Synthesis and Characterization of Polyindole Derivatives for Removal of Organic Dyes: Methyl Orange and Methylene Blue

ABSTRACT New derivatives of polyindole were synthesized through polymer‐analogous transformation and studied using a range of physicochemical methods. They have been successfully used for the adsorption of anionic and cationic dyes from water. The influence of pH, adsorption time, dye concentration, and reusability was investigated. The optimal pH for the adsorption of methyl orange (MO) is 7, while for methylene blue (MB) it ranges from 7 to 12. Adsorption is well described by the Langmuir model, and the kinetics follow a pseudo‐second‐order model. Notably, the maximum adsorption capacity of the polyindole derivatives for the cationic dye MB from 46.00 to 87.16 mg/g, more than twice that observed for the anionic dye MO (18.39 to 22.50 mg/g). This significant difference in adsorption performance is likely attributed to stronger electrostatic interactions between the negatively charged nitrogen atoms in the polyindole backbone and the cationic MB molecules. Thermodynamic parameters further confirmed the spontaneous and endothermic nature of the adsorption process. Importantly, the m ‐OMePIn polymer maintained high adsorption efficiency even after multiple reuse cycles: after five consecutive adsorption–desorption cycles, dye removal efficiency remained above 57%. These findings highlight a convenient synthetic approach for the development of polyindole‐based adsorbents with high adsorption capacity, offering promising potential for the removal of dye pollutants from aqueous environments.