Evaluation of nanoscale zero valent iron particles for the removal of cesium from aqueous solutions

The great East Japan Earthquake occurred in 2011 and seriously damaged Fukushima Daiichi Nuclear Power Plant. Large amounts of cesium was released into the environment following this accident. Therefore, this study for the first time assessed the applicability of nanosccale zero valent iron (nZVI) for the removal of cesium from aqueous solutions. The removal of cesium was investigated in a batch system with respect to initial cesium concentration, contact time, pH, temperature, competing cations and dosage of nanoparticles. The obtained results showed that nZVI displayed effective performance for removal of cesium. The removal efficiency exceeded 99% at initial cesium concentration of 1 mg/L and 1 g/L dose. The removal of cesium was largely depending on the solution pH and temperature. The current work proved the potential utility of the nZVI in the treatment of cesium contaminated water generated after the Fukushima nuclear accident.


Introduction
Following the accident at the Fukushima Daiichi Nuclear Power Plant in 2011, radioactive nuclides were released into the environment in large amounts and heavily contaminated seawater, groundwater and drinking water [1]. Japanese soils were also contaminated with radionuclides. One of the various radionuclides produced by a nuclear accident, radioactive cesium ( 137 Cs) is the most hazardous element of radionuclides due to its abundance in nuclear fallout and radioactive wastewater and the hazards presented by its long half-life (about 30 years) and high emission of beta and gamma particles [2]. In food chain, radioactive liquid waste is the primary pathway of radionuclides; therefore, technologies for removing dangerous radioactive isotopes from liquid waste have received much attention.
Until today, the recovery of hazardous cesium is an unsolved problem. Different cesium adsorbents, including zeolites, Prussian blue, bentonite and aluminum molybdophosphate, were intensively investigated to remove cesium from contaminated water [3][4][5]. However, separation of these adsorbents from environment after use is very difficult. In recent decades, magnetic nanoparticles have been widely used in the fields of medicine, biotechnology, diagnostics and catalysis [6]. Magnetic nanoparticles compose of a magnetic based core and an outer functional shell that can sorb contaminants, which have been extensively studied for environmental remediation applications due to their quite small particle size and large surface area to volume ratio [2]. In addition, the magnetic nanoparticles can be recovered and separated easily from medium by applying an external magnetic field due to the intrinsic magnetic feature of the nanoparticles [7].

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To date, nanoscale zero valent iron (nZVI) has been shown to be efficient environmental remediator of a wide variety of contaminants involving chlorinated organics and inorganic anions [8,9]. In addition, nZVI has also been shown to considerably remediate solutions contaminated with a variety of transitions metals, such as: Cr, Co, Cu, Mo, Zn, Ni, Ag and V; post transition metals, such as: Pb, Cd and metalloids, such as: As and Se [10][11][12]. However, studies for the treatment of radionuclides remain less widely examined and are limited to Ba, U, Pu and TcO 4 [13,14].
In present study, the efficiency of nZVI for the removal of cesium from aqueous solutions was evaluated. The sorption of Cs + was studied in a batch system with respect to initial cesium concentration, contact time, pH, temperature, competing cations and dosage of nanoparticles. To the best of our knowledge, the feasibility of using nZVI for the removal of cesium from contaminated waters was reported in this paper for the first time.

Synthesis of nZVI
nZVI was synthesized following the method first proposed by Wang and Zhang [15], based on chemical reduction of FeCl 3 6H 2 O using NaBH 4 as depicted in the following reaction: Briefly, NaBH 4 (98%, 0.74 M) was pumped slowly into FeCl 3 6H 2 O (99%, 0.15 M) at a rate of 1 L/h using a roller pump in 500 mL four-neck glass flask. A continuous flow of nitrogen gas was maintained during synthesis to create an anaerobic condition. The synthesis was conducted with vigorous stirring at 250 rpm and kept under constant temperature 25 ± 0.5 ºC using a water bath. To complete the reaction, the synthesis was left 20 min as aging time. After reduction, the synthesized nanoparticles were washed with deionized deoxygenated water and ethanol at least three times, filtered by vacuum filtration and applied immediately in batch experiments.

Batch studies
The sorption experiments were performed under kinetic and equilibrium conditions using a batch technique at 298 K. 1 g of nZVI was contacted with 50 mL of 100 mg/L cesium solution and stirred on a magnetic stirrer at 1000 rpm for 2 h to attain equilibrium. After the experiments, the water samples were collected and filtered through 0.2 µm cellulose acetate filter. The obtained liquid were diluted with deionized water to an acceptable concentration range prior to analysis using inductively coupled plasma 3

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International Conference on Materials Engineering and Science IOP Publishing IOP Conf. Series: Materials Science and Engineering 454 (2018) 012104 doi:10.1088/1757-899X/454/1/012104 mass spectrometry (ICP-MS) technique. To assess the effect of cesium concentration, the experiments were conducted at different initial concentrations of 1, 5, 50, 100, 150 and 200 mg/L. To investigate the effect of the solution pH on the cesium uptake, the initial pH was adjusted at 3-12 using dilute solution of HCl or NaOH. To examine the effect of temperature on the cesium removal, the experiments were conducted at four different temperatures ranging from 278 to 343 K. In order to investigate the effect of nanoparticles content on the sorption capacity, a series of dosage from 1 to 30 g/L was prepared. To evaluate the effect of competitive cation ions on the sorption process of cesium, different simulated waste solutions were prepared separately in the presence of Na + , K + , Mg 2+ and Ca 2+ . All batch experiments were carried out in triplicate and the mean values were used to evaluate sorption parameters. The amount of sorbed cesium by unit mass of the nanoparticles, (mg/g), was calculated by the following equation: The percent removal of cesium, , was also calculated using the following formula: where and are the initial and final cesium concentrations (mg/L) respectively, is the amount of the sorbent (g) and is the volume of the solution (L).

Effect of pH
The effect of pH on the removal of cesium by nZVI was investigated over pH range from 3 to 12. As shown in Figure 1, it is obvious that cesium sorption process is a pH dependent process and the initial pH of the aqueous solution has high impact on cesium removal by nZVI. At initial pH 3, the sorption capacity of cesium by nZVI was 1.93 and the cesium removal was only 38.52%. The cesium removal by nZVI increased with the increase in the pH and the maximum removal was observed at pH 8 with cesium removal of 53.58%. At higher pH values, cesium removal by nZVI decreased to only 41.52% and the sorption capacity was 2.08. The cesium removal was inhibited in the acidic medium because the H + ions compete with Cs + ions for the sorption sites. In addition, the degradation of some fraction of the nanoparticles may occur under acidic conditions. In the strong alkaline environment, the formed passive layer of iron hydroxide precipitates can hinder further oxidation of nZVI [16].

Effect of initial cesium concentration and contact time
The effect of contact time on sorption of cesium ions onto nZVI was performed at 298 K and different initial concentrations of 1, 5, 50, 100, 150 and 200 mg/L and the results are presented in Figure 2. It is clear that the cesium sorption rate was initially very fast and the sorption amount of cesium increased with increasing the initial cesium concentration. The amount of sorbed cesium onto nZVI increased with time attaining a maximum value after 20 min. It was observed that the equilibrium time was independent on the initial cesium concentrations investigated in this study (1 to 200 mg/L). The values of maximum removal of cesium onto nZVI at 1, 5, 50, 100, 150 and 200 mg/L were 99.95%, 70.33%, 42.09%, 46.49%, 49.15% and 47.93%. The higher sorption capacity of the nanoparticles at high cesium concentrations could be attributed to higher possibility of collision between cesium ions and the nanoparticles. This behavior could also be related to the ratio of initial cesium concentration to the available reactive sites on the surface of nanoparticles.

Dosage effect
The removal of cesium by nZVI was studied by a series of nanoparticles dosage in the range from 1 to 30 g/L at initial cesium concentration of 100 mg/L, initial pH of 6 and temperature of 298 K. The relation between dosage and removal of cesium is shown in Figure 3. From Figure 3, cesium removal increased with increasing the amount of the nanoparticles that more active sites were available to sorb more cesium. With 1 g/L of nZVI, 29.54% of cesium was removed and the sorption capacity attained its maximum which was 29.54 mg/g. The removal of cesium was up to 33.43% when the dosage of nZVI increased to 2 g/L and the sorption capacity was 16.71 mg/g. These results proposed that the sorption capacity of nZVI was saturated when the dosage of nZVI was below 2 g/L in the treatment system. With 5 g/L of nZVI, the removal of cesium achieved 36.76% with the sorption capacity of 7.35 mg/g. When the addition of nZVI increased to 10 g/L, the removal of cesium achieved 47.27% and the sorption capacity was 4.73 mg/g. In the treatment system of 20 g/L of nZVI, 46.49% of cesium was removed and the removal capacity was 2.32 mg/g. The removal of cesium increased to 54.02% after the addition of 30 g/L of nZVI and the amount of cesium sorbed onto the nanoparticles was 1.80 mg/g.

Effect of temperature
The effect of temperature on cesium sorption by nZVI was investigated at four different temperatures of 298, 313, 328 and 343 K (Figure 4). The sorption capacity decreased with increasing the temperature, suggesting that nZVI was favorable for cesium sorption at low temperatures and the sorption process was exothermic. The values of removal of cesium onto nZVI at 298, 313, 328 and 343 K were 46.49%, 45.53%, 36.36% and 21.13% and the sorption capacity was 2.32, 2.28, 1.82 and 1.06 mg/g, respectively. Results are consistent with the greater sorption of Cs + on natural clays [17] and Ba 2+ on nZVI [18] with decreasing temperature.

Matrix effect
The effect of competing cation ions on cesium removal by nZVI at 298 K and initial pH of 6 was examined and the results are presented in Table 1. Four simulated waste solutions were prepared separately in the presence of similar concentration (20 mg/L) of Na + (0.87 mM), K + (0.51 mM), Mg 2+ (0.82 mM) and Ca 2+ (0.50 mM) with initial cesium concentration of 100 mg/L . It is evident that the removal of cesium were highly decreased in the presence of the competing cation. These findings confirmed that the cation ions can compete and lower the sorption of cesium on the nanoparticles. It was noted that the effect of K + ions on cesium removal was higher than Na + ions. This could be due to the close similarity in the hydration radii of K + and Cs + rather than to Na + [3]. In the presence of Mg 2+ and Ca 2+ ions, passivated precipitates such as Mg(OH) 2 and CaCO 3 formed on the nanoparticles surfaces resulting in blocking the electron transfer from the nanoparticles cores [19].

Conclusion
The current work evaluates the feasibility of using nZVI for cesium removal from aqueous solutions. nZVI demonstrated desirable performance for cesium removal. A minimum removal efficiency of 99% was reached at initial cesium concentration of 1 mg/L and a dosage of 1 g-nZVI/L. Solution pH is an important factor affecting sorption of cesium by nZVI. Lower temperatures are favored for enhanced removal of cesium. The obtained results demonstrated that nZVI can be used as efficient materials for