Electrochemical treatment on a pilot scale of a mixture with high concentrations of dyes from the tanning/textile industry

Abstract

Violet LR (VLR), Brown RD (BRD), and Green A (GA) dyes are widely used in the tannery industry for dyeing leather. These types of compounds can be toxic and, in many cases, carcinogenic. In this context, electrochemical oxidation (EO) and electro-Fenton (EF) processes are the most popular electrochemical advanced oxidation processes (EAOPs) to degrade persistent/toxic water pollutants. The efficiency of these processes largely depends on the nature of the electrode materials, among other operation parameters. This study evaluates Ti/IrO2-SnO2-Sb2O and BDD anodes for dye degradation in a pilot plant of 4.0 L of total volume. It was found that DSA IrO2-SnO2-Sb2O performed better than BDD for the discoloration of a mixture of the dyes (kDSA=0.0970 min−1 and kBDD=0.0370 min−1) in a medium containing 50 mM NaCl and 10 mM Na2SO4 (j = 50 mA cm−2, Q=12 L min−1 and pH 3). Such an effect was due to the combined production of heterogeneous •OH at the anode surface by H2O oxidation and reactive chlorinated species in the bulk solution via Cl- oxidation. In addition, •OH quantification showed a higher production by DSA ([BDD(•OH)] = 3,8 × 10-15 M vs [DSA(•OH)] = 6.6 × 10-15 M). The efficiency was enhanced when applying EF (BDD cathode promoting H2O2 production) in the presence of Fe2+ ions under similar experimental conditions. The color was almost totally removed in 50 min with kEF-DSA=0.1584 min−1, while COD was removed by 80 % in 80 min. The EC was 0.082 kWh (g COD)-1. Such results highlight the potential of EO and EF with accessible Ti/IrO2-SnO2-Sb2O anodes as a pretreatment unit within a treatment train for tanning or textile wastewater management.