Parabens are widely used industrial preservatives, routinely found in wastewater along with major inorganic ions like sulfate and chloride. This work investigates the oxidation ability of three electrochemical processes in tank reactors equipped with an air-diffusion cathode to electrogenerate H2O2 on site, namely electro-oxidation (EO-H2O2), electro-Fenton (EF) and UVA photoelectro-Fenton (PEF), to degrade aqueous solutions of methylparaben (MeP) at pH 3.0. Their performance using boron-doped diamond (BDD), Pt or two kinds of dimensionally stable anodes (DSA ® ) has been compared from the analysis of mineralization profiles and decay kinetics in the presence of sulfate and/or chloride ions. The use of BDD ensured the overall mineralization in all three processes according to the sequence: PEF > EF > EO-H2O2, thanks to the contribution of BDD( • OH), • OH and UVA light. Pt and DSA ® became an interesting alternative in PEF, with slower organic matter removal but similar final mineralization percentages, being much less powerful than BDD in EO-H2O2. The presence of Cl − was beneficial in the latter process, due to the formation of active chlorine as an additional oxidant that caused a much faster decay of MeP.Conversely, it became significantly detrimental in EF due to the partial destruction of H2O2 and• OH in the bulk by active chlorine and Cl − , respectively. The oxidation power of PEF was so high that similar fast, complex decay kinetics was found in all media regardless of the anode, although the mineralization was decelerated owing to the accumulation of chlorinated by-products. GC-MS and HPLC analysis allowed the identification of up to seven aromatic MeP derivatives in sulfate + chloride mixtures, including three non-chlorinated compounds also found in pure sulfate medium. These molecules were gradually transformed into oxalic acid, along with four chlorinated aliphatic carboxylic acids in Cl − -containing media.

Aqueous mixtures of methyl, ethyl and propyl paraben (MeP, EtP and PrP) prepared in real urban wastewater with low conductivity were treated by solar photoelectro-Fenton (SPEF) process at low input current (j = 10 mA cm -2 ) using a pre-pilot plant with an electrochemical reactor equipped with an air-diffusion cathode to electrogenerate H2O2 and a boron-doped diamond (BDD) or RuO2-based anode. Comparative trials in simulated water matrices with or without Cl − in the absence of natural organic matter (NOM) always led to a slower decay of parabens concentration and total organic carbon (TOC). This was mainly due to the superior regeneration of Fe 2+ from photoreduction of Fe(III) complexes formed with NOM in real wastewater compared to that from Fe(OH) 2+ . In all matrices, a catalyst concentration as low as 0.20 mM Fe 2+ was enough to ensure the production of • OH in the bulk from Fenton's reaction. SPEF with BDD yielded a complete removal of parabens in 180 min and 66% mineralization at 240 min. This gave rise to the greatest mineralization current efficiencies reported so far, up to 1000%, with a low energy consumption of 84 kWh (kg TOC) -1 . The synergy between homogeneous and heterogeneous catalysis, which allowed the efficient dosage of • OH and M( • OH) at low j, with simultaneous action of high UV power from sunlight justified such a good performance. Analogous apparent rate constants were determined for MeP, EtP and PrP. Slower decays were found with RuO2-based anode due to its lower oxidation power. As a result, the MCE was 425% as maximum, but a lower energy consumption of 52 kWh (kg TOC) -1 was needed. Since the role of active chlorine was of minor importance, the formation of toxic, refractory chloroderivatives was minimized. All byproducts were transformed into malic, formic and oxalic acids prior to total mineralization.

No abstract

No abstract