# Recovery of Polyphenols from Agri-Food By-Products: The Olive Oil and Winery Industries Cases
## 5 From Agri-Food Wastes to Polyphenols: Processing Techniques
### Recovery of Polyphenols by Adsorption Technologies  
Due to their low toxicity, chemical stability, selectivity, high adsorption capacity, and ease of regeneration, resins are an attractive option for the purification of phenolic compounds [127; 128; 129]. Resins are synthetic polymeric adsorbents that include materials such as polystyrene-divinylbenzene copolymers, polymethacrylate, etc., and can be functionalized with ion-exchange groups [128]. The adsorption properties of resins are related to a polymer composition, surface properties, pore structure, and solubility of adsorbents [130]. bonds, ionic interactions, or hydrophobic interactions. Adsorption should be reversible when it comes to recovering solute [127]. Adsorption isotherms inform about the adsorption capacity of the resin and how it interacts with the adsorbent. The isotherm provides a relationship between the concentration of the phenolic compounds in solution and the concentration of phenolic compounds adsorbed on the solid phase when both phases are in equilibrium. Langmuir, Freundlich, Temkin, BET, Redlich-Peterson or Dubinin-Radushkevich isotherm models are used to fit the data [131]. For polyphenol recovery, both non-functionalized and ion exchange resins have been proposed--non-ionic resins being the most applied [132]. [PERSON] et al. [133] reported that, in general, non-ionic resins show a better performance than ion-exchange resins, but this mode can be advantageous for some specific applications [134]. The nature of the solvent has a strong influence on adsorption. Water or water containing a low percentage of an organic solvent, such as ethanol, are well suited for the loading step. pH may also have a relevant impact on polyphenol adsorption/desorption since it determines polyphenols speciation; at low pH, phenolic compounds are undissociated, while at higher pH values dissociation of carboxylic and phenolic groups can occur. Consequently, the mechanisms of interaction of the resin-polyphenyl system are pH-dependent. Ionic strength may also affect the interactions, especially in the ion-exchange mode [128]. Regarding the desorption step, it is important to consider the solubility of the target compounds, but also the compliance with regulations applying to the potential application sectors: food additives, nutraceutical, etc. Thus, most applications use ethanol or ethanol-water mixtures with a high percentage of ethanol as desorption reagents. A large variety of resins, such as amberlite XAD 4 [52,115], XAD 16N [115], XAD7 HP [52,115], XAD 16 [52,133,135], IRA-958 Cl [133], IRA-67 [133,134], FPX-66 [52], Lewatit AF5, AF6, AF7, MonoPlus M800, K6387, VP OC 1600, MonoPlus SP112, CAL I [52], Macronet MN-202 [114], Amberlyst A26 [134], have been reported for the recovery of polyphenols in olive mill wastewater. For instance, [PERSON] et al. [114] proposed a combination of membrane tangential filtration (UF, NF, and RO) and adsorption techniques at a pilot scale for the treatment of wastewater from oil mills and the recovery of a polyphenol concentrate stream. For membrane filtration, polysulfine spiral-wound polymeric membranes were used, with an antifouling treatment. The NF concentrate was sent to the section of adsorption on a resin, for concentration and purification. A non-ionic, highly cross-linked, polymeric adsorbent (polystyrene-divinylbenzene) was used (Macronet MN-202), characterized by a high specific surface area (800-1100 m\({}^{2}\) g\({}^{-1}\)). The phenolic compounds retained on the column during the adsorption phase were desorbed with ethanol. As a result, they produced 62% purified water which could be reused in industrial processes, and a concentrated phenolic extract (9.5% of total phenols), with the capacity to be used in the food, cosmetic or pharmaceutical industries. [PERSON] et al. [136] investigated the adsorption of phenolic compounds from white wine viruses, onto non-ionic resins, with the objective of recovering phenolic compounds from the effluent.