Adsorption of Anionic and Cationic Dyes on Different Biochars

This research compared the adsorption behavior of anionic dye methyl orange (MO) and cationic dye methylene blue (MB) by five carbon materials, i.e. rice husk biochar (RH), sawdust biochar (SD), activated carbon (AC), bamboo biochar (BB) and palm biochar (PB). Scanning electron microscopy (SEM), Boehm titration, Brunauer-Emmett-Teller (BET) analysis and zeta potential were used to characterize the physicochemical properties of the adsorbents. The pH had a great influence on the adsorption processes. The adsorption processes for two dyes followed the pseudo-second order kinetic model. The Langmuir, Freundlich and Temkin isotherm models all showed good fit to the experimental adsorption data. The thermodynamic studies revealed that all the reactions of MO were exothermic processes with decrease in entropy, while those of MB were just the opposite. The adsorption capacity of RH was the highest, which may be due to its physicochemical properties. The adsorption capacities of SD were the smallest, and the poor physiochemical properties of PB caused that the dyes adsorption onto PB was non-spontaneous and non-feasible. All adsorbents showed higher adsorption capacity for MB than that for MO, which may be due to the additional electrostatic attraction effect between adsorbents and cationic MB dye.