3.2.1. Distribution Ratio (Kd)

The distribution coefficient (Kd) 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 <sup>−</sup><sup>1</sup> 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 (qe) 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 <sup>99</sup>Mo 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 (∆*H*<sup>0</sup> ), standard entropy change (∆*S* 0 ), and Gibbs free energy change (∆*G* 0 ).

#### 3.2.4. Effect of Contact Time

In order to investigate the <sup>99</sup>Mo adsorption rate on the studied metal oxides NPs, we monitored the progress of the uptake capacity of <sup>99</sup>MoO<sup>4</sup> 2– ions (50 mg·<sup>L</sup> <sup>−</sup><sup>1</sup> 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 <sup>99</sup>Mo capacity (q<sup>t</sup> ) in mg·g <sup>−</sup><sup>1</sup> at each time (t).

#### *3.3. Calculations*

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

$$\mathbf{U}\% = \frac{(\mathbf{A}\_{\mathrm{i}} - \mathbf{A}\_{\mathrm{f}})}{\mathbf{A}\_{\mathrm{i}}} \times 100 \tag{8}$$

$$\mathbf{q}\_{\mathbf{e}} = \frac{\mathbf{U}\%}{100} \times \mathbf{C}\_{0} \times \frac{\mathbf{V}}{\mathbf{m}} \quad (\mathbf{m}\mathbf{g} \cdot \mathbf{g}^{-1})\tag{9}$$

$$\mathbf{C\_e = A\_i - \left(A\_i \times \frac{\mathbf{U\%}}{100}\right) \pmod{\mathbf{L}^{-1}}}\tag{10}$$

$$\mathbf{K\_{d}} = \frac{\mathbf{A\_{i}} - \mathbf{A\_{f}}}{\mathbf{A\_{i}}} \times \frac{\mathbf{V'}}{\mathbf{m}} \text{ (mL} \cdot \mathbf{g^{-1}}\text{)}\tag{11}$$

where A<sup>i</sup> and A<sup>f</sup> are the initial and final <sup>99</sup>Mo radioactivity in counts/min. <sup>C</sup><sup>0</sup> (mg·<sup>L</sup> −1 ) is the initial concentration of CA-99Mo, V (L) and V 0 (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 <sup>99</sup>Mo. 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 (∆*H*<sup>0</sup> ), entropy change (∆*S* 0 ), and free energy change (∆*G* 0 ) 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 <sup>99</sup>Mo/99mTc generator using LAS <sup>99</sup>Mo for radiopharmaceutical applications. Since the available specific activity of LAS <sup>99</sup>Mo 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.

**Author Contributions:** Conceptualization, M.F.N.; methodology, M.F.N. and A.F.E.-D.; software, M.F.N., A.F.E.-D. and A.A.; validation, M.F.N., A.F.E.-D., A.A., M.A.S. and A.T.; formal analysis, M.F.N., A.F.E.-D. and A.A.; investigation, M.F.N.; data curation, M.F.N., A.F.E.-D. and A.T.; writing original draft preparation, M.F.N.; writing—review and editing, A.F.E.-D., A.A., M.A.S. and A.T.; visualization, M.F.N. and A.F.E.-D.; supervision, A.T.; project administration, M.F.N. and A.T.; funding acquisition, M.F.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research was funded by the Swiss National Science Foundation (grant number CR-SII5\_180352). Mohamed F. Nawar gratefully acknowledges the funding support of the Swiss Government Excellence fellowships program (fellowship No: 2017.1028).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data is contained within the article.

**Acknowledgments:** The authors would like to express their sincere thanks to Marcel Langensand, Managing Director, Medeo AG, CH-5040 Schöftland, Switzerland, for his valuable support in supplying <sup>99</sup>Mo/99mTc generators to conduct this research study.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

