Electrochemical treatment of Acid Red 1 by electro-Fenton and photoelectro-Fenton processes

Abstract

Small volumes (100 mL) of acidic aqueous solutions with 30-200 mg L^-1 TOC of the toxic azo dye Acid Red 1 (AR1) have been comparatively treated by various electrochemical advanced oxidation processes (EAOPs). The electrolytic system consisted of a BDD anode able to produce ^·OH and an air-diffusion cathode that generated H₂O₂, which subsequently reacted with added Fe²⁺ to yield additional ^·OH from Fenton’s reaction. Under optimized conditions (i.e., 1.0 mM Fe²⁺, 60 mA cm^-2, pH 3.0, 35 ºC), the analysis of the initial rates for decolourization and AR1 decay assuming a pseudo-first-order kinetics revealed a much higher rate constant for photoelectro-Fenton (PEF, ~ 2.7x10^-3 s^-1) compared to electro-Fenton (EF, ~ 0.6x10^-3 s^-1). Mineralization after 180 min was also greater in the former treatment (90 % vs 63 %). The use of UV radiation in PEF contributed to Fe(III) photoreduction as well as to photodecarboxylation of refractory intermediates, yielding a mineralization current efficiency as high as 85% during the treatment of solutions of 200 mg L^-1 TOC. Primary reaction intermediates included three aromatic derivatives with the initial naphthalenic structure and four molecules only featuring benzenic rings, which were totally mineralized in PEF.