We report a new method for treating high-molecular-weight cellulose with 60Co 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 60Co gamma ray irradiation technique described herein is a flexible, stable and highly efficient method for the preparation of functionalized cellulose products.
Research Article
Synthesis of 2-methylacrylamide/high-molecular-weight Cellulose for Removing Anionic Dyes from Water by Irradiation with Gamma Rays
https://doi.org/10.21203/rs.3.rs-1033113/v1
This work is licensed under a CC BY 4.0 License
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Aerogel
Colored wastewater
High-molecular-weight cellulose
Gamma ray irradiation
Solubility
Bio-based functional products have attracted attention because of concerns about the environmental problems associated with non-degradable, petroleum-based resources (Imre et al. 2019; Mortada et al. 2015; Mortada and Abdelghany, 2020; Philp et al. 2013). As an inexhaustible polymeric raw material, cellulose has great potential to meet the increasing demand for green high-value products (Dai et al. 2019; Pan et al. 2019; Sanjay et al. 2018). Cellulose aerogels are of particular interest because they have large specific surface areas and adsorption capacities, good biocompatibility and modifiability, and high stability in most organic solvents (Sun et al. 2021; Budtova, 2019).
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 al. 2016; Cai and Zhang, 2005; Budtova and Navard, 2016). Ionic liquids are also frequently used as solvents for the dissolution of cellulose (Khoo et al. 2021; Zhu et al. 2006; Zhang et al. 2016). Even mixtures of metal chlorides and ionic liquids can only dissolve cellulose with a DP of approximately 800 (Li et al. 2019). Thus, high-molecular-weight (HMW) 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; Wan et al. 2019; 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 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 (Zhou et al. 2021).
Recently, cellulosic biomass has been irradiated with 60Co gamma rays to modify its physicochemical properties and extend its usefulness (Le Moigne et al. 2017; Misra et al. 2020). The physicochemical properties of the irradiated materials differ greatly from those of the virgin cellulosic biomass (Sluijs and Church, 2013; Le Moigne et al. 2017). Various functional groups have been grafted onto the molecular chains of cellulose via irradiation to promote its mechanical, optical, and antibacterial properties (Tataru et al. 2020; Söylemez et al. 2018; Barsbay and Güven, 2019). However, limited information is available regarding the irradiation of cellulosic biomass for the preparation of functional cellulose aerogels. For example, little is known about the solubility of irradiated HMW-cellulose in solvents, or the chemical structure of the surface of irradiated cellulose. For this reason, the effect of irradiation on HMW-cellulose is an important topic for the application of plant fibers.
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 60Co gamma rays. The influence of the 60Co 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 (SDS) of the final 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 SDS 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 purification properties. Therefore, it is potentially very useful for the removal of industrial dyes from wastewater.
2.1. 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 filter with pore size of 0.22 µm was purchased from Shanghai Aladdin Reagent Corporation Ltd. (Shanghai, China). Ultrapure water was purified with a U10 water system (Miaozhiyi Electronic Technology, Nanjing, China).
2.2. Irradiation of samples
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 60Co irradiator (BFT4, Xiyue Technology, Nanjing, China) at ambient temperature. The irradiation doses were determined using a B3 radiochromic film 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.
2.3. 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 (Hu et al. 2018; Yao et al. 2020; Ruan et al. 2016).
\(\mathbf{C}\mathbf{r}\mathbf{I} {\%}=\frac{\sum {{A}}_{{c}{r}{y}{l}}}{\sum {{A}}_{{c}{r}{y}{l}}+\sum {{A}}_{{a}{m}{p}{h}}}\times 100\) (1),
where, \(\sum {{A}}_{{c}{r}{y}{l}}\) is the integrated area of all crystalline peaks at approximately 14˚, 16˚, 23˚, and 34˚ and \(\sum {{A}}_{{a}{m}{p}{h}}\) 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 modifications (Wang et al. 2008). Briefly, 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:
\(\mathbf{S}\mathbf{o}\mathbf{l}\mathbf{u}\mathbf{b}\mathbf{i}\mathbf{l}\mathbf{i}\mathbf{t}\mathbf{y}=10-{{W}}_{1}\) (2),
where, W1 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 Scientific, 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).
The SDS of the obtained MA/HMW-cellulose was calculated by elemental analysis using the following equation (De Oliveira et al. 2020; Le Gars et al. 2020):
\({{S}}_{{D}{S}}=\frac{{{M}}_{{R}{G}{U}}\times {{X}}_{{N}}}{{{M}}_{{N}}-{{M}}_{{g}{r}{a}{f}{t}}\times {{X}}_{{N}}}\) (3),
where, MRGU is the molar mass of one repeating glucose unit (162 g·mol−1), XN is the mass fraction of nitrogen in the sample from the elemental analysis result, MN is the molar mass of grafted nitrogen atoms according to the irradiation reaction shown in Figure 1 (28 g·mol−1) and Mgraft 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 60Co gamma rays were investigated by scanning electron microscopy (SEM, EVO-LS10, ZEISS, Oberkochen, German). Each sample was coated with an 8-nm-thick gold film and the images were obtained at a magnification of 500× under a high vacuum.
2.4. 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 Scientific, 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 film, and the images were obtained at a magnifications of 5000× under a high vacuum.
The specific 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 fine powder and dispersed in 20 mL of the pre-prepared PBS buffer to determine its zeta potential.
The adsorption experiments were performed at 20°C. Model pollutant solutions with various concentrations and different pH values were prepared by dissolving certain amount of AG 50 or AB 1 in PBS buffer solution (0.01 M, pH range 2-10). All adsorption removal experiments were carried out by adding 0.005 g of the aerogel to 100 mL of the model pollutant solution (100 mg·L−1), then shaking at 100 rpm for 24 h. The aerogel was then removed by centrifugation at 1000 rpm for 5 min. The concentrations of the residual model pollutant in the supernatants were determined using a Nano Drop 2000 instrument (Thermo Scientific, Massachusetts, USA) (Liu et al. 2020; Liu et al. 2015). The removal capacity for each organic dye was calculated using the following equation:
where, qt (mg·g−1) is the removal capacity of the aerogel after time t, c0 (mg·L−1) is the initial concentration of the model pollutant, ct (mg·L−1) is the concentration at time t, V (L) is the solution volume, and W (g) is the aerogel mass.
All the adsorption isotherm experiments were carried out by adding 0.005 g of the aerogel to 10 mL of the model pollutant solution, then shaking at 100 rpm for 24 h. The concentrations of AG 50 or AB 1 in the PBS buffer solution (0.01M) with a pH of 5 were 50, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650 and 700 mg·L−1. The concentrations of AG 50 or AB 1 in the PBS buffer solution (0.01M) with a pH of 8 were 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 220 and 240 mg·L−1. After saturation adsorption, the aerogel was removed by centrifugation at 1000 rpm for 5 min.
The Langmuir and Freundlich models were used to calculate the isotherm parameters for the adsorption of the organic dyes. The equation for the Langmuir model is:
where, Ce (mg·L−1) is the equilibrium concentration of the model pollutant, qe (mg·g−1) is the adsorption capacity of the aerogel at equilibrium, qmax (mg·g−1) is the maximum adsorption capacity of the aerogel, and KL (L·mg−1) is the Langmuir constant. The KL is related to the affinity of the model pollutant for the binding site.
The equation for the Freundlich model is:
\({L}\mathbf{n}{{q}}_{{e}}={L}{n}{{K}}_{{F}}+{L}{n}{{C}}_{{e}}/{n}\) (6),
where, KF (mg·g−1) and n are Freundlich constants related to the adsorption capacity and adsorption intensity of the aerogel, respectively.
The reusability of the aerogel was assessed with ten consecutive adsorption cycles. For each cycle, 100 mL of the model pollutant solutions (100 mg·L−1) were used. After adsorption for 24 h, the MA/HMW-cellulose aerogels were desorbed in 0.1 M sodium hydroxide aqueous solution containing 10% ethanol (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.
3.1. 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 60Co 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 fitting 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 celluloseⅠβ 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 fitting peaks (Figure 2B-G). The first peak, at approximately 23˚, corresponded to the crystalline domain, whereas a broad peak appeared at approximately 21˚ that was attributed to the amorphous domain (Yao et al. 2020; French, 2020; Meng et al. 2019; Štefelová et al. 2017; Hao et al. 2012). The CrI % of the HMW-cellulose from cotton was 83.3%, and this decreased to 74.6% (a decrease of 10.4%) after irradiation with a 10-kGy dose of gamma rays. When the 60Co gamma ray irradiation dose was increased, the CrI decreased further to 64.0% (a 20-kGy dose corresponded to a decrease of 23.0%), 58.8% (a 30-kGy dose corresponded to a decrease of 29.4%), 53.1% (a 40-kGy dose corresponded to a decrease of 36.3%), and 42.5% (a 50-kGy dose corresponded to a decrease of 49.0%). This tendency was attributed to the destruction of the crystalline regions as the 60Co gamma ray irradiation dose was increased (Fei et al. 2020; Ling et al. 2019). Thus, 60Co gamma ray irradiation treatment effectively decreases the crystallinity of HMW-cellulose and promotes its dissolution.
3.2. Solubility
Good solubility is vital to the preparation of functional cellulose aerogels. With regard to the HMW-cellulose 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 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 fixed 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 HMW-cellulose, 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 confirmed 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 HMW-cellulose and MA/HMW-cellulose after irradiation at doses above 40-kGy exceeded 4%. Therefore, according to these results, and taking into consideration efficiency, 40-kGy dose is the optimal irradiation dose for preparing HMW-cellulose aerogels.
3.3. 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 60Co 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 confirm that irradiation with 60Co 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 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 products comprise numerous amide groups. Compared with the HMW-cellulose irradiated with a 40-kGy dose, the obtained MA/HMW-cellulose samples generated two new weak adsorption peaks at 1510-1700 cm−1 (Figure 3B). These two peaks increased in intensity as the mass of MA increased. The peak at 1570 cm−1 may be attributed to –NH flexural vibration. The peak at 1660 cm−1 may be attributed to –C=O stretching vibration. Both peaks were also present in the MA spectrum (De Oliveira et al. 2020; Barrera-Andrade et al. 2020; Liu et al. 2020). Compared with the MA spectrum, these two weak peaks shifted slightly owing to irradiation-induced oxidation. These results indicate that MA had been successfully grafted onto the cellulose to form the amide functionalized HMW-cellulose.
The irradiation-induced grafting reaction between HMW-cellulose and MA was further elucidated by XPS. The XPS overview spectra of the MA/HMW-cellulose features an N1s peak at a binding energy of 400 eV (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. The C1s core-level spectrum of the 40-kGy-irradiated HMW-cellulose (Figure 4C) features two peaks at 284.8 and 286.4 eV, which are attributable to C-C/C-H and C-O groups (Liu et al. 2020; Tatary et al. 2020). The C1s core-level spectrum of the MA/HMW-cellulose (AC 1/5, Figure 4B) features three peaks at 285.0, 286.3 and 287.1 eV, which are attributable to C-C/C-H, C-O and carbonyl (C=O) groups, respectively (Tataru et al. 2020; Peng et al. 2021). According to the irradiation-induced grafting reaction (Figure 1), there will be carbonyl and amide groups on the obtained MA/HMW-cellulose. Thus, the XPS results for the MA/HMW-cellulose further proved that MA had been successfully grafted onto the HMW-cellulose.
The SDS of the MA/HMW-cellulose was determined by elemental analysis (Figure 4D, Supplementary Table 3). The SDS of the MA/HMW-cellulose was calculated using Eq. (3). The SDS increased markedly as the mass ratio of MA to HMW-cellulose increased from 1:50 to 1:10. After the mass ratio of MA to HMW-cellulose exceeded 1:10, the value of SDS began to slowly increase. The optimal mass ratio of MA to HMW-cellulose—that is, 1:10—produced a relatively high SDS value and required relatively little functional monomer.
3.4. 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-dose-irradiated 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 60Co gamma rays irradiation partly destroy the molecular chain of HMW-cellulose, 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 m2·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/P0 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 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.
3.5. Effect of pH
The pH of a solution can significantly 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 (pHzpc) of the adsorbing material (Li et al. 2018). The pHzpc of the AC 1/10 aerogel reached 6.02—that is, markedly higher than the pHzpc of 40-kGy-dose irradiated HMW-cellulose aerogel, which was 1.74 (Figure 6A). The higher pHzpc 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 60Co gamma rays irradiated HMW-cellulose, and then formed an amide functionalized HMW-cellulose aerogel with a higher pHzpc.
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 SO32− 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 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 significant potential for the removal of anionic dye pollutants from surface water (pH 5-8).
3.6. 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 fixed at pH 5 and 8 (Figure 8 and Table 1).
|
Langmuir isotherm model |
Freundlich isotherm model |
||||||
|
KL (L·mg−1) |
qmax (mg·g−1) |
R2 |
KF (mg·g−1) |
n |
R2 |
||
|
pH5 |
AG 50 |
1.27×10−2 |
714.29 |
0.98 |
36.97 |
2.04 |
0.87 |
|
AB 1 |
3.82×10−2 |
769.23 |
0.99 |
164.02 |
3.70 |
0.87 |
|
|
pH 8 |
AG 50 |
8.65×10−3 |
294.12 |
0.99 |
4.93 |
1.38 |
0.97 |
|
AB 1 |
6.77×10−3 |
340.14 |
0.98 |
5.59 |
1.47 |
0.95 |
|
In the Langmuir isotherm model, qmax is the monolayer adsorption capacity of the adsorbent. At pH 5, the maximum adsorption capacities of the AC 1/10 aerogels were 714.29 mg·g−1 for AG 50 and 769.23 mg·g−1 for AB 1. At pH 8, the maximum adsorption capacities of the AC 1/10 aerogels also reached 294.12 mg·g−1 (AG 50) and 340.14 mg·g−1 (AB 1). All the KL values were in the range of 0< KL< 1, which indicated that the AC 1/10 aerogel performed well with regard to the adsorption of anionic dyes (Chen et al. 2018; Song et al. 2017). These results prove that the AC 1/10 aerogel exhibited higher uptakes of the dyes in a surface water environment.
In the Freundlich isotherm model, KF is a measure of the capacity for multiple adsorption of a model pollutant on a heterogeneous surface. At pH 5 the KF values of AC 1/10 aerogels were 36.97 mg·g−1 for AG 50 and 164.02 mg·g−1 for AB 1, and at pH 8 the KF values were 1.38 mg·g−1 for AG 50 and 1.47 mg·g−1 for AB 1. In addition, the values of n ranged from 1 to 10 at either pH 5 or at pH 8, indicating a suitable adsorption process (Yang et al. 2019; Wan et al. 2019).
Comparison of the results obtained from the two isotherm models revealed that the Langmuir model produced higher R2 values and fitted the data better. This can be explained by the homogeneous three-dimensional 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.
3.7. 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 adsorption-desorption cycles, the AG 50 removal capacity decreased slightly from 587.3 to 504.2 mg·g−1 (a decrease of 14.15%) at pH 5, and from 237.2 to 209.8 mg·g−1 (a decrease of 11.55%) at pH 8. With regard to AB 1, the removal capacity decreased from 675.5 to 589.4 mg·g−1 (a decrease of 12.75%) at pH 5, and from 296.3 to 264.8 mg·g−1 (a decrease of 10.63%) at pH 8. These results confirm that the AC 1/10 aerogel is stable and can be readily reused in a surface water environment.
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 fibers, we devised a new method for treating HMW-cellulose with 60Co 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 sol-gel 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 60Co gamma ray irradiation technique described herein is a flexible method for the preparation of functionalized HMW-cellulose.
Declaration of interests
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.
Acknowledgements
We thank Ms. Min Wang for determination of FTIR and XRD, and Mr. Cunfa Xu for detection of SEM image and BET.
Funding
This work was financially supported by Jiangsu Agricultural Science and Technology Innovation Fund (Grant No. CX(19)3082), the Six Talent Peaks Project of Jiangsu Province, China (Grant No. NY-034), the Jiangsu province Natural Sciences Foundation (BK20190003), National Natural Science Foundation of China (31872481 and 31802167), and the Open Foundation of State Key Laboratory of Analytical Chemistry for Life Science of Nanjing University (No. SKLACLS1902).
- Abou EI-Reash YG, Abdelghany AM, Abd Elrazak A (2016) Removal and separation of Cu (II) from aqueous solutions using nano-silver chitosan/polyacrylamide membranes. Int J Biol Macromol 86: 789-798. https://doi.org/10.1016/j.ijbiomac.2016.01.101
- Aljohani M, Pallipurath AR, McArdle P, Erxleben A (2017) A comprehensive cocrystal screening study of chlorothiazide. Cryst Growth Des 17(10): 5223-5232. https://doi.org/10.1021/acs.cgd.7b00745.
- Barrera-Andrade JM, Rojas-Garía E, García-Valdés J, Valenzuela MA, Albiter E (2020) Incorporation of amide functional groups to graphene oxide during the photocatalytic degradation of free cyanide. Mater. Lett. 280: 128538. https://doi.org/10.1016/j.matlet.2020.128538.
- Barsbay M, Güven O (2019) Surface modification of cellulose via conventional and controlled radiation induced grafting. Radiat Phys Chem 160: 1-8. https://doi.org/10.1016/j.radphyschem.2019.03.002.
- Budtova T (2019) Cellulose Ⅱ aerogels: A review. Cellulose 26(1): 81-121. https://doi.org/10.1007/s10570-018-2189-1.
- Budtova T, Navard P (2016) Cellulose in NaOH-water based solvents: a review. Cellulose 23(1): 5-55. https://doi.org/10.1007/s10570-015-0779-8.
- Cai J, Zhang L (2005) Rapid dissolution of cellulose in LiOH/Urea and NaOH/Urea aqueous solutions. Macromol Biosci 5(6): 539-548. https://doi.org/10.1002/mabi.200400222.
- Chen Y, Lin YC, Ho SH, Zhou Y, Ren N (2018) Highly efficient adsorption of dyes by biochar derived from pigments-extracted macroalgae pyrolyzed at different temperature. Bioresource Technol 259: 104-110. https://doi.org/10.1016/j.biortech.2018.02.094.
- Dai L, Cheng T, Duan C, Zhao W, Zhang W, Zou X, Aspler J, Ni Y (2019) 3D printing using plant-derived cellulose and its derivatives: A review. Carbohyd Polym 203: 71-86. https://doi.org/10.1016/j.carbpol.2018.09.027.
- De Oliveira JC, Rigolet S, Marichal C, Roucoules V, Laborie MP (2020). Grafting Diels-Alder moieties on cellulose nanocrystals through carbamation. Carbohyd Polym 250: 116966. https://doi.org/10.1016/j.carbpol.2020.116966.
- Fei XH, Jia WB, Wang JQ, Chen T, Liang YS (2020) Study on enzymatic hydrolysis efficiency and physicochemical properties of cellulose and lignocellulose after pretreatment with electron beam irradiation. Int J Biol Macromol 145: 733-739. https://doi.org/10.1016/j.ijbiomac.2019.12.232.
- French AD (2014) Idealized powder diffraction patterns for cellulose polymorphs. Cellulose 21: 885-896. https://doi.org/10.1007/s10570-013-0030-4.
- French AD (2020) Increment in evolution of cellulose crystallinity analysis. Cellulose 27: 5445-5448. https://doi.org/10.1007/s10570-020-03172-z.
- French AD, Cintrón MS (2013) Cellulose polymorphy, crystallite size, and the Segal Crystallinity Index. Cellulose 20: 583-588. https://doi.org/10.1007/s10570-012-9833-y.
- Garba ZN, Lawan I, Zhou W, Zhang M, Wang LW, Yuan ZH (2020) Microcrystalline cellulose (MCC) based materials as emerging adsorbents for the removal of dyes and heavy metals - A review. Sci Total Environ 717: 135070. https://doi.org/10.1016/j.scitotenv.2019.135070.
- Hao Y, Peng J, Ao Y, Li J, Zhai M (2012) Radiation effects on microcrystalline cellulose in 1-butyl-3-methylimidazolium chloride ionic liquid. Carbohyd Polym 90: 1629-1633. https://doi.org/10.1016/j.carbpol.2012.07.042.
- Harings JAW, Yao YF, Graf R, Van Asseten O, Broos R, Rastogi S (2009) Erasing conformational limitations in N,N’-1,4-Butanediyl-bis(6-hydroxy-hexanamide) crystallization from the superheated state of water. Langmuir 25(13): 7652-7666. https://doi.org/10.1021/la900318n.
- Hokkanen S, Bhatnagar A, Sillanpää M (2016) A review on modification methods to cellulose-based adsorbents to improve adsorption capacity. Water Res 91: 156-173. https://doi.org/10.1016/j.watres.2016.01.008.
- Hu Y, Acharya S, Abidi N (2019) Cellulose porosity improves its dissolution by facilitating solvent diffusion. Int J Biol Macromol 123: 1289-1296. https://doi.org/10.1016/j.ijbiomac.2018.10.062.
- Hu Y, Thalangamaarachchige VD, Acharya S, Abidi N (2018) Role of low-concentration acetic acid in promoting cellulose dissolution. Cellulose 25(8): 4389-4405. https://doi.org/10.1007/s10570-018-1863-7.
- Imre B, Garcia L, Puglia D, Vilaplana F (2019) Reactive compatibilization of plant polysaccharides and biobased polymers: review on current strategies, expectations and reality. Carbohyd Polym 209: 20-37. https://doi.org/10.1016/j.carbpol.2018.12.082.
- Khoo KS, Tan XF, Ooi CW, Chew KW, Leong WH, Chai YH, Ho SH, Show PL (2021) How does ionic liquid play a role in sustainability of biomass processing. J Clean Prod 284: 124772. https://doi.org/10.1016/j.jclepro.2020.124772.
- Le Moigne N, Sonnier R, Ei Hage R, Rouif S (2017) Radiation-induced modifications in natural fibres and their biocomposites: Opportunities for controlled physico-chemical modification pathways. Ind Crop Prod 109: 199-213. https://doi.org/10.1016/j.indcrop.2017.08.027.
- Li X, Li HC, You TT, Chen XM, Ramaswamy S, Wu YY, Xu F (2019) Enhanced dissolution of cotton cellulose in 1-Allyl-3-methylimidazolium chloride by the addition of metal chlorides. ACS Sustain Chem Eng 7(23): 19176-19184. https://doi.org/10.1021/acssuschemeng.9b05159.
- Li Y, Xiao H, Pan Y, Wang L (2018) Novel composite adsorbent consisting of dissolved cellulose fiber/microfibrillated cellulose for dye removal from aqueous solution. ACS Sustain Chem Eng 6(5): 6994-7002. https://doi.org/10.1021/acssuschemeng.8b00829.
- Ling Z, Wang T, Makarem M, Cintrón MS, Cheng HN, Kang X, Bacher M, Potthast A, Rosenau T, King H, Delhom CD, Nam S, Edwards JV, Kim SH, Xu F, French AD (2019) Effects of ball milling on the structure of cotton cellulose. Cellulose 26: 305-328. https://doi.org/10.1007/s10570-018-02230-x.
- Liu J, Chen TW, Yang YL, Bai ZC, Xia LR, Wang M, Lv XL, Li L (2020) Removal of heavy metal ions and anionic dyes from aqueous solutions using amide-functionalized cellulose-based adsorbents. Carbohyd Polym 230: 115619. https://doi.org/10.1016/j.carbpol.2019.115619.
- Liu L, Gao ZY, Su XP, Chen X, Jiang L, Yao JM (2015) Adsorption removal of dyes from single and binary solutions using a cellulose-based bioadsorbent. ACS Sustain Chem Eng 3(3): 432-442. https://doi.org/10.1021/sc500848m.
- Luo XG, Zeng J, Liu SL, Zhang LN (2015) An effective and recyclable adsorbent for the removal of heavy metal ions from aqueous system: magnetic chitosan/cellulose microspheres. Bioresource Technol 194: 403-406. https://doi.org/10.1016/j.biortech.2015.07.044.
- Meng F, Zhang X, Yu W, Zhang Y (2019) Kinetic analysis of cellulose extraction from banana pseudo-stem by liquefaction in polyhydric alcohols. Ind Crop Prod 137: 377-385. https://doi.org/10.1016/j.indcrop.2019.05.025.
- Misra N, Rawat S, Goel NK, Shelkar SA, Kumar V (2020) Radiation grafted cellulose fabric as reusable anionic adsorbent: A novel strategy for potential large-scale dye wastewater remediation. Carbohyd Polym 249: 116902. https://doi.org/10.1016/j.carbpol.2020.116902.
- Mortada WI, Abdelghany AM (2020) Preconcentration of lead in blood and urine samples among bladder cancer patients using mesoporous strontium titanate nanoparticles. Biol Trace Elem Res 193(1): 100-110. https://doi.org/10.1007/s12011-019-01704-8.
- Mortada WI, Kenawy IMM, Abdelghany AM, Ismal AM, Donia AF, Nabieh KA (2015) Determination of Cu2+, Zn2+ and Pb2+ in biological and food samples by FAAS after preconcentration with hydroxyapatite nanorods originated from eggshell. Mat Sci Eng C 52: 288-296. https://doi.org/10.1016/j.msec.2015.03.061.
- Pan Y, Xie H, Liu H, Cai P, Xiao H (2019) Novel cellulose/montmorillonite mesoporous composite beads for dye removal in single and binary systems. Bioresource Technol 286:121366. https://doi.org/10.1016/j.biortech.2019.121366.
- Peng B, Yao Z, Wang X, Crombeen M, Sweeney DG, Tam KC (2020) Cellulose-based materials in wastewater treatment of petroleum industry. Green Energy Environ 5(1): 37-49. https://doi.org/10.1016/j.gee.2019.09.003.
- Peng W, Yan Y, Zhang D, Zhou Y, Na D, Xiao C, Yang C, Wen G, Zhang J (2021) Preparation of thermal stable supported metal (Cu, Au, Pd) nanoparticles via cross-linking cellulose gel confinement strategy. Colloid Surface A 624: 126809. https://doi.org/10.1016/j.colsurfa.2021.126809.
- Philp JC, Ritchie RJ, Allan JEM (2013) Biobased chemicals: the convergence of green chemistry with industrial biotechnology. Trends Biotechnol 31(4): 219-222. https://doi.org/10.1016/j.tibtech.2012.12.007.
- Ruan C, Zhu Y, Zhou X, Abidi N, Hu Y, Catchmark JM (2016) Effect of cellulose crystallinity on bacterial cellulose assembly. Cellulose 23(6): 3417-3427. https://doi.org/10.1007/s10570-016-1065-0.
- Sanjay MR, Madhu P, Jawaid M, Senthamaraikannan P, Senthil S, Pradeep S (2018) Characterization and properties of natural fiber polymer composites: A comprehensive review. J Clean Prod 172: 566-581. https://doi.org/10.1016/j.jclepro.2017.10.101.
- Sluijs MH, Church JS (2013) The effect of quarantine-level gamma irradiation on cotton fiber and its subsequent textile processing performance. Text Res J 83(2): 197-207. https://doi.org/10.1177/0040517512458341.
- Song K, Xu H, Xu L, Xie K, Yang Y (2017) Cellulose nanocrystal-reinforced keratin bioadsorbent for effective removal of dyes from aqueous solution. Bioresource Technol 232: 254-262. https://doi.org/10.1016/j.biortech.2017.01.070.
- Söylemez MA, Barsbay M, Güven O (2018) Preparation of well-defined erythromycin imprinted non-woven fabrics via radiation-induced RAFT-mediated grafting. Radiat Phys Chem 142: 77-81. https://doi.org/10.1016/j.radphyschem.2017.01.041.
- Štefelová J, Slovák V, Siqueira G, Olsson RT, Tingaut P, Zimmermann T, Sehaqui H (2017) Drying and pyrolysis of cellulose nanofibers from wood, bacteria, and algae for char application in oil absorption and dye adsorption. ACS Sustain Chem Eng 5(3): 2679-2692. https://doi.org/10.1021/acssuschemeng.6b03027.
- Sun Y, Chu Y, Wu W, Xiao H (2021) Nanocellulose-based lightweight porous materials: A review. Carbohyd Polym 255: 117489. https://doi.org/10.1016/j.carbpol.2020.117489.
- Tataru G, Guibert K, Labbé M, Léger R, Rouif S, Coqueret X (2020) Modification of flax fiber fabrics by radiation grafting: Application to epoxy thermosets and potentialities for silicone-natural fibers composites. Radiat Phys Chem 170: 108663. https://doi.org/10.1016/j.radphyschem.2019.108663.
- Vegas VG, Beobide G, Castillo O, Reyes E, Gómez-García CJ, Zamora F, Amo-Ochoa P (2020) A bioinspired metal-organic approach to cross-linked functional 3D nanofibrous hydro- and aero-gels with effective mixture separation of nucleobases by molecular recognition. Nanoscale 12(27): 14699-14707. https://doi.org/10.1039/D0NR04166A.
- Wan C, Jiao Y, Wei S, Li X, Tian W, Wu Y, Li J (2019) Scalable top-to-bottom design on low tortuosity of anisotropic carbon aerogels for fast and reusable passive capillary absorption and separation of organic leakages. ACS Appl Mater Inter 11(51): 47846-47857. https://doi.org/10.1021/acsami.9b13686.
- Wan C, Jiao Y, Wei S, Zhang L, Wu Y, Li J (2019) Functional nanocomposites from sustainable regenerated cellulose aerogels: a review. Chem Eng J 359: 459-475. https://doi.org/10.1016/j.cej.2018.11.115.
- Wan C, Jiao Y, Tian W, Zhang L, Wu Y, Li J, Li X (2020) A holocellulose framework with anisotropic microchannels for directional assembly of copper sulphide nanoparticles for multifunctional applications. Chem Eng J 393: 124637. https://doi.org/10.1016/j.cej.2020.124637.
- Wang S, Lu A, Zhang L (2016) Recent advances in regenerated cellulose materials. Prog Polym Sci 53: 169-206. https://doi.org/10.1016/j.progpolymsci.2015.07.003.
- Wang Y, Zhao Y, Deng Y (2008) Effect of enzymatic treatment on cotton fiber dissolution in NaOH/urea solution at cold temperature. Carbohyd Polym 72: 178-184. https://doi.org/10.1016/j.carbpol.2007.08.003.
- Yang Y, Zheng L, Zhang T, Yu H, Zhan Y, Yang Y, Zeng H, Chen S, Peng D (2019) Adsorption behavior and mechanism of sulfonamides on phosphonic chelating cellulose under different pH effects. Bioresource Technol 288: 121510. https://doi.org/10.1016/j.biortech.2019.121510.
- Yao W, Weng Y, Catchmark JM (2020) Improved cellulose X-ray diffraction analysis using Fourier series modeling. Cellulose 27(10): 5563-5579. https://doi.org/10.1007/s10570-020-03177-8.
- Zhang JM, Luo N, Zhang XY, Xu LL, Wu J, Yu J, He JS, Zhang L (2016) All-cellulose nanocomposites reinforced with in situ retained cellulose nanocrystals during selective dissolution of cellulose in an ionic liquid. ACS Sustain Chem Eng 4(8): 4417-4423. https://doi.org/10.1021/acssuschemeng.6b01034.
- Zhou L, Ke K, Yang MB, Yang W (2021) Recent progress on chemical modification of cellulose for high mechanical-performance poly(lactic acid)/cellulose composite: A review. Compos Commun 23: 100548. https://doi.org/10.1016/j.coco.2020.100548.
- Zhu SD, Wu YX, Chen QM, Yu ZN, Wang CW, Jin SW, Ding YD, Wu G (2006) Dissolution of cellulose with ionic liquids and its application: a mini-review. Green Chem 8(4): 325-327. https://doi.org/10.1039/B601395C.
- Zugenmaier P (2021) Order in cellulosics: Historical review of crystal structure research on cellulose. Carbohyd Polym 254: 117417. https://doi.org/10.1016/j.carbpol.2020.117417.