Photo-Fenton oxidation of 3-amino-5-methylisoxazole: a by-product from biological breakdown of some pharmaceutical compounds

The present study aims to assess the removal of 3-amino-5-methylisoxazole (AMI), a recalcitrant by-product resulting from the biological breakdown of some pharmaceuticals, applying a solar photo-Fenton process assisted by ferrioxalate complexes (SPFF) (Fe³⁺/H₂O₂/oxalic acid/UVA-Vis) and classical solar photo-Fenton process (SPF) (Fe²⁺/H₂O₂/UVA-Vis). The oxidation ability of SPFF was evaluated at different iron/oxalate molar ratios (1:3, 1:6, and 1:9, with [total iron] = 3.58 × 10⁻² mM and [oxalic acid] = 1.07 × 10⁻¹, 2.14 × 10⁻¹ and 3.22 × 10⁻¹ mM, respectively) and pH values (3.5–6.5), using low iron contents (2.0 mg Fe³⁺ L⁻¹). Additionally, the use of other organic ligands such as citrate and ethylenediamine-N,N′-disuccinic acid (EDDS) was tested. The oxidation power of the classical SPF was assessed at different pH values (2.8–4.0) using 2.0 mg Fe²⁺ per liter. Furthermore, the effect of AMI concentration (2–20 mg L⁻¹), presence of inorganic ions (Cl⁻, SO₄ ²⁻, NO₃ ⁻, HCO₃ ⁻, NH₄ ⁺), and radical scavengers (sodium azide and D-mannitol) on the SPF method at pH 3.5 was also assessed. Experiments were done using a lab-scale photoreactor with a compound parabolic collector (CPC) under simulated solar radiation. A pilot-scale assay was conducted using the best operation conditions. While at near neutral pH, an iron/oxalate molar ratio of 1:9 led to the removal of 72 % of AMI after 90 min of SPFF, at pH 3.5, an iron/oxalate molar ratio of 1:3 was enough to achieve complete AMI degradation (below the detection limit) after 30 min of reaction. The SPF process at pH 3.5 underwent a slower AMI degradation, reaching total AMI degradation after 40 min of reaction. The scale up of SPF process showed a good reproducibility. Oxalic and oxamic acids were identified as the main low-molecular-weight carboxylic acids detected during the pilot-scale SPF reaction. Graphical abstract ᅟ