Thermodynamic analysis of batch adsorption isotherms of different types of olive pomace

The main purpose of this work is to identify the hygroscopic equilibrium and also determine the product–water relationship in olive pomace. In this context, experimental, modeling and thermodynamic analysis of adsorption isotherms of four types of agricultural wastes (olive pomace), have been carried out. Two samples were separately de-oiled by the maceration method and the hydrothermal carbonization process. Besides, adsorption isotherms of these biomass samples were obtained experimentally at three oven temperatures (30, 40 and 50 °C). The findings showed that the de-oiled samples are more hydrophobic than those containing residual oil, especially when water activity ( a w ) values are higher than 0.4. Moreover, adsorption isotherms were modeled by five mathematical models available in the literature. In the thermodynamic point of view, the net isosteric heat and the differential entropy of adsorption were estimated for each sample. The highest value of the net isosteric heat (around 65 kJ mol −1 ) is obtained for no de-oiled samples. Also, it has been found that the development of monolayer adsorption requires more net isosteric heat to ensure the water molecules adhesion to the biomass surface unlike the formation of multilayer adsorption. In addition, the enthalpy/entropy compensation theory for all examined biomasses was confirmed, and their free energy values are positive (between 3479.8 and 4155.6 J mol −1 ). Finally, optimal water activity values were also estimated and ranged from 0.35 to 0.49.