Removal of Technetium ( 99 Tc) from Aqueous Waste by Manganese Oxide Nanoparticles Loaded into Activated Carbon

Technetium-99 is a radioactive isotope with a half-life of 2.13 × 10 5 year. 99 Tc is a significant contaminant of concern to the world. For this reason, a de-tailed understanding of technetium chemistry is essential for the protecting the public and the environment especially after increasing the various applications and uses of isotopes in the medical practices. Therefore, treatment of waste increases prior to the safe discharge to the environment or the storage. The sorption of technetium in the form of pertechnetate on a nano manganese oxide loaded into activated carbon has been investigated. Nano manganese oxide (NMO) was synthesized from manganese chloride and potassium permanganate by co-precipitation and forming a new composite by loading a nanoparticle into a modified activated carbon by different ratios. Modifica-tions of activated carbons using different concentrations of HNO 3 (4 M, 6 M and 8 M) are used in prepared composites. Fourier transform infrared spectroscopy (FT-IR), X-Ray Diffraction (XRD) and Scanning Electron Microscope (SEM) were used to characterize the prepared composites. The adsorption of 99 4 TcO − anions from low level radioactive aqueous waste was examined using batch technique. Different parameters affecting on the adsorption process were studied for the removal of 99


Abstract
Technetium-99 is a radioactive isotope with a half-life of 2.13 × 10 5 year. 99 Tc is a significant contaminant of concern to the world. For this reason, a detailed understanding of technetium chemistry is essential for the protecting the public and the environment especially after increasing the various applications and uses of isotopes in the medical practices. Therefore, treatment of waste increases prior to the safe discharge to the environment or the storage. The sorption of technetium in the form of pertechnetate on a nano manganese oxide loaded into activated carbon has been investigated. Nano manganese oxide (NMO) was synthesized from manganese chloride and potassium permanganate by co-precipitation and forming a new composite by loading a nanoparticle into a modified activated carbon by different ratios. Modifications of activated carbons using different concentrations of HNO 3  TcO − . The results revealed that NMO/AC (4 M, 6 M and 8 M) has a high adsorption efficiency (93.57%, 90.3% and 90.3%) respectively compared to NMO and AC which have a lower adsorption efficiency (41% and 38.9%) respectively. Moreover, the adsorption isotherm belonged to Freundlich model, the adsorption data followed pseudo-second order model and the thermodynamic study indicated that the adsorption of 99 4 TcO − on Nano-composites was an exothermic and spontaneous process.

Introduction
Technetium-99 ( 99 Tc) is a pure β-emitter radionuclide with a half-life of 2.13 × 10 5 years. It is formed in the thermal neutron fission of 235 U with high yield (6%) [1] [2] and by spontaneous fission of 238 U in the earth's crust [3]. It also appears environmentally from the decay of the medical radioisotope 99m Tc (6.0 h half-life) [4]. In uranium enrichment facilities, waste streams containing a high concentration of 99 Tc result from the decontamination of process equipment. Technetium-99 is very mobile in groundwater due to its existence dominantly as the anionic species, pertechnetate 4 TcO − [5]. The pertechnetate 4 TcO − is not immobilized by most common minerals or inorganic sorbents because it is repulsed by their negative charge [6].
A large amount of 99 Tc has been produced and released to the environment from nuclear installations, which makes 99 Tc the only environmentally significant Tc isotope. Meanwhile, 99 Tc can also be produced through neutron activation of 99 Mo. Figure 1 shows the formation scheme of 99 Tc. The higher-water solubility of Tc (in the form of 4 TcO − ), the longer half-life of 99 Tc, and thus long residence time in the oceans, make 99 Tc an ideal oceanographic tracer for investigation of movement, exchange and circulation of water masses. All these investigations and applications require an accurate determination of 99 Tc in various types of samples [7]. Technetium-99 is of particular concern to the U.S. Department of Energy (DOE), the Environmental Protection Agency (EPA), and the Nuclear Regulatory Commission (NRC) because of its persistence and mobility in the environment [8]. Technetium-99 is particularly mobile once released from the waste, since 99m Tc is present in aqueous solutions in its stable heptavalent state as the pertechnetate anion, 99 4 TcO − . This oxo-anion is highly soluble in groundwater under oxidizing conditions and difficult to eliminate, posing concern as a significant environmental hazard [9]. Each nuclear facility generates about 40 kg of 99 Tc every year, most of which is disposed of as nuclear waste material [10]. 99 Tc has been widely distributed in the environment as a result of fallout from ballistics testing [11]. Activated carbon has been shown to retain pertechnetate ions efficiently [12] investigated the adsorption behavior of pertechnetate, 99m 4 TcO − from various acid and salt concentrations. The mechanism behind the adsorption is not exactly known but ion-exchange with functional groups on the active carbon has been suggested by Gu et al., in 1996 [13].
Nanoparticles have received an increasing amount of research interest. This is due to the unique size-dependent properties of nanoparticles, which are often thought as separate and intermediate state of the matter lying between individual atoms and bulk material [14]. Nanoparticles consisting of a large range of transition metals and metal oxides have found to exhibit advantageous size-dependent catalytic properties and are being investigated intensively [15] [16] [17] [18]. Nanomaterials [19] are having a length scale less than 100 nm which are having their potential applications of fascinating electrical, magnetic and catalytic properties [20].
In the present work, we applied the Co-precipitation method to synthesize MnO 2 nanoparticles (NMO) loaded with modified activated carbon after the treated with HNO 3 by different concentrations (4 M, 6 M and 8 M). The prepared composites are used as a low-cost adsorbent for the adsorption of Technetium-99 from aqueous solutions. The influence of various experimental parameters on 99 4 TcO − adsorption process, in addition the optimum adsorption conditions were studied, sorption kinetics and the sorption capacity of NMO/ AC was evaluated using different isotherm models.

Chemicals and Reagents
All chemicals were analytical grade, purchased and used as received without further purification: Potassium Permanganate KMnO 4 ; Sigma-Aldrich, Manganese Chloride MnCl 2 •4H 2 O; Merk, Activated Carbon (AC) commercial-grade provided by El-Gomhoria company for trade service (Cairo), 99 Tc radionuclide (activity 257.4 CPM/ml) was extracted from residual 99m Tc columns from the technetium generator used in nuclear medicine centers, Ultima Gold TM (AB LSC-cocktail) from Packard bioscience company, concentrated acids of A.R HNO 3 , HCl and NaOH were used throughout the investigations as required and freshly bi-distilled water was used through all experiments.

Sorbent Preparation
A variety of sorbents were prepared for 99 4 TcO − removal from the aqueous waste. Sorbent material includes 1) Nanosized manganese oxide (NMO) by co-precipitation method, at which the prepared 0.2 M KMnO 4 stirred with a magnetic stirrer, the pH was adjusted between pH 8 -9 using 2 M NaOH then dropwise 0.3 M MnCl 2 . The suspension is stirred for 60 min then the precipitate is left overnight to settle. The solution is decanted to the extent possible, and the remaining suspension is centrifuged. The sample washed with bi-distilled water, dried under vacuum for two days and finally heated at 90˚C till constant weight [21]. 2) Activated carbon (AC) was soaked overnight in three different HNO 3 concentrations (4 M, 6 M and 8 M) followed by heating at 80˚C -90˚C for 6 hours with refluxing. The treated samples washed with bi-distilled water. The treated AC samples were dried under vacuum for two days and finally, heated at 900˚C till constant weight, grinding and homogenized. 3) Mix the sorbents to evaluate the synergistic effects of modified activated carbon and NMO.

Batch Sorption Experiment
The batch experiments were performed by 0.05 gm of sorbent which added to 5 ml of radioactive waste solutions into a polypropylene centrifuge tube. The suspensions were placed on a slow-moving shaker rotates at 200 rpm for 2 hours. Each suspension was passed through a 0.45 µm cellulose membrane filter with a diameter (25 mm). The effect of initial pH, contact time, initial concentration of adsorbate ( 99 Tc), ionic strength and temperature were investigated. The pH of the solution adjusted at the desired value by adding 0.1 M NaOH or HCl. The radionuclide concentrations of the filtrates were analyzed by a liquid scintillation counter (LSC) where 5 ml of the filtrate was added to 5 ml Packard Ultima Gold TM AB LSC-cocktail for effective beta discrimination LSC, for measuring the activity the LSC model Packard TRI CARB 2770 was used. The adsorption capacity of 99 Tc q t (CPM/g) calculated by Equation (1).
Where: C 0 (CPM/L) is the initial concentration of 99 Tc, C t (CPM/L) is the concentration of 99 Tc at time t, V (L) is the volume of 99 Tc solution, and m (g) the mass of sorbent.
The percent uptake or the removal efficiency (R%) was calculated by Equation The equilibrium adsorption capacity of adsorbent, q e (CPM/g) can be determined by Equation (3).
where: C e (CPM/g) is the 99 Tc concentration at equilibrium in the supernatant after separation of the adsorbent and V, m, and C 0 have the same meaning as in

Structural Characterization
-X-Ray diffraction X-ray diffraction (XRD) analysis was employed to study the phase composition of the synthesized composites, using a diffractometer (Bruker; D8 advance) with monochromator Cu-Kα target wavelength 1.54178 , 40 kV, and 40 mA Germany. Figure [30] and carbon at 2θ = 37.594 related to (JCPDS 26-1076) [31]. It was obvious that sharp diffraction beaks for carbon at 2θ = 26.726 indexed to (JCPDS 26-1077) in their diffraction patterns as depicted in Figure 2(e). The subsequent phase transition into different valences of manganese oxide possibly owing to the oxidative atmosphere used at the beginning of the synthesis provides a pathway to oxidize some Mn 2+ present in Mn(OH) 2 to Mn 3+ and Mn 4+ during the drying step, the reducing reaction with these carbon matrix or the acidic treatment of carbon, then it seems that a partial amount of Mn 3+ can be oxidized leading to where, λ (nm) is the X-ray radiation wavelength, β is the diffraction peak full width at half maximum (FWHM), and θ B is the Bragg diffraction angle.
-FT-IR analysis Fourier Transform Infra-Red spectroscopy (FTIR) model (BRUKER; VERTEX 70v) was used at room temperature in Egyptian Nuclear and Radiological Regulatory Authority for obtaining information about the functional groups in the structure. The FTIR spectra of samples NMO, NMO/(4M AC), NMO/(6M AC), NMO/(8M AC) and AC are depicted in Figure 3 and the bands illustrated as follows. In 400 -4000 cm −1 , the bands at 414 -592 cm −1 should be ascribed to the Mn−O vibrations, and the bands at 750 -1640 cm −1 are usually attributed to the -OH bending vibration with Mn atoms [34] [35], meanwhile, bands can be observed at 3400 -3678 cm −1 which attributed to the O−H stretching vibration [36] [37]. The bands 1500 -1900 cm −1 due to C = O stretching [37] [38], where the bands at 2846, 2912 cm −1 correspond to C-H symmetric and asymmetric respectively [37] [39]. It was found that bands at 3700 -3899 cm −1 attributed to hydrogen bonds (H 2 O…H…H 2 O) + , which were formed between water and protons of acidic groups [40]. Finally, the wave number at bond 2356 cm −1 may be due to forming a new functional group NC = O which can be turned into an electron structure [39].

Morphology Study
The Scanning Electron Microscope (SEM) with a microscope (JEOL JSM-6510LA) was used to study the surface morphology and the particle diameter of NMO, NMO/(4 M AC), NMO/(6 M AC) , NMO/(8 M AC) and AC investigated by field emission Figure 4 indicate the different morphology between the used adsorbents. Figure 4(a) showed that NMO is foamy, spongy and agglomerated. The size of agglomerates is about 50 µm. Figure 4 Figure 4(e) the SEM micrograph showed that the obtained nanoparticle has a flower, mushrooms and agglomerated, with an average size of agglomerates about 50 µm.

Batch Adsorption Experiments
-Effect of solution pH The pH of the solution is an important parameter that controlling the surface charge of the adsorbent and it has a significant change in the behavior of 4 TcO − ions. HNO 3 or HCl and NaOH were used to adjust the pH and reduction of Tc is favored at low pH and alkaline media oxidizes [41]. The results of the adsorbed pertechnetate   TcO − increases with time to reach a saturation level for one-hour prolonged adsorption 24 hours did not a significant change of the equilibrium capacity for any of the investigated sorbents. Figure 6 shows that there was no significant difference between the removal of The significant increase in the adsorption capacity for the anionic type of adsorbates could be attributed to the interaction of the Nano-manganese oxide loaded into the AC by 4 M, 6 M and 8 M [43]. This variation may be due to the initial large number of vacant sites on the adsorbent surface is available for adsorption and high gradient of solute concentration [44] [45]. The slow rate of adsorption may be due to decreasing in the number of vacant surface sites of adsorbent and remaining vacant surface sites are difficult to be occupied due to repulsive force between the solute molecules on the solid and bulk phases [46].
-Effect of temperature The temperature of the reaction has a significant effect on the rate of the adsorption process. The adsorption process was carried out at different temperatures (25˚C, 30˚C, 40˚C and 50˚C) for 99 4 TcO − ions using 0.05 gm of   fact that more adsorption sites were being covered as the pertechnetate ions concentration increased [47].

Adsorption Isotherms
An adsorption isotherm is a curve describing the phenomenon governing the retention or mobility of a substance from the aqueous porous media or aquatic environments to a solid-phase at a constant temperature and pH [48] where: C e (CPM/L) is the equilibrium concentration of the adsorbate, q e (CPM/g) is the amount of adsorbate adsorbed per unit mass of adsorbent, K L and Q m are Langmuir constants and the adsorption capacity in (CPM/g) respectively. C e /q e is plotted against C e , a straight line with slope (1/Q m ) and intercept (1/Q m K L ) is obtained. The essential characteristics of Langmuir isotherm can be expressed by a dimensionless constant called separation factor or equilibrium parameter, R L , defined by Weber and Chakkravorti Equation (6) [56].
Lower in R L value reflects that adsorption is more favorable. In explanation, R L value indicates the adsorption nature to be either unfavorable if (R L > 1), linear (R L = 1), favorable (0 < R L < 1) or irreversible (R L = 0). Table 1 illustrates R L values near to zero, which indicate irreversible adsorption process.
-Freundlich isotherm model Freundlich isotherm model assumes that the adsorption takes place on a heterogeneous surface of the adsorbent and the linearized form of this model can be given by Equation (7) where: K f is the Freundlich constant (CPM/g) which represents the relative adsorption capacity of the adsorbent. (1/n) is the heterogeneity factor and it is a function of the strength of adsorption in the adsorption process and (n) has various values depending on the heterogeneity of the sorbent. if (n) lies between one and ten, this indicates a favorable sorption process [25]. (1/n) and lnK f values were calculated from the slope and intercept of the linear plots of lnq e versus lnC e which shows Freundlich isotherm as in Figure 10 and Table 1. The plot of lnq e versus lnC e gave a straight line with a slope of (1/n) and intercept of lnK F (Equation (7)). The slope ranges between 0 and 1 is a measure of adsorption intensity or surface heterogeneity, becoming more heterogeneous as its value gets closer to zero. Whereas, a value below unity implies chemisorption's process where (1/n) above one is indicative of cooperative adsorption [59].
According to the results presented in Table 1 the calculated correlation coefficients (R 2 ) for Freundlich Model were closer to unity so, this model may be the more appropriate than Langmuir Model. Moreover, the values of (1/n) obtained  from Freundlich Isotherm, equal 1.935, 1.6514 and 1.6513 for NMO/4 M AC, NMO/6 M AC and NMO/8 M AC respectively, (1/n) above unity indicate bi-mechanism and cooperative adsorption [60]. Therefore, 99 M AC could be believed to be complex. The adsorption sites were most likely heterogeneous while the adsorption was not limited to be monolayer formation.

Kinetics Adsorption Models
To predict the adsorption kinetic model for adsorption 99 4 TcO − from its radioactive liquid waste onto composites MnO 2 /AC (4, 6, 8 M). The kinetics models: pseudo-first and second order models were applied to the experimental data. The best fit model was selected based on the linear regression correlation coefficient (R 2 ), which is a measure of how the predicted values from a forecast model match with the experimental data.
-Pseudo-first order model It assumes that the rate of change of the solute uptake with time is directly proportional to the difference in the saturation concentration and the amount of solid uptake with the time, i.e. the rate of occupation sites is directly proportional to the number of unoccupied sites. Probably the earliest known and one of the most widely used kinetic equations so far for the adsorption of a solute from a liquid solution is the Lagergren equation or the pseudo-first order equation [61]. The pseudo-first order kinetics is calculated by Equation (8).
where: K 1 is the rate constant of pseudo-first-order adsorption (min −1 ), and q e and q t denote the amounts of adsorbed 99 4 TcO − anions at equilibrium and at time t (Bq•g −1 ) respectively. Plot of ( ) log e t q q − versus t should give a straight line to confirm the applicability of the kinetic model, logq e should be equal to the intercept and K 1 the slope as disputed in Figure 11 and Table 2.
-Pseudo-second order model It assumes that the rate of occupation of adsorption sites is proportional to the square of the number of unoccupied sites. Where the rate limiting step may be chemical sorption involving valence forces through sharing or exchange of electrons between 4 TcO − and sorbent [62]. The pseudo-second order equation is applied in the given form Equation (9).  where: k 2 (CPM•g −1 •min −1 ) is the rate constant of the pseudo-second order adsorption. Additionally, the initial adsorption rate h (CPM•g −1 •min −1 ) can be determined using Equation (10) [63].
Plot of t/q t versus t should give a straight line, 1/q t equal the intercept and 2 2 1 e K q the slope as shown in Figure 12. Table 2

Adsorption Thermodynamic
The concept of thermodynamic assumes that in an isolated system where the energy cannot be gained or lost, the entropy change is the driving force [64]. Van't hoff Equation (11) was used to calculate the thermodynamic parameters such as change in enthalpy (∆H˚), a change in entropy (∆S˚) and change in Gibbs free energy (∆G˚). Adsorption of 99 4 TcO − on the investigated materials, at different temperatures was determined by using Equations (12) & (13) and illustrated in Figure 13.
where: R (8.314 J/mol K) is the universal gas constant, T (K) the absolute solution temperature and K d is the equilibrium constant which can be calculated as: The obtained values of thermodynamic parameters, (∆H˚, ∆S˚ and ∆G˚), are presented in Table 3    TcO − from low level radioactive liquid waste.