Fabrication of Nanopolyaniline-Grafted Epichlorohydrin for pH-Responsive Adsorption of Organic Dyes

Dyes, which have mutagenic, embryotoxic, and carcinogenic effects on terrestrial animals and resistance to degradation in the presence of air and light, have emerged as a potent pollutant of water today. Out of all developed methods to treat contaminated water, adsorption remains propitious due to its advantages such as simple process, low cost, ease, eco-friendliness, and reusability. Herein, the adsorbent epichlorohydrin-grafted nanopolyaniline (PANI-g-EC) was synthesized by the ring-opening reaction of epichlorohydrin by nanopolyaniline, where the nanopolyaniline acted as a nucleophile. The grafting of nanopolyaniline was confirmed by Fourier transform infrared (FTIR). The morphology and particle size of PANI-g-EC were examined by field emission-scanning electron microscopy (FESEM) and transmission electron microscopy (TEM), showing a sponge-like structure with an average particle size of 42 nm. The adsorption parameters such as dosage of the adsorbent, initial dye concentration, pH, temperature, and thermodynamic parameters (ΔG^o, ΔH^o, ΔS^o) of the adsorption process were extensively studied for potential application. The maximum adsorption of dyes occurs at 35 °C in the presence of 0.16 g·L^-1 adsorbent for 120 min at their respective pH. PANI-g-EC shows pH-selective adsorption of both cationic and anionic dyes with a maximum adsorption capacity of 588.55 mg.g^-1 for methylene blue at pH 8 and 317.89 mg.g^-1 for methyl orange dye at pH 2. The adsorption process follows the pseudo-second-order reaction kinetics and Langmuir adsorption isotherm. The electrostatic force of attraction is found to be responsible for the adsorption of dyes.