RECOVERY OF OLIVE MILL WASTEWATER POLYPHENOLS THROUGH LIQUID-SOLID EXTRACTION

The recovery of polyphenols from olive mill wastewaters (OMWs) conjugates the reduction of the waste phytotoxicity to the obtainment of high-added value molecules. To this aim, liquid-solid extraction can represent an economical and environmental friendly approach, since it can combine simple procedures to the use of non-toxic desorbing solvents. In this work, the adsorption and desorption features of four resins, namely Amberlite XAD7 and XAD16, IRA96 and Isolute ENV+, towards the phenolic fraction of an OMW were studied. Methanol and ethanol, also acidified with HCl 0.5% v/v, were tested as the desorbing phase. The OMW had a total phenol content (on Folin-Ciocalteu method basis) of about 3.4 g/L. Adsorption isotherms related to total phenols and to the single monocyclic detected ones (measured via reverse-phase HPLC-DAD) were determined by filling 50 mL-flasks with 10 mL of an OMW dilution (10÷100%) and with 0.7 g of resin (dry weight) and by shaking the flasks at 180 rpm for 2 hours at room temperature. Freundlich and Langmuir isotherms were used to interpolate the experimental data, which better fitted with the former model. The highest phenolic fraction adsorption was achieved with resin ENV+ resin (up to about 80%), which allowed the almost complete adsorption of hydroxytyrosol, i.e. the highest added-value detected phenol in the OMW (0.59 g/L). The efficiencies of the desorbing step were evaluated by recovering the resins employed in the dephenolization of the not-diluted OMW: after liquid-solid phases separation, the flasks were refilled with 14 mL of solvent and shacked under the same conditions reported above. Comparable results were observed by employing ethanol or methanol. The highest desorption was obtained with acidified ethanol, by which almost all the adsorbed phenolic fraction was recovered from resins ENV+, XAD7 and XAD16, while about its 82% was recovered from IRA96. Considering the integrated adsorption-desorption processes, the 84, 69, 65 and 59%, respectively, of the OMW total phenols were obtained; however, XAD16 gave rise to the highest specific recovery of hydroxytyrosol (77%). The obtained results were validated by employing other two OMWs in identical adsorption-desorption experiments, carried out with ethanol and acidified ethanol as the solvent: again, the latter desorbing phase allowed higher polyphenol recoveries and ENV+ was confirmed to be the most performing resin.