Detoxification of olive oil mill wastewaters by liquid-liquid extraction with natural deep eutectic solvents

Olive oil mill wastewaters (OOMWs) are the main waste stream of olive processing into olive oil and, although the polyphenolic fraction contained therein can find numerous uses as a nutraceutical and cosmeceutical ingredient, they still have a very hard time to be considered a byproduct rather than a waste. The deployment of this large volume resources has, so far, been hindered by the fact that the polyphenolic mixture that can be extracted from it also contained phenol, which is highly toxic for humans, thus requiring a purification step which adds up to the complexity and cost of the separation. A method capable of separating the toxic compound directly from the liquid stream would therefore be highly desirable. In this work the hydrophobic character of three hydrophobic natural deep eutectic solvents (NaDES) recently developed by Florindo et al. (2017 and 2018), which are composed by pairs of substances among C8 and C12 fatty acids and menthol, has been used to investigate Liquid-Liquid Extraction for detoxifying olive mill wastewaters (OMW) from endogenous phenol. Explorative experiments were carried out on a synthetic mixture containing water, tyrosol (representing desired polyphenols) and phenol, representing a model OMW, at different pH of the treated mixture. Experimental results show that the C8:C12 NaDES exhibits the most favourable extraction features among the three solvents and that neutral pH yields an optimal selectivity.