Recovery of high added value natural polyphenols from actual olive mill wastewater through solid phase extraction

The recovery of polyphenols from olive mill wastewaters (OMWs) provides the double opportunity to obtain high-added value biomolecules and to reduce the phytotoxicity of the effluent. In this work, a solid phase extraction procedure for the recovery of OMW phenolic compounds recently developed on water solutions was applied and assessed on two different actual site OMWs. To this aim, the four most promising resins employed in the previous work, namely Amberlite XAD7, XAD16, IRA96 and Isolute ENV+, were tested and compared for their adsorption and desorption features towards the natural phenols occurring in the two real OMWs. Water, methanol and ethanol, also acidified with HCl 0.5 ml/100 mL, were tested as possible desorbing agents. Adsorption isotherms related to total phenols and to hydroxytyrosol, i.e. the most abundant (about 0.6 g L-1) and valuable phenolic compound detected in the employed OMWs, were determined. Generally, all experimental data were well fitted with Freundlich isotherm model. The highest total phenols adsorption was achieved with ENV+ resin, which adsorbed almost completely the OMW hydroxytyrosol. The highest desorption ratios were generally obtained with acidified ethanol, which mobilized almost all polyphenols adsorbed onto resins ENV+, XAD7 and XAD16. Considering the integrated adsorption-desorption processes, ENV+ allowed to achieve a) the highest recovery of total phenols when elution was done with acidified ethanol and b) the highest specific recovery of hydroxytyrosol when non-acidified ethanol was used as the desorbing phase.