Synthesis of 2-methylacrylamide/high-molecular-weight Cellulose for Removing Anionic Dyes from Water by Irradiation with Gamma Rays

We report a new method for treating high-molecular-weight cellulose with 60 Co gamma rays to simultaneously graft functional groups onto the natural polymer and promote its solubility. After exposing cellulose to a 40-kilogray dose of gamma rays in the presence of 2-methylacrylamide, numerous amide groups were grafted onto the cellulose chain and its solubility increased markedly. Amide-functionalized aerogels were prepared via the sol-gel method using the irradiated product as a raw material. Compared with 40-kGy-irradiated cellulose aerogel, the amide-functionalized aerogels had relatively high zero-point charge pH values and excellent adsorption capacities with regard to anionic dyes over the pH range 2-10. They were also stable in terms of reusability. Therefore, the 2-methylacrylamide/high-molecular-weight cellulose aerogel has great potential for use in the treatment of colored surface wastewater. The 60 Co gamma ray irradiation technique described herein is a exible, stable and highly ecient method for the preparation of functionalized cellulose products.


Introduction
Bio-based functional products have attracted attention because of concerns about the environmental problems associated with non-degradable, petroleum-based resources ( Aerogels are generally prepared via the sol-gel process and subsequent drying to remove the solvent. However, the intermolecular and intramolecular hydrogen bonds in cellulose chains make this biopolymer relatively insoluble in common solvents (Peng et al. 2020; Wang et al. 2016). An aqueous sodium hydroxide (NaOH)/urea system is commonly used to dissolve cellulose, although it is only effective when the degree of polymerization (DP) is less than 900 (Wang et  cellulose such as cotton cellulose (DP >5000) remains relatively insoluble and has a slow dissolution rate (Hu et al. 2019;Hu et al. 2018). Therefore, a simple and rapid method for improving the solubility of HMW-cellulose is urgently required.
To promote the adsorption capacities of cellulose aerogels, researchers have grafted functional groups onto cellulose via physical or chemical crosslinking during gelation (Vegas et al. 2020; ; Hokkanen et al. 2016). Although physical crosslinking is a simple, green, and effective treatment method, the obtained functional cellulose aerogels have low structural stability. Compared with physical crosslinking, chemical crosslinking via covalent crosslinking or polymerization crosslinking is more Loading [MathJax]/jax/output/CommonHTML/jax.js effective for adjusting the pore structure, improving the mechanical performance, and increasing the number of stable adsorption sites (Sun et al. 2021;Hokkanen et al. 2016). However, chemical crosslinking involves tedious pre-preparation, requires large quantities of chemical reagents, and is time-consuming ).
Recently, cellulosic biomass has been irradiated with 60 Co gamma rays to modify its physicochemical properties and extend its usefulness (Le Moigne  To promote the industrial application of HMW-cellulose, in the present study 2-methylacrylamide (MA) was grafted onto the molecular chain of HMW-cellulose by simultaneously irradiating both materials with 60 Co gamma rays. The in uence of the 60 Co gamma rays on the HMW-cellulose was assessed with particular focus on the crystallinity, solubility in a NaOH/urea system, and the surface degree of substitution (S DS ) of the nal product. The irradiated HMW-cellulose had a lower degree of crystallinity and greater solubility in the NaOH/urea system than the untreated cellulose. Moreover, the optimal S DS of the MA/HMW-cellulose reached 0.123. After preparing an MA/HMW-cellulose aerogel via the sol-gel method, anionic dyes were used as model pollutants to assess the removal capacity of the aerogel in a surface water environment (pH 5-8). The MA/HMW-cellulose aerogel had excellent puri cation properties. Therefore, it is potentially very useful for the removal of industrial dyes from wastewater.

Materials
Commercially grown China Si-Za 3 cotton produced during the 2019/2020 season under standard growing conditions was used in the present work. NaOH, urea, hydrochloric acid (HCl), anhydrous ethanol, toluene, acetic acid, and MA were of analytical reagent grade and were purchased from Nanjing Reagent Corporation Ltd. (Nanjing, China). Acid Green 50 (AG 50, purity > 90%) and Acid Black 1 (AB 1, purity > 90%) were purchased from the Sinopharm Chemical Co. (Shanghai, China). A cellulose acetate lter with pore size of 0.22 µm was purchased from Shanghai Aladdin Reagent Corporation Ltd. (Shanghai, China).
Ultrapure water was puri ed with a U10 water system (Miaozhiyi Electronic Technology, Nanjing, China).

Irradiation of samples
Loading [MathJax]/jax/output/CommonHTML/jax.js Before irradiation, the Si-Za 3 cotton was underwent Soxhlet extraction for 6 h in toluene/ethanol (2:1, v/v). After thorough washing and vacuum-drying, each 50-g cellulose sample was mixed with 1 L of an aqueous solution of MA at room temperature. The concentrations of the MA aqueous solutions were 0, 1, 2, 5 and 10 g·L −1 . The samples were then placed in sealed polyethylene bottles and irradiated with a 60 Co irradiator (BFT4, Xiyue Technology, Nanjing, China) at ambient temperature. The irradiation doses were determined using a B3 radiochromic lm dosimeter (GEX Co., Colorado, USA). The irradiation was applied at doses of 0 (no irradiation), 10, 20, 30, 40 and 50 kilogray (kGy), and at a dose rate of 10.0 ± 0.2 Gy·min −1 . After irradiation, each sample was washed with excess anhydrous alcohol and ultrapure water to remove unreacted MA, thoroughly dried in a vacuum oven, and stored at ambient temperature until required.

Characterization of the samples
The crystallinity of the irradiated samples was assessed by X-ray diffraction (XRD; XRD-6000, Shimadzu, Tokyo, Japan) at ambient temperature. The scan speed was 1°·min −1 at 40 kV and 30 mA with a 2θ range of 5° to 50°. The crystallinity index (CrI) was calculated using the following equation ( where, ∑ A cryl is the integrated area of all crystalline peaks at approximately 14˚, 16˚, 23˚, and 34˚ and ∑ A amph is the integrated area of the amorphous peak at approximately 21˚. The solubility of the irradiated samples in the NaOH/urea system was determined in accordance with the procedure descried by Wang et al. with several modi cations (Wang et al. 2008). Brie y, 10 g of each sample was added to 100 g of an aqueous solution containing 7 wt% NaOH and 12 wt% urea. After stirring at 1000 rpm for 30 min at room temperature, the mixture was placed in a freezer at -12°C for 30 min, then removed from the freezer and stirred at 1000 rpm for 60 min at room temperature. Any undissolved samples were removed by centrifugation at 5000 rpm. After drying at 50°C under vacuum for 48 h, the undissolved samples were weighted and the solubility was calculated using the following equation: where, W 1 is the mass (g) of the undissolved samples. All samples were tested in triplicate.
The surface groups of the irradiated samples were detected by Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). FTIR spectra were acquired using a Nicolet iS50 instrument (Thermo Scienti c, Massachusetts, USA). The FTIR spectrum of each sample was obtained by averaging the data from 32 scans. The XPS data were obtained with a PHI Quantera II XPS instrument (Ulvac-Phi Co., Chigasaki, Japan).
where, M RGU is the molar mass of one repeating glucose unit (162 g·mol −1 ), X N is the mass fraction of nitrogen in the sample from the elemental analysis result, M N is the molar mass of grafted nitrogen atoms according to the irradiation reaction shown in Figure 1 (28 g·mol −1 ) and M graft is the molecular weight of the grafted molecules (85 g·mol −1 ).
The microstructures of HMW-cellulose and MA/HMW-cellulose samples that had been irradiated with a 40-kGy dose of 60 Co gamma rays were investigated by scanning electron microscopy (SEM, EVO-LS10, ZEISS, Oberkochen, German). Each sample was coated with an 8-nm-thick gold lm and the images were obtained at a magni cation of 500× under a high vacuum.

Preparation and characterization of cellulose based aerogels
The undissolved samples were removed following the dissolution process described above, and the obtained solutions were used to prepare hydrogels by the sol-gel method (Luo et al. 2015). The obtained hydrogels were washed with excess ultrapure water to remove residual NaOH and urea until pH 7 was reached. The resulting aerogels were prepared by freeze-drying the hydrogels at -35°C for 20 h at a chamber pressure of 0.111 kPa using a VirTis AdVantage Plus freeze dryer (SP Scienti c, New York, USA).
The microstructures of the resulting aerogels were also investigated by SEM (EVO-LS10). Each aerogel was coated with an 8-nm-thick gold lm, and the images were obtained at a magni cations of 5000× under a high vacuum.
The speci c surface areas of the obtained aerogels were determined using a surface area and pore size analyzer (NOVA 1000e, Quantachrome Instruments, Boynton Beach, Florida, USA). Each aerogel (2g) was placed in the sample cell and degassed for 12 h. The samples were analyzed with NOVA enhanced data reduction software using the Brunauer-Emmett-Teller (BET) theory of surface area determination.
The zeta potentials of the obtained aerogels were determined by dynamic light scattering at room temperature using a Malvern Zetasizer Nano ZS90 instrument (Malvern Instruments Ltd., Worcestershire, UK). Before detection, phosphate-buffered saline (PBS) solution (0.01 M) with pH values of 2, 3, 4, 5, 6, 7, 8, 9 and 10 were prepared. Each aerogel (0.1 g) was then milled to form a ne powder and dispersed in 20 mL of the pre-prepared PBS buffer to determine its zeta potential. (v/v). After the level of eluent absorbance had reached zero, the MA/HMW-cellulose aerogels were washed thoroughly with ultrapure water for the next adsorption cycle.

Crystallinity
Cellulose macromolecules may comprise up to 70% highly ordered crystalline regions, which greatly restricts their dissolution (French and Cintrón, 2013;French, 2014;Zugenmaier, 2021). Thus, the effect of 60 Co gamma ray irradiation on the crystallinity of HMW-cellulose from cotton was determined by XRD (French, 2020). The XRD spectra of the irradiated HMW-cellulose samples and the corresponding deconvolution spectra are shown in Figure 2. The relative parameters of the tting results are shown in Supplementary Table 1.
The XRD spectrum shown in Figure 2A reveals that the HMW-cellulose from cotton has a strong crystalline peak at approximately 23˚, and three weak crystalline peaks at approximately 14˚, 16˚, and 34( Figure 2A). This proves that the HMW-cellulose from cotton comprises a typically cellulose β crystal type (Ling et al. 2019). As the irradiation dose increased, the intensity of the three weak peaks at approximately 14˚, 16˚, and 35˚ decreased. As the irradiation dose increased to 30-kGy, these three peaks disappeared. The deconvolution spectrum of each sample featured two Gaussian tting peaks ( Figure  2B

Solubility
Good solubility is vital to the preparation of functional cellulose aerogels. With regard to the HMWcellulose samples irradiated in the absence of MA, our results show that only 0.14 g of the HMW-cellulose dissolved per 100 g of the 7 wt% NaOH/12 wt% urea aqueous solution (i.e., 0.14% dissolved). As the irradiation dose increased, the percentage of the samples that dissolved increased as follows: 1.21% (10-kGy dose), 2.98% (20-kGy dose), 3.73% (30-kGy dose), 5.16% (40-kGy dose) and 5.68% (50-kGy dose). Figure 1 is a representation of the irradiation-induced grafting reaction between the HMW-cellulose and the MA. According to the scheme, when the reaction takes place in the presence of MA, the obtained Loading [MathJax]/jax/output/CommonHTML/jax.js products comprise numerous amide groups. The hydrophilic amide groups increased the solubility of the HMW-cellulose. We determined the solubility of the MA/HMW-cellulose samples following irradiation with various doses. In the present work, the mass ratios of MA to HMW-cellulose were xed at 1:5. We found that when the MA/HMW-cellulose was irradiated with a 10-kGy dose of gamma rays irradiation, 1.37 g dissolved per 100 g of the 7 wt% NaOH/12 wt% urea aqueous solution. As the irradiation dose increased, the percentage of MA/HMW-cellulose that dissolved increased as follows: 3.13% (20-kGy dose), 3.98% (30-kGy dose), 5.31% (40-kGy dose), and 5.91% (50-kGy dose). Compared with the irradiated HMWcellulose, the MA/HMW cellulose was more soluble in the 7 wt% NaOH/12 wt% urea aqueous solution at the same irradiation dose. The increased solubility also con rmed that the functional monomer MA had been grafted onto the molecular chain of the HMW-cellulose.
Some researchers reported that a NaOH/ urea aqueous solution containing 4% cellulose can be used to fabricate cellulose hydrogel adsorbents (Luo et al. 2015). In the present work, the solubility of HMWcellulose and MA/HMW-cellulose after irradiation at doses above 40-kGy exceeded 4%. Therefore, according to these results, and taking into consideration e ciency, 40-kGy dose is the optimal irradiation dose for preparing HMW-cellulose aerogels.

Surface chemical structure
The FTIR spectra of the HMW-cellulose samples irradiated in the absence of MA had similar adsorption bands ( Figure 3A). Compared with the HMW-cellulose (0-kGy), the irradiated samples generated a broad peak at 1510-1700cm −1 , which may be attributed to the stretching vibration of -C=O (Wan et al. 2020).
These results can be attributed to the 60 Co gamma ray-induced oxidation of HMW-cellulose in air (Barrera-Andrade et al. 2020). Thus, the intensity of the -C=O stretching vibration peak increased as the irradiation dose increased. These results also con rm that irradiation with 60 Co gamma rays at a dose of 50-kGy or below did not induce any obvious chemical changes in the cellulose macromolecules. Therefore, according to the FTIR results ( Figure 3A), and taking into consideration the solubility results, 40-kGy is the optimal dose for grafting the functional monomer MA.  Figure 4A) in addition to C1s and O1s peaks. The appearance of a peak attributable to nitrogen element indicated that MA had been successfully grafted onto the HMW-cellulose forming the amide functionalized HMW-cellulose.
Deconvolution of the high-resolution C1s core-level spectra of the MA/HMW-cellulose was used to further clarify the irradiation grafting reaction between HMW-cellulose and MA. The contributions of carbon element from the various chemical functionalities after deconvolution are listed in Supplementary Table  2

Surface morphology
The surface morphologies of the HMW-cellulose, the 40-kGy-dose-irradiated HMW-cellulose, and AC 1/10 were determined using SEM images. The surface of the HMW-cellulose appeared smooth and round ( Figure 5A). There was no obvious difference between the surface morphology of the 40-kGy-doseirradiated HMW-cellulose and that of the HMW-cellulose, except for a slight deformation ( Figure 5B). The AC 1/10 sample clearly had a rough surface and marked deformation ( Figure 5C). The surface became rough because certain doses of 60 Co gamma rays irradiation partly destroy the molecular chain of HMWcellulose, and because numerous MA molecules had been grafted onto it.
Subsequently, AC 1/10 sample was used as the raw material in the preparation of an amide functionalized HMW-cellulose hydrogel via the sol-gel method. After freeze-drying at -35°C for 20 h, the inner microstructure of the obtained AC 1/10 aerogel was investigated by SEM and BET surface area determination. The AC 1/10 aerogel has a homogeneous three-dimensional porous structure ( Figure 5D) that provided a large surface area (70.304 m 2 ·g −1 for BET analysis) and plentiful sites for the adsorption of pollutants. The nitrogen adsorption-desorption isotherms of the AC 1/10 aerogel are shown in Figure  5E. The negligible adsorption uptake in the P/P 0 range between 0 and 0.6 indicates very small micropores (< 2 nm) in the AC 1/10 aerogel. During this process, nitrogen molecules were gradually adsorbed on the internal surfaces of porous from single to multilayer structures. The adsorption uptake then increased Loading [MathJax]/jax/output/CommonHTML/jax.js quickly in the range of 0.6-1 and a hysteresis loop formed, indicating the presence of mesopores (2-50 nm) (Wan et al., 2019). The speculations were supported by the observed pore size distribution. Figure 5F shows that the pore sizes of the AC 1/10 aerogel were within the 5-40 nm range.

Effect of pH
The pH of a solution can signi cantly affect the adsorption capacity of a material (Garba et al. 2020; Abou EI-Reash et al. 2016). The adsorption of negatively charged species is favored when the pH of a solution is less than the pH at zero-point charge (pH zpc ) of the adsorbing material (Li et al. 2018). The pH zpc of the AC 1/10 aerogel reached 6.02-that is, markedly higher than the pH zpc of 40-kGy-dose irradiated HMW-cellulose aerogel, which was 1.74 ( Figure 6A). The higher pH zpc of the AC 1/10 aerogel indicates that it could obviously remove anionic pigment pollutants in aqueous solutions with pH values less than 6.02. The results further prove that the functional monomer MA were successfully grafted onto the 60 Co gamma rays irradiated HMW-cellulose, and then formed an amide functionalized HMW-cellulose aerogel with a higher pH zpc .
The ability to remove anionic dyes was used to further assess the effect of solution pH on the adsorption properties of the AC 1/10 aerogel. An AG 50 aqueous solution (100 mg ·L −1 ) and an AB 1 aqueous solution (100 mg·L −1 ) were used as working solutions over the pH range of 2-10. The results showed that the AC 1/10 aerogel had marked anionic dye uptake capacity in solutions with pH values lower than 6 ( Figure 6B and 6C). The maximum removal capacities of the AC 1/10 aerogel at pH 2 were 650.6 mg·g −1 for AG 50 and 803.1 mg·g −1 for AB 1. Over the pH range 6-10, the AC 1/10 aerogel also exhibited higher uptakes of AG 50 and AB 1 than 40-kGy-dose-irradiated HMW-cellulose aerogel.
These results can be attributed to the change of in the mechanism by which the aerogels adsorbed the pollutants (Figure 7). Compared with 40-kGy-dose-irradiated HMW-cellulose aerogel, the AC 1/10 aerogel had numerous amide groups (Figure 1, 3 and 4). At solution pH values lower than 6, the amide groups on the AC 1/10 aerogel were protonated and had strong electrostatic interaction for the negatively charged SO 3 2− groups on the AG 50 and AB 1. Thus, the AC 1/10 aerogel had a marked capacity for the removal of AG 50 and AB 1. As the solution pH values increased from 6 to 10, the amide groups on the AC 1/10 aerogel were deprotonated, which in turn reduced the electrostatic interaction such that the adsorption capacities of the AC 1/10 aerogel for anionic dyes decreased. However, the numerous amide groups on the AC 1/10 aerogel provided hydrogen bonding interactions with the amino and dimethyl-amino groups on the AG 50 and AB 1 (Aljohani et al. 2017;Liu et al. 2020;Harings et al. 2009). Therefore, the AC 1/10 aerogel also exhibited higher uptakes of AG 50 and AB 1 than the 40-kGy-dose-irradiated HMW-cellulose aerogel, even over the pH range 6-10.
The anionic dyes removal capacities of 40-kGy-dose-irradiated HMW-cellulose aerogel increased as the solution pH increased. These results can be attributed to an increase in negatively charge as the solution Loading [MathJax]/jax/output/CommonHTML/jax.js pH value increased from 2 to 10 ( Figure 6A), which increased the electrostatic interaction between the aerogel and the amino and dimethyl-amino groups on the AG 50 and AB 1.
These results clearly prove that MA/HMW-cellulose aerogel has signi cant potential for the removal of anionic dye pollutants from surface water (pH 5-8).

Adsorption isotherms
Adsorption isotherms are often used to determine the interactions between adsorbing materials and pollutants. In the present work, Langmuir and Freundlich models were used to calculate isotherm parameters for the adsorption of the anionic dyes AG 50 and AB 1 by the AC 1/10 and HMW-cellulose aerogels. To simulate the removal of anionic dyes from a surface water environment, the working pH values were xed at pH 5 and 8 ( Figure 8 and Table 1). Comparison of the results obtained from the two isotherm models revealed that the Langmuir model produced higher R 2 values and tted the data better. This can be explained by the homogeneous threedimensional porous structure of the AC 1/10 aerogel, which had pores that were far larger than the AG 50 and AB 1 molecules. Therefore, during the adsorption process, these small molecules readily migrated into the large pores.

Reusability
Reusability is an important property for adsorbents. The reusability and stability of the AC 1/10 aerogels were evaluated by subjecting it to multiple adsorption-desorption cycles (Figure 9). After ten adsorptiondesorption cycles, the AG 50 removal capacity decreased slightly from 587.3 to 504.

Conclusion
High-molecular-weight cellulose is quite insoluble owing to its relatively long molecular chains and the close packing of its chains, which results from numerous hydrogen bonds. To promote the industrial application of plant bers, we devised a new method for treating HMW-cellulose with 60 Co gamma rays to simultaneously graft functional amide groups onto the natural polymer and promote its solubility. After irradiation with a 40-kGy dose of gamma rays, numerous MA molecules were grafted onto the chains of the HMW-cellulose, and the degree of substitution on its surface reached 0.172. The solubility of the irradiated product increased to 5.31-i.e., adequate for the preparation of cellulose hydrogels via the solgel method. The fabricated MA/HMW-cellulose aerogel had a relatively high pH value of zero-point charge and an excellent adsorption capacity for anionic dyes over a pH range 2-10. The aerogel was also stable in terms of reusability. Therefore, the MA/HMW-cellulose aerogel has great potential for the treatment of colored wastewater in surface water environment. The 60 Co gamma ray irradiation technique described herein is a exible method for the preparation of functionalized HMW-cellulose.

Declarations Declaration of interests
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper. Figure 1 Scheme representing ideal irradiation grafting reaction between high-molecular-weight (HMW) cellulose   Fourier-transform infrared spectroscopy (FTIR) spectra of A) irradiated high-molecular-weight (HMW)cellulose; B) irradiated 2-methylacrylamide (MA)/HMW-cellulose. The mass ratios of MA to HMWcellulose were 1:5, 1:10, 1:25 and 1:50, and the MA/HMW-cellulose samples are herein referred to as AC 1/5, AC 1/10, AC 1/25, and AC 1/50, respectively.

Figure 4
A) X-ray photoelectron spectroscopy (XPS) overview spectra of the irradiated samples; B) high-resolution C1s XPS spectra of AC 1/5; C) high-resolution C1s XPS spectra of 40-kGy-irradiated HMW-cellulose; D) the SDS of the irradiated samples.

Figure 7
Mechanism by which the aerogels adsorb pollutants.