Dissolved Oxygen Changes in Wastewater During Sulfamethoxazole Degradation by Photo-Fenton Treatment

Abstract

This study examines the degradation of sulfamethoxazole (SMX) in water using the photo-Fenton process, focusing on dissolved oxygen (DO) dynamics, organic matter mineralization, and water quality improvement. The results show that SMX degradation follows a rapid kinetic pattern, achieving complete removal within 30 min. However, total organic carbon reduction occurs more gradually, indicating the persistence of organic intermediates before full mineralization into CO₂ and H₂O. DO evolution follows a biphasic trend: an initial decline due to oxidative consumption, followed by an increase due to H₂O₂ decomposition into O₂. Initially, at [H₂O₂]₀ ≥ 3.0 mM, DO sharply increases, while at [Fe(II)]₀ = 5.0 mg/L, DO reaches a minimum of 0.3 mg/L due to higher reactive oxygen species (ROS) production. Water quality parameters such as color, turbidity, and aromaticity were also monitored. Aromaticity significantly decreases within 30 min, confirming SMX ring cleavage. Color and turbidity initially intensify and increase due to intermediate formation but later decrease as mineralization progresses. Optimal conditions (1 mol SMX: 10 mol H₂O₂: 0.05 mol Fe(II)) ensure efficient degradation with minimal oxygen depletion without excessive scavenging effects. These findings confirm that the photo-Fenton process effectively removes SMX while improving water quality, making it a sustainable alternative for pharmaceutical wastewater treatment.

1. Introduction

In recent years, the increasing presence of human antibiotics in the environment has raised concerns about the health risks these emerging contaminants (ECs) may pose [1,2]. Defined daily doses (DDDs), the measure by which the World Health Organization defines drug consumption, are increasing dramatically and are estimated to be almost four times higher by 2030 than they were in 2000 [3]. The widespread overuse of antibiotics has serious repercussions on the environment, as these compounds are difficult to break down naturally and tend to accumulate in ecosystems [4]. Their presence alters the biological balance of water, directly affecting aquatic organisms and, indirectly, humans [5]. Within the wide variety of antibiotics present in our daily lives, which have been detected in water, sulfamethoxazole is one of the most recurrent [6].

Sulfamethoxazole (SMX, C₁₀H₁₁N₃O₃S) is an antibiotic of the sulfonamide group [7] that is widely used because of its broad spectrum of antibacterial activity, high stability, and low cost, making it an essential tool in livestock and medicine [8]. However, after ingestion by humans and animals, only a small proportion can be metabolized [9,10], and the remaining 45–70% is excreted into the environment via feces or urine, as sulfamethoxazole or as metabolites [11]. SMX can remain in the environment for more than a year, which poses a number of risks [12]. Its presence can disrupt entire ecosystems, impact biogeochemical cycles, and contribute to the development of antibiotic-resistant bacteria, which represents a public health challenge [13,14]. It has been detected in wastewater treatment plant (WWTP) effluents at concentrations between 10 and 1500 ng/L, indicating its persistence in these systems at relatively high levels [15]. This underscores the importance of implementing effective removal strategies to mitigate its negative impact on the environment and human health [16].

As conventional water treatment methods have proven to be ineffective in the treatment of persistent organic contaminants [17], it is necessary to develop new physical, chemical, or biological technologies for their removal [18]. Among the various options, advanced oxidation processes (AOPs) have attracted a great deal of interest from researchers around the world because of their high degradation efficiency, even with large amounts of organic matter and low pH conditions, and their environmentally friendly nature [19,20]. These processes are based on the generation of free radicals, such as hydroxyl radicals, which under optimal conditions are capable of degrading organic contaminants present in wastewater [21,22]. Free radicals can be generated through different processes, including ozonation, Fenton-based reactions, electrochemical oxidation, and ultrasound cavitation [23]. However, the best results have been obtained in those involving an oxidant, such as hydrogen peroxide, together with ultraviolet (UV) light and a catalyst, as in the case of the photo-Fenton process [24].

The photo-Fenton process emerged as an improvement of the traditional Fenton process, in which hydrogen peroxide and ferrous ions are used to generate the hydroxyl radicals that degrade organic contaminants. This Fenton process has disadvantages such as the excessive use of H₂O₂ and the generation of iron sludge, which can complicate its large-scale application. The photo-Fenton process overcomes these problems with the addition of ultraviolet (UV) light, which helps regenerate ferrous ions, reducing the amount of hydrogen peroxide required and minimizing sludge formation, making the process more efficient and environmentally friendly [25,26,27].

The most commonly measured parameters to assess the efficiency of the photo-Fenton process include total organic carbon (TOC), color, turbidity, and concentration of specific pollutants [28,29,30]. These indicators provide key information on organic matter removal and pollutant mineralization. However, dissolved oxygen is a parameter that has not been extensively studied in the photo-Fenton process, despite its relevance as an indicator of treatment efficiency [31,32]. Its evolution reflects the dynamics of reactive oxygen species (ROS) generation and consumption, as well as the regeneration of Fe²⁺ in the catalytic cycle. In the early stages of the process, the concentration of dissolved oxygen may decrease due to its participation in the formation of superoxide and hydroperoxyl radicals. However, at later stages, it is common to observe an increase in its concentration, especially when excessive doses of hydrogen peroxide are used. This is due to the decomposition of residual hydrogen peroxide into water and molecular oxygen, which not only reduces the efficiency of the process, but also indicates the suboptimal use of the reagent [33]. Therefore, dissolved oxygen monitoring can provide crucial information to optimize hydrogen peroxide dosing and improve overall process performance.

Therefore, it is worth highlighting that this study will expand existing knowledge on SMX degradation by focusing on the role of DO dynamics in the photo-Fenton process. While previous research has established the efficacy of the photo-Fenton process in the degradation of SMX and other drugs, this study provides a more detailed analysis of DO behavior, linking it to reactive oxygen species (ROS) production and hydrogen peroxide decomposition. Key contributions, compared to previous studies, include analyzing the potential of DO as a surrogate marker of treatment efficiency and ROS activity. Previous studies have primarily focused on pollutant removal and TOC reduction but have not comprehensively analyzed DO behavior. Furthermore, this research will provide insight into how different concentrations of H₂O₂ and Fe(II) influence DO levels, refining optimal conditions to ensure effective SMX degradation while minimizing excessive oxygen production or reagent waste. Furthermore, unlike previous work, this study evaluates water quality parameters such as turbidity, color, and aromaticity, confirming the overall efficacy of the treatment beyond simple SMX removal. Finally, this study highlights the persistence of organic intermediates before complete mineralization, highlighting the gradual reduction in TOC. This contrasts with studies that primarily report the rapid decomposition of contaminants without detailing the fate of byproducts. By integrating DO dynamics with SMX degradation and water quality improvements, this study offers a more complete understanding of the photo-Fenton process, making it a valuable contribution to research on pharmaceutical wastewater treatment.

2. Materials and Methods

The experimental assays were conducted in a 1.0 L photocatalytic reactor, provided with a 150 W UV lamp (mercury lamp of medium pressure, TQ 150-Heraeus, 230 V/50 Hz, 2.5 A), homogenized by a magnetic stirrer (see Figure 1). The lamp’s emission range was adequate, with 95% transmission between 300 and 570 nm. Synthetic aqueous solutions of sulfamethoxazole (Fragon, Rotterdam, The Netherlands, C₁₀H₁₁N₃O₃S, 100%) of initial concentration [SMX]₀ = 50.0 mg/L were added into the reactor. In order to study the changes in dissolved oxygen when a photo-Fenton treatment is applied to an SMX solution, two sets of experiments were set up: one in which a constant amount of [Fe(II)]₀ = 0.5 mg/L (Panreac, FeSO₄·7 H₂O, 99.0%) and a range of [H₂O₂]₀ = 0–3.0 mM (Panreac, Castellar del Vallès, Spain, 30% v/v) were used to study the effect of the amount of hydrogen peroxide used and another in which the amount of [H₂O₂]₀ = 2.0 mM was set and the amount of [Fe(II)]₀ = 0–5.0 mg/L was varied to study the effect of the ferrous catalyst. The initial pH of the water samples was adjusted by diluted HCl (0.1 M). The pH was monitored throughout the reaction using a pH meter (Kent EIL9142). Temperature of the system was kept constant at around [T] = 25 °C using a 1.150 W cryo-thermostat bath (Frigiterm-10 Selecta), and water was recirculated through the reactor sheath where the UV lamp is inserted [34].

The following parameters were measured during the reaction: water aromaticity at λ = 254 nm ([Aromaticity], AU) [34] and color at λ = 455 nm ([Color], AU) with a UV/Vis spectrophotometer (model V-630, Jasco, Madrid, Spain) [35]. It is worth noting that aromaticity in the context of AOPs is usually assessed by UV absorbance at 254 nm (UV254), because many aromatic compounds, such as SMX, absorb strongly at this wavelength due to their benzene or heterocyclic ring structures, and it is a rapid and indirect method for monitoring the destruction of aromatic structures during oxidative treatment. The H₂O₂ used as an oxidant in the process has very low absorbance in the UV region, especially around 254 nm. Its absorption spectrum begins to become significant below 200 nm, but at 254 nm, its contribution is practically zero or negligible at the concentrations typically used in AOPs. Therefore, the UV254 signal is dominated by the absorption of SMX (and its aromatic products). H₂O₂ does not contribute a significant signal at this wavelength. The concentrations used in this study are low enough that, even if some were absorbed, it would not significantly alter the UV 254 reading. In addition, the following were measured: water turbidity ([Turbidity], NTU) with a nephelometric turbidimeter (model HI88703, Hanna Instruments S.L., Eibar, Spain); dissolved oxygen ([DO], mg/L) with a dissolved oxygen meter (model HI 9142, Hanna Instruments S.L., Eibar, Spain); and total carbon ([TOC], mg/L).

SMX concentration was measured using high-performance liquid chromatography (Model 2695, Waters Cromatografía S.A., Cerdanyola del Vallès, Spain) with a Dual λ Absorbance Detector (Model 2487, Waters Cromatografía S.A., Cerdanyola del Vallès, Spain). A Zorbax Eclipse PAH column (150 mm, 4.6 mm, particle size 5 μm) and a guard column Zorbax Eclipse PAH (4.6 mm, 12.5 mm) supplied by Agilent (Santa Clara, CA, USA) were used. The mobile phase consisted of water and acetonitrile (ACN), at a flow rate of 0.8 mL/min. Initial gradient conditions were set at 20% ACN and increased from 50% v/v ACN for 3 min, then held for 6 min, and finally decreased to 20% v/v ACN for 1 min. The total run time was 10 min. The injection volume was 50 μL, and all separations were performed at room temperature. The identification of SMX was performed by comparison to the standard. Detection was carried out at the following wavelength: 275 nm.

3. Results

3.1. Changes in Dissolved Oxygen During SMX Oxidation

Figure 2 shows the kinetic evolution of sulfamethoxazole concentration (SMX, mg/L), dissolved oxygen (DO, mg/L), and total organic carbon (TOC, mg/L) during the oxidation of SMX via the photo-Fenton process. As observed, the concentration of SMX decreases rapidly, with a sharp decline during the first 20 min of the reaction, leading to complete degradation after approximately 30 min. These results indicate that the photo-Fenton process is a highly efficient process for SMX degradation.

The reduction in TOC reflects the removal of organic matter from the solution over time. The TOC concentration exhibits a slower and more gradual decline compared to SMX. This indicates that, although SMX degrades rapidly, intermediate products still contain organic carbon, which takes longer to completely mineralize. The complete mineralization of organic carbon requires a longer reaction time due to the formation of intermediate byproducts before the final conversion of SMX into CO₂ and water. Although the rapid removal of SMX demonstrates the kinetic effectiveness of the photo-Fenton process, it cannot be assumed that this guarantees environmental improvement without evaluating the byproducts and their toxicity. The evaluation of the final quality of the treated water must go beyond the removal of the initial contaminant and also consider the nature and impact of the residual compounds [36].

Analyzing the dissolved oxygen concentration, we observed an initial rapid decrease within the first 15 min, due to the high oxygen consumption in the photo-Fenton reaction. Subsequently, DO begins to increase after 45 min, reaching a maximum around 90 min, followed by a slight decrease. This behavior may be attributed to oxygen generation as a reaction byproduct. The DO behavior confirms the involvement of reactive oxygen species in the reaction and the possible generation of oxygen during the process.

The evolution of DO is linked to the reactions occurring in the photo-Fenton process. Its behavior can be divided into two main stages. In the first stage, a rapid initial decrease in DO occurs (first 10–20 min) due to oxygen consumption in the photo-Fenton reaction and other oxidation processes, such as the oxidation of SMX and its intermediates, as well as the production of radical species like HO₂^• and superoxide (O₂^−•). Figure 3 shows a diagram of SMX degradation pathways by a photo-Fenton process and its relationship with dissolved oxygen dynamics [37].

As shown in Figure 3, the main responsible reactions are as follows:

Hydroxyl radical (HO^•) generation via the Fenton system: This reaction consumes hydrogen peroxide but can also deplete dissolved oxygen due to Fe(III) reduction (Equation (1)) [38].

$${Fe}^{2+}+H_2{O}_2 \to {Fe}^{3+}+H{O}^{•}+H{O}^{-}$$

(1)

Fe(II) regeneration under reductive conditions: During this phase, species such as superoxide radicals from dissolved oxygen (O₂^−•) and HO₂^• can form (Equations (2) and (3)) [39], consuming dissolved oxygen to generate hydrogen peroxide (H₂O₂) (Equation (4)) [40]. This is followed by the reduction of the ferric ion (Fe(III)) by the superoxide radical (Equation (5)). Finally, the ferric ion is reduced by hydrogen peroxide (Equation (6)). Once the Fe(II) has been regenerated, it reacts with hydrogen peroxide to generate the hydroxyl radical, which is highly reactive for the degradation of sulfamethoxazole (Equation (1)). These reactions allow the regeneration of Fe(II), ensuring the continuity of the photo-Fenton process and promoting the degradation of the contaminant.

$$O_2+e^{-} \to O_2^{-•}$$

(2)

$$O_2^{-•}+H^{+} \to H{O}_2^{•}$$

(3)

$$O_2^{-•}+O_2^{-•}+2H^{+} \to H_2{O}_2+O_2$$

(4)

$$F{e}^{3+}+O_2^{-•} \to F{e}^{2+}+O_2$$

(5)

$${Fe}^{3+}+H_2{O}_2+h\nu \to {Fe}^{2+}+{HO}_2^{•}+H^{+}$$

(6)

Oxidation of organic intermediates: SMX degradation generates intermediate compounds that may react with oxygen, contributing to DO depletion (Equation (7)) [41]:

$$Reaction intermediates+O_2 \to Products$$

(7)

In the second stage, a progressive increase in DO occurs after 40 min of reaction once most of the SMX has degraded. At this point, the production of reactive oxygen species may exceed their consumption, leading to an overall DO increase. This DO rise is primarily caused by hydrogen peroxide decomposition into oxygen and the conversion of reactive oxygen species into oxygen. This behavior is characteristic of advanced oxidation processes such as the photo-Fenton process, where oxygen can act as both a reactant and a product in different reaction stages. The reactions explaining this behavior are as follows:

Photocatalysis and Fe(II) regeneration: UV light facilitates ferrous ion regeneration, allowing the Fenton cycle to continue without directly consuming oxygen (Equations (2)–(6)).

Hydrogen peroxide decomposition into molecular oxygen: As organic compounds decrease, excess hydrogen peroxide decomposes into molecular oxygen, contributing to the DO increase (Equation (8)) [42].

$$2 H_2{O}_2 \to O_2+2H_{2}O$$

(8)

Superoxide radical (O₂^−•) oxidation into dissolved oxygen: This reaction generates molecular oxygen and contributes to the DO increase after most of the SMX has degraded (Equation (9)).

$$2O_2^{-•}+2H^{+} \to H_2{O}_2+O_2$$

(9)

Figure 4 shows the evolution of color (in the graph scale, numerical values are represented as ×100 in absorbance units), turbidity (NTU), and aromaticity (AU) of water during the oxidation of SMX by the photo-Fenton process. Comparing these results with Figure 1, it can be observed that SMX degradation and TOC mineralization affect water quality in terms of these three properties. Aromaticity is an indicator of the presence of aromatic compounds in water, which may be degradation byproducts of SMX. In Figure 2, aromaticity continuously decreases over time, with a significant reduction within the first 30 min. This behavior aligns with Figure 1, where SMX degrades completely within the same time frame. This indicates that the photo-Fenton oxidation breaks the aromatic rings of SMX and its intermediates, reducing the amount of aromatic compounds in the solution. The degradation of SMX (Figure 1) is accompanied by a reduction in aromaticity (Figure 2), confirming the breakdown of aromatic rings during the photo-Fenton process.

The water color intensifies during the first 10 min, reaches a maximum around 15–20 min, and then gradually weakens. This initial intensification suggests the formation of degradation intermediates with high absorbance in the visible spectrum, possibly quinones or oxidized SMX-derived products. The subsequent color reduction indicates the transformation and eventual mineralization of these intermediates, which correlates with the gradual TOC decrease observed in Figure 1.

Turbidity follows a similar trend to color, increasing within the first 10 min, peaking around 15–20 min, and then progressively decreasing. The initial turbidity increase may be due to the formation of colloidal particles, such as aggregates of intermediate compounds or iron Fe(III) precipitates. The turbidity reduction after 20–30 min suggests that these aggregates either decompose or settle, improving water clarity. This trend is also related to the increase in DO in Figure 1 after 30 min, as oxygen production may enhance the oxidation and removal of these intermediates.

Therefore, according to the obtained results, color and turbidity initially increase due to the formation of degradation intermediates and iron precipitates but gradually decrease as these products are transformed or removed. The results demonstrate that the photo-Fenton process not only eliminates the contaminant (SMX) but also improves water quality by reducing coloration, turbidity, and the presence of aromatic compounds.

The observed color, turbidity, and aromaticity trends show strong correlations with SMX degradation and DO evolution, making them potential alternative indicators of reaction efficiency in the photo-Fenton process. Aromaticity decreases continuously over time, with a significant reduction in the first 30 min. This behavior closely aligns with the sharp drop in SMX concentration, confirming the cleavage of aromatic rings and the oxidation of intermediates. The decrease in aromatic compounds suggests efficient oxidation by hydroxyl radicals, which correlates with the initial decrease in DO (due to oxygen consumption in the oxidation reactions). After 30–40 min, when DO begins to increase, aromaticity is already minimal, indicating advanced degradation and mineralization. Therefore, aromaticity serves as a reliable indicator of SMX degradation and the oxidation of intermediates.

Color and turbidity initially increase (first 10–20 min) before gradually decreasing. This increase is attributed to the formation of degradation intermediates (e.g., quinones, oxidized byproducts of SMX) and colloidal aggregates (e.g., Fe(III) precipitates). The peak at 15–20 min coincides with the maximum degradation of SMX, when most of the original compound is decomposed but intermediates are still present. Color and turbidity decrease after 30 min, reflecting the increase in DO, suggesting that the intermediates are further oxidized (reducing color). The colloidal aggregates then decompose or precipitate (reducing turbidity). The decomposition of H₂O₂ to O₂ contributes to DO recovery. Therefore, the decrease in color and turbidity after its peak correlates with DO recovery, reinforcing their role as indicators of intermediate transformation and mineralization.

Therefore, aromaticity can be considered a direct indicator of the decomposition of organic pollutants, particularly SMX ring cleavage. Color and turbidity provide information on the formation and transformation of degradation intermediates, including colloidal iron species and organic byproducts. Their initial increase and subsequent decrease reflect the progression of oxidation and mineralization, making them valuable complementary indicators of DO. The evolution of DO reflects the dynamics of ROS and the progress of mineralization. Aromaticity tracks the oxidation of organic pollutants.

The degradation process of SMX in the photo-Fenton system occurs through a series of advanced oxidation reactions involving hydroxyl radicals and other reactive oxygen species. These radicals attack the SMX molecule, breaking bonds and generating organic intermediates that can contribute to water coloration and turbidity before complete mineralization into CO₂ and water. The mechanism can be divided into several stages:

Initial attack by hydroxyl radicals: The photo-Fenton reaction generates hydroxyl radicals (Equation (1)), which are highly reactive oxidizing species that attack SMX at various sites, initiating its degradation (Equation (10)) [43]. The main attack sites include the amino group (-NH₂), which can be oxidized to compounds such as quinones and nitro derivatives, the isoxazole ring, which can undergo ring opening to form smaller fragments, and the benzene ring, which can undergo hydroxylation, producing phenolic compounds [44].

$$SMX+{HO}^{•} \to Degradation products$$

(10)

Formation of colored intermediates and turbidity: As SMX degrades, byproducts are formed that contribute to water color and turbidity (Equation (11)) [34,35]. The hydroxylation of the benzene ring in SMX generates phenolic derivatives, such as hydroquinones and catechols, which can absorb in the visible spectrum, causing the initial color intensification observed in the first few minutes. The oxidation of phenolic compounds leads to quinones (Equation (12)), which have high absorbance in the visible range and contribute to the initial color intensification. The cleavage of the isoxazole ring produces aldehydes and organic acids, which do not contribute to coloration but may increase turbidity if colloidal compounds are formed. Additionally, in the photo-Fenton process, iron forms species such as Fe(OH)₃, which can increase turbidity in the first few minutes. These aggregates can adsorb organic intermediates, further contributing to turbidity.

$$SMX+{HO}^{•} \to Fenols+Quinones$$

(11)

$$Fenols+{HO}^{•} \to Quinones+H_{2}O$$

(12)

Elimination of intermediates and reduction in color and turbidity: After approximately 30 min, intermediates begin to mineralize into simpler compounds, explaining the reduction in color and turbidity observed in Figure 2. Over time, quinones degrade into non-colored compounds (Equation (13)), reducing absorbance in the visible region. Additionally, organic intermediates are fully oxidized to CO₂ and H₂O through successive reactions with hydroxyl radicals, leading to the mineralization of organic compounds. Finally, colloidal particles may undergo sedimentation or dissolution, as precipitated iron and organic aggregates can either settle or dissolve, reducing turbidity.

$$Quinones+{HO}^{•} \to Organic acids+{CO}_2$$

(13)

In summary, the initial increase in color and turbidity is due to the formation of phenolic compounds, quinones, and iron aggregates with oxidized intermediates. The subsequent reduction in these parameters occurs as the intermediates degrade into simpler products that do not absorb in the visible spectrum and as precipitated iron is removed by sedimentation or dissolution. Therefore, the SMX degradation mechanism in the photo-Fenton process is proven to be efficient, as it not only eliminates the contaminant but also improves water quality by reducing color and turbidity over time.

3.2. Effect of Oxidant Dosage on Changes in Dissolved Oxygen

Figure 5 illustrates the evolution of normalized DO during the oxidation of SMX using the photo-Fenton treatment, evaluating different initial concentrations of hydrogen peroxide dosed into the process.

Based on the obtained data, a general DO trend is observed for all tested H₂O₂ concentrations, where DO initially decreases over time, indicating oxygen consumption in the early stages of the reaction. This is consistent with the reaction of hydrogen peroxide with Fe(II) in the photo-Fenton process, which generates hydroxyl radicals and consumes dissolved oxygen in the medium. The main reaction governing this initial decrease is shown in Equation (1). Additionally, Fe(III) can be photo-reduced under UV radiation, regenerating Fe(II) and producing additional hydroxyl radicals, following Equation (8). When operating with low to moderate H₂O₂ concentrations, DO remains low after the initial decrease, suggesting continuous oxygen consumption without significant regeneration. This could indicate a balance between oxygen production and consumption, where peroxide continues to react with Fe(II) and other intermediates, preventing the accumulation of free oxygen. The oxygen consumption also results from side reactions involving organic intermediates formed during the SMX degradation, which scavenge hydroxyl radicals and contribute to DO depletion (Equation (11)).

However, when operating with high initial concentrations of [H₂O₂]₀ ≥ 3.0 mM (1 mol SMX: 15 mol H₂O₂), a different behavior is observed. After approximately 50 min, DO begins to increase significantly, reaching values much higher than the initial ones. This suggests that, under excess oxidant conditions, H₂O₂ may decompose into molecular oxygen (O₂), explaining the recovery and increase in DO. The decomposition of hydrogen peroxide into oxygen occurs via the reaction shown in Equation (9). This implies that operating with an excess of H₂O₂ could reduce treatment efficiency due to competitive reactions with the Fenton process, leading to molecular oxygen generation (Equation (8)) instead of hydroxyl radicals. Additionally, an excess of H₂O₂ can act as a scavenger for hydroxyl radicals, reducing their availability for SMX oxidation (Equation (14)) [45]. Then, the efficiency of the photo-Fenton process for degrading SMX is highly dependent on the generation and availability of hydroxyl radicals, which are the primary oxidizing species responsible for the breakdown of organic pollutants. However, excess hydrogen peroxide can lead to a paradoxical effect, reducing overall treatment efficiency rather than enhancing degradation due to radical scavenging and iron regeneration. This highlights the need for precise oxidant dosing strategies to optimize water treatment performance.

$$H_2{O}_2+{HO}^{•} \to {HO}_2^{•}+H_{2}O$$

(14)

As a result, while moderate H₂O₂ concentrations effectively enhance pollutant degradation via hydroxyl radical production, excessive dosing may lead to inefficient oxidant use and increased oxygen evolution, potentially lowering treatment performance.

3.3. Effect of Iron Catalyst Dosage on Changes in Dissolved Oxygen

Figure 6 shows the evolution of normalized DO concentration during the oxidation of SMX via the photo-Fenton process, using different initial concentrations of ferrous ion added as a catalyst to the system.

Figure 6a shows that irradiating the water with UV light leads to a stable behavior over time during the first 50 min of reaction. After 50 min, the DO concentration in the water decreases until it reaches a stable value close to zero (DO = 0.2 mg/L) from 70 min onwards. This behavior can be explained by the fact that UV radiation may induce the photolysis of certain species present in the medium, but its impact on dissolved oxygen is minimal. The stability observed in the first 50 min suggests that no significant oxygen-consuming reaction occurs in the solution during this initial stage. After 50 min, the photolysis of SMX or other organic compounds generated as oxidation intermediates of SMX may take place, potentially generating radicals and reactive species that consume oxygen. The formation of oxidation byproducts could promote the reduction of dissolved oxygen in the water. Additionally, the presence of auto-oxidation processes and oxygen transfer must be considered, as in UV-irradiated aqueous systems, some species may be photo-activated and react with dissolved oxygen, reducing its concentration. Therefore, the observed behavior indicates that DO remains stable for a period before being consumed by UV radiation-induced reactions, possibly related to the photolysis of organic compounds and the generation of reducing species.

Figure 6a shows that enhancing the effect of UV light with hydrogen peroxide results in a decrease in dissolved oxygen from the start of the reaction until 30 min into the oxidation of SMX, at which point it reaches a stable value that persists over time (DO = 1.2 mg/L). This result can be explained by an initial oxygen consumption due to its reaction with H₂O₂. Hydrogen peroxide can decompose under UV irradiation, generating hydroxyl radicals, which are highly reactive and play a key role in SMX oxidation. However, in this process, dissolved oxygen is also consumed through the reactions shown in Equations (14) and (15) [45] and Equation (3). The HO₂^• and O₂^•− radicals can react with DO, leading to its initial decrease in the first 30 min. After 30 min, DO stabilizes at 1.2 mg/L, indicating that oxygen consumption reaches an equilibrium with its replenishment in the system. This behavior suggests that the degradation of SMX and other organic compounds reaches a steady-state regime where radical generation and its interaction with DO are balanced.

$$H_2{O}_2+h\nu \to 2{HO}^{•}$$

(15)

Figure 6b shows that, when iron is added in combination with UV and H₂O₂ (photo-Fenton system), there is an increase in dissolved oxygen during the first 10 min of SMX oxidation. Subsequently, dissolved oxygen rapidly decreases, reaching a minimum value at 20 min, with levels close to [DO] = 0.3 mg/L under certain conditions. After this point, dissolved oxygen concentration remains low and stable in most cases, suggesting sustained oxygen consumption over time. It is important to note that when operating with low Fe(II) concentrations (0.5–1.0 mg/L), oxygen gradually recovers after the first 30 min. At higher Fe(II) doses (3.0–5.0 mg/L), DO remains low and does not recover, indicating more aggressive oxygen consumption due to the increased generation of reactive species. Specifically, at [Fe(II)]₀ = 5.0 mg/L, DO reaches its lowest level and shows minimal recovery, suggesting that the system is highly reactive and that oxygen consumption is maximized.

Based on these observed results, the DO behavior during SMX oxidation via the photo-Fenton process can be summarized in three main phases: an initial increase in DO, a rapid decrease to minimal values, and stabilization at low levels or partial recovery depending on Fe(II) concentration. The initial increase in dissolved oxygen (first 10 min) can be explained by the generation of reactive oxygen species (ROS), which not only degrade the organic pollutant load but can also induce oxygen release from hydrogen peroxide via the Fenton reaction (Equations (1) and (2)). The formation of HO₂^• radicals may contribute to a higher initial oxygen concentration in solution due to their decomposition into molecular oxygen.

The rapid decrease in dissolved oxygen (10–20 min) to minimal values (~0.3 mg/L in some cases) during the first 20 min is due to the intense oxygen consumption in the oxidation reactions of the organic pollutant load in the water and the regeneration of ferrous ion, which enhances hydroxyl radical production (Equations (16) and (17)). The continuous regeneration of ferrous ion and the high reactivity of the photo-Fenton system lead to an accelerated oxygen consumption, explaining the drastic DO reduction. The main reactions responsible for oxygen consumption in the photo-Fenton system are due to the regeneration of ferrous ion by ferric ion reduction (Equation (6)), the reaction of ferrous ion with molecular oxygen to form hydrogen peroxide (Equation (16)), the conversion of the superoxide radical into hydrogen peroxide (Equation (17)), and the formation of hydroxyl radicals (Equation (1)) [38,46,47]:

$${F{e}^{2+}+O}_{2 }\to F{e}^{3+}+O_2^{-•}$$

(16)

$$O_2^{-•}+H_2{O}_2 \to O_2+{HO}^{•}+{HO}^{-}$$

(17)

The DO behavior observed after 30 min, as a function of the iron concentration used in the treatment, can be explained as follows: when operating with low Fe(II) concentrations (0.5–1.0 mg/L), a gradual DO recovery occurs because oxygen consumption is moderate, and oxygen regeneration via gas transfer processes (exchange with the atmosphere) is sufficient to partially restore DO levels. However, at high Fe(II) concentrations (3.0–5.0 mg/L), DO remains low and does not recover, as oxygen consumption is more aggressive due to the increased generation of reactive oxygen species (ROS). At [Fe(II)]₀ = 5.0 mg/L, DO reaches its lowest level and shows minimal recovery, suggesting that the system is extremely reactive and that oxygen is constantly consumed in the oxidation of the organic pollutant load present in the water.

3.4. Analysis of the Quality of Treated Water

Figure 7 shows the relationship between the initial hydrogen peroxide concentration used in the treatment and the amount of DO analyzed in water samples oxidized through the photo-Fenton process (see

Table 1. DO analyzed in treated water samples. Experimental conditions: [SMX]₀ = 50.0 mg/L; [pH]₀ = 3.0; [UV] = 150 W; [T] = 25 °C.

[H2O2]0 mM	[Fe(II)]0 mg/L	[DO] mg/L	[H2O2]0 mM	[Fe(II)]0 mg/L	[DO] mg/L
0.00	0.50	0.40	0.00	0.00	0.20
0.25	0.50	0.10	2.00	0.00	1.20
0.35	0.50	0.20	2.00	0.50	2.00
0.50	0.50	0.30	2.00	1.00	1.10
1.00	0.50	0.20	2.00	2.00	0.30
1.50	0.50	0.10	2.00	3.00	0.30
2.00	0.50	0.10	2.00	4.00	0.20
3.00	0.50	4.60

).

The results indicate that increasing the H₂O₂ concentration up to approximately 2.0 mg/L leads to a decrease in DO to values close to zero. This occurs because H₂O₂ acts as an oxidizing agent in the Fenton reaction, generating hydroxyl radicals, which are highly reactive species that consume oxygen during the degradation of SMX (Equations (1) and (17)). Additionally, in the presence of ferrous ion and UV radiation, hydrogen peroxide can also act as a competitor in reactions with hydroxyl radicals, potentially inhibiting dissolved oxygen recombination and reducing its concentration in solution.

The sudden increase in DO at higher H₂O₂ concentrations (above 2.0 mg/L) may be attributed to the direct decomposition of H₂O₂ into molecular oxygen (O₂), particularly when there is an excess of peroxide in the solution (Equation (9)). In this case, hydrogen peroxide ceases to be consumed by the Fenton process and begins to decompose spontaneously, releasing oxygen into the solution. This suggests the existence of an optimal hydrogen peroxide dosage for the photo-Fenton process, approximately [H₂O₂]₀ = 2.0 mM (1 mol SMX: 10 mol H₂O₂), which would prevent inhibition due to excess and ensure efficient SMX degradation without completely depleting dissolved oxygen in the solution. Within this range, hydrogen peroxide is efficiently utilized for hydroxyl radical generation without causing excessive oxygen consumption. Furthermore, the inhibition of the process due to excess H₂O₂ is minimized, preventing it from acting as a competitor for hydroxyl radicals or decomposing directly into molecular oxygen without contributing to SMX degradation. This ensures improved oxidation kinetics of SMX without negatively affecting dissolved oxygen availability in the solution.

Similar results to this optimal concentration ([H₂O₂]₀ = 2.0 mM to degrade 50.0 mg/L of pollutant) have been obtained by degrading other contaminants such as caffeine operating at similar experimental conditions [48]. This phenomenon is due to the fact that the degradation mechanism of both contaminants follows similar pathways in the photo-Fenton process. That is, they share degradation mechanisms within the photo-Fenton process and rely on a proper balance between hydroxyl radical production, dissolved oxygen availability, and the prevention of inhibitory effects caused by an excess of hydrogen peroxide.

Figure 8 shows the evolution of DO as a function of the initial ferrous ion concentration in the treatment of SMX-containing water by the photo-Fenton process (see Table 1). The results indicate that operating at low Fe(II) concentrations (≤0.5 mg/L) leads to an increase in dissolved oxygen, reaching a maximum value close to [DO] = 2.0 mg/L when operating with 0.5 mg/L of Fe(II). This suggests that, within this range, the generation of reactive oxygen species is efficient, and dissolved oxygen is not aggressively consumed in the solution. Additionally, hydroxyl radical generation is sufficient to degrade SMX without depleting DO. When operating at moderate Fe(II) concentrations (0.5–1.0 mg/L), a progressive decrease in DO is observed, indicating that oxygen consumption becomes more significant. The oxidation of Fe(II) to Fe(III) and its subsequent regeneration consume oxygen, contributing to this reduction. The production of free radicals remains effective, but DO begins to decrease due to increased catalytic activity. At high Fe(II) concentrations (>1.0 mg/L), a drastic decline in dissolved oxygen is observed, reaching values close to [DO] = 0.2 mg/L for Fe(II) concentrations above 2.0 mg/L. This behavior suggests sustained oxygen consumption due to the increased production of hydroxyl radicals and the formation of intermediate species such as residual hydrogen peroxide, which also consumes oxygen during its decomposition. Additionally, high Fe(II) concentrations may promote undesired reactions, such as hydroxyl radical recombination, reducing process efficiency.

This study allows for the estimation of an optimal Fe(II) operating point, approximately [Fe(II)]₀ = 0.5 mg/L (1 mol SMX: 0.05 mol Fe(II)), where dissolved oxygen would reach its maximum value ([DO] = 2.0 mg/L), indicating efficient radical generation without excessive oxygen consumption. At higher concentrations, dissolved oxygen decreases drastically, suggesting increased consumption due to a more aggressive reaction. At concentrations of [Fe(II)]₀ ≥ 3.0 mg/L, DO remains very low and stable, indicating a highly reactive process with maximum oxygen consumption. This analysis suggests that to prevent a drastic decrease in DO and maximize the efficiency of the photo-Fenton treatment for SMX removal, it is advisable to operate with Fe(II) concentrations around 0.5–1.0 mg/L. Therefore, the optimal Fe(II) concentration range is between 0.5 and 1.0 mg/L, as it allows for high SMX degradation efficiency and adequate TOC reduction, without excessive DO consumption, indicating that hydroxyl radical production is sufficient to oxidize the contaminant without promoting scavenger reactions. At concentrations above 1.0 mg/L, the system becomes highly reactive, but with high DO consumption, which could compromise process efficiency and its full-scale application.

4. Conclusions

The results obtained in this study confirm that the photo-Fenton process is highly efficient for the degradation of SMX in water. Rapid elimination of the contaminant was observed within the first 30 min of treatment, accompanied by a progressive decrease in TOC, indicating a gradual mineralization of the generated byproducts.

The behavior of DO during the reaction exhibited two main stages: an initial decrease due to oxygen consumption by reactive oxygen species (ROS) generated in the process, followed by a progressive increase after 40–50 min, attributed to the decomposition of hydrogen peroxide into molecular oxygen. This phenomenon suggests that DO can serve as an indirect indicator of the reaction extent and ROS formation within the system.

The dosage of both the oxidant and catalyst significantly influences the evolution of DO. At moderate H₂O₂ concentrations, dissolved oxygen remains low due to its continuous consumption in the photo-Fenton reaction. However, at high hydrogen peroxide concentrations (≥3.0 mM), DO increases significantly after 50 min, suggesting the direct decomposition of excess H₂O₂ into molecular oxygen. Similarly, Fe(II) concentration determines DO dynamics, with a trend toward depletion and stabilization at low levels when using high catalyst concentrations (>3.0 mg/L), indicating aggressive oxygen consumption due to the continuous generation of reactive species.

The analysis of treated water quality reveals significant improvements in its physicochemical properties. The reduction in aromaticity indicates the breakdown of SMX aromatic rings and its intermediates, while the reduction in turbidity and color after the first 30 min suggests the progressive mineralization of degradation byproducts and the removal of colloidal aggregates. In conclusion, the photo-Fenton process not only efficiently removes SMX but also improves the quality of the treated water. The evolution of dissolved oxygen provides valuable insights into the reaction dynamics and allows for the optimization of treatment conditions to maximize the efficiency of contaminant degradation and mineralization.

This study demonstrates that the photo-Fenton process is an effective method for SMX degradation and has high potential for industrial and municipal applications. Integrating this process into existing wastewater treatment systems would help reduce antibiotic contamination and its environmental impacts. Key applications include use in pharmaceutical facilities and in hospital and laboratory effluents to prevent antibiotic accumulation in sewage systems and improve the removal of pharmaceutical residues from WWTP effluents. Since SMX is widely used in veterinary medicine, it can enter water bodies through animal waste, where the photo-Fenton process could be applied to treat wastewater from farms and animal breeding facilities. Although not a primary method for water purification, this process could be used as an advanced oxidation step to remove traces of antibiotics from drinking water sources. However, scaling up the process requires optimized reactor design, reduced operating costs, catalyst recovery strategies, and regulatory compliance.