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Article

Synthesis of Nickel Cobaltite/Multiwalled Carbon Nanotubes Composites and Their Application for Removing Uranium (VI)

by
Xiaofei Zhang
1,
Binshan Ni
2,
Xiaoxuan Li
1,
Xin Guan
1,
Wandong Xia
1,
Jiabin Hao
1 and
Lichao Tan
3,4,*
1
Department of Chemical Engineering, Chengde Petroleum College, Chengde 067000, China
2
Department of Computer and Information Engineering, Chengde Petroleum College, Chengde 067000, China
3
Key Laboratory of Green Chemical Engineering and Technology of College of Heilongjiang Province, College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150080, China
4
Institute of Carbon Neutrality, Zhejiang Wanli University, Ningbo 315100, China
*
Author to whom correspondence should be addressed.
Crystals 2022, 12(12), 1712; https://doi.org/10.3390/cryst12121712
Submission received: 16 October 2022 / Revised: 13 November 2022 / Accepted: 21 November 2022 / Published: 25 November 2022
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Abstract

A facile hydrothermal method has been developed to prepare a nickel cobaltite/multiwalled carbon nanotubes (NiCo2O4/MWCNTs) composite. The structure and morphology of NiCo2O4/MWCNTs were tested by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscope, and nitrogen sorption isotherm. The nitrogen sorption isotherms of the NiCo2O4/MWCNTs composite indicate that NiCo2O4/MWCNT is a typical mesoporous material. The average pore width of NiCo2O4/MWCNTs is 10.0 nm. When serving as the adsorbent for uranium, the NiCo2O4/MWCNTs composite exhibits a high adsorption capacity, suggesting their potential use in water treatment. The influences of pH, temperature, and time on the adsorption efficiency of uranium by NiCo2O4/MWCNTs were studied. The experimental results show that the maximum adsorption capacity of NiCo2O4/MWCNTs for uranium(VI) is 247.4 mg g−1, suggesting that it is an effective uranium(VI) sorbent in water treatment. Thermodynamic analysis results show that the process is endothermic in nature. As the adsorption capacity does not increase significantly with the increase in T, the uranium adsorption by NiCo2O4/MWCNTs can be carried out at room temperature to reduce energy consumption. The NiCo2O4/MWCNTs composite can be a good alternative to remove uranium(VI).

1. Introduction

As the demand for energy increases, uranium exploitation has been strengthened, and the water pollution problem has become more serious. Uranium is a dangerous heavy metal because of its radiological and biological toxicity [1]. Therefore, it is urgent to take adequate measures to remedy uranium-contaminated wastewater [2].
Thus far, several methods have been used to treat uranium(VI) wastewater. However, most methods have different degrees of shortcomings [3,4,5]. The adsorption method has many advantages, such as high removal efficiency, environmental protection, and low cost, and it is considered to be an effective treatment method. Adsorbents like activated carbon [6], phosphate rock apatite [7], Hematite [8], zeolites [9], coir biomass [10], and maleic anhydride-allyl propionate-styrene terpolymer [11] have been investigated. The key to the adsorption method is high-efficiency adsorbents, so researchers are looking for new high-efficiency adsorbents or improving the performance of existing adsorbents.
In recent decades, multiwalled carbon nanotubes (MWCNTs) have been investigated in photo electronics, catalysis degradation, adsorption, electric heaters, and chemical/biological sensing due to their large specific surface area, rich surface chemical functionalities, and nanosized stability [12,13,14,15,16,17,18,19,20,21]. MWCNTs are considered potential adsorbents. Many studies have reported the adsorption potential of MWCNTs, such as perfluorinated compounds, inorganic pollutants, and organic vapors [22,23].
However, due to the high van der Waals interaction between MWCNTs, the dispersion of MWCNTs in solvents is poor, which seriously hinders their application. It is an effective method to modify MWCNTs by introducing inorganic nanoparticles, improving their dispersibility and properties. Bang et al. [24] composited CuO/Cu2O nanoparticles with multi-walled carbon nanotubes and studied their sensing performance of H2S gas. Madihi-Bidgoli et al. [25] loaded Fe2O3 on multi-wall carbon nanotubes and applied it to activate PMS under UVA-LED irradiation.
NiCo2O4, as one of the binary metal oxides, is a promising material owing to its low cost, abundant resources, good electronic conductivity, and environmental friendliness [26,27,28]. Therefore, NiCo2O4 has been applied in supercapacitors, Li-ion batteries, degradation, and transparent conductive films [29,30,31,32]. Besides, NiCo2O4 was proved to be the most effective adsorbent in the family of MCo2O4 (M = Cu, Zn, Mn, Ni). Bao et al. [33] reported that NiCo2O4 nanosheets had good adsorption performance for Congo red.
This work reported a simple hydrothermal preparation of a NiCo2O4/MWCNTs composite. The obtained NiCo2O4/MWCNTs composite material was used to remove uranium, and the experimental results show that the NiCo2O4/MWCNTs composite material has a good removal effect on uranium. Meanwhile, the key parameters, thermodynamics, and kinetics of uranium adsorption by the NiCo2O4/MWCNTs composite were also studied.

2. Materials and Methods

2.1. Synthesis of NiCo2O4/MWCNTs Samples

First, 30 mg MWCNTS were dispersed in 40 mL deionized water and treated with ultrasound for 10 min. Then, 75 mg of nickel nitrate hexahydrate, 150 mg of cobalt nitrate hexahydrate, 7.25 mg of trisodium citrate, and 35 mg of hexamethylenetetramine (HMT) were added to the above solution and reacted at 90 °C for 6 h. The sediment was collected, cleaned with ethanol, and finally dried for one day at 60 °C. The prepared NiCo-precursor NSs/MWCNTs was then calcined for 2 h under nitrogen protection in a muffle furnace at 300 °C to produce NiCo2O4/MWCNTs [34]. In addition to the addition of multi-walled carbon nanotubes, and the NiCo2O4 were synthesized using the same steps. All the chemical reagents were purchased from Tianjin Kermel Chemical Reagents Development.

2.2. Adsorption Experiments

Next, 0.01 g of NiCo2O4/MWCNTs composite was added to 25 mL of the metal nitrate solution. Before and after the adsorption, the concentration of uranium ions was measured by Bruker 820-MS ICP-MS instrument (Bruker Corporation, Billerica, MA, USA). The pH was adjusted with a dilute solution of sodium hydroxide or nitric acid.

2.3. Characterization

The structures and morphology of NiCo2O4/MWCNTs composite were characterized via an X-ray diffraction analysis (XRD, Rigaku D/max-IIIB diffractometer with Cu Kα irradiation, λ = 1.54178 Ǻ, Rigaku Corporation, Tokyo, Japan), X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi with Al Kα radiation, Thermo Scientific Corporation, Waltham, MA, USA), transmission electron microscopy (TEM, FEI Tecnai G2 20 S–TWIN, FEI Company, Hillsboro, OR, USA), and nitrogen adsorption isotherms (Micromeritics ASAP 2010 analyzer, Micromeritics Company, Norcross, GA, USA).

3. Results and Discussion

3.1. Characterization of the NiCo2O4/MWCNTs Composite

Figure 1 shows the XRD test results of MWCNTs, NiCo2O4, and NiCo2O4/MWCNTs. MWCNTs have dominant peaks centered at 26.0 and 42.9°, corresponding to the (002) and (100), respectively, which is attributed to the hexagonal graphite structures of CNTs [35]. For pure NiCo2O4 and NiCo2O4/MWCNTs, well-defined diffraction peaks of NiCo2O4 (2θ = 20.8, 36.7, 44.6°) of both samples were observed to correspond to their indices ((111), (311) and (400)). The diffraction peaks of the NiCo2O4 are in good accordance with spinel NiCo2O4 (JCPDS No: 20-0781) [36]. In the case of NiCo2O4/MWCNTs, the diffraction peaks are observed at 26.0 and 42.9°, corresponding to (002) and (100) peaks of MWCNTs. This result confirms the existence of MWCNTs after the surface modification step.
The morphology of the NiCo2O4/MWCNTs composites was determined by TEM analysis, as shown in Figure 2. Figure 2 shows that NiCo2O4 nanosheets grow on MWCNTs. The size of NiCo2O4 nanosheets is approximately 5–10 nm. The high-resolution TEM image (Figure 2C) shows NiCo2O4 with clear lattice fringes. The measured fringe spacing is about 0.242 nm, which can be well fitted with the theoretical interplanar spacing of spinel NiCo2O4 (311) planes. Furthermore, the selected area electron diffraction (SAED) pattern of NiCo2O4/MWCNTs composite (Figure 2D) shows well-defined rings of (311), (400), and (440), indicating the polycrystalline nature of the modified NiCo2O4 nanoparticles (JCPDS No: 20-0781).
The NiCo2O4/MWCNTs composite was further characterized by XPS. Figure 3A is the XPS survey spectra of NiCo2O4/MWCNTs. The O 1s, C 1s, Co 2p, and Ni 2p core photoionization signals are displayed in the XPS survey scan spectrum. In specification, the peak at 284.9 eV should derive from the honeycomb carbon ring in the MWCNTs. The Ni 2p spectrum (Figure 3B) displays two spin-orbit doublets for Ni 2p3/2 at 854.9 eV and Ni 2p1/2 at 873.6 eV, each accompanied by a shake-up satellite at 861.7 and 880.0 eV, respectively [37]. The Co 2p spectrum (Figure 3C) shows four peaks at 780.8, 796.4, 786.2, and 803.2 eV, corresponding to Co 2p3/2, Co 2p1/2, and their shake-up satellite. These results align with the XRD result, indicating that there is the existence of NiCo2O4 nanocrystals on the surface of MWCNTs.
Figure 4 depicts the nitrogen sorption isotherms of the NiCo2O4/MWCNTs composite. The specific surface area of NiCo2O4/MWCNTs is 81.0 m2 g−1. The curve is a typical type IV isotherm, and the hysteresis ring is type H3, indicating that NiCo2O4/MWCNT is a typical mesoporous material. Moreover, it can also be seen from Figure 4B that the material has a mesoporous structure. The average pore widths of NiCo2O4/MWCNTs were determined as 10.0 nm. This result indicates the formation of a porous structure, which will help improve the adsorption function.

3.2. Uranium(VI) Sorption by the NiCo2O4@MWCNTs Composite Experiments

3.2.1. pH Influence

As shown in Figure 5, the pH value is an essential parameter for uranium(VI) adsorption by NiCo2O4/MWCNTs. When the pH value increases from 2 to 10, the qe of NiCo2O4/MWCNTs to uranium (VI) increases and decreases. The maximum occurs at pH 5. This phenomenon is because, at low pH, the solution contains many hydrogen ions, which compete for adsorption with positively charged uranium species, resulting in the reduction of uranium (VI) adsorbed by NiCo2O4/MWCNTs. With the increase in pH value, the surface of the adsorbent is deprotonated, thus improving the adsorption capacity of NiCo2O4/MWCNTs for uranium(VI). The different forms of uranium in the solution have great influence on the adsorption performance. The concentration of hydroxide and dissolved carbonate ions at high pH increases gradually. Uranium(VI) is combined with hydroxide and dissolved carbonate in anion form (UO2(CO3)34−, (UO2)2CO3(OH)3− and UO2(OH)3−, etc.), and the adsorbent surface is negatively charged, resulting in reduced adsorption capacity [38,39,40].

3.2.2. Effect of Contact Time

In the kinetic study, the change curve of the adsorption performance of NiCo2O4/MWCNT with time was studied, as given in Figure 6. The adsorption rate of NiCo2O4/MWCNTs to uranium is high, indicating that the adsorbent has a strong adsorption influence on uranium (VI). With the adsorption sites occupied on the composite, the adsorption tends to be balanced.
In order to further explore the process of uranium adsorption by NiCo2O4/MWCNT, the kinetic model was studied. The fitting diagram of uranium adsorbed by NiCo2O4/MWCNT and the calculated results are shown in Figure 7 and Table 1.
The curve of t/qt versus t is linear with R2 of 0.99, indicating that it is suitable to explain the uranium (VI) adsorption on NiCo2O4/MWCNTs. The adsorption process is consistent with the pseudo-second-order kinetic equation. The rate control step of uranium adsorption by NiCo2O4/MWCNTs might be electrostatic interaction between NiCo2O4/MWCNTs and uranium (VI) [41].

3.2.3. Effect of Temperature

Figure 8 shows the curves of uranium adsorption by NiCo2O4/MWCNTs at different temperatures of 25, 35, and 45 °C. The qe of NiCo2O4/MWCNTs on uranium(VI) increased with the temperature increase from 25 to 45 °C, suggesting that the process of uranium adsorption by NiCo2O4/MWCNTs is endothermic. As the adsorption capacity does not increase significantly with the temperature rising from 25 to 45 °C, it can be adsorbed at room temperature to reduce energy consumption.

3.2.4. Adsorption Isotherms

To investigate the isothermal adsorption model of NiCo2O4/MWCNTs, the process of uranium adsorption by NiCo2O4/MWCNTs was further analyzed by the Langmuir adsorption isothermal model (Figure 9).
Parameters were calculated according to the linear graph, and the relevant parameters were displayed in Table 2. From the information in Table 2 and Figure 9, the adsorption of uranium(VI) by NiCo2O4/MWCNTs conforms to the Langmuir model, indicating that uranium(VI) adsorption by NiCo2O4/MWCNTs is monolayer adsorption. The qm of NiCo2O4/MWCNTs on uranium(VI) was 253.8 mg g−1 at 25 °C.

3.2.5. Comparison of Adsorbent Performance

To evaluate the adsorption efficiency of NiCo2O4/MWCNTs, the capacity of NiCo2O4/MWCNTs was compared with those of other adsorbents reported in the literature. Table 3 summarizes previous research on adsorbents for uranium(VI) adsorption [3,6,42,43,44,45,46]. Compared with other adsorbents reported, the qe of NiCo2O4/MWCNTs for uranium(VI) in this study was relatively large. This result indicates that the NiCo2O4/MWCNTs are promising adsorbents for water purification.

4. Conclusions

In conclusion, we demonstrated a simple solvothermal method for preparing NiCo2O4/MWCNTs composites. The nitrogen sorption isotherms showed that the NiCo2O4/MWCNTs had a mesoporous structure. The average pore widths of NiCo2O4/MWCNTs were determined as 10.0 nm. This result indicates the formation of a porous structure, which will help improve the adsorption function. The qe of NiCo2O4/MWCNTs on uranium(VI) was 247.4 mg g−1, showing good adsorption performance. Thermodynamic studies show that the process of uranium adsorption by NiCo2O4/MWCNTs is endothermic. As the adsorption capacity does not increase significantly with the temperature rising from 25 to 45 °C, it can be adsorbed at room temperature to reduce energy consumption. The removal rate of uranium(VI) on NiCo2O4/MWCNTs is fast, and the adsorption process is consistent with the pseudo-second-order kinetic equation. This study shows that NiCo2O4/MWCNTs are a promising material for removing uranium (VI).

Author Contributions

Conceptualization, X.Z. and L.T.; methodology, X.Z. and L.T.; investigation, B.N., X.L. and J.H.; data curation, X.G. and W.X.; writing—original draft preparation, X.Z. and B.N.; writing—review and editing, X.L. and J.H.; supervision, X.G. and W.X.; project administration, X.Z.; funding acquisition, X.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Science Foundation of Hebei Province, China, grant number E2022411007.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We wish to express our gratitude to the members of our research team, Xiaofei Zhang, Binshan Ni, Xiaoxuan Li, Xin Guan, Wandong Xia, Jiabin Hao, and Lichao Tan.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 12 01712 g001
Figure 1. XRD test results of MWCNTs, NiCo2O4, and NiCo2O4/MWCNTs.
Figure 1. XRD test results of MWCNTs, NiCo2O4, and NiCo2O4/MWCNTs.
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Crystals 12 01712 g002
Figure 2. (A,B) TEM, (C) HRTEM, and (D) SAED pattern of NiCo2O4/MWCNTs.
Figure 2. (A,B) TEM, (C) HRTEM, and (D) SAED pattern of NiCo2O4/MWCNTs.
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Crystals 12 01712 g003aCrystals 12 01712 g003b
Figure 3. (A) XPS survey spectra of NiCo2O4/MWCNTs, (B,C) high-resolution spectra of Ni 2p and Co 2p.
Figure 3. (A) XPS survey spectra of NiCo2O4/MWCNTs, (B,C) high-resolution spectra of Ni 2p and Co 2p.
Crystals 12 01712 g003aCrystals 12 01712 g003b
Crystals 12 01712 g004
Figure 4. (A) N2 sorption isotherms and (B) pore size distributions of NiCo2O4/MWCNTs.
Figure 4. (A) N2 sorption isotherms and (B) pore size distributions of NiCo2O4/MWCNTs.
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Crystals 12 01712 g005
Figure 5. Variation curve of uranium (VI) adsorption capacity of NiCo2O4/MWCNTs with pH. temperature, 25 °C; C0, 150 mg L−1; pH, 2.0–10.0; the mass of NiCo2O4/MWCNTs, 0.01 g.
Figure 5. Variation curve of uranium (VI) adsorption capacity of NiCo2O4/MWCNTs with pH. temperature, 25 °C; C0, 150 mg L−1; pH, 2.0–10.0; the mass of NiCo2O4/MWCNTs, 0.01 g.
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Crystals 12 01712 g006
Figure 6. The change curve of adsorption performance of NiCo2O4/MWCNTs with time. Temperature, 25 °C; C0, 150 mg L−1; pH, 5.0; the mass of NiCo2O4/MWCNTs, 0.01 g.
Figure 6. The change curve of adsorption performance of NiCo2O4/MWCNTs with time. Temperature, 25 °C; C0, 150 mg L−1; pH, 5.0; the mass of NiCo2O4/MWCNTs, 0.01 g.
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Figure 7. The kinetic fitting diagram of uranium adsorbed by NiCo2O4/MWCNT at 25 °C.
Figure 7. The kinetic fitting diagram of uranium adsorbed by NiCo2O4/MWCNT at 25 °C.
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Crystals 12 01712 g008
Figure 8. The curves of uranium adsorption by NiCo2O4/MWCNTs as a function of temperature. pH, 5.0; temperature, 25–45 °C; the mass of adsorbents, 0.01 g.
Figure 8. The curves of uranium adsorption by NiCo2O4/MWCNTs as a function of temperature. pH, 5.0; temperature, 25–45 °C; the mass of adsorbents, 0.01 g.
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Crystals 12 01712 g009
Figure 9. The fitting diagram of NiCo2O4/MWCNT isothermal adsorption model. pH, 5.0; temperature, 25–45 °C; the mass of adsorbents, 0.01 g.
Figure 9. The fitting diagram of NiCo2O4/MWCNT isothermal adsorption model. pH, 5.0; temperature, 25–45 °C; the mass of adsorbents, 0.01 g.
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Table 1. Kinetic parameters of uranium adsorption by NiCo2O4/MWCNTs.
Table 1. Kinetic parameters of uranium adsorption by NiCo2O4/MWCNTs.
Kinetic ModelT (°C)qe (exp)
(mg g−1)		qe (cal)
(mg g−1)		k2ads (g mg−1 min−1)R2
Pseudo-second order25247.4254.52.65 × 10−40.99
Table 2. Langmuir constants of NiCo2O4/MWCNTs.
Table 2. Langmuir constants of NiCo2O4/MWCNTs.
TempLangmuir
(°C)b (L mg−1)qm (mg g−1)R2
250.965253.80.99
351.369259.10.99
451.611265.30.99
Table 3. Comparison of uranium adsorption performance of NiCo2O4/MWCNT and other adsorbents.
Table 3. Comparison of uranium adsorption performance of NiCo2O4/MWCNT and other adsorbents.
AdsorbentsCapacity (mg g−1)Ref.
Activated carbon (Merck)28.30[6]
GO-S197.5[42]
Carboxylate-functionalized poly(hydroxy ethylmethacrylate)-grafted lignocellulosics109.6[43]
Polypyrrole87.72[3]
Ni(OH)2-loaded Amberlite IR120439[44]
Co(OH)2-loaded Amberlite IR120451[44]
Amorphous silica58[45]
Hematite3.36[46]
NiCo2O4/MWCNTs247.4This work
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Zhang, X.; Ni, B.; Li, X.; Guan, X.; Xia, W.; Hao, J.; Tan, L. Synthesis of Nickel Cobaltite/Multiwalled Carbon Nanotubes Composites and Their Application for Removing Uranium (VI). Crystals 2022, 12, 1712. https://doi.org/10.3390/cryst12121712

AMA Style

Zhang X, Ni B, Li X, Guan X, Xia W, Hao J, Tan L. Synthesis of Nickel Cobaltite/Multiwalled Carbon Nanotubes Composites and Their Application for Removing Uranium (VI). Crystals. 2022; 12(12):1712. https://doi.org/10.3390/cryst12121712

Chicago/Turabian Style

Zhang, Xiaofei, Binshan Ni, Xiaoxuan Li, Xin Guan, Wandong Xia, Jiabin Hao, and Lichao Tan. 2022. "Synthesis of Nickel Cobaltite/Multiwalled Carbon Nanotubes Composites and Their Application for Removing Uranium (VI)" Crystals 12, no. 12: 1712. https://doi.org/10.3390/cryst12121712

APA Style

Zhang, X., Ni, B., Li, X., Guan, X., Xia, W., Hao, J., & Tan, L. (2022). Synthesis of Nickel Cobaltite/Multiwalled Carbon Nanotubes Composites and Their Application for Removing Uranium (VI). Crystals, 12(12), 1712. https://doi.org/10.3390/cryst12121712

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