Enhanced methylene blue removal by ethylenediamine-modified cotton waste activated carbon: Isotherm, kinetic, thermodynamic, and mechanistic insights

Cotton waste is an abundant agricultural by-product that can be converted into value-added adsorbents for wastewater treatment. In this study, activated carbon prepared from Gossypium hirsutum L. waste biomass was modified with ethylenediamine (EDA) to improve its ability to remove methylene blue (MB) from aqueous solutions. The adsorbents were characterized by BET, FTIR, SEM, XRD, TGA, CHNS analysis, and pHₚzc measurements. EDA modification increased the specific surface area from 308 to 481 m2 g−1 and introduced nitrogen-containing functional groups, providing additional adsorption sites. The adsorption performance was evaluated by examining the effects of solution pH, adsorbent dosage, initial dye concentration, contact time, and temperature. Under the optimum conditions (pH 10, 0.1 g adsorbent, 100 mg L−1 MB, 25 °C, and 120 min), the maximum Langmuir adsorption capacity increased from 81 mg g−1 for CWBAC2 to 115 mg g−1 for EM@CWBAC6. The adsorption data were well described by the Langmuir isotherm and the pseudo-second-order kinetic model, while the thermodynamic analysis indicated that the process was spontaneous and energetically favorable. EM@CWBAC6 also maintained high adsorption efficiency after five adsorption–desorption cycles, demonstrating good stability and reusability. The improved adsorption performance was attributed to the combined effects of electrostatic attraction, hydrogen bonding, π–π interactions, and pore filling. These results show that EDA-modified cotton waste-derived activated carbon is a simple, low-cost, and sustainable adsorbent with considerable potential for the treatment of dye-contaminated wastewater.