Solar photoelectro-Fenton treatment of a mixture of parabens spiked into secondary treated wastewater effluent at low input current

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 Fe2+ 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 Fe2+ was enough to ensure the production of center dot 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 center dot OH and M(center dot 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 by-products were transformed into malic, formic and oxalic acids prior to total mineralization.