**4. Conclusions**

The study presents low-cost deep eutectic solvents (DES) as potential new sorption materials that enable one-step, e ffective biogas upgrading. This is a significant advantage compared to the currently used sorbents that are dedicated to removing only selected groups of impurities, which does not guarantee that the biogas will be of su fficient quality. The use of developed sorbents under absorption conditions results in biogas with high-methane gas parameters that meet the parameters for gas injected into the transmission network and transport fuel. In addition, the use of new sorbents based on DESs are highly advantageous from an economic and ecological point of view because the sorbents are synthesized from inexpensive, easily available materials that can be regenerated many times without loss of absorption capacity.

In this study, 23 low-cost DESs composed of quaternary ammonium salts and organic components were investigated. Based on the basic thermodynamic properties, i.e., the activity coe fficient, excess enthalpy, and Henry's constant, two DESs (ChCl:U (1:2) and ChCl:OA (1:2)) were selected because they showed the highest dissolution potential of the siloxanes, CO2, and H2S. The high a ffinity of both DESs to all of the main biogas contaminations was confirmed by means of σ-profiles and σ-potential analysis. It was shown that the electrostatic interactions between biogas impurities and DESs are the main driving force of the absorption process. For the best DESs, economic analysis simulation was conducted in order to evaluate and compare ChCl:U (1:2) and ChCl:OA (1:2) to each other and to currently available industrial absorbents. The unit cost of DESs depend mainly on the DES structure, which is responsible for its absorption capacity, and regeneration cycles. The unit cost of obtaining 1 m<sup>3</sup> of high-quality bio-methane using DESs absorption is comparable to the costs of currently used technologies. However, the proposed biogas upgrading technology o ffers the possibility of removing CO2, H2S, and siloxanes in one step. This is a significant advantage compared to other commonly used technologies that only remove individual impurities. The obtained results show the grea<sup>t</sup> potential of DESs to improve biogas to high-quality bio-methane with properties comparable to natural gas. Such bio-methane could be injected into the natural gas network or used as an alternative to compressed natural gas fuel. However, further experimental research is needed to confirm the obtained results.

**Author Contributions:** Conceptualization, P.M., and E.S.; methodology, P.M., and E.S.; investigation, P.M., and E.S.; data curation, P.M., and E.S.; writing—original draft preparation, P.M., E.S., J.G.; writing—review and editing, P.M.; visualization, P.M., and E.S.; supervision, J.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
