Performance of indirect electrochemical processes using transition metal oxide electrodes for the abatement of food dye in aqueous solution

Due to escalating industrialization and modernization the use of food dyes as promising coloring candidates is also mounting with time scale, which is an undeniable fact. These synthetic dyes are used to enhance the visual appeal of various foods, arousing appetite and esthetic charm. In 2010, global annual production totaled 2.1 million metric tons, valued at $14.4 billion. However, about 1–2% is lost in production, and 10–15% are discharged as wastewater (Drumond Chequer et al. 2013; Hussain et al. 2020). The presence of even small amounts of these dyes in water poses significant risks to aquatic ecosystems (Husain 2006; Stevens et al. 2014).

Tartrazine (TTZ), also recognized as acid yellow 23 and food yellow 04 (C.I. # 19,140, E.I. # 102), is a lemon-hued azo dye. It finds broad application in sports drinks, bakery items, jams, instant puddings, sauces, soups, chewing gums, marmalades, cosmetics, and pharmaceuticals (Hussain et al. 2019). TTZ demonstrates noteworthy stability, evidenced by a mere 1% degradation after 48 h of UV irradiation (Rana et al. 2020). Its thermal resilience extends up to 200 °C in air/water and 300 °C in an inert atmosphere, attributed to its extensive conjugation (Leulescu et al. 2018). Its presence in water diminishes clarity, impeding photosynthesis and lowering dissolved oxygen levels, impacting water's reoxygenation capacity. As an azo dye with an aromatic structure, TTZ often contains benzidines, known carcinogens. Linked to allergies and adult attention-deficit/hyperactivity disorder in children, TTZ degradation results in hazardous compounds like carcinogenic aromatic amines, triggering genotoxic and micro-toxic effects, immune suppression, hyperactivity, and water eutrophication (Amin et al. 2010; Ratna & Padhi 2012). Consequently, due to its alarming repercussions, the removal of TTZ from wastewater is imperative (Moutinho et al. 2007).

Dye pollution reduction has gained global attention among environmental scientists. Advanced oxidation processes (AOPs) are commonly used for wastewater color removal, including methods like adsorption, ozonation, O₃/UV photocatalysis, TiO₂/UV, H₂O₂/UV, and Fenton's reagent (H₂O₂/Fe⁺²) (Pereira et al. 2016; Garcia-Segura & Brillas 2017; Hussain et al. 2021; Khan et al. 2021). Fenton's methods are highly effective and applicable to a wide range of dyes due to their non-selectivity. AOPs achieve high color removal efficiency, along with photocatalytic degradation of dyes. Likewise, biological techniques, such as activated sludge and aerobic/anaerobic microbial degradation, have gained attention for their effectiveness, simplicity, and cost feasibility. However, these processes are hindered by slow kinetics and challenges in degrading large, complex, and conjugated benzene-like structures (Zhang et al. 2015).

Over the last decade, there has been significant attention directed toward electrochemical techniques. These methods hold considerable promise due to their exceptional adaptability, energy efficiency, ease of use, and potential as environmentally friendly solutions for wastewater discoloration (Liang et al. 2018). The Ti/Ru metal oxide anode, commonly known as dimensionally stable anode (DSA), has become increasingly popular due to its economical nature, versatility, lack of selectivity, ease of preparation, stability, extended operational lifespan, minimal energy consumption (EC) during electrolysis, and exceptional effectiveness in discoloring wastewater, particularly when employed with the appropriate electrolyte like NaCl (Pacheco-Álvarez et al. 2023). This anode possesses resistance to corrosion, remains thermally stable, and exhibits notable resilience against easy degradation (Trasatti 2000; Zeb et al. 2018). The electrode materials primarily influence the efficiency and selectivity of electrochemical oxidation processes, whether the electrode is an active or inactive electrode.

The comparatively less intense effectiveness of DSA in contrast to more reactive electrodes such as boron-doped diamond (BDD) or PbO₂ concerning the degradation of organic substances can be significantly counterbalanced by selecting appropriate supporting electrolytes like NaCl and by carefully adjusting optimally tuned reaction circumstances (Li et al. 2016; Khan et al. 2022). During redox reactions, the metal oxides of ruthenium and titanium are capable of conveniently altering their oxidation states (e.g., Ru_III → Ru_IV) at relatively low applied potentials (ranging from +0.5 to +1.2 V for ruthenium oxide), leading to a notable breakdown of dye molecules with minimal EC.

Active anodes, such as DSA, facilitate the indirect oxidation of organic compounds via the generation of potent oxidants such as hydroxyl radicals, aqueous chlorine (Cl₂(aq)), hypochlorous acid (HOCl), or hypochlorite ions (OCl⁻), ⁠, ⁠, and (Luna-Trujillo et al. 2020; Santos et al. 2020). DSA anodes also possess specialized electrocatalytic surfaces that facilitate the generation of oxidative compounds while minimizing undesired side reactions that may produce harmful products. This selectivity promotes a more ecologically friendly approach to water treatment in contrast to conventional techniques. Additionally, the electrochemical procedure can be fine-tuned and enhanced by considering variables like current density, electrolyte composition, and electrode constitution, which suit diverse water origins and dye kinds (Pointer Malpass & de Jesus Motheo 2021).

This study aims to integrate electrochemical oxidation using a DSA anode (Ti/Ru_0.3Ti_0.7O₂) for efficient degradation of TTZ food dye. Factors such as electrolyte composition, pH, current density, and NaCl concentration are analyzed. Moreover, TTZ electrochemical behavior in different contexts, EC, and total organic carbon (TOC) removal were evaluated.

The electrolytes – NaCl (99.5% purity), H₂SO₄ (98% purity), and Na₂SO₄ (99% purity) – were obtained from SIGMA-ALDRICH at analytical grade, while NaOH was procured from MERCK. The TTZ dye, a food-grade substance, was acquired from a local market in Swabi KP in Pakistan. It was chosen as the representative pollutant for this research. All solutions were prepared using pure distilled water.

The bulk oxidation technique was employed to conduct electrochemical degradation experiments on TTZ dye in an undivided glass electrochemical reactor with a 500 mL effective volume per trial. All dye degradation tests followed galvanostatic conditions, utilizing a controlled direct current (DC) power source (Model EL 843000, GPO-3403S, GW-INSTEK CO-Limited, South Korea). The power supply maintained a constant current, with a voltage range of 0–30 V and a current density range of 0–03 A. Within the electrochemical cell, the vertically positioned working electrode (anode) and counter electrode (cathode) shared a parallel orientation. The inter-electrode distance was 1 cm, set in a 500 mL dye sample solution with an appropriate supporting electrolyte.

The anode, sourced from De Nora Do Brazil, Avenida Jerome (Case No. 1959 18087–220 Eden, Sorocaba – SP Brazil), a rectangular titanium-based plate, featured a coating of mixed oxides of titanium and ruthenium (Ti/Ru_0.3Ti_0.7O₂). It measures 2 cm × 7 cm on two sides, providing a total effective surface area of 28 cm² (14 cm² per face). To optimize mass transport and ensure consistent electrolyte strength, a magnetic stirrer maintained a constant agitation of 300 rpm. An illustrative depiction of the experimental setup is presented in Figure S1 of the Supplementary Materials. To maintain a specific pH in the reaction mixture, 0.05 M solutions of either H₂SO₄ (sulfuric acid) or NaOH (sodium hydroxide) were added throughout the course of the electrolysis process. The pH of the solution is monitored through a Hanna pH meter (HI2210-01) and the readings are recorded every 5 min during electrolysis. Before each electrochemical degradation experiment, the electrode underwent activation and cleaning by immersion in 0.05 M of H₂SO₄ and application of a current density of 10 mA cm⁻² for 5 min, without the addition of dye solution.

Here, TOC_o and TOC_t refer to the concentration of TOC at initial and time ‘t’, respectively.

The mixed metal oxide anode underwent X-ray crystallography (XRD) analysis utilizing an XRD diffractometer 3000 PTS from Seifert, Germany. The analysis involved irradiation with Cu-Kα radiation (wavelength: 1.5405 Å) and continuous scanning on a 2θ scale (0°–90^o). Scanning electron microscope (SEM) images of the anode material were captured with a tungsten filament-equipped XL-30 Philips SEM micrograph from Japan. Surface morphological assessment of the Ti/Ru_0.3Ti_0.7O₂ anode employed energy dispersive X-ray spectroscopy (EDX) through the same device as an SEM analyzer, yielding insights into its elemental composition.

Cyclic voltammetry (CV) was conducted at room temperature (25 °C) using the GAMRAY-Potentiostat/Galvanostate Model G-750 (USA) to probe redox reactions at the metal oxide electrode-solution interface. The experimental setup comprised a conventional three-electrode configuration: a calomel reference electrode (CRE) as the reference electrode, a stainless-steel counter electrode, and a working electrode in the form of a typical DSA with a 14 cm² surface area. Tests were carried out using 0.033 M Na₂SO₄ and 0.1 M NaCl as background electrolytes. The model dye pollutant TTZ was included, and scans were performed at a scan rate of 50 mV s⁻¹, preceded by a 30-s stabilization period.

The investigation involved studying the variation in HOCl concentration during the optimized electrochemical degradation of TTZ. A 100 mg L⁻¹ TTZ dye solution, the target pollutant, was prepared with 0.08 M NaCl as the supporting electrolyte at pH 3. Electrolysis of the dye solution lasted 25 min, with an applied current density of 10 mA cm⁻². Samples were drawn from the electrolytic reactor every 5 min and titrated with 0.1 M sodium thiosulfate solution in the presence of a starch indicator, using 10 mL glacial acetic acid and 0.8 g KI. Iodometric nitration analyzed the HOCl concentration in each withdrawn sample (Neodo et al. 2012; Chianca et al. 2014).

The voltammogram displays a significant current decrease when TTZ is present (red curve, Figure 1(a)), attributed to the adsorbed dye molecules partially deactivating the anode surface and obstructing active sites on the anode. This change suggests an interaction between the anode surface and dye pollutants. A more pronounced current decrease is seen in the cyclic voltammogram (blue) with 0.1 M NaCl (black curve, Figure 1(a)). Higher anodic and cathodic current densities near +1.4 V with NaCl suggest effective degradation of dye pollutants (cyancolour, Figure 1(a)) by in situ electro-generated chlorine and production of reaction byproducts (Trasatti & Buzzanca 1971; Malpass et al. 2007).

The XRD pattern of the Ti/Ru_0.3Ti_0.7O₂ mixed oxide electrode is displayed in Figure 1(b). The pattern exhibits distinct peaks at various 2θ values, signifying a well-defined crystalline structure within the electrode material's lattice. The XRD spectrum of the electrode highlights a notable peak at 2θ = 28.13, corresponding to the TiO₂ XRD Spectrum Figure 1(b). Similarly, a split peak around 2θ = 36 indicates the coexistence of titanium (Ti) and TiO₂ within the electrode lattice. Furthermore, a sharp peak at 2θ = 40 signifies metallic Ti presence in the electrode lattice. Additionally, a high-intensity peak at 2θ = 55 confirms ruthenium oxide (RuO₂) within the electrode lattice. The XRD results are in agreement with some literature data (Trasatti & Buzzanca 1971; Soni et al. 2017).

The evaluation of electrode performance in pollutant electrolysis necessitates a crucial analysis of electrode surface morphology, which is achieved by coating Ti and Ru metal oxides onto a Ti substrate. Figure 1(c) reveals a surface structure resembling ‘cracks within a dried mud-like pattern.’ This distinctive ruggedness, featuring fissures and cavities, is characteristic of this specific electrode type, with these cracks forming during the electrode's calcination phase and subsequent solvent evaporation (Malpass et al. 2006). These cracks, crevices, and pores on the electrode's surface serve as active sites for interacting with the electrolyte during the electrochemical degradation of pollutants, thereby accelerating degradation rates due to the increased surface area. These morphological traits of metal oxide electrodes significantly enhance both electrochemical stability and reactivity toward pollutant degradation (Soni et al. 2017). Figure 1(d) depicts the EDX spectrum for the given electrode, illustrating a surface composition of around 29–30% RuO₂ and 70–71% TiO₂. This composition closely mirrors the electrode's formula, corroborating findings from another study (Malpass et al. 2006).

Hypochlorous acid (HOCl) is the most potent oxidizing agent in this context. The concentration of HOCl during TTZ's electrochemical oxidation is shown in Figure S2, demonstrating a gradual decrease as electrolysis progresses due to the CER onset at the anode. HOCl involvement in dye molecule degradation, generating chlorinated intermediates during initial electrochemical decay, contributes to this decline.

The degradation kinetics of TTZ were explored using various electrolytes. Initially, the pseudo-first-order kinetic equation was applied to the electrochemical oxidation of the dyes with Na₂SO₄ as a supporting electrolyte, and the significance of the model was validated using computed squared correlation (R²) values. A direct correlation between reaction time and the change in dye concentration was revealed as shown in Figure 2(b) indicating that TTZ degradation followed a pseudo-first-order kinetic model with all three tested electrolytes. Data for rate constant ‘k’ half-lives ‘t_1/2’ and R² for TTZ across the three electrolytes are detailed in Table 1. Notably, with 0.033 M Na₂SO₄ at a current density of 10 mA cm⁻², the pseudo-first-order model displayed the highest R² value of 0.998 (Table 1), indicating TTZ's electrochemical degradation adhered to a pseudo-first-order kinetic pattern. Rate constants with Na₂SO₄, H₂SO₄, and NaCl as supporting electrolytes were 6.9 × 10⁻³, 14.5 × 10⁻³, and 65.1 × 10⁻³ min⁻¹, respectively. Notably, switching from Na₂SO₄ to NaCl boosted degradation rates tenfold, reducing the half-life from 100.43 to 10.64 min, signifying a substantial decrease in reaction time.

Figure 2(c) illustrates the impact of initial pH variations on TTZ dye's electrochemical degradation in a chloride medium. The observed trend indicates that as the solution's pH rises, the efficacy of color removal diminishes progressively. Electrochemical degradation trials occurred with a current density of 05 mAcm⁻², utilizing 0.02 M NaCl as a supporting electrolyte. Within 30 min of electrolysis, discoloration rates were 46, 22, and 16% at initial pH values of 3, 6, and 10, respectively. The order of dye removal efficiency followed this sequence: pH 3 > pH 6 > pH 10. The electrochemical oxidation of organic contaminants in a chloride environment using a DSA anode is widely recognized. This process involves active chlorine species like Cl₂, HOCl, OCl⁻¹, and ⁠, which mediate oxidation. The prevailing active chlorine oxidant among these species is determined by the initial pH of the solution. Thus, the solution's pH governs the dominance of distinct chlorinated oxidants (Garcia-Segura et al. 2015). These active chlorine species exhibit diverse oxidation potentials vs standard hydrogen electrodes (SHE). Consequently, they exhibit varying degrees of electrochemical oxidation on dyes, leading to different levels of dye removal at different pH ranges. Table S1 illustrates different chlorinated oxidants, their compatible pH ranges in solution, and their relative oxidation potentials compared to SHE.

It is evident that integrating RuO₂-based anode materials reduces the formation of ClO₃⁻¹ and ClO₄⁻¹ in water (Martínez-Huitle & Brillas 2009; Alves et al. 2010; Brillas & Martínez-Huitle 2015). Additionally, TTZ is an acidic dye that gains a negative charge at alkaline pH due to proton loss. Under these conditions, dye adsorption occurs at the anode surface, competing with chloride ion oxidation. This dual process hinders the anode's active sites, substantially reducing degradation efficiency (Hussain et al. 2017). Figure 2(d) illustrates the electrochemical degradation kinetics of TTZ at various pH levels. A plot of ‘lnC/C_o’ against reaction time generates a linear graph with a negative slope representing the reaction's rate constant and intercept. Table 1 provides the rate constants and half-lives at different pH values. Notably, rate constants increase as the dye solution's pH decreases, ranging from 6.23 × 10⁻³ to 20.49 × 10⁻³ min⁻¹ when pH decreases from 10 to 3. This phenomenon arises because pH below 7 fosters the presence of the potent oxidizing agent HOCl, while pH above 7 prompts HOCl ionization, resulting in less effective hypochlorite ions (OCl⁻¹) and subsequently slower degradation rates and reduced rate constants. These findings align closely with existing literature (Malpass et al. 2008; Khataee et al. 2014).

Figure 3(b) illustrates the kinetics of TTZ dye's electrochemical oxidation under varying applied current densities. Plotting the natural logarithm of concentration alteration against treatment time yielded straight lines, strongly suggesting adherence to a pseudo-first-order kinetic model. For TTZ dye, rate constants surged from 6.01 × 10⁻³ to 133.52 × 10⁻³ min⁻¹ as current density elevated from 2.5 to 10 mA cm⁻², causing half-lives to decrease from 115.30 to 5.19 min (Table 1).

Figure 3(c) depicts the influence of different NaCl concentrations on TTZ dye at a consistent dye concentration (100 mg L⁻¹) and constant current density of 10 mA cm⁻². The results reveal that the degradation efficiency of TTZ dye correlates with NaCl concentration. Elevated chloride levels in the solution lead to a shorter time for % dye removal and a heightened degradation rate, which is attributed to increased chloride ion concentrations, enhancing conductivity and current density. Consequently, the voltage required to attain a given current density decreases, leading to reduced overall electrical EC during electrochemical oxidation (Modirshahla et al. 2013). Maintaining an ionic strength of μ = 0.1 M for each NaCl solution involved adding appropriate Na₂SO₄ quantities. At a 10 mA cm⁻² current density, 20 min of electrolysis resulted in dye removal rates of 99, 95, and 68% for NaCl electrolyte concentrations of 0.08, 0.04, and 0.02 M, respectively.

Figure 3(c) notably demonstrates that polynomial functions most accurately represent the observed TTZ removal kinetics at 0.08 M NaCl, effectively reflecting the stabilization of the removal process observed in the experimental data. The high R² value of 0.933 further supports the suitability of the polynomial fit for these conditions. Figure 3(c) indicates that at NaCl electrolyte concentrations of 0.04 and 0.08 M, the dye degradation rates are significantly high, primarily due to the sufficient in situ generation of chlorinated oxidants. During the first 10–12 min of electrolysis, approximately 95–99% of the dye is degraded. Beyond this point, the rate of dye removal stabilizes, and the degradation profile becomes nearly parallel to the time axis. Although the fitting curve suggests a slight decline, the experimental data points show that the degradation percentage remains constant after 12 min of electrolysis (Hussain et al. 2015). Higher NaCl concentrations led to faster degradation rates under a constant current density, attributed to the effective generation of active chlorine species like aqueous chlorine, HOCl, and OCl⁻¹ (Rajkumar et al. 2007; Alves et al. 2010).

The impact of varying initial concentrations from 25 to 100 mg L⁻¹ of TTZ at a constant current density of 5 mA cm⁻² and employing supporting electrolyte NaCl (0.02 M) influenced the TTZ degradation (Figure 3(d)). Notably, degradation rates displayed an inverse correlation with the initial dye concentrations as the removal percentages of 17, 45, and 84% were achieved for initial TTZ concentrations of 100, 50, and 25 mg L⁻¹, respectively. This phenomenon can be attributed to reduced electro-generation of active chlorine species (oxidants such as Cl₂, HOCl, or OCl⁻¹) at higher dye concentrations. In cases of elevated concentrations within the cell, dye molecules tend to adsorb onto the anode surface, leading to blockage of active sites and subsequently causing a significant decline in degradation rates. Furthermore, while the number of dye molecules increased at higher concentrations in the solution, the active chlorine species remained relatively constant (Idel-aouad et al. 2011). Furthermore, the reduction in discoloration rate as initial dye concentration rises can be attributed to the nonselective nature of active chlorinated species formed at the anode or in the solution bulk. This effect is particularly notable when utilizing a Ti/Ru_0.3Ti_0.7O₂ type anode in a chloride-rich environment. Despite this, the consistent production rate of active chlorine species remains constant. These species tend to preferentially react with dye intermediate compounds rather than the main chromophore dye molecules. Consequently, the overall rate of discoloration significantly diminishes as the initial dye concentration is increased (Rajkumar et al. 2005, 2007).

The process of electrochemical degradation entails the elimination of organic contaminants from wastewater through the utilization of electrical energy to initiate the degradation procedure. Herein, the EC values (kWh·m⁻³) were computed, considering diverse operational factors such as a variety of electrolytes, NaCl concentrations, and current density, as summarized in Table 1. For the TTZ dye, the EC values (kWh·m⁻³) were determined as 0.550 with 0.033 M Na₂SO₄, 0.479 with 0.033 M H₂SO₄, and 0.464 with 0.02 M NaCl as the supporting electrolyte (Table 1 and Figure S3). Notably, the addition of 0.02 M NaCl as a supporting electrolyte to the electrochemical system resulted in a significant enhancement of TTZ removal, despite a concomitant decrease in EC. This improvement can be attributed to the in situ generation of active chlorine oxidants, facilitated by the presence of NaCl. These oxidants exhibit higher reaction rates with the dye compounds than those observed with other electrolytes, leading to more efficient degradation and removal (Figure S3(a)). Additionally, it shows that increasing the NaCl electrolyte concentration resulted in a significant decrease in EC, and higher NaCl concentrations resulted in higher TTZ removal (Figure S3(b)). This was due to the higher generation of active chlorine oxidants in concentrated solutions, accelerating the electrochemical degradation process and subsequently lowering EC. Furthermore, a direct correlation between current density and both TTZ removal efficiency and EC was observed. As the current density increased from 2.5 to 10 mA cm⁻², TTZ removal efficiency rose from 15.08 to 94.78%, while EC increased from 0.12 to 0.68 kWh m⁻³ (Figure S3(c)). This is likely due to the fact that higher current densities correspond to elevated applied potentials, leading to an increased influx of charge into the electrochemical reactor. This heightened charge input facilitates the degradation of dye pollutants, consequently raising the EC of the overall process. Figure S3(d) illustrates the significant influence of pH on TTZ removal and EC. Slightly acidic conditions were found to be more conducive to TTZ removal with lower EC. This may be attributed to the protonation of TTZ at lower pH values, which can enhance its adsorption onto the electrode surface or facilitate its degradation through electrochemical reactions. These analyses demonstrate that EC increased with higher current densities, alkaline conditions, and lower NaCl electrolyte concentrations. The overall energy efficiency, defined as the ratio of TTZ removed to the energy consumed varied depending on the experimental conditions.

The present study has revealed the applicability of Ti/Ru_0.3Ti_0.7O₂-based electrochemical processes for the abatement of food dye TTZ. The following conclusions can be drawn from this work.

• The Ti/Ru_0.3Ti_0.7O₂ anode significantly enhanced TTZ dye degradation via bulk electrolysis, with 97% efficiency in 40 min at 10 mA cm⁻². The optimal concentration of NaCl electrolyte was 0.08 M. Using NaCl during electrolysis for 30 min led to the best degradation and mineralization, generating strong chlorinated oxidants with high oxidation potential. Effective electrochemical degradation occurred at pH 3–6, producing a powerful electrically generated chlorinated oxidant HOCl. Lower dye concentrations showed better degradation than higher amounts.

• The kinetic assessment showed that TTZ degradation adhered to pseudo-first-order kinetics. Using the Ti/Ru_0.3Ti_0.7O₂ anode with NaCl as an electrolyte improved degradation rates, lowered t_1/2, and reduced energy use. These aspects are vital for considering large-scale industrial application feasibility.

• The optimal conditions for effective electrochemical degradation of TTZ required specific parameters: 0.08 M NaCl electrolyte, 10 mA cm⁻² current density, pH 3–6, and 25 mg L⁻¹ initial TTZ concentration.

• EC values (kWh·m⁻³) were computed based on various factors (electrolytes, NaCl, current density). NaCl lowered EC through chlorine oxidants, while higher current density increased EC. The novelty of this electrochemical degradation is that here the target dye pollutant is degraded using a Ti/Ru_0.3Ti_0.7O₂ electrode at least energy consumptions (Table 1) in (kWh m⁻³) rendering it more economical and suitable for practical viability in the future.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.