructured MIL-53 (Al)-F127 by the one-step solvothermal method, where Al is the central skeleton metal atom. Al is more eco-friendly than other metals used in other MOFs, such as MOF-199 (Cu)³⁰ and ZIF-8 (Zn),³⁷ which contain heavy metal ions. And the BC/MIL-53 (Al)-F127 nanofibrous membrane was fabricated using the vacuum filtration method. The BPA adsorption performance of the composite membrane in aqueous solutions was systematically explored by sorption kinetics, and the mechanism of adsorption was investigated in respect of the influences of pH and temperature. Remarkably, BC/MIL-53 (Al)-F127 nanofibrous membranes in this study exhibited excellent adsorption capacity on BPA, which shows promising prospects for the sewage disposal.

Materials and methods

Materials

Yeast extract; D-mannitol; tryptone; sodium hydroxide; N,N-dimethylformamide (DMF); Aluminum nitrate (Al (NO₃)₃·9H₂O); and absolute ethyl alcohol were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). And BPA was purchased from Aladdin Chemistry Co., Ltd (Shanghai, China). Terephthalic acid was purchased from Macklin Biochemical Co., Ltd. PoloxaMer F127 was purchased from Shanghai Vita Chemical Reagent Co., Ltd. Deionized water was utilized in all aqueous solutions. And all materials were used without any further purification.

Preparation of bacterial cellulose

The Gluconacetobacter xylinus (ATCC 10245) bacterial strain was used to prepare the BC membranes. A culture medium was prepared by mixing yeast extract (5 g/L), D-mannitol (25 g/L), and tryptone (3 g/L). It was autoclaved at 121°C for 15 min. Then, the culture medium containing the Gluconacetobacter xylinus bacterial strain was inoculated to the nutrient medium to attain a volume ratio of 10%. Then, the BC was produced after being cultivated for 10 days at 30°C. In order to clear away the bacteria, the obtained BC was washed by immersing it into 4 wt% sodium hydroxide solution at 80°C for 1 day. And the BC was repeatedly washed by deionized water until the BC became neutral.

Preparation of MIL-53 (Al)-F127

Al (NO₃)₃·9H₂O (0.995 g) and terephthalic acid (0.294 g) were dissolved in the mixed solvent containing 13 mL DMF, 4.9 mL H₂O and 3.6 mL absolute ethyl alcohol. Then, PoloxaMer F127 (3.34 g) was put into the mixed solution while stirring. After stirring for 2.5 h at room temperature, the mixture was moved to a Teflon-lined steel autoclave to react in the air oven for 48 h at 130°C. After the reaction was completed, the obtained product was filtered, followed by washing with deionized water and absolute ethyl alcohol. And the washing process was repeated three times to remove the nitric acid formed during the synthesis process. Subsequently, in order to remove surfactant PoloxaMer F127, the obtained samples soaking into ethanol were refluxed for 12 h at 70°C. Then, the products were soaked in DMF for 24 h to remove the residual terephthalic acid. Finally, the samples were dried in the air oven at 150°C for 8 h. The sample obtained was named MIL-53 (Al)-F127.³⁸

Preparation of BC/MIL-53 (Al)-F127 nanofibrous membranes

In order to prepare a uniform BC suspension, the obtained BC films were smashed using a homogenizer at high speed until a uniform suspension was obtained. Subsequently the MIL-53 (Al)-F127 powders (0.5 g) were added into the suspension, and then the mixture was continuously stirred for 7 h, so the hydrogen bonding might be formed between Al (OH) of MIL-53 (Al)-F127 and -OH of BC. And then the mixture formed a composite membrane by vacuum filtration. Finally, the BC/MIL-53 (Al)-F127 composite membrane was obtained after freeze drying for 3 days.

Measurements and characterization

The surface morphologies and microstructure of samples were observed by scanning electron microscopy (SEM, SU8010), and the electron beam acceleration voltage is 5 kV during observation. Fourier transform infrared spectroscopy (FT-IR, Nicolet is10) was collected over the range of 4000–500 cm⁻¹ to analyze the chemical structure. X-ray diffraction (XRD, D2 PHASER) was employed to determine the crystal structure over the 2θ range of 5–70°. The Brunauer–Emmett–Teller surface area of samples were determined using the TriStar II 3020 by nitorgen adsorption–desorption isotherm, and the pore volume and the distributions of pore size were characterized by the Barrett–Joyner–Halenda model. The BPA concentration was determined by ultraviolet-visible spectrophotometer (SHIMADZU UV-2600) at 276 nm according to the value of the UV absorption degree.

Adsorption experiments

To evaluate the removal efficiency and sorption capability of BPA for BC/MIL-53 (Al)-F127 nanofibrous membranes, BC and MIL-53 (Al)-F127 changed over time and BPA solution (0.1 mmol/L) was obtained with deionized water at pH 6. Then, BC/MIL-53 (Al)-F127 nanofibrous membranes with sizes of 3 × 3 cm and 50 mL BPA solution (0.1 mmol/L) were put into several 60 mL brown reagent bottles. Then, the bottles were sealed and put in a shaker with the constant temperature at 40°C, with the shaking speed of 100 rpm. And the solution was taken at different intervals till the solution reached adsorption equilibrium. Then SHIMADZU UV-2600 was employed to measure the UV absorption degree of BPA aqueous solutions at 276 nm. And then according to the results, the adsorption kinetics curves of the adsorbent were plotted.

For comparison, the sorption performance of BC (3 × 3 cm) and MIL-53 (Al)-F127 (20 mg) materials were also studied according to the same method as BC/MIL-53 (Al)-F127 membranes, respectively.

The two equations were used to calculate the adsorption amount of BPA at time t (q_t, mg/g) and at equilibrium (q_e, mg/g), which are shown in the following equations

q_{e} = \frac{(C_{0} - C_{e})}{m} V

(1)

q_{t} = \frac{(C_{0} - C_{t})}{m} V

(2)

in which C₀ (mg/L) presents the initial BPA concentrations, and C_e (mg/L) and C_t (mg/L) present the equilibrium concentrations, and the BPA concentration value at specific time t, respectively, m (g) is the mass of BC/MIL-53 (Al)-F127 menbranes, and V (L) is the volume of the BPA aqueous solution.

The adsorption kinetics was simulated by the pseudo–first-order and the pseudo–second-order model, as shown in the following equations

\ln (q_{e} - q_{t}) = \ln q_{e} - k_{1} t

(3)

\frac{t}{q_{t}} = \frac{1}{k_{2} q_{e}^{2}} + \frac{t}{q_{e}}

(4)

in which k₁ (min⁻¹) presents the pseudo–first-order model constant of adsorption rate, k₂ (g (mg min⁻¹)) presents the pseudo–second-order model constant of adsorption rate and T (min) presents the time of sorption.

In order to estimate the effect of solution pH ranging from 2 to 12 on the adsorption of BPA, BC/MIL-53 (Al)-F127 nanofibrous membranes with sizes of 1.5 × 3 cm were added in 25 mL BPA aqueous solutions at 40°C. And three experiments were carried out and the results were averaged.

To determine the effect of temperature on the sorption process, BC/MIL-53 (Al)-F127 nanofibrous membranes with sizes of 1.5 × 3 cm was added in 25 mL BPA aqueous solutions at pH 6 and at 10, 20, 30, 40, 50, 60°C, respectively. And three experiments were conducted and the results were averaged.

Recyclability of BC/MIL-53 (Al)-F127 nanofibrous membranes

Methanol was used to wash the BC/MIL-53 (Al)-F127 membrane after adsorbing BPA to determine the reusability of the membrane.³⁹ The 1.5 × 3 cm BC/MIL-53 (Al)-F127 membrane, after adsorption process, and 25 mL methanol were put into a 50 mL beaker. And the beaker was shaken in the constant temperature shaker at 25°C for 90 min; after that, it was dried in the oven for 1 h at 80°C. The regenerated BC/MIL-53 (Al)-F127 nanofibrous membranes were put into 60 mL brown reagent bottles with 25 mL BPA solutions (0.1 mmol/L) at pH 6, and then the mixture solution was shaked at 40°C with 100 rpm for 90 min. And then, the BPA concentration after absorption was measured by the UV absorption degree. The recyclability of BC/MIL-53 (Al)-F127 nanofibrous membranes was studied after five cycles.

Results and discussion

Morphology analysis of BC/MIL-53 (Al)-F127 nanofibrous membrane

Figure 1 shows SEM images of BC, MIL-53 (Al)-F127, and BC/MIL-53 (Al)-F127 nanofibrous membranes, respectively, and the digital photo of the BC/MIL-53 (Al)-F127 membrane. The fibrous network was clearly observed in the BC membrane by the vacuum filtration method as shown in Figure 1(a). The sphere-like surface morphology of MIL-53 (Al)-F127 was observed in Figure 1(b), which is basically consistent with that reported in the literature.⁴⁰ The obtained BC/MIL-53 (Al)-F127 composite membrane via vacuum filtration is shown in Figure 1(c), which demonstrates MIL-53 (Al)-F127 particles were located uniformly on the fibers of the membrane. And the membrane still maintained its porous structure.

Figure 1.

SEM image of (a) bacterial cellulose, (b) MIL-53 (Al)-F127, and the (c) BC/MIL-53 (Al) -F127 membrane.

Fourier transform infrared and X-ray diffraction analysis

Figure 2(a) shows the FT-IR spectra of the BC/MIL-53 (Al)-F127 composite membrane, MIL-53 (Al)-F127, and BC. As shown in Figure 2(a), a broad adsorption peak at 3344 cm⁻¹ indicates the -OH on pure BC⁴¹ And the adsorption peak for the symmetric bending of CH₂ appears at 1427 cm⁻¹.⁴² The characteristic peak for the C-O-C asymmetric stretching is observed at 1162 cm⁻¹.⁴¹ And the characteristic peak at 1033 cm⁻¹ was ascribed to the C-O vibrations .⁴³ And for MIL-53 (Al)-F127, a broad characteristic peak at 3380 cm⁻¹ is attributed to -OH groups. The characteristic peak for carbonyl groups (-C=O) appears at 1694 cm⁻¹ due to the free terephthalic acid molecules that are encapsulated in the pores as their protonated form (-CO₂H). The adsorption peaks for the O-C-O anti-symmetrical stretching vibration were observed at 1594 and 1508 cm⁻¹, and the characteristic peaks at 1508 and 1414 cm⁻¹ are for O-C-O symmetrical stretching vibration, confirming the existence of dicarboxylic acid groups, which can make the MIL-53 (Al) (Supplementary Figure S3(a)) network bind to hydroxyl groups and connect AlO₆ octahedrons into infinite chains.^44,45 The spectrum of the BC/MIL-53 (Al)-F127 membrane contains all of the characteristic peaks of BC and MIL-53 (Al)-F127. And it shows no new peaks or other obvious change, demonstrating that only physical interaction existed between MIL-53 (Al)-F127 and BC.

Figure 2.

(a) The FT-IR spectra of pure BC, MIL-53 (Al)-F127, and the BC/MIL-53 (Al)-F127 composite. (b) X-ray diffraction patterns of BC, MIL-53 (Al)-F127, and the BC/MIL-53 (Al)-F127 membrane. BC: bacterial cellulose.

X-ray diffraction patterns of BC, MIL-53 (Al)-F127, and BC/MIL-53 (Al)-F127 membranes are illustrated in Figure 2(b). As for MIL-53 (Al)-F127 spectrum, the main diffraction peaks are observed at 9.8°, 17.4°, and 21.5°, corresponding to the (100), (111), and (220) crystal planes, which are consistent with the literature,³⁸ indicating MIL-53 (Al)-F127 is successfully synthesized. However, the intensive peaks and dispersed peaks are shown in the MIL-53 (Al)-F127 pattern, indicating that the crystalline phase and amorphous phase coexist in MIL-53 (A1)-F127 (Supplementary Figure S3(b)). There are three main characteristic peaks at 14.9°, 17.1°, and 22.7° in the BC pattern, corresponding to the (1 10), (1 10), and (2 00) crystallographic planes of cellulose I, respectively.⁴⁶ The pattern of the BC/MIL-53 (A1)-F127 membrane exhibits all the characteristic peaks of MIL-53 (A1)-F127 and BC, indicating the successful combination of MIL-53 (Al)-F127 with BC.

N₂ adsorption–desorption analysis

Figure 3 illustrates the nitrogen adsorption–desorption isotherm and pore diameter distribution of the composite membrane. The adsorption curve in Figure 3(a) reveals that the largest and lowest surface area are MIL-53 (Al)-F127 and BC, respectively. As for the BC/MIL-53 (Al)-F127 membrane, the surface area was about 91.6537 m²/g, which was much larger than BC without MIL-53 (Al)-F127 (26.6687 m²/g), although lower than MIL-53 (Al)-F127 (247.105 m²/g). Apparently, the combination of MIL-53 (Al)-F127 had a determining influence with BC, and the high surface area of the composite membranes obtained in this work was due to the introduction of MIL-53 (Al)-F127. Besides, it can be found that the main pore size is about 4 nm according to the pore diameter distribution curve, indicating the existence of mesostructure in MIL-53(Al)-F127. More details of surface properties are listed in Supplementary Table S1.

Figure 3.

Nitrogen adsorption–desorption isotherm of (a) bacterial cellulose, MIL-53 (Al)-F127, and BC/MIL-53 (Al)-F127. (b) Pore diameter distribution.

Adsorption properties and adsorption kinetics

The BPA removal efficiency and adsorption capability by the as-prepared composite membrane are indicated in Figure 4(a) and (b) (Supplementary Figure S5 shows the adsorption kinetics of BC/MIL-53 (Al)-F127 with various sizes (1 × 1, 2 × 2, 2.5 × 2.5, 3 × 3, 3.5 × 3.5 cm)). As shown in Figure 4(a) and (b), the adsorption process could reach equilibrium within 120 min, and the removal efficiency and sorption capability of BPA for the BC/MIL-53 (Al)-F127 membrane are approximately 94 (± 2.17)% and 8.3 (± 0.77) mg/g, respectively. As for MIL-53 (Al)-F127, the removal efficiency and sorption capability are 78.9 (± 1.38)% and 70.2 (± 1.23) mg/g, respectively. Besides, the removal efficiency and sorption capability for BC is 1.2 (± 0.43) % and 0.1 (± 0.04) mg/g, and with the time increased, BC absorbed more water than BPA, so the BPA concentration of the solution increased, and the removal efficiency of BPA was negative. MIL-53 (Al)-F127 was the main material that adsorbed BPA. Figure 4(b) shows that the sorption capability for the BC/MIL-53 (Al)-F127 membrane is lower than that for MIL-53 (Al)-F127. The main reason is that MIL-53 (Al)-F127 was the main material for adsorption, and BC is the supporting substrate. And the equilibrium adsorption capacity was calculated by concentration of BPA and the mass of composite membranes, so the mass of BC accounts for a large proportion of the total mass of the composite membrane which has a great influence on the calculation of the value of the equilibrium adsorption amount. Figure 4(a) reveals the sorption rate is high at the incipient stage, and then it gradually levels off, which is attributed to the abundant vacant adsorption sites exposed by the adsorbents in the initial stage and then the gradual decrease of adsorption sites and the electrostatic repulsion between BPA molecules with increased time, leading to the lower sorption rate and even equilibrium.

Figure 4.

The sorption kinetics of the BC/MIL-53 (Al)-F127 membrane, bacterial cellulose, and MIL-53 (Al)-F127. Effect of contact time on (a) the BPA removal efficiency and (b) the equilibrium sorption amounts, (c) the pseudo–first-order kinetics model and (d) pseudo–second-order kinetics model of BPA adsorption on the BC/MIL-53 (Al)-F127 membrane. BPA: bisphenol A.

The pseudo–first-order and pseudo–second-order kinetics model plots of BPA adsorption by the BC/MIL-53 (Al)-F127 composite membrane are illustrated in Figure 4(c) and (d), which are utilized to simulate the experimental data to describe the adsorption mechanism of the BC/MIL-53 (Al)-F127 membrane (more details in Supplementary Figure S5). The correlation coefficients R² obtained from the BC/MIL-53 (Al)-F127 pseudo–first-order curve fitting is 0.927. And R² obtained from pseudo–second-order curve fitting is 0.954. The q_e values obtained by fitting the pseudo–first-order and pseudo–second-order kinetic curves are 12.853 mg/g and 10.7446 mg/g, respectively (more details in Supplementary Table S2). The theoretical value (q_e) obtained by the pseudo–second-order model is well consistent with the experimental value, demonstrating that the experimental data of BPA adsorption are better fitted with the pseudo–second-order kinetic model.

Sorption capacities of BPA by other sorption materials reported in the other literatures are listed in Table 1. The sorption capacities with other sorbents may be tested under different conditions, but it still provides some references in terms of adsorption capacity. The removal efficiency of BPA for the composite membrane BC/MIL-53 (Al)-F127 with a size of 3 × 3 cm could also reach 94%. In addition, the composite membrane presented the advantage to separate freely without a complex filtration system. The results indicated that the BC/MIL-53 (Al)-F127 membrane was a potential sorbent for water treatment.

Table 1.

Sorption capacities of bisphenol A by other sorption materials.

Adsorbent	Removal efficiency (%)	Sorption capacity (mg/g)	Reference
Commercial chitosan	55	27.02	19
Graphite oxide	78	17.27	14
Fe3O4@polyaniline core shell nanomaterials	97	23.09	47
Commercial granular activated carbon	96	3.54	48
MIL-100 (Fe)	—	55.60	49
MIL-101 (Cr)	—	252.50	49
MIL-53 (Al)-F127	79	70.20	This work
BC/MIL-53 (Al)-F127	94	8.30	This work

Effects of solution pH on bisphenol A adsorption

The pH of the solution is a significant element for BPA adsorption since it defines the form of BPA molecules in the solution phase, which affects the adsorption process and adsorption capacity. A comparison study between the BC/MIL-53 (Al)-F127 membrane and pure BC was carried out to determine the optimum reaction pH value. Figure 5 shows that the optimum pH of adsorption is six for BC/MIL-53 (Al)-F127. As for pure BC, the concentration of BPA solution was changed due to the hydrophilicity of BC. The comparison study also indicates that the mesostructured MIL-53 (Al)-F127 exhibited good adsorption properties for BPA. In acidic conditions, as the pH decreased, the adsorption capacity gradually decreased because Al (OH)₄ on the adsorbent was occupied by H⁺, so hydrogen bonds with BPA molecules became less. So π–π bonds would be the main interaction for the BPA adsorption. In the optimum pH condition, there was a synergistic effect with π–π bonds and hydrogen bonding to adsorb BPA. However, as the pH of the BPA solution increased, the removal efficiency decreased almost linearly, which may be due to the ionization of BPA molecules into monovalent or divalent anions (BPA⁻ and BPA^2-), so the hydrogen bonds between adsorbent molecules and BPA disappeared under alkaline conditions.⁵⁰ In fact, this experiment also included the adsorption of BPA at pH 10 and 12, but the absorption peak by the ultraviolet spectrophotometer has changed, which is contributed to the change of BPA molecules. The absorption degree of the BPA solution cannot be determined.

Figure 5.

Removal efficiency of bisphenol A at different solution pH.

Effects of solution temperature on bisphenol A adsorption

Temperature is an important factor for BPA adsorption which is involved in Gibbs free energy. Figure 6 presents the influence of temperature on BPA adsorption. The results indicate that 40°C is the optimal temperature for the BPA adsorption process in the range of 10–60°C. When the temperature was below or above the optimal temperature, the removal efficiency for BPA decreased. Above the optimal temperature, the amount of adsorption decreased with increasing temperature because the interaction between the BPA and the adsorbent molecules was weakened due to the enhanced thermal movement of the molecules. Additionally, the adsorption process is exothermic; when the temperature is high, BPA tends to be desorbed from the solid phase to the aqueous phase.³⁹ Nevertheless, BPA molecules move slowly at the temperature of 10°C or lower, which may limit BPA molecules to diffuse to the pores of the adsorbent. Therefore, the adsorption amount of the adsorbent to BPA decreases below the optimal temperature.

Figure 6.

Removal efficiency of bisphenol A at different solution temperature.

Recyclability of BC/MIL-53 (Al)-F127 nanofibrous membranes

The reusability of adsorbents is actually significant for many practical applications. As shown in Figure 7, the removal efficiency of BC/MIL-53 (Al)-F127 membranes remains at 45.8 $\pm$ 1.4% (the initial removal efficiency = 50.2 $\pm$ 1.2%) toward BPA after reusing 5 times. The removal efficiency of BPA is well maintained at 90% even after five repeated cycles. The results indicate that the as-prepared membranes had an excellent reusabiliity.

Figure 7.

Cycling stability of BC/MIL-53 (Al) -F127 membranes.

Adsorption mechanism

MIL-53 (Al)-F127 is formed by self-assembly of Al ions and terephthalic acid under the solvothermal method. The hollow structure of MIL-53 (Al)-F127 is formed using structure-directing agents.³⁸ Some research works have demonstrated that the breathing phenomena are formed mainly by the structural transformation induced by the MIL-53 (Fe, Cr, Al) family,⁵¹ which gives the possibility for some molecules larger than the pore size to enter. BPA molecules could enter the pores of MIL-53 (Al)-F127 because of the breathing action, even though the dynamic diameter of BPA molecules is larger than the hollow wall of MIL-53 (Al)-F127. After BPA is adsorbed into pores, the interactions between BPA molecules and functional groups on MIL-53 (Al)-F127 molecules are formed. π–π bonds and hydrogen bonding are the main forces during the BPA adsorption on the adsorbent. BPA molecules includes the benzene ring and phenolic group. Therefore, π–π bonds are formed between the benzene ring on the BPA molecule and MIL-53 (Al)-F127 containing benzene rings.⁵² The other interaction for BPA adsorption on the adsorbent is hydrogen bonding. The BPA molecule including two –OH groups is the existence form for BPA in aqueous solution under acidic or neutral conditions. So hydrogen bonds are formed between Al(OH) on MIL-53 (Al)-F127 molecules and -OH on BPA. In alkaline conditions, BPA molecules are deprotonated and ionized into monovalent or divalent anions (BPA⁻ and BPA²⁻), so the hydrogen bonds between adsorbent molecules and BPA disappear.⁵⁰ To better understand the adsorption mechanism for MIL-53 (Al)-F127 on BPA, more detailed work is needed.

Bacterial cellulose nanofibrous membranes are hydrophilic, which contributed to adsorption of BPA in aqueous solution. Through the above comparative experiments, it is revealed that BC has no or less BPA adsorption but is hydrophilic, which further proves that the MOF structure MIL-53 (Al)-F127 has an excellent ability to adsorb BPA. And Figure 8 displays the schematic illustration of the adsorption mechanism of BPA for the BC/MIL-53 (Al)-F127 membrane.

Figure 8.

Schematic illustration of the adsorption mechanism of bisphenol A on the BC/MIL-53 (Al)-F127 membrane.

Conclusion

The BC/MIL-53 (Al)-F127 composite membrane that is synthesized by vacuum filtration in this work showed excellent adsorption behavior for BPA in wastewater. According to the experimental results, the optimum pH and temperature were 6 and 40°C, while the BC/MIL-53 (Al)-F127 adsorbent size was 3 × 3 cm, respectively. Remarkably, BPA could be adsorbed by 94% within 120 min under optimal reaction conditions, and the adsorption kinetics studies indicated that the adsorption process followed the pseudo–second-order kinetic model. In addition, the membrane could be easily reused washed by methanol. Cycling stability experiments revealed that the removal efficiency of BPA was well maintained at 90% even after five repeated cycles. Besides, the absorption mechanism of BPA was speculated due to the breathing phenomena of MIL-53 (Al)-F127. Adsorption affinities between the adsorbents and BPA are π–π bonds and the hydrogen bonds. BC nanofibrous membranes are hydrophilic, which contributed to adsorption of BPA in aqueous solution. These results suggest that BC/MIL-53 (Al)-F127 could be a promising, conveniently prepared, and effective material for BPA adsorption.

Supplemental Material

sj-pdf-1-jit-10.1177_15280837211041769 – Supplemental Material for Eco-friendly sorbent of bacterial cellulose/metal–organic framework composite membrane for effective bisphenol a removal

Supplemental Material, sj-pdf-1-jit-10.1177_15280837211041769 for Eco-friendly sorbent of bacterial cellulose/metal–organic framework composite membrane for effective bisphenol a removal by Yue Sun, Xin Li, Dawei Li, Huizhen Ke and Qufu Wei in Journal of Industrial Textiles

Footnotes

Author contributions

Yue Sun: Conceptualization, methodology, conducting experiments, data analysis, and writing—original draft preparation.

Xin Li: Visualization, methodology, writing— review and editing. Dawei Li: Investigation, writing—review and editing.

Huizhen Ke: Visualization and investigation.

Qufu Wei: Visualization, supervision, writing—review and editing.

All authors read and approved the final article.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was financially supported by the Natural Science Foundation of Jiangsu Province (BK20180628), the National Natural Science Foundation of China (51803078), the China Postdoctoral Science Foundation (2019M661723), the national first-class disc.

Data availability

The datasets supporting the conclusions of this article are included within the article and its supplementary information files.

ORCID iD

Yue Sun

Qufu Wei

Supplemental material

Supplemental material for this article is available online.

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Supplementary Material

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Eco-friendly sorbent of bacterial cellulose/metal–organic framework composite membrane for effective bisphenol a removal

Abstract

Keywords

Introduction

Materials and methods

Materials

Preparation of bacterial cellulose

Preparation of MIL-53 (Al)-F127

Preparation of BC/MIL-53 (Al)-F127 nanofibrous membranes

Measurements and characterization

Adsorption experiments

Recyclability of BC/MIL-53 (Al)-F127 nanofibrous membranes

Results and discussion

Morphology analysis of BC/MIL-53 (Al)-F127 nanofibrous membrane

Fourier transform infrared and X-ray diffraction analysis

N2 adsorption–desorption analysis

Adsorption properties and adsorption kinetics

Effects of solution pH on bisphenol A adsorption

Effects of solution temperature on bisphenol A adsorption

Recyclability of BC/MIL-53 (Al)-F127 nanofibrous membranes

Adsorption mechanism

Conclusion

Supplemental Material

sj-pdf-1-jit-10.1177_15280837211041769 – Supplemental Material for Eco-friendly sorbent of bacterial cellulose/metal–organic framework composite membrane for effective bisphenol a removal

Footnotes

Author contributions

Declaration of conflicting interests

Funding

Data availability

ORCID iD

Supplemental material

References

Supplementary Material

N₂ adsorption–desorption analysis