Novel Cellulose Nanofibers/Polyvinyl Alcohol/Polyethyleneimine Nanoparticle for Cu2+ Removal in Water


 In this study, environmentally friendly CNF/PVA/PEI nanoparticle was obtained by assembling PEI (polyethyleneimine) into CNF/PVA aerogels, which were prepared by freeze-drying method with the help of glutaraldehyde. FTIR results showed that PEI likely assembled into the CNF/PVA aerogel due to appearances of bending vibration of the CNF/PVA/PEI nanoparticle at 1615cm− 1. BET results further demonstrated that PEI have successfully assembled into aerogel since the specific surface area (22.93m2/g) of CNF/PVA/PEI nanoparticle was lower than that (56.37m2/g) of CNF/PVA aerogel. SEM results also showed that PEI could obviously regulate the morphology of CNF/PVA aerogel. TGA indicated that CNF/PVA/PEI nanoparticle was structurally stable at 216.4°C. The adsorption kinetics of the CNF/PVA/PEI nanoparticle for Cu2+ removal presented good correlations with the Pseudo-second-order model. The adsorption and desorption results showed that the removal rate of 2g/L CNF/PVA/PEI nanoparticle for Cu2+in water could reach more than 93% when the concentration of Cu2+ ranged from 20 to 80mg/L in one hour, and it could still retain more than 80% after 3 cycles.


Introduction
Nowadays, with the rapid growing of economic and industrialization, heavy metal ions has been the most serious problem in water environment due to their toxicity and incompatibility (Fu and Wang 2011). Heavy metal ions,such as copper (Cu)(II) ,hydrargyrum(Hg)(II), chromium (Cr)(VI), plumbum (Pb)(II) and so on have posed a great threat to human survival due to its undegradability and toxicity in water (Uddin 2017).
There are many traditional methods to solve the problems caused by heavy metal ions, such as adsorption, coagulation, membrane separation, oxidation and ion exchange(Wan Ngah and Hana ah 2008). Among them, adsorption is considered to be one of the most popular strategies for heavy metal ion removal because of the high removal e ciencies, exibility in the design and application of adsorbents, and low cost (Qin et  It is reported that the heavy metal Cu 2+ usually accumulates in human liver and cause serious anemia and hemolysis (Stern 2010).Tang et al.(2020a) used 3-glycidyloxypropyl) trimethoxy silane (GPTMS) as a cross-linking agent to cross-link PEI onto the cellulose nano bril skeleton and quickly freeze-dried with liquid nitrogen to obtain aerogel beads. Its maximum adsorption capacity for Cu 2+ reached 163.4mg/g. Zhang et al.(2016) used glutaraldehyde as a cross-linking agent to cross-link PEI onto TOCN and then freeze-dried to obtain TOCN-PEI adsorbent with a maximum adsorption capacity of 52.32 mg/g for Cu 2+ . Mo et al.(2019) used Trimethylolpropane-tris-(2-methyl-1-aziridine) propionate (TMPTAP)as a cross-linking agent and cross-linked PEI and TO-CNF to obtain TO-CNF/TMPTAP/PEI aerogel. Its maximum adsorption capacity of Cu 2+ could reach 485.44mg/g. It was reported that PVA could be used as environmentally friendly adsorption material due to its good biocompatibility and biodegradability (Niu et al. 2018;Zheng et al. 2014). In this study, we prepared CNF/PVA/PEI nanoparticle through a freeze-drying method. The structures and properties of CNF/PVA/PEI nanoparticle were analyzed by FTIR, SEM, TGA and fully automatic gas adsorption analyzer. The adsorption performance of CNF/PVA/PEI nanoparticle on Cu 2+ was investigated, and its reusability was also being explored.
The TEMPO oxidized cellulose nano bers(TOCNF) were prepared according to previously reported methods (Qin et al. 2011). Weigh 4gBSP accurately, disperse in 400ml deionized water, add 0.016g TEMPO, 0.16g NaBr, 80ml 10% NaClO solution. After that, 0.1MHCl was used to adjust the optimal pH to 10-10.4. In the reaction process, the pH of the solution will gradually decrease with the formation of carboxyl group. 0.5MNaOH solution is used to adjust the pH to 10 until the pH no longer drops, and 5mL ethanol is used to terminate the reaction.

Preparation of TOCNF/PVA aerogel
The PVA solution (12.5mL, 8wt%), CNF solution (100g, 1wt%) were mixed together in a ask under vigorous stirring for 4h. The weight ratio between the PVA and CNF were 1:1 (Zheng et al. 2014).At the nal stage, CNF/PVA solution was placed for 24h to remove bubbles. The freeze-drying process was maintained at -30°C to obtain CNF/PVA aerogel.
Preparation of CNF/PVA/PEI nanoparticle Add 2g CNF/PVA aerogel into 100ml 10% PEI/methanol (w/v) solution and stir at 30°C for 24h. Then wash with methanol to remove unreacted PEI. Then disperse the precipitate in 100mL deionized water, then slowly add 4mL of 25% glutaraldehyde aqueous solution, then adjust the pH to 8 with 0.4M NaOH. Stir for one hour to make the two react, and nally, wash the product with deionized water and freeze Dry to obtain CNF/PVA/PEI adsorbent. Characterization FTIR Fourier transform infrared spectroscopy (IR Prestige21 Shimadzu Corporation) spectra were obtained using the Alpha infrared spectrometer with a scanning number of 32 from 250 to 4250 cm -1 .

SEM
Scanning electron microscope (Regulus 8220, Japan) was used to characterize the morphology of materials. All the specimens were coated with Au before observation.

TGA
Thermogravimetric analysis (TGA) was conducted with a synchronous thermal analyzer (STA449, USA) test. Samples were heated at a constant heating rate of 10°C/min, from room temperature (RT) to 650°C, under inert (N 2 ) atmosphere.

BET
The speci c surface aeras were measured by the Brunauer-Emmett-Teller (BET) method based on N 2 adsorption. The determinations were carried out from freeze-dried samples with an analyzer (Micromeritics, USA). Prior to testing, the samples should be degassed at 120°C for 4h.

Adsorption experiments
The initial concentrations of Cu 2+ was set to be 1000mg/L. The whole contact time was 2 h under Temersionoxcillator registration (Noki)at room temperature for all batch experimental process. Residual concentration of Cu 2+ were measured by ame atomization atomic absorption spectrometry (AAS, GGX-600, China). The amount of Cu 2+ adsorbed onto the adsorbent was calculated from the difference between the initial (C 0 ) and the remaining (Ce) metal concentration in solution(Anirudhan et al. 2019).
Where C 0 is the initial concentration of Cu 2+ (mg/L) and Ce is the concentration of Cu 2+ at equilibrium time.

Adsorption kinetics
The known Pseudo-rst-order kinetics, Pseudo-second-order kinetics, Intra-particle diffusion model and where Qt is the adsorption capacity after time t and Q 0 is the saturated adsorption capacity of Cu 2+ ; k 1 and k 2 are the constants of the rst-order and second-order kinetics respectively; ki and C are the constants of the Intra-particle diffusion model; ks and A are the constants of the Elovich equation.

The effect of pH on adsorption capacity
The initial pH value of the copper solution has a crucial in uence on the absorption performance of the absorbent, because it determines the form of the copper ion species and the electrostatic repulsion between the absorbent and Cu 2+ ions. Due to the presence of amino groups in CNF/PVA/PEI, the surface charge will change with the change of pH, thereby affecting the adsorption capacity. In addition, when the pH exceeds 6.5, Cu 2+ ions may be converted to Cu(OH) 2 precipitation(Shao et al. 2017), it may cause deviations in the adsorption process. Therefore, the pH of the solution is set to 2, 3, 4, 5, 6. The sample is 0.05g. Take 50ml of 20mg/L Cu 2+ ion standard solution and adjust the pH to 2, 3, 4, 5, and 6, respectively. A sodium hydroxide aqueous solution (NaOH, 0.1mol/L) was used for pH adjustments.
The effect of adsorption time on adsorption capacity Take 0.1g sample and add it to the copper ion solution with pH 6, the concentration is 20mg/L, the sampling time is 10, 20, 30, 60, 120, 180, 240 minutes to determine the time required for adsorption to reach equilibrium.

Adsorption experiments
The standard solution of Cu 2+ is dispersed in deionized water for later experiments (20, 40, 60, 80 mg/L for adsorption experiments and thermodynamic curve study). 0.1g CNF/PVA/PEI nanoparticle were added to 50ml Cu 2+ solution. Shake in a constant temperature shaking box for 24 hours at a temperature of 25°C, then take the supernatant and determine the adsorption capacity with an atomic absorption analyzer to determine the adsorption capacity.

Desorption experiments
The 100mg CNF/PVA/PEI nanoparticle adsorbed with Cu 2+ were dispersed in 1M HCl solution and stirred at room temperature for 1 hour to remove Cu 2+ ions. The nanoparticle was then submerged into deionized water to remove the excess acid until the pH reached 7, and the nanoparticle were dried for the next adsorption process.

Results And Discussions
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 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-CH 2 -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 akes, CNF aerogel, CNF/PVA aerogel and CNF/PVA/PEI nanoparticle were examined by SEM (Fig.2.) The overall and local ber morphology of bleached softwood pulp was observed at 1K and 60K, respectively. It can be clearly seen that the length of cellulose ber 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. Speci cally, 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 pro les 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 signi cant 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 rst 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 rst 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 e uents can be warm or hot (Silva et al. 2020).
Speci c 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 N 2 adsorption-desorption curve of the sample at a temperature of 77K. The curve of CNF/PVA/PEI nanoparticle shown in the gure is type 4, which also veri es that the nanoparticle have a mesoporous structure ). As can be seen from Table 1, the BET speci c surface area of CNF aerogel is 35.52m 2 /g, and the BET speci c surface area of CNF/PVA aerogel obtained by freeze-drying after being compounded with PVA solution is 56.37m 2 /g, and the speci c surface area is greatly increased. After PEI modi cation, the speci c surface area decreased, and the BET speci c surface area of CNF/PVA/PEI nanoparticle became 22.93m 2 /g, which indicated that PEI was successfully assembled into the CNF/PVA aerogel.
Effect of pH on Cu 2+ adsorption As shown in the Fig. 4. (a), the removal rate of CNF/PVA/PEI nanoparticle measured under ve different pH conditions of 2, 3, 4, 5, 6. The removal rate CNF/PVA/PEI nanoparticle to Cu 2+ 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 nally stabilized, which may be due to the reduction of effective adsorption sites. Therefore, 120min is selected as the best adsorption time for Cu 2+ . Fig.4. (c-e) show digital photographs of the Cu 2+ adsorption in 20mg/L of the adsorbent. Firstly, 0.1gCNF /PVA/PEI nanoparticle are placed in Cu 2+ 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 Cu 2+ 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 could be used to describe the adsorption mechanism ( Fig.4.(b)). It shows the timedependent 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 Cu 2+ 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 Cu 2+ ions from aqueous solutions.
Four curve models, Pseudo-rst-order kinetics, Pseudo-second-order kinetics, Intra-particle diffusion model and Elovich equation, are used to explain the adsorption mechanism of Cu 2+ in aqueous solution. The Table 4andFig.5. (a-d) show the tted curve and result, respectively. At four different concentrations, the correlation coe cient (R 2 s) of the Pseudo-second-order model is all above 0.99. However, for the Pseudo-rst-order model, the R 2 s is between 0.9006-0.9677. The tting results of the Intra-particle diffusion model are not very good, and the R 2 s of the four concentrations are all less than 0.87. For the tting results of Elovich equation, the R 2 s 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 Cu 2+ 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 speci c 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 Cu 2+ 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 Cu 2+ 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 nal 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-brillated 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 Cu 2+ 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 signi cantly is superior than other samples (Table 3).

Conclusion
To improve the removal rate and adsorption capability of CNF based adsorbents for Cu 2+ , we introduce a novel and green approach to assemble CNF/PVA/PEI nanoparticle, which reaches fast adsorption of Cu 2+ . Its adsorption for Cu 2+ is very consistent with the pseudo-second-order model among four kinds of adsorption models. It is evident that the chemical adsorption is the main rate-limiting step when the CNF based adsorbent is carried out to remove Cu 2+ in water. We can assemble the environmentally friendly and effective CNF based adsorbents for the removal of Cu 2+ by doping new chemical agents and controlling the structure in the near future. Overall, this study not only reveals adsorption mechanism of CNF/PVA/PEI nanoparticle for Cu 2+ removal, but also provides a promising pathway to regulate the CNF based adsorbents for the removal of the other heavy metal ions in water.

Declarations
Authors' contribution All authors contributed as the main contributors of this work. All authors performed the literature search and analysis. All authors had drafted, revised the work and approved the nal paper.  Table 1 Thermal stability parameters        (a) Plots of Pseudo-rst-order, (b) Pseudo-second-order, (c) Elovich, (d) Intra-particle diffusion models of 20, 40, 60, 80mg/L