Journal of the Science of Food and Agriculture J Sci Food Agric 87:783–788 (2007) Effect of salts on the solubility of phenolic compounds: experimental measurements and modelling Adel Noubigh, 1 Arbi Mgaidi, 3 Manef Abderrabba, 1 Elise Provost 2∗ and Walter F ¨ urst 2 1 Laboratoire de Physico-chimie des mat ´ eriaux, IPEST, Bp51, 2070 La Marsa, Tunisia 2 Laboratoire Chimie et proc ´ ed ´ es, ENSTA, 32 de Boulevard Victor, 75739 Paris, Cedex 15, France 3 Laboratoire de Chimie Min ´ erale Industrielle, Facult ´ e des Sciences de Tunis, Tunisia Abstract: Phenolic compounds can be found in industrial or agricultural waste, such as waste water from olive mills. Because of the environmental problem caused by this residue from olive oil production, the aim of this work was to model the effect of salts on the solubility of four phenolic compounds. For this purpose, the solubilities of gallic acid, protocatechuic acid, vanillic acid and vanillin have been measured in various chloride solutions at constant temperature. The Clegg–Simonson–Pitzer activity coefficient, together with the dissociation constant obtained from the literature, has been used to represent the equilibrium solid-liquid data. Good agreement (3.4%) between the experimental and calculated values for solubility was obtained.  2007 Society of Chemical Industry Keywords: phenolic compounds; solubility; salt effect; activity coefficient; modelling INTRODUCTION Olive mill wastewaters (OMWW) are agricultural waste materials that result from the production of olive oil. They are produced on a large scale, mainly in the Mediterranean basin (more than 30 million m 3 year −1 ), where most of the world’s production of olive oil is located. Typical OMWW composition by weight is 83 – 94% water, 4–16% organic compounds and 0.4–2.5% mineral salts. The organic fraction contains 2–15% of phenolic compounds (PhCs), corresponding to a concentration up to 10 g L −1 . 1,2 The main character- istics of these aqueous by-products are a high chemical oxygen demand (COD) and biological oxygen demand (BOD), an acidic pH (4–5) and a ionic conductivity of about 10 mS cm −2 due to the presence of ions (K + , Cl − , Ca 2+ and Mg 2+ ). The chemical composition of OMWW depends on several factors: the oil extrac- tion process (a two-phase or three-phase system), the age of the olives when gathered and their storage time. 3,4 Though the phenolic fraction accounts for a relatively low contribution to COD (roughly 14%), it is generally acknowledged that it is responsible for the high toxicity of OMWW. 5 The PhCs are antioxidant, phytotoxic and toxic to bacteria used in common bio- logical wastewater treatment. 6–8 On the other hand, their antioxidant properties make them very interesting products from a nutritional point of view. In many production areas, there is no specific treatment for OMWW and the standard practice continues to be the unrestrained dumping of OMWW in rivers, streams and soil and sometimes ends up reaching the sea. Even though the solubility of the PhCs studied here is low, 12 it can be modified by salts found in OMWW or in the area used for dumping. However, no experimental or theoretical study concerning the effect of salt on the solubility of PhCs has been reported in the literature. The first step of this work was to obtain experimental values for the solubility – in pure water and in some chloride solutions at 25 ◦ C – of several PhCs generally present in OMWW. According to Mantzavinos and Kalogerakis, 9 gallic acid (3,4,5- trihydroxybenzoic acid), protocatechuic acid (3,4- dihydroxybenzoic acid), vanillic acid (4-hydroxy-3- methoxybenzoic acid) constitute the second major family of PhCs commonly found in olive mill effluents. The solubility of these acids, as well as that of vanillin (4-hydroxy-3-methoxybenzaldehyde), has been measured in chloride solutions. Figure 1 shows the structure of each compound. Three chloride solutions have been considered: NaCl, because it is present in sea water; KCl, because it is present in OMWW; and LiCl in order to observe the effect of a more solvated cation. Li + has been considered because it is much more solvated than K + or Na + . The second step of this work was to represent the solubility data using an activity coefficient thermody- namic model based on the Clegg–Simonson–Pitzer’s formalism. 10,11 Determination of the activity coeffi- cients of organic and inorganic salts can be successfully extrapolated to predict other equilibrium conditions such as liquid–liquid equilibrium and to evaluate partition coefficients between organic phases (fauna, ∗ Correspondence to: Elise Provost, Laboratoire Chimie et proc ´ ed ´ es, ENSTA, 32 de Boulevard Victor, 75739 Paris, Cedex 15, France E-mail: elise.provost@ensta.fr (Received 29 July 2005; revised version received 18 April 2006; accepted 29 May 2006) Published online 5 February 2007; DOI: 10.1002/jsfa.2762  2007 Society of Chemical Industry. J Sci Food Agric 0022–5142/2007/$30.00