Adsorption of phenolic compounds from olive mill wastewaters on spent coffee grounds: isotherms, kinetics, and pure phenol adsorption

The purpose of the present study, besides the reutilization of spent coffee grounds (SCG) as an innovative adsorbent for the depollution of olive mill wastewaters (OMW), which was proved in our previous study, is the possible interpretation of adsorption behavior of OMW. Sorption characteristics of OMW's pure phenolics (p-coumaric acid, caffeic acid, tyrosol, hydroxytyrosol, oleuropein) on SCG were examined and correlated with their physicochemical properties (molecular weight, aqueous solubility, pKa, and octanol-water partition coefficient). The adsorption of OMW on SCG, with an efficiency of 47.38% at the optimum conditions, was compared with a model solution which simulates the phenolic profile of OMW (adsorption efficiency of 40.91%). In addition, the combination of these pure phenolics produced a synergistic adsorption result (efficiency of 40.91%) that was lower than the cumulative sum of the effect that pure phenolic compounds produced individually (efficiency of 57.15%). Finally, different kinetic and isotherm models were developed to further study the adsorption mechanism. The sorption followed the intra-particle diffusion model and was explained more appropriately by the Langmuir sorption isotherm.