NaOCl and UVC Mediated Heterogeneous Fenton Type Processes for Efficient COD Removal of Acid Orange 8

This study investigates the removal of chemical oxygen demand (COD) and color from Acid Orange 8 (AO8) using heterogeneous and photo‐Fenton‐type processes. Two catalysts, nano‐ferrite ( n ‐Fe 2 O 3 ) and pyrite (FeS 2 ), were employed with NaOCl as the oxidant, both under dark and UV‐illuminated conditions. Under optimized parameters ([ n ‐Fe 2 O 3 ] = 4.0 g L −1 ; [FeS 2 ] = 2.0 g L −1 ; [NaOCl] = 2.5 mM; pH = 3.0; equilibrium period = 30 min), n ‐Fe 2 O 3 /NaOCl achieved a higher initial COD removal (91.3% at 5 min) compared to FeS 2 /NaOCl (72.8%). However, UV‐assisted systems exhibited superior performance, with FeS 2 /NaOCl/UV and n ‐Fe 2 O 3 /NaOCl/UV, achieving complete COD removal within 60 and 120 min, respectively, as opposed to 300 min for non‐UV systems. The presence of SO 4 2− , NO 3 − , Cl − , and Br − inhibited COD removal, whereas F − enhanced it in the n ‐Fe 2 O 3 /NaOCl system. Kinetic analysis revealed that the Behnajady–Modirshahla–Ghanbary (BMG) model best described the degradation behavior. Among all combinations, the FeS 2 /NaOCl/UV system demonstrated the highest synergistic effect and cost efficiency, achieving nearly 100% COD removal. Mechanistic studies indicated that both HO • and Fe(IV) were major contributors in n ‐Fe 2 O 3 ‐based systems, while Fe(IV) dominated in FeS 2 ‐based systems, followed by HO • and reactive chlorine species (RCS), accounting for its superior efficiency.