Removal and recovery of phenolic compounds from olive mill wastewater by cooling crystallization

Phenolic compounds are among, the most complex and the most difficult to remove compounds from the by-products of olive mill wastewaters (OMW). Due to significant properties, including stability and anti-oxidative activity, the recovery of poly-phenols (PP) from OMW is of paramount importance. In the present work, trans-cinnamic acid and ferulic acid, model compounds for the PP present in OMW were removed by cooling crystallization from aqueous solutions. Cooling crystallization experiments were done in a batch reactor kept at constant temperature, Thot (70 °C). Solutions of the two tested PP were prepared in the reactor and were allowed to equilibrate. Crystallization took place on the surface of a cylindrical (Ø 25 mm, L = 120 mm) stainless steel (SS) metal tube immersed into the supercooled solution. The SS surface was cooled at a lower temperature, Tcold, with respect to the respective melting point, in the range between 0 and 25 °C. Preliminary experiments showed that the solubility of the tested PP solutions was 1 g L−1 between 45 and 55 °C for trans-cinnamic acid and 3 g L−1 at 50 and 60 °C for ferulic acid. These were the initial concentration values of the PP used in the cooling crystallization process in this study. For the calculation of the super-cooling with respect to each of the PP studied, the respective solubilities as a function of temperature were measured in the range 0-60 °C. The formation of PP crystal layers on the SS tube surface took place without any appreciable induction time. In the series of experiments done, the temperature and concentration profile in the batch reactor in the presence of the cooled SS surface were calculated on the basis of mass and heat transport equations for unstirred systems. The imposed super-cooling (ΔT = Tmelt − Tcold) from the cold surface to the bulk solution, which is the driving force for the crystallization of the phenolic compounds on the SS surface, was thus estimated.