Enhanced adsorptive removal of cationic and anionic dyes from aqueous solutions by olive stone activated carbon

In this work, four activated carbons were synthetized from natural olive stone (NOS) wastes using ZnCl₂ as an activating agent. These activated carbons (OSAC) were synthetized for a constant mass ratio of ZnCl₂: NOS of 2:1, a contact time of 2 h and four different heating temperatures (300, 400, 450 and 500 °C). The physicochemical characterization of these activated carbons by various analyses including N₂ adsorption–desorption measurements, surface charge evolution versus pH, Boehm titration, Fourier transform (FTIR) and scanning electron microscopy (SEM) showed that the activated carbon produced at a temperature of 400 °C (OSAC 400 °C) exhibited the best properties. Indeed, it has the highest BET surface area, total pore and micropore volumes with values of 740 m²·g⁻¹, 0.57 cm³·g⁻¹ and 0.25 cm³·g⁻¹, respectively. Moreover, it is rich in various acidic and basic functional groups that could react with various common adsorbents. The test of these activated carbons, for the adsorption of a cationic (methylene blue (MB)) and anionic (methyl orange (MO)) dye under various experimental conditions, showed that OSAC 400 °C could be considered as an effective, attractive and promising adsorbent for both the tested dyes. The Langmuir’s adsorption capacities of this adsorbent were assessed to 303.0 and 277.8 mg·g⁻¹ for MB and MO, respectively, which are significantly high compared to other various activated carbons. The retention of the pollutants is mainly chemical including hydrogen bond and electrostatic attraction between the dyes and the activated carbon surface.