Synthesis of hydrogels for adsorption of anionic and cationic dyes in water: ionic liquid as a crosslinking agent

In this work, we synthesized an ionic liquid (IL)—(Dimethylamino)ethyl Methacrylate maleate ([DMAEMA]MA) as the crosslinker, through one-pot to synthesized hydrogels with high adsorption capacity for dye in water. Both anionic dyes (methylene blue, rhodamine B) and cationic dyes (congo red, eosin B) could be adsorbed by this type of hydrogel with different adsorption mechanism, and its adsorption capacity for methylene blue (MB), rhodamine B (RHB), congo red (CR), eosin B (EB) were 489.1, 463.2, 465.5 and 462 mg/g (amount of dye adsorbed per gram of hydrogel), respectively. The surface structure of the hydrogel before and after adsorption was observed and compared by scanning electron microscope (SEM). After studying the adsorption isotherms of the hydrogel adsorbent, it was found that the hydrogel adsorbent had two adsorption mechanisms. This was not found in reported literatures previously.


Introduction
With the rapid development of social science and technology, our life becomes colorful by using of dyes. Dyes were widely used in daily life, such as textiles, coatings, leather, ink, medicine, rubber and so on [1,2]. At the same time, the treatment of dye wastewater was also very difficult to solve. Due to the high color fastness of the dye effluent, the azo and aromatic groups in dye molecule were highly toxic and non-degradable resulting in a serious environmental hazard. In addition, there were carcinogens such as naphthalene and benzidine in dyes, which also caused serious harm to human body [3,4]. Therefore, the treatment of dyes had become the focus of attention. So far, the main methods of dye treatment included flocculation, reverse osmosis, membrane filtration, photodegradation, biological treatment, adsorption and chemical precipitation, etc. [5][6][7] Compared with adsorption, although these treatment methods provide some solutions to dye wastewater, they also have the limitations of low efficiency, high toxicity of decomposition products and high cost [8]. In contrast, the adsorption method was more in line with the needs of industrial production. Through continuous research and improvement of adsorption capacity of adsorbents, the adsorption method in dye treatment has been continuously improved [9]. The earliest adsorbents included carbon nanotubes, graphene oxide, zeolite and natural materials [10][11][12][13][14][15]. Among them, Huang et al. [10] used graphene oxide modified zeolite as a dye adsorbent to adsorb cationic dyes (MB) in water. However, it cannot be used as an effective adsorbent due to its low adsorption capacity. Similarly, other inorganic dye adsorbents have been reported in many literatures, but their adsorption capacity were not satisfactory [11][12][13][14][15]. Therefore, they cannot be selected as efficient dye adsorbents.
Hydrogel is a kind of polymer material with threedimensional network structure which capable of undergoing large volume deformation by absorbing water [16][17][18][19]. When the hydrogels are allowed to swell in a dye solution, the aqueous solution will transfer dye molecules to the surface of the hydrogel and adsorb through van der Waals interactions or hydrogen bonding [20]. From what has been discussed above, we synthesized PAM/ PAA/[DMAEMA]MA hydrogel adsorbent with extremely high swelling rate by using ionic liquids as crosslinking agents. After adsorption experiments, it was found that the adsorbent could reach adsorption equilibrium for only about 30 min. In contrast, Sarmah et al. [21] synthesized double network hydrophobic starch based amphoteric hydrogel to adsorb MB and CR. The experimental results showed that it took longer to reach adsorption equilibrium. Most of hydrogel adsorbents reported in the literatures were single selective adsorbents for cation or anion. For example, Jiang et al. [22] reported a composite chitosan/β-Cyclodextrin polymer hydrogel adsorbent had a selective adsorption capacity of 392 mg/g for anionic dyes (methyl orange, MO). Similarly, Yang et al. [23] synthesized a graphene oxide/montmorillonite composite hydrogel adsorbent for the cationic dye MB, but it could only adsorb at high concentrations. Besides, with the progress of research, hydrogel adsorbents which could adsorb cationic and anionic dyes at the same time were also reported. Shukla et al. [24] reported that the amphoteric hydrogel adsorbent could adsorb cationic dyes (MB) and anionic dyes (orange G, OG) simultaneously. It realized the simultaneous adsorption of cationic and anionic dyes. However, the hydrogel adsorbent had low adsorption capacity.
In this work, the PAA/PAM/[DMAEMA]MA hydrogel adsorbent could adsorb anionic and cationic dyes at the same time and had outstanding adsorption capacity. However, the adsorption mechanism in this study was different from previous reports, and both two adsorption modes played a role together. Physical adsorption was realized through the electrostatic interaction between cation and anion. Chemisorption depended on the chelation between the carboxyl group, the amino group and the dye molecules in hydrogel adsorbent. It was also confirmed by adsorption isotherm and adsorption kinetics. Besides, the influence of pH and temperature on dye adsorption was also tested. It was found that the hydrogel adsorbent could adsorb dye at room temperature and near neutral conditions.

Characterization
1 H NMR spectra was recorded at 400 MHz on a Bruker avance III spectrometer (Bruker Daltonics Inc., America) with tetramethylsilane as the internal standard. The Fourier transform infrared (FTIR) spectrum was studied by Nicolet IS5 FTIR spectrometer coupled with an attenuated total reflectance (ATR) accessory. The samples were first mixed with dried KBr pallets before analysis and spectra of each sample was obtained in the range of 4000-500 cm −1 . Surface morphology of hydrogels was detected on scanning electron microscope (SEM, Zeiss Sigma). Uvmini-1240 UV-visible spectrophotometer (Shimadzu, Japan) was used to measure the adsorption value of aqueous solution after hydrogel adsorption the dyes.

Synthesis of [DMAEMA] MA
Under N 2 protection, added 6.29 g (0.04 mol) of DAMEMA and 10.00 g of methanol to a 100 mL three-necked flask, stirred until all DMAEMA was dissolved, then added 2.32 g (0.02 mol) of MA, and reacted at 40 °C for 2 h. After the completion of the reaction, the reaction solution was transferred to a beaker, cooled, and crystallized at low temperature for 2 h, the reaction solution was filtered under reduced pressure and the filter cake was washed 3 times with methanol. The filter cake was dried at 50 °C for 3 h to obtain white needle-like crystal powder.

Preparation of PAA/PAM/[DMAEMA]MA hydrogel
Appropriate amounts of acrylic acid (AA), acrylamide (AM) (m AA :m AM = 7:3), sodium hydroxide(neutralizing: 60%) and distilled water are mixed by ultrasound and stirred to room temperature. Then added a certain amount of crosslinking agent (0.8 wt%) [DMAEMA]MA, stirred magnetically at room temperature for 30 min. N 2 was introduced to drain the oxygen in the flask, APS (0.4 wt%) was added and sealed. The polymerization was carried out in a water bath at 60 °C for 4 h. Unreacted monomer on hydrogel surface was treated with distilled water, dried to constant weight at 60°, and pressed into powder for storage. The synthetic process of PAA/PAM/[DMAEMA]MA hydrogel was showed in Fig. 1.

Hydrogel swelling study
The swelling behavior of PAA/PAM/[DMAEMA]MA hydrogels were investigated by immersion of 0.1 g of the hydrogel in 100 mL distilled water at room temperature and the hydrogels reached swelling equilibrium for 12 h. The influence of pH on the swelling behavior was tested by HCl and NaOH. The hydrogels were weighed before and after they reached the maximum swelling. Equation (1) calculates the percentage of hydration.
where m w is the mass of the swollen sample at time t and m d is the weight of the dry sample.

Dye adsorption experiments
Dry hydrogel (dry until constant weight) powder 0.1 g was put into a 100 mL Conical flask, added 50 mL of dyes (MB, RHB, CR, EB) of different concentrations, Stirred at 25 °C for 12 h. After the adsorption equilibrium was reached, the decreased dye concentration was determined by measuring absorbance at max of each dye using a UV-Vis spectrophotometer. Q e and removal ratio (RR%) were calculated using the following Eqs. (2) and (3): where Q e (mg/g) represents the equilibrium removal capacity of hydrogel, C 0 (mg/L) is the initial and C e (mg/L) is the equilibrium concentrations of dyes in liquid phase, V (L) is the volume of dye solution and M (g) is the weight of dried hydrogel.

FT-IR analysis
For PAA/PAM/[DMAEMA]MA hydrogel (Fig. 2b), the strong absorption bands of amide groups at 3358 and 3196 cm −1 , belong to the -NH 2 stretching vibration peaks which shifted from 3359 to 3195 cm −1 of AM (Fig. 2a); The C=O stretching vibration absorption peak at 1652 cm −1 , belong to the C=O stretching vibration absorption peak shifted to this point in the AM infrared spectrum at 1677 cm −1 ; 1558 and 1448 cm −1 correspond to the C=O stretching vibration peaks of the carboxy anion (-COO − ) in AA. The above data showed that the copolymerization of AA and AM successfully took place, and PAA/PAM/[Vim]Br 2 hydrogels were synthesized.

SEM analysis of PAA/PAM/[DMAEMA]MA hydrogel
As shown in Fig. 3a, after freeze drying, the prepared PAA/ PAM/[DMAEMA]MA hydrogel had a uniform distribution of internal pore diameters. This porous network structure increased the effective contact areas between the hydrogel and the dyes, which was conducive to the adsorption of dyes by hydrogels. As shown in Fig. 3b, the surface of the hydrogel without adsorption was pure and smooth. As shown in Fig. 3c-f, there were many flocculated impurities on the surface of the hydrogel after adsorption of MB and RHB; after CR and EB were adsorbed, a large area of circular pitted structure appeared on the hydrogel surface, which may be related to the crystal structure of CR and EB.

Effect of adsorbent dosages
The effect of the addition amount of hydrogel on RR (%) was shown in Fig. 4. The resulted showed that the RR (%) increased rapidly with the increase of hydrogel dosage, which mainly increased the surface area and more active sites of hydrogel with the addition of hydrogel, and reached maximum values of 97.1%, 93.2%, 92.1%, 91.3% for MB, RHB, CR, EB at the amount of hydrogel addition was 2 g/L. However, when the hydrogel dosage was 3 g/L, the saturation phenomenon appeared, and then the RR (%) increased slowly with the continuous increased of the amount of hydrogel. Therefore, 2 g/L was selected as the best dosage for further adsorption experiments.

Effect of pH on Q e
The initial pH value of aqueous solution had an important influence on the adsorption behavior of hydrogels, which not only affected the ionic form of dyes aqueous solutions, but also affected the degree of protonation of adsorbents [25]. Figure 5 showed the effect of pH value on the adsorption of PAA/PAM/[DMAEMA] hydrogel on MB, RHB, CR and EB. The Q e in strong acid (pH = 3) and strong alkaline (pH = 11) environment was significantly lower than that in neutral environment, this was because too much H + made the -COO − and -NH 2 on the hydrogel structure protonated to form -COOH and -NH 4 + . Meanwhile, H + also competed with dyes ions and took up more adsorption sites. When the pH value increased gradually, the hydrogels were deprotonated on -NH 4 + and -COOH, and a large number of -NH 2 and -COO − chelate with dyes ions, so that the hydrogel increased the Q e of dyes ions [26]. Under strong alkaline conditions, the carboxyl and amino groups of the main chains all existed in the form of ions, due to the increase of ionic strength of external solutions, the osmotic pressure inside and outside the hydrogel network decreased, which led to the decrease of Q e .

Effect of temperature on Q e
As shown in Fig. 6, for anionic dyes CR and EB, when the temperature rose from 15 to 30 °C, the Q e of dyes ions  adsorbed by the hydrogel gradually increased. When the temperature exceeded 30 °C, the Q e of the dye ions decline slowly. This showed that when the temperature was from 15 to 30 °C, the adsorption process of the hydrogel to the dye was mainly an endothermic reaction, therefore, an appropriate increase in temperature could effectively improve the adsorption of the hydrogel to CR and EB dyes. When the temperature exceeded 30 °C, the excessively high temperature destroyed the three-dimensional network structure inside the hydrogel, weakened the chelating effect of dye ions and hydrogel functional groups, thereby reduced the ability of the hydrogel to adsorb dyes.  The adsorption kinetics of PAM/PAA/[DMAEMA]MA hydrogel on four dyes were fitted by pseudo first order kinetics and two stage kinetic equation, the kinetics of adsorption reaction of hydrogel on four dyes were obtained. The equation expression is as follows (4) and (5) respectively.

Adsorption kinetics of
where t is the adsorption time (min), Q e (mg/g) and Q t (mg/g) are respectively the adsorption capacity of PAA/ PAM/[DMAEMA]MA hydrogel to dyes ions during the process and when adsorbed at equilibrium time, k 1 and k 2 are quasi one and quasi two stage adsorption rate constants, respectively.
The results of pseudo first order and second order reaction kinetics fitting curves of four dyes were shown in Fig. 8 and Table 1. The fitting results showed that: compared with the pseudo first order adsorption kinetics fitting results [R 1,MB = 0.792, Fig. 8a; R 1,RHB = 0.823, Fig. 8c; Fig. 8e; R 1,EB = 0.819, Fig. 8g], the pseudo second order adsorption kinetic model fitting was more consistent[R 2,MB = 0.9959, Fig. 8b; R 2,RHB = 0.9996, Fig. 8d; R 2,CR = 0.9983, Fig. 8f; R 2,EB = 0.9991, Fig. 8h]. Therefore, the adsorption of four dyes ions were a multistep process. First, the dyes ions adhered to the material surface, and then entered the hydrogel through the channel of the hydrogel to further spread.

Adsorption Isotherms of PAA/PAM/[DMAEMA] MA Hydrogels for MB, RHB, CR and EB
The adsorption isotherms of MB, RHB, CR and EB dyes ions on PAA/PAM/[DMAEMA]MA hydrogels were shown in Fig. 9. When the concentration of dyes increased from 20 to 1000 mg/L, the maximum Q e of four dyes ions were 489.2, 463.51, 465.48 and 462.2 mg/g, respectively. When the concentration of dyes continued to increase, the Q e of hydrogel increased slowly and tended to be saturated. Therefore, the proper increase of initial concentration of dyes could enhanced the Q e of hydrogels to dyes.
The adsorption isotherms of Freundlich and Langmuir were studied. The Freundlich isotherm was a heterogeneous, multilayer adsorption system, the absorption process took place on an active heterogeneous surface. The Langmuir isotherm was a homogeneous, single molecular layer adsorption system, each binding site on the absorption surface adsorbed the same energy, and each binding site occupied only one dye ion. The two models are presented in Eqs. (6) and (7): where C e , Q e , Q m are the initial equilibrium concentration (mg/L) of dyes ion solution, the adsorption capacity of PAA/PAM/[DMAEMA]MA hydrogel to dyes ions (mg/g), and the saturated adsorption capacity of hydrogel to dyes ions (mg/g). K F and K L are Freundlich and Langmuir equilibrium constants respectively, and n is the concentration index.
The Freundlich and Langmuir adsorption isotherm models were fitted to the initial concentration and equilibrium adsorption capacity of the PAA/PAM/[DMAEMA]MA hydrogel to adsorb four dyes ions. The results were shown in Fig. 10 and Table 2. Freundlich and Langmuir adsorption isothermal models can better fit the adsorption process of hydrogels for four dyes ions, and the linear coefficients R 2 of the fitting equation were all more than 0.98. However, the Freundlich adsorption isotherm model fits the MB dye better, and the Langmuir adsorption isotherm model fits the RHB, CR and EB dyes molecules better. Besides, the concentration index 1 < n < 2 obtained by the Freundlich adsorption isotherm model fitting indicated that the adsorption process of the four dyes by the hydrogel was easy to perform.
In addition, we also compared the adsorption capability of the hydrogel with those reported in the previous literature, in order to evaluate the adsorption performance of hydrogels on dyes in this paper. The comparison of Table 3 shows that the PAA/PAM/[DMAEMA]MA hydrogel has excellent adsorption properties for anionic and cationic dyes. e and f CR was fitted by pseudo first and quasi two order kinetic models. g and h EB was fitted by pseudo first and quasi two order kinetic models   Fig. 10 a and

Dye adsorption mechanism
As shown in Fig. 11, the adsorption of the adsorbent for the cationic dyes (MB) was physical. In contrast, the adsorption for cationic dyes (RHB) and anionic dyes (CR, EB) was chemisorption. The possible reason was that the molecular structure of MB dyes was linear chain and the cations were relatively small in size. So, the cations of MB dyes were likely to interact electrostatically with the anions and cations of the ionic liquids, thus causing the adsorption of MB on hydrogels to become physical adsorption. Since the molecular structures of RHB, CR and EB were more complex than MB, their anions and cations were relatively large in size and were more likely to chelate and coordinate with the functional groups inside the hydrogel, resulting to chemisorption with the RHB, CR and EB dye molecules. In addition, ionic liquid would cause the change of pore structure of hydrogel [32,33], which also affected the adsorption of dye by hydrogel adsorbent. The Innovative adsorption mechanisms of hydrogel adsorbent on anionic and cationic dyes have not been reported in the literature.

Conclusions
In this work, we used ionic liquids as crosslinking agents to synthesize PAM/PAA/[DMAEMA]MA hydrogels through one pot process. The preparation process was relatively simple. In addition to the excellent adsorption properties, the hydrogel could adsorb different types of dyes simultaneously, and the adsorption process was found to be carried out by two different adsorption mechanisms, which was different from previous studies. In summary, the synthesized hydrogel could make a significant contribution to existing hydrogel research. This kind of double adsorption had been extended the adsorption range, further studies focusing on the preferential adsorption and recycling of this hydrogel are currently underway. We also hope that our research work will contribute to the treatment of dye wastewater.

Declarations
Ethical approval This article does not contain any studies with human participants or animals performed by any of the authors.
Informed consent Informed consent was obtained from all individual participants included in the study.

Conflict of interest
All authors declare that they have no conflict of interest.
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