In this research, Cnicus Benedictus roots powder (CBRP) was utilized as a low-cost adsorbent for removing methylene blue (MB) as a cationic dye from aqueous solution. Characterization of CBRP by Boehm titration method, Fourier transform infrared (FT-IR) and environmental scanning electron microscope (ESEM) indicates the presence of different types of functional groups and a very rough surface filled with cavities, suitable for MB adsorption. The effect of various parameters such as CBRP dose (0.2–2 g/1L), contact time (0–180 min), initial pH (1–10), coexisting anions/cations and the initial MB concentration (100–500 mg L−1) on MB adsorption were examined, the results showed that MB uptake depends on the pH of solution, the electrostatic attraction forces, the formation of H-bonds and π–π interactions are the possible CBRP–MB interaction mechanisms. Adsorption kinetic of MB on CBRP was best represented by linear and non-linear form of the pseudo-second order model. The experimental equilibrium data were best described by linear and non-linear forms of Langmuir and Redlich–Peterson isotherm models (R 2> 0.98) with a maximum MB adsorption capacity of 85.47 mg g−1 at 25 °C, the adsorption efficiency R (%) was greater than 90 (%). MB adsorption-desorption experiments showed the reusability of CBRP for five consecutive cycles. According to the obtained results, CBRP can be used as a natural adsorbent material for cationic dyes removal from aqueous solution.
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