Electrochemical advanced oxidation discoloration and removal of three brown diazo dyes used in the tannery industry

Abstract

Three different advanced oxidation processes, electrochemical oxidation (EO), electro-Fenton (EF) and photoelectro-Fenton (PEF) were tested for the treatment of individual and mixed solutions of Bismark Brown G, Bismark Brown R and Brown DGI dyes. The experiments were performed in a 500 mL stirred undivided cell with a 25 cm2 BDD anode, using 0.050 M Na2SO4 as electrolyte and pH 3.0. The cathodes used for the different processes were a stainless-steel plate in EO and another BDD electrode in EF and PEF. In the two latter processes, pumped air was bubbled for H2O2 generation and 0.50 mM Fe2+ was added as the Fenton's reagent. For PEF, the solution was irradiated using an UVA lamp. The oxidation power of the processes increased in the order: EO < EF < PEF. This trend is explained by the prevalent formation of hydroxyl radicals at the anode surface from water oxidation and in the bulk from Fenton's reaction. The best performance of PEF is considered to be related with the photolytic action of UVA. In all cases, the discoloration presents pseudo-first order kinetics. The performance of PEF treatment of individual dye solutions was improved by raising diazo dye concentration and current density. When PEF was tested with a mixture with 60 mg L−1 of each dye, 80%, 92% and 97% discoloration, and 58%, 76% and 95% chemical oxygen demand decay were found after 60 min using 20 mA cm−2. Small concentrations of succinic, malic and oxalic acids were detected at the end of the PEF process.