Adsorption of Toxic Methylene Blue Dye from Aqueous Solutions Using Thymbra spicata L., Optimization by Taguchi Method and Investigation of Its Antioxidant Properties

Abstract

This study aimed to effectively evaluate various applications of Thymbra spicata L. biomass used in a zero-waste context. The study aimed to better understand the biomass’s potential multifunctional uses in environmental remediation by evaluating its adsorption efficiency, methylene blue (MB) dye removal processes, and antioxidant properties. T. spicata L. biomass was characterized by Fourier transform infrared-attenuated total reflectance (FTIR-ATR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and point of zero charge (pHpzc). The optimum physicochemical parameters were optimized using the Taguchi method. The dye selectivity studies were conducted using various anionic and cationic dyes. Adsorption tests were carried out by adjusting the concentration (10–50 mg/L), period (0–120 min), pH (2–12), and dose (0.01–0.20 g/50 mL). We used equilibrium data to non-linear Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherm models. The results stated that the removal process occurred in a monolayer on the homogeneous surface of T. spicata L. biomass and the $$\:{q}_{m}$$ value was determined as 65.03 mg/g at 298 K. Kinetic results showed that MB adsorption on T. spicata L. biomass followed a pseudo-second order. Thermodynamic tests showed that the process was exothermic ( $$\:\varDelta\:{H}^{o}=-14.92\:kJ/mol$$ ) and spontaneous ( $$\:\varDelta\:{G}^{o}=-28.31\:kJ/mol$$ ). In the second part, antioxidant capacity was determined by various methods using the extract of T. spicata L. biomass. The results of the study indicated that T. spicata L. could be a viable adsorbent for cationic dyes in aqueous solutions and a natural antioxidant source. Furthermore, compared to similar biomasses reported in the literature, it was found to be an alternative adsorbent.