Effective Removal of Methylene Blue Dye Using Low-Cost Essential Oil Industry Waste: Novel Hybrid Design Approach, Kinetic and Thermodynamic Evaluation

This study utilizes spent carrot seed waste (SCSW) as an adsorbent for the removal of methylene blue dye from synthetic wastewater. The influence of reaction parameters—temperature, pH, time, dye concentration, adsorbent dose, and RPM—is studied to maximize the adsorption capacity and dye removal. The hybrid optimization design approach (DoE + manual) reveals that dye concentration and adsorbent dose are the primary influencing factors. SCSW exhibits a maximum adsorption capacity of 234 mg/g, achieving approximately 98% dye removal. Langmuir, Freundlich, and Temkin isotherms are employed, with the best fit observed at 50 °C. Kinetic analysis reveals that the adsorption process adheres to the pseudo-first and pseudo-second-order processes, as well as the Elovich model at all dye concentrations. The equilibrium models suggest that the adsorption of Methylene Blue dye molecules is monolayer and multilayer onto the heterogeneous surface of SCSW. The kinetic and thermodynamic studies imply that the adsorption process is exothermic and physico-chemical in nature. Finally, regeneration of dye-coated SCSW is performed to check its reusability and found that thermochemical regeneration is the appropriate method for regeneration.