Effect of various inorganic anions on the degradation of Congo Red, a di azo dye, by the photo-assisted Fenton process using zero-valent metallic iron as a catalyst

Abstract

The present research focuses on the heterogeneous advanced photo-Fenton processes of the type Fe⁰/H₂O₂/UV and Fe⁰/ammonium persulfate (APS)/UV as a potential technique to degrade Congo Red (di azo dye). Both the oxidants H₂O₂ and APS showed comparable efficiencies on the iron surface for the mineralization of Congo Red (CR) under UV light. The influence of various reaction parameters like pH of the solution, catalyst loading, concentration of the oxidants (H₂O₂/APS), influence of hydroxyl radical scavenger and the concentration of the substrate dye molecules are investigated and the optimum conditions are reported. The influences of various inorganic anions that are commonly present in the industrial effluents are studied using the above processes. All the inorganic anions used shows inhibitive effect on the degradation rate. Inorganic anions like chloride (Cl^-) and sulfate (SO^2- ₄) inhibit the degradation rate by forming complexes with Fe²⁺/Fe³⁺ ions in the solution and they also quench the generated hydroxyl radicals. Anions like nitrate (NO^- ₃), carbonate (CO^2- ₃) and bicarbonate (HCO^- ₃) suppress the degradation rate mainly by scavenging the generated hydroxyl radicals. Quinol, hydroquinone and naphthalene derivative were the major intermediates obtained during the mineralization process. The experiments were extended successfully to treat the industrial effluent containing CR dye. APS proved to be a better oxidant than H₂O₂ for treating the effluent. The present method is advantageous as it is a simple and cost-effective technique for the mineralization of non-biodegradable di azo dye.

Keywords:

• Advanced oxidation process (AOP)
• Photo-Fenton type reaction
• Zero-valent metallic iron surface
• Inorganic anions
• Congo Red