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

https://doi.org/10.1016/j.cej.2011.05.056Get rights and content

Abstract

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.

Highlights

► We assessed a solid phase extraction procedure for the recovery of natural polyphenols from olive mill wastewaters. ► Comparison among the adsorption/desorption performances of four commercially available resins was done. ► Isolute ENV+ resin allowed the highest overall phenol recovery. ► The chemical features of olive mill wastewaters can affect the process productivities.

Introduction

Olive mill wastewater (OMW) is one of the most environmentally concerning food processing effluents of the Mediterranean area. Such effluents are containing high concentrations of polyphenols, up to 10 g L−1 [1]. Such chemicals are able to exert acute toxic effects towards microorganisms [2], [3], plants [4], [5], [6] and pluricellular organisms belonging to different trophic levels [7]. On the other hand, they are natural antioxidants [8], [9] of special relevance for several industrial sectors, such as cosmetic [10], [11], food preservation [9] and medicine [12]. Thus, the selective recovery of phenolic compounds from OMWs can accomplish both the reduction of the intrinsic wastewater environmental toxicity and the obtainment of high added value molecules. In particular, hydroxytyrosol (HT) and tyrosol are among the most economically relevant components of OMW phenolic fractions [13], [14].

The technologies dedicated to the recovery and purification of bioactive compounds from natural effluents and biowastes are under implementation [8], [15], [16]. The recovery of polyphenols from OMWs was mostly proposed by membrane processes [7], [17], [18], [19] and liquid–liquid extraction [8], [20], [21], often assisted by surfactants [22], solid phase extraction (SPE) can represent a feasible alternative, since it relies on simple, effective and cheap procedures [23]. An effective extraction of polyphenols from different organic residues was obtained through SPE approaches; in particular, the acrylic ester-based Amberlite XAD7 and the styrene-divinylbenzene-based XAD16 resins were successfully employed in the solid phase recovery of polyphenols from grape pomace [24], [25], [26] and apple wastes [23]. The same adsorbents were also used in the selective recovery of polyphenols from OMWs [13].

In the perspective of developing an optimized SPE process addressed to recover phenolic compounds from OMWs, we recently characterized the adsorption and desorption features of a large spectrum of commercially available resins, including the above mentioned XAD7 and XAD16, towards a synthetic mixture of ten prominent low molecular weight phenolic compounds (stock phenol solution, SPS) typically occurring in OMWs [27]. Among the tested resins, Amberlite IRA96 and Isolute ENV+, never employed before in the recovery of natural phenols, displayed interesting results. In particular, the latter, which is a non-polar adsorbent, gave rise to the best overall process productivity (i.e., it allowed the recovery of almost 60% of the overall phenols originally occurring in the phenols solution), when ethanol was used as the desorbing phase [27].

This study was undertaken to test and further assess the most promising SPE protocols developed in the previous study on real OMWs, where the native phenols are coexisting, often adsorbed onto solid materials, together with high concentration of a variety of biogenic molecules and macromolecules. Two OMWs obtained from two different Italian olive milling facilities and displaying a different chemical composition were used in the study. To the very best of our knowledge, this is the first work in which the adsorption/desorption properties of different kind of resins towards the polyphenols of actual site OMWs are jointly investigated and assessed.

Section snippets

Chemicals and olive mill wastewaters (OMWs)

Two OMWs, named OMW1 and OMW2, were employed in the research. They were kindly purchased by the Sant’Agata d’Oneglia (Imperia, Italy) and Grassanese (Matera, Italy) three phase olive mills, respectively, during the 2009–2010 production campaign. Four resins, namely Amberlite XAD7, XAD16 and IRA96 (all purchased by Sigma–Aldrich, Milan, Italy) and Isolute ENV+ (Stepbio, Bologna, Italy), were employed as the solid adsorbing phase. Their main physical properties are reported in Table 1. The

Results and discussion

Four commercial resins, namely, Amberlite XAD7, XAD16 and IRA96 and Isolute ENV+, were recently tested and compared for their ability to recover 10 low molecular weight phenolic compounds of OMWs combined in a defined water solution (Stock Phenol Solution, SPS [27]). In particular, the latter two adsorbents, which were never employed before in the field, offered very encouraging evidences, since ENV+ provided the most promising results in terms of process productivity and IRA96 allowed the

Conclusions

The employment of solid phase extraction procedures appears a feasible option for the environmental sustainable and effective recovery of natural polyphenols from real industrial OMWs. Among the applied commercial resins, ENV+ gave rise to the best overall recovery productivities. The elution of the adsorbed OMW total phenols was successfully carried out especially through the employment of acidified ethanol, whereas that of more polar low molecular weight phenols (including HT) was better

Acknowledgements

The authors thank the Olive mills Sant’Agata d’Oneglia (Imperia, Italy) and Grassanese (Matera, Italy) for having kindly provided OMW1 and OMW2, respectively. This research was financially supported by the Fondazione Del Monte di Bologna e Ravenna (Bologna, Italy).

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