A Solar-Enabled Strategy for Degrading Allura Red AC Using a Laddered Heterojunction Nanocomposite

Allura Red AC is a synthetic azo dye that is widely used in the food and textile industries. It raises significant environmental and health concerns due to its chemical stability, toxicity, and resistance to conventional water treatment methods. In this study, we demonstrate the effective photocatalytic degradation of Allura Red AC using a solar-active nanocomposite consisting of Ag-coated Co²⁺-substituted ZnFe₂O₄ integrated with ZnO (Ag-CZFO/ZnO), synthesized via a microwave-assisted solvothermal method. The nanocomposite exhibited an extended light absorption up to 925 nm, enabling efficient utilization of both visible and near-infrared light. Under optimized conditions (pH 5.89, catalyst loading 1 g/L, dye concentration 50 μM), the system achieved 96% degradation of the dye within 150 min, with a pseudo-first-order rate constant of 0.0188 min^–1. The introduction of electron and hole scavengers demonstrated that photogenerated holes played a crucial role in the oxidation process. Hydroxyl and superoxide radicals were also significant contributors to the degradation pathway, as confirmed by reactive oxygen species probe experiments. This process resulted in a substantial reduction of chemical oxygen demand from 31 to below 5 ppm, indicating nearly complete mineralization. Additionally, the photocatalyst maintained high activity over five reuse cycles, confirming its stability and potential for repeated use. Our laddered heterojunction solar photocatalyst (i) significantly reduced the charge recombination as observed by a 100-fold enhancement in the photocurrent and (ii) enhanced the charge transfer process as observed by the smallest semicircle in the Nyquist plots. These findings demonstrate that this solar-driven photocatalytic approach is a sustainable and effective method for degrading persistent organic pollutants such as Allura Red AC. This offers a promising solution for the large-scale treatment of dye-contaminated wastewater using sunlight as an energy source.