Developing a Chromatographic 99mTc Generator Based on Mesoporous Alumina for Industrial Radiotracer Applications: A Potential New Generation Sorbent for Using Low-Specific-Activity 99Mo

The commercial low-pressure column chromatographic 99Mo/99mTc generator represents a reliable source of onsite, ready-to-use 99mTc for industrial applications. These generators use fission-produced 99Mo of high specific activity, posing serious production challenges and raising proliferation concerns. Therefore, many concepts are aimed at using low-specific-activity (LSA) 99Mo. Nonetheless, the main roadblock is the low sorption capacity of the used alumina (Al2O3). This study investigates the feasibility of using commercial alumina incorporated with LSA 99Mo to develop a useful 99Mo/99mTc generator for industrial radiotracer applications. First, the adsorption profiles of some commercial alumina sorbents for LSA 99Mo were tested under different experimental conditions. Then, the potential materials to develop a 99Mo/99mTc generator were selected and evaluated regarding elution yield of 99mTc and purity. Among the sorbents investigated in this study, mesoporous alumina (SA-517747) presented a unique sorption-elution profile. It demonstrated a high equilibrium and dynamic sorption capacity of 148 ± 8 and 108 ± 6 mg Mo/g. Furthermore, 99mTc was eluted with high yield and adequate chemical, radiochemical, and radionuclidic purity. Therefore, this approach provides an efficient and cost-effective way to supply onsite 99mTc for radiotracer applications independent of fission-produced 99Mo technology.


Introduction
Short-lived radionuclides have proved their crucial role in developing different industrial applications [1][2][3][4]. Their contribution helps to provide effective malfunction detection and process optimization. Accordingly, this reduces production costs, enhances process efficiency, and improves product quality [2,3,5]. Notably, 99m Tc received considerable attention in multi-disciplinary fields due to its accessible availability and favorable nuclear properties, such as its short half-life of 6.01 h and the emission of low-energetic photons (140 keV) [6][7][8][9][10][11][12][13]. 99m Tc is widely available from 99 Mo/ 99m Tc radionuclide generators. These generators are based on retaining the parent, 99 Mo, and then the radioactive-decaygenerated daughter, 99m Tc, can be periodically collected using an isotonic saline solution as an eluent at desired time slots [10,14,15].
It is possible to obtain 99 Mo in two different product qualities; high-and low-specificactivity products. High-specific-activity 99 Mo is produced from the neutron-induced uranium fission method. This approach is the most widely used one for large-scale 99 Mo supply. Over 95% of all 99 Mo used for the production of 99m Tc is available from the fission

Effect of Solution pH
Batch experiments were conducted to verify the usefulness of some commercial alumina for CA-99 Mo sorption from aqueous solutions. In order to design a successful sorption process, investigating the optimum pH is of crucial concern. The solution pH mainly governs the sorption behavior, as it controls the existing Mo species in the solution and the degree of charge that appears on the sorbent surface [30]. To demonstrate the influence of solution pH on the efficiency of the sorption process, equilibrium studies were conducted at a pH ranging from (1 to 8). The results presented in Figure 1a uptake values are observed at an initial pH of around 3. Beyond this value, it can be seen that Mo uptake starts either to decrease very slightly (almost consistent) for M-Sauer, AA-11501, AA-46064, SA-199966, SA-517747 and SA-799300, or sharply, in the case of M-Neutral, AA-11502, SA-267740, SA-769290, SA-199974 and SA-544833. This behavior can be explained based on determining the surface charge of the solid phase and the distribution of the molybdate species in the solution.
On the one hand, the isoelectric point (pHIEP) of alumina sorbents varies from (pH 4-6.5), as reported in the literature [31,32]. Consequently, the surfaces of the sorbents carry a positive charge at pH < 4-6.5 and are negatively charged at pH > 4-6.5. On the other hand, the distribution of Mo species at different solution pH values was investigated using the PHREEQC software (version 3) (Figure 1c). At low pH values, different molybdenum anions and polyanions may exist due to the polymerization of the monomeric molybdate anions, MoO4 2-. Molybdate species of MoO4 2− , [Mo7O24] 6− , and [Mo8O26] 4− are the most predominant species in this region. These polyanions have higher molybdenum content. Therefore, an electrostatic attraction between negatively charged molybdate anions and positively charged alumina surfaces occurs [33].
The slight and sharp decrease in the uptake affinity of the sorbents can also be assumed based on the isoelectric point (pHIEP) of the studied alumina sorbents and the equilibrium pH values (Figure 1d). The sorbents with equilibrium pH values below or within (4-6.5) show a slight decrease in the uptake values. Meanwhile, those with an equilibrium pH > 6.5 (surpassing pHIEP) show a sharp decrease in their uptake behavior.

Thermodynamic Studies
The amount of sorbed CA-99 Mo was examined as a function of reaction temperature (T). The thermodynamic parameters include Gibbs free energy ΔG° (kJ/mol), standard enthalpy change ΔH° (kJ/mol), and standard entropy change ΔS° (J/mol•k). These were investigated at different temperatures (298, 313, 323, and 333 K) using Equations (1) and (2) [34,35] and are summarized in Table 1: On the one hand, the isoelectric point (pH IEP ) of alumina sorbents varies from (pH 4-6.5), as reported in the literature [31,32]. Consequently, the surfaces of the sorbents carry a positive charge at pH < 4-6.5 and are negatively charged at pH > 4-6.5. On the other hand, the distribution of Mo species at different solution pH values was investigated using the PHREEQC software (version 3) (Figure 1c) are the most predominant species in this region. These polyanions have higher molybdenum content. Therefore, an electrostatic attraction between negatively charged molybdate anions and positively charged alumina surfaces occurs [33]. The slight and sharp decrease in the uptake affinity of the sorbents can also be assumed based on the isoelectric point (pH IEP ) of the studied alumina sorbents and the equilibrium pH values (Figure 1d). The sorbents with equilibrium pH values below or within (4-6.5) show a slight decrease in the uptake values. Meanwhile, those with an equilibrium pH > 6.5 (surpassing pH IEP ) show a sharp decrease in their uptake behavior.

Thermodynamic Studies
The amount of sorbed CA-99 Mo was examined as a function of reaction temperature (T). The thermodynamic parameters include Gibbs free energy ∆G • (kJ/mol), standard enthalpy change ∆H • (kJ/mol), and standard entropy change ∆S • (J/mol·k). These were investigated at different temperatures (298, 313, 323, and 333 K) using Equations (1) and (2) [34,35] and are summarized in Table 1: where R is the universal gas constant (8.314 J/mol·k), T is the absolute temperature (K), and K d (mL/g) is the distribution coefficient. Linear plots of ln K d versus (1/T) are deployed and presented in Figure 2. The calculated ∆G • values at each temperature for all sorbents are ∆G • < 0, which indicates that the sorption processes of CA-99 Mo were spontaneous in nature and all the reactions were feasible. The values of ∆G • decrease with increasing temperature, implying that the more the temperature increases, the more the spontaneity degree can be improved. In addition, since the Gibbs free energy lies between −20 < ∆G • < 0, this indicates the occurrence of a physisorption process [36]. The values of ∆S • are positive, which states that random sorption occurs at all alumina adsorbents and Mo(VI) interfaces. The values of ∆H • are negative (∆H • < 0) for both M-Sauer and AA-11501 adsorbents, implying that Mo(VI) adsorption at their surfaces is exothermic [36,37]. Meanwhile, for SA-267740, SA-199966, SA-517747, SA-544833, and SA-799300 adsorbents, the change in enthalpy (∆H • ) is positive (∆H • > 0), suggesting that the adsorption of Mo(VI) at their surfaces is endothermic [38].

Adsorption Isotherms
Generally, the ion sorption mechanism for solids can occur either through chem bond formation (chemisorption) due to the formation of an inner-sphere surface com or through electrostatic attraction that mainly results from the formation of an o sphere surface complex. Moreover, the reactions with solid particles may involve the etration of the sorbent material or include the formation of precipitates on the adsor surface (usually time-dependent). Sorption can be described by empirical sorption

Adsorption Isotherms
Generally, the ion sorption mechanism for solids can occur either through chemical bond formation (chemisorption) due to the formation of an inner-sphere surface complex or through electrostatic attraction that mainly results from the formation of an outer-sphere surface complex. Moreover, the reactions with solid particles may involve the penetration of the sorbent material or include the formation of precipitates on the adsorbate surface (usually time-dependent). Sorption can be described by empirical sorption isotherms, where the relationship between the concentration of solute in solution (C e ) and the concentration of solute adsorbed on the surface of adsorbent q e is presented as an X-Y graph. Several models have been proposed to describe the observed trend of ion sorption on solid surfaces (sorbents); the most commonly used sorption isotherms are described below [39].
In 1926, Freundlich developed a general power equation to describe the sorption behavior of radionuclides onto different adsorbent materials [39,40]. It has the form shown, as follows: where q e (mg/g) is the concentration of Mo (spiked with 99 Mo) adsorbed and C e (mg/L) is the concentration of Mo remaining in the solution. K f (mg 1-n L n /g) and n f (dimensionless) are constants unique to each combination of adsorbent and adsorbate. Langmuir (1918) proposed an equation to describe the sorption of gases on the surface of solid sorbents. Afterward, this equation was used to describe the sorption of adsorbate onto different sorbent matrices in aqueous solutions [39,41]. This equation has the following form: where q e (mg/g) is the total concentration of solute adsorbed, K L (L/mg) is an equilibrium constant, and n L (mg/g) is the adsorption capacity. Temkin adsorption isotherms were initially used to describe hydrogen adsorption on platinum electrodes in an acidic solution as a chemisorption process [42]. The Temkin adsorption isotherm model considers the interaction between adsorbate and adsorbent in the range of intermediate concentrations, assuming that the adsorption heat depending on temperature, varies linearly (decline) with adsorbate-adsorbent overlap degree [43]. This relationship is described in the following equation: where A T is the Temkin isotherm equilibrium binding constant (L/g), b T is the Temkin isotherm constant, R is the universal gas constant (8.314 J/mol·K), and T is the temperature (K). The measured adsorption data (C e versus q e ) of CA-99 Mo on the alumina sorbent system were modeled using the non-linear forms of Freundlich, Langmuir, and Temkin. Adsorption isotherm models were applied, and both measured and modeled data are displayed in Figure 3. Adsorption parameters were optimized using the add-in Solver function in Microsoft Excel. Freundlich parameters (K f and n f ), Langmuir parameters (K L and n L ), Temkin parameters (A T and b T ), and the goodness of fit of the modeled lines to the experimental data (R 2 ) are shown in Table 2. The regression coefficient values tabulated in Table 2 demonstrate that the Langmuir model failed to fit the equilibrium sorption isotherm of Mo(VI) on commercial alumina sorbents, as lower R 2 values were obtained. On the contrary, good-to-excellent correlation values were obtained between the experimental results and the fitted data of the Freundlich isotherm model for all commercial alumina sorbents under investigation except for the M-Sauer and SA-799300 adsorbents, which best fit the Temkin model. 4 mg Mo/g were reached for CA-Mo by using M-Sauer, AA-11501, SA-267740, SA-199966, SA-517747, SA-544833, and SA-799300, respectively.
The obtained results reveal that out of the commercial alumina investigated in this study, SA-517747 exhibited a unique sorption profile and demonstrated a higher sorption capacity than the conventional alumina currently used in 99 Mo/ 99m Tc generators. Therefore, it can be considered a promising sorbent for developing a 99m Tc generator based on LSA 99 Mo for industrial applications.   These results indicate that adsorption of CA-99 Mo on the sorbents (AA-11501, SA-267740, SA-199966, SA-517747, and SA-544833) occurred mainly through multilayer adsorption at heterogeneous surfaces [39,44]. The Freundlich adsorption constant (n f ) represents the adsorption intensity, for example: (i) n f < 1 (a chemical process), (ii) n f = 1 (linear adsorption), and (iii) n f > 1 (physisorption) [44]. The n f values presented in Table 2 are higher than 1, indicating that the CA-99 Mo adsorption on commercial alumina sorbents used in this study was physisorption and favorable under the current experimental conditions. In addition, the closer the 1/n values are to 0 than to unity (ranging from 0.19 to 0.25), the more heterogeneous the surface is, implying a broad distribution of adsorption sites on the adsorbent surface [33,38]. Furthermore, Mo(VI) adsorption onto M-Sauer and SA-799300 shows a higher correlation with the Temkin model. This finding suggests that the reaction occurs in heterogeneous multilayer adsorption with a decrease in the heat of adsorption with increasing the overlap degree with Temkin constant (A T ) 1.57 and 1.98 L g −1 for M-Sauer and SA-799300 adsorbents, respectively.
The obtained results reveal that out of the commercial alumina investigated in this study, SA-517747 exhibited a unique sorption profile and demonstrated a higher sorption capacity than the conventional alumina currently used in 99 Mo/ 99m Tc generators. Therefore, it can be considered a promising sorbent for developing a 99m Tc generator based on LSA 99 Mo for industrial applications.
In order to establish an efficient sorption process, the equilibrium time and the kinetic sorption parameters were investigated. Therefore, the contact time needed for a complete uptake of CA-99 Mo onto SA-517747 was monitored. The obtained data shows a rapid and instantaneous removal of Mo from the solution, and equilibrium was established within the first minute. The results indicate that the equilibrium time was already reached at the very beginning of the sorption process. Accordingly, using the current methodology, such data cannot be modeled to sorption kinetic models.

Preparation of 99 Mo/ 99m Tc Generator
The distribution ratio (K d ) is a helpful indicator for investigating the selective sorption behavior of mesoporous alumina (SA-517747) for the parent 99 Mo from aqueous solutions and the feasible elution of its daughter 99m Tc. Larger K d values indicate that a more significant amount of ions can be retained on the sorbent material. The K d data for 99 Mo and 99m Tc on mesoporous alumina (SA-517747) are depicted in Figure 4. The figure shows that mesoporous alumina exhibits significant sorption affinity for CA-99 Mo and negligible affinity for 99m Tc. The K d of CA-99 Mo shows high values at around pH 4, which is optimum for 99 Mo sorption. Nonetheless, Kd values were kept consistent along the investigated pH range. To better understand this behavior, the isoelectric point (pH IEP ) of mesoporous alumina was determined experimentally and found to be 7.1 ± 0.5. In addition, the measured equilibrium pH values of the aqueous solution did not exceed this value (Figure 1d). Therefore, optimum conditions were maintained for Mo sorption onto SA-517747 at almost all the investigated pH ranges [19,22,33].
Furthermore, the distribution ratio (K d ) of the molybdate (MoO 4 2− ) and the pertechnetate ( 99m TcO 4 − ) anions on mesoporous alumina was monitored in 0.9% NaCl solution. The obtained data show high K d values of CA-99 Mo and lower K d values of 99m Tc in 0.9% NaCl solution. The low K d value of 99m TcO 4 − in 0.9% NaCl indicates its feasible elution from the column matrix [19,22]. The 99 Mo sorption-99m Tc elution mechanisms are discussed in [22,33]. Under column conditions, the dynamic capacity profile of CA-99 Mo onto mesoporous alumina (SA-517747) was studied and is depicted in Figure 5. The figure illustrates that the breakthrough capacity reaches 44 ± 3 mg Mo/g. After reaching this value, 99 Mo starts to appear in the effluent solution. The calculated dynamic sorption capacity at C/C0 = 0.5 is 108 ± 6 mg Mo per gram of sorbent material. These values are higher than the capacity of the conventional alumina (2-20 mg Mo/g of alumina) [22,24]. The data obtained from the distribution ratio (Kd) and the dynamic sorption profile of CA-99 Mo are promising for developing a 99 Mo/ 99m Tc generator based on mesoporous alumina using a higher amount of activity, about 500 MBq (13.5 mCi). Then, the column was washed and conditioned for the elution of the generated 99m Tc by passing two consecutive solutions, namely acetate buffer solution and 0.9% saline solution, through the column. Eventually, the 99m Tc elution profile was further studied. Figure 6 shows the 99m Tc typical elution profile from a mesoporous alumina column. The elution process was periodically conducted using 10 mL of 0.9% NaCl solution at a 1 mL/min flow rate. It can be observed that about 95% of the 99m TcO4 -radioactivity is concentrated in the first 5 mL of the eluate, which indicates a sharp elution profile and that high concentrations of radioactive 99m TcO4 -could be obtained. Furthermore, the elution Under column conditions, the dynamic capacity profile of CA-99 Mo onto mesoporous alumina (SA-517747) was studied and is depicted in Figure 5. The figure illustrates that the breakthrough capacity reaches 44 ± 3 mg Mo/g. After reaching this value, 99 Mo starts to appear in the effluent solution. The calculated dynamic sorption capacity at C/C 0 = 0.5 is 108 ± 6 mg Mo per gram of sorbent material. These values are higher than the capacity of the conventional alumina (2-20 mg Mo/g of alumina) [22,24].  Under column conditions, the dynamic capacity profile of CA-99 Mo onto mesoporous alumina (SA-517747) was studied and is depicted in Figure 5. The figure illustrates that the breakthrough capacity reaches 44 ± 3 mg Mo/g. After reaching this value, 99 Mo starts to appear in the effluent solution. The calculated dynamic sorption capacity at C/C0 = 0.5 is 108 ± 6 mg Mo per gram of sorbent material. These values are higher than the capacity of the conventional alumina (2-20 mg Mo/g of alumina) [22,24]. The data obtained from the distribution ratio (Kd) and the dynamic sorption profile of CA-99 Mo are promising for developing a 99 Mo/ 99m Tc generator based on mesoporous alumina using a higher amount of activity, about 500 MBq (13.5 mCi). Then, the column was washed and conditioned for the elution of the generated 99m Tc by passing two consecutive solutions, namely acetate buffer solution and 0.9% saline solution, through the column. Eventually, the 99m Tc elution profile was further studied. Figure 6 shows the 99m Tc typical elution profile from a mesoporous alumina column. The elution process was periodically conducted using 10 mL of 0.9% NaCl solution at a 1 mL/min flow rate. It can be observed that about 95% of the 99m TcO4 -radioactivity is concentrated in the first 5 mL of the eluate, which indicates a sharp elution profile and that high concentrations of radioactive 99m TcO4 -could be obtained. Furthermore, the elution The data obtained from the distribution ratio (K d ) and the dynamic sorption profile of CA-99 Mo are promising for developing a 99 Mo/ 99m Tc generator based on mesoporous alumina using a higher amount of activity, about 500 MBq (13.5 mCi). Then, the column was washed and conditioned for the elution of the generated 99m Tc by passing two consecutive solutions, namely acetate buffer solution and 0.9% saline solution, through the column. Eventually, the 99m Tc elution profile was further studied. Figure 6 shows the 99m Tc typical elution profile from a mesoporous alumina column. The elution process was periodically conducted using 10 mL of 0.9% NaCl solution at a 1 mL/min flow rate. It can be observed that about 95% of the 99m TcO 4 − radioactivity is concentrated in the first 5 mL of the eluate, which indicates a sharp elution profile and that high concentrations of radioactive 99m TcO 4 − could be obtained. Furthermore, the elution performance studies prove that the 99m Tc elution yield is reproducible and does not depend on the elution frequency of the generator (84 ± 0.73%) over two weeks. performance studies prove that the 99m Tc elution yield is reproducible and does not depend on the elution frequency of the generator (84 ± 0.73%) over two weeks. In order to evaluate the effectiveness of the radiochemical separation process, the generated eluates were examined based on their radiochemical, radionuclidic, and chemical purity. To investigate the radiochemical purity, 99m TcO4 -species were separated using chromatography paper (Whatman No. 1) developed in 85% methanol medium. Figure 7 shows the radio-chromatogram obtained for the 99m Tc eluates. In all chromatograms, the radiochemical (RC) purity was >99%, and only one peak was detected at R ≈ 0.8 corresponding to 99m TcO4 - [45,46]. This value agrees with the recommended specifications for the preparation of 99m Tc-labelled compounds [31,47,48]. Since 99 Mo is the prime impurity that may interfere with the eluate, determining its impurity level is a priority. The eluate purity was evaluated using gamma-ray spectrometry (HPGe detector coupled with a multichannel analyzer). The 99m Tc eluate was analyzed immediately after elution and after 60 h from elution, respectively. We observed that no energy peaks corresponding to 99 Mo were detected, and only the energy peak of 99m Tc (140 keV) was detected in the eluates. The absence of any 99 Mo gamma-energy peak in the collected samples indicated that the 99m Tc was obtained in an adequately pure form. The In order to evaluate the effectiveness of the radiochemical separation process, the generated eluates were examined based on their radiochemical, radionuclidic, and chemical purity. To investigate the radiochemical purity, 99m TcO 4 − species were separated using chromatography paper (Whatman No. 1) developed in 85% methanol medium. Figure 7 shows the radio-chromatogram obtained for the 99m Tc eluates. In all chromatograms, the radiochemical (RC) purity was >99%, and only one peak was detected at R f ≈ 0.8 corresponding to 99m TcO 4 − [45,46]. This value agrees with the recommended specifications for the preparation of 99m Tc-labelled compounds [31,47,48]. performance studies prove that the 99m Tc elution yield is reproducible and does not depend on the elution frequency of the generator (84 ± 0.73%) over two weeks. In order to evaluate the effectiveness of the radiochemical separation process, the generated eluates were examined based on their radiochemical, radionuclidic, and chemical purity. To investigate the radiochemical purity, 99m TcO4 -species were separated using chromatography paper (Whatman No. 1) developed in 85% methanol medium. Figure 7 shows the radio-chromatogram obtained for the 99m Tc eluates. In all chromatograms, the radiochemical (RC) purity was >99%, and only one peak was detected at R ≈ 0.8 corresponding to 99m TcO4 - [45,46]. This value agrees with the recommended specifications for the preparation of 99m Tc-labelled compounds [31,47,48]. Since 99 Mo is the prime impurity that may interfere with the eluate, determining its impurity level is a priority. The eluate purity was evaluated using gamma-ray spectrometry (HPGe detector coupled with a multichannel analyzer). The 99m Tc eluate was analyzed immediately after elution and after 60 h from elution, respectively. We observed that no energy peaks corresponding to 99 Mo were detected, and only the energy peak of 99m Tc (140 keV) was detected in the eluates. The absence of any 99 Mo gamma-energy peak in the collected samples indicated that the 99m Tc was obtained in an adequately pure form. The Since 99 Mo is the prime impurity that may interfere with the eluate, determining its impurity level is a priority. The eluate purity was evaluated using gamma-ray spectrometry (HPGe detector coupled with a multichannel analyzer). The 99m Tc eluate was analyzed immediately after elution and after 60 h from elution, respectively. We observed that no energy peaks corresponding to 99 Mo were detected, and only the energy peak of 99m Tc (140 keV) was detected in the eluates. The absence of any 99 Mo gamma-energy peak in the collected samples indicated that the 99m Tc was obtained in an adequately pure form. The analysis of the decayed samples of 99m Tc verified that the 99 Mo breakthrough in the eluate was ≤0.1%. Furthermore, the decay curve of 99m Tc eluate was investigated, and the data show that the eluate decays with a half-life of ≈6 h, which verifies the high purity of the 99m Tc eluate [49,50].
The presence of chemical impurities hinders the labeling efficacy of 99m Tc. These impurities may originate from the column bed matrix. Therefore, 99m Tc eluates were chemically analyzed using ICP-MS to detect the presence of Al. The results revealed that Al concentrations were in the range of <1 µg/mL. The pH of 99m Tc eluates was also measured by using pH paper. The pH values were found to be in the range of 6-6.5. These values are in agreement with the recommended value for 99m Tc eluates [19,45,50]. Table 3 displays the elution performance data of 99m Tc eluates from the prepared 99 Mo/ 99m Tc generator. Since the generator under a particular activity is no longer helpful for industrial applications, we attempted to remove the adsorbed 99 Mo for fast and safe disposal. Figure 8 shows the desorption profile of 99 Mo from the mesoporous alumina column. The figure shows that 99 Mo can be desorbed under alkaline conditions. Furthermore, nearly all of the loaded 99 Mo can be quickly recovered in the first 4-5 mL of 2 M NaOH, with a total recovery yield of >95%. analysis of the decayed samples of 99m Tc verified that the 99 Mo breakthrough in the eluate was ≤0.1%. Furthermore, the decay curve of 99m Tc eluate was investigated, and the data show that the eluate decays with a half-life of ≈ 6 h, which verifies the high purity of the 99m Tc eluate [49,50]. The presence of chemical impurities hinders the labeling efficacy of 99m Tc. These impurities may originate from the column bed matrix. Therefore, 99m Tc eluates were chemically analyzed using ICP-MS to detect the presence of Al. The results revealed that Al concentrations were in the range of <1 µg/mL. The pH of 99m Tc eluates was also measured by using pH paper. The pH values were found to be in the range of 6-6.5. These values are in agreement with the recommended value for 99m Tc eluates [19,45,50]. Table 3 displays the elution performance data of 99m Tc eluates from the prepared 99 Mo/ 99m Tc generator. Since the generator under a particular activity is no longer helpful for industrial applications, we attempted to remove the adsorbed 99 Mo for fast and safe disposal. Figure 8 shows the desorption profile of 99 Mo from the mesoporous alumina column. The figure shows that 99 Mo can be desorbed under alkaline conditions. Furthermore, nearly all of the loaded 99 Mo can be quickly recovered in the first 4-5 mL of 2 M NaOH, with a total recovery yield of >95%.

Materials
All chemicals were of analytical grade purity (A. R. grade) and were used without further purification. Milli-Q water was used for preparing solutions and washings. Sodium hydroxide and nitric acid were purchased from Merck, Darmstadt, Germany. The aluminum oxides were purchased from different suppliers (Table 4).

Instrumentation
Radiometric identifications and measurements were carried out by using a multichannel analyzer (Inspector 2000 model, Canberra Series, Meriden, CT, USA.) coupled with a high-purity germanium coaxial detector (HPGe). Samples of constant geometry were counted at a low dead time (<5%). The radionuclide levels were determined by quantifying the 140 and 740 keV photo peaks corresponding to 99m Tc and 99 Mo, respectively. A pH-meter with a microprocessor (Mettler Toledo, Seven Compact S210 model, Greifensee, Switzerland) was used to adjust the pH values. A thermostated shaking water bath (Julabo GmbH, Seelbach, Germany) was used for all batch equilibrium studies. Zeta potential (ζ) measurements were performed using a zeta-sizer Nano ZS (Malvern, UK) for isoelectric point (pH IEP ) measurements. The chemical analyses to determine trace levels of metal contaminations were performed using inductively coupled plasma-mass spectroscopy (NexION 2000s ICP-MS, PerkinElmer, Waltham, MA, USA).

Static Equilibrium Studies
The batch equilibrium experiments were conducted to investigate the sorption performance of carrier-added (CA) 99 Mo (Mo(IV) spiked with 99 Mo) under different experimental conditions, such as solution pH, initial Mo concentration, temperature, and reaction time.
In the first step, the impact of solution pH was evaluated over a broad range of pH values ranging from 1 to 8 by equilibrating 200 mg of each alumina sorbent with 20 mL of 50 mg/L CA-99 Mo solutions. The pH value was adjusted by adding a few drops of HNO 3 or NaOH. Each run of vials was kept at 25 ± 1 • C and shaken for 24 h in a thermally-controlled water bath shaker at a speed of 180 rpm. Then, the supernatant was decanted and centrifuged at 4000 rpm for 10 min. After that, 1 mL was pipetted and measured using a γ-ray spectrometer. Moreover, the pH of the Mo(IV) solution before and after reaching equilibrium was measured with a bench-style pH meter. In the second step, the effect of temperature on the CA-99 Mo sorption behavior onto each sorbent was investigated at four reaction temperatures, namely 298, 313, 323 and 333 K. The CA-99 Mo concentration was 1000 mg/L, and all other reaction parameters were kept constant. In the third step, equilibrium isotherm studies were conducted, applying the same previous experimental procedure while varying the initial molybdate concentration (50-5000 mg/L) and adjusting the initial Mo solution pH to (pH ≈ 3). Other parameters, such as reaction temperature and time, were kept at 25 ± 1 • C and 24 h, respectively. In the fourth step, the maximum adsorbent sorption capacity was determined by repeatedly equilibrating CA-99 Mo with different alumina sorbents under optimum reaction conditions. This procedure was repeated several times until complete saturation of each sorbent material with CA-99 Mo was achieved, and no further uptake occurred. Finally, the progress of CA-99 Mo uptake as a function of agitation time was monitored at different time intervals for an initial Mo concentration of 50 mg/L (pH ≈ 3), using a sorbent dose of 200 mg, and the reaction temperature was adjusted to 25 ± 1 • C.

Data Presentation
The sorption data of CA-99 Mo, such as uptake percent (U%), distribution coefficient (K d ), sorption equilibrium capacity (q e ), Mo(IV) equilibrium concentration (C e ), and maximum sorption capacity (q max ) were calculated according to the following equations: where A o and A e are the initial and equilibrium 99 Mo radioactivity (counts/min), respectively. v 1 (mL) and v (L) are the liquid phase volume. m is the sorbent weight (g) and C o is the equilibrium Mo(IV) concentration in (mg/L).

Application of Mesoporous Alumina in Preparing a 99 Mo/ 99m Tc Generator
To evaluate the dynamic sorption capacity of mesoporous alumina for CA-99 Mo under column conditions, we packed 1 g of the sorbent material in a column of dimensions (12 cm length × 0.4 cm i.d.) with a sintered disc at the bottom. Then, the column matrix was treated with 10 −3 M HNO 3 . Subsequently, sodium molybdate solution (5 mg Mo/mL), spiked with 370 kBq (10 µCi) of 99 Mo tracer, was passed through the columns at a flow rate of 0.25 mL/min. In order to monitor the adsorption pattern, 2 mL of the mother feed solution was kept as a reference (C o ). Likewise, the effluent volume was collected in fractions of 2 mL aliquots (C). Then, the count rate ratio of each fraction to the count rate of the mother feed solution was determined by measuring the 740 keV γ-ray peak of 99 Mo in a HPGe detector. Eventually, the capacity was calculated by using the following equation: where C o is the initial Mo ion concentration in its feeding solution, V 50% is the effluent volume (mL) at C/C 0 = 0.5, and m (g) is the weight of the column matrix.
To design a 99 Mo/ 99m Tc generator, a column with 1 g of mesoporous alumina (SA-517747) was packed and conditioned with HNO 3 . Then, it was loaded with 130 mg Mo spiked with 500 MBq of 99 Mo (pH ≈ 3) by using the previously mentioned protocol. Subsequently, the column was washed with 50 mL of acetate buffer solution and 100 mL of 0.9% saline solution. Finally, the column was left for about 24 h before the 99m Tc elution.

Elution Performance of 99m Tc Eluate
In order to investigate the 99m Tc elution performance, the generator was eluted with 0.9% NaCl solution at a 1 mL/min flow rate at different time slots. The eluates were collected in equal fractions (1 mL each) and immediately analyzed. In order to identify the contribution of foreign radionuclidic contaminants in the 99m Tc solution, the eluates were radiometrically analyzed immediately after elution and subsequently after 60 h. Additionally, the radionuclidic purity of 99m Tc eluate was studied by following its radioactive decay.
The radiochemical purity of the eluted 99m Tc (percentage of 99m TcO 4 − to the total activity of the eluate) was determined by ascending paper chromatography using Whatman no. 1 paper and a mixture of (85% methanol + 15% H 2 O) as developing solvent. The radioactivity distributions were monitored using a γ-ray spectrometer to determine R f . The measured radioactivity was plotted as a function of the traveled distance from the starting line. The R f value was calculated according to the following equation: The distance (Cm)from the starting line to the radioactivity peak position The distance (Cm)from the starting line to the solvent front (12) Any possible impurities of aluminum were determined in the 99m Tc eluates originating from the column matrix. The aluminum level was measured by using inductively coupled plasma-mass spectroscopy (ICP-MS). All tests were performed after complete decay of 99 Mo and 99m Tc in the eluates.

Recovery of 99 Mo from the Spent Generator
The exhausted generator was rinsed with 20 mL of 0.9% saline. Then, Mo was desorbed using a 2 M NaOH solution at a 0.5 mL/min flow rate. The desorbed solution was recovered in fractions of 1 mL each. Subsequently, each fraction was measured, and the total recovery yield of 99 Mo was investigated.

Summary and Conclusions
The objective of this paper was to investigate the feasibility of using commercial alumina incorporated with LSA 99 Mo to develop a useful 99 Mo/ 99m Tc generator for industrial radiotracer applications. From the research conducted in this study, we found that molybdenum is selectively adsorbed on a mesoporous alumina (SA-517747) column, and the sorbent exhibits a high equilibrium and dynamic sorption capacities for LSA 99 Mo (148 ± 8 and 108 ± 6 mg Mo/g). Moreover, 99m Tc could be eluted with high yield and adequate chemical, radiochemical, and radionuclidic purity. The available specific activity of LSA 99 Mo can reach 5 Ci/g Mo. Therefore, based on our findings, it is possible to build a 99m Tc generator of 18.5 GBq (500 mCi) at calibration time using 1 g of mesoporous alumina (SA-517747). Consequently, mesoporous alumina is a viable option for developing a 99m Tc generator based on 99 Mo of LSA. This method provides an efficient and cost-effective way to supply onsite 99m Tc for radiotracer applications independent of fission-produced 99 Mo technology.