## 2 Materials and Methods  
The experiments were conducted using a photocatalytic reactor equipped with a 150 W medium-pressure mercury lamp (Heraeus, 85.8 V, 148.8 W, 1.79 A, 95% transmittance within the 300-570 nm wavelength range) and a magnetic stirrer (Figure 1). The reactor temperature was maintained at a constant 30 \({}^{\circ}\)C using a recirculating cooling bath (Frigiterm-P, Selecta, Barcelona, Spain). Photocatalytic oxidation experiments were performed by introducing 1.0 L of an aqueous sulfamethoxazole solution (50.0 mg/L; SMX, Fragon, Rotterdam, The Netherlands; C\({}_{10}\)H\({}_{11}\)N\({}_{3}\)O\({}_{3}\)S) into the reactor. Subsequently, varying initial concentrations of ferrous ion [Fe(II)]\({}_{0}\) = 0-5.0 mg/L (Panreac, FeSO\({}_{4}\cdot\)7H\({}_{2}\)O, 99%) were added to investigate their catalytic effect. The pH of the solution was adjusted to 3.0 by adding a few drops of 0.1 M hydrochloric acid (Panreac, Castellel dell Valles, Spain; HCl, 37% \(v/v\)), and monitored using a pH meter (Kent EIL9142, Cambridge, UK). At this stage, baseline measurements were recorded for SMX concentration, color, turbidity, and aromaticity. The UV lamp was then switched on, initiating the reaction, followed by the addition of hydrogen peroxide at an initial concentration of [H\({}_{2}\)O\({}_{2}\)]\({}_{0}\) = 2.0 mM (Panreac, Castellel dell Valles, Spain; H\({}_{2}\)O\({}_{2}\), 30% \(w/v\)). The photocatalytic process was carried out for 120 min, during which time sequential measurements of water quality parameters were taken to monitor the progression of the reaction.  
Maintaining the reaction medium at pH 3.0 is essential to ensure high oxidation efficiency in the Fenton-based degradation of SMX. Operating within this acidic range maximizes the generation of hydroxyl radicals due to the favorable redox cycling of Fe(II) to Fe(III), which is critical for initiating the decomposition of hydrogen peroxide [32]. Deviations from this optimal pH can significantly impact the degradation efficiency of SMX. At pH values above 4, iron solubility decreases, leading to the precipitation of ferric hydroxide, which reduces the availability of catalytic Fe(II) in solution. Additionally, hydrogen peroxide tends to decompose non-productively, generating oxygen and water rather than reactive oxygen species. Conversely, at pH levels below 2.5, the stability of hydrogen peroxide may be compromised, and competitive side reactions can occur, both of which hinder the formation of hydroxyl radicals.  
Using a UV-Vis spectrophotometer (model V-630, Jasco, Madrid, Spain), aromaticity ([Aromaticity], AU) was measured at a wavelength of 254 nm and color ([Color], AU) at a wavelength of 455 nm. A multiparameter photometer with COD (Hl83399 model, Hanna Instruments S.L., Eibar, Spain) was used for the measurement of sulfates, with a sulfate reagent (HI93751-01, Hanna Instruments S.L., Eibar, Spain), and nitrates, with a nitrate reagent (HI93728-0, Hanna Instruments S.L., Eibar, Spain). SMX degradation was measured using a High-Performance Liquid Chromatograph (HPLC, Model 2695, Waters Cromatografia S.A., Cerdanyola del Valles, Spain) coupled to a dual wavelength absorbance detector (Model 2487, Waters Cromatografia S.A., Cerdanyola del Valles, Spain). The determination was carried out with the use of a Zorbax Eclipse PAH column (150 mm, 4.6 mm, particle size 5 \(\upmu\)m) and a guard column Zorbax Eclipse PAH (4.6 mm, 12.5 mm), both supplied by Agilent (Santa Clara, CA, USA). The chromatographic separation was carried out at room temperature, with an injection volume of 50 \(\upmu\)L and under gradient mode with a mobile phase composed of water and acetonitrile (ACN) at 0.8 mL/min. Initial gradient conditions were set at 20% ACN and increased from 45% ACN for 3 min, then held for 6 min, and finally decreased to 20% ACN at 1 min. Total run time was 10 min. Detection was carried out at 275 nm, and for the identification of the SMX, a standard was used.