Evaluating the Sorption Affinity of Low Specific Activity ⁹⁹Mo on Different Metal Oxide Nanoparticles

Abstract

⁹⁹Mo/^99mTc generators are mainly produced from ⁹⁹Mo of high specific activity generated from the fission of ²³⁵U. Such a method raises proliferation concerns. Alternative methods suggested the use of low specific activity (LSA) ⁹⁹Mo to produce ^99mTc generators. However, its applicability is limited due to the low adsorptive capacity of conventional adsorbent materials. This study attempts to investigate the effectiveness of some commercial metal oxides nanoparticles as adsorbents for LSA ⁹⁹Mo. In a batch equilibration system, we studied the influence of solution pH (from 1–8), contact time, initial Mo concentration (from 50–500 mg∙L⁻¹), and temperature (from 298–333 K). Moreover, equilibrium isotherms and thermodynamic parameters (changes in free energy ΔG⁰, enthalpy change ΔH⁰, and entropy ΔS⁰) were evaluated. The results showed that the optimum pH of adsorption ranges between 2 and 4, and that the equilibrium was attained within the first two minutes. In addition, the adsorption data fit well with the Freundlich isotherm model. The thermodynamic parameters prove that the adsorption of molybdate ions is spontaneous. Furthermore, some investigated adsorbents showed maximum adsorption capacity ranging from 40 ± 2 to 73 ± 1 mg Mo∙g⁻¹. Therefore, this work demonstrates that the materials used exhibit rapid adsorption reactions with LSA ⁹⁹Mo and higher capacity than conventional alumina (2–20 mg Mo∙g⁻¹).

1. Introduction

⁹⁹Mo/^99mTc radioisotope generators have a growing importance in nuclear medicine investigations. They are the primary source of supplying ^99mTc radionuclide for diagnostic purposes [1,2,3]. ^99mTc is considered the workhorse of all nuclear medicine applications [4,5]. It is involved in more than 80% of all in vivo diagnostic procedures because of its ideal nuclear characteristics, such as the short half-life of 6 h, absence of beta particles, and emission of a mono-energetic photon with low energy at 140 keV [3,6]. Therefore, this leads to less radiation exposure dose to the patients, and it produces a high-quality image for better diagnosis aspects. Furthermore, its unique labeling chemistry allows the use of a wide range of ^99mTc-labelled compounds to visualize different body organs [7,8]. For instance, ^99mTc-DTPA and ^99mTc-MAG3 are used to monitor renal functions [9]. In addition, ^99mTc-tetrofosmin, ^99mTc-sestamibi, and ^99mTc-teboroxime are utilized for the diagnosis of cardiac disease [10]. Moreover, ^99mTc-lidofenin is applied for liver diagnostics [11]. Furthermore, ^99mTc-medronate, ^99mTc-propyleneamineoxime, and ^99mTc-MDP (methylene diphosphonate) are involved in skeletal imaging, cerebral perfusion, and diagnosis of bone metastases, respectively [12,13,14,15].

Among the developed ⁹⁹Mo/^99mTc generators, the chromatographic column type is the most widely used system [3,16]. This system is based on adsorbing ⁹⁹Mo on a column filled with a suitable material from which ^99mTcO₄⁻ can be easily eluted while ⁹⁹Mo remains adsorbed [3]. The differences between these generators include the column material and the origin of the parent, ⁹⁹Mo. The main practical difficulties linked to the preparation of ⁹⁹Mo/^99mTc generators are the low sorption capacity of the bulk conventional inorganic sorbents usually used. These sorbents have low sorption capacity (2–20 mg Mo/g) due to the low availability of active sites and relatively limited surface area [3]. Consequently, such sorbents require a parent of high specific activity to prepare a useful generator of a proper radioactivity level. A high specific activity parent can be produced from the fission of ²³⁵U. Fission-produced ⁹⁹Mo faces some critical difficulties. For example, sophisticated infrastructures and well-qualified personnel are needed to separate and purify ⁹⁹Mo from the irradiated ²³⁵U target and other fission products. In addition, a considerable level of radioactive waste is generated during the manufacturing process, which increases the cost of production [17,18]. Alternatively, research studies focused on developing clinical-grade chromatographic ⁹⁹Mo/^99mTc generators based on ⁹⁹Mo of low specific activity (LSA) [3,19,20]. However, this proposal demands using high-capacity sorbents to compensate for the LSA ⁹⁹Mo and make it more reliable from the economic point of view [17,21].

The use of advanced nanomaterials has generated a growing interest in developing diagnostic ^99mTc generators [3]. Nawar and Türler [3] highlighted several nanomaterial adsorbents that have been developed for ⁹⁹Mo/^99mTc generator application. This class of sorbents possesses appreciable adsorption capacity and unique performance [20]. In this regard, the utilization of advanced commercial metal-oxide nanoparticles is an exciting idea due to their improved properties. In contrast to traditional sorbents, these nano-adsorbents have large surface-to-volume ratios, enhanced porosity, improved surface reactivity, and significant radiation resistance and chemical stability [21,22]. Therefore, they show high adsorption efficiency and selectivity [23].

In this study, we intend to evaluate the sorption efficiency of some commercially available nano-metal oxides towards LSA ⁹⁹Mo. To achieve this goal, we investigated the adsorption behavior of the selected materials for LSA ⁹⁹Mo under different experimental conditions. These conditions include the pH, initial concentration of molybdate ions, contact time, and temperature. In addition, to better understand their sorption behavior, the sorption kinetics, equilibrium isotherms, and thermodynamic behavior were evaluated.

2. Results and Discussion

2.1. Effect of Solution pH

The solution pH has a profound impact on the efficiency of the adsorption process. The influence of pH can be clarified by understanding its role in varying the ionic state of the functional groups on the adsorbent surface. Moreover, it affects the ionization and/or the dissociation of the studied ions [24]. In this context, a batch equilibration experiment was conducted at a pH range from 1 to 8 to determine the optimum pH value that shows the maximum ⁹⁹Mo retention on each adsorbent. Figure 1a depicts the distribution coefficients (K_d) of CA-⁹⁹Mo at different pH values. The data presented in this figure show that higher K_d values are observed at pH values (2–4). Beyond this region, the K_d values decrease with increasing the solution pH, which agrees with previously published studies [25].

Since the adsorbents are metal oxides, they might have similar surface chemistry. Moreover, since the adsorption process depends mainly on the aqueous phase’s pH values and the adsorbent material’s surface characteristics, we investigated the isoelectric point (pHI_EP) of each adsorbent (

Table 1. Description of the analyzed commercial metal oxides NPs *.

No.	Name	Description	Particle Size, (nm)	Surface Area, (m2∙g−1)	Isoelectric Point (pHIEP)
CeO2-544841	Cerium oxide-SA-544841	Molecular formula: CeO2 Molecular weight: 172.11 Density: 7.13 g∙mL−1 at 298 K	<25	N.A	5
ZrO2-544760	Zirconium oxide-SA-544760	Molecular formula: ZrO2 Molecular weight: 123.22 Density: 5.89 g∙mL−1 at 298 K	<100	≥25	6.1
TiO2-637254	Titanium oxide-SA-637254	Molecular formula: TiO2 Molecular weight: 79.87 Density: 3.9 g∙mL−1 at 298 K	<25	45–55	6.6
SnO2-549657	Tin oxide-SA-549657	Molecular formula: SnO2 Molecular weight: 150.71 Density: 6.95 g∙mL−1 at 298 K	≤100	20.1	3.8
SiO2-637246	Silicon oxide-SA-637246	Molecular formula: SiO2 Molecular weight: 60.08 Density: 2.2–2.6 g∙mL−1 at 298 K	5–20	590–690	2.5
AlCeO3-637866	Cerium aluminium oxide-SA-637866	Molecular formula: AlCeO3 Molecular weight: 215.1	≤80	N.A	4.8
Al2TiO5-634143	Aluminium titanium oxide-SA-634143	Molecular formula: Al2TiO5 Molecular weight: 181.83	<25	N.A	6.4
Al2TiO5-14484	Aluminium titanium oxide-AA-14484	Molecular formula: Al2TiO5 Molecular weight: 181.86	100 mesh	N.A	6.5
CeO2/ZrO2-634174	Cerium zirconium oxide-SA-634174	Molecular formula: (CeO2)·(ZrO2) Molecular weight: 295.34 Density: 6.61 g∙mL−1 at 298 K	<50	N.A	6.7
SiO2/Al2O3-643653	Aluminosilicate-SA-643653	Molecular formula: (SiO2)x(Al2O3)y pore volume: 0.8–1.1 cm3∙g−1 mesostructured, pore size: 2–4 nm	4.5–4.8	900–1100	6
CeO2-700290	Cerium oxide-SA-700290	Molecular formula: CeO2 Molecular weight: 172.11 Density: 7.13 g∙mL−1 at 298 K	<50	30	4.5

* The information was provided by the supplier. Only the isoelectric point data were determined experimentally. Abbreviations: AA: Alfa Aesar (Kandel, Germany); N.A: Not Available; SA: Sigma-Aldrich (Buchs, Switzerland).

). The pHI_EP measurements help to clarify the sorption mechanism. The sorbent surface carries a positive charge at pH < pHI_EP, zero charge at pH~pHI_EP, and is negatively charged at pH > pHI_EP. Consequently, there is a change in the pHI_EP of the sorbent with the pH of an aqueous solution. Nawar et al. [22] reported that this behavior might occur because amphoteric hydroxyl groups cover the adsorbent surface. Hence, based on the pH of the medium, these groups develop different reactions in different pH media, resulting in positive or negative charges appearing on the adsorbent surface. Herein, at pH < pHI_EP, they are protonated, and the surface develops a positive charge as follows:

$$\mathrm{Adsorbent}-{\mathrm{OH}}_{\mathrm{Surface}}+{ \mathrm{H}}_{\mathrm{solution}}^{+} \rightleftharpoons \mathrm{Adsorbent}-{\mathrm{OH}}_2^{+}$$

(1)

The data presented in Figure 1a can be interpreted by considering the speciation diagram of molybdenum shown in Figure 1b [22]. The speciation data are generated using the PHREEQC software (version 3) to determine the predominant Mo species at different pHs for the following conditions: C₀ = 50 mg∙L⁻¹ at 298$$1 K and using the built-in database of stability constants [22]. At acidic medium, the molybdate anionic species exist and polymerize, increasing the molybdenum content per unit charge as follows:

$$7 {}_{}{}^{99}\mathrm{M}{\mathrm{oO}}_4^{2-}+8{ \mathrm{H}}^{+} \rightleftharpoons {}_{}{}^{99}\mathrm{M}{\mathrm{o}}_7{\mathrm{O}}_{24}^{6-}+4{ \mathrm{H}}_2\mathrm{O}$$

(2)

Consequently, this results in favorable interactions between negatively charged molybdenum polyanions and positively charged adsorbents surfaces [26]. At higher pH values, the speciation shifts to less negatively charged Mo species, and the density of hydroxyl groups (OH⁻) increases in solution. These hydroxyl anions compete with less negatively charged molybdenum anions to retain the available active sites on adsorbents surfaces, explaining the low K_d distribution values at higher pH values [22,27].

Moreover, based on the isoelectric point (pH_IEP) of each sorbent material (Table 1) and the measured final solution pH (Figure 1c), it can be observed that K_d values start to decrease when the final solution pH exceeds the sorbent’s pH_IEP, which can be attributed to the expected change in the surface charge of the sorbent material. As previousely mentioned, at solution pH values above the pH_IEP, the sorbent surface becomes predominately negatively charged. As a result, repulsion between the negatively charged sorbent surface and the negatively charged molybdenum polyanions takes place, leading to the observed decrease in K_d values [22,28].

It can also be observed that both silicon oxide and aluminosilicate nanoparticles possess small particle sizes (5–20 and 4.5–4.8 nm) and high surface area (590–690 and 900–1100 m²∙g⁻¹), respectively (Table 1). However, both adsorbents show a weak affinity for Mo species. This behavior may be attributed to their poor stability with increasing pH values. At high pH values, the dissolution of silica occurs, resulting in the formation of monomeric ortho-silicic acid (H₄SiO₄), which can be explained due to the presence of more hydroxyl groups. These hydroxyl groups are chemisorbed on the adsorbent surface, which increases the number of coordination bonds around the silicon atom to more than four bonds. Consequently, it may lead to Si-O bond rupture, and the silicon atom dissolves as Si(OH)₄ and ortho-silicic acid [29].

According to the obtained results, we selected six adsorbents that showed high distribution coefficient values towards CA-⁹⁹Mo for the subsequent investigations. These adsorbents are CeO₂-544841, ZrO₂-544760, TiO₂-637254, Al₂TiO₅-634143, CeO₂/ZrO₂-634174, and CeO₂-700290.

2.2. Adsorption Isotherm

Equilibrium isotherms are essential in describing the adsorption mechanisms for the interaction of Mo(VI) ions with the surfaces of the investigated metal oxides NPs. These mechanisms describe the adsorption process successfully. Here, we investigated equilibrium data obtained for adsorption of CA-⁹⁹Mo on CeO₂-544841, ZrO₂-544760, TiO₂-637254, Al₂TiO₅-634143, CeO₂/ZrO₂-634174, and CeO₂-700290 with various isotherm models to find out which one is the most suitable for describing the obtained adsorption equilibrium data.

2.2.1. Freundlich Isotherm

Many studies have utilized the Freundlich adsorption isotherm model proposed as a general power equation used to describe the adsorption of radionuclides in a large number of studies [30,31,32]. The Freundlich isotherm has the form shown as follows:

$${\mathrm{q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{f}}{\left({\mathrm{C}}_{\mathrm{e}}\right)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{${\mathrm{n}}_{\mathrm{f}}$}\right.}$$

(3)

where q_e (mg∙g⁻¹) is the concentration of CA-⁹⁹Mo adsorbed and C_e (mg∙L⁻¹) is the concentration of Mo remaining in the solution. K_f (mg¹⁻ⁿLⁿ∙g⁻¹) and n_f (dimensionless) are constants unique to each combination of adsorbent and adsorbate.

2.2.2. Langmuir Isotherm

Langmuir (1918) developed an equation to describe the adsorption of gases on a solid surface that was subsequently adapted to describe the adsorption of solutes onto solids in aqueous solutions [31,33,34], as shown in Equation (4):

$${\mathrm{q}}_{\mathrm{e}}=\frac{{\mathrm{n}}_{\mathrm{L}}{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}{1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}$$

(4)

where q_e (mg∙g⁻¹) is the total concentration of solute adsorbed, K_L (L∙mg⁻¹) is an equilibrium constant, and n_L (mg∙g⁻¹) is the adsorption capacity.

Figure 2 presents the experimental adsorption equilibrium data obtained for Mo ions on the investigated metal oxide adsorbents as a plot of adsorption equilibrium capacity (q_e) against initial concentration (C₀). It is observed that there is an increase in the amount of Mo ions taken up with the increase in the initial metal ion concentration. This increase in the adsorbate uptake can be explained by the driving force for mass transfer [34].

The non-linear forms of both isotherm models were applied to the measured adsorption data (C_e versus q_e), and the data were displayed in Figure 3. Adsorption parameters were optimized using the add-ins “Solver” function in Microsoft Excel.

Table 2. Isotherm parameters calculations for the adsorption of CA-⁹⁹Mo on different metal oxides NPs.

Isotherm Model	Parameter	CeO2-544841	ZrO2-544760	TiO2-637254	Al2TiO5-634143	CeO2/ZrO2-634174	CeO2-700290
Langmuir	nL (mg∙g−1)	26.704	16.814	36.980	10.207	23.470	19.603
	KL (L∙mg−1)	0.407	0.993	0.311	0.0907	3.537	0.254
	R2	0.911	0.957	0.930	0.836	0.870	0.954
Freundlich	KF (mg1−nLn∙g−1)	10.514	9.058	13.064	6.364	11.506	8.876
	nf	5.010	8.294	4.079	10.325	6.346	6.644
	R2	0.982	0.968	0.989	0.898	0.955	0.966

gives the Freundlich parameters (K_f and n_f), Langmuir parameters (K_L and n_L), and the goodness of fit of the model lines to the experimental data (R²). Based on the regression coefficient values reported in Table 2, it is observed that good to excellent correlations between the experimental results and the fitted data of the Freundlich isotherm model were obtained for all the investigated sorbents. In contrast, the Langmuir model failed to fit any equilibrium sorption isotherm of the CA-⁹⁹Mo on all tested adsorbents; lower R² values were obtained.

These findings suggest that CA-⁹⁹Mo adsorption on metal oxide nanomaterials under investigation mainly occurred through multilayer adsorption at heterogeneous surfaces [31,35]. The Freundlich adsorption constant (n_f) is usually used as a measure of adsorption intensity as follows; (i) n_f < 1 indicates that adsorption takes place via a chemical process, (ii) n_f = 1 shows linear adsorption, (iii) while n_f > 1 indicates physisorption [35]. The n_f values displayed in Table 2 were higher than 1, indicating that CA-⁹⁹Mo adsorption on the materials used in this study was physisorption and favorable under the investigated conditions. Furthermore, the closer the 1/n value to 0 than unity (ranging from 0.10 to 0.25), the more heterogeneous the surface is, implying a broad distribution of adsorption sites on the adsorbent surface [32,33].

2.3. Thermodynamic Studies

We determined the amount of CA-⁹⁹Mo adsorbed on the surface of the materials investigated in the current study as a function of temperature (T) using adsorption thermodynamic parameters. These parameters include the Gibbs free energy ΔG⁰ (kJ∙mol⁻¹), the standard enthalpy change ΔH⁰ (kJ∙mol⁻¹), and the standard entropy change ΔS⁰ (J∙mol⁻¹∙K⁻¹). They were investigated at different temperatures (298, 313, 323, and 333 K) using Equations (5) and (6) [34,36,37] and are tabulated in

Table 3. Thermodynamic parameters for the sorption of CA-⁹⁹Mo on different metal oxides NPs.

Adsorbent	Temperature (K)	ΔG0 (kJ∙mol−1)	ΔH0 (kJ∙mol−1)	ΔS0 (J∙mol−1∙K−1)
CeO2-544841	298	−9.8 ± 2.9	−5.1 ± 1.5	16.0 ± 4.7
	313	−10.1 ± 3.0
	323	−10.2 ± 3.0
	333	−10.4 ± 3.1
ZrO2-544760	298	−7.8 ± 1.4	−1.4 ± 0.7	21.3 ± 2.2
	313	−8.1 ± 1.4
	323	−8.3 ± 1.4
	333	−8.5 ± 1.5
TiO2-637254	298	−10.9 ± 3.1	4.7 ± 1.5	52.0 ± 5.0
	313	−11.7 ± 3.1
	323	−12.2 ± 3.2
	333	−12.7 ± 3.2
Al2TiO5-634143	298	−7.2 ± 2.0	−4.5 ± 1	9.1 ± 3.2
	313	−7.4 ± 2.0
	323	−7.5 ± 2.0
	333	−7.6 ± 2.0
CeO2/ZrO2-634174	298	−9.0 ± 1.4	1.3 ± 0.7	34.6 ± 2.2
	313	−9.5 ± 1.4
	323	−9.9 ± 1.4
	333	−10.2 ± 1.4
CeO2-700290	298	−8.2 ± 1.9	−0.1 ± 0.9	27.2 ± 3.0
	313	−8.6 ± 1.9
	323	−8.9 ± 2.0
	333	−9.1 ± 2.0

:

$$∆G^0=-{\mathrm{RTlnK}}_{\mathrm{d}}$$

(5)

$$\ln {\mathrm{K}}_{\mathrm{d}}=\frac{∆S^0}{\mathrm{R}}-\frac{∆H^0}{\mathrm{RT}}$$

(6)

where R is the universal gas constant (8.314 J∙mol⁻¹∙K⁻¹), T is the absolute temperature (K), and K_d (mL∙g⁻¹) is the distribution coefficient.

Figure 4 shows linear plots of ln K_d versus (1/T). The calculated ΔG⁰ values at each temperature for all nano-adsorbents are ΔG⁰ < 0, which implies that the Mo(VI) adsorption process on the surfaces of all adsorbents is spontaneous and the reaction is feasible. Likewise, ΔG⁰ values decrease with increasing temperature, indicating that the degree of spontaneity can be enhanced by increasing the temperature. Furthermore, the adsorption process is physisorption (−20 < ΔG⁰ < 0) [38]. The positive values of ΔS⁰ (ΔS⁰ > 0) report random adsorption reactions of CA-⁹⁹Mo at all adsorbents surfaces. The values of ΔH⁰ are positive (ΔH⁰ > 0) for both TiO₂-637254 and CeO₂/ZrO₂-634174, implying that CA-⁹⁹Mo adsorption at their surfaces is endothermic [39]. While for CeO₂-544841, ZrO₂-544760, Al₂TiO₅-634143, and CeO₂-700290, the change in enthalpy (ΔH⁰) is negative (ΔH⁰ < 0), indicating that the adsorption of CA-⁹⁹Mo at their surfaces is exothermic [38,40].

2.4. Determining the Maximum Sorption Capacity

In order to evaluate the maximum sorption capacity of each adsorbent, the equilibrations of CA-⁹⁹Mo with each adsorbent were performed separately. Batch equilibrations were repeated until no further ⁹⁹Mo(IV) uptake was observed, and the adsorbents became fully saturated with ⁹⁹Mo. After each equilibration, 1 mL aliquot was decanted, centrifuged, and counted. Ultimately, the maximum sorption capacity $\left({\mathrm{q}}_{\max }\right)$ for each material was calculated by applying the following equation:

$${\mathrm{q}}_{\max }=\frac{\sum \mathrm{U}\%}{100}\times {\mathrm{C}}_{\mathrm{o}}\times \frac{\mathrm{V}}{\mathrm{m}}\hspace{1em}\hspace{1em}\hspace{1em}\left(\mathrm{mg}·{\mathrm{g}}^{-1}\right)$$

(7)

where U% is the uptake percent of CA-⁹⁹Mo, C₀(mg∙L⁻¹) is the starting Mo(IV) concentration, V (L) is the liquid phase volume, and $\mathrm{m}$(g) is the adsorbent weight. Figure 5 shows CA-⁹⁹Mo maximum sorption capacity on different studied metal oxides NPs. It can be concluded that the studied metal oxide NPs show better sorption capacity than conventional alumina currently used in ⁹⁹Mo/^99mTc generators. Nonetheless, the obtained capacities are insufficient for developing a clinical-grade ^99mTc generator based on LSA ⁹⁹Mo.

2.5. Effect of Contact Time

The effect of contact time on the uptake percent of CA-⁹⁹Mo was monitored for an initial Mo(IV) concentration of 50 mg∙L⁻¹ (pH~3), using an adsorbent dose of 200 mg. The reaction temperature was adjusted to 298 ± 1 K. The results are shown in Figure 6. The results show that the Mo uptake sharply increased at the beginning of the adsorption process and reached a constant value (a plateau value) in the first two minutes. This behavior indicates a rapid and almost instantaneous removal of CA-⁹⁹Mo from the solution, and a dynamic equilibrium is established under the given experimental conditions. In order to design an effective adsorption process, determining the kinetic parameters is crucial. The kinetic data shown in Figure 6 revealed that the equilibrium for adsorption of Mo on metal oxide nano-adsorbents is already reached at the very beginning of the adsorption process. Consequently, using the current methodology, such data cannot be modeled with adsorption kinetic models.

3. Materials and Methods

3.1. Materials

All chemicals are of analytical grade purity (A. R. grade) and were used without further purification. Milli-Q water was used for the preparation of solutions and washings. Sodium hydroxide and nitric acid were purchased from Merck, Darmstadt, Germany. The metal oxide nanomaterials were purchased from different suppliers (Table 1).

⁹⁹Mo radiotracer solution was obtained by eluting a 40 GBq fission ⁹⁹Mo alumina-based ⁹⁹Mo/^99mTc generator (Pertector, manufactured by National Centre for Nuclear Research, POLATOM, Otwock, Poland) with 5 mL of 1 M NaOH solution after ~7 d from the calibration date. The total ⁹⁹Mo radioactivity was measured with a Capintec Radioisotopes Calibrator (model CRC-55tR Capintec, Inc., Florham Park, NJ, USA). The ⁹⁹Mo eluate solution was passed through a 0.45 micro-Millipore filter to retain alumina particles. Then, the ⁹⁹Mo solution was treated with nitric acid to attain the desired pH value.

3.2. Batch Equilibrium Studies

A batch equilibration experiment was conducted to investigate the adsorption behavior of carrier-added (CA) ⁹⁹Mo (Mo(IV) treated with ⁹⁹Mo) on several commercial metal oxide nanoparticles (NPs) under different conditions. These conditions included the influence of pH, contact time, reaction temperature, and initial adsorbate concentration. In a series of clean glass bottles, we added 200 mg of each adsorbent to 20 mL of ⁹⁹Mo(IV) solution of a given concentration and pH value. Subsequently, the mixtures were shaken in a thermostatic shaker water bath (Julabo GmbH, Seelbach, Germany) at 298 ± 1 K for 24 h. Eventually, the supernatant solution was collected, centrifuged, and 1 mL was separated for radiometric measurements. For all radiometric identifications and γ-spectrometry, we used a multichannel analyzer (MCA) of Inspector 2000 model, Canberra Series, Mirion Technologies, Inc., Meriden, CT, USA, coupled with a high-purity germanium coaxial detector (HPGe). All samples have fixed geometry and were counted at a low dead time (<2%). The measurements were done by using an appropriate gamma-ray peak of 740 keV.

3.2.1. Distribution Ratio (K_d)

The distribution coefficient (K_d) values of CA-⁹⁹Mo were investigated at a wide range of pH (from 1–8). For adjusting the desired pH value of the solutions, few drops of 0.5 M nitric acid or 0.5 M sodium hydroxide were added. The pH values of the solutions were measured before and after reaching the equilibrium state. pH values were determined using a pH-meter with a microprocessor (Mettler Toledo, Seven Compact S210 model, Greifensee, Switzerland).

3.2.2. Adsorption Isotherm

In order to determine the sorption isotherms, we used different initial molybdate ion concentrations from 50 to 500 mg∙L⁻¹ while keeping the adsorbent amount constant. Moreover, the solution pH, equilibrium time, and reaction temperature were kept at pH~3, 24 h, and 298 ± 1 K, respectively. In addition, the equilibrium adsorption capacity (q_e) was calculated. Finally, we used the obtained results to determine the sorption isotherm model.

3.2.3. Thermodynamic Studies

The reaction temperature effect on the uptake of carrier-added ⁹⁹Mo was studied at four different reaction temperatures (298, 313, 323, and 333 K). At each temperature, we added 20 mL of CA-⁹⁹Mo solution (pH 3) in contact with 200 mg of the adsorbent material for 24 h. From the resulting data, we calculated different thermodynamic parameters, namely the standard enthalpy change (ΔH⁰), standard entropy change (ΔS⁰), and Gibbs free energy change (ΔG⁰).

3.2.4. Effect of Contact Time

In order to investigate the ⁹⁹Mo adsorption rate on the studied metal oxides NPs, we monitored the progress of the uptake capacity of ⁹⁹MoO₄^2– ions (50 mg∙L⁻¹ and pH~3) at different time slots. The adsorption of CA-⁹⁹Mo was followed with time until the equilibrium was established. Finally, we calculated the ⁹⁹Mo capacity (${\mathrm{q}}_{\mathrm{t}}$) in mg∙g⁻¹ at each time (t).

3.3. Calculations

The adsorption data of CA-⁹⁹Mo include uptake percent (U%), distribution coefficient (K_d), equilibrium capacity (q_e), and equilibrium concentration (C_e). These data were calculated according to the following equations:

$$\mathrm{U}\%=\frac{({\mathrm{A}}_{\mathrm{i}}-{\mathrm{A}}_{\mathrm{f}})}{{\mathrm{A}}_{\mathrm{i}}}\times 100$$

(8)

$${\mathrm{q}}_{\mathrm{e}}=\frac{\mathrm{U}\%}{100}\times {\mathrm{C}}_0\times \frac{\mathrm{V}}{\mathrm{m}}\hspace{1em}\hspace{1em}\hspace{1em}\left(\mathrm{mg}·{\mathrm{g}}^{-1} \right)$$

(9)

$${\mathrm{C}}_{\mathrm{e}}={\mathrm{A}}_{\mathrm{i} }-\left({\mathrm{A}}_{\mathrm{i}}\times \frac{\mathrm{U}\%}{100}\right)\hspace{1em}\hspace{1em}\hspace{1em}\left(\mathrm{mg}·{\mathrm{L}}^{-1}\right)$$

(10)

$${\mathrm{K}}_{\mathrm{d} }=\frac{{\mathrm{A}}_{\mathrm{i}}-{ \mathrm{A}}_{\mathrm{f}}}{{\mathrm{A}}_{\mathrm{i}}}\times \frac{{\mathrm{V}}^{\prime }}{\mathrm{m}}\hspace{1em}\hspace{1em}\hspace{1em}\left(\mathrm{mL}·{\mathrm{g}}^{-1}\right)$$

(11)

where A_i and A_f are the initial and final ⁹⁹Mo radioactivity in counts/min. ${\mathrm{C}}_0$ (mg∙L⁻¹) is the initial concentration of CA-⁹⁹Mo, $\mathrm{V}$ (L) and ${\mathrm{V}}^{\prime }$ (mL) represent the volume of liquid phases, and m (g) is the weight of the solid phase.

4. Summary and Conclusions

The main objective of this study was to evaluate the adsorption affinity of different commercial metal oxides NPs purchased from different suppliers towards LSA ⁹⁹Mo. All experiments were conducted at static equilibrium conditions. We studied the distribution ratio of CA-⁹⁹Mo in a pH range of 1 to 8. The optimum adsorption pH was found to be in the range of pH 2 to 4. In addition, the Freundlich isotherm model fitted the experimental data of the CA-⁹⁹Mo on all adsorbent materials investigated in this study. Moreover, we determined the values of enthalpy change (ΔH⁰), entropy change (ΔS⁰), and free energy change (ΔG⁰) at the different reaction temperatures. Furthermore, the maximum adsorption capacities were evaluated, and the best adsorbents showed a capacity of 40 ± 2 to 73 ± 1 mg Mo∙g⁻¹. Summing up the results, it can be concluded that the adsorption behavior of the materials investigated depends on the solution pH, contact time, initial metal ion concentration, and temperature. Furthermore, the investigated materials showed higher static sorption capacities than conventional alumina (2–20 mg Mo∙g⁻¹). Nonetheless, they are not suitable to build a useful ⁹⁹Mo/^99mTc generator using LAS ⁹⁹Mo for radiopharmaceutical applications. Since the available specific activity of LAS ⁹⁹Mo is 2.5–5 Ci/g Mo, approximately 20–25 g of each material would be required to prepare a ^99mTc generator of 37 GBq (1 Ci). Using such a massive amount of sorbent material per generator would deteriorate the elution performance and the radioactive concentration of the produced ^99mTc.