Study of the degradation of monoazo dye Acid Red 1 using electrochemical anodic oxidation, electro-Fenton and photoelectro-Fenton processes.

Abstract

A comparative study of the degradation of the monoazo dye Acid Red 1 (AR1) was carried out to elucidate the effectiveness of electrochemical anodic oxidation (AO), electro-Fenton (EF) and photoelectro-Fenton (PEF) processes, three representatives of the so-called electrochemical advanced oxidation processes (EAOPs). Three different aspects were taken into account in order to assess degradation levels achieved, i.e. mineralization, decolouration and species released into the solution. Electrolytic trials were performed using an air-diffusion PTFE/carbon cathode and Pt or boron-doped diamond (BDD) anodes alternately, the latter yielding the best performance. The mineralization level was quantified through the electrolytic processes by means of total organic carbon (TOC) measurements. PEF process performs the best in mineralization terms. EF follows with a medium-high performance while AO proves to be the least effective process, especially when Pt anode is used. Spectrophotometric measurements allowed decolouration monitoring. EF and PEF processes present the same behaviour, reaching complete decolouration in minute-scale. AO has a poorer performance; decolouration is completed after several hours of electrolysis or, in some cases, not even achieved. Short-chained carboxylic acids and inorganic ions were detected and quantified through HPLC techniques. Oxalic and oxamic acids are the main carboxylic species found in EF and PEF processes, while only low concentrations are noticed in AO. NH4+, NO3- and SO42- ions are found in solution. NH4+ is the majority inorganic N-specie in EF and PEF processes; NO3- is it in AO. All sulphur atoms contained in AR1 are released as SO42- for all the three methods assessed. The main conclusion is that EAOPs tested stand as clean, easy and powerful processes to degrade complex aromatic molecules like AR1.