Fabricating Cationic Lignin Hydrogels for Dye Adsorption

Due to the low content of adsorption-active groups in lignin, its application in the field of adsorption is limited. Herein, we first prepared cationic kraft lignin acrylate, from which a cationic lignin (CKLA) hydrogel was further prepared by cationic kraft lignin acrylate, acrylamide, and N, N’-methylenebisacrylamide. The morphology, compression properties and swelling properties of CKLA hydrogels were investigated. The prepared CKLA hydrogel was applied as an adsorbent for Congo red. The effect of CKLA hydrogel dosages, initial concentration of Congo red, and pH on adsorption efficiency was investigated. The maximum Congo red removal efficiency was obtained at the initial concentration of Congo red of 50 mg/L, pH 7, and 5 mg dosage of CKLA hydrogel with 20% cationic lignin content. After five cycles of adsorption, the adsorption efficiency of the hydrogel for Congo red still reached more than 80%. The CKLA hydrogel showed pseudo-second-order adsorption kinetics for Congo red adsorption. These results demonstrate the potential of the CKLA hydrogel as an adsorbent for water treatment.


Introduction
With the discharge of a large amount of dye wastewater, the textile, printing and dyeing industries have caused serious water pollution [1,2]. Anion dyes are widely used in the dyeing process due to their simple synthetic process, low cost, and better staining performance [3,4]. During the process of anionic dyeing, the fabric needs to be washed several times to improve its dyeing fastness, resulting in the generation of a large amount of anionic dye wastewater [5], causing oxygen deficiency and jeopardizing the viability of aquatic animals and plants [6,7].
Several methods such as chemical methods (flocculation [8], advanced oxidation [9], etc.), biological methods (anaerobic, aerobic [10], etc.), membrane separation methods [11], and physical methods (adsorption, etc.) have been developed to treat dye wastewater. Adsorption [12] stands out among the above-mentioned methods because of its many advantages, e.g., low cost, easy operation, less secondary pollution, and good treatment effect. Hydrogels have a 3D cross-network structure, containing hydrophilic and reactive functional groups [13]. Classic superabsorbent hydrogels composed of polymerized acrylic acid and acrylamide are gradually being replaced due to being non-sustainable, non-biodegradable, and costly [14]. Compared to conventional adsorbents, biomass-based adsorption materials such as cellulose [15], lignin [16], corn [17], and sugarcane [18] have the advantages of wide source, low price, easy availability, and biodegradability, providing the excellent prospect for developing environmentally friendly materials [16].
Many bio-based materials, including lignin, cellulose and chitosan, have been used as adsorbents [19,20]. However, cellulose and chitosan are expensive and difficult to modify due to poor solubility in solvents. Lignin is the second most abundant bioresource on our planet [21], which has phenylpropane nits with methoxyl, phenolic and aliphatic hydroxyl, and carboxyl groups [22,23]. Therefore, lignin has been used to prepare different value-added products, including functional hydrogels, due to its rich availability, renewable source, low cost, and unique physiochemical properties. Lignin hydrogels are significantly porous with a larger specific surface area and exhibit high mechanical strength and chemical stability [24,25].
Considering the fast-swelling properties and porous structure of lignin hydrogels, they can absorb dye molecules through chemical, electrostatic interactions and hydrogen bonding [26]. A based biosorbent hydrogel (AML) was prepared by grafting alkali lignin with methylamine and formaldehyde via Mannich reaction, which displayed with fast adsorption of Pb 2+ and adsorption capacity of 60.5 mg/g [27]. Domínguez-Robles et al. Crosslinked lignin with a mixed solution of ammonium hydroxide, methyl vinyl ether, and maleic acid through ester bonds to obtain superabsorbent lignin hydrogels, showing good adsorption efficiency for methylene blue [28]. Noteworthy, lignin does not contain cationic groups and is only used as a crosslinking agent while preparing hydrogels.
In this work, a functional cationic lignin hydrogel (CKLA) has been prepared from the polymerization of cationic lignin acrylate, acrylamide and N, N'-methylenebisacrylamide, and the adsorption properties of the hydrogels for Congo red (Fig. 1) were investigated. The effect of the amount of lignin used for preparing hydrogel on the swelling rate of hydrogels was investigated. The optimized hydrogel was assessed at different time intervals for its adsorption under different pH and congo red initial concentrations. Furthermore, kinetics and isotherm studies were conducted to assess the viability of CKLA (biomasses cationic lignin hydrogel) as an adsorbent for the removal of congo red.

Material
Kraft lignin was provided by Nanjing Shanhu Chemical Co., Ltd. (Nanjing, China). Glycidyl trimethyl ammonium chloride, N, N'-methylenebisacrylamide were obtained from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Acrylylchloride, acrylamide (AM), and Congo red were supplied by Aladdin Reagent (Shanghai) Co., Ltd. (Shanghai, China). Sodium bicarbonate was provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the solvents and chemicals used in the syntheses and analyses were of analytical grade, obtained from commercial suppliers, and used without further purification.

Preparation of Cationic Lignin Acrylate
First, Kraft lignin (15 g), 2,3-epoxypropyl ammonium chloride (7.8 g) and benzyltrim-ethy-lammonium chloride (0.3 g) were dissolved in 30 g of N, N'-dimethylformamide, heated to 140°C and kept for 2 h, followed by cooling to room temperature. Next, NaHCO 3 (7.77 g) was added and mixed evenly. Acryloyl Figure 1: The chemical structure of congo red chloride (8.38 g) was added dropwise to the above solution over 4 h. After the reaction, the mixture was poured into ethyl acetate for precipitation. Finally, the precipitate was vacuum-dried to obtain 19.70 g of cationic lignin acrylate.

Preparation of Cationic Lignin-Based Hydrogel
Different masses of cationic lignin acrylate, acrylamide and N, N'-methylenebis(2-propenamide) (MBA) were dissolved in water with respect to the V-50 catalyst, retaining the total mass of monomers be 10 wt%, under a nitrogen atmosphere for 15 min to remove oxygen. The mixture was reacted at 50°C for 4 h, affording the CK cationic lignin-based hydrogel (CKLA hydrogel).

Characterization
FTIR spectra were obtained using a spectrometer (Nicholas iS50, U.S.) in the scanning range of 4000-500 cm −1 . 31 P NMR spectra were recorded using a Bruker 500 MHz NMR spectrometer. UV-vis spectral investigations were carried out using a UV-vis spectrometer (UV-2450, Shimadzu). Elemental analysis was performed using an elemental analyzer (2400, PerkinElmer).

Adsorption Experiment of Congo red
Congo red aqueous solutions of various concentrations (10, 30, 50, 70, 100, and 150 mg/L) were prepared. The prescribed amount of CKLA hydrogels was added in 25 mL congo red aqueous solutions, and the resulting mixture was shaken for 24 h. Once the adsorption equilibrium was reached, the absorbance of the supernatant was measured by UV-vis spectroscopy, and the adsorption efficiency was calculated as follows: where C 0 and C e are initial and equilibrated dye concentrations (mg/L), respectively.
The amount of adsorption was calculated as follows: where C 0 and C e are initial and equilibrated dye concentrations in the solution (mg/L), respectively; V is the volume (L) of the congo red solution, and M is the weight (g) of the adsorbent.

Adsorption Kinetics
The adsorption kinetics was studied to investigate can reveal the equilibrium state of the adsorption process and inspect the important parameters that provide the suitability of the CKLA hydrogel as an adsorbent. Therefore, during the adsorption kinetics experiment, the following optimal reaction conditions were used: the initial concentration of the CR solution was 50 mg/L, the pH was 7, and the dosage was 5 mg/25 mL for a time of 1000 min. The adsorption experiments were evaluated using the adsorption capacity, and the adsorption kinetic models were analyzed by the pseudo-first-order adsorption kinetic model, the pseudo-second-order adsorption kinetic model, the Webber-Morris intraparticle diffusion model, and the Elovich kinetic model.

Characterization of Kraft Lignin and Cationic Lignin-Based Acrylates
Various hydroxyl groups on lignin were studied by the 31P NMR spectrum. The various hydroxyl groups on lignin and their concentrations are shown in Fig. 2  The cationic lignin was obtained by reaction between the hydroxyl group in Kraft lignin and 2,3epoxypropyl ammonium chloride, resulting in an epoxy ring-opening reaction to introduce a quaternary ammonium salt structure. Subsequently, the CKLA was prepared by the reaction of cationic lignin and acryloyl chloride (Fig. 4). The FTIR spectra of kraft lignin and CKLA are shown in Fig. 3. Compared with lignin, CKLA exhibited a noticeable decrease in the OH absorption peak at 3308 cm −1 . After the reaction of Kraft lignin with 2,3-epoxypropylammonium chloride and acryloyl chloride, an ester group peak at 1724 cm −1 and an unsaturated double bond peak at 1650 cm −1 appeared in the FT-IR spectra of CKLA. Furthermore, through elemental analysis (Table 2), the N content in kl was obtained to be 0, while it was 3.15% in CKLA, proving the successful synthesis of CKLA.

Preparation and Swelling Properties of CKLA Hydrogel
The preparation route of the cationic lignin-based hydrogel is delineated in Fig. 4. Concisely, CKLA hydrogels were prepared by copolymerization of cationic lignin-based acrylate, acrylamide, and N, N'-methylene bisacrylamide in an aqueous solution [29].
The swelling properties of CKLA hydrogels with different lignin content are shown in Fig. 5. The swelling ratio of all CKLA hydrogels increases gradually, then slowly reaches the swelling equilibrium. It is evident that the hydrogels reached the swelling equilibrium (the equilibrium swelling ratios of   Fig. 5). With the increased CKLA content, the quaternary ammonium salt group content also increased, resulting in higher equilibrium swelling ratios of hydrogels. However, Seq-5 is higher than Seq-15 and Seq-10; this is because the content of CKLA is low, leading to a low cross-link density [30].

Effect of the Dosage of CKLA Hydrogels on CR Adsorption
The effect of the dosage of various CKLA hydrogels on the adsorption of CR is shown in Fig. 6. The CR removal efficiency of hydrogels initially increased with the increasing amount of cationic lignin-based hydrogel and then reached the maximum. The adsorption efficiency obtained was the best for 20% lignin content, while the hydrogel dosage was 5 mg. The main reason affecting the adsorption is that with the   increase of the content of CKLA, the quaternary ammonium salt on the CKLA hydrogel also increases, which provides more adsorption sites and thus reduces the required dosage of CKLA hydrogels. A decrease in adsorption efficiency was observed for the CKLA hydrogel with 25% CKLA content due to the increase of quaternary ammonium content, which causes electrostatic repulsion [31].

Effect of CR Initial Concentration on Adsorption Efficiency
The effect of the initial concentration of Congo red on the removal efficiency of the prepared hydrogels is shown in Fig. 7. Upon increasing the concentration of Congo red solution progressively, the removal efficiency of cationic lignin-based hydrogel first showed an increasing trend. It reached the maximum for 50 mg/L Congo red aqueous solution, and further concentration increments resulted in the decline of the removal efficiency. A higher initial dye concentration will provide an enhanced propulsion force to accelerate the mass transfer resistance of dye molecules from the liquid phase to the solid phase (adsorbent), thereby improving the adsorption capacity of the hydrogel [32]. At the beginning of the adsorption process, there is a mass of vacancies on the surface of the adsorbent. The repulsive force between the Congo red molecules adsorbed on the surface of the adsorbent and present in the solution phase gradually weakens the effective adsorption sites of the adsorbent [33]. Thus the CKLA hydrogels

Effect of pH on Adsorption Efficiency
The effect of pH on the adsorption efficiency of cationic lignin-based hydrogels is shown in Fig. 8. It was observed that with the increase of pH value, the removal efficiency for Congo red by CKLA hydrogels first increased and then showed a decreasing trend. The maximum adsorption efficiency was obtained at pH 7. Under acidic conditions, the conversion of sodium sulfonate in Congo red to sulfonic acid groups weakens the electrostatic interaction with quaternary ammonium salt groups, resulting in low adsorption efficiency [34,35]. At pH > 7, the electrostatic repulsion between hydroxide ions and Congo red molecules [36], reduces removal efficiency.

Adsorption Kinetics
During the adsorption process, the diffusion of the adsorbate from the solution to the adsorbent and the rate at which the adsorbate accumulates on the surface of the adsorbent determine the kinetics of the adsorption process and the adsorption efficiency [37]. The adsorption time can indicate whether the whole adsorption process has reached an equilibrium state and evaluate the adsorption capacity of the adsorption process [38]. The effect of adsorption time on the adsorption of CR by CKLA hydrogels is illustrated in Fig. 9. Evidently, CKLA hydrogels reached adsorption equilibrium at 600 min.
To describe the adsorption process more clearly, the adsorption kinetics of CR for CKLA hydrogel was investigated by various kinetic models: pseudo-first-order model: i.e., pseudo-second-order model, Webber-Morris Intral-particle diffusion model, and the Elovich model.  ln q e À q t ð Þ¼ln q e À k 1 t The pseudo-second-order model can be described by Eq. (4): q e is the amount of CR adsorbed at equilibrium (in mg/g), qt represents the CR adsorbed at time t (in mg/g), and k 1 and k 2 are the pseudo-first-order model and pseudo-second-order model constants, respectively.
The Webber-Morris Intral-particle diffusion model can be given by Eq. (5): The Elovich model can be given by Eq. (6): where q e and q t are the same as previous. k i is the rate constant for the Webber-Morris Intral-particle diffusion model, whereas α and β are the Elovich model rate constants. Table 3 summarizes the three kinetic model fitting parameters and mass transfer process models for the adsorption of CR by CKLA. The linear fitting form of these models is displayed in Fig. 10. The pseudosecond-order kinetic model fitted the maximum adsorption capacity of 125.63 mg/g, close to the experimental value. Moreover, the pseudo-second-order kinetic model R2 had the best coefficient. Therefore, the adsorption process of CKLA hydrogel for Congo red was more consistent with the pseudo-second-order kinetic model, showing that the adsorption process is mainly chemical adsorption. Furthermore, the intra-particle diffusion model demonstrated that the adsorption process was the internal mass transfer.

Reusability of CKLA Hydrogel
The reusability of CKLA hydrogel is shown in Fig. 11. The results showed that after five cycles of adsorption, the adsorption efficiency of the hydrogel for Congo red still reached more than 80%. This proved that the hydrogel had excellent reusability.

Possible Mechainism for the Adsorption of Congo Red by CKLA Hydrogel
The adsorption mechanisms tend to be electrostatic interactions between the quaternary ammonium group in the hydrogel and the sulfonate in Congo red [39]. Besides, Congo red is adsorbed through hydrogen bonding, van der Waals forces and hydrophobic interactions [40,41].

Conclusion
In this work, cationic kraft lignin acrylate (CKLA) was prepared by reacting Kraft lignin with 2,3epoxypropyl ammonium chloride and acryloyl chloride. Cationic lignin hydrogels were further synthesized by polymerization of acrylamide, MBA, and CKLA. The effect of dosage of CKLA hydrogels, initial concentration of Congo red, and pH on the adsorption efficiency of the prepared hydrogels was investigated. The optimum Congo red removal efficiency was obtained for the hydrogel with 20% lignin content, 5 mg hydrogel dosage, 50 mg/L concentration of Congo red aqueous solution, and pH = 7. Adsorption kinetics studies revealed that the adsorption of CR by CKLA hydrogels followed pseudo-second-order kinetics.
Funding Statement: This study was supported by Fundamental Research Funds of CAF (CAFYBB2020MB002).

Conflicts of Interest:
The authors declare that they have no conflicts of interest to report regarding the present study.