Zeinab F. Akl
Egyptian Atomic Energy Authority, P.O. Box 11762, Cairo, Egypt. E-mail: eltasneem2007@yahoo.com
First published on 8th December 2021
In this study, novel Cyanex-923-coated magnetite nanoparticles (Fe3O4@Cyanex-923) were prepared, comprehensively characterized, and employed for uranium(VI) ion adsorption from aqueous solutions. FTIR and TGA data confirmed that Fe3O4 has successfully gained Cyanex-923 surface functionality. Particle size and morphological studies via DLS, HR-TEM, and SEM showed uniform-dispersed quasi-spherical nanoparticles with a mean diameter of ca. 44 nm. Magnetism measurement data revealed the superparamagnetic properties of the Fe3O4@Cyanex-923 nanoadsorbent. The effect of different experimental settings on the adsorption efficiency was studied to determine the best operational conditions. The experimental results were analyzed using Langmuir, Freundlich, and Temkin isotherms; where the adsorption data obeyed the Langmuir model showing a theoretical adsorption capacity of 429.185 mg g−1 at 298 K. Kinetics data analysis revealed a fast adsorption process that could reach equilibrium within 60 min and is well-fitted to the pseudo-2nd-order model. Temperature affected the adsorption process and the thermodynamic data indicated that uranium(VI) adsorption was spontaneous and exothermic. Fe3O4@Cyanex-923 nanoparticles displayed a good regeneration behavior over three sequential adsorption–desorption cycles. The Fe3O4@Cyanex-923 nanoadsorbent showed a high uranium adsorption capacity, fast equilibration time, economic nature, good reusability, and easy separation; making it a promising candidate for uranium(VI) removal from nuclear waste streams.
Among magnetic iron oxides, magnetite (Fe3O4) is widely investigated for the environmental remediation as well as other numerous applications including biomedicine, sensors, catalysis, energy storage devices, and magnetic resonance imaging.5 Owing to their inexpensive production strategy, availability, physiochemical properties, nontoxic character, ease of modification, biocompatibility, ecological suitability, stability, superparamagnetism, recoverability, and recyclability; Fe3O4-based adsorbents have a considerable potential for removing a wide range of organic and inorganic contaminants from water streams.6 The nanoscale dimension of Fe3O4 can be developed leading to an enhanced performance under various environmental circumstances.7 The high surface area of Fe3O4 nanoparticles along with their susceptibility to the magnetic fields; have made them attractive materials for efficient removal of water impurities.4
Uranium is the most common naturally occurring radionuclide that possesses different forms and oxidation states.8 Excessive uranium quantities are discharged to the environment from many anthropogenic activities e.g., mining, milling, spent fuel reprocessing, improper nuclear waste management, nuclear testing, nuclear accidents, and phosphate fertilizers industry.8 Such activities increase the potential of uranium contamination in natural waters (ground and surface water) that may be adsorbed and concentrated by plants and consequently reach the food chain.8 Accordingly, uranium would hand over to humans and could finally accumulate in various organs to cause acute or chronic health problems; such as tubular necrosis, skin corrosion, leukemia, bone cancer, kidney failure, liver or brain damage, and even death.9 As a result, uranium removal from water streams has become one of the main environmental remediation issues nowadays.
Various technologies have been applied to remove uranium from contaminated water, nevertheless; adsorption is considered an effective method that has gained wide interest among the nuclear industry scientists. Over the last decades, developing efficient uranium adsorbents has been a research focus. Numerous organic and inorganic materials have been developed and assessed for their affinity towards uranium such as amidoxime-based,10 carbon-based,11 biochar-based,12,13 metal organic frameworks-based,14 covalent organic frameworks-based composites15,16 and nanocomposites.17 Among them, nanometric scale adsorbents are of particular concern due to their inimitable properties, as they have high specific surface area, elevated reactivity, high adsorption efficiency and capacity, fast adsorption rate, and acceptable reusability. In this context, diverse nanomaterials have been developed to remove uranium from aqueous media, including carbon nanotubes,18 graphene nanocomposites,19 nanoscale zero-valent iron,20 and functionalized Fe3O4 nanoparticles.21
During the last decades, bare and coated Fe3O4 nanoparticles have been widely investigated for uranium entrapment.22–25 Utilization of Fe3O4 nanoparticles for uranium remediation conforms to the environmental and human safety considerations; as they can be easily and remotely controlled and separated and consequently reduces the radiation exposure.6 Fe3O4 nanoparticles can be readily coated with a variety of functional groups, where their affinity towards uranium can be enhanced by their surface modification with materials having a good binding capability for uranium.24 Up to now, several studies have investigated the removal of uranium by Fe3O4 nanoparticles coated with various organic and inorganic materials.22–28
Organophosphorus compounds have good affinity and selectivity for actinides and were extensively tested for uranium extraction from nuclear waste streams.29,30 In particular, trialkylphosphine oxides are cost-effective neutral extractant with demonstrated high extraction capacity and high stability in strong acidic media, which makes them are excellent ligands to functionalize various adsorbents for uranium capture.31,32 Cyanex-923 is a promising phosphine oxide extractant which revealed high radiation stability, good hydrolysis resistance, poor water solubility, and non-corrosive nature;33,34 thus, it could be a potential candidate to be impregnated onto the Fe3O4 surface for the efficient uranium adsorption.
Although coating of Fe3O4 nanoparticles is a well-established method to enhance their adsorption efficiency, there is no work carried out on coating the surface of Fe3O4 nanoparticles by Cyanex-923, to the author's knowledge. While plentiful studies have been reported on uranium(VI) adsorption via various adsorbents, the feasibility of uranium(VI) adsorption on Fe3O4@Cyanex-923 nanoadsorbent has not been tested yet. Thus, motivated by combining the features of simple magnetic separation and efficient uranium extraction; this work reports Cyanex-923 utilization for coating the Fe3O4 nanoparticles in order to design a novel uranium(VI) nanoadsorbent. Uranium remediation from water systems was investigated in relation to the medium acidity, Fe3O4@Cyanex-923 dose, initial concentration, and temperature. Besides, the adsorption isotherms, kinetics, and thermodynamics studies were conducted.
Cyanex-923 coated Fe3O4 nanoparticles were obtained by dispersing a known weight of the prepared Fe3O4 nanoparticles in Cyanex-923 ethanolic solution (0.150 mol L−1, 50 mL) followed by overnight shaking at room temperature. Then, the product was collected using an external magnet, washed with ethanol, and vacuum-dried to a fixed weight.
The adsorption efficiency of uranium(VI) on Fe3O4@Cyanex-923 (EA%) was determined from the difference of the starting and equilibrium uranium(VI) concentrations according to the following equation:
![]() | (1) |
The uranium(VI) quantity adsorbed through Fe3O4@Cyanex-923 unit weight (qe, mg g−1) was determined from the subsequent expression:26
![]() | (2) |
To evaluate the pH effect on uranium adsorption, the adsorption experiments were carried out while varying the solution pH from 2 to 9. Briefly, aliquots of pH-adjusted uranium(VI) solutions with identical concentrations were added to a constant weight of Fe3O4@Cyanex-923 and stirred at 25 °C for a fixed time. Finally, the uranium content remaining in the solution was spectrophotometrically measured.
The adsorption behavior of bare Fe3O4 nanoparticles towards uranium(VI) was also investigated under the same conditions.
The experimental results were treated by the pseudo-1st-order and pseudo-2nd-order kinetics models given by the following formulas, respectively:17
![]() | (3) |
![]() | (4) |
The chi-square test (χ2) was further used to validate the kinetic models fitting to the experimental data and was determined as follows:36
![]() | (5) |
Moreover, the pseudo-2nd-order fitting data was used to compute the initial adsorption rate (h, mg g−1 min−1) as follows:35
h = k2qe2 | (6) |
The experimental results were analyzed by three adsorption isotherm models; namely Langmuir, Freundlich, and Temkin. The Langmuir isotherm model is expressed as:27
![]() | (7) |
The essential characteristics of Langmuir model are described through a non-dimensional parameter known as separation factor (RL) that explains the favorability of the adsorption process and can be calculated from the subsequent formula:36
![]() | (8) |
Freundlich isotherm model is given by the following equation:28
![]() | (9) |
Temkin isotherm model is expressed as:37
qe = B![]() ![]() ![]() ![]() | (10) |
![]() | (11) |
ΔG° = RT![]() ![]() | (12) |
![]() | (13) |
TEM analysis was conducted to further investigate the particles size, structure, and surface morphology of Fe3O4@Cyanex-923. As is indicated from Fig. 1(b), Fe3O4@Cyanex-923 nanoparticles have a quasi-spherical structure of an average diameter of ∼34.4 nm. The observed partial agglomeration in Fig. 1(b) could be due to the drying process and the tendency of the nanoparticles to aggregate that result from their large surface-to-volume ratio.39 As can be noted, the average diameter observed by TEM measurement is relatively smaller than that obtained by DLS analysis. This is due to the fact that DLS calculates the particles diameter in liquid media, thus the hydrodynamic size measurements are usually greater than the actual nanoparticles size. This originates from the presence of extra solvent layers associated with the nanoparticles surface.40 The surface morphology of Fe3O4@Cyanex-923 nanoadsorbent was further observed by SEM. The micrograph of Fe3O4@Cyanex-923, Fig. 1(c), shows nearly homogeneous grains with a smooth surface where most of the nanoparticles are quasi-spherical in shape.
The functional groups of Fe3O4@Cyanex-923 were identified by FTIR spectroscopy as shown in Fig. 2(a). The signals at 454 cm−1 and 588 cm−1 correspond to the characteristic Fe–O vibrations of the Fe3O4 core.41 The signals at 1150 cm−1, 1289 cm−1, and 1460 cm−1 arise from PO, O–P–O, and P–C stretching vibrations, respectively.42 The signals at 1378 cm−1 and 2954
cm−1 represent the –CH3 symmetric and dissymmetric stretching frequency, whereas the signals at 2854
cm−1 and 2924 cm−1 represent –CH2 symmetric and dissymmetric stretching vibrations, respectively.43 These results indicate that Cyanex-923 was successfully coated onto the Fe3O4 surface. The appearance of a broad peak around 3400 cm−1 could result from the water molecules adsorbed from the atmospheric moisture.
![]() | ||
Fig. 2 (a) FTIR spectrum, (b) TGA curve, and (c) magnetization curve of Fe3O4@Cyanex-923 nanoadsorbent. |
To investigate the thermal stability and quantify the surface coating of the Fe3O4 nanoparticles, TGA was performed and represented in Fig. 2(b). The TGA curve of Fe3O4@Cyanex-923 shows two-step degradation with a total mass loss of 35.5%, which indicates that the magnetic content of Fe3O4@Cyanex-923 could be up to 64.5%. Fig. 2(b) reveals an initial 2.5% weight loss observed in the temperature range 88–180 °C which could be attributed to the loss of the adsorbed water. This is followed by a second 33% weight loss starts from 180 to 386 °C which is attributed to the decomposition of Cyanex-923 molecules onto the surface of Fe3O4 nanoparticles. No mass loss appears at T > 400 °C, as the Fe3O4 residue is stable beyond 400 °C. TGA results confirm the successful coating of Cyanex-923 on Fe3O4 surface, since the TGA of bare Fe3O4 nanoparticle shows only 6% weight loss at 160 °C due to dehydration.44
Magnetization is a promising feature of Fe3O4-based adsorbents that allows for their facile separation from the treated solution. Thus, the magnetic properties of Fe3O4@Cyanex-923 were examined and the magnetic hysteresis loop was represented in Fig. 2(c). The nonlinear variation in magnetization as a function of the magnetic field (S shape) reveals that Fe3O4@Cyanex-923 nanoparticles are constituted by a single magnetic domain with negligible coercivity or remanence. This indicates the superparamagnetic character of Fe3O4@Cyanex-923. The relatively high value of the saturation magnetization (Ms) of Fe3O4@Cyanex-923 nanoadsorbent (64.5 emu g−1) is good enough to enable its easy separation and recovery from the aqueous solutions by the conventional magnets.45 Compared to bare Fe3O4 nanoparticles (Ms ∼72 emu g−1),35 Fe3O4@Cyanex-923 shows a relatively lower Ms value. This is caused by the decrease of Fe3O4 fraction after Cyanex-923 coating and the surface spin effect on Fe3O4 nanoparticles that subsequently decrease the Ms value.41
The results of uranium adsorption on the bare magnetite nanoparticles demonstrate that the adsorption efficiency is dependent on the solution pH as shown in Fig. 3(a). This can be attributed to the influence of the medium acidity on the functional groups at the Fe3O4 surface. Fe3O4 is an amphoteric solid that is capable to develop charges via the protonation and deprotonation of –FeOH sites on its surface with the pH change as follows.47,48
–FeOH + H+ ⇔ –FeOH2+ | (14) |
–FeOH2+ − H2O ⇔ –Fe+ | (15) |
–FeOH2+ ⇔ –FeOH + H+ | (16) |
–FeOH ⇔ –FeO− + H+ | (17) |
At low pHs, the H+ ions exist in excess that causes the protonation of the –OH on the surface of Fe3O4 and accordingly the –FeOH2+ content increase, resulting in electrostatic repulsion with positively charged uranium(VI) ions.47 With the pH increase, more adsorption sites on the Fe3O4 surface turn into the more reactive deprotonated and negatively charged forms, thereby leading to a higher uptake of uranium(VI) ions.48 With further pH value increase, the anionic uranium species are formed that do not tend to bind to the negatively charged Fe3O4 surface.
![]() | ||
Fig. 4 (a) Effect of contact time on uranium adsorption, (b) pseudo-1st-order, and (c) pseudo-2nd-order kinetics models fitting curves. |
As can be noticed from Fig. 4(a), Fe3O4@Cyanex-923 shows a faster equilibrium time (60 min) compared to bare Fe3O4 (180 min). The fast equilibrium state achieved by Fe3O4@Cyanex-923 is caused by the strong chelation of uranium ions with phosphine oxide functional groups. Additionally, the adsorbent nanoscale structure gives a share in the fast diffusion. According to the kinetic results, Fe3O4@Cyanex-923 could have real-world applications potential for uranium removal from sizable amounts of water.
To describe the rate-controlling step of uranium(VI) adsorption on the prepared nanoadsorbent, the pseudo-1st-order and pseudo-2nd-order models were employed and the corresponding fitting curves were illustrated in Fig. 4(b) and (c), respectively. The related kinetic parameters calculated from these models were summarized in Table 1. According to the results, the pseudo-2nd-order model reveals a better regression analysis for Fe3O4@Cyanex-923 experimental data (R2 = 0.997), thus the uranium(VI) adsorption process on Fe3O4@Cyanex-923 obeys the pseudo-2nd-order kinetic equation. Additionally, the adsorption capacity obtained from the pseudo-2nd-order model (108.695 mg g−1) is nearer to the experimental value (99.862 mg g−1) compared to that obtained from the pseudo-1st-order model (127.938 mg g−1). The lower χ2 value obtained for the pseudo-2nd-order model further validates that this model is ideal for explaining the uranium(VI) adsorption behavior on Fe3O4@Cyanex-923. These results indicate that the dominant interaction of uranium(VI) ions with the active sites on the Fe3O4@Cyanex-923 surface is a chemical one,50 where the adsorption rate is governed by chemical chelation via sharing or exchanging electrons at the solid/solution interface.
Adsorbent | qe,exp (mg g−1) | Pseudo first-order | Pseudo second-order | h (mg g−1 min−1) | ||||||
---|---|---|---|---|---|---|---|---|---|---|
qe,cal (mg g−1) | k1 (min−1) | R2 | χ2 | qe,cal (mg g−1) | k2 × 10−3 (g mg−1 min−1) | R2 | χ2 | |||
Fe3O4@Cyanex-923 | 99.862 | 127.938 | 0.052 | 0.974 | 8.453 | 108.695 | 0.719 | 0.997 | 0.949 | 8.503 |
Fe3O4 nanoparticles | 13.601 | 13.365 | 0.025 | 0.996 | 0.063 | 16.666 | 1.534 | 0.981 | 0.691 | 0.427 |
Contradictory, data in Table 1 points out that adsorption kinetics of uranium(VI) on bare Fe3O4 nanoparticles could be better explained by the pseudo-1st-order model. It can be noted that qe value of uranium adsorption on bare Fe3O4 calculated by the pseudo-1st-order model is more consistent with the experimental value. Moreover, pseudo-1st-order model reveals a better fitting with higher correlation coefficient (R2) than the pseudo-2nd-order.
![]() | ||
Fig. 5 (a) Effect of initial uranium concentration on the adsorption capacity, (b) Langmuir, (c) Freundlich, and (d) Temkin isotherms fitting curves. |
To understand the adsorption mechanism and predict the maximum adsorption capacity; the isotherm studies were carried out using Langmuir, Freundlich, and Temkin equations; and the obtained results were represented in Fig. 5(b), (c), and (d), respectively. The corresponding fitting parameters were tabulated in Table 2.
Adsorption isotherm | Fitting parameters at 25 °C, pH 5 | |
---|---|---|
Fe3O4@Cyanex-923 | Fe3O4 nanoparticles | |
Langmuir | R2 = 0.997 | R2 = 0.927 |
qmax = 429.185 (mg g−1) | qmax = 29.850 (mg g−1) | |
KL = 0.099 (L mg−1) | KL = 0.041 (L mg−1) | |
Freundlich | R2 = 0.958 | R2 = 0.994 |
KF = 57.003 (mg g−1) | KF = 4.611 (mg g−1) | |
n = 2.402 (L mg−1) | n = 2.792 (L mg−1) | |
1/n = 0.416 | 1/n = 0.358 | |
Temkin | R2 = 0.988 | R2 = 0.805 |
AT = 2.559 (L g−1) | AT = 1.496 (L g−1) | |
B = 52.849 (J mol−1) | B = 4.590 (J mol−1) |
Langmuir isotherm assumes single-layered chemisorption onto the adsorbent surface that has similar binding positions and equivalent adsorption energies with no steric hindrance. In contrast, Freundlich isotherm assumes a multiple-layered adsorption at heterogonous surfaces where there are interactions between adsorbed molecules.28 Fitting the experimental results of uranium adsorption on Fe3O4@Cyanex-923 to the Langmuir and Freundlich models gave R2 values of 0.997 and 0.958, respectively. It can be noted that Freundlich model showed a lower R2 value compared to the Langmuir model, suggesting that Langmuir model is better fitted with the Fe3O4@Cyanex-923 adsorption data and a homogenous single-layered adsorption dominates the process. On the contrary, the fitting coefficient for bare magnetite nanoparticles is greater for Freundlich isotherm than Langmuir isotherm, suggesting the suitability of Freundlich model to the adsorption data of brae Fe3O4 and the existence of heterogeneous adsorption sites.
The maximum single-layer adsorption capacity of uranium(VI) on Fe3O4@Cyanex-923 and bare Fe3O4 nanoparticles was calculated by Langmuir isotherm as 429.185 and 29.850 mg g−1, respectively. The results clearly indicate the higher affinity of Fe3O4@Cyanex-923 towards uranium(VI) ions, hence it could be considered as a promising material for uranium(VI) adsorption from aqueous solutions. The RL values were less than unity for both Fe3O4@Cyanex-923 and bare Fe3O4 (0.048–0.993), indicating that the adsorption process is suitable for both magnetic nanoadsorbents.
The higher value of Freundlich constant for Fe3O4@Cyanex-923 (KF = 57.003 L g−1) compared to bare Fe3O4 (KF = 4.611 L g−1) indicates the higher adsorption capacity of Fe3O4@Cyanex-923 that results from the strong affinity of Fe3O4@Cyanex-923 to uranium(VI) ions.52 The n values of Fe3O4@Cyanex-923 and bare Fe3O4 lies between 1 and 10, indicating a favorable adsorption process. The n value is slightly higher for bare Fe3O4 nanoparticles, denoting the increased heterogeneity of the adsorption.53
Temkin isotherm is another model that assumes the dependence of the adsorption free energy on the adsorbent surface coverage, as the adsorption heat linearly decreases because of the adsorbent–adsorbate interactions. The plot of Temkin isotherm fitted quite well to the experimental data obtained for Fe3O4@Cyanex-923 (R2 = 0.988) which further elucidates the chemisorption adsorption process.36 The poor-fit of the uranium(VI) adsorption data on bare Fe3O4 nanoparticles to Temkin model indicates the unsuitability of this model to describe the uranium(VI) interaction with bare Fe3O4. The B value calculated for Fe3O4@Cyanex-923 is 52.849 J mol−1 which confirmed that the uranium(VI) adsorption process onto Fe3O4@Cyanex-923 is a chemisorption one.54 The positive value of B made it clear that the adsorption is exothermic for Fe3O4@Cyanex-923 supporting the obtained thermodynamics results.55
Generally, the thermodynamic parameters offer in-depth data concerning the spontaneity and feasibility of the adsorption process. Thus, the van't Hoff plot, Fig. 6, was used to calculate the thermodynamic parameters of uranium adsorption on Fe3O4@Cyanex-923 and bare Fe3O4 nanoparticles and the data was listed in Table 3. It is clear that low temperature is favorable for uranium adsorption on Fe3O4@Cyanex-923 and bare Fe3O4, as interpreted from the decrease of ΔG° values with temperature increasing. It is noticed that the Kc values decrease with the temperature rise, signifying an exothermic process.54 The negative ΔG° values indicate the spontaneity and feasibility of uranium adsorption on Fe3O4@Cyanex-923 and bare Fe3O4 under the studied temperatures.54 The negative ΔH° values further affirm the exothermic character of the adsorption process. ΔS° positive values suggest the increase of randomness at the solid/solution interface resulting from the decrease in molecular orderliness during the adsorption process.28
ΔH° (kJ mol−1) | ΔS° (kJ mol−1 K−1) | Temp. (K) | ΔG° (kJ mol−1) | |
---|---|---|---|---|
Fe3O4@Cyanex-923 | −64.381 | 0.189 | 298 | −8.296 |
308 | −5.457 | |||
318 | −3.870 | |||
328 | −2.576 | |||
Fe3O4 nanoparticles | −20.061 | 0.147 | 298 | −1.256 |
308 | −0.782 | |||
318 | −0.233 | |||
328 | −0.075 |
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Fig. 7 Effect of desorption agents on the desorption efficiency of U(VI) from Fe3O4@Cyanex-923 and bare Fe3O4 for three continues cycles. |
It can be indicated from the results that higher recovery of U(VI) species from the loaded Fe3O4@Cyanex-923 and bare Fe3O4 could be obtained using 0.5 mol L−1 Na2CO3 solution.
Adsorbent | pH | Temperature °C | qmax (mg g−1) | Time | Ref. |
---|---|---|---|---|---|
Fe3O4 nanoparticles | 7.0 | 25 | 5.5 | 4–6 h | 22 |
Fe3O4@GO | 5.5 | 25 | 69.5 | 4 h | 23 |
Amidoxime modified Fe3O4@SiO2 | 5.0 | 25 | 105 | 24 h | 24 |
PAAM-FeS/Fe3O4 | 5.0 | 20 | 311 | 1 h | 25 |
Magnetic oxine | 7 | 25 | 125 | 4 h | 26 |
Fe3O4@C@Ni-Al LDH | 6 | 25 | 227 | 3 h | 27 |
Fe3O4@MS | 5.5 | 25 | 242.5 | 5 h | 28 |
Fe3O4@TiO2 | 6.0 | 25 | 118.8 | 4 h | 56 |
Fe3O4@SiO2 | 6.0 | 25 | 52 | 57 | |
Fe3O4@C-KO | 6.0 | 25 | 38.7 | 2 h | 58 |
Fe3O4@C@ASA | 4 | 25 | 46.2 | 4 h | 59 |
Fe3O4@PAM | 5 | 20 | 220.9 | NA | 60 |
Fe3O4@phosphoramide | 6 | 25 | 95.2 | 1.25 | 61 |
Magnetic schiff base | 6 | 25 | 94.3 | 6 h | 62 |
Fe3O4@Cyanex-923 | 5 | 25 | 429.1 | 1 h | This work |
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