Photocatalytic degradation of methylene blue with P25/graphene/polyacrylamide hydrogels: Optimization using response surface methodology

An environment-friendly hydrogel was synthesized by entrapping Degussa P25 on the surface of a reduced graphene oxide (rGO)-polyacrylamide (PAM) matrix.The PAM content of the P25-rGO-PAM (PGP) hydrogel considerably influenced the adsorption and photocatalytic degradation of methylene blue (MB), and the optimal PAM content was 10% (w/v). Furthermore, rGO not only enhanced the adsorption capacity of the hydrogel by increasing the surface area but also increased the photodegradation efficiency synergistically by separating electron-hole pairs. The reaction kinetic constant for MB degradation by the hydrogel was 0.0276 min⁻¹, which was three and five times the reaction kinetic constants of P25-PAM and rGO-PAM hydrogels, respectively. The synthesized PGP showed high stability and its MB degradation efficiency was considerably high up to five consecutive cycles under UV-irradiation. The eco-friendly nature of the hydrogel was evaluated on the basis of bacterial inactivation, and the treated water was found to be safe for use. Three key operating parameters (initial MB concentration, temperature, and pH) were optimized for maximizing MB removal using a response surface methodology. The complete MB removal efficiency was obtained for the optimal conditions of pH 9.4, a temperature of 31.2 °C, and an initial MB concentration of 5.2 mg/L.