Alginate-assisted self-optimizing heterojunction photocatalyst for sustainable wastewater remediation

The persistent challenge of organic dye pollution necessitates the development of innovative and sustainable photocatalytic solutions. This study presents a self-optimizing hydrogel bead photocatalyst, comprising of graphene oxide-copper oxide (GO-CuO) nanocomposite encapsulated within a calcium alginate (CaAlg) matrix leading to the formation of CaAlg/GO-CuO, for enhanced degradation of methylene blue (MB) under UV-Vis light irradiation. Notably, the system enables in-situ reduction of Graphene oxide (GO) to reduced graphene oxide (rGO), significantly enhancing photocatalytic efficiency while allowing for facile catalyst recovery. The composite was characterized using Fourier Transform Infrared (FT-IR) Spectroscopy, X-ray Diffraction (XRD), Raman spectroscopy, Scanning Electron Microscopy-Energy Dispersive X-ray Analysis (SEM-EDAX), and UV-Vis Diffuse Reflectance Spectroscopy (UV-Vis DRS) techniques. Photocatalytic tests demonstrated a remarkable 99 % degradation of MB within 120 min using CaAlg/GO-CuO, compared to 74 % with GO-CuO. Kinetic analysis revealed a 2.5-fold increase in the degradation rate constant, attributed to improved charge separation and enhanced mass transfer facilitated by the alginate matrix. Mechanistic investigation uncovered a dual-phase photocatalytic process: an initial type-II heterojunction-driven mechanism (0-60 min), followed by a Schottky junction-dominated pathway (60-120 min), triggered by in-situ photoreduction of GO to rGO. This transition improved electrical conductivity, suppressed charge recombination, and optimized electron transport pathways.