Electrochemical Generation of Reactive Chlorine Species via Chloride Oxidation on –COOH-Modified Graphite Electrode to Attain Dye Degradation

Abstract

In this study, we investigate the electrochemical performance of a carboxyl-functionalized pencil graphite (CFPG) electrode for chloride ion oxidation and its subsequent application in dye degradation. The graphite electrode was chemically modified using acetic acid to introduce –COOH functional groups, enhancing surface polarity and chloride adsorption capacity. Surface characterization by SEM, EDX, and XPS confirmed morphological changes and oxygen enrichment following functionalization. Electrochemical measurements demonstrated a positive shift in open circuit potential (OCP) and significantly enhanced chloride oxidation activity, as evidenced by cyclic voltammetry (CV) in 0.1 M KCl. The functionalized electrode facilitated the in situ generation of reactive chlorine species (RCS), with spectral features near ~240 nm consistent with HOCl/ClO⁻ and a broader band around ~450 nm attributable to chlorine-derived intermediates rather than exclusively to molecular chlorine. These species played a central role in degrading structurally diverse dyes—Kenacid Green and Brilliant Green—via oxidative pathways. The results highlight the potential of low-cost, –COOH-modified graphite electrodes as effective platforms for the RCS-mediated electrochemical treatment of organic contaminants.

1. Introduction

The anodic oxidation of chloride ions (Cl⁻) on conductive electrode surfaces confers several advantages for electrochemical advanced oxidation processes (EAOPs), especially in treating persistent organic pollutants [1]. The electrochemical activation of Cl⁻ leads to the formation of RCS including Cl₂, HOCl, and ClO⁻, depending upon solution pH and applied electrode potential [2]. These RCS possess strong oxidation potentials (Cl₂/Cl⁻ = 1.36 V; HOCl/Cl⁻ = 1.49 V vs. SHE), enabling non-selective oxidation via both direct electron transfer and indirect oxidative pathways [3]. The reactivity of RCS directly influences the complex molecular environment rich in π electron clouds. For instance, Cl₂ and HOCl can disrupt conjugated π-systems in complex dyes, while HOCl’s strong electrophilicity facilitates nucleophilic attack on amino and phenyl moieties in azo and cationic dyes [4,5]. Continuous Cl⁻ oxidation allows for the regeneration of active chlorine species, offering sustained oxidative capacity ideal for flow-based electrochemical systems [6]. Complementary reviews and mechanistic studies have highlighted how chlorine-mediated electrochemical advanced oxidation processes (EAOPs) contribute to pollutant degradation but also present risks associated with chlorinated organic and inorganic byproducts [7,8,9]. Overall, chloride oxidation represents a cost-effective, scalable, and efficient method for targeting diverse organic contaminants in water treatment [10]. However, the effectiveness and selectivity of Cl⁻-mediated EAOPs are strongly influenced by electrode material, applied potential, pH, and chloride concentration [11,12]. Recent studies have advanced understanding of reactive chlorine species (RCS) electrogeneration and its practical implications. Investigations on boron-doped diamond (BDD) and Ti₄O₇ electrodes demonstrated that operating parameters strongly influence the formation of byproducts such as chlorate and perchlorate, underscoring the importance of process control in chloride-containing systems [7,8,9]. Graphite has appeared as a pioneering low-cost anode, suitable for chloride oxidation due to its high electrical conductivity and capacity for surface modification [13]. Despite its lower overpotential and oxidative stability relative to boron-doped diamond (BDD) and dimensionally stable anodes (DSAs), graphite is appealing for scalable and economically viable systems, particularly when surface-functionalized to improve adsorption and catalytic behavior [14,15,16]. Specifically, the introduction of oxygen-containing moieties such as carboxyl (–COOH) groups onto graphite surfaces enhances chloride adsorption and promotes indirect oxidation for RCS generation [17].

Chemical and electrochemical oxidation using acetic or nitric acid has proven to introduce polar functionalities like –COOH onto carbon electrodes, thereby increasing their hydrophilicity, electroactivity, and ionic adsorption capacity [4]. For instance, Liu et al. demonstrated that surface –COOH groups significantly improve the electroactive surface area and foster a microenvironment conducive to ionic adsorption and charge transfer—ultimately enhancing anodic oxidation reactions [15]. Chen et al. further reported that mildly oxidized carbon featuring carboxyl and hydroxyl functionalities exhibits enhanced wettability and double-layer capacitance, which improves access for electrolyte ions and accelerates electrochemical kinetics [18]. In the context of Cl⁻-mediated oxidation, Treviño-Reséndez et al. showed that acetic acid-pretreated graphite electrodes exhibited superior electrochemical performance in chloride-containing media, suggesting that surface –COOH moieties facilitate Cl⁻ accumulation and activation under anodic conditions [15]. Theoretical insights from density functional theory (DFT) calculations by Ana S. Dobrota et al. demonstrated that Cl atoms adsorb more strongly onto oxygen-functionalized graphene—especially at carboxyl and hydroxyl edge sites—than on pristine surfaces, suggesting that surface functional groups such as –COOH can promote chloride accumulation and facilitate enhanced RCS formation at the molecular level [19]. Although RCS-based EAOPs using chloride electrolytes have been extensively studied, most research has focused on noble-metal electrodes or composite coatings, often overlooking the potential of chemically tailored carbon electrodes. Additionally, recent reviews and mechanistic studies have shown that chlorine-mediated electrochemical advanced oxidation processes (EAOPs) are effective for pollutant degradation, but they also generate undesirable chlorinated organic and inorganic byproducts [3,4,5]. In parallel, pilot-scale studies, such as the electrochemical treatment of chelated wastewaters and solar-assisted EAOPs for rural sewage, have demonstrated that these processes can be scaled beyond laboratory conditions [6,7]. To our knowledge, systematic exploration of how –COOH functionalization impacts Cl⁻ oxidation and subsequent dye degradation remains unexplored. Herein, we address this gap by integrating surface engineering, electrochemical characterization (OCP and cyclic voltammetry), spectroscopic verification of RCS generation, and application to the degradation of structurally diverse dyes—Kenacid Green and Brilliant Green. Our findings elucidate the mechanistic contribution of carboxyl functional groups on graphite toward facilitating chloride ion oxidation and the subsequent generation of RCS. The presence of these surface functionalities enhances the electrode’s redox activity, thereby promoting selective interfacial reactions that yield RCS capable of initiating the oxidative degradation of dye molecules. Therefore, the present work does not aim to develop a new functionalization strategy but instead demonstrates the mechanistic role of carboxyl functionalities in driving the generation of RCS and their role in dye degradation. This focus on mechanistic validation distinguishes the study from earlier reports that primarily addressed electrode modification methods without directly correlating surface chemistry to RCS formation and contaminant removal. Additionally, this study is not intended to quantify degradation kinetics or removal efficiencies; instead, it establishes mechanistic evidence linking electrode surface chemistry to RCS formation and verifies their practical role as reactive oxidants in contaminant degradation. By clarifying this fundamental relationship, the present work provides a scientific basis for advancing carboxyl-functionalized electrodes as sustainable and cost-effective platforms, while laying the groundwork for future investigations aimed at systematically optimizing RCS generation and quantitatively evaluating dye degradation pathways in complex water matrices. Importantly, several pilot-scale demonstrations reported in 2023–2024 have shown the feasibility of chlorine-mediated EAOPs for treating real effluents, including chelated industrial wastewater and rural sewage, highlighting both scalability and sustainable energy coupling [20,21]. Despite challenges with byproduct formation, these advances illustrate how process optimization can enable practical deployment. Within this context, the present study provides mechanistic evidence that carboxyl-functionalized graphite facilitates RCS generation and dye degradation. However, full time-resolved COD and TOC profiles were not obtained; end-point analyses showed 98% TOC reduction and 92% COD removal, demonstrating the effectiveness of the carboxyl-functionalized graphite electrode and establishing a basis for benchmarking against conventional anodes in future studies and future pilot-scale application.

2. Results and Discussion

2.1. Fabrication of –COOH-Functionalized Pencil Graphite Electrode

Tailoring electrode surfaces with oxygen-containing groups has emerged as a pivotal strategy to enhance chloride-mediated electrochemical processes. Herein, the functionalization of a bare graphite electrode was carried out by immobilization of the –COOH group. The OCP measurement (Figure 1) supported the –COOH functionalization on the electrode surface. After treating with acetic acid, for the same graphite electrode, the OCP shifted markedly to 0.325 V from the 0.05 V for bare graphite electrode in KCl medium, indicating the significant modification of the graphite surface was accomplished.

The pronounced anodic shift is plausibly attributed to the incorporation of electron-withdrawing carboxyl functionalities can facilitate the stabilization of the positive charges on the modified electrode surface. The incorporated –COOH groups contribute the enhancement of the overall oxidation state of the electrode surface and the electron-withdrawing character of the interface, resulting in an anodic shift in the OCP. Additionally, morphological studies were carried out to examine the –COOH functional groups on the surface of the graphite electrode. Figure 2 shows the SEM image of bare and modified graphite electrodes. Prior to functionalization (Figure 2A,B), the graphite surface exhibited a layered morphology with continuous sheet-like formations. The surface appeared uneven and irregular, with closely packed graphite structures and visible microgrooves indicative of its native, unmodified state. The SEM image of the –COOH-functionalized graphite (CFG) electrode (Figure 2C,D) exhibits clear morphological modification, including increased surface roughness, flake-like texture, and signs of shedding of surface sheets. These features indicate structural disruption and localized expansion due to oxidative treatment with acetic acid introducing oxygen-containing groups.

Following the morphological evaluation by SEM, energy-dispersive X-ray spectroscopy (EDX) was conducted to assess the elemental composition of the pencil graphite electrode surfaces before and after functionalization (Figure 3). The analysis provides insight into the chemical changes associated with the introduction of –COOH group, resulting from acetic acid treatment. The bare electrode demonstrated in Figure 3A primarily consisted of carbon (87.72%), with minor amounts of oxygen (9.82%) and trace elements such as sulfur, chlorine, and iron, likely originating from the manufacturing process or environmental exposure. Following treatment with acetic acid, the functionalized electrode exhibited a significant increase in oxygen content (31.43%) and a corresponding decrease in carbon (64.35%), indicating the successful incorporation of –COOH functionalities onto the graphite surface (See Figure 3B). This rise in oxygen from –COOH is consistent with chemical oxidation during carboxylic acid treatment. A slight increase in iron (1.60%) may result from trace contamination during handling or sample preparation. The presence of sodium (0.05%), absent in the untreated sample, reflects residual ions from cleaning or the electrolyte environment.

Another approach, the XPS analysis, was applied to identify the source of the oxygen signal observed in the EDX spectra (Figure 3B). Figure 4 represents the XPS in the C 1s and O 1s regions for both bare and functionalized graphite, based on their respective survey scans. In the deconvoluted XPS for the C 1s region shown in Figure 4A, a prominent peak near 284.4 eV is attributed to C–C bonds within the graphite structure [22,23,24]. The spectra for both kinds of electrode demonstrate a peak around ~286.8 eV which can be attributed to the C–O–C bond of the graphite structure. Notably, the spectrum of the functionalized graphite exhibits an additional peak at approximately 288 eV, which is absent in the bare graphite. This peak is characteristic of O–C=O bonds, indicating the presence of –COOH functional groups on the graphite surface [23,24,25,26]. Next, the deconvoluted XPS for the O 1s regions for both the electrodes are displayed in Figure 4B. The main peak at around 531–532 eV for bare graphite (intensity multiplied by 10) is very weak, indicating small oxygen contribution of the bare graphite compared to the functionalized graphite. These prominent peaks at 531.0 eV and 532.1 eV for bare and functionalized electrodes, respectively, feature the presence of C=O oxygen species [27]. The slightly higher binding energy value for the functionalized electrode confirms the formation of new oxygen-containing C=O groups with greater electron-withdrawing character. The peak at ~534 eV can be typically attributed to organic C–O oxygen species as well as adsorbed oxygen species.

The present XPS analysis confirms the incorporation of oxygen-containing functionalities after acetic acid treatment, primarily attributed to carboxyl groups, substantiated by the appearance of new peak at 288.4 eV in the C 1s region of XPS of the functionalized graphite and the primary O 1s peak shifting from 531.0 eV to a more positive binding energy value (532.1 eV) with increased intensity.

2.2. Oxidation of Chloride on CFG Electrode

A 0.1 M KCl solution was used to carry out the adsorption of the Cl⁻ on the CFG electrode. The role and function of surface-bound –COOH groups in expediting the Cl⁻ oxidation was examined via monitoring the Cl⁻ concentration and pH effect. To confirm the relative Cl⁻ oxidation, cyclic voltametry was performed for both the functionalized and bare graphite electrodes under identical conditions in 0.1 M KCl at a scan rate of 0.1 V s⁻¹. As shown in Figure 5, the bare graphite electrode exhibited a negligible anodic current within the potential window (blue line) corresponding to chloride oxidation, indicating an absence of sufficient adsorption sites and the electrochemical passivity of the bare electrode surface toward Cl⁻ oxidation. In contrast, the CFG electrode displayed a distinct anodic peak at approximately +0.3 V vs. Ag/AgCl, accompanied by a substantial current response of ~5 mA. This well-defined signal indicates the enhanced adsorption and electrochemical oxidation of Cl⁻ ions at the modified electrode. The improved electrochemical performance of the CFPG electrode can be ascribed to the synergistic effects of increased surface polarity and the introduction of electrochemically active sites. Specifically, the deprotonation of surface –COOH groups into carboxylate (–COO⁻) species that facilitate Cl⁻ accumulation via cation-bridging interactions with K⁺ ions (Equation (1)) [28,29]. In association with this, residual –COOH may be involved through forming hydrogen bonds with Cl⁻. These combined interactions including transient ion-pairing complex formation and electrostatic force effectively concentrate Cl⁻ at the electrode interface, thereby promoting its anodic oxidation while the –COOH/–COO⁻ groups themselves do not undergo direct redox transformation.

R–COO−⋅⋅⋅K⁺⋅⋅⋅Cl^–

(1)

Mechanistically these ion-bridging effects with the weak hydrogen bond contribute to improving the formation of a structured electrical double layer schematically, as demonstrated in Figure 6, which can modulate the local ionic distribution and promote the selective oxidation of Cl⁻ at anodic potentials [28,30]. This localized enrichment of Cl⁻ at the electrode–electrolyte interface assists its electrochemical oxidation, as evidenced by the observation of an anodic response in CV near 0.3 V vs. Ag/AgCl.

In accordance with the literature, under anodic conditions, –COOH group functionalization drives RCS generation in the following way, as shown in Equations (2)–(4) [31,32].

$$\mathrm{Chloride} \mathrm{ion} \mathrm{oxidation}: 2{\mathrm{Cl}}^{-}\to {\mathrm{Cl}}_2+2{\mathrm{e}}^{-}$$

(2)

$$\mathrm{Chlorine} \mathrm{gas} \mathrm{hydrolysis}: {\mathrm{Cl}}_2+{\mathrm{H}}_2\mathrm{O}\to \mathrm{HClO}+\mathrm{HCl}$$

(3)

$$\mathrm{Hypochlorous} \mathrm{acid} \mathrm{dissociation}: \mathrm{HClO}\rightleftharpoons {\mathrm{H}}^{+}+{\mathrm{ClO}}^{-}$$

(4)

In situ time-variant UV–Vis spectral data was recorded for 0.1 M KCl at 5 volt using the CFG electrode to confirm the formation of RCS. The resulting spectra (Figure 7) of UV-Vis exhibits two distinct absorption bands centered at ~240 nm and ~450 nm. The strong absorbance peak at ~240 nm is characteristics of hypochlorous acid (HClO) or its conjugate base hypochlorite (ClO⁻), due to n→π transitions in the Cl–O bond, which are typically known to arise from the hydrolysis of electrochemically generated chlorine gas [33,34,35].

The broader absorption band with lower intensity centered around ~450 nm is attributed to presence of chlorine-derived reactive species, consistent with the initial oxidation product of Cl⁻. However, established spectroscopic data indicate that the primary absorption maximum of Cl₂ occurs at ~325–330 nm [36,37]. Thus, the absorption band at 450 nm cannot be attributed solely to Cl₂. Instead, this feature is more plausibly associated with chlorine-derived reactive species, such as chlorinated intermediates or other RCS formed in situ [2]. While the presence of Cl₂ in the system cannot be excluded, UV–Vis spectroscopy at 450 nm does not provide definitive evidence for its identification. Therefore, in this work, the 450 nm band is interpreted as a marker of chlorine-derived reactive species rather than as a direct fingerprint of molecular chlorine [33,38]. Additionally, using a Cu electrode, the cyclic voltametric response demonstrated the presence of chlorine-derived species in the solution condition developed after the anodic electrolysis of Cl^-. Cyclic voltammetry in 0.1 M KCl revealed a pronounced cathodic peak on the Cu electrode at approximately −1.2 V (vs. Ag/AgCl), with a current density of nearly −8 mA cm⁻² (See Figure 8). This feature is convincingly attributed to the reduction of dissolved hypochlorous acid (HOCl) generated during the preceding anodic sweep. During cyclic voltametric potential cycling, the HOCl undergoes reduction as shown in Equation (5). Furthermore, no significant background signal was obtained from the voltametric scan indicating the responsive current is the resultant of HOCl reduction [39].

$${\mathrm{OCl}}^{-}+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to {\mathrm{Cl}}^{-}+2{\mathrm{OH}}^{-}$$

(5)

This evidence of spectral features and voltametric responses shows a correlation with the surface-mediated oxidation mechanism, wherein initial adsorbed Cl⁻ undergoes electrochemical anodic oxidation at 5 volts to generate chlorine-derived species, and subsequently hydrolyzes to HClO and ClO⁻ depending on the solution pH is demonstrated by Equations (2)–(4) [40,41,42,43]. The simultaneous observation of both species in the UV–Vis spectrum indicates a dynamic equilibrium between chlorine-derived species and their hydrolysis products, thereby supporting the role of the functionalized electrode surface in promoting Cl⁻ adsorption and subsequent oxidation. Meanwhile, the progression of chlorine-based oxidation products in 0.1 M KCl was monitored by recording the spectra at 0, 6, 12, 18, 24, and 30 s, allowing product selectivity and the optimum time-resolved operating window required for the transformation of chlorine-derived species including Cl₂ into HOCl and ClO⁻.

The spectra collected at different time intervals consistently showed that the absorbance near ~240 nm was markedly more intense than the broad feature observed around ~450 nm. This trend indicates that while chlorine-derived species are generated at the electrode surface through chloride oxidation, molecular Cl₂ is unlikely to persist as the dominant product in aqueous solution. Instead, rapid hydrolysis and subsequent speciation yield hypochlorous acid (HOCl) and hypochlorite (ClO⁻), which are more stable and therefore represent the predominant reactive species under the conditions employed.

When, later, a CV was recorded on Cu, these oxidants were cathodically reduced and produced a clear reductive peak at around −1.2 V vs. Ag/AgCl. It should be noted that the electrochemical oxidation of chloride may under certain conditions also yield secondary products such as organochlorine compounds or chloramines, as reported in previous studies [44,45,46]. Although the present work is limited to establishing the mechanistic role of carboxyl-functionalized graphite in promoting RCS generation, the potential formation of such by-products cannot be excluded and warrants systematic investigation in future research.

To investigate the influence of electrolyte concentration on the formation and distribution of RCS, electrochemical experiments were conducted at four different lower concentrations of KCl: 0.01 M, 0.025 M, 0.05 M, and 0.1 M, under a constant applied potential of 5.0 V using a CFPG electrode. The evolution of RCS was monitored by UV–Vis spectroscopy at various time intervals depicted in Figure 9A–D and observed no change in band position at all concentrations of KCl indicating the formation of chlorine-derived species, such as HOCl/ClO⁻ and possibly Cl₂, remains consistent even at lower chloride concentrations.

At 0.1 M KCl, the absorbance at 240 nm was consistently the highest across all time intervals, indicating an elevated accumulation of HClO/${\mathrm{ClO}}^{-}$ in solution. This enhancement is due to increased Cl⁻ availability at the anode surface, facilitating the electrochemical oxidation of Cl⁻ to chlorine-derived species. At higher electrolyte concentrations (0.1 M KCl), the increased ionic strength improves solution conductivity and reduces ohmic resistance resulting in more efficient charge transfer and enhanced oxidation rates [47]. Consequently, the hydrolysis of chlorine-derived species was also accelerated, promoting higher HClO accumulation and the corresponding increase in absorbance at 240 nm [35].

In contrast, the 450 nm peak associated with chlorine-derived species was significant at all concentrations, indicating the balance between Cl⁻ oxidation and hydrolysis favors the persistence of molecular chlorine in solution [48]. The chlorine-derived species formation remains efficient at all concentration while the rate of its conversion to HClO is comparatively slower than in 0.1 M KCl. Additionally, lower concentrations of KCl may foster a favorable electric double-layer structure at the electrode interface, enabling the transient stabilization of Cl₂ and delaying hydrolysis [49,50]. Conversely, at 0.1 M KCl, the higher reaction kinetics and increased local acidity near the anode surface accelerate Cl₂ consumption via hydrolysis, thereby diminishing its steady-state concentration in bulk solution. The relationship between chloride concentration and the absorbance response was examined to evaluate the chloride dependence of reactive chlorine species (RCS) formation. At a fixed observation time, the absorbance (A) at the monitoring wavelength increased with increasing [Cl⁻], indicating that chloride availability governs the generation of oxidizing species in the system. The data were fitted to a power-law model of the form

$$\log \mathrm{A}=\log \mathrm{K}+n\log \left[{\mathrm{Cl}}^{-}\right]$$

(6)

where K is a proportionality constant and n represents the apparent reaction order with respect to chloride concentration. Linear regression of log A versus [Cl⁻] (Figure 10) yielded an apparent order of n ≈ 1.1 with a correlation coefficient R² = 0.997. The near-unity order demonstrates that RCS generation under the applied conditions follows an approximately first-order dependence on chloride concentration. This analysis confirms that chloride ions play a direct role in controlling the extent of RCS formation at the electrode surface, consistent with their participation in the oxidation pathway leading to chlorine-derived oxidants. While the present dataset does not provide time-resolved kinetics, the concentration-dependent behavior offers mechanistic evidence that RCS production is chloride-limited under the studied conditions.

To understand the influence of pH on the electrochemical formation of RCS, time-resolved UV–Vis spectroscopy was further explored at controlled pH values of 1.0, 5.5, 7.0, and 13.0 following the electrochemical oxidation of Cl⁻ under constant potential. At pH 1.0, both absorbance bands appeared at 240 nm and 450 nm and demonstrated comparable intensities during the all-measurement period, representing the co-existence of Cl₂ and partially hydrolyzed products (Figure 11A). In this context, the suppressed hydrolysis kinetics favored more stability of Cl_2, thermodynamically leading to significant absorbance at 450 nm. On the other hand, a pronounced dominance of the 240 nm band is still observed at all time intervals as pH increases to 5.5, consistent with rapid Cl₂ hydrolysis toward the formation of HClO (Figure 11B). However, the absorbance intensities of both peaks (240 nm and 450 nm) were lower at pH 7.0 and 13.0 compared to pH 1 and 5.5, represented in Figure 11C,D. The partial dissociation of HClO can form ClO⁻, with a lower optical extinction coefficient at the neutral pH 7.0, and remained dominant at pH 13.0, which was responsible for the overall drop in absorbance [51]. Furthermore, hypochlorite disproportionation with electrode-generated OH⁻ species may further reduce detectable RCS concentration under alkaline conditions [52,53]. These results validate that pH can govern the distribution of chlorine species thermodynamically to generate RCS at different rate. Additionally, the enhancement of Cl₂ persistence and HClO accumulation occur in acidic conditions (pH ≤ 5.5) favoring a higher overall UV–Vis response; neutral to strongly basic conditions are more favorable toward the generation of less optically active and less stable ClO⁻.

2.3. Electrocatalytic Dye Degradation

To evaluate the practical applicability of the CFG electrode, its electrocatalytic performance toward the oxidative degradation of model dye pollutants was systematically investigated in the presence of in situ-generated RCS. Two cationic dyes, Brilliant Green and Kenacid Green, were employed to investigate the electrochemical degradation in 0.1 M KCl at 5 V. UV–Vis spectra (Figure 12A,C) were recorded to monitor the degradation and the possible mechanism (Figure 12B,D) involved in the degradation. According to the obtained UV-Vis spectra, one new band in the range of 600–650 nm and two recurring bands at ~240 nm and ~450 nm were observed for both dyes. Mechanistically, RCS, as in situ-generated products, behave as primary oxidants performing the role of degrading the dyes. The continuous generation of RCS throughout the entire reaction time frame indicates their sustainability and availability to interact with the dyes for efficient degradation. The band appeared in the visible region at 600–650 nm due to the extended chromophores presents in both dyes. Figure 12A,C show the gradual decrease in the adsorption band as the process degradation for both Brilliant Green and Kenacid Green dyes at 610 nm and 630 nm progresses, suggesting the stepwise disruption of the π-conjugation moieties through the contribution of an electrophilic oxidative pathway [54,55]. The degradation is hypothesized to follow a two-step sequential pathway (Figure 12B,D). In the initial stage (Phase I), electrophilic species such as Cl₂ and HClO initiate degradation through aromatic ring substitution, N-dealkylation, and hydroxylation, leading to the disruption of the dye’s conjugated structure [56,57]. In the subsequent stage (Phase II), these oxidized intermediates undergo aromatic ring cleavage and are further oxidized to form low-molecular-weight carboxylic acids, along with complete mineralization to CO₂, NH₄⁺, and Cl⁻ [58,59]. The comparable spectroscopic responses of Kenacid Green and Brilliant Green indicate a degradation mechanism mediated by RCS, although slight variations in degradation kinetics and chromophore resilience are evident during the time-dependent degradation performance. These variations are likely due to structural differences, as the increased polarity and distinct electron distribution of Kenacid Green may enhance its susceptibility to initial oxidative attack, resulting in a more rapid decline in visible-region absorbance relative to Brilliant Green. Thus, the degradation pathways—though influenced by structural differences, are mechanistically convergent and driven by the same RCS-mediated oxidative framework. In the present circumstances, the TOC and COD measurements were taken and 98% TOC reduction and 92% COD removal were obtained with the progressive decrease in absorbance. This spectral change was supported by bulk water quality parameters including TOC decreasing by 98%, indicating near-complete mineralization of organic carbon, and COD decreasing by 92%, demonstrating extensive oxidation of residual organic matter. The agreement among absorbance, TOC, and COD results confirms that the carboxyl-functionalized graphite electrode facilitated both decolorization and mineralization through the action of RCS. Overall, in this study, the obtained UV–Vis spectra confirmed the degradation of dyes in 0.1 M KCl solution during electrolysis at 5.0 V, demonstrating that in situ-generated RCS actively disrupt the chromophoric structures. The present work emphasizes establishing the mechanistic relationship between electrode functionalization, RCS generation, and dye degradation, rather than performing a detailed analysis of degradation intermediates or final products. The assessment of product identity and toxicity represents an important next step for future investigations. Accordingly, the current findings were regarded as a proof-of-concept for RCS-driven dye degradation, and the primary validation of water purification strategy.

3. Experimental Section

3.1. Chemicals

All reagents used in this study were of analytical grade and used without further purification. Potassium chloride (KCl) was purchased from Sigma-Aldrich, St. Louis, MO, USA. Acetic acid (-COOH), Kenacid Green, and Methylene Blue dyes were purchased commercially from Merck, Darmstadt, Germany and used as received. Millipore Milli-Q water (resistivity ≈ 18.2 MΩ·cm) was used throughout for solution preparation and rinsing procedures. Commercially available Graphite rod substrate was collected to use for the experimental investigations.

3.2. Instruments and Equipment

All real time UV–Visible absorption spectra were recorded using an AvaLight-D(H)-S spectrophotometer (Avantes, Apeldoorn, The Netherlands). pH values were measured using a digital pH meter (PH800, Apera Instruments, Columbus, OH, USA) calibrated with standard buffer solutions of pH 4, 7, and 10. Electrode-cleaning and solution-mixing procedures were facilitated by a laboratory sonicator. Electrochemical measurements were carried out using a CHI660E electrochemical workstation (CH Instruments, Austin, TX, USA) and a Potentiostat PGSTAT 128N (Metrohm Autolab, Utrecht, The Netherlands). A programmable DC Power Supply (Vantek DPS3305P) was used to apply a constant voltage during oxidation and degradation studies. The surface morphology of both bare and functionalized graphite was examined using field emission scanning electron microscopy (FESEM, JSM-7610F, Akishima, Tokyo, Japan). Elemental composition was analyzed through energy-dispersive X-ray spectroscopy (EDX) with a TM3030Plus miniscope (Hitachi Ltd., Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) was carried out using a delay-line detector (DLD) spectrometer (Kratos Axis Ultra, Kratos Analytical Ltd., Manchester, UK) with an Al Kα radiation source operating at 1486.6 eV. A precision 4-digit analytical balance (Shimadzu AUW220D, Kyoto, Japan) was used for weighing chemicals. Data analysis and plotting were performed using SigmaPlot version 10.0.

3.3. Electrode Functionalization and Electrochemical Measurements

The working electrode was prepared from commercially available graphite rods (~1 cm diameter), which were first cleaned by sonication in Milli-Q water and air-dried prior to functionalization. The surface was then polished with emery paper, rinsed thoroughly with deionized water, and treated with acetic acid for 4 h to introduce –COOH functional groups. After functionalization, the electrodes were rinsed with deionized water and dried at room temperature before use. For electrochemical measurements, the graphite rod was fixed in a Teflon holder and secured using a stainless-steel clip to ensure firm electrical contact with the potentiostat lead. All electrochemical measurements were carried out in a conventional two-compartment glass cell equipped with a three-electrode system, consisting of the –COOH-functionalized graphite working electrode, an Ag/AgCl (saturated KCl) reference electrode, and a counter electrode. While detailed porosity and microstructural parameters of the graphite were not provided by the manufacturer, the electrochemical behavior reported herein is primarily governed by the surface modification process. A platinum wire was employed as the counter electrode during cyclic voltammetry owing to its inertness and stability, whereas a stainless-steel plate was used for bulk electrolysis experiments to demonstrate a cost-effective and scalable alternative. Open-circuit potential (OCP) and cyclic voltammetry (CV) were performed to evaluate the electrochemical surface properties. Data fitting and graphical analysis were conducted using SigmaPlot software.

3.4. Chloride Ion (Cl⁻) Oxidation

The electrochemical oxidation of chloride ions was studied in a one-compartment cell containing 0.1 M KCl solution. A two-electrode configuration was employed, using the –COOH-functionalized graphite electrode as the working electrode and a stainless-steel electrode as the counter electrode. A constant potential of 5.0 V was applied using the DC Power Supply instrument. Electrolysis was carried out at an applied potential of 5.0 V to provide a sufficient driving force for chloride oxidation, ensuring stable operation and reproducible generation of reactive chlorine species (RCS) under the present conditions. The generation of reactive chlorine species was monitored using real time UV–Vis spectroscopy. This experiment was repeated using KCl solutions of variable concentrations (0.01–0.1 M) and across different pH values to evaluate the effects of electrolyte strength and pH on chloride oxidation efficiency.

3.5. Electrochemical Dye Degradation

The electrochemical degradation of dyes was performed using the same two-electrode system as described above. In this case, the electrolyte concentration was 0.1 M KCl, and dye (Kenacid Green, Brilliant green, or Methylene Blue) concentration was 5 ppm in 100 mL analyte system, where the functionalized graphite electrode served as anode. The catholyte separated with membrane was KCl only, where a Pt (1 cm²) served as a counter electrode. The anode and cathode were kept 0.5 cm apart from each other. During electrolysis, the cylindrical shaped bare and functionalized graphite electrodes with a diameter of 1 cm were dipped inside the 100 mL 5 ppm dye solution by about 2 cm. The whole system was placed above a magnetic stirring system, where the rotation speed was set to 100 rpm during the electrolysis. After the application of a fixed voltage of 5.0 V, a current magnitude of about 0.85 A was achieved during the electrolysis process. The degradation process was monitored until the complete disappearance of the characteristic dye absorbance peak. The UV–Vis absorption spectrum of the solution was recorded prior to electrolysis. A constant volt of 5.0 V was then applied, and real-time spectral data were collected at regular intervals using the UV–Vis spectrophotometer. The degradation process was monitored until the complete disappearance of the characteristic dye absorbance peak.

4. Conclusions

This work demonstrates that carboxyl-functionalized graphite electrodes, prepared via simple acetic acid treatment, exhibit significantly improved electrochemical activity for chloride oxidation followed by dye degradation. The introduction of –COOH groups enhances surface charge density, promotes chloride adsorption, and facilitates the generation of reactive chlorine species (Cl₂, HOCl), which are responsible for the efficient oxidative degradation of persistent dye molecules. The system showed consistent performance across variable electrolyte concentrations and pH conditions, confirming its adaptability. SEM and EDX analyses supported the successful surface modification, while UV–Vis spectroscopy confirmed RCS generation and dye degradation through the structural breakdown under the influence of RCS. Overall, the study provides mechanistic insight into the synergy between surface functionalization and electrochemical oxidation, offering a cost-effective and scalable approach for water purification and pollutant degradation.