**4. Conclusions**

The present work studied the adsorption of TODGA, a common chelating agent, on HCP in an SC CO2 medium. The measurements were made by two methods: on-line SFC and gravimetrical analysis. Both methods showed almost identical results, but the first proved to be more reliable when measuring adsorption from concentrated solutions.

Isotherms of TODGA adsorption on the MN202 and MN270 sorbents were measured at 313 K and 20 MPa. Based on the capacity of the sorbents determined by isotherm approximation, the MN202 sorbent was chosen as a suitable carrier for TODGA.

Varying the fluid density during impregnation within the range 0.78–0.91 g/mL in isothermal conditions did not have a significant effect on TODGA adsorption. In contrast, the temperature parameter has a significant influence on the MN202 TODGA adsorption capacity. Semi-empirical models based on the Langmuir equation were used to make a general description of the adsorption dependence on concentration and temperature. The temperature increase from 313 K to 343 K at the medium density of 0.78 g/mL changed the maximum amount of adsorbed TODGA from ~0.68 mmol/g to ~0.49 mmol/g.

In conditions of equilibrium adsorption from methanol it is inappropriate to prepare impregnated TODGA/HCP adsorbents, due to the small slope value of the isotherm. The SC CO2 application in the impregnation procedure allowed us to reduce the process time and eliminate toxic organic solvents. The samples of the sorbent impregnated in methanol and SC CO2 have identical chemical resistance to nitric acid solutions, so there is no difference between two impregnation media with respect to this property.

**Author Contributions:** Conceptualization, M.K. and O.P.; methodology, M.K.; investigation, M.K. and O.P.; writing—original draft preparation, M.K.; writing—review and editing, O.P.; supervision, O.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Ministry of Science and Higher Education of the Russian Federation as part of the State Assignment of the Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors express their gratitude to Sergey Lyubimov from INEOS RAS (Moscow, Russia) for providing the HCP samples, Andrey Shadrin from JSC Bochvar VNIINM (Moscow, Russia) for providing TODGA.

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

**Sample Availability:** Not available.
