Advancing wastewater sustainability through electro-refineries: Oxalic acid valorization

The conventional approach to treating wastewater contaminated with organic pollutants focuses on complete mineralization to CO 2 , a product with no economic value. In contrast, this study proposes a valorization-oriented strategy by targeting oxalic acid (OA), a key and persistent intermediate in advanced oxidation processes, as a platform molecule for value generation within emerging electro-refinery frameworks. Herein, we present a systematic side-by-side investigation of two complementary reduction pathways for OA: direct electrochemical reduction and catalytic hydrogenation, evaluated under representative operating conditions. The electrochemical route, studied using Cu and Ni cathodes over a wide range of current densities, selectively produces glycolic and glyoxylic acids, with copper exhibiting higher conversion efficiency and lower energy demand. Mechanistic insights derived from electrochemical and spectroscopic analyses reveal distinct reduction pathways depending on cathode material. In contrast, catalytic hydrogenation over a Ni-based catalyst at 450 °C and 50 bar achieves near-quantitative selectivity toward ethylene glycol as the sole liquid-phase product. By directly comparing these two routes within a unified experimental framework, this work highlights how reaction pathways, selectivity, and current utilization can be deliberately directed through process design. These findings demonstrate the potential of OA valorization to shift wastewater treatment from carbon mineralization toward selective generation of value-added chemicals, providing new insights for the development of sustainable organic electro-refineries.