Synthesis of zeolite X from waste basalt powder and its efficient adsorption of uranyl ions in solution

A new type of zeolite X with high purity and crystallinity was prepared from waste basalt powder by alkaline hydrothermal fusion and its adsorption performance as an adsorbent on uranium-containing wastewater was investigated. The results show that the equilibrium adsorption capacity of zeolite X for uranyl ions is 228.4 mg g−1. The adsorption process is spontaneous and exothermic. Langmuir isotherm model and quasi-second-order model are suitable for describing the adsorption process. Studies on the adsorption mechanism have shown that there is an ion exchange between UO22+ and zeolite during the adsorption process.


Introduction
With the rapid economic growth, people consume more and more energy [1,2]. Nuclear energy has been vigorously developed as a clean and efficient green energy source [3,4]. Uranium is a major component of nuclear fuel, and large amounts of uranium-containing wastewater are generated in uranium mining and smelting [5,6]. This radioactive uranium-containing wastewater would pose a serious hazard to humans and the environment if it is discharged directly into the environment without treatment [7][8][9]. Therefore, how to treat uranium-containing wastewater in an efficient manner has become a focus of attention. At present, there are many methods to treat uranium-containing wastewater, such as chemical precipitation [10,11], evaporation [12], ion exchange [13], condensation-flocculation [14], extraction [15], membrane permeation [16], electrochemistry [17], and adsorption [18]. Among the above methods, adsorption method has become an important research direction for the treatment of uranium-containing wastewater because of its easy operation, low price and high efficiency [19][20][21].
As a silica-aluminate mineral, zeolite has a unique pore structure, large specific surface area, strong ion exchange capacity, and high thermal and hydrothermal stability. As an excellent adsorbent material, zeolites play a great role in environmental treatment. As demand increases, the current production of mined zeolite is struggling to meet the demands of production, coupled with the increasingly high cost of raw materials [22]. This has limited its industrial application. Basalt is abundant in China, and a large amount of basalt waste powder is produced during the mining process [23]. Using basalt waste powder as a raw material to synthesize zeolite can greatly reduce production costs, increase the potential economic value and achieve the goal of treating waste with waste [24]. The main methods for synthesizing zeolites are hydrothermal, Alkali fusion method, and microwave heating. The factors affecting the synthesis of zeolites are mainly the type of raw material, the reaction temperature, the reaction time, and the type and amount of alkali chosen to be added.
In this study, high purity zeolite X was prepared by alkaline hydrothermal fusion (AFH) using basalt waste powder as raw material and used as an adsorbent to treat uranium-containing wastewater. The aim was to combine the two methods, hydrothermal and alkaline melting. The effects of parameters such as pH, C 0 (Initial concentration of uranyl ions), T(Reaction temperature), m(Adsorbent 1 3 dosage), and t(Adsorption time) on the adsorption performance of the prepared zeolite X for uranium ions were investigated. The thermodynamics and kinetics of the adsorption process were also investigated and the adsorption mechanism was described.
X-Max EDS instrument (Oxford) was used to analyze the element content of the zeolite X. A Nicolet-460 Fourier Transform Infrared Spectrometer (FTIR) (Thermo Fisher Science, USA) was used to analyze the zeolite X in the range of 4000-400 cm −1 . The T6 Xinrui visible spectrophotometer (Beijing General Analytical Instrument Co., Ltd.) was used to determine the concentration of uranium in the solution. X-ray diffraction (XRD) (Kα radiation, Cu target, Rigaku company, Japan) was performed on the zeolite X using D/Max-RB X-ray diffractometer. A scanning electron microscope (SEM) (JSM-7500F SEM analyzer) was used to characterize the shape of the zeolite X. X-ray fluorescence (XRF) (Axios PW4400, Holland) was used to analyze the chemical composition of Waste basalt powder.

Synthesis of zeolite X
Basalt powder (10.0 g) and NaOH (10.0 g) were homogeneously mixed in a crucible and calcined in a muffle furnace at 560 ℃ for 3 h for alkali fusion treatment. The cooled sample was ground to a powder using a mortar and pestle. The powder and 50 mL of deionized water were added to a flask and stirred magnetically for 10 h. The mixture was placed in a hydrothermal reactor lined with PTFE and crystallized at 120 ℃ for 12 h, then cooled naturally to room temperature. The crystallized product was washed with deionized water to neutralize. After washing and filtration, the filter cake is placed in a surface dish and dried in a drying oven at 60 ℃ for 12 h to give zeolite X.

Preparation of the uranium standard solution, buffer solution, and arsenazo III color reagent
Dilute uranium nitrate hexahydrate (2.1091 g) to 1 L with deionized water to obtain 1 g L −1 uranium standard solution. Dilute chloroacetic acid (4.7241 g) and sodium acetate (4.1055 g) with deionized water to a 250 mL volumetric flask to obtain chloroacetic acid-sodium acetate buffer. Dilute Arsenazo III (0.1250 g) deionized water into a 250 mL volumetric flask to obtain Arsenazo III color reagent.

Adsorption process
Add a certain volume of uranium standard solution into the Erlenmeyer flask, and add deionized water to make the initial concentration of the uranium solution 35 mg L −1 . Add a certain mass (2-14 mg) of sorbent to a conical flask containing 35 ml of U(VI) solution at the designed concentration. Adjust the pH of the solution to 3.0-10.0 with dilute nitric acid or sodium hydroxide solution. The above solution is shaken at a certain temperature (25-45 °C) in a water bath shaker to carry out the adsorption reaction. After adsorption for some time (30-840 min), the solution supernatant was separated. The supernatant of the solution was filtered through a 0.45 μm microporous membrane and the concentration of uranyl ions was determined using a visible spectrophotometer. The details are as follows: 0.5 mL test solution is placed in a cuvette, 0.5 mL arsenazo III colorimetric agent is added, and then chloroacetic acid-sodium acetate is added to make the volume to 5 mL, and it is allowed to stand for 5-10 min. The absorbance was measured at 652 nm with a visible spectrophotometer.
The adsorption capacity of Zeolite X for uranyl ions (Q) and the removal rate of uranyl ions in solution (R) were calculated as follows: where Q is the adsorption capacity of Zeolite X for uranyl ions (mg g −1 ), C 0 (mg L −1 ) and C e (mg L −1 ) are the initial concentration and equilibrium concentration of UO 2 2+ in the solution, respectively, and V (mL) is the volume of the solution, m (g) is the dosage of zeolite X, R is the removal rate of uranyl ions in solution.
To investigate the reusability of zeolite X, we carried out desorption experiments. The loaded UO 2 2+ adsorbent after the above adsorption experiment was collected with a centrifuge, and the 30 mL of 2.0 mol L −1 NaOH was carried out as the eluent to desorb UO 2 2+ from the loaded UO 2 2+ zeolite X adsorbent. The supernatant after each adsorption experiment uranium content was checked with the T6 Xinrui visible spectrophotometer, and repeat the above adsorption experiment.

Chemical composition of waste basalt powder
In this study, the XRF method was used to determine the chemical composition of waste basalt powder. The results are shown in Table 1, indicating that the oxides of Si and Al in the sample accounted for 47.90% and 19.17%, respectively. They are the basic components of silicon and aluminum needed for the synthesis of zeolite. The above explanation shows that zeolite can be prepared by using basalt powder as raw material. Table 2 shows the BET characterization results of the samples. It shows that the specific surface area of synthetic zeolite X is 623.395 m 2 g −1 , indicating that zeolite X prepared from basalt has a large specific surface area and has good adsorption potential.

SEM analysis
The microstructure of zeolite X was characterized by SEM. Figure 1a is the SEM picture of zeolite X. Figure 1a shows that zeolite X has a polyhedral structure with overall integrity and smooth crystalline surfaces. Figure 1b shows an SEM image of zeolite X after adsorption of uranyl ions. It can be seen that zeolite X can still maintain its intact polyhedral structure after adsorption, but the crystalline surface becomes rough.

XRD analysis
In Fig. 2, curve a is the XRD pattern of zeolite X, and curve b is the XRD pattern of zeolite X adsorbing uranyl ions. As can be seen from curves a, the XRD diffraction peaks of the samples show ten characteristic diffraction peaks (  purity and complete structure can be successfully prepared by alkali melting method using waste basalt powder as raw material. Compared with curve a, curve b has two characteristic peaks (220) (311) at 2θ of 46.98° and 55.72°. Comparing the two characteristic peaks with the uranium standard card (JCPDS: 41-1422), the characteristic peak of UO 2 indicates that the prepared zeolite X successfully adsorbed uranyl ions. At the same time, at 2θ of 10.02° (220), 11.78° (311), 23.32° (533), 33.54° (664), the intensity of the characteristic peaks decreased. This may be due to the large amount of uranium attached to the surface of the material. The scattering between the hole walls is reduced, and the characteristic peak intensity is weakened. Curve a is the FT-IR spectrum of zeolite X, and curve b is the FT-IR spectrum of zeolite X adsorbing uranyl ions. In curve a, the characteristic peak at 3463.85 cm −1 corresponds to the O-H tensile vibration peak, the characteristic peak at 1635.30 cm −1 corresponds to the bending vibration peak of adsorbed water, the characteristic peak at 1464.23 cm −1 corresponds to the -COOH vibration peak, the characteristic peak at 975.56 cm −1 corresponds to the peak of asymmetric stretching vibration in the tetrahedron, the characteristic peak at 564.92 cm −1 corresponds to the vibration peak of the double six-membered ring bond. Comparing curve a and curve b in Fig. 3, it can be seen that the O-H tensile vibration peak moved from 3463.85 cm −1 to 3452.01 cm −1 , and the -COOH vibration peak moved from 1464.23 cm −1 to 1421.30 cm −1 . The reason for the change in peak position may be that after zeolite X adsorbs uranyl ions, the H + originally bound to the active site is replaced by uranyl ions, which changes the chemical composition of zeolite X, leading to changes in the peak positions of O-H and -COOH Figure 4 shows the results of the EDS analysis of the samples. It shows that the atomic percentage of the O element in zeolite X is 51.36%, the atomic percentage of Al is 4.15%, and the atomic percentage of Si is 5.39%, indicating that zeolite X is mainly composed of O, Al and Si elements. After the adsorption experiment, the atomic percentage of uranium in the samples obtained reach 2.91%. Combined with the XRD analysis results, the successful adsorption of uranyl ions by zeolite X was confirmed.

Effect of adsorbent dosage
Under the condition that the reaction temperature (T) was 30℃, the reaction time (t) was 720 min, pH was 6, and the initial concentration of uranyl ions (C 0 ) was 35 mg L −1 , the volume of uranium-containing wastewater (V) was 35 mL, the effect of m on the adsorption of uranyl ions by zeolite X was investigated, as shown in Fig. 5. R increases with the increase of m, and the change of Q is exactly the opposite to that of R, and Q decreases with the increase of m.
When m is 5.0 mg, R is 91.20% and Q is 219.07 mg g −1 . When m is 11.0 mg, R is 96.49% and Q is 105.53 mg g −1 . This is mainly because the content of uranyl ions in the uranyl solution is constant, with the increase of m, the number of effective adsorption active sites will increase, and R will increase. However, the number of uranyl ions adsorbed by zeolite X per unit mass decreased, which resulted in a decrease of adsorbent adsorption per unit mass. Although R increased with the increase of m, it was slow. Conversely, the adsorption capacity of adsorbent per unit mass decreased rapidly. From the optimal point of view, m of 5.0 mg is chosen as the optimal dosage in 35 mL uranium-containing wastewater.

Effect of the initial concentration of uranyl ions
Under the condition that T was 30℃, t was 720 min, pH was 6, and m was 5 mg, V was 35 mL, the effect of C 0 on the adsorption of uranyl ions by zeolite X was investigated, the results are shown in Fig. 6.
As can be seen from Fig. 6, when C 0 is low, the uranyl ions in the solution are completely absorbed by the zeolite X. With the continuous increase of C 0 , the active adsorption sites on zeolite X are constantly occupied. When the uranyl ion concentration reaches 60 mg L −1 , the maximum adsorption capacity of zeolite X for uranyl ions reaches 225.83 mg g −1 . Uranium-containing wastewater from China Nuclear 272 Uranium Industry Co., Ltd. was studied. The main pollutant in the wastewater was the radionuclide uranium, of which the main anions included Cl − , CO 3 2− and SO 4 2− . The investigation found that the mass concentration of uranyl ions in the untreated uranium-containing wastewater discharged from the factory was about 35 mg L −1 . Therefore, in the follow-up study, the uranium-containing wastewater with C 0 = 35 mg L −1 was selected as the research object.

Effect of pH
The effect of pH on the adsorption of uranyl ions by X zeolite was investigated at T of 30 °C, t of 720 min, C 0 of 35 mg L −1 , m of 5 mg, and V of 35 mL. To investigate the surface charge properties of zeolite X, zeta potential analysis was performed and the results are shown in Fig. 7. When the pH of the solution increases from 3.0 to 6.0, Q increases with the increase of pH, but Q decreases with the increase of the pH in the range of 6.0 to 10.0. This is because when the pH < 4, U(VI) mainly exists in the form of UO 2 2+ and UO 2 OH + , and the functional groups on the surface of the zeolite X are protonated, which makes the zeolite X positively charged, and there is a strong electrostatic repulsion between the two, which weakens the adsorption force of the material [25]. The zeta potential analysis likewise shows that at lower pH the surface of zeolite X is positively charged. In the process of increasing the pH to 6, U(VI) will be hydrolyzed to form UO 2 (OH) + and (UO 2 ) 3 (OH) 5 + , and the positive charge on the surface of the zeolite X begins to decrease. At this time, the complexation reaction of U (VI) with the functional groups on the surface of the zeolite X. With the further increase of pH, the concentration  5+ , which reduce the adsorption capacity of the adsorbent to uranyl ions. Similarly, zeta potential analysis confirms this experimental result. Hence, the solution pH of 6 was chosen as the optimal reaction condition [26].

Effect of adsorption time
Under the condition that T was 30℃, pH was 6, C 0 was 35 mg L −1 , and m was 5 mg, V was 35 mL, the effect of t on the adsorption of uranyl ions by zeolite X was investigated, the results are shown in Fig. 8. Q increases rapidly with the increase of t for the first 6 h. After that, R became slower and slower. The adsorption time increases from 720 to 840 min while the amount of adsorption remains almost constant, therefore at 720 min, the adsorption can be considered to have reached equilibrium. At this time, the maximum adsorption capacity is 220.93 mg g −1 , and R is 93.78%.

Effects of temperature
Under the condition that t was 720 min, pH was 6, C 0 was 35 mg L −1 , and m was 5 mg, V was 35 mL, the effect of T on the adsorption of uranyl ions by zeolite X was investigated, the results are shown in Fig. 9. As can be seen from Fig. 9, when T is 25-30 ℃, Q increases with the increase of T. When T is 30 ℃, R is 90.69% and Q is 222.18 mg g −1 . After that, with the further increase of T, Q and R decreased rapidly. Hence, 30 ℃ were chosen as the optimal reaction temperature.

Adsorption thermodynamics
The thermodynamic parameters are calculated by formulas (3)(4)(5). where: Q e is the adsorption capacity at equilibrium, mg g −1 . K d is the adsorption distribution coefficient, L/mg. C e is the mass concentration at equilibrium, mg L −1 . T is the thermodynamic temperature, K. R is the ideal gas constant, 8.314 J/(mol·K) −1 . ΔH is the change in enthalpy, kJ mol −1 . ΔG is the Gibbs free energy, kJ mol −1 . ΔS is the change in entropy, J/(mol·K) −1 . lnK d is used to plot 1/T. The slope and intercept are ΔH/R and ΔS/R respectively. As can be seen from Fig. 9, the change in sorption from 20 °C to 30 °C is relatively large. In contrast, the variation in adsorption capacity fluctuates less in the range of 30 °C, 35 °C, and 40 °C and therefore these three temperatures were chosen to calculate the thermodynamic parameters. The result is shown in Fig. 10 and Table 3. ΔH < 0 and ΔS < 0, indicating that the reaction is a process of exothermic and entropy reduction. With the increase of the reaction temperature, ΔG gradually increased, indicating that the decrease of temperature is favorable for zeolite X to adsorb uranyl ions. When the temperature is low, increasing the temperature can effectively improve the adsorption efficiency and adsorption effect. However, with increasing temperature, the adsorption effect becomes progressively worse. The adsorption thermodynamic calculations show that the adsorption process is exothermic, which is consistent with the experimental results. Combining the experimental results, 30 °C was chosen as the optimum reaction temperature.

Adsorption kinetics
The quasi-first-order kinetic equation (Eq. 6) and the quasisecond-order kinetic equation (Eq. 7) were used to fit the The results are shown in Fig. 11 and Table 4. where, Q e and Q are the adsorption capacity at the time of adsorption equilibrium and time t, (mg g −1 ), K 1 (min −1 ) and K 2 (g mg −1 min −1 ) are the two kinetic reaction constants, respectively.
It was shown that the fit of the quasi-second-order kinetic equation (R 2 2 ) was 0.9967 greater than that of the quasifirst-order kinetic equation (R 1 2 ), which was 0.7239, and the theoretical value Q e obtained from the quasi-second-order   kinetic model was very close to the actual value q e . The results indicate that the adsorption of uranium ions by zeolite X is more consistent with the quasi-second-order kinetic model, suggesting that the adsorption and removal process of uranium ions by zeolite X is mainly controlled by the chemisorption process [27].

Adsorption isotherm
Langmuir, Freundlich and Dubinin-Radushkevic isothermal models were used to analyze the experimental data.
Dubinin-Radushkevich (D − R) adsorption isotherm: where, q e represents the adsorption capacity of zeolite X to uranyl ions at equilibrium adsorption, (mg g −1 ). C e is the concentration of uranyl ions in the solution at adsorption equilibrium, (mg L −1 ). q m is the maximal adsorption capacity of zeolite X to uranyl ions, (mg g −1 ). K L is the Langmuir constant, (L mg −1 ). K F is the Freundlich constant. the value of 1/n indicates the strength of adsorption. β represents the activity constant. R is the ideal gas constant, 8.314 J/ (mol·K) −1 , and E represents the adsorption free energy. Figure 12 is the fitting curve of each isotherm equation, and the specific parameter results are shown in Table 5. It can be seen from Table 5 that the degree of fitting of the Langmuir isotherm adsorption equation (R 2 = 0.998) is higher than the degree of fitting of the Freundlich isotherm adsorption equation (R 2 = 0.583) and the fitting degree of the D-R isotherm adsorption equation (R 2 = 0.971). Therefore, the adsorption isotherm of zeolite X on uranium ions is more appropriately described by Freundlich adsorption isotherm ∶ ln q e = lnK F + 1 n ln C e (10) lnq e = lnq m − 2 (11) = RTln 1 + 1 C e (12) E = (2 ) −0.5 the Langmuir isotherm model. The adsorption of U(VI) by zeolite X is a homogeneous monomolecular layer. The experimental data fitted by the D-R isotherm model showed that the free energy of adsorption E(1125.49 kJ mol −1 ) > 8 kJ mol −1 , indicating that the adsorption process is chemisorption. Table 6 compares zeolite X with  other typical adsorbents for U(VI) decontamination. The efficient adsorption of uranium makes zeolite X a very promising material.

XPS analysis
The changes of functional groups of zeolite X before and after adsorption were analyzed by XPS characterization, and the adsorption mechanism of uranyl ions on zeolite X was explored. As shown in Fig. 13a, the zeolite X showed a clear U 4f binding energy peak after adsorption, which indicated that the zeolite X successfully adsorbed uranyl ions. In addition, Fig. 13a also shows that the Na1s binding energy peak intensity decreases after adsorption, so there is ion exchange during the adsorption process. From the O1s XPS spectrum in Fig. 13b, we can get the binding energy peaks at 530.99 eV and 530.16 eV, respectively, corresponding to -OH and C=O. After the adsorption of uranyl ions, the O1s peak shifts to a lower level. The binding energy peaks changed to 530.78 eV and 529.88 eV, respectively. The shift of peak position and the change of peak area show that -OH and C=O play an indispensable role in the adsorption process. As shown in Fig. 13c, by comparing the characteristic peaks of C 1 s in the XPS spectra before and after adsorption, it can be observed that the characteristic peaks of the binding energy of C=O and C-OH have changed after zeolite X adsorbs uranyl ions. This is due to the chemical adsorption of uranyl ions with -OH and -COOH [28]. In summary, the adsorption of uranyl ions by zeolite X is firstly through the complexation of -COOH and -OH with uranyl ions, and secondly, ion exchange occurs between zeolite X and uranyl ions. Desorption and reusability of basalt-based zeolite X Using 30 mL 2.0 mol L −1 NaOH as eluate to desorb uranyl ions from zeolite X after adsorption reaction, the recycling performance of the prepared zeolite X adsorbent to adsorb uranyl ions was investigated, and the results are shown in Fig. 14. The prepared zeolite X adsorbent has a removal rate of 93.06% for uranyl ions when it is used for the first time, and the removal rate of uranyl ions for the fifth cycle is still 80.13%. After five cycles of adsorption and regeneration, the adsorption performance of zeolite X to uranyl ions did not decrease significantly. The above data indicates zeolite X is a reusable adsorption material after regeneration.

Effect of coexisting ions
To investigate the effect of coexisting ions on the adsorption of U(VI) by zeolite X, the adsorption performance of zeolite X on U(VI) in the presence of anions at pH = 6.0 and T = 303 K was investigated. As can be seen from Fig. 15, the adsorption capacity of zeolite X for U(VI) is significantly reduced when PO 4 3− , CO 3 2− , and SO 4 2− ions coexist with U(VI). The reason for this is that in a solution at pH = 6, U(VI) will be present as UO 2 HPO 4 (aq), UO 2 CO 3 (aq), and UO 2 SO 4 (aq). PO 4 3− , CO 3 2− and SO 4 2− ions will combine with UO 2 2+ to form insoluble complexes. Therefore, the presence of PO 4 3− , CO 3 2− , and SO 4 2− reduces the adsorption effect. The presence of Cl − , Br − , and NO 3 − does not interfere with the adsorption of U(VI) by zeolite X. This is because these ions can have a different valence and ionic radius to the uranyl ion without affecting the adsorption effect. In the presence of certain interfering ions, zeolite X can maintain good adsorption performance, which proves that zeolite X has potential in the field of uranium removal.

Conclusion
Using waste basalt powder as raw material, high-purity zeolite X is prepared by the alkaline fusion hydrothermal method, and its specific surface area is as high as 623.395 m 2 g −1 . The adsorption performance of zeolite X for uranyl ions in aqueous solution was investigated. The maximum adsorption of uranium ions by zeolite X was 228.4 mg g −1 when the adsorption time was 720 min, the adsorption temperature was 30 °C, the initial uranium ion concentration was 35 mg L −1 , the pH was 6.0 and the amount of adsorbent was 5 mg. The adsorption of uranium ions by zeolite X was following the quasi-second order kinetic equation and the Langmuir adsorption isotherm model. The adsorption process is homogeneous single molecular layer adsorption. the D-R isotherm model and the quasi-second order kinetic model indicate that the adsorption process is chemisorption. ΔG < 0, ΔS < 0, ΔH < 0 indicate that the adsorption reaction is a spontaneous exothermic process. Zeolite X has good adsorption properties for uranyl ions and is a good performing environmentally friendly adsorption material.