Photo-assisted electrochemical production of HClO and Fe2+ as Fenton-like reagents in chloride media for sulfamethoxazole degradation

Abstract

The photoelectro-Fenton (PEF)-like process based on the replacement of H2O2 by HClO remains quite unexplored. Its ability to mineralize solutions containing 0.208 mM of the antibiotic sulfamethoxazole (SMX), either in 25 mM Na2SO4 + 35 mM NaCl or 45 mM Na2SO4 + 15 mM NaCl media, has been evaluated. The assays were performed in a 3 L pre-pilot flow plant composed of a filter-press FM01-LC reactor, which was equipped with an Ir-Sn-Ru oxide anode and a stainless steel cathode, coupled to an annular photoreactor containing a 160 W UVA lamp. A higher amount of active chlorine (HClO) was produced as the electrolysis was prolonged and the current density was increased. The accumulated HClO concentration dropped down more rapidly at a greater Fe2+ content due to the enhanced oxidant decomposition that yielded ¿OH in the bulk. A continuous production of this radical was ensured from the effective Fe2+ regeneration, being favored by cathodic reduction and photoreduction of dissolved Fe(III) species. The SMX degradation was faster in 25 mM Na2SO4 + 35 mM NaCl mM because of the quicker HClO generation, although the larger proportion of recalcitrant chloro-derivatives and iron-chlorocomplexes decelerated the mineralization process. In 45 mM Na2SO4 + 15 mM NaCl, the best PEF-like treatment was attained at 0.4 mM Fe2+ and 15 mA cm-2, achieving the complete antibiotic disappearance at 60 min and 59.7% mineralization after 420 min, with an energy consumption of 0.776 kWh (g TOC)-1. Sixteen intermediates were detected by gas chromatography-mass spectroscopy, ten of which were chlorinated, allowing the proposal of a reaction route for SMX. Since the final solutions did not contain chloro-organics, the partial mineralization was mainly related to the accumulation of refractory non-chlorinated carboxylic acids that could not form photoactive Fe(III)-carboxylate complexes because of the presence of iron-chlorocomplexes.