Polyphenols from olive mill wastewaters: large scale sustainability of their recovery, and selective extraction by means of molecularly imprinted polymers

The use of agroindustrial wastes as potential renewable resources for the production of high added-value compounds, biomolecules or bioenergy has been largely investigated in recent years. To date, integrated multi-step physicochemical biotechnological processes are allowing the extensive valorization of an increasing number of biowastes. In this respect, the design of a biorefinery for the case study of olive mill wastewaters (OMWs) has been recently proposed [1]. Particular attention was given to polyphenols and their recovery through a solid phase extraction procedure. Several commercial adsorbing agents were tested (namely, Amberlite XAD resins, as well as IRA96 and Isolute ENV+). First investigations on their adsorption and desorption capacities were carried out with synthetic solutions made of 10 polyphenols among those commonly found in OMWs, using ethanol as desorbing solvent [2]. Performances of such adsorbing agents were then tested with actual site OMWs [3], showing that Amberlite XAD16 was the most effective, particularly when valuable polyphenols such as hydroxytyrosol and tyrosol were considered. Notwithstanding the fact that solid phase extraction procedure may be very practical, yet proofs of the feasibility of its scale-up have to be shown once the process economy is considered. As a matter of fact, fully activated Amberlite resins may cost some 600 euro per kg, and solvent price may be too high for industrial scale. To this aim, sustained recycling of both the resin and the solvent (i.e., ethanol) was tested [4]. The same amount of Amberlite XAD16 was used in 10 consecutive cycles for polyphenols recovery from actual site OMWs. At the end of each cycle, resin was re-activated using fully biocompatible solutions (i.e., ethanol and water). Sustained reuse of the resin had no negative effects on adsorption capacity, while desorption one was only slightly reduced. Ethanol regeneration was carried out using a rotary evaporator. As a result, Folin Ciocalteu's and HPLC analyses showed no trace of polyphenols in the regenerated ethanol while, notably, the HPLC profile of the concentrated polyphenolic mixture, its anti-radical and antioxidant capacity remained constant. This approach would significantly reduce overall operational costs, and determine a theoretically infinite reuse of the solvent employed. However, if on the one hand intensive recovery of polyphenolic mixtures appears feasible, on the other hand recovery of target molecules from complex substrates appears very difficult yet. In this respect, molecularly imprinted polymers (MIPs) are known to represent a promising solution. First attempts to selective extract and purify a single phenol (i.e., gallic acid, GA) were conducted [5]. Imprinting efficiency was evaluated in different water/ethanol solutions by using pyrogallic acid as the template analogue. When actual site OMWs were employed, recovery values were between 85 and 97%. However, provided that the highest recognition with MIPs usually occurs with pure ethanol, selective recovery of target polyphenols using the concentrated alcoholic mixture as obtained after rotary evaporator is underway.