Solid-Phase Extraction Method by Magnetic Nanoparticles Functionalized with Murexide for Trace U(VI) from Sea Water Prior to Spectrophotometric Determination

In this study, magnetic nanoparticles (Fe3O4/SiO2/APTES) functionalized with murexide were used for the determination of uranium(VI) in sea water by spectrophotometric method in perchloric acid medium using Arsenazo-III as chromogenic reagent. The effects of some analytical parameters, such as pH, contact time, and eluent volume, on the recovery of uranium(VI) were examined in synthetic sea water. The optimum conditions were achieved with a 15 min adsorption time and 2 min elution time with 1 mL of 5 mol L–1 HClO4 at pH of 6.5 and 25 mg of the magnetic sorbent. The linear range, detection limit, and precision (as RSD%) of the method were found to be 0.02–4.0 mg L–1, 0.001 mg L–1 and 3.0%, respectively. The proposed method is simple, rapid, and cost-effective for the determination of U(VI) in sea water, with a total analysis time of approximately 30 min. The adsorption isotherm was well fitted to the Langmuir model, with a correlation coefficient of 0.9997 and Qmax value was found to be 77.51 mg g–1. The magnetic sorbent was successfully used for the rapid determination of trace quantities of U(VI) ions in different sea waters, and satisfactory results were


Introduction
In recent years, environmental pollution with toxic elements, such as uranium, has increased considerably.2][3][4] Furthermore, it can cause respiratory diseases, such as fibrosis and emphysema, and even cause irreversible effects in some tissues, such as the kidneys.Uranium is found in sea water at 3 µg L -1 and at approximately 0.0004% in the Earth's crust. 5In many countries, the uranium concentration in drinking water is determined to be 0.03 mg L -1 , according to the United States Environmental Protection Agency. 6Currently, the determination of uranium in environmental samples is crucial due to applications of uranium in areas, such as in the products of nuclear energy, catalysis, and nuclear weapons.The determination of trace uranium in complex samples and natural waters is a challenging task.Most instruments are not sensitive enough to allow for its determination at very low concentration levels in complex matrix such as sea water.For example the heavy salt matrix reduces sensitivity in direct determinations from sea water (ca.3.5% salt).7][8][9] Preconcentration/ separation techniques, such as solid phase extraction (SPE), 3,[10][11][12][13] liquid-liquid microextraction (LLME), 14 and cloud point extraction (CPE) [15][16][17] are used for the determination of uranium in various samples.SPE has commonly been used as a technique for preconcentration/separation due to its higher enrichment factor and practicality.9][20][21] Most of these sorbents have disadvantage such as low sorption capacities or efficiencies.Recent studies show that nanomaterials exhibit perfect sorption capacity.But the high dispersibility of nanomatereials in aqueous solutions makes it difficult to separate sorbents from aqueous phase after saturated sorption, which limits their real application in large volumes of waters. 22Recently, nanosized iron oxide particles have become an important absorbent in SPE because they show magnetic properties, as well as the general properties of nanomaterials.Furthermore, the use of magnetic nanoparticles in SPE has many advantages compared to other adsorbents.4][25][26][27] Aside from these advantages, raw Fe 3 O 4 nanoparticles have several disadvantages, such as oxidation, aggregation tendencies, and low selectivity.However, magnetic nanoparticles can be modified by special ligands to overcome these problems.9][30] Murexide is one of these ligands. 18,31n this study, for the quantitative determination of uranium in seawater, a simple and rapid method was developed using an Fe 3 O 4 nanoparticles modified with murexide.Several experimental parameters, such as pH, contact time, eluent concentration, and sorption capacity, were examined, and the developed method was then applied to real sea water samples.

Apparatus
The UV-Vis spectra were recorded using a Shimadzu 3600 spectrophotometer.A Selecta brand pH metre was used for all pH measurements.A Biosan multi rotator was employed for the effective mixing of sorbent and solution.

3. Synthesis of Murexide Functionalized Magnetic Nanoparticles
Fe 3 O 4 nanoparticles (Fe 3 O 4 NPs) were synthesized with an eco-friendly method, modified from Gautam et al.Briefly, FeCl 3 • 6H 2 O (6.1 g) was dissolved in deionized water (100 mL), followed by the addition of a few drops of concentrated HCl to prevent Fe(OH) 3 precipitation.FeSO 4 • 7H 2 O (4.2 g) was then added to the mixture and heated to 90 °C, followed by the rapid addition of NH 4 OH (10 mL, 27%), with the solution kept at a pH of 10.0.The mixture was stirred at 90 °C for 30 min and cooled to room temperature.The resulting solid black substance was collected with a strong magnet and washed several times with ethanol and deionized water.The Fe 3 O 4 NPs were then dried under vacuum at 60 °C.
To prepare core-shell nanoparticles (Fe 3 O 4 /SiO 2 ), the Fe 3 O 4 nanoparticles (0.50 g) were dispersed in a solution of ethanol (80 mL) and deionized water (20 mL) by sonicating for 30 min.Then, ammonia solution (5 mL, 27 wt %) and TEOS (4 mL) were added sequentially.The mixture was stirred and allowed to react for 6 h at room temperature.The product, Fe 3 O 4 /SiO 2 , was collected by a magnet, washed several times with deionized water, and dried under vacuum at 60 °C for 8 h.Fe 3 O 4 /SiO 2 nanoparticles (1 g) were dispersed in 50 mL of toluene in a flask.After 1 h, APTES (4 mL) was added to the mixture, stirred continuously, and refluxed at 125 °C for 12 h.The magnetic nanoparticles (Fe 3 O 4 /SiO 2 /APTES) were separated with a strong magnet and washed several times with deionized water and ethanol, then dried at 70 °C for 8 h.
In the third step, Mu (0.1 g) was dissolved in DMSO (50 mL), and Fe 3 O 4 /SiO 2 /APTES (1 g) was added to the reaction mixture and refluxed at 200 °C for 24 h.The resulting product was separated, washed several times with methanol, and dried at room temperature.

4. Procedure
The method was tested with synthetic sea solutions prior to its application to real sea samples.For this purpose, the synthetic solutions containing the main components present in synthetic sea water (SSW) were prepared at the following concentrations: Na + = 10569 mg L -1 ; Mg 2+ = 1270 mg L -1 ; K + = 379 mg L -1 ; Ca 2+ = 397 mg L -1 ; BO 2 -= 18 mg L -1 ; Cl -= 18990 mg L -1 ; HCO 3 − = 139 mg L -1 ; SO 4 2-= 2648 mg L -1 ; Br -= 65.5 mg L -1 ; and F − = 14 mg L -1 . 13e 3 O 4 /SiO 2 /APTES (25 mg) was transferred to a 50-mL volumetric flask, and synthetic sea water solutions (40 mL) were added (U(VI): 0.05 mg L -1 ).The pH was adjusted to 6.5 with 0.01 M CH 3 COOH/NH 3 .The solutions were shaken and allowed to stand for 15 min at room temperature.The magnetic sorbent was separated from the suspension using a powerful magnet and supernatant was decanted.1.0 mL of 5 mol L -1 HClO 4 was added to the magnetic sorbent with shaking for 2 min to elute the U(VI) ion.The magnetic sorbent was separated from the eluent using a magnet.U(VI) ion in eluent was determined spectrophotometrically in perchloric acid medium using Arsenazo-III as chromogenic reagent. 13To this end, an Arsenazo-III solution (0.1 mL, 0.1%) was added to eluent solution, and the absorbance of the uranium(VI)-Arsenazo-III complex was measured spectrophotometrically (653 nm).Finally, the magnetic sorbent was washed with deionized water for reuse.

1. Characterization of Fe 3 O 4 /SiO 2 /APTES Functionalized with Murexide
Scanning electron microscopy (SEM) studies were performed on a Tescan Mira 3XMU with an Oxford EDS analysis system.As shown in Figure 1 Elemental analysis showed the presence of C, N and Si in the structure of the modified magnetic nanosorbent.
Infrared absorption measurements of Fe 3 O 4 and Fe 3 O 4 / SiO 2 /APTES were carried out using a Fourier Transform Infrared (FTIR) spectrophotometer (Bruker Optics -Alpha).The FTIR spectra were obtained in the wavenumber range 500-4000 cm −1 using single bounce ATR with selenium crystal.The absorption peaks at 550 cm −1 (Fe-O) in the spectra of Fe 3 O 4 NPs confirmed the synthesis of Fe 3 O 4 nanoparticles. 32,33n the other hand, the peaks observed at 1045 cm −1 (Si-O), 1450 cm −1 (C=N), 1530 cm −1 (C=C) and 1630 cm −1 (C=O) in the spectra of Fe

2. Effect of pH
In the SPE, an important parameter for obtaining the quantitative adsorption and recovery of trace elements is pH.For this purpose, the adsorption of uranium ions on Mu-functionalized Fe 3 O 4 /SiO 2 /APTES sorbent was studied as a function of pH.The pH of the model solutions (40 mL, SSW) containing 50 µg L -1 of U(VI), was adjusted to a pH range of 4-8 by the use of relevant buffer solutions; the retained uranium ions were eluted by HClO 4 (1 mL, 5 mol L -1 ).The graph of retention as a function of pH is shown in Fig. 4. The quantitative recovery (≥ 95%) for the uranium ions studied was obtained at a pH of 6-7.Therefore, a pH of 6.5 was chosen as an optimum pH for subsequent experiments.

Effect of Eluent Concentration and Volume
In this study the elution of uranium was studied to find the optimum amount of HClO 4 in the range of 2-5 M and volume of 0.5 to 2 mL. 1 mL of 5 M HClO 4 was found to be satisfactory for elution of uranium (recovery ≥ 95%).
Therefore, 1 mL of 5 M HClO 4 as eluent was chosen for the following experiments.

4. Effect of Matrix Components
The effects of matrix ions, which are found at high concentrations in real samples, on the recovery of metal ions were studied.Various concentrations of Fe 3+ , Cd 2+ , Pb 2+ , Co 2+ , Ni 2+ , Cu 2+ , Cr 3+ , Al 3+ , and Zn 2+ , as their chloride, nitrate and sulfate salts, were added individually to a model solution of 50 mL containing 0.05 mg L -1 U(VI).The described method was applied under optimum conditions.The results are given in Table 1.The most significant interferences were found with 1 mg L -1 of Cr 3+ and Ni 2+ when determining the presence of uranium.These interferences were prevented by using 0.02 M EDTA.Besides, EDTA can be used as a masking agent for many elements such as Th, Zr, because EDTA forms stable complex with these elements, and unstable complex with U(VI). 34ble 1.Tolerance limits for interference ions on the determination of U(VI) (n = 3 0,05 mg L -1 U, 1 mg L -1 of metal ions)

Ion
Interference ion to Recovery %, metal ion/ratio U(VI)

5. Effect of Adsorption and Elution Time
The rate of U(VI) adsorption by Fe 3 O 4 /SiO 2 /APTES/ Mu was studied (50 mL, 0.05 mg L −1 ) with 25 mg of the sorbent over a series of varying shaking times (5-30 min).The results showed that the extraction percentage of U(VI) at 15 min was higher than 98%.The rate of elution of U(VI) by Fe 3 O 4 /SiO 2 /APTES /Mu was studied (50 mL, 0.05 mgL −1 ) with 25 mg of the sorbent and an adsorption of 15 min over a series of varying shaking times (1-5 min).Therefore, 15 min and 2 min, respectively, were used in all subsequent experiments for quantitative sorption and elution of U(VI).

6. Sorption Capacity
The maximum sorption capacity of Fe 3 O 4 /SiO 2 / APTES/Mu was obtained from the batch methods.A total of 25 mg of Fe 3 O 4 /SiO 2 /APTES/Mu was added to a 40-mL Oymak: Solid-Phase Extraction Method by Magnetic Nanoparticles ... solution containing different amounts of U(VI) ions (0.8-8 mg) at pH 6.5.After shaking for 1 h, the mixture was separated with the use of a magnet.The supernatant solutions were then measured by UV-Vis spectrophotometry after dilution.Many isotherm models have been proposed to explain the adsorption equilibrium, such as the Langmuir and Freundlich isotherms, which are the most commonly used for the clarification of adsorption of molecules from the liquid phase.The Langmuir equation is given as follows: (1) where Q max (mg g -1 ) is the maximum adsorption capacity; Q e is the amount of solute adsorbed per unit weight of adsorbent (mg g -1 ) at equilibrium; C e is the equilibrium solute concentration (mg L -1 ) in solution and K is the Langmuir constant (L mg -1 ).
The Freundlich isotherm equation is given below: 35,36 (2 where Q e is the amount of adsorbed U(VI) per mass of adsorbent, K f is the Freundlich constant, C e is the equilibrium U(VI) concentration and 1/n is a constant related to the adsorption intensity. 37s shown in Table 2, the adsorption mechanism was well-suited to the Langmuir model, with a correlation coefficient of 0.9997.The Q max value was found to be 77.51mg g -1 .The n value was 3.87, calculated from the Freundlich isotherm, which is higher than 1.The n value indicated the favourable adsorption of U(VI) on Fe 3 O 4 /SiO 2 / APTES/Mu.The Langmuir and Freundlich isotherm parameters are shown in Table 2.

7. Analytical Performance and Applications to Real Sea Water Sample
The limit of detection (LOD) study was performed by applying the described method to ten blank solutions of 40 mL.The limit of detection calculated as the ratio of the three standard deviations of the blank to the slope of plot was 0.001 mg L -1 with a preconcentration factor of 40.The relative standard deviation was calculated as 3.0% at 0.05 mg L -1 of U(VI) (n = 7) and the linear range in final eluate was 0.02-4.0mg L -1 of uranium(VI).
The method was successfully applied to sea water.The accuracy of the developed method for sea water was tested by adding the known amounts of U(VI).After applying the separation/ preconcentration procedure, quantitative recovery (≥95%) was found for U(VI).The results of the analysis of sea water samples are shown in Table 3.

8. Reusability of the Adsorbent
The reusability of the Fe 3 O 4 /SiO 2 /APTES/Mu adsorbent was investigated by adsorption and desorption cycling experiments.The results have shown that the sorbent was stable up to 86 cycles without an obvious decrease in the recoveries.The mean recovery ± standard deviation from 86 runs was found to be 97.6 ± 3.6%.This result indicates that the adsorbent possessed a perfect reusability.

Conclusions
In this paper, Fe 3 O 4 /SiO 2 /APTES/Mu was prepared.Then SPE procedure was developed by using these magnetic nanoparticles.The proposed SPE method is simple, fast, practical, and low-cost.The SPE method has a good potential for the extraction of uranium(VI) from sea water.Significant advantages of this method are a short analysis time and satisfactory results in sea water, which has a high salt concentration.In comparison to other SPE methods, the presented method has a low consumption of time, with a total analysis time of approximately 30 min, including the enrichment/separation procedure and the measurement by spectrophotometry.The adsorbent has considerable reusability.The initially synthesized Fe 3 O 4 /SiO 2 / APTES/Mu was used for optimization studies and for a sample application.The adsorbent was reused for 86 cycles.The obtained results show that Fe 3 O 4 /SiO 2 /APTES/ Mu has a good adsorption capacity (77.51 mg g -1 ).As a result, Fe 3 O 4 /SiO 2 /APTES/Mu is indeed an efficient scav-

Acknowledgement
This work is supported by the Scientific Research Project Fund of Cumhuriyet University under the project number ECZ-040.
, the spherical structure of the Fe 3 O 4 NPs changed after modification.The surface of Fe 3 O 4 /SiO 2 /APTES functionalized with murexide had a rough morphology compared with Fe 3 O 4 .SEM images of Fe 3 O 4 and Fe 3 O 4 /SiO 2 /APTES functionalized with murexide are shown in Figure 1.

3 O 4 /
SiO 2 /APTES/Mu have shown the successful modification of Fe 3 O 4 with silan agents and Mu.33

Table 3 .
The results for determination of U(VI) in sea water

Table 2 .
Langmuir and Freundlich isotherm parameters for U(VI) in sea water in terms of its fast sorption time, large sorption capacity, selectivity, easy separation and good reusability of the material.