New study of the adsorption mechanism of different dye molecules by high porous sludge activated carbon produced from a dairy-treatment effluent plant

In this study, a high-quality porous sludge-activated carbon from a dairy-treatment effluent plant in México (ACSDM) was produced, characterized, and investigated in the adsorption of 8 different dyes. The main focus was establishing how the dyes' molecular characteristics affect adsorption. So, the intrinsic characteristics of the different dye molecules were correlated with the experimental adsorption data. ACSDM proved high quality, considering their textural properties and physical-chemical characteristics. Adsorption was efficient using the natural pH of each dye solution and 1 g L−1of ACSDM. The highest adsorption capacities were obtained in the following order: Methylene Blue (MB) (998.44 mg g−1), Crystal Violet (CV) (795.65 mg g−1), Fuchsine (F) (342.52 mg g−1), Direct Blue 86 (DB-86) (306.36 mg g−1), Alizarin Green (AG) (303.25 mg g−1), Reactive Black 5 (RB-5) (259.82 mg g−1), Methanil Yellow (MY) (252.81 mg g−1) and Amaranth Red (AR) (251.12 mg g−1). These results were obtained at 55 °C (endothermic process), except for MY, where the adsorption was exothermic (25 °C). Freundlich was the model that best fitted the DB-86, MB, and CV equilibrium data. While the Langmuir and Tóth models best fit the RB-5, AR, and MY isotherms. Tóth also best represented the F and AG data. Kinetic curves demonstrated that adsorption occurred faster for dyes MB, F, CV, MY, AR, and RB-5, respectively, reaching 90 %-99 % of their saturations in the first 30 minutes. Meanwhile, the adsorption of AG and DB-86 only about 85 % of the saturation occurred. The study indicated that molecular volume is the major variable in the dye adsorption process, as well as some individual chemical characteristics of the molecule.