Selective and Adjustable Removal of Phenolic Compounds from Water by Biquaternary Ammonium Polyacrylonitrile Fibers.

A series of biquaternary ammonium-functionalized fibers were developed to efficiently realize selective removal of phenolic compounds from water. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were employed to determine the successful preparation of functionalized fibers. Scanning electron microscopy, X-ray diffraction (XRD) patterns, and elemental analysis were used to analyze the microstructure and composition. First, the adsorption result shows that a fiber with a three-carbon alkyl chain (PANBQAS-3F) has the maximum adsorption capacity for 2,4-dinitrophenol (2,4-DNP) (406 mg g-1). Electrostatic attraction and π-π interaction are the main forces in adsorption. The adsorption kinetics studies display that PANBQAS-3F can rapidly adsorb 2,4-DNP in 10 min and achieve equilibrium within 20 min. The adsorption process of 2,4-DNP by PANBQAS-3F follows the Langmuir model, demonstrating that the process is more consistent with monolayer adsorption. What is more, the adsorbent PANBQAS-3F can be reused after 10 adsorption/desorption cycles and still maintains an excellent removal rate (99%). Otherwise, PANBQAS-3F was used in a continuous flow process and exhibited a removal rate of more than 96%, which certifies that PANBQAS-3F is an excellent adsorbent and can be utilized in practice.

Introduction

2,4-Dinitrophenol (n class="Chemical">2,4-DNP) is widely used in industries processes, such as dyes, insecticides, preservatives, explosives, and paper bleaching.[1] Therefore, a great deal of 2,4-DNP wastewater is produced in the manufacturing process and trace 2,4-DNP is also contained by all of abovementioned products. Due to the low biodegradability, high solubility, and stability, 2,4-DNP has become a typical pollutant and has been banned by the American Environmental Protection Agency and the European Union. In particular, the ingestion of 2,4-DNP may cause harmful effects on human health, like skin allergy, cardiovascular diseases, and low concentration carcinogenesis to human health.[2] Therefore, it is significant to develop efficient methods for the removal of 2,4-DNP from wastewater before being discharged to the environment.

To achieve this purpose, many methods such as photocatalysis,[3,4] adsorption,[5−7] electrochemical degradation,[8,9] and biochemical reactions[10] have been explored. Among these approaches, adsorption methods have aroused considerable interest owing to their simplicity, efficiency, and low cost.[11] Till now, various adsorbents including silica gels,[5,12] n class="Chemical">metal–organic frameworks,[6] and activated carbon[13,14] have been used to remove 2,4-DNP from an aqueous solution. However, some of these adsorbents have the disadvantages of high cost, a tedious preparation process, and unsatisfactory recyclability, which hinder their practical applications. Thus, there still remains a great challenge to develop effective adsorption materials to overcome these defects.

A polyacrylonitrile fiber (n class="Chemical">PANF) is a low-cost material with a large surface and high mechanical strength and contains numerous nitrile groups, which can be easily transformed into various functionalized moieties.[15,16] To date, many kinds of functionalized polyacrylonitrile fibers have been prepared and employed to remove organic pollutants and to absorb and recognize metal.[17−22] However, there are few reports on the application of functionalized PANF on the removal of 2,4-DNP from the aqueous solutions, so it is of great significance and challenge to prepare efficient fiber absorbents for 2,4-DNP removal.

Results and Discussion

Synthesis and Characterization of Functionalized Fibers

The modification degree of functionalized fibers can be expressed by weight gain (%) and functionality (mmol g–1). The corresponding expressions are eq (23,24) and eq ,[25] respectivelywhere W1 and W2 represent the weight of before and after modification of the fiber, respectively, and M is the molecular weight caused by functional organic molecules (for example, in the preparation of PANPF, M is 103 g mol–1). Table shows the weight gain and corresponding functionality of different functionalized fibers.

Table 1

Modification Degree of Different Functionalized Fibers

entry	fibers	weight gain (%)	functionality (mmol g–1)
1	PANPF	24.5	1.91
2	PANQAS-1F	38.6	1.63
3	PANQAS-2F	24.1	1.79
4	PANBQAS-2F	54.0	1.08
5	PANBQAS-3F	73.6	1.21
6	PANBQAS-4F	62.8	1.10
7	PANBQAS-5F	64.2	1.07
8	PANBQAS-6F	40.3	0.76

Characterization of Functionalized Fibers

Fourier Transform Infrared Spectroscopy (FTIR)

The PANF, n class="Chemical">PANPF, PANBQAS-3F, PANBQAS-3F-1 (fiber recycled once), and PANBQAS-3F-10 (fiber recycled 10 times) samples were pulverized by cutting and then prepared into KBr pellets. The FTIR spectra are presented in Figure . The FTIR spectra of PANF (Figure a) show that there are two obvious peaks at 2241 and 1736 cm–1, corresponding to the stretching vibrations of C≡N and C=O,[26] which are the characteristic peaks of polyacrylonitrile fiber C≡N and C=O in methyl methacrylate or methyl acrylate, respectively. After modification, PANPF (Figure b) and PANBQAS-3F (Figure c) showed a wide absorption band between 3600 and 3100 cm–1, which corresponds to N–H stretching vibrations. This indicates that N,N-dimethyl-1,3-propanediamine was successfully grafted onto the fiber surface. Compared to PANF (Figure a) and PANPF (Figure b), the absorption peak at 1736 cm–1 shifted to 1661 cm–1, which further indicated that the ester group on the surface of the fiber was transformed into an amide group with higher stability. In addition, PANBQAS-3F shows new peaks at 735 and 701 cm–1, which are the stretching vibration peak and the out-of-plane vibration peak of C–H on a benzene ring, respectively, proving the successful immobilization of small molecules of a quaternary ammonium salt. After thePANBQAS-3F was used 10 times, the FTIR spectra of PANBQA-3F-10 (Figure d) are basically unchanged compared with those of PANBQAS-3F, which proves that the functionalized fiber is quite stable.

Figure 1

FTIR spectra of (a) PANF, (b) PANPF, (c) PANBQAS-3F, (d) PANBQAS-3F-1, and (e) PANBQAS-3F-10.

Elemental Analysis (EA)

The elemental analysis (EA) was used to demonstrate the successful preparation of PANBQAS-3F and the stability during the application process. The elemental analysis data of n class="Chemical">PANF, PANPF, PANBQAS-3F, PANBQAS-3F-1, and PANBQAS-3F-10 are shown in Table . Compared with PANF, the content of C and N in PANPF (Table , entry 2) decreased and the content of H increased. This is due to the introduction of N,N-dimethyl-1,3-propanediamine, which contains less carbon and nitrogen and more hydrogen than PANF. After quaternary ammonium salt modification, caused by the introduction of Br atoms in the small molecules, the contents of C, H, and N in PANBQAS-3F decreased compared with PANPF, and the sum of C, H, and N elements (71.4%) decreased significantly (Table , entry 3). It is worth mentioning that the elemental analysis data (Table , entries 4–5) of PANBQAS-3F reused once and reused 10 times have no significant changes compared with original PANBQAS-3F, which indicates that the fiber has high stability and reusability.

Table 2

Elemental Analysis Data

entry	fiber	C (%)	H (%)	N (%)
1	PANF	66.15	5.74	24.29
2	PANPF	53.90	6.83	15.64
3a	PANBQAS-3F	51.09	6.79	13.52
4	PANBQAS-3F-1	51.00	6.76	13.64
5	PANBQAS-3F-10	51.31	6.73	13.75

Weight gain of PANBQAS-3F is 73.6%.

X-ray Photoelectron Spectroscopy (XPS)

To prove the successful functionalization, the chemical composition of the original fiber and the modified fiber were analyzed by XPS. As shown in Figure , the XPS full-scan spectra of PANF, n class="Chemical">PANPF, and PANBQAS-3F show three peaks at 530.89, 397.96, and 284.38 eV, corresponding to the characteristic peaks of O 1s, N 1s, and C 1s, respectively. In addition, a new peak of PANBQAS-3F appears at 68.56 eV,[27] attributed to the characteristic peak of Br 3d, which indicates that small molecules of bromine-containing quaternary ammonium salt were successfully grafted onto a tertiary amine-functionalized fiber.

Figure 2

XPS full-scan spectra of (a) PANF, (b) PANPF, and (c) PANBQAS-3F.

The XPS high-resolution spectra of C 1s, O 1s, N 1s, and Br 3D of n class="Chemical">PANBQAS-3F are shown in Figure . The three groups of peaks in the C 1s high-resolution spectrum of PANBQAS-3F show three absorption peaks corresponding to the functionalized groups C–O/C–N (284.57 eV), C≡N (285.99 eV), and C=O (288.37 eV) (Figure a).[28,29] In the N 1s high-resolution spectrum of PANBQAS-3F, the binding energies are 400.20 and 402.10 eV belonging to N–C=O/N (C)3 and N+ groups, respectively,[30] which further proves the successful grafting of a quaternary ammonium-functionalized fiber (Figure c).

Figure 3

High-resolution XPS spectra of (a) C 1s, (b) O 1s, (c) N 1s, and (d) Br 3d of the PANBQAS-3F.

Mechanical Strength

Mechanical strength is one of the important indicators to measure the physical stability of the fiber. The results in Table show that PANF (10.90 cN) has high mechanical strength; the strength of n class="Chemical">PANPF (9.15 cN, modified by N,N-dimethyl-1,3-propanediamine) is slightly lower than that of PANF. The mechanical strength of PANBQAS-3F still retains 71% of the fracture strength of PANF. In conclusion, the strength of the fiber is slightly reduced during the modification, but the mechanical strength is still retained high. Moreover, the adsorption operation is generally carried out at room temperature; the adsorption process is mild and the damage to the fiber is minimal (Table , entries 4 and 5). The mechanical strength loss of PANBQAS-3F is less and still has high stability after 10 adsorption/analytical cycles, indicating that PANBQAS-3F has high stability and excellent recyclability.

Table 3

Mechanical Properties of Different Fibers

entry	fiber	BS (cN)	RBS (%)a
1	PANF	10.90	100
2	PANpF	9.15	84
3	PANBQAS-3F	7.74	71
4	PANBQAS-3F-1	6.81	62
5	PANBQAS-3F-10	6.51	60

Retention of breaking strength (RBS) based on PANF (10.90 cN).

Scanning Electron Microscopy (SEM)

The SEM images of PANF, PANPF, PANBQAS-3F, PANBQAS-3F-1, and PANBQAS-3F-10 at different magnifications are presented in Figure . Compared with PANF, the surface of PANPF (Figure b) and PANBQAS-3F (Figure c) was rougher, and their diameters increased gradually with modification. This is because the fiber swells in the process of modification. It is worth mentioning that both the SEM images of PANBQAS-3F-1 and PANBQAS-3F-10 (Figure d,e) still maintain the analogous morphology like PANBQAS-3F, which proves the excellent cycling capacity of the PANBQAS-3F.

Figure 4

SEM images of (a) PANF, (b) PANPF, (c) PANBQAS-3F, (d) PANBQAS-3F-1, and (e) PANBQAS-3F-10.

Adsorption Properties of Functionalized Fibers

The adsorption capacities were calculated by the formulae provided in the Supporting Information.

Adsorption Capacities of Functionalized Fibers

The 4-NP was selected as a model compound to investigate the adsorption capacities of different functionalized fibers. The adsorption results of n class="Chemical">4-NP by various functionalized fibers are shown in Figure . The fiber PANPF has a certain adsorption capacity for 4-NP, which may be caused by the hydrogen bond between the amino group of PANPF and 4-NP or the electrostatic attraction between the protonated amino group and 4-NP. For a monoquaternary ammonium-functionalized fiber, the adsorption capacity of PANQAS-1F for 4-NP is slightly higher than PANQAS-2F, which may be due to the π–π interaction of the benzene rings in PANQAS-1F with 4-NP. For a biquaternary ammonium-functionalized fiber, PANBQAS-2F, PANBQAS-3F, PANBQAS-4F, PANBQAS-5F, and PANBQAS-6F all showed good adsorption for 4-NP. Among them, PANBQAS-3F has the best adsorption capacity attributed to the stable six-membered ring structure formed by two nitrogen cations in PANBQAS-3F and phenolic oxygen anions, which can stabilize the transition state better, so PANBQAS-3F shows better adsorption performance. Therefore, in the subsequent experiments, the adsorption properties of PANBQAS-3F were studied systematically.

Figure 5

Adsorption of 4-NP by different functionalized fibers.

Adsorption Selectivity of PANBQAS-3F

Benzene, n class="Chemical">toluene, nitrobenzene, naphthalene, 4-methylphenol, phenol, naphthalene-1-phenol, 2-dichlorophenol, and 4-NP were selected as target compounds to systematically investigate the adsorption selectivity of PANBQAS-3F for different substances (Table ). The experimental results are shown in Figure . First of all, the adsorption capacity of PANBQAS-3F for phenolic compounds is obviously better than that of aromatic hydrocarbons (phenol > benzene; 4-methylphenol > toluene; 4-NP > nitrobenzene; naphthalene-1-phenol > naphthalene). This is due to the fact that phenolic compounds can be dissociated into phenolic oxygen anions in an aqueous solution and can bind to the positive ion adsorption sites on biquaternary ammonium-functionalized fibers. Second, the adsorption capacity of PANBQAS-3F for phenolic compounds increases with acidity enhancement of these compounds (4-NP > 2,4-dichlorophenol > naphthalene-1-phenol > phenol > 4-methylphenol). This is because the higher the acidity, the easier it is for compounds to ionize hydrogen ions to form phenoxy anions in an aqueous solution, and the stronger the electrostatic attraction is to show the better adsorption capacity. In addition, comparing the adsorption ability of PANBQAS-3F to different aromatic compounds, the adsorption amount of naphthalene is greater than benzene and naphthalene-1-phenol is greater than phenol, which indicates that there is a π–π interaction between the functionalized fiber and the target compound.

Table 4

Compounds Used in Adsorption Selectivity of PANBQAS-3F

Figure 6

Adsorption selectivity of PANBQAS-3F to different compounds.

From the abovementioned experiment, it can be seen that the acidity or the pKa of the compound may be the main factor affecting the adsorption of the functionalized fiber to the target compound. So more acidic phenolic compounds n class="Chemical">2,4-DNP (pKa = 4.08) and 2,4,6-dinitrophenol (pKa = 0.38), were used to explore the adsorption mechanism of functionalized fibers. The results are shown in Figure . As expected, the adsorption of phenolic compounds by PANBQAS-3F increases with a decrease in pKa, which illustrates that the electrostatic attraction of positive and negative charges is the main force of adsorption. In the follow-up research, 2,4-DNP, which has a larger adsorption capacity and is relatively easy to obtain, was selected as the target compound.

Figure 7

Adsorption capacity of PANBQAS-3F to phenolic compounds with different pKas.

Regulating the Adsorption Selectivity of PANBQAS-3F by pH

In the paper, adsorption properties of PANBQAS-3F to n class="Chemical">phenolic compounds were investigated at different pH values, and the results are shown in Figure . The adsorption selectivity of the fiber PANBQAS-3F for phenolic compounds can be regulated by changing the pH values of the solution, which determine the existing forms of phenolic compounds in water. First, PANBQAS-3F has better adsorption capacity for 2,4-DNP than other compounds at different pH values because of the strong acidity and a relatively high degree of dissociation of 2,4-DNP. In both acidic and alkaline conditions, 2,4-DNP exists in the form of a phenoxy anion, which can interact well with functionalized fibers. On the other hand, the acidity of 2,4-dichlorophenol and 4-NP is relatively weak, and the degree of dissociation is also weak under acidic conditions. They exist mostly in a molecular form, and the adsorption performance is poor under acidic conditions. However, negative ions can be formed under alkaline conditions, which enhances the interaction with fibers and improves the adsorption capacity of the fiber. Phenol has the weakest acidity and still exists in the form of a molecule under weak alkaline conditions, so the adsorption properties under different pH values are very weak.

Figure 8

Adsorption capacity of PANBQAS-3F for various phenols under different pH values.

Effect of Weight Gain of PANBQAS-3F on Adsorption Capacity

A series of PANBQAS-3Fs with different weight gains (6.9, 12.1, 20.9, 38.9, 54.3, and 73.6%) were prepared, and their adsorption capacities for n class="Chemical">2,4-DNP are shown in Figure . With increased weight gain, the adsorption capacity performance was better exhibited. In addition, the mechanical strength tests have shown that the fibers can maintain higher mechanical strength at a greater weight gain (73.6%). Therefore, considering the adsorption capacity and the strength of the functionalized fiber, the PANBQAS-3F fiber with a weight gain of 73.6% was selected for the experiment.

Figure 9

Effect of different weight gains of PANBQAS-3F on adsorption of 2, 4-dinitrophenol.

Adsorption Kinetics

The adsorption kinetics of PANBQAS-3F for 2,4-dinitrophenol was measured, and the results are shown in Figure . It can be seen that the adsorption rate of PANBQAS-3F was rapid; it took only 10 min for the fiber to almost reach its saturated adsorption equilibrium (405 mg g–1). The adsorption capacity did not further increase after 20 min.

Figure 10

Effect of adsorption time of the adsorption capacity of 2,4-DNP by PANBQAS-3F.

Pseudo-first-order and pseudo-second-order models were used to interpret the kinetics characteristics of the 2,4-DNP adsorption process (Figure S12). The pseudo-first-order eq (31) and pseudo-second-order eq (32) are given followingwhere t (min) is the adsorption time, qe (mg g–1) is the equilin class="Chemical">brium adsorption of the adsorbent, qt (mg g–1) is the amount of the 2,4-DNP adsorbed by PANBQAS-3F at a given time, and k1 (min–1) and k2 (g mg–1 min–1) are pseudo-first-order and pseudo-second-order rate constants, respectively. It can be observed that the qe,exp value obtained from the experiment (406.0 mg g–1) and the qe,cal value calculated based on pseudo-second-order (403.2 mg g–1) are more close to each other than that of pseudo-first-order, and the values of R2 for the pseudo-second-order (0.99989) are greater than the pseudo-first-order model (0.9641), which indicates that the pseudo-second model is more suitable for fitting experimental data. Therefore, the adsorption of 2,4-DNP from aqueous to PANBQAS-3F should be a chemisorption process (Table ).

Table 5

Kinetic Parameters for the Adsorption of 2,4-DNP by PANBQAS-3F

	pseudo-first-order model			pseudo-second-order model
qe, exp (mg g–1)	k1 (min–1)	qe,cal (mg g–1)	R2	k2 (g (mg min)−1)	qe,cal (mg g–1)	R2
406.0	0.259	191.1	0.96481	0.006424	403.2	0.99989

Adsorption Isotherms

The effect of the initial concentration of 2,4-DNP on adsorption by n class="Chemical">PANBQAS-3F was examined (Figure ). It can be seen that the adsorption amount of PANBQAS-3F to 2,4-DNP gradually increases with an increase in the initial concentration until the adsorption equilibrium is reached. However, when the concentration of 2,4-DNP reached 200 mg L–1, the adsorption capacity gradually stabilized.

Figure 11

Effect of temperature on the adsorption for 2,4-DNP by PANBQAS-3F.

Langmuir and Freundlich isotherm models were then used to fit the equilibrium adsorption data (Figure S13). The Langmuir isotherm model[33] assumes that the adsorption process is monolayer adsorption, that there is no interaction between the adsorbates and they are independent, and the surface of the adsorbent is uniform. The Freundlich isotherm model is not only suitable for single molecular layer adsorption but also for multilayer adsorption or uneven surface adsorption. All equations were expressed as[33−36]The[33] corresponding fitting parameters are listed in Table . It can be seen from Table that the R2 value of the Langmuir model is 0.9992 larger than that of the Freundlich model R2 (0.7969), indicating that the adsorption process of n class="Chemical">PANBQAS-3F to 2,4-DNP is more consistent with the Langmuir model and closer to monolayer chemical adsorption. The result shows that the adsorption is due to the interaction of electrostatic attraction and hydrogen bonding. The maximum adsorption capacity (qmax) calculated by the Langmuir isothermal model is 429.0 mg g–1, which indicates that PANBQAS-3F has more advantages than other adsorbents (Table S2).

Table 6

Parameters for Langmuir and Freundlich Models

model	Langmuir			Freundlich
equation
parameters	KL (L mg–1)	qmax (mg g–1)	R2	KF ((mg g–1)(L mg–1)1/n)	n	R2
value	0.2980	429.0	0.9992	102.87	3.320	0.7969

Adsorption Thermodynamics

The effect of temperature on the adsorption capacity of 2,4-DNP by n class="Chemical">PANBQAS-3F was investigated at 298, 318, and 338 K (Figure ). The adsorption capacities of 2,4-DNP gradually decreased as the temperature increased from 298 to 338 K. Vander Hoff equations provided in the Supporting Information were used to calculate the thermodynamic parameters such as ΔH0 and ΔS0 (Table ).

Figure 12

(a) Effect of temperature on the adsorption of 2,4-DNP by PANBQAS-3F. (b) Thermodynamic fitting diagram.

Table 7

Thermodynamic Parameters for the Adsorption of 2,4-Dinitrophenol by PANBQAS-3F

T (K)	ln K	ΔH (kJ mol–1)	ΔS (J mol–1)	ΔG (kJ mol–1)
298	1.0581			–2.624
318	0.8674	–7.708	–17.06	–2.283
338	0.6897			–1.9417

Desorption and Reusability

Recyclability is an important factor of an adsorbent in practical application. The desorption curve of 2,4-DNP from n class="Chemical">PANBQAS-3F using a NaBr solution as a desorbent is shown in Figure a. It is found that the desorption rate reached 91.99% after 30 min. When time increased to 240 min, 2,4-DNP could be completely desorbed from the fiber. Therefore, 240 min was chosen as the optimal desorption time. After 10 times adsorption–desorption cycles (Figure b), the adsorption and desorption rate still remained above 99%. The excellent reusability proved the great potential of the PANBQAS-3 adsorbent in real application.

Figure 13

(a) Desorption curve of 2,4-DNP on the PANBQAS-3F fiber. (b) Reusability of PANBQAS-3F in a continuous flow condition.

Adsorption Mechanism

Based on the abovementioned experimental results, a possible adsorption mechanism of 2,4-DNP by n class="Chemical">PANBQAS-3F is proposed and described in Scheme . The electrostatic force between a phenoxy anion and a quaternary ammonium group on the fiber is the main adsorption force, so the smaller the pKa of the adsorbent, the easier it is to ionize phenoxy, which can enhance adsorption capacity. The electrostatic attraction between phenolic compounds and functionalized fiber increases with the increase in adsorption sites, so the adsorption capacity of biquaternary ammonium-functionalized fibers is better than single quaternary ammonium-functionalized fibers. At the same time, two nitrogen cations in the PANBQAS-3F structure of the biquaternary ammonium-functionalized fiber can synergistically interact with phenoxy anions and form a stable six-membered ring structure, which can better stabilize the transition state, so it has the best adsorption performance. In addition, the π–π interaction between phenolic compounds and the benzene ring on the fiber is also one of the adsorption interaction forces. Therefore, the adsorption is enhanced in the phenolic hydroxyl group, and the nitrogen atom of the residual amino group in the PANBQAS-3F fiber plays an auxiliary role in the adsorption process.

Scheme 1

Possible Adsorption Mechanism

Conclusions

A series of monoquaternary ammonium- and biquaternary n class="Chemical">ammonium-functionalized fibers were successfully prepared and used to remove 4-nitrophenol from water. The results showed that the biquaternary ammonium-functionalized fiber (PANBQAS-F, n = 2, 3, 4, 5, and 6) exhibited better adsorption of 4-nitrophenol than monoquaternary ammonium-functionalized fiber (PANQAS-F, n = 1, 2). Then, PANBQAS-3F was studied to investigate adsorption properties for 2,4-DNP. PANBQAS-3F shows excellent adsorption capacity (406.0 mg g –1) and reusability (removal efficiency >99% after 10 adsorption/desorption cycles). The adsorption experiment data fitted well with the pseudo-second-order kinetic model and the Langmuir adsorption isotherm model, indicating that the adsorption process is chemisorption of a monolayer. In addition, PANBQAS-3F exhibited remarkable and obvious performance under continuous flow conditions, which can be referred to in the video in the Supporting Information.

Experimental Section

Reagents and Instruments

Reagents

A polyacrylonitrile fiber with a diameter of 20 ± 0.5 μm (purchased from the Fushun Petrochemical Corporation of China) was cut into lengths of 5–10 cm before use. n class="Chemical">N,N-dimethyl-1,3-propanediamine, 1,2-dibromoethane, 1,3-dibromopropane, 1,4-dibromobutane, 1,5-dibromopentane, 1,6-dibromohexane, N,N-dimethylbenzylamine, benzyl bromide, bromoethane, ethanol, diethyl ether, acetonitrile, phenol, 4-chlorophenol, 4-nitrophenol, and ethyl acetate were all analytical grade and used without further purification. The water was deionized (24.08 μS cm–1).

Instruments

An AVATAR360 FTIR spectrometer (Thermo Nicolet) was employed to obtain FTIR spectra of the fibers. Elemental analysis (EA) data of the original and functionalized fibers were obtained using an ElementarVario EL instrument. X-ray photoelectron spectroscopy (XPS) was obtained using a VersaProbe spectrometer (model PHI-5000). X-ray powder diffraction (XRD) patterns were recorded on a D/MAX-2500 X-ray diffractometer (Rigaku Corporation). The thermal stability of fibers was investigated using an STA409PC TGA/DSC simultaneous thermal analyzer (Netzsch company, Germany). The mechanical properties of the different fiber samples were tested using an electronic single fiber strength tester LLY-6 (Laizhou Electronic Instrument Corporation, China). The surface morphology of the fibers was observed using a microscope (Hitachi, model S-4800). The pH values were determined using a pH meter (Model PHS-25). A TU-1901 dual-beam ultraviolet visible spectrophotometer (Persee) was employed to determine the concentrations of various n class="Chemical">phenol solutions. 1H NMR (600 MHz) spectra were recorded on a JEOL JNM ECZ600R instrument using tetramethylsilane as an internal standard. 1H NMR (400 MHz) spectra were recorded on a BRUKER-AVANCE III instrument using tetramethylsilane as an internal standard.

Synthesis of the Functionalized Fiber

The quaternary ammonium salt-functionalized fiber PANQASFs were prepared through a two-step reaction, as shown in Scheme . First, n class="Chemical">N,N-dimethyl-1,3-propanediamine was grafted onto the PANF to obtain the aminated fiber (PANPF). Then, the fiber further reacted with halogenated hydrocarbon through quaternization to get different quaternary ammonium salt-functionalized fibers PANQASFs. The detailed preparations of functional organic molecules (RBr) and functionalized fibers are described in the Supporting Information (Scheme S1).

Scheme 2

Preparation of the Functionalized Fibers

Step 1: Dried PANF (1.0 g), n class="Chemical">N,N-dimethyl-1,3-propanediamine (20 mL), and deionized water (10 mL) were added to a three-neck flask. The mixture was stirred and refluxed for 4.5 h. After that, the functionalized fiber was filtered and washed with hot water (60–70 °C) until neutral. The fiber was dried overnight at 60 °C under vacuum to give PANpF (Scheme S2).

Step 2: Dried PANPF (1.00 g), corresponding Rn class="Chemical">Br (2 mmol), and ethanol (20 mL) were added to a three-neck flask. The mixture was stirred and refluxed for 4.0 h. After the reaction, the functionalized fiber was filtered and washed with ethanol in a soxhlet extractor to remove unreacted small molecules. The fiber was dried overnight at 60 °C under vacuum to give PANQAS-1F, PANQAS-2F, and PANBQAS-Fs (Schemes S3 and S4).

Adsorption Experiments

Adsorption Properties of Different Functionalized Fibers for 4-Nitrophenol

Dried functionalized fibers (PANPF, PANQAS-1F, PANQAS-2F, and PANBQAS-Fs) (15 mg) were immersed in 25 mL of n class="Chemical">4-nitrophenol (4-NP) (200 mg L–1, pH = 8) and stirred for 12 h. The concentrations of 4-NP before and after adsorption were determined by UV–vis spectroscopy.

Adsorption Kinetics, Isotherm, and Thermodynamics

Dried PANBQAS-3F (15 mg) was immersed in 40 mL of a n class="Chemical">2,4-DNP solution (200 mg L–1, pH = 6) and then the mixture was stirred at required time and temperature. For kinetics research, after being stirred for a certain time at room temperature, the fiber was filtered and the remaining concentration of 2,4-DNP was measured by UV–vis spectroscopy. Similarly, the isotherm and thermodynamic experiments were carried out at different initial concentrations (25–400 mg L–1) and temperatures (298, 303, and 308 K), respectively.

Flow Adsorption Experiment

Dried PANBQAS-3F (0.25 g) was filled into a cylindrical tube with a length of 100 mm and a diameter of 5.7 mm, and then the solution of n class="Chemical">2,4-DNP with an initial concentration of 200 mg L–1 and pH = 6 was pumped through the cylindrical tube with the functionalized fiber at a flow rate of 2 mL min–1 using a peristaltic pump, and the liquid was collected. The concentration of 2,4-DNP in the effluent was determined by UV–vis spectroscopy.

Desorption and Reversibility of PANBQAS-3F

Adsorption: First, dried PANBQAS-3F (0.20 g) was filled into a cylindrical tube with a length of 100 mm and a diameter of 5.7 mm, and then a n class="Chemical">2,4-DNP solution (V = 250 mL, Co = 200 mg L–1, pH = 6) was pumped through the cylindrical tube containing the functionalized fiber at a flow rate of 2 mL min–1, and the effluent was collected. The absorbance of 2,4-DNP in the effluent was determined by UV–vis spectroscopy.

Desorption: A NaBr solution (0.1 mmol L–1) was used as an eluent and pumped through a cylindrical tube containing the adsorbed functionalized fiber at a flow rate of 2 mL min–1. The liquid was collected in a receiving bottle at the same time interval, and the desorption ratio of the fiber at different times was studied. The concentration of n class="Chemical">2,4-DNP in the effluent was determined by UV–vis spectroscopy. Such adsorption/desorption recycling is used to determine the reuse performance of the functionalized fiber.