Effect of Solar Irradiation on the Electrooxidation of a Dye Present in Aqueous Solution and in Real River Water

Abstract

This study investigates the performance of an electrooxidation (EO) process employing Sb₂O₅-doped RuO₂–ZrO₂|Ti anodes integrated into a concave-cover solar still for the degradation of Allura Red dye in aqueous solution and real river water. The anode was synthesized and characterized via scanning electron microscopy (SEM) and X-ray diffraction (XRD) to confirm its porous morphology and crystalline structure. Operational parameters—including supporting electrolyte concentration, initial solution pH, and current density—were systematically optimized. Under optimal conditions (pH 2–3 and 5 mA cm⁻²), the EO process was evaluated under natural solar irradiation. Sunlight exposure increased the solution temperature from approximately 20 °C to 50 °C, enhancing molecular diffusion and mass transport, thereby accelerating decolorization kinetics. Compared to EO performed under laboratory conditions, the solar-assisted system achieved an additional 20% increase in chemical oxygen demand (COD) removal and a fast reduction in color. When applied to real Lerma River water samples under these optimal conditions, the treatment achieved approximately 50% reduction in both COD and true color, demonstrating its applicability to complex environmental matrices. These results confirm that coupling electrooxidation with solar thermal input significantly improves pollutant degradation efficiency and energy performance, establishing this integrated approach as a promising and sustainable technology for advanced wastewater treatment.

1. Introduction

Electrochemical advanced oxidation processes (EAOPs) are new and powerful technologies currently used to abate recalcitrant organic contaminants in wastewater. They rely on the in situ generation of highly reactive oxidants, principally hydroxyl radicals (•OH), in chloride-containing matrices in the presence of active chlorine species such as HOCl/OCl⁻. These oxidants can mineralize pollutants to CO₂, H₂O, thereby overcoming the limited biodegradability of complex molecules such as synthetic dyes and pharmaceuticals [1,2,3].

Electrochemical oxidation proceeds via two principal pathways: direct oxidation at the anode and indirect oxidation by electrogenerated mediators, which takes place in solution.

In direct oxidation, organics are oxidized at the electrode surface either by outer-surface electron transfer or by surface-bound hydroxyl species—commonly described as physisorbed •OH (•OHads) on “non-active” anodes (e.g., boron-doped diamond, BDD) and chemisorbed oxo/lattice oxygen species (MO_x+1) on “active” anodes (e.g., RuO₂, IrO₂). In indirect oxidation, oxidants—predominantly active chlorine formed from chloride—are generated in situ (Equations (1)–(3)). Chlorine species have a lower standard potential than hydroxy radical (1.36, 1.49 and 0.89 V for Cl₂, HClO and ClO⁻, respectively), but they are more selective and can react rapidly with specific electron-rich groups in contaminants. The predominant species are pH dependent: Cl₂ at pH < 3, HClO between 3 and 8, and ClO⁻ at pH > 8 [2,4,5].

$${2\mathrm{C}\mathrm{l}}^{-}\to {\mathrm{C}\mathrm{l}}_2+2\mathrm{e}$$

(1)

$${\mathrm{C}\mathrm{l}}_2+{\mathrm{H}}_2\mathrm{O}\to \mathrm{H}\mathrm{C}\mathrm{l}\mathrm{O}+{\mathrm{H}}^{+}+{\mathrm{C}\mathrm{l}}^{-}$$

(2)

$$\mathrm{H}\mathrm{C}\mathrm{l}\mathrm{O}\leftrightarrow {\mathrm{H}}^{+}+{\mathrm{C}\mathrm{l}\mathrm{O}}^{-}$$

(3)

Two canonical classes of anodes are recognized in EAOPs: active and nonactive. Active anodes—such as graphite, IrO₂/Ti, RuO₂/Ti, and Pt/Ti—exhibit strong interaction with reaction intermediates; surface •OH is rapidly converted into chemisorbed “active oxygen” (MO_x+1), and lower-potential inorganic mediators are formed, so oxidation tends to be selective/partial.

In contrast, nonactive anodes—including PbO₂/Ti, SnO₂/Ti, and boron-doped diamond (BDD)—possess a high oxygen-evolution overpotential, enabling sustained generation of weakly bound physisorbed hydroxyl radicals (•OHads) that promote extensive mineralization to CO₂. Among nonactive materials, BDD routinely affords the highest mineralization efficiencies, albeit at substantially higher capital cost [5,6,7].

Alternatively, mixed-metal-oxide (MMO/MO_x) coatings—exemplified by RuO₂-based layers deposited on Ti—are widely employed as dimensionally stable anodes (DSAs) owing to their high catalytic activity, large electrochemically active surface area, and excellent corrosion resistance. Their predominant reaction depends on the electrolyte: in chloride-free media they favor the oxygen evolution reaction (OER), whereas in chloride-containing electrolytes they catalyze the chlorine evolution reaction (CER). Incorporation of dopants such as Sb₂O₅ can enhance electronic conductivity and operational stability, thereby extending service life, while addition of ZrO₂—despite its inert, poorly conductive character—improves corrosion resistance and thermo-chemical robustness, markedly increasing dimensionally stable anode (DSA) durability under OER conditions [1,8,9,10].

Recent investigations into the degradation of organic pollutants using DSA electrodes have reported moderate removal efficiencies. For instance, methylene blue achieved up to 73% COD removal [5], whereas tartrazine exhibited only a 71% reduction in color [11], and Red 23 showed a 25% decrease in total organic carbon (TOC) [12]. In the case of phenol, COD removal reached approximately 40% [1], while olive oil wastewater achieved about 60% COD reduction [13]. These findings highlight the inherent recalcitrance of such contaminants, which are not readily mineralized under standard electrochemical conditions and thus pose a persistent environmental risk when discharged into riverine systems without adequate treatment.

Solar energy, recognized as a clean and renewable source, drives most natural processes, and can be applied to water purification through solar stills, which artificially replicate the hydrological cycle by inducing water evaporation and subsequent condensation, thereby enabling the separation of dissolved salts and the removal of microorganisms [14,15,16,17].

In Mexico, average daily solar radiation ranges from 4.4 to 6.3 kWh m⁻²·day. In Toluca, the annual average is about 5.4 kWh m⁻²·day, equivalent to 500–600 W m⁻² during effective sunshine hours, with peaks of 900–1100 W m⁻² at midday. The UVA radiation averages of 1.7–2.3 mW cm⁻², with daily maxima of 5.4–6.4 mW cm⁻² [18,19].

This study evaluates the effectiveness of the electrochemical oxidation (EO) using a RuO₂–ZrO₂/Ti anode doped with Sb₂O₅, coupled to a solar distiller (solar still), as a strategy for degrading the azo dye Allura Red (AR) in aqueous solution. Allura Red is widely used in foods, cosmetics, textiles, and other consumer products; however, it exhibits mutagenic, genotoxic, and carcinogenic effects, making it an emerging contaminant of concern. Its presence in wastewater, surface water, and groundwater—primarily originating from industrial effluents and landfill leachates—poses significant risks to aquatic ecosystems. Reported effects include interference with algal photosynthesis, reductions in dissolved oxygen levels, and chronic toxicity in aquatic organisms, such as oxidative stress and tissue damage [20]. Due to its azo structure and the presence of sulfonic groups, its oxidative degradation is particularly challenging, and AR has been shown to display high resistance to light, temperature, and environmental conditions, with degradation times ranging from 48 to 100 h under UV irradiation [21,22].

The Lerma River is one of the most important hydrological systems in Mexico; however, its water quality has progressively deteriorated over recent decades due to continuous discharges from municipal, industrial, and agricultural sources. Once the optimal electrooxidation operating conditions were established under controlled laboratory tests, the process was applied to real river water samples to evaluate its treatment efficiency under natural conditions. This experiment allowed the assessment of the system’s performance in the presence of the complex matrix and background pollutants characteristic of the Lerma River.

Custom electrodes were fabricated and integrated into the solar distillation system [23]. Process performance was studied, considering current density, NaCl concentration (as the supporting electrolyte), initial pH, and temperature. Under optimized conditions, specific energy consumption (SEC) and chemical oxygen demand (COD) removal were quantified as the principal performance metrics.

The main aim of this work is to demonstrate that coupling a mixed-metal-oxide (RuO₂–ZrO₂–Sb₂O₅/Ti) dimensionally stable anode with a solar unit can significantly enhance contaminant removal efficiency. Although conventional DSA-based electrooxidation typically achieves only moderate degradation of recalcitrant pollutants, the synergistic integration of electrochemical oxidation with solar-assisted heating and irradiation has not been thoroughly investigated. This hybrid configuration improves reaction kinetics through the combined effects of elevated temperature and photonic activation under solar irradiation, thereby reducing energy consumption and positioning this approach as a promising and sustainable environmental treatment technology.

2. Results and Discussion

2.1. Anode Surface Characterization and Structural Analysis

The surface morphology of the Sb₂O₅-doped RuO₂–ZrO₂|Ti electrode is shown in Figure 1, which includes two scanning electron microscopy (SEM) images at magnifications of 1000× and 3000×. The micrographs reveal a highly porous cracked surface, which significantly enhances the electrode’s active surface area. This morphological structure is attributed to the presence of ZrO₂, which acts as a dispersing agent, and Sb₂O₅, which functions as a dopant within the mixed metal oxide matrix.

Electrodes with this type of morphology typically exhibit two distinct surface characteristics: (i) an internal surface composed of interconnected pores and mud-crack structures, and (ii) an external, relatively flat surface that maintains direct contact with the electrolyte [12]. Energy-dispersive X-ray spectroscopy (EDS) analysis of the outer active layer confirmed the presence of oxygen (73.2 wt%), titanium (1.2 wt%), antimony (1.3 wt%), zirconium (15 wt%), and ruthenium (9.3 wt%). Previous characterization of electrodes produced in our group has demonstrated that the used procedure results in a high degree of dispersity [24].

Figure 2 shows the X-ray diffraction (XRD) pattern of the Sb₂O₅-doped RuO₂-ZrO₂|Ti electrode. The prominent diffraction peaks at 2θ ≈ 28° and 36° correspond to the rutile-type tetragonal phase of RuO₂ (space group: P4₂/mnm), indicating its successful incorporation into the oxide layer. Additional reflections at 2θ ≈ 31°, 39°, and 41° are attributed to tetragonal ZrO₂ (P4₂/mnm) and hexagonal Ti (P6₃/mmc), confirming the presence of these crystalline phases. Notably, no distinct peaks related to Sb₂O₃ were observed, likely due to its low molar concentration and high dispersion within the oxide matrix. The identification of these diffraction features aligns with previously reported data by Palma-Goyes [10], supporting the structural integrity and multiphase composition of the electrode coating.

2.2. Electrochemical Anode Characterization

The cyclic voltammetry of the Sb₂O₅-doped RuO₂–ZrO₂|Ti electrode used as anode in our experiments showed the signature of chloride oxidation after 1.25 V, indicating that active chloride species are generated; therefore, the reaction rate for Cl₂-active formation is high with this ternary oxide (Figure 3a). On the other side, the EIS showed after fitting analysis using a [R(RQ)(RQ] circuit gave a value of Rp of 448 Ω (Figure 3b). Both experiments exhibit characteristics similar to those of the previously reported anodes prepared by the group.

Estimation of the electrochemically active surface area (ECSA) using EIS cannot be obtained from the fitting of experimental EIS spectra. As previously reported, complex interfacial processes occur alongside double-layer formation, attributed to the large polycrystalline Ti plates relative to typical electrode sizes and to the mixture of co-deposited phases (e.g., RuO₂, ZrO₂, Sb₂O₅) with different capacitive and charge-transfer properties [24].

2.3. pH Effect During Dye Degradation

The influence of initial pH on the electrocatalytic degradation of Allura Red (AR) dye is presented in Figure 4. As shown in Figure 4a, color removal efficiencies after 15 min of electrolysis were 98.7%, 96.9%, 96.7%, and 98.6% at initial pH ranges of 2–3, 5–6, 7–8, and 9–10, respectively. Beyond the 15 min mark, only minimal improvements in decolorization efficiency were observed up to 60 min. Figure 4b shows the UV-Vis spectra of AR before and after 15 min of electrolysis at different pH levels. A notable absorption peak at approximately 316 nm corresponds to the π → π* transition of the azo group conjugated with aromatic rings. The primary absorption maximum at 505 nm, characteristic of the azo chromophore, decreases significantly during treatment, indicating the cleavage of the –N=N– bond.

Figure 4c depicts the evolution of pH over time. At pH < 3, the system becomes enriched in H⁺ ions, promoting the generation of free chlorine (Cl₂) via chloride hydrolysis. In the pH range 3–8, hypochlorous acid (HOCl) predominates, functioning as a strong oxidant that degrades chromophores and organic compounds. Furthermore, in dimensionally stable anode (DSA) systems, pH plays a pivotal role in the oxygen evolution reaction (OER). Under acidic conditions, the generation of adsorbed hydroxyl radicals (•OH_ads) is favored over O₂ formation, enhancing oxidation efficiency [2].

2.4. Current Density Effect During Dye Degradation

Current density is a critical experimental parameter that significantly influences the performance of the electro-oxidation (EO) process, as the generation of hydroxyl radicals (OH) largely depends on the amount of electrical energy delivered to the anode surface [25]. To investigate the influence of current density on dye effluent oxidation, the electrocatalytic degradation was carried out with current densities ranging from 5 to 13.33 mA cm⁻² at 1 L of dye aqueous solution containing 100 mg L⁻¹ of RA dye.

To evaluate its effect on dye degradation, electrochemical experiments were conducted using current densities ranging from 5.00 to 13.33 mA cm⁻² in a 1 L aqueous solution containing 100 mg L⁻¹ of Allura Red (AR) dye.

Figure 5a presents the color removal efficiencies achieved under different current densities (5.00, 6.67, 10.00, and 13.33 mA cm⁻²). Remarkably, color removal efficiencies exceeding 98% were obtained after only 15 min of electrolysis across all current densities tested. As shown in Figure 5b,c, moderate current densities (10.00 and 13.33 mA cm⁻²) led to more rapid degradation kinetics during the initial phase of the process. However, prolonged electrolysis at higher current densities may enhance the formation of undesirable oxidation by-products. Therefore, operating at lower current densities appears to be more suitable for maintaining high efficiency while minimizing secondary contamination in this EO system.

2.5. NaCl Concentration Effect During Dye Degradation

In this study, sodium chloride (NaCl) was employed as the supporting electrolyte. The generation rate of oxidizing species such as chlorine gas (Cl₂) and hypochlorite ions (ClO⁻) is closely linked to NaCl concentration. To assess this effect, the influence of NaCl concentration on color removal efficiency was evaluated using three electrolyte concentrations: 0.05, 0.10, and 0.20 mol L⁻¹. The experiments were conducted with an initial dye concentration of 100 mg L⁻¹, an initial pH of 2–3, and an applied current density of 10 mA cm⁻².

As shown in Figure 6a, higher NaCl concentrations accelerated the color removal rate during the first 5 min of electrolysis. However, after 10 min, the removal efficiencies converged, with all conditions achieving values above 98%. Notably, the 0.05 M NaCl condition was sufficient to promote the generation of oxidizing species, ensuring efficient dye degradation without detectable by-product formation.

2.6. Optimal Conditions Under Ambient Temperature for Dye Degradation

According to the previous evaluations, the optimal operating conditions for the degradation of a 100 mg L⁻¹ Allura Red dye solution were determined to be: an initial pH of 2–3, a supporting electrolyte concentration of 0.05 M NaCl, and an applied current density of 5 mA cm⁻². Under these conditions, a color removal efficiency of 99% and a chemical oxygen demand (COD) reduction of 71% were achieved after 60 min of electrolysis (Figure 7b,c). Figure 7a present the UV-Vis absorption spectra recorded at different time intervals throughout the electrolysis process. A 99% decrease in dye concentration was observed within the first 15 min, corresponding to a substantial reduction in the absorbance peak at 505 nm. Additionally, in the wavelength regions of 220–300 nm and 300–400 nm, a progressive decrease in absorbance was observed, indicating the stepwise degradation of intermediate by-products over 60 min.

The high removal efficiency obtained in this study—99% decolorization and 71% COD reduction under optimal EO conditions—closely aligns with previous research on electrochemical oxidation using dimensionally stable anodes (DSA). For example, Abilaji S et al. (2023) reported COD removal values of 60–85% for azo dye wastewater using RuO₂-Ti anodes at comparable current densities and NaCl concentrations [26]. Similarly, Montenegro-Ayo et al. (2023) demonstrated that low to moderate current densities (5–10 mA cm⁻²) enhance the formation of active chlorine species and hydroxyl radicals (•OH_ads), resulting in efficient degradation of organic pollutants while minimizing energy use and by-product formation [27].

Mechanistically, the electrooxidation process involves direct and indirect pathways. On the anode surface, adsorbed hydroxyl radicals are generated via water oxidation:

$${\mathrm{H}}_2\mathrm{O}\to \cdot \hspace{0.17em}\mathrm{O}{\mathrm{H}}_{\mathrm{ads}}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$

(4)

These •OH_ads species act as potent oxidants and can directly degrade dye molecules. Simultaneously, chloride ions in the electrolyte are electrochemically oxidized [28,29]:

$$2\hspace{0.17em}{\mathrm{C}\mathrm{l}}^{-}\to {\mathrm{C}\mathrm{l}}_2+2{\mathrm{e}}^{-}{\mathrm{C}\mathrm{l}}_2+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{H}\mathrm{O}\mathrm{C}\mathrm{l}+{\mathrm{H}}^{+}+{\mathrm{C}\mathrm{l}}^{-}$$

(5)

The resulting free chlorine species (Cl₂, HOCl, OCl⁻) further contribute to pollutant breakdown via indirect oxidation. This dual mechanism enables fast decolorization and partial mineralization, as observed in the progressive reduction of UV-Vis absorbance and COD values. These findings confirm that electrooxidation with Sb₂O₅-doped RuO₂-ZrO₂|Ti is a highly effective strategy for the treatment of dye-contaminated water [30,31].

2.7. Effect of Solar Radiation on Dye Effluent Degradation

To assess the influence of solar radiation on the electrochemical oxidation of Allura Red dye, experiments were conducted under previously optimized conditions. The tests were performed between 8:00 and 11:00 a.m., during which the incident solar irradiance ranged from 900 to 1100 W m⁻², and the UV (UVA and UVB) intensity was between 7 and 9 mW cm⁻². During the first hour, the water temperature increased gradually, reaching 50 °C, at which point electrolysis was initiated and maintained at 45–55 °C.

Noticeable changes were observed within the first 5 min of the process, characterized by a rapid decolorization of the solution. After 15 min, the treated water became completely transparent. For comparison, an additional experiment was conducted under identical thermal conditions using a heating plate (under dark conditions, without solar radiation). This test exhibited a similar decolorization trend to that obtained under solar radiation, as shown in Figure 8a. However, some differences were evident during the first 15 min of electrolysis when compared with the process carried out at ambient temperature.

Furthermore, the chemical oxygen demand (COD) removal efficiency was evaluated for the three systems: ambient temperature (AT), heating plate (HP), and solar radiation (SR), yielding values of 71%, 77.6%, and 87.4%, respectively (Figure 8b). These differences in COD removal are also reflected in the UV-Vis spectra of each process (Figure 8d), particularly in the wavelength range of 200–300 nm. Likewise, Figure 8c shows the energy consumption, where the systems operated at higher temperatures (HP and SR) exhibit lower energy demand.

In aqueous dye systems exposed to light or oxidative treatment, an increase in temperature from ~20 °C toward 35–50 °C can accelerate decolorization kinetics via several interrelated effects. Elevated temperature enhances molecular mobility and diffusion rates, reducing mass-transfer limitations to the catalytic or reactive surfaces. It also increases the collision frequency of dye molecules with reactive species (e.g., •OH, radicals) and facilitates desorption of intermediates from electrode surfaces, thus maintaining active sites [32,33].

Increasing temperature can improve charge-carrier mobility and interfacial charge transfer, thereby lowering recombination rates and improving oxidant generation efficiency (e.g., •OH formation). However, above an optimal threshold, higher temperature may reduce the lifetime or solubility of reactive radicals, or promote non-productive radical recombination, thus limiting further gains. For example, Chen et al. (2021) observed that increasing the temperature up to ~55 °C improved photodegradation rates of organic pollutants over Ag/TiO₂, but beyond that, performance declined due to radical instability and thermal desorption effects [34].

Groeneveld et al. (2023) reviewed that for many nanoparticle systems, moderate temperature rises increase dye photodegradation rates, though for some morphologies (e.g., nanofibers) the effect may reverse at higher temperatures [35].

These findings suggest that a moderate temperature elevation (20 → 35–50 °C) can favor decolorization in dye-containing aqueous systems. However, this must be balanced against possible radical decay or non-productive side reactions at higher temperatures.

Exposing the dye-containing aqueous solution to solar irradiation induces three synergistic effects:

(a)

Thermal enhancement, whereby the gradual rise in solution temperature (from ambient to ~50 °C) improves molecular motion and diffusion rates. This reduces external mass-transfer limitations and facilitates more effective interaction between dye molecules and reactive sites at the electrode surface, enhancing COD removal from 71% to 77.6%.

(b)

Photo activation involves UV and near-UV solar radiation which stimulates direct photolysis of dye chromophores and promotes the generation of reactive oxygen species (ROS), such as hydroxyl radicals (•OH), via photochemical reactions in water or through excitation of surface-bound species. Even in the absence of photocatalysts, dyes such as azo compounds may undergo bond cleavage upon UV exposure, particularly in the presence of auxiliary oxidants or chloride ions that yield secondary oxidants (e.g., Cl•, Cl₂•⁻, HOCl). This results in a significantly improved degradation pathway, increasing COD removal to 87.4% and making the treated water visually transparent [36,37]. The combined effect of thermally enhanced diffusion and photo-induced oxidation mechanisms highlights the advantages of using natural solar energy for water treatment applications.

(c)

The architecture of the Ti|RuO₂–ZrO₂–Sb₂O₅ anode enables solar irradiation to promote the generation and extraction of useful carriers, thereby enhancing the formation of reactive oxidizing species and improving pollutant degradation efficiency. The inclusion of ZrO₂ and Sb₂O₅ increases the dispersion of the active layer, decreases crystallite size, and improves coating morphology, resulting in a larger electroactive area [24]. Although RuO₂ is not a classical semiconductor capable of generating electron–hole pairs from direct band-to-band excitation, under illumination it acts as an efficient conductive co-catalyst. RuO₂ facilitates the extraction and transport of photogenerated carriers arising from interfacial or electrolyte-induced photo-processes, thus minimizing recombination and enhancing the overall electro-photocatalytic activity [38].

2.8. Evaluation of Electrooxidation Coupled to Solar Radiation on Real Wastewater Degradation

The electro-oxidation process assisted by solar radiation was evaluated for pollutant degradation of real wastewater under the previously optimized conditions established for the synthetic dyed effluent (LRP1: pH 2–3, 0.05 M NaCl, j = 5 mA cm⁻²). Additionally, the process was assessed and compared using the original properties of the wastewater (LRP2: pH 7–8, j = 5 mA cm⁻²) and without the addition of a supporting electrolyte (LRP3: pH 2–3, j = 5 mA cm⁻²). Figure 9 presents the UV-Vis spectra obtained at the end of the process (60 min). In comparison with the initial wastewater, the system operated without the supporting electrolyte and at an initial pH of 2–3 exhibited superior degradation performance. The evaluated water quality parameters are summarized in

Table 1. Water quality results.

Sample	% COD Removal	pH	Color (Pt-Co Scale)
Lerma River	-	7.23	424
LRP1 (pH 2–3, 0.05 NaCl, 5 mA cm−2)	23%	7.3	200
LRP2 (pH 7–8, 5 mA cm−2)	15%	7.85	183
LRP3 (pH 2–3, 5 mA cm−2)	47%	7.5	102

.

The evaluated sample exhibited a conductivity of 507 µS cm⁻¹. In the Lerma River, chloride is part of the natural ionic composition, with an average concentration of 1.974 meq L⁻¹ (≈70 mg L⁻¹) [39]. When NaCl is added, the chloride concentration increases, shifting the redox chemistry toward greater accumulation of HOCl/Cl₂ and resulting in decreased electrochemical efficiency, with COD removal reduced to 23% [40]. Conversely, when operating at the natural conductivity (without added NaCl), moderate Cl⁻ levels promote the formation of reactive chlorine species (RCS), enhancing degradation rates for specific organic contaminants [41].

Beyond chloride, the Lerma River contains an ionic mixture including Ca²⁺, Mg²⁺, Na⁺, K⁺, CO₃²⁻, HCO₃⁻, and SO₄²⁻, with HCO₃⁻ as the dominant anion. These species can act as radical scavengers, reducing the contribution of ·OH and other reactive oxidant species, thereby shifting degradation pathways. They may also inhibit or alter active-chlorine formation mechanisms on DSA-type anodes [42,43].

3. Materials and Methods

3.1. Chemicals

Hydrogen chloride (HCl, CAS:7647-01-0, analytical grade), sodium chloride (NaCl, CAS: 7647-14-5, analytical grade), and Allura Red Ac dye content 80% (CAS: 25956-17-6) were obtained from Sigma-Aldrich (St. Louis, MO 63178, USA). Distilled water was used to prepare all synthetic solutions.

3.2. Anode Preparation, Characterization and Morphological Studies

The mixed-oxide catalytic layer (Sb₂O₅-doped RuO₂–ZrO₂) was synthesized in our laboratory and deposited onto a perforated circular Ti substrate using the Pechini method, as detailed in reference [24]. The resulting electrocatalytic coating was homogeneously deposited and subsequently characterized as described below. The surface morphology and elemental composition and Ti electrodes were characterized using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS). Imaging and microanalytical measurements were conducted with a JEOL JSM-6510LV (3-1-2 Musashino, Akishima, Tokyo 196-8558, JAPAN) scanning electron microscope operated at an accelerating voltage of 15 kV, equipped with a Bruker XFlash 6|10 EDX detector. XRD analyses were performed using a Panalytical X’Pert Pro (Malvern Panalytical B.V., Lelyweg 17602 EA Almelo, The Netherlands) diffractometer configured for GIXRD, with a step size of 0.02°. The measurements were carried out over a 2θ range of 20–80°, using Cu Kα radiation (λ = 1.5406 Å).

3.3. Experimental Setup

The electrolysis of the aqueous solution with Allura red dye (at 100 mg L⁻¹) was performed in a solar still with a volume of 1 L using two electrodes, Sb₂O₅-doped RuO₂-ZrO₂|Ti (Centro Mexicano para la Producción más Limpia IPN, Ciudad de Mexico, Mexico) as anode and Ti (TEPSA, México) as cathode. Each electrode has an approximate geometric area of 150 cm² and an interelectrode distance of 0.5 cm (Figure 10). Electrolysis was carried out at constant current using a power supply (SPS-W3010, 30 V, 10 A). Before the test, the electrodes were subjected to electrochemical activation with 0.50 M of NaCl, adjusting the pH in a range of 2 to 3 with HCl, applying a current density (j) of 0 mA cm⁻² for 30 min. The Ag/AgCl electrode was used as a reference electrode to maintain a stable potential between 1.2 V and 1.4 V. The objective of the activation was to form a more stable and conductive surface layer of RuO₂, create active sites for the absorption and oxidation of Cl⁻ ions, and avoid passive or poorly formed zones in the electrode that reduce the efficiency of the Cl evolution reaction.

The solar still was instrumented with two DS18B20 digital temperature sensors connected to an Arduino Uno microcontroller to monitor water and internal air temperatures. Ambient environmental parameters—including air temperature, relative humidity, and wind speed—were measured using a Metoluar MXDAP-001 (VentDepot, Ciudad López Mateos, Mexico) portable weather station, which offers a temperature accuracy of ±1 °C and a relative humidity accuracy of ±5%. Incident solar radiation (W m⁻²) was quantified using an SM206 solar power meter. See Figure 10.

3.4. Dye Mineralization in the EO Process

The electrochemical oxidation (EO) process using an Sb₂O₅-doped RuO₂–ZrO₂|Ti anode was evaluated in a solar still system for the treatment of aqueous solutions containing Allura Red (AR) dye at an initial concentration of 100 mg L⁻¹, prepared in distilled water. Color removal was assessed through measurements of chemical oxygen demand (COD) and reductions in UV-Vis absorbance. Electrolysis was performed for 60 min under varying current densities (5.00–13.33 mA cm⁻²), pH ranges (2–3, 5–6, 7–8, 9–10), and NaCl concentrations (0.05, 0.10, and 0.20 M) as the supporting electrolyte. The experiments aimed to compare the efficiency of the EO process under solar irradiation versus non-irradiated conditions, particularly in terms of AR dye mineralization. Tests under solar exposure were conducted over a 3 h period (08:00 to 11:00), during which operational parameters such as water temperature, incident solar radiation, ambient temperature, and relative humidity were continuously monitored. To isolate the thermal effect associated with the increase in kinetic energy during electrolysis, experiments were conducted under previously established optimal conditions. The water temperature was maintained between 45 and 50 °C to simulate heating induced by solar radiation, using a heating plate as the thermal source.

3.5. Monitoring of Color Removal

During the electrochemical treatment, the concentration of Allura Red (AR) dye was monitored by measuring the decrease in absorbance at the maximum wavelength (λ_max = 505 nm), using a UV-Visible spectrophotometer (PerkinElmer Lambda 365). The color removal efficiency (%) was calculated based on the relative change in absorbance before and after treatment, according to the following equation:

$$\% \mathrm{C}\mathrm{o}\mathrm{l}\mathrm{o}\mathrm{r} \mathrm{r}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l}={\displaystyle \frac{{\mathrm{A}}_0-{\mathrm{A}}_{\mathrm{t}}}{{\mathrm{A}}_0}}\times 100$$

(6)

where A₀ is the initial absorbance and A_t is the absorbance at time t. This method allowed for a rapid, non-destructive quantification of dye removal efficiency throughout the treatment process.

3.6. Monitoring of Degradation by Chemical Oxygen Demand (COD)

The COD of the samples was determined using Merck COD tubes. To do this, 3 mL of sample was taken, placed in a COD tube, and heated in a digester (HACH) at 150 °C for 120 min. After cooling, the COD value was read directly using a DR/6000 spectrophotometer (HACH). The COD removal efficiency and energy consumption (Wh mg⁻¹_COD) were calculated by using the following formulas:

$$\% \mathrm{C}\mathrm{O}\mathrm{D} \mathrm{r}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l}={\displaystyle \frac{{\mathrm{C}\mathrm{O}\mathrm{D}}_0-{\mathrm{C}\mathrm{O}\mathrm{D}}_{\mathrm{t}}}{{\mathrm{C}\mathrm{O}\mathrm{D}}_0}}\times 100$$

(7)

$${\mathrm{E}\mathrm{C}}_{\mathrm{C}\mathrm{O}\mathrm{D}}={\displaystyle \frac{{\mathrm{E}}_{\mathrm{c}\mathrm{e}\mathrm{l}\mathrm{l}}\times \mathrm{I}\times \mathrm{t}}{\mathrm{V}\left({\mathrm{C}\mathrm{O}\mathrm{D}}_0-{\mathrm{C}\mathrm{O}\mathrm{D}}_{\mathrm{t}}\right)}}$$

(8)

where COD₀ is the COD value of initial aqueous solution (mg L⁻¹), COD_t is the COD value after electrochemical treatment (mg L⁻¹), Ecell was the cell voltage (V), I the actual current (A), t the electrolysis time (h) and V the solution volume (L).

3.7. Real Wastewater Treatment

A wastewater sample was collected from the Lerma River, located in the State of Mexico (19°17′11″ N, 99°31′18″ W). The sample was collected in plastic containers, maintained at 4 °C during transport, and subsequently stored in the laboratory for characterization and treatment using the electrochemical process. The process is proposed as a tertiary treatment for the removal of recalcitrant contaminants, such as dyes, as well as for reducing residual COD. For this reason, the suspended solids present in the water were removed by filtration prior to the treatment. The evaluation of water quality was determined by COD removal efficiency, color (Pt/Co scale) and pH, as indicated in NMX-AA-038-SCFI-2001, NMX-AA-008 SCFI-2016, NMX-AA-093-SCFI-2000 and NMX-AA-030/2-SCFI-2011 standards [44,45,46,47].

4. Conclusions

In this study, the electrochemical degradation of Allura Red dye was systematically investigated using an Sb₂O₅-doped RuO₂–ZrO₂|Ti anode integrated into a solar-assisted electrooxidation system. The effects of current density, initial pH, and supporting electrolyte concentration were evaluated to determine the optimal operational parameters for pollutant degradation. Under the best experimental conditions—current density of 5 mA cm⁻², initial pH of 2–3, NaCl concentration of 0.05 M, and electrolysis time of 60 min—the process achieved 87.4% chemical oxygen demand (COD) removal and 99% color elimination. The gradual disappearance of the absorption peaks in the UV-Vis spectra confirmed the oxidative breakdown of the dye’s chromophoric and aromatic structures.

When the optimized electrooxidation process was applied to real Lerma River water samples, the system achieved approximately 50% reduction in both COD and true color, highlighting its practical applicability to complex environmental matrices. These findings confirm that the Sb₂O₅-doped RuO₂–ZrO₂|Ti anode exhibits high electrocatalytic activity and stability, enabling efficient mineralization of organic pollutants. Overall, the integration of solar energy with electrooxidation provides a sustainable, energy-efficient, and scalable approach for the remediation of industrial and natural water systems impacted by persistent contaminants.