Investigation of adsorption and mechanism of the cationic methylene blue by the polymeric Rubia tinctorum seeds from environment wastewater: kinetic equilibrium and thermodynamic studies

This study focuses on the removal of methylene blue (MB), an organic dye, through adsorption on the Rubia tinctorum seeds (RTS) which are considered a new economical and efficient adsorbent for the removal of MB. This efficiency is proven by the microscopic analyses performed using Fourier-transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy/energy-dispersive X-ray spectroscopy analysis techniques. These were performed before and after dye fixation and showed that the prepared surface of the RTS is made up of layers and pores, which could increase the total surface area and provide better fixation for the MB adsorbing cationic dye. To investigate the variation of the adsorption capacity, various parameters were tested such as the mass of the biosorbent, temperature, concentration, pH. The main parameters for maximum adsorption of MB were 40 min contact time and pH 5.00 and a percentage removal of 89.14% was obtained. Under optimal conditions, the monolayer adsorption capacity reached 188.68 mg/g. In addition, modeling studies showed that the adsorption process was best described by the pseudo-secondorder kinetic model and the isotherm of Langmuir. Finally, the thermodynamic study revealed the endothermic mode of the MB removal process by RTS.