Valorization of Bioelectrochemically Derived Hydrogen Peroxide through Catalytic Sulfoxidation

In this work, we demonstrate a combined bioelectrochemical and inorganic catalytic system for resource recovery from wastewater. We designed a microbial electrolysis cell (MEC) for continuous hydrogen peroxide (H₂O₂) production and used this bioelectrochemically-derived H₂O₂ as a green oxidant for sulfoxidation, a representative oxidation reaction used industrially for chemical synthesis and oxidative desulfurization of transportation fuels. We continuously operated a MEC equipped with a gas diffusion electrode cathode for six months, achieving a peak current density above 1.4 mA/cm² with 60% acetate removal efficiency at an anodic coulombic efficiency of 67%. During operation, we evaluated several cathode buffers for solubility and pH compatibility with downstream catalytic systems and identified a 200 mM phosphate buffer that allowed us to achieve high effluent H₂O₂ concentrations (3.0 g/L) at a low energy input (1.6 Wh/g H₂O₂) and pH (10) in 24-hour batch tests. The MEC-derived H₂O₂ solution was used directly as an oxidant for a representative catalytic sulfoxidation of 4-hydroxythioanisole over a solid niobium(V)-silica catalyst. We achieved complete selective conversion of 50 mM 4-hydroxythioanisole to 4-(methylsulfinyl)-phenol on a 0.5 mol% catalyst loading in 100 minutes in aqueous media. Reactivity was within 20% for batch experiments performed with commercial H₂O₂ and batch and CSTR reactors operated with MEC catholyte. Our results demonstrate a new and versatile approach for valorization of wastewater through continuous production of H₂O₂ and its subsequent use as a green oxidant.