Electro- and photo-electrooxidation of 2,4,5-trichlorophenoxiacetic acid (2,4,5-T) in aqueous media with PbO2, Sb-doped SnO2, BDD and TiO2-NTs anodes: A comparative study

Abstract

Abstract In this work, the 2,4,5-T degradation in aqueous media was studied by electro- (EC) and photoelectrochemical (PEC) oxidation methods, which are two of the most important advanced oxidation processes (AOPs) for wastewater treatment. Both EC and PEC experiments were carried out in a single compartment cell under galvanostatic conditions (30 mA cm−2) at 298 K by using 0.05 M Na2SO4 + 200 ppm 2,4,5-T at pH 3 and pH 9, as model solutions. EC oxidation was performed using Sb-doped SnO2, PbO2 and boron doped diamond (BDD) as anodic materials. Besides these anodes, TiO2 in a nanotubular structure (TiO2 NTs) with or without PbO2 dispersed nanoparticles (TiO2::PbO2) were also used for PEC oxidation process. In all cases, the electrolyzed solutions were periodically analyzed by UV–vis spectrophotometry (232 or 235 nm) and liquid chromatography (HPLC) for 2,4,5-T as well as the oxidation by-products. The mineralization level was occasionally measured by using total organic carbon (TOC). For PEC experiments the photocurrent response variations with potential in 0.5 M H2SO4 were obtained by the linear scanning voltammetry (LSV) method at a slow potential sweep (5 mV s−1). Results clearly showed that at 30 mA cm−2, the 2,4,5-T was 100% oxidized on Sb-doped SnO2 and 95% on PbO2 after 120 min of electrolysis, which allow to assume the EC as an adequate treatment method to remove this herbicide from wastewaters. HPLC results showed the formation of aromatic intermediates such as 2,4,5-trichlorophenol (2,4,5-TCP) and 2,5-dihydroxyquinone (2,5-DHQ) followed by the formation of carboxylic acid such as hydroxyacetic acid. For these cases, alkaline media (pH 9) was better than acidic media (pH 3) in about one magnitude order in the apparent rate constant (k). On the other hand, photochemical activity results showed that PEC on TiO2-NTs::PbO2 had higher photocurrent density values than on naked TiO2 NTs as photoanodes. These electrodes also had the better photo conversion efficiencies (η), which strongly suggests the occurrence of a “co-catalysis” effect due to an electronic transfer assisted by the closed contact between TiO2 and PbO2. This fact was supported by the PEC oxidation of 2,4,5-T in aqueous solution, whose k value was slightly lower than that observed for bulk material.