Oxidant Scavenging Properties of Urea Enable Selective Oxidation of Pharmaceuticals and Suppression of Oxyhalide Formation during Electrolysis of Fresh Human Urine

Abstract

<p>Many pharmaceutical compounds are excreted unchanged or as active metabolites that pass through conventional wastewater treatment processes, presenting a risk to aquatic ecosystems and humans. Advanced oxidation processes (AOPs) are able to degrade these compounds but often lead to the formation of oxidation byproducts (OBPs) including chlorate, perchlorate, and halogenated organics at hazardous concentrations. Point-source remediation is desirable as it provides an avenue to destroy pharmaceuticals before dilution with wastewater. However, the high concentration of urea and chloride in fresh human suggests low selectivity of pharmaceutical oxidation and high OBP formation are likely. Here, we show that the high urea content of fresh human urine suppresses the formation of oxychloride byproducts by inhibiting formation of HOCl/OCl<sup>‒</sup> during electrolysis, while still enabling the oxidation of pharmaceuticals by <sup>•</sup>OH due to the slow rate of urea oxidation by hydroxyl radicals. This (primarily indirect) electrochemical oxidation scheme is shown to degrade cyclophosphamide and sulfamethoxazole with surface-area-to-volume-normalized pseudo-first-order observed rate constants greater than 0.08 cm/min in authentic fresh human urine matrixes. It results in two orders-of-magnitude decrease in pharmaceutical concentrations in 2 hours while generating three orders-of-magnitude lower oxychloride byproduct concentrations in synthetic fresh urine as compared to synthetic hydrolyzed aged urine matrixes. Importantly, this proof-of-principle shows that simple and safe electrochemical methods can be used for point-source-remediation of pharmaceuticals in fresh human urine (before dilution with other wastewater) without formation of significant oxychloride byproducts.</p>