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Abstract
The high concentrations of organic and inorganic compounds in human urine present both challenges and opportunities for its treatment using advanced oxidation processes. This study evaluates Fenton oxidation for selectively degrading endogenoous organic metabolites in urine while preserving critical nutrients such as urea. Using targeted metabolomics, over 200 organic metabolites were identified in acidified 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 and reaching equilibrium 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.
Supplementary materials
Title
Supplementary Information
Description
The supporting information provides detailed experimental, modelling, and metabolomics data to support the findings of this study. It includes the compositions of real and synthetic urine used in the experiments (Table S1), experimental design of the synthetic urine trials (Table S2), thermodynamic modelling results predicting the forms of iron and phosphorus precipitates at varying pH levels (Figure S1), the kinetics of H₂O₂ activation in synthetic urine under different conditions (Figure S3), alongside the fate of ammonium and nitrate concentrations in treated urine (Table S3) are presented. The correlation between COD and UV₂₅₄ removal is also shown to highlight the role of UV₂₅₄ as a surrogate parameter for dissolved organic carbon in urine (Figure S4). Comprehensive metabolomics data, including over 200 organic metabolites, their molecular properties, concentrations, and degradation efficiencies, are provided (Table S4). A PCA biplot highlighting molecular property descriptors and degradation patterns across treatments is also included (Figure S5).
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