Synthesis and Utilization of rGO/Ultrathin Nanotube Bi₅O₇I for Photodegradation of Methylene Blue and Photoreduction of Cr⁶⁺ to Cr³⁺ toward Detoxification of Water

The reduced graphene oxide/ultrathin nanotube Bi₅O₇I (rGO/UN-Bi₅O₇I) was well synthesized via a simple method to investigate the charge transfer and light-harvesting ability and its efficient application in wastewater problems such as organic (Methylene blue) and inorganic (Cr⁶⁺) pollution. The analysis shows the synergistic effect of the graphitic structure of rGO and the ultrathin nanotube structure of Bi₅O₇I, leading to efficient light harvesting and charge transfer. The efficient photocatalytic activity of this photocatalyst was achieved with 20% rGO. The optimum pH, ionic strength, and time for the photodegradation of MB were 12.0, 0.05 M, and 4 min, and those for the photoreduction of Cr⁶⁺ were 2.0, 0.10 M, and 38 min, respectively. In addition, the experimental data for MB investigation show that the kinetic surface adsorption model follows the Langmuir isotherm [<i>Q</i>_max = 350.00 (mg/g)] model. The kinetic isotherm of the surface adsorption is observed by pseudo-first-order kinetics [<i>Q</i>_e.cal = 136.87 (mg/g)]. Also, the photodegradation efficiency of UN-Bi₅O₇I and rGO/UN-Bi₅O₇I was compared, and the results showed that the kinetic rate constant of rGO/UN-Bi₅O₇I for the photodegradation of MB was 14 times, and that for the photoreduction of Cr⁶⁺ was 4.5 times higher than UN-Bi₅O₇I. The scavenger test showed that the hole (h⁺) and superoxide radical (^•O₂^–) have the main role in the photodegradation of MB. The rGO, due to its functional groups, improved the surface adsorption of pollutants and caused the photodegradation of MB by h⁺ and the photoreduction of Cr⁶⁺ to Cr³⁺ by electrons. Consequently, rGO/UN-Bi₅O₇I can be efficiently utilized in environmental pollutant remediation.