Citrullus colocynthis seeds activated with H₂O₂/NaOH as a promising efficient methyl green stain-removing biosorbent

Organic pollutants, such as dyes, which are widely employed in many industries, have contributed to serious environmental contamination, particularly water pollution, necessitating the development of innovative technologies for their removal from water. In the present study, a two-step H₂O₂/NaOH activation technique was applied to enhance the capacity of Citrullus colocynthis seeds to adsorb methyl green (MG), a basic cationic dye from aqueous solution using batch adsorption method. The prepared material was characterized using various techniques like BET surface area, FTIR, SEM–EDX, and TGA. The optimum adsorption conditions were pH = 6.5 ± 0.2, adsorbent mass = 0.02 g, MG initial concentration = 30 ppm, and temperature = 328 K. The results of adsorption kinetics indicated that the pseudo-second order was the most pertinent model to describe the MG adsorption process onto the activated Citrullus colocynthis seeds (ACCSs) with a correlation factor R² = 0.998. The isotherms of adsorption data were analyzed using three models in their linearized forms namely Langmuir, Freundlich, and Radushkevich. The results showed that the MG biosorption onto ACCSs was well fitted to the Langmuir isotherm (R² > 0.999) with a maximum adsorption capacity 164.7 mg/g at 328 K. In addition, thermodynamic parameters (ΔH^º, ΔS^º, ΔG^º) were assessed. Their values highlighted the endothermic and spontaneous nature of the studied process accompanied by a randomness rise at the adsorbent/solution interface. The above results demonstrated that ACCSs can be used as an effective and environmentally friendly adsorbent for extracting organic dyes from wastewater.