Upcycling of petcoke to graphene oxide-ZnO heterojunctions for continuous photocatalytic wastewater remediation: Reactor design and sustainability impact

Continuous-flow photocatalytic systems offer a promising approach for the practical implementation of advanced oxidation processes in water treatment; however, catalyst cost remains a critical challenge for large-scale implementation. In this study, a thermochemical procedure was used to turn petroleum coke into graphene oxide (GO), as a sustainable process for turning waste into useful materials. GO was loaded with ZnO nanoparticles via a hydrothermal method and characterized using TEM, SEM, EDX, XRD, N₂-sorption, Raman, and FTIR. In a continuous column-bed design, the photocatalytic activity of GO-ZnO was assessed for methylene blue (MB) degradation under visible light. The GO-ZnO nanocomposite exhibited high photocatalytic activity with 2.93 eV band gap, achieving substantial MB degradation within 30 min of irradiation. Improved charge separation, as evidenced by EIS with R_ct of 48.2 Ω and TRPL with lifetime analysis (τ = 14.21 ns), as well as greater hydroxyl radical production in alkaline environments, is responsible for the improved performance. Over 10 cycles, the photocatalyst achieved > 90% degradation efficiency while maintaining excellent stability. This work demonstrates an effective waste-to-resource strategy and highlights the potential of GO-ZnO nanocomposites for sustainable wastewater treatment, supporting environmental and economic sustainability. The continuous-flow system enabled sustained pollutant removal and efficient catalyst utilization, highlighting its potential for process intensification and scalable water remediation.