Different Adsorption Behaviors and Mechanisms of Anionic Azo Dyes on Polydopamine–Polyethyleneimine Modified Thermoplastic Polyurethane Nanofiber Membranes

Abstract

Considering the notable mechanical properties of thermoplastic polyurethane (TPU), polydopamine–polyethyleneimine (PEI) -modified TPU nanofiber membranes (PDA/PEI-TPU NFMs) have been developed successfully for removal of anionic azo dyes. The adsorption capacity of PDA/PEI-TPU NFMs was evaluated using three anionic dyes: congo red (CR), sunset yellow (SY), and methyl orange (MO). Interestingly, it exhibited different adsorption behaviors and mechanisms of CR on PDA/PEI-TPU NFMs compared with SY and MO. With the decrease in pH, leading to more positive charges on the PDA/PEI-TPU NFMs, the adsorption capacity of SY and MO increased, indicating electrostatic interaction as a main mechanism for SY and MO adsorption. However, wide pH range adaptability and superior adsorption have been observed during the CR adsorption process compared to SY and MO, suggesting a synergistic effect of hydrogen bonding and electrostatic interaction, likely as a critical factor. The adsorption kinetics revealed that chemical interactions predominate in the CR adsorption process, and multiple stages control the adsorption process at the same time. According to the Langmuir model, the maximum adsorption capacity of CR, SY and MO were reached 263, 17 and 23 mg/g, respectively. After six iterations of adsorption–desorption, the adsorption performance of the PDA/PEI-TPU NFMs did not decrease significantly, which indicated that the PDA/PEI-TPU NFMs have a potential application for the removal of CR molecules by adsorption from wastewater.

1. Introduction

Dye contamination has become one of the most severe aspects of global water pollution [1,2]. Azo dyes account for more than two-thirds of all synthetic dyestuffs [3]. Presumably, at least 2000 different types and over 700,000 tons of azo dyes have been produced annually worldwide for the textile dyeing, pharmaceutical and paper industries [4]. However, about 10–15% of these azo dyes are unutilized and discharged into the environment through wastewater [5]. Congo red (CR), sunset yellow (SY), and methyl orange (MO) are three typical anionic dyes, which have been widely applied in the experimental analysis, food processing, plastics, rubber, printing and optoelectronics industries [6,7,8]. Under special circumstances, these azo dyes can be bio-transformed to release aromatic amines with toxicity, mutagenicity and persistence, which can alter DNA transcription, lead to lesions and bring on cancer once they are ingested by animals [9]. Hence, it is required to treat azo dye wastewater before its final discharge into water bodies.

Undoubtedly, adsorption technology has been widely used for the removal of dye stuffs, owing to its simplicity, high efficiency and low cost [10]. In recent decades, a multitude of adsorption nanoscale-materials, including carbon-based nanomaterials [11], nanocomposites [12] and nanofiber membranes [13,14], have been developed to remove organic dyestuffs. Among them, nanofiber membranes have attracted substantial interest for the removal of micropollutants due to their very large specific surface areas, high porosity and highly controlled surface characteristics [15]. Compared with other materials dispersed in the wastewater during the water treatment process, the nanofiber membrane, after adsorption, can be taken out from the wastewater as a whole or directly used as an adsorption filter membrane, thus avoiding the secondary removal of the adsorbed materials and improving the treatment efficiency. However, most of the polymer nanofiber membranes have poor mechanical properties that restrict their universal practicability [16,17].

Polyurethane (PU) is an engineered polymer material that is widely used in medical dressings, architectural coatings and fabric production. Some prominent features, such as durability, tensile fatigue resistance, shear resistance and elasticity are worth mentioning [18,19]. Therefore, it is possible that the mechanical properties of the nanofiber membranes prepared by PU can be improved compared to other membrane materials. In recent years, there has been an increasing amount of literature related to the application of PU foam-based adsorbents for oil–water separation [20,21,22], heavy metals [23,24] and dye [25,26] removal, which shows that this material has great potential for pollutant removal. As adsorbents, the external functional groups on PU play a vital role in adsorption, influencing the adsorption mechanisms. The molecule dopamine (DA) can securely connect to substrates by self-polymerization to produce a polydopamine (PDA) coating under mild alkaline conditions [27]. Therefore, DA has been used extensively in surface modification of the material to remove organic dyestuffs from polluted water [28,29]. Polyethyleneimine (PEI) is a typical hyperbranched polymer with excellent adsorption ability for organic dyes, as its polymer chains contain a large quantity of primary and secondary amine groups [30,31,32]. Functional PDA-PEI coating can be constructed by PEI-modified PDA coating, based on Michael addition or Schiff base reaction [33].

As our goal was to removal anionic azo dyes, TPU nanofiber membranes (TPU NFMs) were used, as they are soft and stretch-resistant, which can make up for the shortcomings of traditional film materials. PDA/PEI co-deposition can occur under mild conditions. This represented an easier, greener and less expensive material preparation process. Thus, we selected DA and PEI to construct a functional PEI-PDA coating in order to modify the TPU NFMs. We hypothesized that the modified TPU NFMs would exhibit good stability and adsorption capacity for anionic azo dyes. The TPU nanofiber membranes were, firstly, prepared by electrospinning. Then, TPU NFMs were modified by PDA/PEI co-deposition to obtain PDA/PEI-TPU NFMs. The membranes were characterized by X-ray photoelectron spectrometer (XPS), contact angle instruments, Fourier transform infrared spectrometer (FTIR) and scanning electron microscopy (SEM). Three typical anionic azo dyes, including CR, SY and MO, were selected as the main adsorption objects to explore the adsorption behavior of the prepared PDA/PEI-TPU NFMs.

2. Experimental Sections

2.1. Chemicals and Materials

All chemicals were used as obtained without additional purification: polyethyleneimine (PEI, M_W = 10,000, 30% solution in water) and dopamine hydrochloride (DA, 98%) were purchased from J&K Scientific Co., Ltd. (Beijing, China). TPU powder was provided by BASF Co., Ltd. (Shanghai, China). N,N-Dimethylformamide (DMF, C₃H₇NO), acetone (C₃H₆O, AR), aqueous hydrochloric acid (HCl, AR), sodium hydroxide (NaOH, AR), congo red (C₃₂H₂₂N₆Na₂O₆S₂, AR) and methyl orange (C₁₄H₁₅N₃NaO₂S, AR) were obtained from Chuandong Chemicals Co., Ltd. (Chongqing, China). Sunset yellow (C₁₆H₁₀N₂Na₂O₇S₂, AR) and Tris(hydroxymethyl)aminomethane (C₄H₁₁NO₃, AR) were obtained from Chengdu Chron Chemicals Co., Ltd. (Chengdu, China).

2.2. Synthesis of TPU and PDA/PEI-TPU NFMs

Figure 1 depicts the adsorptive PDA/PEI-TPU NFMs preparation schematic. First, TPU NFMs were synthesized by the electrospinning method. A specific quantity of TPU powder was added to acetone (DMF (v:v = 80:20)) and stirred for 12 h to prepare 15 wt% TPU spinning solutions. The prepared electrospinning solution was injected into a 5 mL plastic syringe fitted with a steel spinneret (inner diameter: 0.25 mm), and electrospun for 5 h. Other optimized electrospun conditions were voltage (15 kV), injection speed rate (1 mL/h) and receiving distance (10 cm) [34]. The resulting TPU NFMs were cleaned with ethanol and vacuum-dried to use as the support membrane for subsequent modification.

Then, 0.2 g of DA and 0.2 g of PEI were dissolved in 100 mL of Tris buffer solution (pH = 8.5, 50 mM), and approximately 1.0 g of cleaned TPU NFMs were completely immersed in this DA/PEI mixture. The co-aggregation process was implemented at ambient temperature while being continuously shaken at 100 rpm for 12, 24, 36, 48, and 60 h. The time of co-aggregation affected the degree of PDA/PEI grafting on the surface of TPU NFMs. As the reaction proceeded, particles were gradually deposited on the surface, resulting in a darker color of the membranes. Pre-experiments also showed that the maximum adsorption capacity of dyes was found to be reached in the material co-deposited for 48 h, and as the deposition time increased, it did not significantly improve the adsorption of dyes. Therefore, the deposition time of this experiment was chosen to be 48 h. In addition, PDA/TPU NFMs were prepared as comparative adsorbents by adding 0.2 g of DA and 1.0 g of cleaned TPU NFMs to 100 mL of Tris buffer solution. Other conditions were consistent. After the co-aggregation, the obtained PDA-TPU NFMs and PDA/PEI-TPU NFMs were taken out, soaked and cleaned with deionized water for 24 h in order to remove unreacted substances on the surface. They were vacuum-dried at ambient temperature, then stored for subsequent characterization and measurement. Figure 1 also further shows the molecular structure of the co-aggregation reactions involving DA and PEI. Under alkaline conditions, DA oxidation auto-polymerization produces reactive sites, which react with the positively charged PEI molecules to form a stable cross-linked structure by Michael addition and Schiff base reaction [35].

2.3. Characterization of Adsorbents

The surface morphology of the membranes was examined by a scanning electron microscope (SEM, INSPECT F50, MA, USA). The surface chemical composition of the membranes was performed on X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, MA, USA). The functional groups of the membranes were obtained by FT-IR spectrometer (Thermo-Nicolet 670, MA, USA) equipped with an attenuated total reflection (ATR) unit. The static contact angles of the membranes were characterized by contact angle system (HARKE-SPCA-X3, Beijing, China). The concentration of dyes was quantitatively measured on an ultraviolet spectrophotometer (UV-vis-3150, Kyoto, Japan) from 600 to 200 nm, using deionized water for the background correction.

The net weight gain of the modified membranes was calculated by Equation (1):

(1)${\mathrm{W}}_{\mathrm{g}}=\frac{{\mathrm{m}}_1-{\mathrm{m}}_0}{{\mathrm{m}}_0}\times 100\%$

where m₀ (g) is the constant weight of pre-modified TPU NFMs, and m₁ (g) is the constant weight of the membranes after PDA and PEI modification.

2.4. Batch Adsorption Experiments

The initial and residual concentration of dyes were measured by a UV–vis spectrophotometer at the maximum wavelengths of (λ_max) 497 nm, 482 nm and 464 nm for CR, SY and MO, respectively. The adsorption capacity was calculated by Equation (2). Three parallel adsorption experiments were repeatedly carried out, and the average value was used to determine the final result.

(2)$\mathrm{Q}=\frac{\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{t}}\right)\mathrm{V}}{\mathrm{m}}$

where m (g) is the weight of membrane, C₀ (mg/L) is the initial concentration, C_t (mg/L) represents the residual concentrations at different time intervals, respectively, and V (L) is the volume of the dye solution.

2.4.1. The Impact of Initial Solution pH

The influences of original solution pH, which ranged from 4.0 to 10.0 on the capture of three azo dyes (CR, SY and MO) by PDA/PEI-TPU NFMs, were further investigated. The initial concentrations of CR, SY and MO were 100 mg/L each. Constant shaking was performed at room temperature, with 20 mg of adsorbents in 50 mL of CR solution, 20 mL of SY solution and 20 mL of MO solution, for 24 h. In the experiment, 1 M HCl or 1 M NaOH solution was used to adjust the pH.

2.4.2. Equilibrium Isotherms

The effect of initial concentrations of three dyes onto PDA/PEI-TPU NFMs were investigated under neutral conditions by isotherm adsorption experiment. The initial concentration of CR ranged from 50–400 mg/L, while SY and MO ranged from 5–100 mg/L. All the experiments were performed by adding 20 mg of PDA/PEI-TPU NFMs into 20 mL of dyes solution, and then shaking at 150 rpm for 24 h at room temperature to achieve adsorption equilibrium. Moreover, Langmuir and Freundlich adsorption isotherm models (Equations (3) and (4)) were implemented in order to simulate the experimental data and to define the maximum dye adsorption capacity of the PDA/PEI-TPU NFMs [36].

(3)$\mathrm{Langmuir}: {\mathrm{q}}_{\mathrm{e}}=\frac{{\mathrm{q}}_{\max }{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}{1+{ \mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}$

(4)$\mathrm{Freundlich}: {\mathrm{q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{F}}{\mathrm{C}}_{\mathrm{e}}^{\frac{1}{\mathrm{n}}}$

In the present equation, q_max (mg/g) is the maximum equilibrium adsorption capacity of the PDA/PEI-TPU NFMs; K_L (L/mg) is Langmuir binding constant; C_e (mg/L) and q_e (mg/g) are the equilibrium concentration and adsorbent uptake capacity at equilibrium, respectively; and K_F ((mg/g)/(mg/L)ⁿ) and n are the empirical Freundlich constant and heterogeneity factor, respectively.

2.4.3. Kinetic Adsorption

The effect of different contact times of three dyes adsorbed onto PDA/PEI-TPU NFMs, from 0 to 48 h and under neutral conditions, were investigated in order to better understand the adsorption process. The initial dye concentration of CR was 300 mg/L; for both SY and MO, the concentration was 100 mg/L. Then, 200 mg of PDA/PEI-TPU NFMs were added to 200 mL of dye solution, and then shaken at 150 rpm. In order to further examine the adsorption process, the pseudo-first-order model, pseudo-second-order model and intra-particle diffusion model were used to analyze the kinetic data (Equations (5)–(7)) [37].

(5)$\mathrm{Pseudo-first-order} \mathrm{model}: {\mathrm{q}}_{\mathrm{t}}={\mathrm{q}}_{\mathrm{e}}\left(1-{\mathrm{e}}^{-{\mathrm{k}}_1\mathrm{t}}\right)$

(6)$\mathrm{Pseudo-second-order} \mathrm{model}: {\mathrm{q}}_{\mathrm{t}}=\frac{{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2\mathrm{t}}{1+{ \mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}\mathrm{t}}$

(7)$\mathrm{Intraparticle} \mathrm{diffusion} \mathrm{model}: {\mathrm{q}}_{\mathrm{t}}={\mathrm{k}}_{\mathrm{i}}\times {\mathrm{t}}^{\frac{1}{2}}+\mathrm{b}$

Herein, q_e (mg/g) and q_t (mg/g) are the adsorption capacity at the equilibrium time and the desired time, respectively. k₁ (1/h)), k₂ (g/(mg h)) and k_i (mg/(g h^1/2)) are the pseudo-first-order model rate constant, the pseudo-second-order model rate constant and the intraparticle diffusion rate constant, respectively, and b is the intercept of the linear portion of the equation.

2.4.4. Desorption Experiments

A desorption experiment was set up to evaluate the recycling performance of materials. In the experiment, 0.05 M NaOH solutions were employed as desorption solutions. The PDA/PEI-TPU NFMs, after reaching adsorption equilibrium, were taken out and placed in 20 mL of 0.05 M NaOH solution for ultrasonic inspection for 60 min, and then the CR concentration in the desorption solution was measured. The PDA/PEI-TPU NFMs, after desorption, were collected and rinsed sufficiently. Finally, they were dried in an oven before the next cycle. The adsorption capacity and desorption capacity were calculated using Equation (2).

3. Results and Discussion

3.1. The Grafted Yield and Stability of PDA/PEI-TPU NFMs

The grafted yields of PDA and PDA/PEI on TPU NFMs were calculated by the constant weight method. The weight loss of the membranes after ultrasonic treatment was also studied to roughly evaluate the stability of PDA-TPU NFMs and PDA/PEI-TPU NFMs. It can be seen in Figure 2a that when modified only by PDA, the grafted yield of the PDA-TPU NFMs was 5.7%. By adding PEI, the grafted yield of the PDA/PEI-TPU NFMs increased notably, to 10.2%. As shown in Figure 2b, no weight loss of PDA-TPU NFMs or PDA/PEI-TPU NFMs was observed even, after being ultrasonically treated at a power of 200 W for 60 min, demonstrating the high stability of PDA-TPU NFMs and PDA/PEI-TPU NFMs. Obviously, the PDA/PEI-TPU NFMs have good adsorption stability and a possibility of long-term application due to their high adhesion stability.

3.2. Morphology and Microstructure of the Nanofiber Membranes

The morphology and microstructure of the electrospun TPU NFMs, PDA-TPU NFMs and PDA/PEI-TPU NFMs were analyzed by SEM. As shown in Figure 3a,b, we can see that the pure TPU nanofibers exhibited quite smooth surfaces and relatively slim fiber contours, the average diameter of which was uniformly distributed at about 160 nm. After being coated by a PDA layer (Figure 3d,e), some nanoparticles were deposited on the surface, resulting in a rougher surface, and the average diameter of PDA-TPU nanofibers was slightly increased to 263 nm. Figure 3g,h show the morphology and microstructure of PDA/PEI-TPU NFMs. Obviously, PDA/PEI-TPU NFMs have a much different morphology compared with the TPU and PDA-TPU NFMs. A rougher fiber surface and larger average diameter (502 nm) were observed, which indicated significant adhesion of PDA/PEI cross-linked aggregates on the modified membrane surface. Some studies have reported that DA is able to trigger oxidative self-polymerization reactions in a weakly alkaline environment, leading to homogeneous coating on any polymer membranes or nanofibers [38,39,40]. PEI and DA are covalent cross-linked, based on both Michael addition and Schiff reaction, for forming robust aggregates on the TPU NFMs surface. In addition, PEI has a large number of cationic amino groups. While the intermediate product of PDA, catechol, has a strong conjugation system, the cation-π interaction between them will enhance the internal cohesion of co-aggregated coating, thus improving the PEI grafted amounts on the surface of TPU NFMs [39].

The ATR-FTIR was used to study the surface functional groups of the TPU NFMs, PDA-TPU NFMs and PDA/PEI-TPU NFMs; the results are shown in Figure 4. The typical characteristic absorption bands of TPU NFMs at 3355, 1727 and 1061 cm⁻¹ were assigned to the N-H stretching vibration, C = O stretching vibration and C-O-C stretching vibration, respectively [41]. After being deposited only with PDA, the intensity of a broad characterization absorption band at 3100–3650 cm⁻¹ increased compared with TPU NFMs, which can be attributed to the stretching vibration of the O-H and N-H groups. For the PDA/PEI-TPU NFMs, the new absorption band was observed at 1659 cm⁻¹, and can be ascribed to the formation of C = N bonds stretching vibrations between PEI and PDA [33]. The creation of a C = N bond suggested that PEI was successfully grafted onto TPU NFMs surface by reacting with the active site generated through DA oxidative self-polymerization.

XPS investigation further showed the elemental compositions and chemical combined state of TPU NFMs, PDA-TPU NFMs and PDA/PEI-TPU NFMs. From the XPS full spectrum (Figure 5a) and the elemental percentages of C, O and N (Table 1), it can be clearly observed that the elemental N content in both PDA-TPU NFMs and PDA/PEI-TPU NFMs are increased compared with TPU NFMs, but the PEI modification has a considerably higher amount of the N element. This is mainly attributed to a small amount of terminal amino group in the PDA and the great quantity of amino groups in PEI. Meanwhile, the high-resolution spectrum of the element N in membranes is analyzed. The core spectral curves of N1s, fitted by XPS Peak software, are shown in Figure 5b,d. The original TPU NFMs surface contained few C-N structures due to the urethane and urea ester bonds in TPU NFMs [42,43]. After introducing PDA, a significant increase in the peak intensity of the C-N structure occurred. While PEI introduced the deposition process, the C-N structure intensity on the surface of PDA/PEI-TPU NFMs also dramatically increased (Figure 5d). A new peak also appeared, located in 402.1 eV, which was attributed to the C = N structures formed by PEI and DA [39]. These results further indicate that the co-aggregation process occurred successfully in the TPU NFMs.

PDA and PEI with hydrophilic properties, when introduced onto the PDA/PEI-TPU NFMs, can improve the hydrophilicity of the membrane surface. Therefore, contact angle measurement was used to confirm the change in hydrophilicity of the membrane surface. The images of water drop on the nanofiber membrane surfaces are shown in Figure 6. Within 3 s, the water contact angle of TPU NFMs changed insignificantly, while PDA-TPU NFMs and PDA/PEI-TPU NFMs showed better water permeability, with the water contact angle decreasing from 31.1° and 19.5° at the beginning to less than 10°. These phenomena indicate that the presence of PDA and PEI greatly improves the hydrophilicity of the PDA-TPU NFMs and PDA/PEI-TPU NFMs compared to TPU NFMs. This mainly contributed to the abundance of polar groups in PDA that can attract water molecules, as well as the many -NH⁺ and -NH₂ groups induced by co-aggregation of PEI and PDA. The enhanced hydrophilicity of the PDA/PEI-TPU NFMs facilitates the removal of contaminants by this adsorbent in the aqueous environment.

3.3. Dye Adsorption Ability

In order to study the adsorption performance of the membranes (TPU NFMs, PDA-TPU NFMs and PDA/PEI-TPU NFMs) towards various anionic azo dyes, we chose three classical anionic dyes (CR, SY and MO) as model molecules (Figure 7). Compared with original TPU NFMs and PDA-TPU NFMs, the adsorption capacities of CR, SY and MO onto PDA/PEI-TPU NFMs were significantly enhanced, which indicated that cationic properties of PEI modification are an effective way to improve the adsorption ability for anionic dyes. Obvious differences were also found in the adsorption capacities of different azo dyes onto PDA/PEI-TPU NFMs. The adsorption ability of CR reached 232.55 mg/g, while for SY and MO, the values were, respectively, only 16.58 and 15.57 mg/g under the same neutral conditions, far less than CR. In addition, the pure TPU NFMs and PDA-TPU NFMs exhibited some adsorption for CR, but almost no adsorption for SY and MO. The different adsorption behavior may be attributed to the different adsorption mechanisms for CR and SY versus MO. The high adsorption capacity for CR also suggests that PDA/PEI-TPU NFMs have great potential in the treatment of CR-dyed wastewater.

Further, we investigated the effect of the solution pH on adsorption capacity of PDA/PEI-TPU NFMs for three dyes. The initial solution pH is very important for the adsorption process, as pH determines the degree of protonation of functional groups, as well as competitive adsorption between contaminants and hydrogen ions [44]. Since CR, MO and SY have good solubility in the pH range of 4–10, this pH range was chosen to study the adsorption performance of PDA/PEI-TPU NFMs. Figure 8 shows the significant differences in the adsorption of CR, SY and MO in the pH range of 4–10. CR has a wide pH adaptability, with more than 235 mg/g adsorption capacity and 95% removal efficiency in the pH 4–8. Even at pH 10, these values can reach 110 mg/g and 41%, respectively. In the determined pH range, the adsorption capacity of SY and MO by PDA/PEI-TPU NFMs was similar, but far lower than CR. With the pH increase, the adsorption capacity of SY and MO decreased gradually. The apparent difference in sorption capacity may be attributed to the difference in sorption mechanisms.

3.4. Possible Adsorption Mechanism

Theoretically, under the same conditions, the charge on the PDA/PEI-TPU NFMs surface is the same. Assuming that electrostatic adsorption is the main mechanism of adsorption, the PDA/PEI-TPU surface will adsorb the same number of anions, and will increase with the increase in positive charges on the PDA/PEI-TPU NFMs surface. In fact, with the increase in H⁺, the amine and imine groups present in PEI of PDA/PEI-TPU NFMs gradually protonate into -NH₃⁺ and -NH₂⁺-, while the sulfonic acid groups of anionic dyes ionize hydrogen ions [45]. The adsorption of SY and MO on PDA/PEI-TPU NFMs is increased with positive charge on the PDA/PET-TPU NFMs, illustrating a positive correlation with electrostatic interaction. Nevertheless, CR is an exception. Unlike SY and MO, it has an excellent adsorption for CR, not only under acidic conditions, but also under neutral and alkaline conditions. By analyzing their chemical structure, we found that, in addition to the sulfonic acid groups (also present in SY and MO), CR has two another reactive amine groups. Considering the large amount of amino groups in PEI and the presence of -C = O(NH) and -COO- group in original TPU NFMs, it is speculated that intermolecular hydrogen bonding may occur between PDA/PEI-TPU NFMs and CR, which may possibly be the main reason for the highest CR adsorption capacity.

In order to confirm this speculation, the FTIR spectra of CR as well as PDA/PEI-TPU nanofibers before and after adsorbing CR molecules are compared in Figure 9, where some new peaks and characteristic bond shifts are observed. For example, the stretching vibrations of -N = N- and -S-O(SO₃-H), stretching vibrations belonging to the CR molecule, can be observed in PDA/PEI-TPU NFMs after adsorption of CR at 1592 cm⁻¹ and 1036 cm⁻¹ [46]. In addition, the stretching vibration peaks of C = N after adsorption of CR shifted from 1659 cm⁻¹ to 1653 cm⁻¹, suggesting that N-H groups (amines) in CR have hydrogen bonding with C = N in PDA/PEI-TPU NFMs [30]. The C = O bond at 1727 cm⁻¹ is shifted to 1731 cm⁻¹, indicating that the carbonyl group in PDA/PEI-TPU NFMs is also a proton receptor [47]. In addition, the wide peak at 3355 cm⁻¹, corresponding to the stretching vibration of the N-H and O-H groups of PDA/PEI-TPU NFMs shifted to 3450 cm⁻¹, suggests that the hydroxyl and amino groups also take part in the adsorption as hydrogen bonding sites [47]. All of these changes confirm that there is hydrogen bonding between PDA/PEI-TPU NFMs and CR.

In fact, according to the structures of adsorbent material and dye molecules, we also take into account π–π interaction and van der Waals force. However, the magnitude of these forces is much weaker than that of hydrogen bonding and electrostatic interaction during dye adsorption. Therefore, the mechanistic analysis focuses on hydrogen bonding and electrostatic adsorption. The main possible adsorption mechanism of PDA/PEI-TPU NFMs for three azo dyes is shown in Figure 10. Electrostatic interaction is considered a main mechanism for SY and MO adsorption. However, synergistic effect of hydrogen bonding and electrostatic interaction is likely a critical factor, leading to significant adsorption of PDA/PEI-TPU NFMs for CR, in a wide pH range.

3.5. Adsorption Kinetics and Adsorption Isotherm for Three Dyes

The adsorption kinetics and adsorption isotherm of PDA/PEI-TPU NFMs were conducted for three dyes in order to explore the possibility of PDA/PEI-TPU NFMs for adsorption in application. According to Figure 11a–c, the adsorption rates of CR, SY and MO were relatively fast in the first few hours. This can be explained by the fact that a great quantity of active adsorption sites on the surface of PDA/PEI-TPU NFMs are completely exposed to the aqueous solution in the initial stage, and the dye molecules in the solution occupy these active sites rapidly. Afterwards, as the adsorption time increases, the adsorption rate of the dye gradually decreases and finally reaches the adsorption equilibrium stage. To better describe the adsorption process, pseudo-first-order and pseudo-second-order kinetic models were used to fit the experimental data. The fit curves are shown in Figure 11a–c, and the fitted parameters are shown in Table 2. Based on the R² values, the pseudo-second-order model can better fit the experimental data, which indicates that chemisorption dominates the adsorption process of the three dyes. This result agrees with our previous assumptions that the hydrogen bonding and electrostatic effects of chemisorption were probably a critical factor leading to good adsorption of PDA/PEI-TPU NFMs for CR, SY and MO. The adsorption rate is controlled by the chemisorption mechanism, involving electron cooption and transfer. In addition, the theoretical q_e values calculated from the pseudo-second-order model (217.99 mg/g, 13.24 mg/g and 8.25 mg/g for CR, SY and MO, respectively) are closer to the experimental q_e values (220.34 mg/g, 14.33 mg/g and 8.42 mg/g, respectively), further indicating that the pseudo-second-order kinetics can have good corroboration with the experimental data.

To explore the diffusion mechanism, an intraparticle diffusion model was applied (the insert of Figure 11a–c). The diffusive process is depicted as Figure 11d. Apparently, the adsorption process of CR, SY and MO can be roughly divided into three linear regions. The first sharp linear phase is related to surface adsorption, and indicates that the dye molecules diffuse from the liquid phase to the adsorbent surface, gradually occupying most of the adsorption sites on the PDA/PEI-TPU NFMs surface. The second linear phase is intraparticle diffusion, indicating that the molecules start to enter the inner surface from the outer, and are adsorbed internally. The last stage is the final adsorption–desorption equilibrium, during which the rate constant is greatly reduced due to the reduction in available adsorption sites and residual concentration of contaminants, which takes a long time to reach [48]. The fitted parameters of the intraparticle diffusion model are shown in Table 2, and the K_id values of CR are much larger than those of SY and MO, which indicates that CR is subjected to intraparticle diffusion at a higher rate than SY and MO. In addition, none of the calculated C-values are past the zero point, indicating that the rate control of dye adsorption is determined by multiple stages, and that intraparticle diffusion is not the only determining stage.

Isothermal experiments were investigated for CR, SY and MO in varying concentrations, at room temperature. Two famous Langmuir and Freundlich isotherm models were able to characterize the equilibrium isotherm parameters. Figure 12 shows that the adsorption capacity of PDA/PEI-TPU NFMs gradually increased as the initial concentration of CR increased from 50 to 400 mg/L. SY increased from 5 to 100 mg/L, as did MO, and then reached adsorption equilibrium. This can be explained by the fact that adsorption sites on membranes are limited to hold only a specific quantity of contaminants, and no more active sites are available for the adsorption of additional contaminants. Although the overall trends for CR, SY and MO are similar, the adsorption capacities exhibited at low concentrations are significantly different. CR shows a vertical increase in adsorption capacity at low concentrations compared to SY and MO, indicating that PDA/PEI-TPU NFMs show high affinity for CR molecules [49]. Moreover, a small amount of CR was detected in the solution at a low concentration, indicating that CR was stably adsorbed on the membrane surface without desorption. It is noteworthy that the faster rate of increase in the adsorption capacity of SY compared to MO can be explained by the presence of a hydroxyl group in SY, which may have a weak hydrogen bonding interaction with the membranes. The fitting parameters of the isothermal models are listed in Table 3. The fitted R² values of the Langmuir model reached 0.999, which is higher than the Freundlich model, indicating that monolayer adsorption is more suitable for explaining the adsorption processes of CR, SY and MO onto PDA/PEI-TPU NFMs. The maximum adsorption capacities of CR (263 mg/g), SY (17 mg/g) and MO (23 mg/g) estimated by the Langmuir model are considerably different. This could be related to the hydrogen bond between amine group in CR or to the specific functional groups on the PDA/PEI-TPU NFMs surface.

In this work, the ultra-high adsorption capacity of CR was obtained by using PDA/PEI-TPU NFMs as adsorbents (q_max = 263 mg/g). To compare the adsorption performance for CR removal among various adsorbents, the adsorption capacities, based on the adsorption isotherm experiment of separation membranes and other adsorbents, are listed in Table 4. It can be concluded that PDA/PEI-TPU NFMs have excellent adsorption and removal capacity for CR, and can be applied for the purpose of efficient removal of CR from aqueous solutions.

3.6. Regeneration of the PDA/PEI-TPU NFMs

The desorption and regeneration ability of the adsorption membrane is one of the most important factors used to determine its recyclability [57]. After 24 h adsorption, the membranes were placed in 0.05 M NaOH solution for ultrasound for 60 min in order to desorb CR completely, and then dried in the oven and reused for the next adsorption. After six iterations of adsorption–desorption, the adsorption performance of the PDA/PEI-TPU NFMs did not decrease significantly (Figure 13), which indicates that the PDA/PEI-TPU NFMs has potential application value for the removal of CR molecules by adsorption from water.

4. Conclusions

In this work, TPU NFMs were prepared by electrostatic spinning and then modified by PDA/PEI co-aggregation to obtain an environmentally friendly and widely available adsorption method: PDA/PEI-TPU NFMs. The PEI coating provided the membranes with high adsorption sites and covalent binding stability, while the PDA offered a strong adhesion, allowing the PEI modified layer to be grafted onto the surface of TPU support membranes. Electrostatic interaction is deemed mainly as a mechanism for SY and MO adsorption on PDA/PEI-TPU NFMs, because the adsorption was changed with the positive charge of the PDA/PEI-TPU NFMs. However, the synergistic effect of hydrogen bonding and electrostatic interaction are likely critical factors, resulting in good adsorption of PDA/PEI-TPU NFMs for CR within a wide pH range (4–10). The maximum adsorption capacity of CR, SY and MO at pH = 7 were 263, 17 and 23 mg/g, respectively. Moreover, the PDA/PEI-TPU NFMs exhibit excellent desorption regeneration performance in NaOH solution. After the 6 iterations of repeated adsorption and desorption, the PDA/PEI-TPU NFMs still maintained high adsorption capacity levels. Eventually, the PDA/PEI-TPU NFMs could be suggested as potential adsorbents for CR wastewater treatment.

Author Contributions

J.S.: conceptualization, methodology, supervision. Y.Z.: investigation, data curation, writing—original draft. X.J.: investigation, methodology. J.F.: supervision, writing—reviewing and editing. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Conflicts of Interest

This research received no external funding.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Schematic synthetic routes of the PDA/PEI-TPU NFMs.

Figure 2. (a) The grafted yield of the membranes after modification. (b) Stability of modified membranes after ultrasound.

Figure 3. SEM images and nanofiber diameter distribution of (a–c) TPU NFMs, (d–f) PDA-TPU NFMs and (g–i) PDA/PEI-TPU NFMs.

Figure 4. ATR-FTIR spectra of the TPU NFMs, PDA-TPU NFMs and PDA/PEI-TPU NFMs.

Figure 5. (a) XPS wide spectra and high-resolution XPS N1s spectra of (b)TPU NFMs, (c) PDA-TPU NFMs and (d) PDA/PEI-TPU NFMs.

Figure 6. The contact angle of (up) TPU NFMs, (middle) PDA-TPU NFMs and (bottom) PDA/PEI-TPU NFMs.

Figure 7. The adsorption capacity of original TPU, PDA-TPU and PDA/PEI-TPU NFMs for CR SY and MO dyes (dyes concentration: 100 mg/L, pH = 7).

Figure 8. The effects of initial solution pH on adsorption capacity (a) and removal efficiency (b) of PDA/PEI-TPU NFMs toward CR, SY and MO (dyes concentration: 100 mg/L).

Figure 9. FTIR spectra of procured CR and PDA/PEI-TPU NFMs before and after adsorbed CR molecules.

Figure 10. The possible main adsorption mechanism of PDA/PEI-TPU NFMs for three azo dyes.

Figure 11. Adsorption kinetics of CR (a), SY (b) and MO (c) fitted with pseudo-first-order and pseudo-second-order models. Inset was intra-particle diffusion model. (d) Schematics of the diffusion process of CR, SY and MO onto PDA/PEI-TPU NFMs.

Figure 12. Different initial concentration of dyes (a) CR, (b) SY and (c) MO sorption onto PDA/PEI-TPU NFMs.

Figure 13. Repeated adsorption–desorption performance of PDA/PEI-TPU NFMs on CR dye.

Table 1

Percentage of surface element composition of the nanofiber membranes analyzed by XPS.

Membranes	C1s	O1s	N1s	N/C Ratio	N/O Ratio
TPU NFMs	71.77	26.81	1.42	0.02	0.052
PDA-TPU NFMs	68.78	25.71	5.51	0.08	0.21
PDA/PEI-TPU NFMs	66.46	22.35	11.19	0.168	0.500

Table 2

Kinetic parameters for adsorption of three dyes onto the PDA/PEI-TPU NFMs.

Kinetic Models	Parameters	CR	SY	MO
Pseudo-first order	qe (mg/g)	198.84	12.63	7.90
	k1 (1/h)	0.38	3.26	3.12
	R2	0.954	0.834	0.867
Pseudo-second order	qe (mg/g)	217.99	13.24	8.25
	k2 (g/(mg h))	0.0023	0.36	0.58
	R2	0.989	0.914	0.95
Intraparticle diffusion	ki 1 (mg/(g h1/2))	77.07	13.48	8.01
	b1	−1.876	0.48	0.44
	R2	0.997	0.937	0.918
	ki 2 (mg/(g h1/2))	26.51	0.91	0.968
	b2	87.33	9.13	5.27
	R2	0.999	0.914	0.933
	ki 3 (mg/(g h1/2))	11.62	0.61	−0.10
	b3	142.06	10.31	9.06
	R2	0.917	0.806	0.250

Table 3

Isotherm parameters for adsorption of three dyes onto the PDA/PEI-TPU NFMs.

Dyes	Langmuir Isothermal Model			Freundlich Isothermal Model
	KL(L/mg)	qmax(mg/g)	R2	KF(mg/g)/(mg/L)n	n	R2
CR	0.772	262.95	0.969	123.39	5.36	0.955
SY	0.583	17.46	0.962	8.345	5.383	0.958
MO	0.042	23.34	0.992	2.509	2.174	0.974

Table 4

Summary of adsorption capacities of CR on different nanofiber adsorbents or others.

Materials	qm (mg/g)	Refs.
the coordination cluster Zn5(H2Ln)6(NO3)4	166.91	[50]
p-PEN/a-CNTs@TA/CC nanofibrous	180	[51]
(CEMNPS)-C/CoFe2O4	43.07	[52]
(PCNFs) carbon nanofibers	218	[53]
PHMG-OCS-PVA nanofibers	76.92	[54]
Chitosan–alginate sponge	121.95	[55]
Fe3O4@bacteria	320.1	[56]
PDA/PEI-TPU nanofibers	262.95	This work

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