Preparation of New Carbonaceous Adsorbents Based on Agricultural Waste and Its Application to the Elimination of Crystal Violet Dye from Water Media

In this work, the Washingtonia palm stems were used as a novel precursor to product biochar (BCW) and activated carbon (CAW). The resulting CAW and BCW samples were characterized by N₂ adsorption−desorption, scanning electron microscopy (SEM) and pH of zero-charge point (pH_PZC). The results show a very large specific surface area and pore volume for CAW (S_BET = 1032 m²/g, V = 1.1 cm³/g) compared to BCW (S_BET = 357 m²/g, V = 0.27 cm³/g). The SEM images show a more or less regular porous shape for both samples. The obtained values of pH_PZC were 5.2 and 6 for CAW and BCW samples, respectively. The ionic strength from 0 to 1.0 M greatly affects the adsorption capacity of Crystal Violet (CV) on BCW unlike that for CAW. The adsorption process occurred quickly and the kinetic data were well described by the pseudo first order (PFO), pseudo second order (PSO), Elovich and Avrami models. The maximum adsorption capacity of CV on CAW and BCW calculated from the Langmuir model was 328 and 93 mg/g respectively at 35°C. The values of the average free energy determined by the Dubinin–Radushkevich model are less than 8 kJ/mol, indicative of physisorption. The thermodynamic study at different solution temperatures (15, 25, and 35°C) shows that the adsorption process occurred spontaneously (∆G° < 0) and was exothermic for CAW and BCW (∆H° = –28.66 and –38.46 kJ/mol, respectively).