Porous cellulose nanofiber (CNF)-based aerogel with the loading of zeolitic imidazolate frameworks-8 (ZIF-8) for Cu(II) removal from wastewater

Abstract

A novel biobased porous aerogel was synthesized using physical mixing, freeze-drying, and in-situ growth methods. Zeolitic imidazolate frameworks-8 (ZIF-8) were grafted onto the surface of the CS/CNF solid composite to form a ZIF-8@CS/CNF aerogel. The structural characteristics and the adsorption potential of the ZIF-8@CS/CNF aerogel were investigated. It was found that the specific surface area of the ZIF-8@CS/CNF aerogel was 206 m2/g, and the water stability of the CNF aerogel was enhanced by incorporating the CS. Meanwhile, the adsorption isotherm and kinetics of the composite aerogel fit the pseudo-second-order kinetic model (R2 = 0.96) and the Langmuir isotherm model (R2 = 0.97) with the copper(II) oxide (Cu(II)) theoretical adsorption capacity of 245 mg/g, respectively. Furthermore, this aerogel, which combined metal-organic frameworks (MOFs) and CNF, was easy to fabricate and it was biodegradable. These characteristics suggest it has a broad potential for wastewater treatment.

Full Article

Porous Cellulose Nanofiber (CNF)-based Aerogel with the Loading of Zeolitic Imidazolate Frameworks-8 (ZIF-8) for Cu(II) Removal from Wastewater

Wanyao Meng, Sijie Wang, Haifeng Lv, Zhenxing Wang, Xuewen Han, Zijing Zhou, and Junwen Pu *

A novel biobased porous aerogel was synthesized using physical mixing, freeze-drying, and in-situ growth methods. Zeolitic imidazolate frameworks-8 (ZIF-8) were grafted onto the surface of the CS/CNF solid composite to form a ZIF-8@CS/CNF aerogel. The structural characteristics and the adsorption potential of the ZIF-8@CS/CNF aerogel were investigated. It was found that the specific surface area of the ZIF-8@CS/CNF aerogel was 206 m²/g, and the water stability of the CNF aerogel was enhanced by incorporating the CS. Meanwhile, the adsorption isotherm and kinetics of the composite aerogel fit the pseudo-second-order kinetic model (R² = 0.96) and the Langmuir isotherm model (R² = 0.97) with the copper(II) oxide (Cu(II)) theoretical adsorption capacity of 245 mg/g, respectively. Furthermore, this aerogel, which combined metal-organic frameworks (MOFs) and CNF, was easy to fabricate and it was biodegradable. These characteristics suggest it has a broad potential for wastewater treatment.

DOI: 10.15376/biores.17.2.2615-2631

Keywords: Cellulose nanofibers; CNF; Metal-organic framework; Chitosan; Aerogel; Heavy metal

Contact information: MOE Engineering Research Center of Forestry Biomass Materials and Bioenergy, Beijing Forestry University, No.35 East Qinghua Road, Haidian District, Beijing 100083, China;

* Corresponding author: jwpu@bjfu.edu.cn

INTRODUCTION

With the vigorous development of industry and agriculture, the discharge of pollutants has posed an enormous challenge to the water environment (Muhamad et al. 2018). Heavy metals exist in industrial emissions. They are toxic, harmful, and cause severe damage to the human and natural environment due to their non-biodegradable and bioaccumulation characteristics (Tang et al. 2019; Li and Xu 2021). For instance, copper(II) (Cu(Ⅱ)) oxide is a common natural substance that is widely used in commercial applications, such as in the electrical industry or in antifouling paints. Meanwhile, Cu(Ⅱ) is also an indispensable trace ion for human beings (Wan et al. 2010; Fu and Wang 2011; Fu and Xi 2020). When the body intakes too much copper, it can be noxious to humans, causing liver function damage and cancer (Uauy et al. 2008; Anush et al. 2020).

Currently, heavy metals are removed from wastewater via flotation (Colic et al. 2007), chemical precipitation (Chen et al. 2018), membrane systems (Liu et al. 2019), and adsorption (Zhao et al. 2011), among others. Among these methods, adsorption is considered the most promising available technique due to it has high removal rate, simplicity, and wide range of adaptation (Zhou et al. 2020). A variety of porous materials have been used for water treatment because of their high surface area. Some examples include activated carbon (Rao et al. 2009), clays (Gu et al. 2019), aerogels (Maleki 2016), metal-organic frameworks (MOFs) (Maleki et al. 2015), graphene (Liu et al. 2020), and polysaccharides (Musarurwa and Tavengwa 2020). However, many adsorbents have limitations, such as a high cost, low adsorption capacity, single synthesis condition, poor controllability, non-biodegradability, and non-renewability. Therefore, a novel, high selectivity adsorbent has been found for the environment and water decontamination. Metal-organic frameworks are a novel high porosity crystal hybrid material. There are numerous advantages with MOFs, such as high specific surface area, easy functionalization, tunable pore structures, simple preparation methods, and chemical stability (Schejn et al. 2014; Jian et al. 2015; Xiao et al. 2019; Xu et al. 2021). Therefore, MOFs can be widely used in drug delivery, gas storage and separation, water purification, and catalysis, which has attracted the attention of many researchers. However, MOFs are powdery nanomaterials. They are easily assembled together and not recycled in the water, which cause another problem of water pollution. In order to avoid the secondary pollution, MOF powders should be loaded with other material by electrostatic action, Van der Waals force, or pi-pi conjugate bonding. Gnanaselvan et al. (2018) embedded MOF into cellulose acetate, PES, and PVDF respectively. for removing heavy metal ions from wastewater. Ma et al. (2019) prepared a lightweight and porous zeolitic imidazolate frameworks-8@cellulose-nanofiber@cellulose foam using a simple in situ green growth method for gas adsorption and heavy metal ions removal.

Cellulose nanofiber (CNF) aerogels display outstanding performance traits, such as a high porosity rate, a low density, and a high specific surface area. In addition, CNF aerogels are biodegradable, bio-compatibly nontoxic, and they possess hydroxyl function groups (Long et al. 2018; Esmaeili et al. 2021). The abundant hydroxyl functional groups in CNF allow it to easily bind to different adsorbents. These kinds of nanocellulose-based composite material have shown excellent adsorption capacity (Tshikovhi et al. 2020). Yu et al. (2020) prepared graphene oxide (GO)/carboxymethyl cellulose nanofibril composite fiber as an adsorbent that is economically competitive with an efficient lead uptake (99.0 mg/g). Jin et al. (2019) synthesized carboxymethyl cellulose/MOF beads for the adsorption of lead ions (Pb²⁺) from an aqueous solution with the maximum adsorption capacity of 132 mg/g. Nevertheless, there is only weak hydrogen bonding between cellulose nanofibers, and these can be directly dispersed in water. Therefore, in order to solve this problem, it is a good choice to introduce new groups to add cross-linking agents (Ngah et al. 2011; She et al. 2018). Tian et al. (2017) used natural CNFs cross-linked with acrylic acid (AA) to remove heavy metal ions, and the CNF-based aerogel formed highly porous networks after cross-linking with the AA. Hong et al. (2021) synthesized a three-dimensional (3D) porous polyethylenimine (PEI)/CNF aerogel for copper removal. The combination of the CNF and PEI provides excellent wet stability. However, most of the crosslinking agents are toxic and have complex reaction conditions. Therefore, the combination of natural crosslinking agents and CNF can be considered, such as chitosan (CS) (Luo et al. 2015), cyclodextrin (Zhang et al. 2015), and sodium alginate (Zhao et al. 2021). These natural crosslinking agents are inexpensive, biodegradable, non-poisonous, and renewable. Chitosan is an environmentally friendly, inexpensive, and readily available biomolecule material (Ahmed et al. 2020; Zhang et al. 2021). Chitosan has two reactive functional groups, amine groups at C₂ and hydroxyl groups at C₃ and C₆ (Yang et al. 2014). Due to the amine groups, chitosan is a positively charged natural polymer (Xing et al. 2019). The amino groups can chelate with certain metal ions and improve the removal rate of the metal ions (Haripriyan et al. 2022). Also, it can form hydrogen bonds with cellulose nanofibers (Bao et al. 2015).

In this paper, zeolitic imidazolate frameworks-8 (ZIF-8) loaded CS/CNF composite was prepared via an initial one-pot synthesis. The ZIF-8 crystals were added by in-situ growing on the CS/CNF composites to form the ZIF-8@CS/CNF aerogel (Fig. 1). The CNF and CS are sustainable structural materials, with no harmful effects on the environment, human health, or ecosystem. In addition, the CS/CNF composites could help disperse and load the ZIF-8 nanoparticles. The hybrid aerogel exhibited excellent adsorption performances, efficiently collecting heavy metal ions by investigating the adsorption capacity. Therefore, this approach provided a green adsorption material that can efficiently remove Cu(Ⅱ).

Fig. 1. Schematic illustration of the formation process of the ZIF-8@CS/CNF aerogel

EXPERIMENTAL

Materials

The nanofibrillated cellulose (CNF) suspension of mechanical lapping (1.2% w/v) was obtained from Tianjin Woodelf Biotechnology Co. (Tianjin, China). The chitosan (CS) was supplied by Sinopharm Chemical Reagent Co. (Shanghai, China). The zinc acetate ((CH₃COOH)₂Zn) and adsorbate copper chloride (CuCl₂) were purchased from Meryer Chemical Technology Co. (Shanghai, China). The 2-methylimidazole (2-H-MeIM, C₄H₆N₂) was purchased from Shanghai Energy Chemicals Co. (Shanghai, China), and the AA (CH₃COOH) was obtained from Beijing Tongguang Fine Chemical Company (Beijing, China). All the reagents mentioned above were analytically pure without any additional treatment.

Fabrication of the CS/CNF@ZIF-8 Aerogel

Firstly, CS was dissolved in a 2% acetic acid aqueous solution to form a 5% CS solution. Next, 1.25 g of the CS solution (5% w/w) was poured into 5.2 g of CNF suspension (1.2% w/w) and stirred for 30 min to obtain a uniform mixture solution. The CS/CNF compound was acquired by using a vacuum freezer dryer (Free Zone 4.5L; Labconco; USA).

The ZIF-8@CS/CNF aerogel was synthesized in the water by the in-situ growth method shown in Fig. 1. Briefly, 2.6 mmol of (CH₃COOH)₂Zn was dissolved to obtain solution A, and 40 mmol 2-methylimidazole was dissolved by deionized water to get solution B. The solid compound of CS/CNF was immersed in solution A for 3 h at room temperature. Next, the CS/CNF composite aerogel was placed into solution A by vacuum pressure processes for 30 min. Subsequently, the Zn²⁺– CS/CNF composite solid was immersed in solution B in the same way. The ZIF-8@CS/CNF composite aerogel was then prepared by a vacuum freeze dryer. As shown in Fig. 1, -OH and -NH on CS/CNF composites were coordination bonds with Zn²⁺ and 2-methyl imidazole formed with Zn²⁺ by coordination bonds which indicated ZIF-8 was grown on CS/CNF composites (Wang et al. 2019).

Characterization

A scanning electron microscope (SEM) (G300; ZEISS, Jena, Germany) was used to examine the surface morphology and structure of the ZIF-8 and the absorbents. Fourier transform infrared spectroscopy (FTIR), ranging 400 to 4000 cm^-1, and X-ray diffraction (XRD) were applied to characterize the existence of functional groups and characteristic peaks of the ZIF-8, CNF, and CS. A PerkinElmer Frontier system (Waltham, MA, USA) and a Bruker D8 Advance system (Billerica, MA, USA) were used for the FTIR and XRD analyses, respectively. The specific surfaces of the ZIF-8, CNF/CS aerogel, and ZIF-8@CS/CNF aerogel were measured by the BET method using a specific surface area analyzer (ASAP 2460; Micrometrics, Atlanta, GA, USA).

Adsorption Study

A series of batch adsorption experiments were carried out to study the adsorption capacity of the CS/CNF composite and ZIF-8@CS/CNF aerogel. At a certain temperature, a series of variables were designed, including the different initial Cu²⁺ concentration (400 to 1,000 mg/L), reaction time (0 to 600 min), and pH (2 to 6). For the Cu(II) adsorption measurements, 0.08 g of ZIF-8@CS/CNF aerogel was added to the stoppered glass tubes that contained 20 mL of Cu(Ⅱ) solution of different concentrations until adsorption equilibrium (600 min). The tubes were then shaken in a thermostatic oscillator (ZWY-2102C; Shanghai Zhicheng Analytical Instrument Manufacturing Co., Ltd; China) at 80 rpm and room temperature.

The concentration of Cu(Ⅱ) solution before and after adsorption was measured by atomic absorption spectrum (AAS), using a Varian SpectrAA 220 instrument (Victoria, Australia) at a wavelength of 324.7 nm. To evaluate the adsorption capacity of the ZIF-8@CS/CNF aerogel, the amount of Cu(Ⅱ) adsorption and the removal efficiency were calculated by Eqs. 1 and 2, respectively (Wang et al. 2020),

(1) (2)

where C₀ is the initial concentration of the Cu(Ⅱ) solution (mg/L), C_e is the Cu²⁺ concentration of the adsorption equilibrium (mg/L), v is the volume (L) of the Cu²⁺ solution, and m is the quality (g) of the adsorbent.

RESULTS AND DISCUSSION

Characterization

The SEM analysis (Fig. 2) illustrated the morphologies of the CS/CNF composites and ZIF-8@CS/CNF aerogel. As can be seen in Fig. 2, the CS/CNF composites and ZIF-8@CS/CNF aerogel exhibited a 3D porous network structure. It appeared to consist of irregularly shaped interconnected pores with thin walls, which can be attributed to irregular ice crystals that were generated by the freeze-drying technique. The highly porous absorbent would facilitate adsorption capacity because the number of adsorption sites increased. Meanwhile, there were ZIF-8 nanoparticles present in the ZIF-8@CS/CNF aerogel in Fig. 2d. In general, the microscopic morphology exhibited a remarkable difference brought about by the in-situ growth of ZIF-8 on the CS/CNF aerogel.

Fig. 2. a) and c) are the SEM images of CS/CNF; b) and d) are the SEM images of the ZIF-8@CS/CNF aerogel; The inset in d) shows the ZIF-8 loaded on the CS/CNF composite aerogel

The N₂ isotherm of the adsorbent can well describe the specific surface area of the material, and its data are shown in Table 1. After loading ZIF-8, the specific surface area of the composites increased significantly. As shown in Fig. 3a, the CS/CNF composite exhibited III type isotherms, indicating that the composite was a macroporous material (Zhao et al. 2020). ZIF-8 nanopaticles and ZIF-8@CS/CNF aerogels showed typical I isotherms, indicating that there was mainly microporous adsorption (Sing and Williams 2005). ZIF-8 nanopaticles constitute a highly organic metal skeleton with high porosity (Ren et al. 2018). The loading will not destroy the morphology and structure of the composite, and it will increase the specific surface area of ZIF-8@CS/CNF composites aerogels (Thunberg et al. 2021). At the same time, the density of the composite aerogels increased with the increase of ZIF-8 loading, while the porosity decreased with the increase of ZIF-8 loading.

Fig. 3. a) N₂ adsorption-desorption isotherms of ZIF-8 nanoparticles, CS/CNF composites, and ZIF-8@CS/CNF aerogels respectively; b) the water stability of the cellulose nanofibers, CS/CNF composite and ZIF-8@CS/CNF aerogel. The photos illustrate the water stability of the nanofibrillated cellulose, CS/CNF composite and ZIF-8@CS/CNF aerogel.

The stability of aerogels in a water environment was investigated. It can be seen from Fig. 3b that CS/CNF composites and ZIF-8@CS/CNF composites aerogels did not undergo deformation and collapse in water, showing good stability. It might be that CNF can form a good combination with the amino and hydroxyl groups of chitosan through hydroxyl (Xiao et al. 2017). In addition, the impregnation solution of the composite aerogels was kept clear, and no turbidity was found between the prepared ZIF-8 materials and the cellulose aerogels. This could be judged to be a stable combination of ZIF-8 and aerogels to a certain extent, and the two strains were not easily separated.

Table 1. Density, Porosity and Specific Surface Area of the Three Materials (ZIF-8 nanoparticles, CS/CNF composite, ZIF-8@CS/CNF aerogel)

The FTIR spectra of the ZIF-8, the CS/CNF composites, and the ZIF-8@CS/CNF aerogel samples are shown in Figs. 4a and 4b. The FTIR spectrum of the CS/CNF composites exhibited the characteristic peak of CNF and CS at 3347, 2923, 1591, and 1372 cm^-1, representing O−H and N−H stretch vibration peaks that overlap to form a broad peak, C−H stretch vibration, −NH₂ bending, and CH₃ and CH₂ stretch vibration, respectively (Lim and Hudson 2004; Wang et al. 2021). After the ZIF-8 was synthesized by in-situ on the CS/CNF composites, new stretching bands at 748 and 673 cm^-1 appeared in the FTIR spectra. These bands represented the typical bands of ZIF-8 molecules, with N−H and out-of-plane bending vibration of imidazole (Bo et al. 2018). Meanwhile, the decreased intensity at 2900 to 3500 cm^-1 demonstrated the interaction between the ZIF-8 and the CNF. The FTIR spectra of the ZIF-8@CS/CNF sample confirmed that the ZIF-8 was successfully grafted onto the CS/CNF composites by the in-situ method.

Fig. 4 The a) FTIR spectra of the pure ZIF-8 nanoparticles, the CS/CNF composite, and the ZIF-8@CS/CNF aerogel; the b) enlargement of the FTIR spectra of the CS/CNF composite and the ZIF-8@ CS/CNF aerogel; the c) XRD pattern of the pure ZIF-8 nanoparticles, the CS/CNF composites, and the ZIF-8@ CS/CNF aerogel; the d) enlargement of the XRD pattern of CS/CNF composites

Figure 4c shows the XRD patterns of the synthesized ZIF-8 nanoparticles, the CS/CNF composites, and the ZIF-8@CS/CNF aerogel. The XRD pattern of the CS/CNF composites displayed a sharp peak at 2θ = 23.4° and two weak peaks at 2θ = 15.4° and 34.7°, exhibiting a typical diffraction pattern of cellulose I (Chook et al. 2015). Meanwhile, Fig. 4d showed the characteristic peaks at 2θ = 11.8° and 21.3°, which indicated the crystalline nature of CS (Sarkar et al. 2017). The XRD spectra of the ZIF-8@CS/CNF aerogel showed the new characteristic peaks at 2θ = 10.7°, 13.0°, 14.9°, 16.7°, 18.3°, and 26.9°, which were the characteristic peaks of ZIF-8, in accordance with the reported literature (Venna et al. 2010). These results indicated that the ZIF-8 was successfully grown on the CS/CNF aerogel.

Copper Adsorption Experiments

The solution pH of the copper solution is a critical parameter for the adsorption behavior of the adsorbent. The pH value can affect the adsorption site and the physicochemical status of heavy metal ions. For example, Cu(Ⅱ) was transformed sediment (Cu(OH)₂) when the pH was greater than 6.5 (Liu et al. 2017). Therefore, a pH range between 2 and 6 was chosen to study its effect on the ZIF-8@CS/CNF aerogel adsorption capacity. The results in Figure 5a showed that the adsorption capacity declined as the pH value decreased from 5 to 2. This might be due to partial amino protonation and structural collapse of ZIF-8 under strong acid condititon (Li et al. 2021). Also, there is protonantion of amino of chitosan under acidic solution conditions. When pH = 6, it can be observed that the adsorption capacity is small, which is possible that there may be a small amount of Cu(OH)₂ precipitation in the solution (Zhang et al. 2016). Therefore, a pH value of 5 was selected as the optimum pH value for further experiments with high uptake performance.

Fig. 5. The a) the effect of the pH on the Cu(II) adsorption with the ZIF-8@CS/CNF aerogel and the b) the adsorption capacity of ZIF-8@CS/CNF and CS/CNF aerogel at different initial concentrations.

Figure 5b shows the adsorption results of ZIF-8@CS/CNF aeogel and CS/CNF composite at different initial concentrations. The results showed that the adsorption capacity of CS/CNF composite doped with ZIF-8 was greatly increased. To help analyze the adsorption of Cu(Ⅱ) solution, the Langmuir and Freundlich models fit the experimental adsorption data. Usually, the equations of the Langmuir isotherm (Eq. 3) and the Freundlich isotherm (Eq. 4) are presented as follows (Gupta and Pathak 2021):

(3)

(4)

where Q_e and Q_max are the adsorption capacity at equilibrium (mg/g) and the maximum adsorption capacity of the absorbent (mg/g), respectively, k_L is the adsorption constant of Langmuir isotherm model, k_F is the Freundlich constant, and 1/n is the empirical parameter in the Freundlich model, which represents the affinity between the adsorbent and the adsorbate.

Figure 6 shows the plots of Langmuir and Freundlich isothermal models of two composites. The calculated parameters of the two models are summarized in Table 2. As is well-known, the n parameter indicates the interaction occurred during the adsorption process. When n is greater than 1, it indicates a chemical interaction, when n is less than 0, it denotes the physical interaction, and when n is equal to 1, it indicates that no adsorption process that will occur. In this work, n greater than 1 represented the chemical interaction in the adsorption process (Chee et al. 2021). Also, it can be seen from Table 2 that the correlation coefficients of Langmuir and Freundlich isothermal models were more than 0.95, which can well reveal the adsorption data. This suggests that the adsorption sites were uniformly distributed in the aerogel and may represent a monolayer adsorption (Spagnol et al. 2012; Wang et al. 2020). According to the Langmuir model, the maximum adsorption capacity of the ZIF-8@CS/CNF aerogel and CS/CNF composite on Cu(Ⅱ) was 245 mg/g and 36.1 mg/g respectively. In addition, the maximum adsorption capacity was compared to these previously reported adsorbents.

Fig. 6. The a) Langmuir adsorption isotherm plots of the ZIF-8@CS/CNF aerogel; the b) Freundlich adsorption isotherm plots of the ZIF-8@CS/CNF aerogel; the c) and d) is Langmuir adsorption isotherm plots and Freundlich adsorption isotherm plots of the CS/CNF composite, respectively

Table 2. Langmuir and Freundlich Parameters for the Adsorption Isotherms of the ZIF-8@CS/CNF Aerogel

The data listed in Table 3 demonstrate that the maximum adsorption capacity of the ZIF-8@CS/CNF aerogel for the removal of Cu(Ⅱ) was higher than recently reported in the literature.

To determine the feasibility and suitability of the ZIF-8@CS/CNF aerogel, the isotherm data separation factor (R_L) was calculated in terms of the Langmuir isotherm data. The separation factor equation can be seen below in Eq. 5,

(5)

If R_L ranges between 0 and 1, it is beneficial to the adsorption process. If R_L is greater than 1, it is disadvantageous. If R_L is equal to 0, the process is irreversible. The results shown in Table 2 indicated that the ZIF-8@CS/CNF aerogel had a beneficial ability to remove Cu(Ⅱ).

Table 3. Maximum Capacity of Various Adsorbents for Cu(II) in the Literature

The ZIF-8@CS/CNF aerogel was used to learn the adsorption kinetics, and Fig. 7a showed the adsorption behaviors of the aerogel under the initial concentration of 500 mg/L.

Fig. 7. The a) pseudo-first-order dynamics and pseudo-second-order dynamics of the ZIF-8@CS/CNF aerogel, the b) intraparticle diffusion of the ZIF-8@CS/CNF aerogel

As can be seen in Fig. 7a, the Cu(Ⅱ) adsorption process exhibited two stages (Qadeer 2013). First, the adsorption capacity increased rapidly during 0 ~ 60 minutes. Secondly, after 300 minutes, the adsorption was close to equilibrium. To explain the adsorption process and efficiency, the experimental results were analyzed based on the pseudo-first-order model (Eq. 6), the pseudo-second-order model (Eq. 7), and the intraparticle diffusion model (Eq. 8) (Shen et al. 2020),

(6)

(7)

(8)

where Q_t is the adsorption capacity (mg/g) at time t (min), k₁ is the rate constant of the pseudo-first-order adsorption kinetic model, k₂ is the rate constant of the pseudo-second-order adsorption kinetic model, and K_id (mg/(g·min^0.5)) is the intraparticle diffusion model rate constant.

The experimental data were fitted with nonlinear curves, which are shown in Fig. 7a. The fitted parameters can be seen in Table 4. The regression coefficients of pseudo-second-order model and pseudo-second-order model were more than 0.9, and comparison of data indicates that the R² of the pseudo-second-order model was better than the pseudo-first-order model. This suggested that the pseudo-second-order equation was consistent with the main kinetic process (Guo et al. 2019). According to the existence of -OH, -NH₂ and -NH- groups of ZIF-8@CS/CNF aerogels, which could be speculate that the adsorption process has both chemical and physical effects (Wang et al. 2019). To further illustrate the uptake process, the intra-particle diffusion model was considered. As can be seen in Fig. 8b and Table 4, R² of intraparticle diffusion model (0.86) express well fitted for the data. The fitting curve did not pass through the origin. This result indicated that intraparticle diffusion was not the only control step of adsorption.

Table 4. Kinetic Modeling Parameters for Adsorption of the ZIF-8@CS/CNF Aerogel

CONCLUSIONS

1. An aerosol of zeolitic imidazolate framework with nanofibrillated cellulose (ZIF-8@CS/CNF) for removing Cu(II) from its aqueous solution was synthesized successfully by a facile method.
2. The specific surface area of the CS/CNF composites increased after it was grafted with the ZIF-8, which enhanced the adsorption capacity of the composite aerogel.
3. The adsorption isotherm Cu(Ⅱ) on the ZIF-8@CS/CNF aerogel obeyed the Langmuir model, and the maximum uptake capacity was 234 mg/g.
4. The ZIF-8@CS/CNF aerogel has a great potential to be applied in adsorbing heavy metals and enlarges the scope of the green substrate for immobilizing ZIF-8 nanoparticles.

ACKNOWLEDGEMENTS

The authors would acknowledge the support from the Beijing Common Construction Project and Beijing Forestry University (Grant No. 2016HXKFCLXY0015).

Declaration of Interest Statement

The authors declare no competing financial interest.

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Article submitted: December 22, 2021; Peer review completed: February 5, 2022; Revised version received and accepted: March 9, 2022; Published: March 21, 2022.

DOI: 10.15376/biores.17.2.2615-2631