poly(NVI-co-AA) hydrogel as an easily recyclable adsorbent for cationic and anionic dyes

Colloids and Surfaces A 588 (2020) 124393

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pH-responsive magnetic graphene oxide/poly(NVI-co-AA) hydrogel as an easily recyclable adsorbent for cationic and anionic dyes

T

Guohong Yao, Wendie Bi, Hui Liu* Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, Hunan, PR China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: pH-responsive Magnetic hydrogel Graphene oxide Easily recyclable adsorbent Dyes Adsorption

pH-responsive magnetic graphene oxide/poly(N-vinylimidazole-co-acrylic acid) hydrogel (MGO/PNA) was synthesized by random copolymerization of N-vinylimidazole (NVI) and acrylic acid (AA) in the presence of ethylenediamine-modified magnetic graphene oxide (MGO). The resulting hydrogel was systematically characterized by mean of Fourier transform infrared spectrometer, X-ray photoelectron spectroscopy, Raman spectrum, Thermogravimetric analysis, and Scanning electron microscope. The swelling percentage of the hydrogel attained the extremely high value at pH = 2 and pH = 12. The resulting MGO/PNA was used as an easily recyclable adsorbent to remove cationic and anionic dyes, and the effects of solution pH, adsorption time, adsorption temperature, and initial concentration on the adsorption process were systematically investigated. The maximum theoretical adsorption capacities of methyl violet (MV), methylene blue (MB), tartrazine (TZ), and amaranth (AR) were 609.8, 625.0, 613.5 and 609.8 mg/g, respectively. The adsorption behaviors of the hydrogel conformed to Langmuir isothermal model and pseudo-second-order model. The mechanism analysis indicated that the addition of Fe3O4 into the hydrogel not only endowed MGO/PNA with easily recyclable characteristic, but also improved the adsorption capacity of the hydrogel to some degree. MGO/PNA hydrogel could maintain good adsorption effect after five adsorption-desorption cycles, so the hydrogel might have the wide application prospect in adsorption science and technology area.

⁎ Corresponding author at: College of Chemistry and Chemical Engineering, Central South University, 932 South Lushan Road, Changsha, 410083, Hunan, PR China. E-mail address: [email protected] (H. Liu).

https://doi.org/10.1016/j.colsurfa.2019.124393 Received 23 October 2019; Received in revised form 20 December 2019; Accepted 23 December 2019 Available online 28 December 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.

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1. Introduction

2. Experimental

With the continuous development of industry, more and more dyes are frequently used in various industries such as weaving, hide, paper and food processing, so the discharge of dye wastewater is also increasing [1]. Depending on the type of charge, dyes can be classified into cationic dyes (for example, methyl violet (MV) and methylene blue (MB)) and anionic dyes (for instance, tartrazine (TZ) and amaranth (AR)), which are ubiquitous in printing and dyeing wastewater [2]. Dyestuff wastewater is characterized by large discharge, complex components, high frequency of water quality change, high content of organic compounds with aromatic compounds as matrix, and poor biodegradability. Therefore, it is necessary to remove harmful dye molecules from wastewater before it is discharged [3]. In recent years, there are many technologies to remove dyes from wastewater, including adsorption [4], ion exchange [5], membrane filtration [6], and bio-removal [7]. Among the above methods, adsorption method is widely used because of its advantages of low budget, simple operation, and high productivity [8]. Porous resins and activated carbon are commonly used as adsorbents [9], but these adsorbents are difficult to recover after use, which limits their widespread application to some degree [10]. Therefore, adsorbents with unique advantages, such as abundant raw materials, simple preparation, high efficiency, low cost, environmental friendliness, and easy recovery, have attracted a lot of attention. Polymer hydrogels with abundant functional groups have come into people's attention since they can efficiently adsorb dye molecules in wastewater by electrostatic interaction, hydrogen bonding, van der Waals, and hydrophobic interactions [11,12]. Although it is an adsorbent toward cationic dyes, polyacrylic acid (PAA) hydrogel still has poor mechanical property and low dye adsorption capacity due to its flexible segments. In order to improve its performance, PAA hydrogel is usually combined with other materials [13,14]. As generally accepted, graphene oxide (GO) has good mechanical property and high adsorption capacity because of a large number of oxygen-containing functional groups (such as −COOH, –O–, and −OH) on its surface [15]. Yang et al. synthesized GO based hyperbranched polymer hydrogels with high water absorption ability and excellent mechanical properties [16]. The composition of dye wastewater including anionic dyes and cationic dyes simultaneously is, however, extremely complicated in the actual treatment process, so it is urgent to improve the applicability of the adsorbents. Since positively charged hydrogels containing imidazole or amino groups can adsorb anionic dyes [17], the introduction of N-vinylimidazole (NVI) into the system may endow the hydrogels with the ability to adsorb anionic dyes. At the same time, from the economic point of view, the introduction of magnetic Fe3O4 nanoparticles into the hydrogel system can improve the recyclability and reusability of materials [18]. Long et al. synthesized an easily recovered magnetic adsorbent Fe3O4@catechol/PEI with high adsorption performance toward anionic dyes [19]. Jv et al. prepared magnetic hydrogel (PAsp-PAA/Fe3O4) with excellent adsorption properties to removal organic dyes [20]. Herein, we prepared a pH-responsive modified magnetic graphene oxide/poly (NVI-co-AA) (MGO/PNA) hydrogel by random copolymerization of negatively charged acrylic acid (AA) and positively charged NVI in the presence of GO in aqueous medium. The swelling properties of the resulting hydrogel at different pH values were investigated. At the same time, the effects of some adsorption parameters on the removal of cationic dyes (MV, MB) and anionic dyes (TZ, AR) were systematically explored. The adsorption kinetics, thermodynamics, isotherm, and reusability of the hydrogel were also studied. The asprepared MGO/PNA hydrogel could have potential applications in complicated treatment environment.

2.1. Materials N,N-methylenebisacrylamide (BIS), ethylenediamine, ammonium persulfate (APS), N-vinylimidazole (NVI, 98 %), and AA were obtained from Aladdin Industrial Inc. China. Ferric chloride hexahydrate (FeCl3·6H2O, 99 %), hydrochloric acid (HCl), sodium hydroxide (NaOH), ferrous chloride tetrahydrate (FeCl2·4H2O, 98 %), and ammonia hydroxide solution (25–28 %) were supplied by Adamas. MB and MV were purchased from Kermel Chemical Industrial Co. Ltd. China. AR and TZ were obtained from Sigma-Aldrich. The graphite powder with particle size lower than 45 μm was purchased from Xfnano. Deionized water was used throughout all the studies. 2.2. Synthesis of MGO/PNA hydrogels MGO/PNA hydrogels were fabricated as follows: GO was prepared by modified Hummers method [21]. Into a Schlenk flask with the mixture of GO (0.1 g), FeCl2·4H2O (2.00 g, 10 mmol), and FeCl3·6H2O (5.46 g, 20 mmol) at 80 °C, ammonia hydroxide solution (4.55 g, 13 mmol) was added under nitrogen atmosphere until the pH was higher than 9. After 1 h, the black magnetic graphene oxide (MGO) was separated by external magnet [22]. Then MGO was modified with ethylenediamine at 70 °C for 12 h, and the precipitate was repeatedly washed with water until the filtrate became neutral [23]. The MGO/PNA hydrogels were prepared via random copolymerization of AA (1 g, 13.88 mmol) and NVI (1 g, 10.62 mmol) on the surface of modified MGO (0.1 g) in the presence of BIS as the cross-linker (0.05 g, 0.32 mmol), APS as the initiator (0.05 g, 0.88 mmol), and water as the solvent (8 mL) [24]. The polymerization was kept at 80 °C for 3 h, and then MGO/PNA hydrogels were immersed and washed with water for 12 h. The final hydrogels were dried overnight and stored for future experiments. The synthetic illustration of MGO/PNA was shown in Scheme 1. 2.3. Synthesis of GO/PNA hydrogels and Fe3O4/PNA hydrogels GO/PNA hydrogels were fabricated similar to MGO/PNA in section 2.2, and the difference was that MGO was replaced by GO in the experiment. A typical process of Fe3O4/PNA hydrogels was prepared as follows: Into a Schlenk flask with the mixture of FeCl2·4H2O (2.00 g, 10 mmol) and FeCl3·6H2O (5.46 g, 20 mmol) at 80 °C, ammonia hydroxide solution (4.55 g, 13 mmol) was added under nitrogen atmosphere until pH was higher than 9. After 1 h, the black Fe3O4 nanoparticles was separated by external magnet. The Fe3O4/PNA hydrogels were prepared via random copolymerization similar to MGO/PNA in section 2.2, and the difference was that Fe3O4 was used as the inorganic particle instead of MGO. 2.4. Instrumental characterization Fourier transform infrared (FTIR) spectra of samples were recorded with a Nicolet model in the range 500–4000 cm−1. X-ray photoelectron spectroscopy (XPS) spectra were obtained using an ESCALAB MKII instrument. Raman spectra were evaluated with Raman spectroscope (Renishaw inVia, at 532 nm). The thermogravimetric analysis (TGA) was observed by SDT Q600 apparatus and the results were recorded from room temperature to 800 °C under nitrogen atmosphere at a heating rate of 10 °C/min. The morphology of GO and the hydrogels was observed by a Mira3 Tescan Scanning electron microscopy (SEM) at the voltage of 10 kV. The magnetization was operated at ± 20.0 kOe with vibrating sample magnetometer (VSM; Model LakeShore 7404). Zeta potential measurement was carried out in a Zetasizer Nano-ZS90 machine. Brunauer-Emmett-Teller (BET) surface area, pore diameter 2

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Scheme 1. Schematic illustration for the synthesis of MGO/PNA hydrogel.

AR, and TZ/AR separately (V1=V2 = 5 mL, and the concentrations of the four dyes in aqueous solution were 100 mg/L). The equilibrium dye mixture concentrations were measured by a Shimadzu UV-2450 Spectrometer at λmax, MB =664 nm, λmax, MV = 576 nm, λmax, TZ =426 nm, and λmax, AR = 520 nm. Also, the qe was calculated according to Eq. (3).

and pore volume were performed using a surface area analyzer (ASAP 2020) at 77 K. 2.5. Measurement of swelling ratio In order to study the pH-sensitive swelling properties of the hydrogels, pH value was adjusted by dropping either 0.1 mol/L HCl or NaOH solution. At different pH values, MGO/PNA hydrogel (10 mg) was immersed in water at room temperature until swelling equilibrium was attained. The hydrogel was separated from the aqueous medium, and the excess water of the hydrogel was wiped off. The swelling ratio (SR) of the hydrogel was determined with Eq. (1) [25].

SR =

(MS − Md ) × 100% Md

2.8. Regeneration ability experiment The reusability of MGO/PNA hydrogel was investigated, MB and MV (at pH = 12) and TZ and AR (at pH = 2) adsorbed in the hydrogels were removed by desorption experiment. Typical experimental step was described as follows: 10 mg hydrogel was added into 10 mL MB aqueous solution at pH = 12 and stirred for 80 min until the adsorption reached equilibrium. After equilibration, the MB dye-loaded hydrogel was separated by filtration through external magnetic field and washed several times with distilled water. For the desorption process, the hydrogels loaded with cationic dyes were immersed in pH = 2 solution for 24 h to remove the adsorbed MB dyes. RP% of the resulting hydrogel was measured according to the same method in Section 2.6. For anionic dyes (TZ and AR), the hydrogels loaded with anionic dyes were transferred into the aqueous solution (pH = 12) for 24 h to remove the adsorbed anionic dyes.

(1)

where Ms and Md are the weights of the swollen gel and dried gel, respectively. 2.6. Adsorption experiments Adsorption experiments were carried out on four different dyes of MB, MV, TZ, and AR. Typical steps of MB adsorption were as follows: the mixture of 10 mg MGO/PNA hydrogel and 10 ml MB aqueous solution (at pH = 12) was stirred to achieve adsorption equilibrium, and then the mixture was centrifuged under the rotation speed of 10,000 rpm. The MB concentration of the supernatant was measured by a Shimadzu UV-2450 Spectrometer at the wavelength of 664 nm. The removal percentage (RP%) and the equilibrium adsorption capacity (qe) (mg/g) were obtained using Eq. (2) and Eq. (3) [26].

C − Ce RP % = 0 × 100 C0

(2)

(C0 − Ce ) V m

(3)

qe =

3. Results and discussion 3.1. Characterization of the hydrogels 3.1.1. FTIR analysis FTIR spectra of GO and MGO/PNA are shown in Fig. 1A. From the GO profile, it can be seen that the peak at 1070 cm−1 is ascribed to the tensile of C–O and the skeleton vibration of the graphite domain is observed at 1632 cm−1. C=O stretching is located at 1735 cm−1. The signal at 1400 cm−1 is caused by C–OH traction, and the signal at 3419 cm−1 is due to O–H drawing vibration [27]. For MGO/PNA, the peak of C–O stretching vibration is found at 1126 cm−1. The peak at 617 cm−1 is certainly owing to the Fe–O bands in Fe3O4 nanoparticles [28]. The peaks at 829 cm−1, 1415 cm−1, 1583 cm−1, and 1649 cm−1 are caused by the C=CH bond vibration of NVI, the tensile vibration of C–N, the –C=N vibration, and the –C=C– vibration of NVI and graphite domain, respectively [29]. C=O bond in carboxylic groups stretching is located at 1725 cm−1. Therefore, the above analysis indicates that the MGO/ PNA hydrogel has been successfully prepared.

where C0 and Ce are the initial and equilibrium concentration (mg/L) of the adsorbate in the solution, respectively, m is the hydrogel mass (g) and V is the dye solution volume (L). The dye concentration measurements of MV (at pH = 12), TZ (at pH = 2), and AR (at pH = 2) aqueous solution were carried out at the wavelength of 576, 426, and 520 nm, respectively. 2.7. Preferential adsorption experiments of dyes by the hydrogel A typical process was prepared as follows: 10 mg hydrogel was added into the dyes mixture of MB/MV, MB/TZ, MB/AR, MV/TZ, MV/ 3

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Fig. 1. (A) FTIR spectra, (B) XPS full survey spectra, (C) Raman spectra, and (D) TGA curves of GO and MGO/PNA.

characteristic peak of γ-Fe2O3 at 719.0 eV is observed, which indicates that the magnetic particle in MGO/PNA hydrogel is not γ-Fe2O3 but Fe3O4 nanoparticle [32].

3.1.2. XPS analysis The results of XPS spectra are shown in Fig. 1B and Table 1. Compared with GO, the oxygen element content of MGO/PNA hydrogel is found to decrease, and it should be caused by the forming polymer with relatively high carbon content. Moreover, the appearance of N1s and Fe2p peaks of MGO/PNA is due to the introduction of NVI unit and Fe3O4 particle into the hydrogel, respectively. The XPS curve of carbon species in GO can be fitted into four peaks (Fig. S1A): C–O–C (286.91 eV), C–C (284.79 eV), C–OH (284.90 eV) and O–C=O (289.00 eV). For the O1s fitting spectrum of GO (Fig. S1B), there are C=O (532.81 eV), O=C–OH (531.70 eV), and C–O (533.89 eV) bonds [30]. In contrast, in the curve of MGO/PNA hydrogel, five peaks are fitted for the carbon species (Fig. S1C) because the MGO/PNA hydrogel produces a new C–O/C–N peak (287.62 eV). Similarly, the oxygen species contained in MGO/PNA material also generates a new peak at 529.54 eV (Fig. S1D), which corresponds to metal oxygen bond (M–O) [31]. In Fig. S1E, the core-level N1s spectrum of MGO/PNA material is fitted with three peaks at 400.63, 398.57, and 399.20 eV, corresponding to positively charged nitrogen (N+), amine (=N–), and imine (–NH–), respectively. The core-level Fe2p spectrum of MGO/PNA material is shown in Fig. S1F, and the peaks of Fe 2p1/2 and Fe 2p3/2 are located at 723.94 and 710.09 eV, respectively. More importantly, no

3.1.3. Raman analysis The results of Raman spectra are shown in Fig. 1C, and it can be clearly seen that both GO and MGO/PNA have obvious characteristic peaks of D and G bands. The peaks of D and G bands represent defects in carbon atom lattices and in-plane stretching vibration of carbon atom SP2 hybridization, respectively [33]. As can be calculated, ID/IG of MGO/PNA (1.05) is higher than that of GO (0.77), which is owing to the aggravation of MGO/PNA structure disorder. 3.1.4. TGA analysis The results of TGA testing are shown in Fig. 1D. The weight loss of GO can be divided into two main stages. The weight loss rate (approximately 45 wt%) from room temperature to 200 °C is mainly due to impurity removal and water evaporation. The weight loss rate (about 35 wt%) ranging from 200 to 800 °C is possibly caused by the removal of CO2, CO, and H2O steam from the interior of GO [34]. The weight loss of MGO/PNA hydrogel is also divided into two main parts. The weight loss of the hydrogel from room temperature to 200 °C is close to 10 wt%, which is owing to the evaporation of adsorbed water. The weight loss rate (approximately 80 wt%) of the hydrogel is occurring between 200 °C and 800 °C, which can be attributed to the breakdown of polymer chains grafted on the surface of magnetic graphene oxide and the decomposition of oxygen functional groups in graphene oxide.

Table 1 Chemical compositions of GO and MGO/PNA hydrogel from XPS results. Samples

C (atomic %)

N (atomic %)

O (atomic %)

Fe (atomic %)

GO MGO/PNA

62.76 65.10

0 9.88

37.24 24.38

0 0.64

3.1.5. SEM and magnetization curve Fig. 2 shows the SEM images of GO and MGO/PNA hydrogel at same 4

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Fig. 2. SEM images of (A) GO and (B) MGO/PNA.

bringing about the enlargement of intramolecular space of hydrogels and the penetration of more water molecules into the hydrogels. In the pH ranging from 4 to 10, PAA is negatively charged, while PNVI is positively charged. Because of the strong electrostatic interaction between the two charges, so the hydrogel exhibits a relatively low swelling ratio [35]. When pH value is located between 10 and 12, the positive charge (N+) of PNVI chain is shielded by high concentration of OH– group, so carboxylic acid groups carries negative charge after the deprotonation. At this situation, the repulsion force among the negative charges (COO–) started to play a major role, so the hydrogel can adsorb more water and the swelling ratio is increased. Generally, the swelling ratio of MGO/DNA hydrogels is affected by pH, and this two-way pHsensitive behavior is relatively rare. Fig. 4B can also verify the pH-responsive swelling mechanism of MGO/PNA hydrogel. The positive or negative of zeta potential represents the category of charge carried by the material. Zeta potential is positive at low pH, which is attributed to the cooperative action of H+ in solution and N+ in hydrogel. When pH value increases, the gradual change of zeta potential from positive to negative is due to the deprotonation of −COOH. When pH is higher than 10, the carboxyl groups are deprotonated completely, and OH− group with high concentration occupies most of the active positions, leading to the continuous increase of negative zeta potential. It can be observed from Fig. 4B that the zero charge point (PZC) of MGO/PNA hydrogel is at about pH=4.2.

magnifications. As shown in Fig. 2A, rough wrinkles are observed on the surface of GO. MGO/PNA has a cross-linked three-dimensional network structure as shown in Fig. 2B. Corresponding to SEM image, the prepared hydrogel is also analyzed by the energy dispersive spectrum (EDS) as demonstrated in Fig. S2. The main element signals of C, O, N, and Fe confirm the formation of Fe3O4 nanoparticles and the random copolymerization of AA and NVI on the surface of GO. At the same time, the elemental analysis data of EDS is basically equal to the XPS analysis data in Table 1. The superparamagnetic properties of MGO/PNA hydrogels are measured by VSM. The hysteresis loop in Fig. 3 indicates that MGO/ PNA magnetic hydrogel has a saturation magnetization of 3.1 emu/g and it is superparamagnetic with almost no coercivity at room temperature. It is also suggested that the hydrogel can be separated from the medium by magnets (the inset of Fig. 3). 3.1.6. pH-responsive swelling studies Fig. 4A shows the pH-responsive swelling behaviors of the hydrogel at various pHS, and the tendency presents a “U-shaped” curve. The swelling ratio is extremely high at pH = 2 and 12, and the possible reason is that carboxylic acid and imidazole-N groups in the structure of hydrogels have different surface charges under different pHs. When the pH value is lower than 4, the carboxylic groups of AA units in the hydrogel are protonated at strong acid environment, and the electrostatic interaction between AA and NVI chains is weakened to some degree. Both –NH– involved in BIS and imidazolic-N groups contained in NVI are protonated, hence the repulsion among polymer chains is enhanced. It should be noted that the repulsion dominates the swelling process,

3.2. Dye adsorption studies The effects of solution pH, adsorption time, adsorption temperature, and initial dye concentration on dye removal are systematically investigated. Fig. S3 shows the digital images of the cationic and anionic dyes (MB, MV, TZ, and AR) aqueous solution adsorbed by MGO/PNA hydrogel under the optimum experimental conditions. 3.2.1. Effect of solution pH Generally, pH has some influence not only on the swelling properties of hydrogels, but also on the removal efficiency of dyes. As shown in Fig. 5A, the RP% of cationic dyes (MB and MV) is the highest at pH = 12, while that of anionic dyes (TZ and AR) is the highest at pH = 2. Therefore, the surface charge of MGO/PNA hydrogel can be changed by adjusting pH to achieve selective adsorption [36]. At low pH, some chromogenic groups of anionic dyes ionized in water are negatively charged owing to the presence of sulfonic group. The carboxyl group in the hydrogel is protonated in acidic solution and it is positively charged due to the positive charge exposed by the shared electrons biased toward oxygen atom. With the increase of solution pH, the protonation ability of imino group in hydrogels decreases gradually. Therefore, the ability of binding anionic dyes by electrostatic attraction decreases, and the RP% is also reduced. When the number of hydroxide

Fig. 3. The magnetization curve of MGO/PNA hydrogel. 5

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Fig. 4. (A) pH-responsive swelling property for the MGO/PNA hydrogel; (B) The zero charge point of the MGO/PNA hydrogel.

hydrogels is very low, because the hydrogel is near to electrically neutral at pH = 4.5. This pH is related to the PZC value of the hydrogel obtained from Fig. 4B.

ions attached to the surface of hydrogel reaches a certain level, the surface electrical property will be reversed. At this time, the hydroxide ions will compete with the anionic dyes for the adsorption site, so that the adsorption capacity is further decreased. The RP% of cationic dyes increases with the increasing pH, because the part of cationic dyes with chromogenic groups is positively charged after ionization in water. The carboxyl groups of PAA segment are deprotonated in alkaline environment, and the cationic dyes are strongly adsorbed by electrostatic attraction [37]. At a specific pH (pH = 4.5), the RP% of four dyes by

3.2.2. Effect of adsorption time The effect of adsorption time on the removal percentage is shown in Fig. 5B. The results show that the RP% of dyes increases and attains a plateau with the prolonging time. At the onset of adsorption, the four dyes interact with the imidazole and carboxyl groups of the polymer

Fig. 5. Effect of (A) solution pH, (B) adsorption time, (C) adsorption temperature, and (D) initial dye concentration on the removal percentage of dyes by MGO/PNA hydrogel. 6

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Fig. 6. Plots of (A) Pseudo-first-order, (B) Pseudo-second-order, (C) Langmuir isotherm, and (D) Freundlich isotherm models for the adsorption of the four dyes by MGO/PNA hydrogel.

3.2.3. Effect of temperature The effect of adsorption temperature on the removal percentage is given in Fig. 5C. It can be observed that the removal efficiency of the dyes first increases and then decreases as the rising temperature. Physical adsorption is usually an exothermic process, and the increasing temperature will lead to the decrease of adsorption capacity. Meanwhile, it is necessary to increase temperature to accelerate the formation of chemical bonding in chemical adsorption [40]. However, excessive temperature will give rise to the breakage of chemical bonds and the decrease of adsorption capacity. In addition, temperature also has effect on the solution viscosity and dye molecular activity. With the increase of temperature, the decrease of solution viscosity and the intensification of dye molecules movement are easy to cause the collision between dyes and adsorbents. Moreover, thermodynamic parameters are further used to explain the effect of temperature on dye removal and the feasibility of adsorption process [41]. The thermodynamic parameters related to adsorption are usually enthalpy change (ΔH°), entropy change (ΔS°), and Gibbs free energy change (ΔG°). The formulas are listed as follows:

chain via non-covalent interactions [38]. As adsorption time increases, the adsorption sites on the surface of hydrogels gradually decrease, and the adsorption equilibrium is finally obtained. For the adsorption kinetics, pseudo-first-order and pseudo-secondorder models are used to fit the experimental data, and the details are listed in the Supporting information. The details of the foregoing parameters are given in Fig. 6A-B and Table 2. As can be observed, the correlation coefficients (R2) of pseudo-second-order model of the four dyes are higher than 0.99, so the curve of the pseudo-second-order model is extremely consistent with the experimental data, indicating that the pseudo-second-order model fits the adsorption process better than pseudo-first-order model. The adsorption capacity calculated by the pseudo-second-order model is also in good agreement with the experimental adsorption capacity, which further verifies the superiority of the pseudo-second-order model. All these evidences indicate that the adsorption process may be driven by the chemical action between some active groups and dye molecules adsorbed by the hydrogel [39]. Table 2 Constants of kinetic models for four dyes adsorption on MGO/PNA hydrogel. Kinetic model

Pseudo-first-order

Pseudo-second-order

Parameters

qe(mg/g) K1 R2 qe(mg/g) K2 R2

ln

qe Ce

=

ΔS° ΔH ° − R RT

(4)

Cationic dyes

Anionic dyes

MB

MV

TZ

AR

ΔG° = ΔH ° − TΔS°

96.2 0.0237 0.984 96.0 0.0070 0.991

91.3 0.0122 0.999 86.3 0.0071 0.999

95.1 0.0115 0.998 89.6 0.0068 0.999

94 0.0098 0.988 86.9 0.0074 0.998

where qe (mg/g) is the adsorption capacity, Ce (mg/L) is the equilibrium concentration of the adsorbate in the solution, R is gas constant (8.314 J/mol•K), and T is the temperature (K). As can be found from Table 3, the negative value of ΔG° adsorbed by the four dyes confirms the spontaneousness of the adsorption process. The positive value of ΔH° and ΔS° indicates that the randomness of the dye molecules 7

(5)

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Table 3 Thermodynamics parameters of MGO/PNA hydrogel at C0 = 100 mg/L. Thermodynamic parameters

Temperature (K) ΔG° (kJ/mol) ΔH° (kJ/mol) ΔS° (J/molK%)

Cationic dyes

Anionic dyes

MB

MV

TZ

AR

313 −8.45 28.57 118.27

313 −4.87 13.11 57.44

313 −6.08 11.13 54.98

313 −5.83 15.41 67.85

Table 4 Adsorption isotherm parameters of MGO/PNA hydrogel. Isotherm model

Langmuir

Freundlich

Parameters

qm (mg/g) b (L/mg) RL R2 1/nF KF (mg/g) R2

Cationic dyes

Anionic dyes

MB

MV

TZ

AR

625.0 0.038 0.208 0.998 0.426 66.011 0.956

609.8 0.018 0.357 0.996 0.485 37.953 0.968

613.5 0.027 0.270 0.993 0.432 55.437 0.977

609.8 0.023 0.303 0.992 0.448 48.605 0.979

Fig. 7. FTIR spectra of MGO/PNA hydrogel before and after dyes adsorption.

increases, which is favorable for dyes adsorption. 3.2.4. Effect of initial dye concentration The effect of various dye concentration (from 100 to 800 mg/L) on the adsorption capacity of MGO/PNA adsorbents is investigated as shown in Fig. 5D and Fig. S4. The dye RP% of the hydrogel has the opposite trend with the initial dye concentration. Since the number of adsorption sites on the adsorbent surface is certain, the amount of adsorption may keep constant when the dye molecules has occupied all the adsorption sites. Therefore, as the concentration increases, the RP% of the dye by the MGO/PNA hydrogel decreases. With the increase of equilibrium concentration, equilibrium adsorption capacity first increases and attains a stable plateau. Adsorption isothermal model is one of the most important methods to describe the interaction and mechanism between adsorbent and adsorbate. So far, the most widely used mathematical models are Freundlich and Langmuir models which are available for the adsorption of dyes by the hydrogel in aqueous solutions, and the details are given in the Supporting information. For the sake of obtaining the parameters in the two models, experimental data of all concentrations (100−800 mg/L) is used for

Table 5 Comparison of some adsorbents discussed in the literatures for MB/MV/TZ/AR adsorption capacity. Adsorbents

qm (mg/g)

KG-cl-poly(AA-co-NVI) Gg-cl-P(AAm-co-MAA) h-XG/SiO2 nanocomposite poly(AAc-SA-AM)/SH Fe3O4/MgO poly(AAm-co-AMPS)/Na-MMT Gk-cl-p(AA-co-AAM) KGNCH-3 MGO/PNA hydrogel

References

MB

MV

TZ

AR

331.5 694.4 497.5 270 – 176 1408.6 165.28 625.0

286.0 543.4 378.8 231 – 155 – 158.73 609.8

201.5 – – – – – – – 613.5

– – – – 38.1 – – – 609.8

[29] [42] [43] [44] [45] [46] [47] [39] This work

Scheme 2. Scheme of possible interaction mechanism between MGO/PNA hydrogel with cationic dyes (A) and anionic dyes (B). 8

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Fig. 8. XPS fitting curves of (A) C1s, (B) O1s, (C) N1s, and (D) Fe2p of MGO/PNA hydrogel before and after the adsorption of cationic dyes.

calculation. To evaluate the fitting degree between isothermal model and experimental data, the correlation coefficient and the calculated qm are compared with the testing data, and the results are listed in Fig. 6CD and Table 4. As can be found, the values of 1/nF →0 and between 0 and 1 of RL indicate that the adsorption process toward four dyes by MGO/PNA hydrogel is easy to occur. Based on the result in Table 4, R2 of Langmuir model for all the four dyes is higher than 0.99. It is concluded that Langmuir model is more suitable than Freundlich model in this experiment and the adsorption of the four dyes are subjected to monolayer adsorption. The maximum adsorption capacity (about 600 mg/g) of the four dyes is higher than most of previously reported adsorbents as shown in Table 5, suggesting that MGO/PNA hydrogel is a promising adsorbent in many application domain.

owing to the electrostatic interaction and hydrogen bonding between the carboxyl groups in acrylic acid and cationic dyes. It is also observed that the band of imidazole –C = N groups of the hydrogel is shifted from 1583 to 1595 cm-1 after adsorption toward anionic dyes. This is also because there are electrostatic interaction and hydrogen bonding between PNVI segment and anionic dyes. These results indicate that the introduction of imidazole can endow the hydrogel with the strong ability to adsorb anionic dyes. In addition to FTIR analysis, XPS analysis of MGO/PNA hydrogels before and after adsorption toward anionic and cationic dyes is performed to further confirm the adsorption mechanism as shown in Figs. (8,9). In the C1s curves in Fig. 8, a binding energy shift of O–C=O peak is observed from 288.54–288.31 eV after the adsorption of cationic dyes by MGO/PNA hydrogel. The binding energy of M–O in O1s curve is shifted from 529.54–531.24 eV, and the two binding energies in Fe2p curve also change after cationic adsorption. Therefore it can be concluded that carboxylic groups and Fe3O4 in the hydrogel interact with cationic dye molecules. Similarly in Fig. 9, some shifts in binding energy of C1s, O1s, N1s, and Fe2p can be also found, and all the changes are listed in Table S1 in the Supporting Information. Hence, it is the imidazole groups and Fe3O4 that interact with anionic dyes in the adsorption process of the hydrogel. To further investigate the functionality of Fe3O4, the adsorption performances of GO, Fe3O4/PNA, GO/PNA, and MGO/PNA are studied as demonstrated in Fig. 10. Compared with that of GO, the RP% of Fe3O4/PNA and GO/PNA are enhanced. This is because the porous structure of the hydrogel can increase the contact area between the adsorbent and dye molecules. In particular, MGO/PNA has higher RP%

3.3. Adsorption mechanism The adsorption mechanism of MGO/PNA hydrogel on anionic and cationic dyes is shown in Scheme 2. Negatively charged (COO–) groups on the hydrogel can interact strongly with = N+– moieties in cationic dyes. Similarly, the positive charge on the hydrogel (=N+–H) is electrostatically attracted by the oxygen atoms of the –SO3– Na+ groups of anionic dyes [48]. In addition, van der Waals, hydrogen bond, π-π stacking interaction, and hydrophobic interaction further improve the adsorption property of hydrogels. As shown in Fig. 7, the existence of these interactions is further confirmed by FTIR spectra of MGO/PNA before and after adsorption. Compared to that of MGO/PNA before adsorption, the peak of C]O groups of the hydrogel is shifted from 1725 to 1718 cm−1 after adsorption toward cationic dyes, which is 9

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Fig. 9. XPS fitting curves of (A) C1s, (B) O1s, (C) N1s, and (D) Fe2p of MGO/PNA hydrogel before and after the adsorption of anionic dyes.

Fig. 11. Removal percentage of MB, MV, TZ, and AR by MGO/PNA hydrogel in different cycles.

Fig. 10. Comparison of removal percentage of MB, MV, TZ, and AR by four different adsorbents.

hydrogel to some degree. This inference can be also confirmed by Fig. S5 and Table S2 in the Supporting information. As can be seen from Fig. S5, the surface of MGO/PNA hydrogel is rougher and more porous than that of GO/PNA hydrogel. From Table S2, the BET specific surface area of MGO/PNA hydrogel is calculated to be 13.64 m2/g. Compared with GO/PNA hydrogel, MGO/PNA hydrogel has larger specific surface area and pore volume. All of these evidences can prove that the introduction of GO and Fe3O4 nanoparticles provides more adsorption sites and improves the adsorption capacity of the hydrogels.

than Fe3O4/PNA and GO/PNA. The introduction of GO and Fe3O4 may form physical cross-linking points, and the interaction between GO sheets and Fe3O4 particles can effectively prevent the redeposition of graphene sheets and the agglomeration of Fe3O4 particles. They can be well dispersed in the solution, and the specific surface area of the composite can be increased [49,50]. Therefore, the addition of Fe3O4 into the hydrogel not only endows MGO/PNA with easily recyclable characteristic, but also improves the adsorption capacity of the 10

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technology area. It was anticipated that this work was contributive to the development of smart hydrogels and provided new insights for water treatment and biomedicine field.

3.4. Preferential studies In a quaternary dye mixture system (X/Y), the selectivity coefficient (αYX ) is used to assess the preferential adsorption of X in the dye mixture (X and Y) as shown in Eq. (6) [51].

αYX =

qX CY CX qY

Notes The authors declare no competing financial interest.

(6)

Authors’ contributions

where, qX is the adsorption capacity (mg/g) of the X dye, and qY is the adsorption capacity (mg/g) of the Y dye. CX is the X dye concentration (mg/L) at equilibrium, and CY is the Y dye concentration (mg/L) at equilibrium. When αYX is greater than 1, the effect of adsorbing X dye is better, and when αXY is less than 1, Y dye is preferentially adsorbed. As can be seen from Table S3 and Fig. S6, for MB/MV, MB/TZ, and MB/AR dye mixture, αXY is greater than 1, indicating that MB is preferentially adsorbed by MGO/PNA hydrogel [52]. All the αXY values of MV/MB, MV/TZ, and MV/AR dye systems are lower than 1, showing that the affinity of hydrogel toward MV is relatively weak. For TZ/AR dye mixture, the αXY value is greater than 1, which means that TZ is more readily adsorbed by the hydrogel than AR. Therefore, the priority adsorption order of four dyes by MGO/PNA hydrogel is: MB > TZ > AR > MV.

Guohong Yao was responsible for the synthesis and adsorption of MGO/PNA hydrogel. At the same time, she also wrote this manuscript. Wendie Bi was responsible for the characterization of the resulting MGO/PNA hydrogel. Hui Liu conceived and designed the study, and he was also responsive for the revision of the manuscript. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.

3.5. Regeneration ability of MGO/PNA hydrogel Acknowledgements In order to explore the regeneration ability of MGO/PNA hydrogel, the desorption behaviors of four dyes by the hydrogel are studied. Contrary to the adsorption behaviors, the desorption process of cationic dyes is carried out at pH = 2. This is because the formation of carboxylic acid leads to the attenuation of electrostatic attraction between carboxylic ions and cationic dyes in strong acidic condition [53]. With regard to anionic dyes, the desorption experiment is performed at pH = 12, because the formation of free imidazole–N in strong alkaline environment is favorable for the release of more anionic dyes from the surface of hydrogel. As can be seen from Fig. 11, the dye RP% is still no lower than 65 % even after five adsorption-desorption cycles although it is slightly reduced with the increasing repetition times. Ethanol as the desorption solvent is used to compared with the desorption solvent selected in the present experiment. As shown in Fig. S7, the desorption solvent selected in this experiment can be used more effectively to achieve the reuse of hydrogels after five adsorption-desorption cycles. The regeneration experimental results verify that MGO/PNA hydrogel can maintain good adsorption effect after multiple adsorption-desorption cycles, so the hydrogel has the wide application prospect in adsorption science and technology area.

This work was financially sponsored by the National Natural Science Foundation of China (Grant no. 21376271), the Hunan Provincial Science and Technology Plan Project, China(Grant no. 2016TP1007), the Fundamental Research Funds for the Central Universities of Central South University (Grant no. 2019zzts447), and the Open sharing Fund for the Large-scale Instruments and Equipments of Central South University (Grant no. CSUZC201928). Wendie Bi thanked the support of the Undergraduates Innovative Training Foundation of Central South University (S201910533529). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2019.124393. References [1] M.T. Uddin, M.A. Islam, S. Mahmud, M. Rukanuzzaman, Adsorptive removal of methylene blue by tea waste, J. Hazard. Mater. 164 (2009) 53–60. [2] M.A.M. Salleh, D.K. Mahmoud, W.A.W.A. Karim, A. Idris, Cationic and anionic dye adsorption by agricultural solid wastes: a comprehensive review, Desalination 280 (2011) 1–13. [3] Z. Yang, F. Wang, H. Liu, Dual responsive spiropyran-ended poly(N-vinyl caprolactam) for reversible complexation with metal ions, J. Polym. Res. 26 (2019) 89. [4] P. Ilgin, H. Ozay, O. Ozay, Selective adsorption of cationic dyes from colored noxious effluent using a novel N-tert-butylmaleamic acid based hydrogels, React. Funct. Polym. 142 (2019) 189–198. [5] S. Hua, X. Yu, F. Li, J. Duan, H. Ji, W. Liu, Hydrogen titanate nanosheets with both adsorptive and photocatalytic properties used for organic dyes removal, Colloid. Surface. A 516 (2017) 211–218. [6] N. Nikooe, E. Saljoughi, Preparation and characterization of novel PVDF nanofiltration membranes with hydrophilic property for filtration of dye aqueous solution, Appl. Surf. Sci. 413 (2017) 41–49. [7] Y. Tang, L. Chen, X. Wei, Q. Yao, T. Li, Removal of lead ions from aqueous solution by the dried aquatic plant, Lemna perpusilla Torr, J. Hazard. Mater. 244-245 (2013) 603–612. [8] G. Zelmanov, R. Semiat, Boron removal from water and its recovery using iron (Fe+3) oxide/hydroxide-based nanoparticles (NanoFe) and NanoFe-impregnated granular activated carbon as adsorbent, Desalination 333 (2014) 107–117. [9] A.M.M. Vargas, A.L. Cazetta, A.C. Martins, J.C.G. Moraes, E.E. Garcia, G.F. Gauze, W.F. Costa, V.C. Almeida, Kinetic and equilibrium studies: adsorption of food dyes acid Yellow 6, Acid Yellow 23, and Acid Red 18 on activated carbon from flamboyant pods, Chem. Eng. J. 181–182 (2012) 243–250. [10] K.A.G. Gusmão, L.V.A. Gurgel, T.M.S. Melo, L.F. Gil, Adsorption studies of methylene blue and gentian violet on sugarcane bagasse modified with EDTA dianhydride

4. Conclusion In this work, pH-responsive magnetic MGO/PNA hydrogel was synthesized by random copolymerization of NVI and AA in the presence of MGO as an easily recyclable adsorbent for cationic and anionic dyes. FTIR, XPS, Raman, SEM, and TGA analysis confirmed the successful synthesis, and the swelling percentage of the hydrogel attained the extremely high value at pH = 2 and pH = 12, respectively. The mechanism analysis indicated that the addition of Fe3O4 into the hydrogel not only endowed MGO/PNA with easily recyclable characteristic, but also improved the adsorption capacity of the hydrogel to some degree. The simulation results showed that the adsorption data followed the Langmuir model, and the adsorption kinetics agreed well with the pseudo-second-order model. Besides, the adsorption thermodynamic data indicated that the adsorption of dyes by hydrogels was a spontaneous process. MGO/PNA hydrogel could maintain good adsorption effect after multiple adsorption-desorption cycles, so the hydrogel might have the wide application prospect in adsorption science and 11

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