Kinetic Modelling of Aromaticity and Colour Changes during the Degradation of Sulfamethoxazole Using Photo-Fenton Technology

Abstract

Sulfamethoxazole (SMX) is an antibiotic that is extensively used in veterinary medicine, and its occurrence in wastewater and surface water can reach up to 20 μg/L. SMX is categorized as a pollutant of emerging concern by the US EPA due to its persistence and effects on humans and the environment. In this study, photo-Fenton technology is proposed for the removal of SMX. Aqueous solutions of SMX (50.0 mg/L) are treated in a 150 W UV photoreactor, using [Fe²⁺]₀ = 0.5 mg/L and varying [H₂O₂]₀ = 0–3.0 mM. During the reaction, colour (AU) was assessed along with SMX (mg/L), turbidity (NTU), and TC (mg/L). SMX degrades to aromatic intermediates with chromophoric groups, exhibiting colour (yellow to brown) and turbidity. As these intermediates are mineralized into CO₂ and H₂O, the colour and turbidity of the water lose intensity. Using a molar ratio of 1 mol SMX:10 mol H₂O₂, the maximum degradation of aromatic species takes place (71% elimination), and colourless water with turbidity < 1 NTU is obtained. A kinetic modelling for aromaticity loss and colour formation as a function of the oxidant concentration has been proposed. The application of this model allows the estimation of oxidant amounts for an efficient removal of SMX under environmentally friendly conditions.

1. Introduction

Antibiotic residues have been a source of concern in recent years due to the environmental pollution they generate. Antibiotics, even at low concentrations, form a part of the group of emerging pollutants that cause detrimental effects on aquatic organisms [1,2] due to their toxic, carcinogenic, and mutagenic nature [3]. The overuse of antibiotics has caused increasing concern for public health and the balance of natural aquatic ecosystems [4,5]. This is because antibiotics are only partially absorbed by the human body and animals and are excreted into water and the environment through urine and faeces [6].

Recent research has found a global antibiotic consumption rate of 14.3 defined daily doses (DDDs) per 1000 people per day in 2018. Considering that this rate was 9.8 DDDs per 1000 people per day in 2000, this represents an increase of 46% [7,8]. In addition, it should be noted that, during the COVID-19 pandemic, global antibiotic use has seen a sharp spike [9]. Therefore, global antibiotic consumption is on the rise, with a 67% increase in antibiotic consumption predicted by 2030 [10]. Among the most commonly used antibiotics are sulfamethoxazole, tetracycline, ciprofloxacin, and trimethoprim, which are found in surface water and groundwater in varying concentrations. Some of their most notable characteristics are that they are toxic, poorly or non-biodegradable, and, consequently, persistent [11,12,13].

Specifically, sulfamethoxazole (SMX) is a common sulphonamide antibiotic that has been widely used for the treatment of bronchitis and other infections [14,15]. About 90% of this drug is excreted in the urine within 24 h of oral administration, which increases its toxicity in environmental matrices [16]. It is important to note that, although SMX concentrations in the environment are below those that can cause a toxic effect on humans [17], its accumulation in fish and birds can lead, even at trace levels, to the development of antibiotic resistance [16]. This would have negative consequences for humans and represent a serious threat to the ecosystems [18,19,20,21]. The use of SMX is so widespread that it has been detected in the outflow of wastewater treatment plants (WWTPs) [12]. The biological processes that are commonly used in wastewater treatment plants do not achieve an efficient removal of this type of contaminant [18,22], which is related to its recalcitrant characteristics. Therefore, there is a need to develop new technologies for the degradation of antibiotic residues in the aquatic environment that guarantee safe water [23]. In this context, advanced oxidation processes (AOPs) represent an efficient and environmentally compatible alternative to eliminate this persistent pollutant.

Advanced oxidation processes (AOPs) are based on the oxidation of organic pollutants by hydroxyl radicals (HO^●), which react rapidly and indiscriminately with various organic compounds. In this case, their main objective is to mineralize SMX into carbon dioxide and water or to transform it into by-products that are less harmful to human health and the aquatic environment. The oxidation capacity of these processes usually increases in the presence of catalysts, leading to higher levels of mineralization [24]. In this context, a typical AOP with the presence of a catalyst for SMX removal is the photo-Fenton process. This process is a variant of the Fenton process, which combines the reaction of hydrogen peroxide and Fe²⁺ under UV irradiation, achieving a more complete degradation of SMX compared to conventional treatments [25,26,27,28].

Various AOPs have been developed and applied for wastewater treatment, but the Fenton and photo-Fenton processes stand out as the most powerful, efficient, and cost-effective options. These methods are particularly effective in treating persistent pollutants, whether used alone or in combination with conventional or biological treatments [29]. The photo-Fenton process offers several key advantages over the conventional Fenton method. By utilizing light energy, it generates more hydroxyl radicals, leading to faster and more efficient pollutant degradation. It also enhances the regeneration of ferrous iron, reducing the need for additional chemicals like hydrogen peroxide and iron. Unlike the traditional Fenton process, which works best in acidic conditions, the photo-Fenton process can operate effectively at a wider pH range, including near-neutral conditions, making it more versatile and environmentally friendly. Based on this, studies reported in the literature show that the photo-Fenton process is an effective treatment both in the degradation of micropollutants and in bacterial inactivation [30], verifying that in the case of the elimination of SMX, both the degradation performance and the speed at which it is eliminated are much greater using the photo-Fenton technology than using the conventional Fenton process [31].

Several authors have proposed kinetic models to describe the degradation of antibiotics by the photo-Fenton process, with the aim of optimising the design and operation of this technology. These models include studies on kinetic constants and the influence of key operational parameters such as pH, hydrogen peroxide concentration, iron concentration, and irradiation time on contaminant degradation [32,33]. Such models are essential for predicting process behaviour under different conditions and maximising pollutant removal efficiency. However, in addition to the removal of SMX, it is important to study the different mechanisms of the reaction that takes place, taking into account the intermediates that are generated throughout the process before complete mineralisation [34,35,36]. These intermediates can contribute to the colour and turbidity of the treated water, as it has been detected in the treatment of other types of medicines [37,38,39,40].

This study aims to complement previous studies on the degradation of SMX with UV light combined with hydrogen peroxide and iron salts. Specifically, the effect of this technology on the organoleptic parameters of water quality, such as colour and turbidity, will be analysed to ensure that the treated water complies with environmental regulations. In addition, based on the results obtained and the SMX degradation mechanisms published in the literature, the theoretical pathways of SMX degradation leading to the formation of intermediate products responsible for the high turbidity and colouration that arise during the process are studied.

2. Results and Discussion

2.1. Parameters Indicating the Water Quality during the SMX Oxidation

Figure 1 shows the kinetics of several of the parameters that indicate water quality when aqueous solutions of SMX are oxidized using the photo-Fenton technology. Figure 1a shows that the photo-Fenton treatment is capable of completely degrading the SMX load present in the water in less than 15 min. As can be observed in the graph, the water’s aromaticity experiences a slight initial increase during the first 5 min of the reaction, followed by a decrease over time until reaching a stable value that persists in the treated water. This is because, in the early stages of the photo-Fenton reaction, hydroxyl radicals attack the aromatic ring of SMX, generating partially oxidized intermediates. In these early stages, the hydroxylation of SMX occurs mainly at the benzene ring and the amino group, producing several oxidized mono- and dihydroxylated intermediates, such as 3-hydroxysulfamethoxazole, 5-hydroxysulfamethoxazole, and the oxidized derivatives of isoxazole [35,41]. These intermediate by-products may have greater aromaticity or even temporarily form more aromatic structures. This increase occurs because, by breaking certain non-aromatic bonds of the compound, intermediate structures with more pronounced or additional aromatic rings are formed.

As the photo-Fenton reaction progresses, hydroxyl radicals continue to attack these aromatic intermediates, leading to the opening of the benzene rings or the complete oxidation of aromatic compounds into simpler products such as organic acids, CO₂, and water. This explains the subsequent decrease in aromaticity, as the aromatic structures are being destroyed. However, in some cases, certain resistant aromatic by-products may form during the process and are not easily degraded by hydroxyl radicals. The presence of certain substituent groups on aromatic molecules such as nitro, sulfonate, or carboxylate groups can affect the reactivity of hydroxyl radicals because they may induce inductive or resonance effects that further stabilize the aromatic structure, making radical attack less likely. These groups can also increase the electron density at certain positions on the ring, protecting the compound from oxidative attack [42]. Furthermore, during the oxidation process, oxidized intermediates are generated that have a very stable structure, such as quinones or aromatic acids that can be especially resistant [38]. These refractory residues are more complex, oxidized, and stable molecules that do not easily react under the conditions of the photo-Fenton treatment. This may be due to the inability of hydroxyl radicals to break certain highly condensed or resistant aromatic structures.

Figure 2a shows the residual aromaticity value of water samples oxidized using different concentrations of hydrogen peroxide. As the concentration of the oxidant used in the treatment increases, the degradation level of aromatic species increases, as indicated by the relationship shown in Equation (1). As confirmed, the maximum degradation of aromatic species (71% removal) is achieved by dosing an oxidant concentration of 2.0 mM, corresponding to a molar ratio of 1 mol SMX:10 mol H₂O₂.

$${\left[\mathrm{Aromaticity}\right]}_{\mathrm{final}}=- 0.0114{ [\mathrm{H}}_2{\mathrm{O}}_2{]}_0^3+0.169{ [\mathrm{H}}_2{\mathrm{O}}_2{]}_0^2-0.6344{ [\mathrm{H}}_2{\mathrm{O}}_2{]}_0+0.7407$$

(1)

$${\left[\mathrm{TC}\right]}_{\mathrm{final}}=1.23{ [\mathrm{H}}_2{\mathrm{O}}_2{]}_{0 }^2- 8.9597{ [\mathrm{H}}_2{\mathrm{O}}_2{]}_0+24.586$$

(2)

On the other hand, when analysing the mineralization of the treated water, Figure 1a shows that the concentration of total carbon (TC, mg/L) decreases as SMX degradation progresses. Figure 2b illustrates the dependence of total carbon in the water on the concentration of the oxidant used in the treatment (Equation (2)). This occurs because, when SMX is attacked by hydroxyl radicals, its C-C and C-H bonds are broken, fragmenting the molecule into simpler compounds such as organic acids, aldehydes, alcohols, and other intermediate species. During the advanced oxidation process, the intermediate products generated (fragments of the initial molecule) continue to be attacked by hydroxyl radicals. If the process is sufficiently efficient, these intermediates are further degraded, eventually transforming into CO₂, H₂O, and inorganic salts (mineralization). The conversion of organic carbon into carbon dioxide contributes to the reduction in total carbon in the solution.

As the photo-Fenton process progresses, more carbon from the initial organic molecules and their intermediate products is oxidized into CO₂, which is released into the atmosphere as a gas. This phenomenon leads to a continuous decrease in total carbon concentration in the aqueous phase. UV light regenerates the Fe²⁺ ion from Fe³⁺ and promotes the continuous production of hydroxyl radicals, which sustains the degradation of organic compounds, thereby accelerating the reduction in total carbon.

A notable phenomenon within this reaction system is that, during the oxidation of SMX, brown colour and high turbidity are generated in the water (see Figure 1b). As observed, both the colour and turbidity in the water increase immediately after the reaction begins, reaching a maximum intensity around 15–20 min into the reaction. Subsequently, they decrease over time until reaching a stable value that remains once the steady state is achieved (see Figure 3a,b). It is confirmed that both parameters are dependent on the concentration of the oxidant used in the treatment, as shown in Equations (3) and (4). Operating with an oxidant concentration of 2.0 mM, which corresponds to a molar ratio of 1 mol SMX:10 mol H₂O₂, results in colourless water with turbidity < 1 NTU, making it suitable for discharge into natural watercourses.

$${\left[\mathrm{Turbidity}\right]}_{\mathrm{final}}=- 0.2027{ [\mathrm{H}}_2{\mathrm{O}}_2{]}_{0 }^3+2.2859{ [\mathrm{H}}_2{\mathrm{O}}_2{]}_{0 }^2- 7.5725{ [\mathrm{H}}_2{\mathrm{O}}_2{]}_0+8.0725$$

(3)

$${\left[\mathrm{Colour}\right]}_{\mathrm{final}}=0.90\times \exp ( - 1.557{ [\mathrm{H}}_2{\mathrm{O}}_2{]}_0)$$

(4)

During the oxidation process, hydroxyl radicals attack the benzene ring (aromatic structure) of SMX, leading to the formation of oxidized aromatic derivatives, such as quinones or polycyclic compounds, which have chromophoric properties. These compounds absorb visible light, resulting in the appearance of colour in the solution. Sometimes, instead of fully decomposing, fragments of organic molecules can react with each other to form larger and more complex compounds, such as condensation products, which may also contain chromophoric groups. These larger intermediate products tend to be more coloured and can temporarily accumulate during SMX degradation. Another group of intermediates that may appear during degradation includes aldehydes and unsaturated organic acids. Some of these compounds may contain conjugated double bonds, which can also contribute to the appearance of colour in the solution. Additionally, SMX contains a heterocyclic ring (a structure that includes sulphur and nitrogen). Oxidation of this type of ring can generate intermediate products with complex structures that also possess chromophoric properties. As these intermediates are mineralized into CO₂ and H₂O, the colour of the water gradually decreases.

The turbidity generated during SMX oxidation is primarily due to the precipitation of iron hydroxides and the formation of colloidal particles and insoluble organic intermediate products that remain suspended in the water. Turbidity decreases at the end of the process, when the intermediates are fully degraded and the iron precipitates are either removed or dissolved. During the process, Fe²⁺ is oxidized to Fe³⁺, which can form precipitates of iron hydroxides (Fe(OH)₃). These hydroxides are solid particles that contribute to turbidity by remaining suspended as colloidal particles in the water. Furthermore, as SMX degrades, intermediates with low water solubility, such as organic acids, aldehydes, or polymeric compounds, are formed. These products can aggregate and form colloidal particles, increasing the turbidity of the solution. In some cases, intermediates can react with each other or with inorganic species present in the water, forming larger aggregates or structures that contribute to the formation of suspended particles. Additionally, products of incomplete oxidation, which have not yet been fully mineralized, can form larger or more complex substances that are less soluble in water, increasing turbidity until they are fully degraded [39].

2.2. Kinetic Modelling of the Parameters Indicating the Water Quality

2.2.1. Kinetic Modelling of Pseudo-First Order for SMX Oxidation

A pseudo-first-order kinetic model has been proposed to predict the degradation kinetics of SMX, where SMX degrades to reaction intermediates according to a kinetic constant k_SMX (1/min) (see Equation (5)).

$$\mathrm{SMX} \stackrel{{ \mathrm{k}}_{\mathrm{SMX} }}{\to } \mathrm{Reaction} \mathrm{intermediates}$$

(5)

By setting up the mass balance (Equation (6)) and integrating, the first-order kinetic equation for SMX degradation is obtained (Equation (7)), where [SMX]₀ is the initial concentration of SMX in the water (=50.0 mg/L), and t is the reaction time (min).

$$\mathrm{d}\left[\mathrm{SMX}\right] / \mathrm{dt}=-{\mathrm{k}}_{\mathrm{SMX} }\left[\mathrm{SMX}\right]$$

(6)

$$\left[\mathrm{SMX}\right]{=\left[\mathrm{SMX}\right]}_0 \times \exp (-{\mathrm{k}}_{\mathrm{SMX}} \mathrm{t})$$

(7)

Figure 4 shows the model predictions, which allow for determining the dependence of the SMX degradation rate constant on the concentration of the oxidant used in the treatment [H₂O₂]₀ in mM (Equation (8)).

Table 1. Kinetic parameters estimated for the SMX oxidation by the photo-Fenton technology. Experimental conditions: [SMX]₀ = 50.0 mg/L; [UV] = 150 W; [Fe²⁺]₀ = 0.5 mg/L; [pH] = 3.0; and [T] = 30 °C.

[H2O2]0 (mM)	kSMX (1/min)	karom (1/min)	kcolour,form (1/min)	kcolour,deg (1/min)	αcolour (-)
0.00	0.15	7.0 × 10−2	1.9 × 10−4	2.9 × 10−2	2.00
0.20	0.20	6.3 × 10−2	3.7 × 10−4	3.0 × 10−2	1.83
0.25	0.22	6.5 × 10−2	3.4 × 10−4	3.0 × 10−2	1.80
0.35	0.22	6.0 × 10−2	3.6 × 10−4	3.5 × 10−2	1.77
0.50	0.25	5.5 × 10−2	4.8 × 10−4	3.5 × 10−2	1.62
1.00	0.35	4.7 × 10−2	8.0 × 10−4	4.0 × 10−2	1.25
1.50	0.35	4.0 × 10−2	9.9 × 10−4	4.6 × 10−2	1.11
2.00	0.36	4.5 × 10−2	1.0 × 10−3	5.5 × 10−2	1.00
2.50	0.35	4.4 × 10−2	9.6 × 10−4	5.8 × 10−2	1.00
3.00	0.29	5.0 × 10−2	9.0 × 10−4	5.8 × 10−2	1.03

presents the estimated values of the rate constants.

$${\mathrm{k}}_{\mathrm{SMX} }=- 0.0627{ [\mathrm{H}}_2{\mathrm{O}}_2{]}_{0 }^2+0.2325 \left[{\mathrm{H}}_2{\mathrm{O}}_2\right]+0.1553{ (\mathrm{r}}^{2 }=0.9777)$$

(8)

2.2.2. Kinetic Modelling for Aromaticity Loss

A pseudo-first-order model has been proposed to predict the kinetics of aromaticity loss in aqueous SMX solutions oxidized by a photo-Fenton treatment. It is important to note that, to simplify the model, the initial increase in aromaticity observed in the water during the first 5 min of the reaction has been neglected, considering only the phase corresponding to the loss of aromaticity. Based on this approach, a mass balance has been proposed, where the initial aromaticity of the water containing 50.0 mg/L of SMX decreases as the aromatic functional groups are broken down by the action of hydroxyl radicals, according to a pseudo-first-order rate constant k_arom (1/min) (see Equation (9)).

$$\mathrm{SMX} \stackrel{{ \mathrm{k}}_{\mathrm{arom} }}{\to } \mathrm{non-aromatic} \mathrm{intermediates}$$

(9)

The mass balance proposed in Equation (10), corresponding to the decrease in aromaticity following first-order kinetics, has been corrected with the term [Aromaticity]_final (AU) (estimated in Equation (1)). This adjustment is necessary because, given the characteristics of the oxidation treatment, the dosed hydrogen peroxide is consumed without fully eliminating the aromatic load. As a result, a refractory aromatic residue remains in the treated water, and its value must be accounted for in the mass balance to improve the accuracy of the model prediction.

$$\mathrm{d}\left[\mathrm{Aromaticity}\right] / \mathrm{dt}=-{ \mathrm{k}}_{\mathrm{arom} }(\left[\mathrm{Aromaticity}\right]-{ \left[\mathrm{Aromaticity}\right]}_{\mathrm{final}} )$$

(10)

By integrating the mass balance, the kinetic equation for the loss of aromaticity in aqueous SMX solutions oxidized by a photo-Fenton treatment is obtained (Equation (11)). A mean value for the initial aromaticity of water containing 50.0 mg/L of SMX can be considered as [Aromaticity]₀ = 1.958 AU.

$$\left[\mathrm{Aromaticity}\right]{=\left(\right[\mathrm{Aromaticity}]}_0-{ \left[\mathrm{Aromaticity}\right]}_0) \times \exp (-{\mathrm{k}}_{\mathrm{arom}}{ \mathrm{t})+[\mathrm{Aromaticity}]}_{\mathrm{final}}$$

(11)

Figure 5 shows the predictions of the proposed model. Based on the experiments conducted, the rate constant for the loss of aromaticity in the water has been estimated as a function of the initial oxidant concentration dosed in the treatment (Equation (12)). Table 1 presents the estimated values of the rate constants.

$${\mathrm{k}}_{\mathrm{arom} }=- 0{.0079 [\mathrm{H}}_2{\mathrm{O}}_2{]}_{0 }^2- 0.0301 {\left[{\mathrm{H}}_2{\mathrm{O}}_2\right]}_0+0.0696{ (\mathrm{r}}^2=0.9679)$$

(12)

2.2.3. Kinetic Modelling for Colour Changes

A series reaction kinetic model has been proposed for the colour changes observed in oxidized water, where SMX is oxidized to coloured intermediates according to a pseudo-first-order kinetic constant k_colour,form (1/min). Next, the intermediates of coloured nature degrade to colourless species according to a pseudo-first-order kinetic constant k_colour,deg (1/min) (Equation (13)).

$$\mathrm{SMX} \stackrel{ \mathrm{k} \mathrm{colour}, \mathrm{form} }{\to } \mathrm{Coloured} \mathrm{intermediates} \stackrel{ \mathrm{k} \mathrm{colour},\mathrm{deg} }{\to } \mathrm{Colourless} \mathrm{species}$$

(13)

Equation (14) shows the proposed mass balance, considering that the colour of the water would maintain a behaviour similar to a reaction intermediate. Since the mass balance uses concentration units for SMX concentration and absorbance units for colour, the term α_colour (AU L/mg) has been introduced to homogenize units. Furthermore, because in several tests the oxidant used in the treatment is consumed without completely degrading into coloured species, leaving a refractory-coloured residue in the treated water, a correction has been made in the mass balance, introducing the term [Colour]_final (AU) estimated in Equation (4).

$$\mathrm{d}\left[\mathrm{Colour}\right] /{ \mathrm{dt}=\mathrm{k}}_{\mathrm{colour},\mathrm{form}} \times { \mathsf{\alpha }}_{\mathrm{colour} } \times \left[\mathrm{SMX}\right]-{ \mathrm{k}}_{\mathrm{colour},\mathrm{deg}} \times \left( \left[\mathrm{Colour}\right]-{\left[\mathrm{Colour}\right]}_{\mathrm{final}} \right)$$

(14)

Integrating the mass balance, the kinetic equation for the colour changes in the aqueous solutions containing SMX oxidized with the photo-to-Fenton technology is obtained (Equation (15)).

$$\left[\mathrm{Colour}\right]{=\left[\mathrm{Colour}\right]}_{\mathrm{final}}+\left[{\displaystyle \frac{{\mathrm{k}}_{\mathrm{colour},\mathrm{form}}{\left[\mathrm{SMX}\right]}_0}{{\mathrm{k}}_{\mathrm{colour},\mathrm{deg} }-{ \mathrm{k}}_{\mathrm{SMX}}}}\right] \times \left[{ \mathsf{\alpha }}_{\mathrm{colour}} \times { \exp (-\mathrm{k}}_{\mathrm{SMX}} \mathrm{t}) -{ \exp (-\mathrm{k}}_{\mathrm{colour},\mathrm{deg}} \mathrm{t})\right]$$

(15)

$${\mathrm{k}}_{\mathrm{colour},\mathrm{form} }=- 0.0002{ [\mathrm{H}}_2{\mathrm{O}}_2{]}_{0 }^2+0.0008 {\left[{\mathrm{H}}_2{\mathrm{O}}_2\right]}_0+0.0001{ (\mathrm{r}}^2=0.9889)$$

(16)

$${\mathrm{k}}_{\mathrm{colour},\mathrm{deg} }=- 0.0017{ [\mathrm{H}}_2{\mathrm{O}}_2{]}_{0 }^2+0.0154 {\left[{\mathrm{H}}_2{\mathrm{O}}_2\right]}_0+0.028{ (\mathrm{r}}^{2 }=0.9744)$$

(17)

$${\mathsf{\alpha }}_{\mathrm{colour}}{=\mathsf{\lambda }}_{455}/{\mathsf{\lambda }}_{260}=0.2098 {\left[{\mathrm{H}}_2{\mathrm{O}}_2\right]}_{0 }^2- 0.9689 {\left[{\mathrm{H}}_2{\mathrm{O}}_2\right]}_0+2.0639 ({\mathrm{r}}^2=0.9932)$$

(18)

The predictions of the proposed model are shown in Figure 6. From the tests carried out, the kinetic constants for the formation and degradation of the colour of the water have been estimated, as well as the correlation parameter of units, depending on the concentration of oxidant used in the treatment (see Equations (16)–(18)). The estimated parameter values are shown in Table 1.

3. Materials and Methods

The oxidation assays by a photo-Fenton treatment were conducted in a photo-catalytic reactor of 1.0 L oxidizing aqueous solutions of sulphamethoxazole [SMX]₀ = 50.0 mg·L⁻¹ (Fragon, Rotterdam, The Netherlands, C₁₀H₁₁N₃O₃S 100.6%) by a 150 W medium-pressure mercury lamp (Heraeus, Hanau, Germany, 85.8 V, 148.8 W, 1.79 A, 95% transmission between 300 and 570 nm) combined with hydrogen peroxide [H₂O₂]₀ = 0–3.0 mM (Panreac, Castellar del Vallès, Spain, H₂O₂ 30% w/v) and ferrous ion dosage [Fe²⁺]₀ = 5.0 mg L⁻¹ (Panreac, FeSO₄·7H₂O 99.0%). The reactor, operating in batch mode, was stirred using a magnetic stirrer. All reactions were performed at pH = 3.0 (using a pH-meter Kent EIL9142, Cambridge, UK) and the temperature was kept at around 30 °C using a heating bath (Frigiterm-P Selecta, Barcelona, Spain).

During 120 min of reaction, the parameters that were measured are as follows: the aromaticity of the water at λ = 254 nm ([Aromaticity], AU) and colour at λ = 455 nm ([Colour], AU) using a UV/Vis Spectrophotometer (Model V-630, Jasco, Madrid, Spain) [39]; the water turbidity ([Turbidity], NTU) with a nephelometric turbidimeter (Model HI88703, Hanna Instruments S.L., Eibar, Spain); the dissolved oxygen ([DO], mg/L) with a dissolved oxygen-meter (Model HI 9142, Hanna Instruments S.L., Eibar, Spain), and the total carbon ([TC], mg/L) using a TOC meter (Shimadzu TOC-V, Shimadzu Corporation, Kyoto, Japan).

The SMX concentration was determined by 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 ZORBAX Eclipse PAH guard column (4.6 mm, 12.5 mm) supplied by Agilent (Santa Clara, CA, USA) were used. The mobile phase consisted of water and acetonitrile (ACN), with a flow rate of 0.8 mL/min. Initial gradient conditions were set at 20% ACN, maintained for 1 min, then increased to 50% v/v ACN over 7 min, and finally returned to 20% v/v ACN in 1 min. Total run time was 9 min. The injection volume was 50 μL, and all separations were performed at room temperature. DCF identification was performed by comparison with a standard. Detection was carried out at 275 nm.

4. Conclusions

The photo-Fenton treatment is capable of completely degrading the SMX load contained in the water. During oxidation, the aromaticity of water experiences a slight initial increase because partially oxidized intermediates are generated that contain greater aromaticity or that can even generate more aromatic structures. In addition, it is observed that a brown colour is being generated in the water. The colour is caused by some of the aromatic intermediates, such as quinones or polycyclic compounds, which are chromophoric in nature. It can also be caused by fragments of organic molecules reacting with each other forming condensation products. On the other hand, it must be considered that SMX contains a heterocyclic ring (containing sulphur and nitrogen) that can generate intermediate products with chromophoric properties. Simultaneously with the colour, turbidity is also generated in the water, which can be caused by the precipitation of iron hydroxides and the formation of colloidal particles and insoluble organic intermediates, which remain suspended in the water. As the photo-Fenton reaction proceeds, hydroxyl radicals continue to attack the aromatic intermediates, causing the opening of the benzene rings or the complete oxidation of the aromatic compounds to simpler products. As these intermediates are mineralized into CO₂ and H₂O, the colour and turbidity of the water lose intensity. It is verified that, using a molar ratio of 1 mol SMX:10 mol H₂O₂, the maximum degradation of aromatic species takes place (71% elimination), and colourless water with turbidity < 1 NTU is obtained, which enables them to be discharged into natural channels. A kinetic modelling of colour formation as a function of the oxidant concentration has been proposed, following a series reaction model. The application of this model allows the estimation of oxidant amounts for an efficient removal of SMX under environmentally friendly conditions.