Mechanism and Chemical Stability of U(VI) Removal by Magnetic Fe3O4 @Biochar Composites

Biochar is typically made via pyrolysis of organic materials under anoxic conditions, due to its high surface area and significant negative charge. It also has the ability to minimize the mass or volume of waste items. As a result, biochar is frequently used in the remediation of environmental contamination. To overcome the shortcoming of biochar in the application, the magnetic Fe3O4@Biochar from walnut shells were prepared. The magnetic Fe3O4@Biochar from walnut shells is used to study the adsorption of U(VI) in the solution. SEM, XRD, and FT-IR are used to determine the properties of magnetic Fe3O4@Biochar. The results revealed that magnetic Fe3O4@Biochar has a fragmented and irregular form. On the surface of magnetic Fe3O4@Biochar, several functional groups can aid in the adsorption of pollutants. The adsorption capacity of U(VI) by magnetic Fe3O4@Biochar is influenced by the contact time and initial concentration of U(VI). For the adsorption of U(VI) in solution by magnetic Fe3O4@ Biochar, the pseudo-first-order kinetic equation and the Langmuir isotherm equation can be fitted. The adsorption of the process is chemical adsorption and monolayer adsorption. The chemical stability of magnetic Fe3O4@Biochar is very well.


INTRODUCTION
Uranium is a heavy metal element with radioactivity and high toxicity, which poses a great threat to the ecological environment and human health (Dong et al. 2017. During the various stages of nuclear power plants and nuclear accidents, a certain amount of uranium-containing radioactive wastewater will be generated (Vogel et al. 2010, Qiu et al. 2018. Therefore, the uranium-containing radioactive wastewater must be treated. Physical or physical-chemical approaches are used in the majority of traditional radioactive wastewater treatment procedures. Chemical precipitation, evaporation, ion exchange, and membrane separation are commonly used to treat low and medium concentrations of radioactive wastewater (Bhat et al. 2008, Qiu & Huang 2017, Hao et al. 2021.
In most circumstances, traditional radioactive wastewater treatment methods have a high removal efficiency. However, several issues remain, such as high chemical consumption, high energy consumption, rapid corrosion and scaling, and secondary pollution (Febrianto et al. 2009, Das 2012. The adsorption method has gradually attracted people's attention due to its characteristics of high efficiency, energy saving, environmental protection, and recyclability.
Because of the high surface area and considerable negative charge, biochar is ordinarily fabricated by the pyrolysis of organic materials under anoxic conditions (Zhang et al. 2013, Dai et al. 2021. Moreover, it can also reduce the mass or volume of waste materials (Han et al. 2016, Yao et al. 2021. Therefore, biochar is applied to the remediation of environmental pollution widely (Qiu and Huang 2017, Jang et al. 2018, Mohanty et al. 2018, Wang et al. 2020. The spectrum of raw materials available to identify affordable biochar resources has been expanding in recent years (Sun et al. 2015, Dutta & Nath 2018. Agricultural by-products and industrial waste, such as peanut shells, straws, walnut shells, and so on, are available in addition to standard high-quality wood and wood chips (Xu et al. 2014, Chang et al. 2017. The annual output of walnuts in China is more than 200,000 tons, and its shell and outer peel are almost discarded as waste. A large number of walnut shells were burned, causing great waste of resources and air pollution. Therefore, biochar is applied to the remediation of environmental pollution widely, such as heavy metals, organic pollution, and inorganic pollution . Biochar, on the other hand, is difficult to recycle in the application. As a result, magnetic biochar may be readily recycled, reused, and costs can be lowered. Vol. 21, No. 1, 2022 • Nature Environment and Pollution Technology detail according to the results of SEM, EDS, XRD, and FT-IR; 3) the adsorption experiments are also carried out; 4) the adsorption mechanism is elaborated according to adsorption kinetics and adsorption isotherm. This study is of great significance for new sources of biochar raw materials. It provides a new way for the resource utilization of crop wastes.

Materials
Walnut shells are obtained from a farm in the City of Linan, Zhejiang Province, P.R. China. Biochar was prepared using walnut shells. The walnut shells were washed three times with water and dried at 378 K to a consistent weight. Then they were pulverized and passed through a 20 meshes sieve. At a temperature of 523 K, 10 g of walnut shell is pyrolyzed for 2 h in a nitrogen atmosphere. It was pulverized through 100 meshes after cooling to room temperature. Then, the biochar from walnut shells is obtained.
1 g of biochar was added to 250 mL Erlenmeyer flask. Then, we added 100 mL of 1 mol.L -1 Fe -3+ and stirred for 30 min under ultrasonic conditions. Finally, we added 100 mL of 3 mol.L -1 NaOH solution and stirred for 60 min under ultrasonic conditions. They were washed with DMF until the supernatant was clear. Then, they were dried under vacuum at 80° for 12 h. The magnetic Fe 3 O 4 @Biochar was obtained for adsorption experiments.

Adsorption Experiment
Adsorption tests were carried out in a set of 250 mL Erlenmeyer flasks containing magnetic Fe 3 O 4 @Biochar and 100 mL U(VI), with initial concentrations in an aqueous solution. The flasks were shaken at 303 K and 150 rpm at a steady temperature. After that, the samples were filtered, and the residual U(V) concentration was determined.

Analytical Methods
The concentration of U(VI) ion in solution was measured with a UV-1600 spectrophotometer. The adsorption capacity of U(VI) and removal rate of U(VI) were calculated as follows: of small particles can be observed on the Where, C 0 and C e (mg.L -1 ) are the initial and equilibrium concentrations of U(VI) in solution respectively. q s (mg.g -1 ) is the adsorption amount per unit mass of the biochar at adsorption equilibrium. V (mL) is the volume of solution, m(g) is the mass of the biochar. R(%) is the removal rate of U(VI) ions in an aqueous solution.
SEM (Ultra 55), X-ray diffraction (Ulitma IV), and Fourier transform infrared spectroscopy spectra (Nicolet 5700) were used to study the physicochemical features of the magnetic Fe 3 O 4 @Biochar made from walnut shells.   preparation, the element of Fe can be determined on the surface of the magnetic Fe 3 O 4 @Biochar.

The Characteristic of the Magnetic Fe 3 O 4 @Biochar from Walnut Shells
The possible function groups of the magnetic Fe 3 O 4 @ Biochar are observed by FT-IR (Fig. 3A). There are seven peaks on the magnetic Fe 3 O 4 @Biochar. They are 3358, 2332, 1608, 1377, 1059, 569 and 405 cm -1 , respectively. They are assigned as -O-H, -C≡C-, -C=C-, -C-H, -C-C, and -C-H, respectively. It suggests that there are a large number of functional groups on the surface of the magnetic Fe 3 O 4 @ Biochar, which can facilitate the adsorption of pollutants. The results of the XRD pattern are shown in Fig. 3B. The characteristic peak of the magnetic Fe 3 O 4 @Biochar can be observed. It is 21.35 o . This result corresponds to previous studies.

Adsorption Experiment
Adsorption experiments are conducted in a set of 250 mL Erlenmeyer flasks. First, the effect of contact time on adsorption capacity is tested. Experimental conditions are followings: C 0 = 60 mg.L -1 , pH = 4.31, Temperature 308 K, rotating speed = 150 rpm, dosage of the magnetic Fe 3 O 4 @Biochar is 0.4 g. The results of the experiment are shown in Fig. 4A and 4B.
At the first stage of adsorption, the adsorption rate increases quickly. It may be the reason that there are a large number of adsorption sites on the surface of the magnetic Fe 3 O 4 @Biochar. Therefore, the adsorption rate increases very quickly. After 30 min, as the contact time increases, the adsorption rate increases slowly. It may be the reason that the adsorption sites on the surface of the magnetic Fe 3 O 4 @ Biochar begin to decrease. As the adsorption time continues to increase, adsorption gradually reaches adsorption equilibrium. The influence of initial solution concentration U(VI) on adsorption capacity is then tested. Experimental conditions are followings: contact time 360 min, pH = 4.31, Temperature 308 K, rotating speed = 150 rpm, the dosage of the magnetic Fe 3 O 4 @Biochar is 0.1 g. The results of the experiment are shown in Fig. 4B. As the adsorption time increases, the adsorption capacity gradually decreases. When the initial concentration of U(VI) in solution is 100 mg.L -1 , the adsorption capacity of U(VI) by the magnetic Fe 3 O 4 @ Biochar reaches 4.71 mg.g -1 .

Adsorption Kinetics
To describe the adsorption kinetic of U(VI) in solution by the magnetic Fe 3 O 4 @Biochar, pseudo-first-order kinetic equation and pseudo-second-order kinetic equation are used in this study. Their equations are as follows (Mellah et al. 2006, Troyer et al. 2016 indicates that Fe3O4 nanoparticles are loaded successfully. This result can be verified by the EDS (Fig. 2). As shown from Fig. 2A and 2B, the elements of C, O, K and Mg appeared on the surface of biochar. After preparation, the element of Fe can be determined on the surface of the magnetic Fe3O4@Biochar.    magnetic Fe3O4@Biochar is 0.1 g. The results of the experiment are shown in Fig. 4B.
As the adsorption time increases, the adsorption capacity gradually decreases. When the initial concentration of U(VI) in solution is 100 mg.L -1 , the adsorption capacity of U(VI) by the magnetic Fe3O4@Biochar reaches 4.71 mg.g -1 .

Adsorption Kinetics
To describe the adsorption kinetic of U(VI) in solution by the magnetic Fe3O4@Biochar, pseudo-first-order kinetic equation and pseudo-second-order kinetic equation are used in this study. Their equations are as follows (Mellah et al. 2006, Troyer et al. 2016): Where (mg.g -1 ) and (mg.g -1 ) are adsorption capacity of U(VI) in solution by the magnetic Fe3O4@Biochar at adsorption time t and adsorption equilibrium respectively. 1 (min -1 ) and 2 (min -1 ) are the adsorption rate constants.

The adsorption kinetic equation of U(VI) ion by the magnetic Fe3O4@Biochar from
walnut shells is shown in Fig. 5A and 5B. As shown in Fig. 5A and 5B, it can be concluded that the adsorption process can be ascribed with a pseudo-second-order kinetic equation (R 2 = 0.9997). It indicates that the adsorption process of U(VI) in solution by the magnetic Fe3O4@Biochar is chemical adsorption. Chemical adsorption is the most common form of adsorption.

…(4)
Where q t (mg.g -1 ) and q s (mg.g -1 ) are adsorption capacity of U(VI) in solution by the magnetic Fe 3 O 4 @Biochar at adsorption time t and adsorption equilibrium respectively. K 1 (min -1 ) and K 2 (min -1 ) are the adsorption rate constants. Fig. 5A and 5B. As shown in Fig. 5A and 5B, it can be concluded that the adsorption process can be ascribed with a pseudo-second-order kinetic equation (R 2 = 0.9997). It indicates that the adsorption process of U(VI) in solution by the magnetic Fe 3 O 4 @Biochar is chemical adsorption. Chemical adsorption is the most common form of adsorption.

Adsorption Isotherms
To describe the adsorption isotherm of U(VI) ion by the magnetic Fe 3 O 4 @Biochar, Langmuir isotherm equation and Freundlich isotherm equation are applied in this study. Their equations are follows (Freundlich 1906, Langmuir 1918):

Adsorption Isotherms
To describe the adsorption isotherm of U(VI) ion by the magnetic Fe3O4@Biochar, Langmuir isotherm equation and Freundlich isotherm equation are applied in this study.
Their equations are follows (Freundlich 1906, Langmuir 1918 The adsorption isotherm of U(VI) ion by the magnetic Fe3O4@Biochar from walnut shells is shown in Fig. 6. According to the value of R 2 , it can be suggested that Langmuir

Adsorption Isotherms
To describe the adsorption isotherm of U(VI) io

Langmuir isotherm equation and Freundlich isoth
Their equations are follows (Freundlich 1906, La The adsorption isotherm of U(VI) ion by the ma shells is shown in Fig. 6. According to the value of isotherm is more suitable for the adsorption of U Fe3O4@Biochar. The adsorption process is the ad Where q e (mg.g -1 ) is the adsorption capacity of U(VI) in solution by the magnetic Fe 3 O 4 @Biochar at adsorption equilibrium. q max (mg.g -1 ) is the maximum adsorption capacity of the adsorbent under specific adsorption conditions. K L (L.mg -1 ) and K F (L.mg -1 ) are the adsorption rate constants. C S (mg.L -1 ) is equilibrium concentrations of U(VI) in solution.
The adsorption isotherm of U(VI) ion by the magnetic Fe 3 O 4 @Biochar from walnut shells is shown in Fig. 6. According to the value of R 2 , it can be suggested that Langmuir isotherm is more suitable for the adsorption of U(VI) ions in solution by the magnetic Fe 3 O 4 @Biochar. The adsorption process is the adsorption of the monolayer.

Stability of Adsorption U(VI) Ions in Aqueous Solution
100 mL of 0.1 mol.L -1 NaOH and 100 mL of 0.01 mol.L -1 HCl were used to rinse the magnetic Fe 3 O 4 @Biochar. Then it's washed again with distilled water for 5 min. The magnetic Fe 3 O 4 @Biochar prepared in this way is used as adsorbents in recycling experiments. The repeated reusability of the magnetic Fe 3 O 4 @Biochar is displayed in Fig.7. It shows that the removal rate of U(VI) decreases from 65.56% to 49.14% during five adsorption and desorption experiments.   7 adsorption capacity is then tested. Experimental conditions are followings: contact time 360 min, pH = 4.31, Temperature 308 K, rotating speed = 150 rpm, the dosage of the magnetic Fe3O4@Biochar is 0.1 g. The results of the experiment are shown in Fig. 4B.
As the adsorption time increases, the adsorption capacity gradually decreases. When the initial concentration of U(VI) in solution is 100 mg.L -1 , the adsorption capacity of U(VI) by the magnetic Fe3O4@Biochar reaches 4.71 mg.g -1 .

Adsorption Kinetics
To describe the adsorption kinetic of U(VI) in solution by the magnetic Fe3O4@Biochar, pseudo-first-order kinetic equation and pseudo-second-order kinetic equation are used in this study. Their equations are as follows (Mellah et al. 2006, Troyer et al. 2016): Where (mg.g -1 ) and (mg.g -1 ) are adsorption capacity of U(VI) in solution by the magnetic Fe3O4@Biochar at adsorption time t and adsorption equilibrium respectively. 1 (min -1 ) and 2 (min -1 ) are the adsorption rate constants.
The adsorption kinetic equation of U(VI) ion by the magnetic Fe3O4@Biochar from walnut shells is shown in Fig. 5A and 5B. As shown in Fig. 5A and 5B, it can be concluded that the adsorption process can be ascribed with a pseudo-second-order kinetic equation (R 2 = 0.9997). It indicates that the adsorption process of U(VI) in solution by the magnetic Fe3O4@Biochar is chemical adsorption. Chemical adsorption is the most common form of adsorption.  The chemical stability of the magnetic Fe 3 O 4 @Biochar is good. isotherm is more suitable for the adsorption of U(VI) ions in solution by the magnetic Fe3O4@Biochar. The adsorption process is the adsorption of the monolayer.

Stability of Adsorption U(VI) Ions in Aqueous Solution
100 mL of 0.1 mol.L -1 NaOH and 100 mL of 0.01 mol.L -1 HCl were used to rinse the magnetic Fe3O4@Biochar. Then it's washed again with distilled water for 5 min. The magnetic Fe3O4@Biochar prepared in this way is used as adsorbents in recycling experiments. The repeated reusability of the magnetic Fe3O4@Biochar is displayed in