Mass Transfer Mechanism and Equilibrium Modelling of Hydroxytyrosol Adsorption on Olive Pit–Derived Activated Carbon

Abstract

The rise in olive oil production coincides with an increase in olive oil side-streams, such as olive mill wastewater and olive pits, enabling the exploitation of higher value-added products through proper processes. Adsorption techniques, common in wastewater treatment, facilitate the recovery of valuable compounds found in these by-products, such as the strong natural antioxidant hydroxytyrosol present in olive mill wastewater. Owing to their chemical composition, olive pits are a promising precursor for activated carbon production intended for adsorption purposes. However, a complementary side-stream strategy requires an in-depth analysis of the equilibrium state and the governing mass transfer mechanism based on sophisticated mathematical models for a reliable investigation of the processes occurring during hydroxytyrosol adsorption on olive pit–derived activated carbon (OPAC). Here we show the technological suitability of OPAC for adsorption of hydroxytyrosol in comparison to an existing commercial activated carbon (CAC). We found that the removal efficiency of OPAC was superior to CAC with increasing initial concentrations of hydroxytyrosol. The Redlich-Peterson isotherm provided the best fit to the adsorption equilibrium data based on the Akaike Information Criterion (AIC). Elucidation of the adsorption kinetics by means of the pore volume and surface diffusion model (PVSDM) showed that intraparticle diffusion resistance dominated hydroxytyrosol adsorption on OPAC. In addition, investigations over a broad temperature and pH range revealed the versatile applicability of OPAC and indicated a physisorption governed interaction. Our results demonstrate the fundamental parameters associated with hydroxytyrosol adsorption on OPAC, crucial for the development of adsorption systems to convert by-products into opportunities.