Optimization of a cationic dye removal by a chemically modified agriculture by-product using response surface methodology: biomasses characterization and adsorption properties

The present study investigates the alkaline modification of raw orange tree sawdust (ROS) for an optimal removal of methylene blue (MB), as a cationic dye model, from synthetic solutions. The effects of operating parameters, namely, sodium hydroxide (NaOH) concentrations, ROS doses in NaOH solutions, stirring times, and initial MB concentrations on dye removal efficiency, were followed in batch mode. The process optimization was performed through the response surface methodology approach (RSM) by using Minitab17 software. The results showed that the order of importance of the followed parameters was NaOH treatment concentrations > stirring times > initial MB concentrations > ROS doses in NaOH solutions. The optimal experimental conditions ensuring the maximal MB removal efficiency was found for a NaOH treatment concentration of 0.14 M, a stirring time of 1 h, a ROS dose in NaOH solutions of 50 g L⁻¹, and an initial MB concentration of 69.5 mg L⁻¹. Specific analyses of the raw and alkali-treated biomasses, e.g., SEM/EDS and XRD analyses, demonstrated an important modification of the crystalline structure of the wooden material and a significant increase in its surface basic functional groups. Kinetic and isotherm studies of MB removal from synthetic solutions by ROS and the alkali-treated material (ATOS) showed that for both adsorbents, the pseudo-second-order and Langmuir model fitted the best the experimental data, respectively, which indicates that MB removal might be mainly a chemical and a monolayer process. Furthermore, thanks to the chemical modification of the ROS, the MB maximal uptake capacity has increased from about 39.7 to 78.7 mg g⁻¹. On the other hand, due to the competition phenomenon, the coexistence of MB and Zn(II) ions could significantly decrease the MB removal efficiency. A maximal decrease of about 32 % was registered for an initial Zn(II) concentration of 140 mg L⁻¹. Desorption experiments undertaken at natural pH (without adjustment: pH = 6) and with different NaCl concentrations emphasized that the adsorbed MB could be significantly desorbed from both the tested materials, offering their possible reuse as efficient adsorbents. All these results confirmed that NaOH-treated orange tree sawdust could be considered as an efficient, economic, and ecological alternative for the removal of cationic dyes from industrial wastewaters.