Equilibrium, kinetic, and thermodynamic studies of the adsorption of anionic and cationic dyes from aqueous solution using agricultural waste biochar

The potential of an agricultural waste (Calophyllum inophyllum seed husk) as a low-cost adsorbent for the removal of dyes from an aqueous solution was studied. Biosorption of Alizarin Yellow (AY), an anionic dye, and Malachite Green (MG), a cationic dye, onto modified C. inophyllum seed husk showed that certain modifications improved the adsorption property of this adsorbent. Modification of the seed husk with a combination of carbonization and microwave irradiation (HCM) worked best for the removal of AY, while a base-treated carbonized form (HCB) of the husk produced the best result for the removal of MG. Batch adsorption studies were conducted, and various parameters such as pH, initial concentration of the adsorbates, adsorbent dose, time, and temperature were optimized to evaluate the adsorption behavior of the biomass. Optimum dye removal was obtained at pH 3 for AY and pH 5 for MG after 100 min of the adsorbent and adsorbate interaction. Kinetic data obtained fitted better into the pseudo-second-order model, thus suggesting a chemisorption mechanism. Equilibrium studies showed that the removal of the dyes is best represented with the Freundlich isotherm model, suggesting a heterogeneous surface of the modified C. inophyllum seed husk and also indicating that surface adsorption is not the rate-determining step. Adsorption capacities of 27.90 mg/g and 31.25 mg/g were obtained for AY and MG, respectively. Thermodynamic parameters with increment delta H-o values of 4139.29 J/mol for AY and 10,089.04 J/mol for MG and increment delta S-o values of 3.61 and 28.38 J/mol/K for AY and MG, respectively, show that the adsorption processes are endothermic in nature, entropy-driven, and nonspontaneous. The results showed that the low-cost biomass possesses the potential for the effective removal of both cationic and anionic dyes from wastewater.