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:
![](data:image/png;base64,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)
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:
![](data:image/png;base64,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)
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.