1. Introduction
Biofouling, which is described as the growth of marine organisms (microorganisms, diatoms, protozoa, and sessile animals) on ship hulls presents significant challenges to the marine environment [1]. It increases hydrodynamic drag, raises fuel consumption, accelerates corrosion, and introduces invasive species [2–6]. To mitigate these issues, antifouling paints have been developed, evolving from early copper and zinc-based formulations to more advanced tributyltin (TBT)-based coatings in the 1960s [1, 7, 8]. Despite their efficacy, TBT-based paints were banned in 2008 due to their environmental toxicity [9, 10], prompting widespread adoption of ‘TBT-free’ formulations using inorganic biocides (e.g., copper oxide) and organic booster biocides (OBBs) [11].
Over 20 different types of OBBs have been developed to date. Among these, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (Diuron), 2-methylthio-4-tert-butylamino-6-cyclopropylamino-s-triazine (Irgarol® 1051), and 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT) are widely adopted [12]. Although designed as safer alternatives, these compounds still exhibit notable toxicity toward non-target marine species and persist in the environment [13–15]. Diuron, a phenylurea-based herbicide, and Irgarol® 1051, a s-triazine-based compound, can inhibit photosystem II in algae and plants, with relatively long aqueous photolysis half-lives (43–2180 days for Diuron [16], 100–350 days for Irgarol® 1051) [17, 18]). Due to their ecotoxicity, a global ban on Irgarol® 1051 has been announced under the International maritime organization (IMO) convention, with enforcement expected in 2028 [19–22]. DCOIT, an isothiazolinone-based biocide, produces free radicals under sunlight exposure and has been shown to cause toxicity to non-target marine species including mussels (Perna perna) and marine medaka (Oryzias melastigma) [23, 24]. Their persistence and toxicity still pose ecological risks, underscoring the urgent need for effective degradation strategies.
Various literature studies have explored the removal of OBBs, namely biological treatment [25, 26], photocatalytic degradation [27], and electrochemical oxidation (EO) [28–30]. EO is particularly effective for removing persistent and toxic organics within relatively short treatment times, as it generates oxidants through both direct and indirect pathways [31] [32]. It has also been applied to treat a wide range of industrial effluents, including textile wastewater, Membrane bioreactor (MBR) pretreatment, tannery effluents, and car wash wastewater [33–36]. Although research on Diuron, Irgarol® 1051, and DCOIT removal on EO has been conducted [37–39], significant gaps remain in our understanding, and the results have been limited, with most prior studies focusing mainly on photodegradation [40, 41] and microbial degradation [42, 43]. Furthermore, despite the potential for significant ecological adverse impacts, comprehensive studies investigating the leaching characteristics of these OBBs and effective remediation strategies are conspicuously absent.
Hence, this study assessed the environmental impact of the representative OBBs such as Diuron, Irgarol® 1051, and DCOIT based on their release behavior and biotoxicity. Their degradation through electrochemical oxidation (EO) was subsequently investigated, and optimal conditions, including current density, electrolyte type, pH, and electrode spacing, were identified to maximize removal efficiency. Subsequently, the balance between current density and energy consumption (to achieve 99.9% removal) was carefully evaluated. The role of reactive oxygen species (ROS) in the degradation process was also examined through scavenger tests using radical-quenching agents. This study is anticipated to propose a more economical and efficient method to degrade OBBs to improve the marine environment.
2. Materials and Methods
2.1. Organic Booster Biocides Release Test
To confirm release behaviors of OBBs, SS400-grade ship steels (cut into 110 mm × 230 mm × 15 mm) were coated with DCOIT-containing antifouling paint (A-LF-Sea 600 Cocoa Brown, Nippon Paint, Japan). Three layers of paint was coated onto the steel plates, each of which was air-dried at room temperature overnight to ensure proper adherence of the coating. The coated steel plates were then placed in 2.0 L beaker. The beaker will be filled with 1.8 L of either seawater or distilled water (DI water) for the release test. To simulate real environmental conditions, the beakers were stirred at 250 rpm using a jar test apparatus. Periodically, DI water will be added to each reactor, respectively, to compensate for any potential evaporation. The samples were collected on 0th, 30th, and 60th day. Before the analysis, the samples were filtered through a 0.45 μm syringe filter and concentrated 1000× before analysis. The concentration of released OBBs was determined using liquid chromatography-mass spectrometry (LC-MS, HPLC Ultimate 3000, MS Q ExactiveTM Plus, Thermo Fisher Scientific, USA) that is equipped with a C18 column. After the experiments were completed, the corrosion behavior on the antifouling-coated steel plates was observed using a scanning electron microscope (SEM, TESCAN CLARA, Czech Republic).
2.2. Biotoxicity Assessment
The biotoxicity test was conducted based on the acute toxicity testing method using water fleas, as specified in Republic of Korea’s water pollution testing standards [44]. For the biotoxicity test, Daphnia magna Straus (provided by the National Institute of Science and Technology, Republic of Korea) was used as the test organism. To ensure consistency of the results, the adult female Daphnia that were acclimated in fresh culture medium the day before the experiment and neonates less than 24-hour were used. The culture medium was prepared with essential minerals, including potassium chloride (KCl), magnesium sulfate (MgSO4), calcium sulfate dihydrate (CaSO4•2H2O), and sodium bicarbonate (NaHCO3) at 8 mg/L, 120 mg/L, 120 mg/L, and 192 mg/L, respectively. Subsequently, a 3000 mg/L stock solution of OBBs was prepared using ethanol as the solvent, and test solutions were prepared at different concentrations (0 mg/L, 0.5 mg/L, 5 mg/L, 10 mg/L, 20 mg/L, and 30 mg/L). The 3000 mg/L stock solution was diluted using the Daphnia culture medium to obtain the desired test concentrations. The tests were conducted at a controlled temperature (19 to 21°C) for 24 hours (light/dark cycle of 16 hours/8 hours). Each test solution (50 mL) contained five Daphnia, and four replicates were conducted for each concentration. After 24-hour exposure, the Daphnia were observed for immobilization and mortality. Mortality was defined as the absence of movement when the beaker was gently tapped, and immobilization was defined as the inability to swim. The EC50 values were calculated using the Trimmed Spearman-Karber method from the dose-response curve [45].
2.3. Electrochemical Oxidation Experiments
2.3.1. Preparation of OBBs solutions
Stock solutions of OBB solution that contains Diuron (Sigma-Aldrich, USA), Irgarol® 1051 (PESTANAL®, Sigma-Aldrich, USA), and DCOIT (AldrichCPR, Sigma-Aldrich, USA) were prepared by dissolving each substance in acetonitrile to achieve a concentration of 3000 mg/L. This solvent is used as it does not act as a scavenger for reactive oxygen species or radicals. Subsequently, experimental solutions were diluted further to 30 mg/L. Diuron, Irgarol® 1051, and DCOIT was diluted with 0/100, 10/90, and 30/70 acetonitrile/ultrapure water mixture, respectively. This is because their solubility in water decreases in the order of Diuron, Irgarol® 1051, and DCOIT. Sodium sulfate (Na2SO4) and sodium chloride (NaCl) were used as electrolytes, with sodium hydroxide (NaOH) and sulfuric acid (H2SO4) were used to adjust the pH of the solution.
2.3.2. Reactor configuration and electrochemical oxidation procedure
The EO reactor, constructed from acrylic with dimensions of 94 mm × 120 mm × 75 mm, was designed with a working volume of 300 mL for the different OBB solutions (Fig. 1). It featured internal grooves allowing the electrode spacing to be adjusted to a 2 mm gap, and a 22 mm space from the bottom was reserved for proper stirring (250 rpm). The anode was a titanium (Ti) electrode coated with platinum (Pt loading: 4.13 mg/cm2; coating thickness: 2 μm), and the cathode was a Ti electrode, both with a surface area of 50 cm2 (71 mm × 71 mm × 1 mm) and designed with 6 mm diameter circular holes to facilitate solution movement.
EO experiments were conducted using a DC power supply (OPE-3010 DI, ODA, Republic of Korea), with current densities set at 20, 40, 60, and 80 mA/cm2. The initial concentration of the solutions was set at 30 mg/L for all three toxic substances, with an initial solution volume of 280 mL. The oxidation tests were conducted over 3 hours, with 6 mL samples collected at 0, 1, 5, 15, 30, 45, 60, 120, and 180 minutes. Furthermore, to identify the optimal conditions for OBB degradation, the degradation performance was evaluated by varying the electrolyte, pH, and spacing between the electrodes.
Electrolyte concentrations of 0.05 M NaCl, and 0.05, 0.1, and 0.15 M Na2SO4 were used to assess the impact of electrolyte strength on the removal efficiency of OBBs. The pH conditions were varied across 3, 6, and 10 to evaluate the influence of acidity and alkalinity on the EO process. Additionally, the spacing between the electrodes was adjusted to 0.2 cm, 1.0 cm, and 3.0 cm to analyze the effect of spacing between electrodes on the oxidation efficiency and energy consumption.
2.3.3. Analytical methods
Following the completion of the EO tests, the residual concentrations of OBB samples were quantified. This analysis was performed using high-performance liquid chromatography (HPLC, LCMS-2050, Shimadzu, Japan) that was equipped with a C18 LC column (4.6 × 150 mm, 5 μm, Agilent, USA). The mobile phase for all three substances was consisted of 60/40 acetonitrile/ultrapure water mixture, with a flow rate set at 1 mL/minutes. The detection wavelengths for Diuron, Irgarol® 1051, and DCOIT were 254 nm, 223 nm, and 210 nm, respectively, with the respective retention times of 2.7 minutes, 5.4 minutes, and 18.8 minutes. The injection volumes for Diuron, Irgarol® 1051, and DCOIT were registered at 20 μL, 20 μL, and 40 μL, respectively. During the experiment, it was observed that larger volume for DCOIT is required to increase the sensitivity of the LC result. Subsequently, the pH of the residual concentration was measured with a pH meter (Orion StarTM A216, Thermo Fisher Scientific, USA). Total organic carbon (TOC) analyzer was used to measure the carbon content in the residual sample (TOC-LCPH, Shimadzu, Japan). Lastly, the energy consumption can be obtained through the current density plot by taking the product between the voltage and current used in the EO test.
2.3.4 Scavenger test
The reactive oxygen species (ROS) considered were hydroxyl radicals (•OH), sulfate radical (•SO4−), superoxide (O2−), and singlet oxygen (1O2). The scavengers used in this experiment are tert-butanol (•OH), absolute ethanol (•OH, •SO4−), p-benzoquinone (O2−), and furfuryl alcohol (1O2) (Supplementary Materials Table S1). The electrochemical oxidation for the scavenger test were based on the optimal condition identified in Section 2.3. However, to ensure accurate assessment of ROS contributions, all experiments were conducted at a fixed current density of 20 mA/cm2, as higher current densities may cause the reactions to proceed too quickly, making it difficult to perform precise evaluation.
3. Results and Discussion
3.1. Release Characteristics of OBBs from Ship Antifouling Paints
The corrosion behavior of ship steel in DI water and seawater was investigated over a 60-day period. As shown in Supplementary Materials Fig. S1a–e, no corrosion was observed in DI water (Supplementary Materials Fig. S1b–c), suggesting that its non-reactive nature contributes to the stability of the antifouling coating. In contrast, significant paint degradation and steel corrosion were evident after seawater exposure (Supplementary Materials Fig. S1d–e), which was further confirmed by SEM analysis. Initially, the steel surface was smooth and intact (Supplementary Materials Fig. S1f), whereas clear cracks appeared after 60 days, indicating substantial corrosion and coating failure (Supplementary Materials Fig. S1g).
The observations from Fig. 2 were further investigated with DCOIT release analysis. While the concentration of DCOIT in DI water remained consistently below 0.16 μg/L over 60 days, drastic increase in the DCOIT concentration was observed when seawater is used. Under the same duration, the concentration of DCOIT in DI water were 0 μg/L and 0.15 μg/L at 30 and 60 days, respectively. In comparison, the concentrations of DCOIT in seawater were registered at 1.46 μg/L and 5.89 μg/L at the same period (Fig. 2). This indicates that DCOIT concentration in seawater after 60 days was c.a. 39-fold higher than in DI water. This released concentration is similar with the reported DCOIT concentrations (3.7 μg/L) in the Mediterranean waters in Catalonia, Spain [46].
These results suggest that ions in seawater enhance solution conductivity and facilitate the release of OBBs, thereby accelerating steel corrosion compared to DI water [47]. Nevertheless, it should be noted that other factors (e.g., pH as well as presence of aquatic pollutants and microorganisms) may influence the corrosion behavior [48].
3.2. Biotoxicity of OBBs
The biotoxicity of three OBBs (Diuron, Irgarol® 1051, and DCOIT) was evaluated based on the survival rate (i.e., viability) of Daphnia magna (Fig. 3). The ethanol-only control confirmed negligible toxicity up to 30 mg/L, validating its use as a solvent.
From Fig. 3, a sharp decline in the survival of Daphnia magna was observed with an increase in Diuron concentration. For instance, the survival rate decreased to c.a. 35% at the concentration of 10 mg/L. This is similar to the reported Diuron exposure based on EC50 calculations (6.12 mg/L vs. 8.6 mg/L in the previous study [49]). Given the low toxicity of ethanol itself, the observed reduction in survival can be attributed primarily to the toxic effects of Diuron. Similarly, the results for Irgarol® 1051 showed a marked decrease in the survival of Daphnia magna under an increased OBB concentrations. At 10 mg/L, the survival rate fell to c.a. 50%. The computed EC50 for Irgarol® 1051 was calculated at 9.30 mg/L, which is closely matched with the reported literature value (10.1 mg/L) [26]. These findings suggested that Irgarol® 1051 has comparatively lower toxicity with reference to Diuron. In comparison, DCOIT exhibited extremely high toxicity, with complete a mortality of Daphnia magna at low concentration (0.05 mg/L). This result is also supported with previous study with the reported caused EC50 of 0.12 mg/L under the same 24-hour exposure time [50]. Hence, these results confirmed the potent toxicity of the studied OBBs, which is deemed threatening to the wildlife in seawater.
3.3. Optimization of Electrochemical Oxidation Conditions
3.3.1. Current density
Fig. 4a–c show the removal efficiencies of each compound under various current densities. As shown in Fig. 4a, Diuron removal efficiency increased markedly with higher current density, rising from 24.0% to 83.9% after 15 minutes. After 45 minutes, removal rates increase to 60.6%, 83.5%, 89.1%, and 93.8%, respectively, indicating a clear enhancement in Diuron removal. Irgarol® 1051 showed rapid degradation, reaching nearly complete removal within minutes even at low current densities, with minimal differences observed beyond 40 mA/cm2. DCOIT was also rapidly removed, with minimal variation across current densities, indicating efficient degradation even at lower levels.
Based on the observation above, it is anticipated that at higher current density, the current that flows across the electrodes increases, thus allowing more electrons to be available at the electrode surface, which enhances the rate and extent of the chemical reactions[51]. While higher current densities led to increased reaction efficiency, excessive current densities can also result in electrode damage and increased energy consumption, reducing the overall economic feasibility of the process. Hence, additional analysis will be performed in Section 3.4 to observe the trade-off and optimize the appropriate current density for EO process, after other process parameters (electrolyte, pH, and electrode spacing) have been optimized accordingly.
3.3.2. Electrolyte
The removal efficiencies of Diuron, Irgarol® 1051, and DCOIT at varying Na2SO4 concentrations are presented in Fig. 5a-c. In the case of Diuron (Fig. 5a), there was no significant difference in removal efficiency with increasing Na2SO4 concentration, suggesting that the concentration of Na2SO4 does not play a major role in the degradation process. This indicates that sulfate radicals (•SO4−) from Na2SO4 are ineffective to remove Diuron.
To further investigate this inefficiency, additional experiments were conducted using NaCl (0.1 M) as an alternative electrolyte (Supplementary Materials Fig. S2). Compared to Na2SO4, NaCl significantly improved Diuron removal, achieving 99.3% removal within 30 minutes, whereas Na2SO4 resulted in only 44%. This enhancement is attributed to the formation of active chlorine species (e.g., chlorine gas (Cl2), hypochlorous acid (HOCl), and hypochlorite ion (ClO−)), which are known to be more effective oxidants for Diuron [52]. Based on this result, NaCl (0.1 M) was selected as the optimal electrolyte for Diuron in the pH experiment (Section 3.3.3), as the interaction between pH and electrolyte type was considered important. While NaCl was also tested for Irgarol® 1051 and DCOIT, no measurable peaks were observed under the same analytical conditions, making it difficult to quantify removal efficiency. Therefore, further discussion of NaCl was limited to Diuron. However, in the electrode spacing experiment (Section 3.3.4), the effect of applied voltage was deemed more significant than electrolyte type. Therefore, additional experiments using NaCl were not conducted, and Na2SO4 was used for all compounds to ensure consistency across conditions.
In contrast, the removal efficiency of Irgarol® 1051 (Fig. 5b) improves with an increase in Na2SO4 concentration. For instance, at Na2SO4 the removal efficiencies after 15 minutes were registered at 75.8%, 81.0%, and 95.0%, respectively for concentration of 0.05 M, 0.1 M, and 0.15 M. The increase in the removal efficiency at higher electrolyte concentrations indicates the presence of •SO4− radicals in Na2SO4 plays an important role in the degradation of Irgarol® 1051. This effect contributes significantly at higher concentrations. Hence, the optimal electrolyte concentration for Irgarol® 1051 was determined to be 0.15 M Na2SO4.
Lastly, in the case of DCOIT (Fig. 5c), the removal efficiency decreases with an increased in Na2SO4 concentrations. This significant contrast can be explained by the saturation of the electrode surface as Na2SO4 concentration increases. The increased Na2SO4 concentration likely limits the availability of active sites on the electrode surface, thereby reducing the efficiency of the electrochemical reactions [53]. Based on the optimization process, 0.05 M Na2SO4 demonstrates the highest removal efficiency. Thus, the optimal electrolyte concentration for DCOIT was determined to be 0.05 M Na2SO4.
3.3.3. pH
pH is also a critical factor in the EO of OBBs. Thus, the removal efficiencies of Diuron, Irgarol® 1051, and DCOIT were investigated at three different pH levels (Fig. 6a-c). NaCl solution was used as the electrolyte for Diuron removal, while Na2SO4 solution was employed for Irgarol® 1051 and DCOIT removal as discussed in Section 3.3.2. Diuron achieved a removal efficiency of 92.8% within 5 minutes at pH 3, while Irgarol® 1051 and DCOIT showed similarly high removal rates of 95.5% and 97.3%, respectively. These results indicated that the highest removal efficiencies within a short period can be achieved at pH 3, as evident from previous study [52]. This suggests that under acidic condition, the strong oxidative activity of chloride ions (Cl−) and •SO4− plays a significant role. In particular, under acidic condition, the generation of peroxydisulfate ions (S2O82−) from Na2SO4 improve the efficiency of EO process [54].
Nevertheless, it should be noted that for the case of Diuron removal, the presence of chlorine in the electrolyte may produce halogenated products, which is detrimental to health and environment [55]. In general, the chemical form of chlorine changes depending on the pH of the electrolyte. Chlorine exists as hypochlorous acid (HClO) and hypochlorite ion (ClO−) at low and high pH, respectively [56]. In comparison, ClO− has a weaker oxidative power than HClO. Hence, less halogenated byproducts are produced under high pH condition. Therefore, while high removal rates can be achieved at low pH, the potential for halogenated byproduct formation due to the increased production of HClO is increased. This highlights the need to balance the efficiency of OBB removal and generation of byproduct.
3.3.4. Electrode spacing
The effect of electrode spacing on removal efficiency was evaluated using Supplementary Materials Fig. S3a–c and applied voltages in Supplementary Materials Fig. S4. For Diuron and Irgarol® 1051, similar removal efficiencies (72–79%) were observed across 0.2, 1.0, and 3.0 cm, with the highest at 0.2 cm. DCOIT showed greater sensitivity to electrode spacing, with 90% removal at 0.2 cm compared to 79% at 3.0 cm. Applied voltage increased with spacing—for DCOIT, from 5.1 V (0.2 cm) to 11.0 V (3.0 cm). These results suggest that smaller electrode gaps enhance removal efficiency and reduce energy consumption, with 0.2 cm being optimal for all compounds.
Previous studies support this, noting that wider gaps reduce conductivity and removal rates[57], while narrower gaps improve efficiency due to higher current density[58]. However, spacing below 0.2 cm may hinder fluid flow and increase resistance[59]. Therefore, optimizing electrode spacing is critical for balancing performance and energy efficiency.
3.4. Performance evaluation of EO process
3.4.1. Trade-off between removal efficiency and energy consumption
EO process is an energy-intensive operation. While high current densities improve the efficiency and removal rates of OBB, energy consumption also increases. Thus, it is crucial to optimize the current density to ensure that both process costs and treatment times can be reduced while ensuring economic feasibility. This section provides an economic evaluation based on the energy consumption results at different current densities (Fig. 7). The analysis includes the energy consumed (Wh/mgremoved), which means the amount of energy required, in watt-hours (Wh), to remove one milligram of OBBs during the EO process. Moreover, 99.9% removal of Diuron, Irgarol® 1051, and DCOIT can be achieved at the specified condition (0.05 M Na2SO4, pH 6, and 1.0 cm electrode spacing).
In the case of Diuron (Fig. 7a), increase in the current density significantly reduces both energy consumption and time required to achieve 99.9% removal. Specifically, the time required to achieve 99.9% removal (at current density of 80 mA/cm2) was 7.4 times shorter, together with 7.2 times reduction in the energy consumption, as compared to 20 mA/cm2. These results indicate that higher current densities lead to faster oxidation and lower energy consumption, suggesting that 80 mA/cm2 the most cost-effective condition for Diuron removal. However, it is anticipated that true optimal condition beyond 80 mA/cm2 may still be possible to be achieved, as the trend display a consistent decrease in both energy consumption and the time to achieve 99.9% removal. This is likely due to Na2SO4 being an unsuitable electrolyte for Diuron removal. As discussed in Section 3.3.2, the use of NaCl, which exhibits superior Diuron degradation efficiency, could allow for much lower current densities and reduced energy consumption.
Similarly, increasing the current density reduces the required time to achieve 99.9% removal for Irgarol® 1051 (Fig. 7b). Based on the result, the required time decreases from 33.4 minutes to 3.9 minutes with the increase in current density from 20 mA/cm2 to 80 mA/cm2. However, beyond 40 mA/cm2, the removal time was fairly stabilized. Thus, the required energy consumption increases drastically from 0.21 Wh/mgremoved to 0.45 Wh/mgremoved at 40 mA/cm2 and 80 mA/cm2, respectively. It should be noted that the required energy consumption was comparable for the current density at 20 mA/cm2 and 80 mA/cm2, respectively. Thus, based on these results, the optimal current density for Irgarol® 1051 is set at 40 mA/cm2. On the contrary, the profile for energy consumption and removal time of DCOIT (Fig. 7c) showed that both parameters increase with the increase in current density. This could be possibly due to the comparable removal rate (6.0 to 7.8 minutes) under the current density between 20 to 80 mA/cm2. Hence, the considering the minimal benefit of incorporating additional energy consumption without improving the removal time, 20 mA/cm2 is the optimal condition for removing DCOIT.
Hence, based on the analysis conducted in Section 3.3 and Section 3.4, the optimal condition for the removal of OBB by taking the minimization of energy consumption as the main consideration were presented in Supplementary Materials Table S2.
3.4.2. Byproduct formation during EO
In the EO process, the formation of byproducts is inevitable due to the involvement of complex chemical reactions. During the disintegration of OBBs, various intermediate and secondary compounds are generated through partial oxidations and various side reactions. These byproducts can vary depending on electrolyte concentration and pH, current density, as well as the studied OBBs. The generation of byproducts can be verified by comparing the difference in the concentration between OBBs and TOC. The experiment was conducted at a low current density (20 mA/cm2) as it was observed that the formation of byproduct is most pronounced. First, all studied OBBs (Diuron, Irgarol® 1051, and DCOIT) are feasible to achieve 100% removal within 180 minutes (Supplementary Materials Fig. S5). However, the TOC removal rates for Diuron, Irgarol® 1051, and DCOIT were corresponded to 51%, 46%, and 34%, respectively. Therefore, although all OBBs were successfully removed, it is likely that byproducts remain present.
The observed difference between OBB and TOC concentrations may suggest the formation of unknown byproducts during the OBB disintegration process, but further investigation is required to confirm this hypothesis. Further analysis on the OBBs (Diuron, Irgarol® 1051, and DCOIT) concentration profile shows that a significant difference between the degradation rate of OBBs and TOC at the initial stage. This potentially suggests the rapid byproduct formation at the early stages of the reaction. It is anticipated that with an increase in current densities, the difference between OBB degradation and TOC concentration decreases rapidly. This suggests that higher current densities may facilitate the byproduct degradation.
Although this study did not specifically identify intermediate byproducts, it is well known that various byproducts are generated during the degradation of OBBs [60, 61]. Future studies would benefit from using advanced analytical techniques, such as LC-MS, to elucidate the structure, toxicity, and environmental persistence of these intermediates. These findings underscore the importance of evaluating the toxicity of these byproducts.
3.5. Effect of Reactive Oxygen Species on OBBs Degradation
Next, the contribution of different ROS species towards the degradation of OBBs through EO was evaluated using the scavenger test (Fig. 8). The scavenger test allows the evaluation of the contribution of different ROS towards the degradation process by comparing the ratio of the initial concentration to the final concentration (C0/C) with the absence and presence of specific scavengers. The impact of each scavenger towards the degradation of OBBs was compared to understand the underlying mechanism of the EO process.
Fig. 8a shows the contribution of each ROS towards the EO for Diuron removal after 60 minutes. 1O2 contributed the most at 83%, followed by O2− and •OH at 29% and 1%, respectively. Hence, these results indicate that 1O2 plays a dominant role in the degradation of Diuron in EO. This is also consistent with the optimization study conducted in Section 3.3, where NaCl is effective electrolyte for Diuron degradation. This is attributed to the enhancement production of both Cl− and 1O2 under acidic condition (pH = 3). This is also evident from previous study on wastewater treatment containing quaternary ammonium compounds (QACs) under high salinity conditions, where Cl− ions promote the formation of ClO− and 1O2 [62]. This is further enhanced by the enhanced formation of 1O2 in acidic condition through the interaction between H+ and OH− ions. In contrast, the overall removal efficiency of Diuron is lower when Na2SO4 is used as the electrolyte due to the less effective generation of 1O2.
In comparison, the contribution of ROS to the EO of Irgarol® 1051 after 60 minutes has observed that 1O2 contributes the most at 90%, followed by O2−, •SO4− and •OH at 31%, 28%, and ≈ 0%, respectively (Fig. 8b). Similar to Diuron, 1O2 is the primary reactive species in the oxidation of Irgarol® 1051. However, the contribution of •SO4− is relatively higher. This is potentially attributed to the use of Na2SO4 as the electrolyte. This allows the creation of •SO4− in EO. Lastly, there are no dominant ROS that specifically contributed towards the removal of DCOIT (Fig. 8c) during 60 minutes duration. O2−, 1O2, •OH and •SO4− contributed at 12%, 11%, 4%, and 2%, respectively. This suggests that the steric hindrance present in DCOIT may influence the reactivity of ROS during the EO process in comparison to Diuron and Irgarol® 1051. Therefore, other oxidants (e.g., ozone (O3) or hydrogen peroxide (H2O2)) may also contribute to the degradation of DCOIT. Overall, the observed differences in ROS contributions across OBBs highlight the importance of understanding the specific mechanisms involved in EO processes.
Given these differences, the role of the electrode material in facilitating ROS generation becomes critical. Among various EO anode materials, dimensionally stable anodes (DSAs) are widely adopted due to their excellent chemical stability and high oxidation potential. These electrodes typically consist of a titanium (Ti) substrate coated with noble metal such as palladium (Pd), ruthenium (Ru), iridium (Ir), or platinum (Pt), formulated as single or mixed oxide layers [51]. While Pd, Ru, and Ir are often selected for their relatively lower cost, recent increases in raw material prices (e.g., Pt = 32.9 USD/g, Pd = 32.0 USD/g, Ru = 14.9 USD/g) have narrowed the economic advantage among these metals.
Despite its higher cost, platinum remains a widely preferred choice for EO applications due to its superior ability to generate reactive oxygen species—particularly hydroxyl radicals and singlet oxygen. In our study, the use of Pt-coated DSAs likely contributed to the high removal efficiencies observed, especially under acidic conditions where ROS formation is enhanced. Additionally, the strong oxidative performance of Pt enabled shorter treatment durations and lower energy demands, as demonstrated in Section 3.4.1. These factors collectively support the technical and economic feasibility of employing Pt-based electrodes for the efficient degradation of persistent OBBs in marine environments.
4. Conclusions
In this study, the release characteristics and biotoxicity of OBBs towards marine and aquatic ecosystems were analyzed. Considering the concentration required to demonstrate clear harmful effect to living organism is low (based on EC50), the removal of OBBs were performed with EO process, where additional optimizations based on the change in electrolyte concentration, pH, electrode spacing and current density were performed to ensure that OBBs can be removed effectively (99.9%) without substantial strain towards the increase in energy consumption. Future works will be emphasized on the understanding of the exact transformation pathways on the creation of byproducts during the OBBs degradation as well as the degradation mechanisms of biocides based on the contributions of various ROS. It is expected that these results provide a solid foundation for the future discharge standards on antifouling paint contaminants to effectively protect aquatic ecosystems.
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