Two-step mineralization of Tartrazine solutions: Study of parameters and by-products during the coupling of electrocoagulation with electrochemical advanced oxidation processes

Abstract

A novel sequential electrochemical treatment consisting in an electrocoagulation (EC) pre-treatment and the subsequent advanced oxidation by photoelectro-Fenton (PEF) process with in-situ H2O2 electrogeneration and UVA light irradiation has been envisaged for the removal of organic pollutants from water, showing a high performance for the decolorization and mineralization of Tartrazine (Acid Yellow 23) solutions. EC has a dual purpose as a coagulation and catalyst source step since it allows the formation of Fe(OH)n (n=2 or 3) coagulant and Fe3+/Fe2+ ions. The influence of the anode material (Fe and Al), supporting electrolyte, pH and current during the individual EC on the abatement of color and total organic carbon (TOC) has been assessed. EC with Fe in 0.05M NaCl yielded the best results. Next, various single electrochemical advanced oxidation processes (EAOPs) such as electro-oxidation (EO), electro-Fenton (EF) and PEF with a Pt or boron-doped diamond (BDD) anode and an air-diffusion cathode (ADE) have been tested. PEF with BDD/ADE yielded the quickest degradation among all the EAOPs in NaCl, due to the action of oxidants like active chlorine as well as •OH formed at the anode surface from H2O oxidation and in the bulk from UV-enhanced Fenton's reaction between cathodic H2O2 and added Fe2+. Therefore, the two-step EC (Fe/steel)/PEF (BDD/ADE) process was the best EC/EAOP combination. The EC treatment in 0.05M NaCl at natural pH 6.3 and 200mA, followed by PEF treatment of the supernatant at pH 3.0 and 200mA, yielded the best conditions with a total decolorization in 15min of EC and total mineralization in ca. 300min of PEF. GC–MS analysis showed the formation of several benzenic by-products during the application of EC/PEF. Independent electrolyses revealed the ability of EC to accumulate soluble chlorobenzene derivatives, which can be completely destroyed in the PEF step under the action of the mixture of oxidants, particularly by successive hydroxylation via •OH largely promoted in the bulk by the Fe2+ ions generated in EC.