Electro-Fenton treatment of real pharmaceutical wastewater paired with a BDD anode: Reaction mechanisms and respective contribution of homogeneous and heterogeneous [rad]OH

Abstract

Even though there is increasing evidence on the benefits of combining electro-Fenton (EF) and anodic oxidation with boron doped diamond anodes (BDD) for the degradation of organic pollutants, the exact contribution of OH, BDD(OH) and their synergy in the paired EF-BDD process has still not been clearly established, taking into account i) the applied current/potential; ii) the reactivity of the pollutants and their intermediates formed during treatment and; iii) the matrix effect in real wastewater. In this paper, we address the above points by conducting the EF treatment of real pharmaceutical wastewater (COD0 = 1200 mg L−1 and BOD5/COD = 0.14). In-depth data analysis revealed a synergy between homogeneous and heterogeneous BDD(OH) that accelerated the mineralization of organics, with homogeneous OH dominating during the first treatment stages, while BDD prevailed in the last stages through electron transfer reactions. EF-BDD achieved the highest mineralization yield (97.1% of TOC removal at 4.17 mA cm−2 and 0.2 mM of Fe2+), as compared to EF with a dimensionally stable anode (DSA), a material not producing heterogeneous OH. However, EF-DSA proved equally effective at increasing the biodegradability of the effluent with lower operational costs (BOD5/COD = 0.68 at US$ 1.46 m−3 in 4 h). Hence, we point out the versatility of EF to reach different targets of water quality depending upon the anode materials: EF-DSA to increase the biodegradability of refractory wastewater vs. more powerful (though costlier) EF-BDD for quasi-complete mineralization.