On the performance of HOCl/Fe2+, HOCl/Fe2+/UVA, and HOCl/UVC processes using in situ electrogenerated active chlorine to mineralize the herbicide picloram

Abstract

Four different treatment methods based on the HO production were assessed to oxidize and mineralize the herbicide picloram (PCL), which is considered very toxic and so is a potential contaminant of surface and ground water. The processes based on the Fenton type (homolysis reaction of HOCl by Fe2+ ions) and photo-Fenton type reaction (using a 9 W UVA light) with in situ electrogenerated HOCl species, using a commercial DSA® anode in the presence of Cl− ions, led to poor mineralization performances in comparison to the HOCl/UVC process. In that case, the homolysis reaction of HOCl mediated by a 5 or 9 W UVC light resulted in almost complete removal of the organic load within 12 h of treatment, from acidic to neutral solutions and using 1 g L−1 of NaCl concentration after optimization of the experimental conditions. When the HOCl/UVC process using a 5 W UVC light is compared to the electrochemical method using a boron-doped diamond anode (electrochemical/BDD), the oxidation and mineralization rates of the HOCl/UVC process were always superior, with ∼95% removal of total organic carbon (TOC) after 12 h treatment. The energy consumption per unit mass of removed TOC remained around 4 and 8 kW h g−1 for the HOCl/UVC and electrochemical/BDD treatment processes after 90% removal of TOC, respectively, even considering the energy consumption of the UVC lamp. In the final treatment stages, high CO2 conversions were obtained using both methods, as the generated intermediates were almost completely eliminated. Finally, the HOCl/UVC process is a reasonable option to treat solutions contaminated with organic pollutants as the common problems associated with the Fenton based (acidic solution, Fe2+ ion recovery, generation of H2O2) and electrochemical/BDD (mass transport) processes can be readily circumvented.