Reverse-engineered Electro-Fenton for the selective synthesis of oxalic or oxamic acid through the degradation of acetaminophen: A novel green electrocatalytic refinery approach

The Electro-Fenton process (EF) has been conventionally applied to efficiently degrade refractory and/or toxic pollutants. However, in this work, EF was used as a reverse engineering tool to selectively synthesize highly value-added products (oxalic or oxamic acid) through the degradation of the model pollutant acetaminophen, a widely used analgesic and antipyretic drug. It was found that the production of either oxalic or oxamic acid is dictated by the applied current density. Hence, oxalic acid is favored at low current densities trough a mechanism dominated by homogeneous ^•OH radical oxidation, while oxamic acid is the majoritarian product at high current densities where electron transfer at the anode surface becomes an important mechanism in combination with ^•OH oxidation. Under optimal reaction conditions (0.71 mA cm^-2 and 100 mg l^-1 of initial total organic carbon (TOC) concentration), up to 227.1 ± 26.3 mg l^-1 of oxalic acid were produced, with high yield and selectivity of 54.9 ± 5.1 % and 94.7 ± 9.9 %, respectively (the TOC removal was 42.0 ± 2.4 %). In the case of oxamic acid, the highest concentration of 33.8 ± 2.1 mg l^-1 was produced at 2.13 mA cm^-2 and an initial TOC concentration of 50 mg l^-1, which represented a yield of 18.7 ± 0.3 % and 60.9 ± 9.3 % selectivity (71.1 ± 4.4 % of TOC removal). It is worth noting that at low current density when oxalic acid is favored, the selectivity for both products was 100 %, meaning that those were the only products remaining in the solution, with oxalic acid as the major product (94.7 ± 9.9 % with initial TOC of 100 mg l^-1, and 98.7 ± 0.9 % with initial TOC of 50 mg l^-1). This is a pioneer work on EF applications to the field of wastewater valorization/refining through the recovery of value-added products within a circular economy.