Crosslinking types in CNF/PVA aerogel and CNF/PVA/PEI nanoparticle
It is clear that softwood bleached pulp cellulose has absorption peaks at 3400cm-1, 1430cm-1, 1370cm-1, 1322cm-1 and 897cm-1(Fig.1), which are O-H group stretching vibration, C-H scissoring vibration, C-H bending vibration, O-H in -plane vibration and C-H deformation vibration of cellulose, respectively(Zhu et al. 2021). (Sirvio et al. 2015; Tarchoun et al. 2019).It can be observed that TO-CNF appears an intense peak in 1606cm-1, which was assigned to the characteristic peak of the carbonyl of the carboxylic acid group.
Compared with CNF, a absorption peak belonging to PVA appeared at 850cm-1 of CNF/PVA aerogel, it may be caused by the formation of intermolecular hydrogen bonds(Takeno et al. 2020). And for CNF/PVA/PEI nanoparticle, the peak in the 1600-1800cm-1 region appeared to be the overlap the three individual peaks, which belonged to N-H bending vibration (1615cm-1), C=O stretching vibration for-COO-(1660cm-1) and for -COOH (1713cm-1), respectively. At the peak of 1660cm-1, there is also a C=N group stretching, which appears in the process of glutaraldehyde crosslinking. In addition, at 2923cm-1 and 2848cm-1 as early as- CH2—stretching vibrations can also prove that PEI likely assembled into the CNF/PVA aerogel by N-H bond(Tang et al. 2020b).
Morphological analysis of CNF, CNF/PVA aerogel and CNF/PVA/PEI nanoparticle
The morphology and structure of BSP, CNF suspension is dispersed on mica flakes, CNF aerogel, CNF/PVA aerogel and CNF/PVA/PEI nanoparticle were examined by SEM(Fig.2.) The overall and local fiber morphology of bleached softwood pulp was observed at 1K and 60K, respectively. It can be clearly seen that the length of cellulose fiber is about 20-30 microns, and the length can reach vertical microns. SEM results show that there are some pores with diameter of 10-20nm on the surface of cellulose. The CNF obtained by homogenizing after TEMPO oxidation has a diameter between 10-50nm and a length of a few microns. It can be seen that CNF has an excellent aspect ratio, which can have a positive effect on subsequent adsorption(Garba et al. 2020). CNF aerogel and CNF/PVA aerogel were observed at 30 times, and honeycomb structure was found on the surface of CNF aerogel, and porous structure was found on the surface of CNF/PVA aerogel. The CNF/PVA aerogel obtained by freeze-drying after being compounded with PVA has a porous network structure on the surface, which provides a larger area for the cross-linking of PEI. Under the action of glutaraldehyde cross-linking agent, PEI is cross-linked on CNF/PVA aerogel, resulting in a change in the morphology of CNF/PVA/PEI. Specifically, due to the recombination of PEI on the surface of the CNF/PVA aerogel during the GA cross-linking process, the surface becomes denser and many granular nanoparticle are produced. Nano Measurer Software was used to count the particle size distribution of CNF/PVA/PEI nanoparticle. According to the calculation, the particle size of CNF/PVA/PEI nanoparticle was mainly distributed between 40 and 80nm, accounting for 83% of the total. These phenomena indicate that PEI plays a great role in changing the surface morphology during the cross-linking process.
Thermal stability of CNF, CNF/PVA aerogel and CNF/PVA/PEI nanoparticle
The thermal degradation profiles of BSP, CNF, CNF/PVA aerogel, CNF/PVA/PEI nanoparticle, were assessed by thermogravimetric analysis (TGA) under nitrogen atmosphere from RT (room temperature) to 650°C (Fig.3. a). During the process from RT to 100°C, due to the loss of water, the three substances have a slight quality degradation (not more than 2%)(Eun et al. 2020; Rani et al. 2014). BSP loses the fastest weight in 280°C-399°C, which is the main stage of cellulose pyrolysis. Within this range, BSP is pyrolyzed into small molecular gases and condensable volatiles of macromolecules, resulting in significant weight loss, and its weight loss rate reaches its maximum at about 384.9°C. The maximum degradation temperature of CNF is 320°C (about 50% loss), which is used for the pyrolysis of the cellulose skeleton(Yao et al. 2017). At the end of the analysis, the mass percentage of CNFs pyrolysis residue was 26%. The CNF/PVA aerogel begins to show its first decline at 221.8°C, and the maximum degradation temperature (loss of ca. 40%) appears at about 324.7°C. CNF/PVA/PEI nanoparticle have their first degradation at 216.4°C, and the maximum degradation temperature is 327.5°C (loss of ca. 52%). At the end of the analysis, the remaining CNF/PVA aerogel and CNF/PVA/PEI nanoparticle were 31% and 16%, respectively. The thermal stability of the adsorbents up to 205°C can be a valuable asset in the industrial sector context where the effluents can be warm or hot(Silva et al. 2020).
Specific surface aera of CNF, CNF/PVA aerogel and CNF/PVA/PEI nanoparticle
In order to better explore the morphological evolution of CNF/PVA/PEI nanoparticle, CNF aerogels, CNF/PVA aerogels and CNF/PVA/PEI nanoparticle obtained by freeze-drying CNF suspension were measured. Fig.3(b) shows the N2 adsorption-desorption curve of the sample at a temperature of 77K. The curve of CNF/PVA/PEI nanoparticle shown in the figure is type 4, which also verifies that the nanoparticle have a mesoporous structure(Wang et al. 2019). As can be seen from Table 1, the BET specific surface area of CNF aerogel is 35.52m2/g, and the BET specific surface area of CNF/PVA aerogel obtained by freeze-drying after being compounded with PVA solution is 56.37m2/g, and the specific surface area is greatly increased. After PEI modification, the specific surface area decreased, and the BET specific surface area of CNF/PVA/PEI nanoparticle became 22.93m2/g, which indicated that PEI was successfully assembled into the CNF/PVA aerogel.
Effect of pH on Cu2+adsorption
As shown in the Fig. 4. (a), the removal rate of CNF/PVA/PEI nanoparticle measured under five different pH conditions of 2, 3, 4, 5, 6. The removal rate CNF/PVA/PEI nanoparticle to Cu2+ increased with the increase of pH value. Due to the protonation of the amino group, it exhibits a lower adsorption capacity at a lower pH. When the pH is 6, the removal rate increased to maximum value (56.55%), so the pH is 6 to learn the adsorption performance of CNF/PVA/PEI nanoparticle.
As shown in the Fig.4. (b), at the beginning, the adsorption capacity increased rapidly and then the increasing trend became slow and finally stabilized, which may be due to the reduction of effective adsorption sites. Therefore, 120min is selected as the best adsorption time for Cu2+.
Fig.4. (c-e) show digital photographs of the Cu2+ adsorption in 20mg/L of the adsorbent. Firstly, 0.1gCNF /PVA/PEI nanoparticle are placed in Cu2+ solution, and the particles will be suspended in water. After adsorption, the particles will gradually settle at the bottom, which is conducive to the determination of Cu2+ and the recycling of CNF/PVA/PEI nanoparticle.
As shown in Fig.4. (b), at four different concentrations, the adsorption equilibrium was reached within two hours, and the removal rates in solutions of 20, 40, 60, and 80 mg/L were 94.1%, 93.6%, 95.7%, and 96.5%, respectively. All are above 93% This indicates that CNF/PVA/PEI nanoparticle may be an excellent adsorbent.
Adsorption kinetics of Cu2+ by the CNF/PVA/PEI nanoparticle.
Adsorption kinetics could be used to describe the adsorption mechanism (Fig.4.(b)). It shows the time-dependent adsorption performance of CNF/PVA/PEI nanoparticle at concentrations of 20mg/L, 40mg/L, 60mg/L, and 80mg/L. The adsorption equilibrum was achieved after 1 hours for all 4 concentrations. At four different concentrations, the removal rate of Cu2+ ions by CNF/PVA/PEI nanoparticle reached more than 93%. This indicates that CNF/PVA/PEI nanoparticle may become a good adsorption material for removing Cu2+ ions from aqueous solutions.
Four curve models, Pseudo-first-order kinetics, Pseudo-second-order kinetics, Intra-particle diffusion model and Elovich equation, are used to explain the adsorption mechanism of Cu2+ in aqueous solution. The Table 4andFig.5. (a-d) show the fitted curve and result, respectively. At four different concentrations, the correlation coefficient (R2s) of the Pseudo-second-order model is all above 0.99. However, for the Pseudo-first-order model, the R2s is between 0.9006-0.9677. The fitting results of the Intra-particle diffusion model are not very good, and the R2s of the four concentrations are all less than 0.87. For the fitting results of Elovich equation, the R2s of the four concentrations are between 0.8065-0.9594. Therefore, the Pseudo-second-order model is considered to be the most suitable data for experimental results. In the adsorption process, both physical adsorption and chemical adsorption exist. The chemical adsorption of Cu2+ is a rate-limiting process. Functional groups such as hydroxyl and amino groups in CNF/PVA/PEI nanoparticle provide adsorption sites for chemical adsorption. Large specific surface area will provide more opportunities for the adsorption of heavy metal ions.
Desorption ability of CNF/PVA/PEI nanoparticle
In addition to desired removal rate, ideal environmental remediation materials should possess excellent regeneration and recyclability (Fig.6). The Cu2+ removal rate of CNF/PVA/PEI nanoparticle remained to 80% after three adsorption-desorption cycles. The removal rate was decreased slightly with the cycle time, which was due to the decrease of effective adsorption sites in CNF/PVA/PEI nanoparticle. There were two reasons to result in the reduction of removal rate. One of that was Cu2+ combined with some active sites of CNF/PVA/PEI nanoparticle with an irreversible way. The other one was due to a few amine groups were protonated during the desorption process (Tang et al. 2020b).
Comparison with reported studies
The final adsorption properties of CNF/PVA/PEI nanoparticle were evaluated and are presented in Table 3 for comparison with reported data. The CNF/PVA/PEI nanoparticle reached adsorption equilibrium within one hour, far better than the 2,3-dialdehyde nano-fibrillated celluloses (DNFCs)(Lei et al. 2019) and the perlite/solid iron(Khudr et al. 2021). The removal rate of CNF/PVA/PEI nano reached 93% in the solution of 20-80mg/ L Cu2+ solution, far higher than the perlite/solid iron. It is worth noting that, the removal rate can still reach 80% after three cycles, However, DNFCS, Perlite/ Solid Ion, Multi-walled Carbon Nanotubes (MWCNT) do not show cyclic performance(Mobasherpour et al. 2014). Which means CNF/PVA/PEI nanoparticle may be an excellent alumina bent can be applied in practical application. The adsorption capacity of CNF/PVA/PEI nanoparticles displayed improved significantly is superior than other samples (Table 3).