Balancing the oxidation of endogenous organics and macronutrient recovery from human urine treated with fenton’s reagent: A targeted metabolomics study

Abstract

Human urine is a promising resource for circular fertiliser production, but its high concentrations of organic and inorganic compounds present both challenges and opportunities for effective treatment. This study evaluates Fenton oxidation for selectively degrading endogenous organic metabolites in acidified, unhydrolysed urine while preserving critical nutrients such as urea. Using targeted metabolomics, over 200 organic metabolites were identified in urine, with creatinine, citric acid, hippuric acid, and methylhistidine comprising half of the total organic metabolite load (ΣOMs = 3.23 g L⁻¹). Under optimised conditions (pH 4.0, 1:1 Fe²⁺: H₂O₂ molar ratio), 59 % of ΣOMs were degraded in unconcentrated urine treated with 1 g H₂O₂ L⁻¹. Increasing the H₂O₂ dose in unconcentrated urine, or treating concentrated urine obtained through evaporative water removal, resulted in higher ΣOMs degradation but also increased urea oxidation, highlighting a trade-off between efficient COD removal and nutrient recovery. COD removal was 38 % at pH 4.0 and 27 % at pH 6.0, suggesting that Fenton oxidation could be applied to H₂O₂ stabilised urine without strict pH adjustment. Real urine differed significantly from synthetic urine, requiring five times more Fe²⁺ catalyst for complete H₂O₂ activation, with peroxide consumption occurring within five minutes compared to two hours in synthetic urine. Organic compounds in urine scavenged Fe³⁺, forming iron-organic complexes that disrupted Fe²⁺ regeneration and contributed to iron precipitation at higher pH values. These findings demonstrate that Fenton oxidation can be optimised to achieve selective degradation of undesirable organics while preserving plant-essential nutrients in urine collected within resource-oriented sanitation systems.