**4. Conclusions**

The adsorption/desorption conditions of the three Amberlite resins were evaluated to optimize the extraction and purification of polyphenols from the ethanol extract of roasted hazelnut skin. Static adsorption and desorption tests showed that 5 g of Amberlite XAD16 resin had the highest adsorption capacity (40.06 ± 0.55 mg GAE/g) and adsorption ratio. The adsorption kinetics were well-fitted by a pseudo-second order model. Among the tested concentrations of desorbing solvent, 70% *v/v* ethanol solution showed the highest desorption ratio (81.17 ± 1.19%). In the dynamic adsorption/desorption processes performed using 5 g of XAD 16, the breakthrough point increased with decreasing adsorption flow rates, whereas the higher flow rate of solvent in dynamic desorption shortened the desorption time, but polyphenol recovery (87.7%) was observed at the lowest flow rate (1.5 BV/h). The purified extract showed higher phenolic compound levels and antioxidant activity than the crude extract and may be useful as a natural source of bioactive compounds for producing functional foods, as well as cosmetic and pharmaceutical preparations.

**Author Contributions:** Conceptualization, N.S.Z.; methodology, N.S.Z.; formal analysis, N.S.Z.; resources, G.Z.; writing—original draft preparation, N.S.Z. and G.Z.; writing—review and editing, N.S.Z. and G.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** Research supported by VALTIFRU 4.0-Valorizzazione delle filiere di frutta a guscio e fresca trasformata ad alto valore aggiunto-ARS01\_01060.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable. **Conflicts of Interest:** The authors declare no conflict of interest.
