*Article* **Evaluating the Sorption Affinity of Low Specific Activity <sup>99</sup>Mo on Different Metal Oxide Nanoparticles**

**Mohamed F. Nawar 1,2,\* , Alaa F. El-Daoushy <sup>2</sup> , Ahmed Ashry <sup>3</sup> , Mohamed A. Soliman 3,4 and Andreas Türler <sup>1</sup>**


**Abstract:** <sup>99</sup>Mo/99mTc generators are mainly produced from <sup>99</sup>Mo of high specific activity generated from the fission of <sup>235</sup>U. Such a method raises proliferation concerns. Alternative methods suggested the use of low specific activity (LSA) <sup>99</sup>Mo 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 <sup>99</sup>Mo. 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 ∆*G* 0 , enthalpy change ∆*H*<sup>0</sup> , and entropy ∆*S* 0 ) 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 <sup>99</sup>Mo and higher capacity than conventional alumina (2–20 mg Mo·g −1 ).

**Keywords:** LSA <sup>99</sup>Mo; thermodynamic parameters; solid-phase extraction; isotherm; metal oxides NPs

**1. Introduction**

<sup>99</sup>Mo/99mTc radioisotope generators have a growing importance in nuclear medicine investigations. They are the primary source of supplying 99mTc radionuclide for diagnostic purposes [1–3]. 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

**Citation:** Nawar, M.F.; El-Daoushy, A.F.; Ashry, A.; Soliman, M.A.; Türler, A. Evaluating the Sorption Affinity of Low Specific Activity <sup>99</sup>Mo on Different Metal Oxide Nanoparticles. *Inorganics* **2022**, *10*, 154. https:// doi.org/10.3390/inorganics10100154

Academic Editor: Roberto Nisticò

Received: 7 August 2022 Accepted: 23 September 2022 Published: 26 September 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

diphosphonate) are involved in skeletal imaging, cerebral perfusion, and diagnosis of bone metastases, respectively [12–15].

Among the developed <sup>99</sup>Mo/99mTc generators, the chromatographic column type is the most widely used system [3,16]. This system is based on adsorbing <sup>99</sup>Mo on a column filled with a suitable material from which 99mTcO<sup>4</sup> <sup>−</sup> can be easily eluted while <sup>99</sup>Mo remains adsorbed [3]. The differences between these generators include the column material and the origin of the parent, <sup>99</sup>Mo. The main practical difficulties linked to the preparation of <sup>99</sup>Mo/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 <sup>235</sup>U. Fission-produced <sup>99</sup>Mo faces some critical difficulties. For example, sophisticated infrastructures and well-qualified personnel are needed to separate and purify <sup>99</sup>Mo from the irradiated <sup>235</sup>U 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 <sup>99</sup>Mo/99mTc generators based on <sup>99</sup>Mo of low specific activity (LSA) [3,19,20]. However, this proposal demands using high-capacity sorbents to compensate for the LSA <sup>99</sup>Mo 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 <sup>99</sup>Mo/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 <sup>99</sup>Mo. To achieve this goal, we investigated the adsorption behavior of the selected materials for LSA <sup>99</sup>Mo 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 <sup>99</sup>Mo retention on each adsorbent. Figure 1a depicts the distribution coefficients (Kd) of CA-99Mo at different pH values. The data presented in this figure show that higher K<sup>d</sup> values are observed at pH values (2–4). Beyond this region, the K<sup>d</sup> values decrease with increasing the solution pH, which agrees with previously published studies [25].

**Figure 1.** Effect of initial pH on (**a**) the distribution coefficients (Kd) of CA-<sup>99</sup>Mo on different metal oxides NPs (C0 = 50 mg*∙*L*–*<sup>1</sup> , V/m = 100 mL*∙*g *–*1 , and temperature = 298 ± 1 K), (**b**) Speciation of molybdenum [22], and (**c**) variation of the final pH values. **Figure 1.** Effect of initial pH on (**a**) the distribution coefficients (Kd) of CA-99Mo on different metal oxides NPs (C<sup>0</sup> = 50 mg·L −1 , V/m = 100 mL·g −1 , and temperature = 298 ± 1 K), (**b**) Speciation of molybdenum [22], and (**c**) variation of the final pH values.

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 (pHIEP) of each adsorbent (Table 1). The pHIEP measurements help to clarify the sorption mechanism. The sorbent surface carries a positive charge at pH < pHIEP, zero charge at pH~pHIEP, and is negatively charged at pH > pHIEP. Consequently, there is a change in the pHIEP 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 < pHIEP, they are protonated, and the surface develops a positive charge as follows: 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 (pHIEP) of each adsorbent (Table 1). The pHIEP measurements help to clarify the sorption mechanism. The sorbent surface carries a positive charge at pH < pHIEP, zero charge at pH~pHIEP, and is negatively charged at pH > pHIEP. Consequently, there is a change in the pHIEP 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 < pHIEP, they are protonated, and the surface develops a positive charge as follows:

$$\text{Adscrbert} - \text{OH}\_{\text{Surface}} + \text{H}^+\_{\text{solution}} \rightleftharpoons \text{Adscrbert} - \text{OH}^+\_2 \tag{1}$$

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*–*<sup>1</sup> 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: 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: C<sup>0</sup> = 50 mg·L <sup>−</sup><sup>1</sup> 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:

$$7\,\mathrm{^{99}MoO\_4^{2-}} + 8\,\mathrm{H^+} \rightleftharpoons \mathrm{^{99}Mo\_7O\_{24}^{6-}} + 4\,\mathrm{H\_2O} \tag{2}$$

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<sup>d</sup> distribution values at higher pH values [22,27].


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

\* 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).

Moreover, based on the isoelectric point (pHIEP) of each sorbent material (Table 1) and the measured final solution pH (Figure 1c), it can be observed that K<sup>d</sup> values start to decrease when the final solution pH exceeds the sorbent's pHIEP, 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 pHIEP, 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<sup>d</sup> 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 m<sup>2</sup> ·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)<sup>4</sup> 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 [30–32]. The Freundlich isotherm has the form shown as follows:

$$\mathbf{q}\_{\mathbf{e}} = \mathbf{K}\_{\mathbf{f}} (\mathbf{C}\_{\mathbf{e}})^{\frac{1}{\mathbf{n}\_{\mathbf{f}}}} \tag{3}$$

where q<sup>e</sup> (mg·g −1 ) is the concentration of CA-99Mo adsorbed and C<sup>e</sup> (mg·<sup>L</sup> −1 ) is the concentration of Mo remaining in the solution. K<sup>f</sup> (mg1−nL n ·g −1 ) and n<sup>f</sup> (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):

$$\mathbf{q}\_{\text{e}} = \frac{\mathbf{n}\_{\text{L}} \mathbf{K}\_{\text{L}} \mathbf{C}\_{\text{e}}}{1 + \mathbf{K}\_{\text{L}} \mathbf{C}\_{\text{e}}} \tag{4}$$

where q<sup>e</sup> (mg·g −1 ) is the total concentration of solute adsorbed, K<sup>L</sup> (L·mg−<sup>1</sup> ) is an equilibrium constant, and n<sup>L</sup> (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 (qe) 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].

data.

2.2.1. Freundlich Isotherm

2.2.2. Langmuir Isotherm

rium constant, and n<sup>L</sup> (mg∙g*–*<sup>1</sup>

where q<sup>e</sup> (mg∙g−<sup>1</sup>

where q<sup>e</sup> (mg∙g*–*<sup>1</sup>

**Figure 2.** The influence of initial molybdate concentration on the equilibrium sorption capacity (qe) of CA-<sup>99</sup>Mo on different metal oxides NPs (pH = 3, V/m =100 mL∙g *–*1 , t = 24 h, and temperature = 298 ± 1 K). **Figure 2.** The influence of initial molybdate concentration on the equilibrium sorption capacity (qe) of CA-99Mo on different metal oxides NPs (pH = 3, V/m =100 mL·<sup>g</sup> −1 , t = 24 h, and temperature = 298 ± 1 K).

find out which one is the most suitable for describing the obtained adsorption equilibrium

q<sup>e</sup> = Kf(Ce)

ber of studies [30–32]. The Freundlich isotherm has the form shown as follows:

) is the concentration of CA-

stants unique to each combination of adsorbent and adsorbate.

tration of Mo remaining in the solution. K<sup>f</sup> (mg1−nL<sup>n</sup>∙g*–*<sup>1</sup>

aqueous solutions [31,33,34], as shown in Equation (4):

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 num-

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

> nLKLC<sup>e</sup> 1 + KLC<sup>e</sup>

) is the total concentration of solute adsorbed, K<sup>L</sup> (L∙mg*–*<sup>1</sup>

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 (qe) 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].

) is the adsorption capacity.

q<sup>e</sup> =

1 nf

<sup>99</sup>Mo adsorbed and C<sup>e</sup> (mg∙L–<sup>1</sup>

<sup>⁄</sup> (3)

) and n<sup>f</sup> (dimensionless) are con-

) is the concen-

) is an equilib-

(4)

The non-linear forms of both isotherm models were applied to the measured adsorption data (C<sup>e</sup> versus qe), and the data were displayed in Figure 3. Adsorption parameters were optimized using the add-ins "Solver*"* function in Microsoft Excel. Table 2 gives the Freundlich parameters (K<sup>f</sup> and nf), Langmuir parameters (K<sup>L</sup> and nL), and the goodness of fit of the model lines to the experimental data (R<sup>2</sup> ). 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 The non-linear forms of both isotherm models were applied to the measured adsorption data (C<sup>e</sup> versus qe), and the data were displayed in Figure 3. Adsorption parameters were optimized using the add-ins "Solver" function in Microsoft Excel. Table 2 gives the Freundlich parameters (K<sup>f</sup> and n<sup>f</sup> ), Langmuir parameters (K<sup>L</sup> and nL), and the goodness of fit of the model lines to the experimental data (R<sup>2</sup> ). 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 R<sup>2</sup> values were obtained. *Inorganics* **2022**, *10*, x FOR PEER REVIEW 7 of 14 for all the investigated sorbents. In contrast, the Langmuir model failed to fit any equilibrium sorption isotherm of the CA-<sup>99</sup>Mo on all tested adsorbents; lower R<sup>2</sup> values were obtained. **Table 2.** Isotherm parameters calculations for the adsorption of CA-<sup>99</sup>Mo on different metal oxides NPs.

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<sup>f</sup> ) is usually used as a measure of adsorption intensity as follows; (i) n<sup>f</sup> < 1 indicates that adsorption takes place via a chemical process, (ii) n<sup>f</sup> = 1 shows linear adsorption, (iii) while n<sup>f</sup> > 1 indicates physisorption [35]. The n<sup>f</sup> 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]. **Isotherm Model Parameter CeO2- 544841 ZrO2- 544760 TiO2- 637254 Al2TiO5- 634143 CeO2/ZrO2- 634174 CeO2- 700290** Langmuir nL (mg*∙*g*–*<sup>1</sup> ) 26.704 16.814 36.980 10.207 23.470 19.603 KL (L*∙*mg*–*<sup>1</sup> ) 0.407 0.993 0.311 0.0907 3.537 0.254 R<sup>2</sup> 0.911 0.957 0.930 0.836 0.870 0.954 Freundlich KF (mg1−nLn*∙*g*–*<sup>1</sup> ) 10.514 9.058 13.064 6.364 11.506 8.876 n<sup>f</sup> 5.010 8.294 4.079 10.325 6.346 6.644 R<sup>2</sup> 0.982 0.968 0.989 0.898 0.955 0.966

**Figure 3.** Adsorption isotherms: (**a**) Langmuir and (**b**) Freundlich of CA-<sup>99</sup>Mo on different metal oxides NPs. **Figure 3.** Adsorption isotherms: (**a**) Langmuir and (**b**) Freundlich of CA-99Mo on different metal oxides NPs.

investigation mainly occurred through multilayer adsorption at heterogeneous surfaces

intensity as follows; (i) nf < 1 indicates that adsorption takes place via a chemical process, (ii) n<sup>f</sup> = 1 shows linear adsorption, (iii) while n<sup>f</sup> > 1 indicates physisorption [35]. The n<sup>f</sup>

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

vestigated in the current study as a function of temperature (T) using adsorption thermo-

). They were investigated at different temperatures (298, 313, 323, and 333 K) using Equa-

<sup>0</sup> = −RTlnK<sup>d</sup>

∆ 0 R − ∆ 0 RT

dynamic parameters. These parameters include the Gibbs free energy Δ*G*<sup>0</sup>

∆

ln K<sup>d</sup> =

(kJ*∙*mol*–*<sup>1</sup>

<sup>99</sup>Mo adsorption on metal oxide nanomaterials under

99Mo adsorbed on the surface of the materials in-

), and the standard entropy change Δ*S*<sup>0</sup>

<sup>99</sup>Mo adsorption on the

(kJ∙mol*–*<sup>1</sup>

(J∙mol*–*<sup>1</sup>

), the

∙K*–*

(5)

(6)

values displayed in Table 2 were higher than 1, indicating that CA-

These findings suggest that CA-

on the adsorbent surface [32,33].

standard enthalpy change Δ*H*<sup>0</sup>

1

We determined the amount of CA-

tions (5) and (6) [34,36,37] and are tabulated in Table 3:

*2.3. Thermodynamic Studies*


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

#### *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 ∆*G* 0 (kJ·mol−<sup>1</sup> ), the standard enthalpy change ∆*H*<sup>0</sup> (kJ·mol−<sup>1</sup> ), and the standard entropy change ∆*S* 0 (J·mol−<sup>1</sup> ·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:

$$
\Delta G^0 = -\text{RTl} \text{lnK}\_{\text{d}} \tag{5}
$$

$$
\ln \mathbf{K\_d} = \frac{\Delta S^0}{\mathbf{R}} - \frac{\Delta H^0}{\mathbf{RT}} \tag{6}
$$

where R is the universal gas constant (8.314 J·mol−<sup>1</sup> ·K −1 ), T is the absolute temperature (K), and K<sup>d</sup> (mL·g −1 ) is the distribution coefficient.

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



**Table 3.** *Cont.*

Figure 4 shows linear plots of ln K<sup>d</sup> versus (1/T). The calculated ∆*G* <sup>0</sup> values at each temperature for all nano-adsorbents are ∆*G* <sup>0</sup> < 0, which implies that the Mo(VI) adsorption process on the surfaces of all adsorbents is spontaneous and the reaction is feasible. Likewise, ∆*G* <sup>0</sup> 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 < ∆*G* <sup>0</sup> < 0) [38]. The positive values of ∆*S* 0 (∆*S* <sup>0</sup> > 0) report random adsorption reactions of CA-99Mo at all adsorbents surfaces. The values of ∆*H*<sup>0</sup> are positive (∆*H*<sup>0</sup> > 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 (∆*H*<sup>0</sup> ) is negative (∆*H*<sup>0</sup> < 0), indicating that the adsorption of CA-99Mo at their surfaces is exothermic [38,40]. *Inorganics* **2022**, *10*, x FOR PEER REVIEW 9 of 14

**Figure 4.** Van't Hoff plot for the sorption of CA-<sup>99</sup>Mo on different metal oxides NPs. **Figure 4.** Van't Hoff plot for the sorption of CA-99Mo on different metal oxides NPs.

#### *2.4. Determining the Maximum Sorption Capacity 2.4. Determining the Maximum Sorption Capacity*

where U% is the uptake percent of CA-

CA-

**qmax**

**(mg.g-1**

**)**

In order to evaluate the maximum sorption capacity of each adsorbent, the equilibrations of CA-<sup>99</sup>Mo 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 (qmax) for each material was calculated by applying the following equation: 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 <sup>99</sup>Mo(IV) uptake was observed, and the adsorbents became fully saturated with <sup>99</sup>Mo. After each equilibration, 1 mL aliquot was decanted, centrifuged, and counted. Ultimately, the maximum sorption capacity (qmax) for each material was calculated by applying the following equation:

<sup>99</sup>Mo, C0(mg∙L*–*<sup>1</sup>

tion, V (L) is the liquid phase volume, and m (g) is the adsorbent weight. Figure 5 shows

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.

$$\mathbf{q}\_{\text{max}} = \frac{\sum \mathbf{U} \%}{100} \times \mathbf{C}\_0 \times \frac{\mathbf{V}}{\mathbf{m}} \quad (\text{mg} \cdot \text{g}^{-1}) \tag{7}$$

**CeO2**

 **ZrO2**

**TiO2**

 **Al2TiO5**

**CeO2**

 **CeO2**

**-544841**

**-544760**

**-634143**

**-700290**

**-634174**

**-637254**

**/ZrO2**

) is the starting Mo(IV) concentra-

99Mo.

**Figure 5.** The maximum sorption capacity of different metal oxide NPs for CA-

where U% is the uptake percent of CA-99Mo, C0(mg·<sup>L</sup> −1 ) is the starting Mo(IV) concentration, V (L) is the liquid phase volume, and 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 <sup>99</sup>Mo/99mTc generators. Nonetheless, the obtained capacities are insufficient for developing a clinical-grade 99mTc generator based on LSA <sup>99</sup>Mo. where U% is the uptake percent of CA-<sup>99</sup>Mo, C0(mg∙L*–*<sup>1</sup> ) is the starting Mo(IV) concentration, V (L) is the liquid phase volume, and m (g) is the adsorbent weight. Figure 5 shows CA-<sup>99</sup>Mo 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.

× C<sup>o</sup> ×

In order to evaluate the maximum sorption capacity of each adsorbent, the equilibra-

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 (qmax) for each material

> V m

<sup>99</sup>Mo with each adsorbent were performed separately. Batch equilibrations

**CeO2-544841 ZrO2-544760 TiO2-637254 Al2TiO5-634143**

 **CeO2-700290**

<sup>99</sup>Mo on different metal oxides NPs.

(mg ∙ g −1

) (7)

**/ZrO2-634174**

**CeO<sup>2</sup>**

*Inorganics* **2022**, *10*, x FOR PEER REVIEW 9 of 14

**0.0030 0.0031 0.0032 0.0033 0.0034**

**1/T**

**Figure 4.** Van't Hoff plot for the sorption of CA-

*2.4. Determining the Maximum Sorption Capacity*

was calculated by applying the following equation:

qmax =

∑ U% 100

**2.5**

tions of CA-

**3.0**

**3.5**

**ln Kd**

**4.0**

**4.5**

**Figure 5.** The maximum sorption capacity of different metal oxide NPs for CA-99Mo.**Figure 5.** The maximum sorption capacity of different metal oxide NPs for CA-99Mo.

#### *2.5. Effect of Contact Time 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. The effect of contact time on the uptake percent of CA-<sup>99</sup>Mo was monitored for an initial Mo(IV) concentration of 50 mg∙L*–*<sup>1</sup> (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-<sup>99</sup>Mo 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**

*3.2. Batch Equilibrium Studies*

*3.1. Materials*

**Figure 6.** Effect of contact time on CA-<sup>99</sup>Mo uptake on different metal oxide NPs (C0 = 50 mg*∙*L*–*<sup>1</sup> , pH = 3, V/m = 100 mL*∙*g *–*1 , and temperature = 298 ± 1 K). **Figure 6.** Effect of contact time on CA-99Mo uptake on different metal oxide NPs (C<sup>0</sup> = 50 mg·<sup>L</sup> −1 , pH = 3, V/m = 100 mL·g −1 , and temperature = 298 ± 1 K).

metal oxide nanomaterials were purchased from different suppliers (Table 1).

the 99Mo solution was treated with nitric acid to attain the desired pH value.

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

99Mo radiotracer solution was obtained by eluting a 40 GBq fission 99Mo aluminabased 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,

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)

.

#### **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).

<sup>99</sup>Mo radiotracer solution was obtained by eluting a 40 GBq fission <sup>99</sup>Mo aluminabased <sup>99</sup>Mo/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 <sup>99</sup>Mo radioactivity was measured with a Capintec Radioisotopes Calibrator (model CRC-55tR Capintec, Inc., Florham Park, NJ, USA). The <sup>99</sup>Mo eluate solution was passed through a 0.45 micro-Millipore filter to retain alumina particles. Then, the <sup>99</sup>Mo 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) <sup>99</sup>Mo (Mo(IV) treated with <sup>99</sup>Mo) 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 <sup>99</sup>Mo(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.
