Visible-Light-Driven Ferrioxalate Activation for Dye Degradation in a Recirculating Photoreactor: LED vs. Fluorescent Light Sources

This study explores the visible-light-driven photolysis of Ferrioxalate complexes for the degradation of Toluidine Blue (TB), a persistent phenothiazine dye, using a 1 L recirculating batch-loop photoreactor. The reactor system incorporated two tubular photochemical units (35 cm × 3 cm each) in series: the first equipped with an immersed blue fluorescent lamp (12 W, 30 cm-tube), and the second with dual external blue LED lamps (18 W total, 30 cm) encasing a double-walled glass cell. Continuous flow between the units was maintained via a peristaltic pump. Experimental investigations were used to evaluate the effects of key parameters such as Fe(III) and oxalate concentrations, initial TB load, pH, light source, flow rate, ligand type, dissolved gas type, external H2O2 addition, and the presence of various inorganic ions. The results demonstrate efficient dye degradation, with ~75% TB removal within 1 h under combined fluorescent and LED irradiation, where each reactor contributing comparably. The optimal performance was achieved at pH 4, with a 10 oxalate-to-Fe(III) molar ratio (1 mM:0.1 mM) and a flow rate of 25 mL s−1. Among various ligands tested (oxalate, acetate, citrate, EDTA), oxalate proved to be the most effective. The presence and type of anions significantly influenced degradation efficiency due to their potential scavenging effects. Although the process achieved high dye removal, TOC analysis indicated only moderate mineralization, suggesting the accumulation of non-colored intermediates. External H2O2 addition moderately improved TOC removal, likely due to enhanced hydroxyl radical generation via the Fenton mechanism. These findings highlight the promise of Ferrioxalate-based photochemical systems under visible light for dye removal, while also emphasizing the need for further research into by-product identification, mineralization enhancement, and toxicity reduction to ensure safe effluent discharge.