Comparative Evaluation of UV-C-Activated Peroxide and Peroxydisulfate for Degradation of a Selected Herbicide

Abstract

Extensive utilization of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) has resulted in contamination of the aquatic environment; this situation requires effective treatment technology. Ultraviolet-based advanced oxidation processes (UV-AOPs) are widely employed for the removal of organic contaminants from water. This study’s aim was to compare the degradation of the pesticide 2,4-D in UV-C-activated peroxide and peroxydisulfate systems. UV-C irradiation alone exhibited a negligible effect on pesticide degradation, whereas the addition of oxidants significantly enhanced the degradation efficiency relative to 2,4-D. Complete pesticide removal was achieved after 15 min of UV/H₂O₂ treatment, while twice as much time was required with the UV/S₂O₈²⁻ process. COD decreased by 74% and 28% for UV-C-activated peroxide and peroxydisulfate, respectively. Both investigated systems demonstrated good performance for 2,4-D dechlorination. Pesticide degradation rates increased with increasing dosages of the applied oxidants. Acidic conditions were more favorable for degradation of 2,4-D, compared to neutral and basic conditions, for both systems studied. The degradation efficiency relative to 2,4-D decreased in the presence of HA, Cl⁻ and HCO₃⁻ in water matrices. The predominant radical for the UV-C-activated peroxydisulfate was determined to be a sulfate radical. These findings are of fundamental and practical significance in understanding UV-C-activated 2,4-D degradation, paving the way for the selection of preferred processes for the optimal removal of pesticides from various aqueous matrices.

1. Introduction

The consumption of pesticides has increased considerably, because they are used on a large scale for pest control. Organochlorine pesticides are frequently used due to their high efficacy and low cost [1]. Among organochlorine pesticides, 2,4-dichlorophenoxyacetic acid (2,4-D) is one of the most widely used in the world [2]. This pesticide is a selectively killing herbicide that is suitable for preventing the growth of broadleaf weeds, and it effectively destroys various types of weeds [3]. The low cost, good accessibility and high water-solubility of this herbicide, together with its low adsorption coefficient in the soil, are taken into account when it is used to control various types of weeds [4]. According to the USEPA, 2,4-D has moderate toxicity to birds and mammals, is slightly toxic to fish and aquatic invertebrates and is practically non-toxic to honeybees. The herbicide 2,4-D was classified as “possibly carcinogenic to humans” in a report of the International Agency for Research on Cancer [5]. The NORMAN SLE database has categorized 2,4-D as a potential endocrine disruptor in humans [6]. 2,4-D can enter the aquatic environment through direct application for weed control, disposal of waste from manufacturing and production facilities, runoff from treated areas and drift during applications [7]. A comprehensive review of Islam et al. showed that 2,4-D has frequently been detected in surface waters and groundwater [8]. The estimated concentrations of 2,4-D in freshwater bodies range from 4 to 24 μg dm⁻³, whereas in agricultural fields, its concentration can reach up to 4000 μg dm⁻³. According to the European Commission guideline, the maximum allowable concentration is 0.1 μg dm⁻³ for individual pesticides and 0.5 μg dm⁻³ as the total of all pesticides present, while the World Health Organization (WHO) has recommended 20 μg dm⁻³ as the maximum permissible concentration of 2,4-D in drinking water [8]. Furthermore, water bodies surrounding rice cropping areas are likely to contain elevated concentrations of the pesticide 2,4-D, as pesticide preparation for crop spraying and the subsequent cleaning of spray tanks are typically conducted in the same field, potentially serving as a point source of 2,4-D input to the environment. Osborne et al. reported that the concentration of 2,4-D in a spray tank prior to the first rinse was 312 mg dm⁻³, which was reduced to 23 mg dm⁻³ after the first rinse and further decreased to 3.3 mg dm⁻³ after the third rinse [9]. Therefore, a large percentage of the pesticide to be applied ends up in the water, thus posing a threat to the exposed vegetation and animal life [10]. In addition, the herbicide 2,4-D is carried by runoff into local river systems, which is a cause for alarm as to the aquatic life. Given the aforementioned evidence, it is imperative to develop strategies for the degradation and removal of this toxic and persistent herbicide in aquatic ecosystems to mitigate environmental contamination.

Currently, various physicochemical techniques have been employed to remove the herbicide 2,4-D from water-based solutions, including adsorption [11], catalytic ozonation [10], photocatalytic oxidation [12] and photo-electrocatalytic degradation [13]. Generally, these approaches can be divided into two main groups: non-destructive methods, such as adsorption, and destructive processes, like oxidation. Advanced oxidation processes (AOPs) are recognized to be efficient methods for fully breaking down harmful, persistent and non-biodegradable organic contaminants. These processes achieve this goal by generating non-selective and reactive radicals, such as hydroxyl and sulfate radicals (^•OH, SO₄^•−). Generally, the oxidation process occurs in two phases. Initially, potent oxidants, such as hydroxyl or sulfate radicals, are generated. Subsequently, these oxidants interact with organic pollutants in the wastewater. Hydroxyl radicals, characterized by their high redox potential (2.8 V) and non-selective nature, serve as powerful oxidizing agents in AOPs for wastewater treatment [14]. Sulfate radicals-based advanced oxidation processes (SR-AOPs) have emerged as a promising alternative to traditional hydroxyl radicals-based AOPs (HR-AOPs). This is due to the numerous benefits offered by sulfate radicals, including their superior standard oxidation potential (2.5–3.1 V), enhanced selectivity and efficiency and ability to effectively react with organic compounds across a broad pH spectrum of 2–8. Additionally, the extended half-lives of sulfate radicals, lasting 30–40 μs, enable improved mass transfer stability and better interaction with target compounds [15]. Both of these radicals are capable of breaking down wastewater contaminants and converting them into less harmful or non-toxic substances, thus offering a comprehensive solution for wastewater purification [16]. The effectiveness of these oxidation technologies is largely determined by the generation of highly reactive free radicals. In addition to this, the degradation efficiency relative to organic pollutants in various AOPs is highly dependent on the chemical structures of the target compounds, since these two radicals possess different levels of selectivity and mechanisms of reaction. ^•OH radicals exhibit low selectivity and can react with organic pollutants by means of hydrogen abstraction or addition, or electron transfer mechanisms, while SO₄^•− radicals have high selectivity and prefer to react with organic pollutants via an electron transfer mechanism [17]. Thus, depending on the chemical structures of the organic pollutants, sulfate radicals may exhibit greater or lesser removal efficiency relative to the organic compounds, compared to the hydroxyl radicals. Recently, the utilization of other oxidants, such as peroxymonocarbonate (HCO₄⁻), which is generated in situ through an equilibrium reaction between H₂O₂ and HCO₃⁻ in aqueous solutions, has increasingly garnered the attention of researchers. The activation of HCO₄⁻ and the subsequent generation of reactive oxygen species (^•OH and CO₃^•−) can be accomplished through heterogeneous catalysis [18,19] and thermal activation [20] and can be applied for efficient degradation of organic pollutants.

Advanced oxidation processes offer several advantages under normal conditions: they effectively break down organic substances, they are cost-effective and eco-friendly, they eliminate the need to transfer contaminants between media, they require minimal chemical usage and they typically have a brief reaction period. According to their mode of oxidation, AOPs can be classified as chemical, photochemical, electrochemical or sonochemical processes [21]. The most-used photochemical AOPs utilize energy sources for the formation of highly reactive radical species via the activation of oxidants. Among them, UV irradiation is considered to be one of the most efficient methods for activating oxidizing agents, due to its high irradiation energy [22]. UV/oxidation processes combine ultraviolet light (particularly UV-C light) with oxidizing agents such as hydrogen peroxide, persulfates and chlorine to remove organic contaminants from water. The mechanisms of UV-C/oxidation can be categorized into two types: direct and indirect oxidation [23]. In direct oxidation, contaminants are degraded through direct exposure to UV-C light and/or oxidized by an oxidizing agent. Indirect oxidation, on the other hand, involves the breakdown of pollutants by the radical species produced when an oxidant undergoes photolysis. Utilizing UV-C light in conjunction with peroxide as an oxidizing agent offers several benefits. Peroxide is water-soluble, stable and readily available commercially. Additionally, this process does not result in sludge formation [24]. Alternatively, persulfates serve as effective oxidants in AOPs due to their advantageous qualities: (1) persulfate salts are easily obtainable; (2) storage and transportation costs are reduced; and (3) sulfate radicals are produced in high quantities and have longer half-lives, compared to other reactive oxygen species [25]. The persulfates commonly used for the generation of sulfate radicals are peroxymonosulfate (HSO₄⁻, PMS) and peroxydisulfate (S₂O₈²⁻, PDS).

Various advanced oxidation processes have been employed for the degradation of the pesticide 2,4-D. However, research has predominantly concentrated on the detailed examination of individual systems, making it challenging to compare results across different processes, due to variations in the systems utilized. To the best of our knowledge, a comprehensive comparative analysis of the UV-C-activated degradation of the pesticide 2,4-D, in conjunction with the various oxidants and the radicals generated (particularly ^•OH and SO₄^•−), has not yet been conducted. Considering the non-selective nature of hydroxyl radicals and the typically significant matrix effect, this comparison could be useful in determining whether peroxydisulfate can serve as an alternative to peroxide in the treatment of specific aqueous matrices. The findings of this study could provide information which would be essential in selecting UV-C-activated processes to remediate water sources contaminated with pesticide compounds such as 2,4-D.

Thus, this study aims to provide a comparative evaluation of the UV-C-activated oxidation of the herbicide 2,4-D, using two different oxidation agents, hydrogen peroxide and peroxydisulfate, under the conditions associated with achieving a high efficiency of degradation. Different oxidants were employed, considering that distinct radicals were to be formed as the primary oxidation species. In the UV-C-activated peroxide system, hydroxyl radicals are present, whereas in the UV-C activation of peroxydisulfate, sulfate radicals are predominantly formed (at high pH values, ^•OH radicals can be generated). The herbicide 2,4-D was selected due to its widespread presence in surface and groundwater, making it suitable for the evaluation of degradation processes. The chosen oxidants were activated with UV-C light emitted at 254 nm. Initially, the effects of oxidant concentration and initial pH value on the removal efficiency with respect to the herbicide were investigated for both processes. Due to the influence of the water matrix on the efficacy of UV-C-based AOPs through UV radiation absorption or free radical consumption, the impacts of natural organic matter and inorganic anions were evaluated. Subsequently, the processes applied were compared in terms of degradation efficiency and mineralization. Furthermore, the specific determination of the dominant radical under various operational conditions was performed for the UV-C-activated peroxydisulfate system.

2. Materials and Methods

2.1. Chemicals

2,4-dichlorophenoxyacetic acid (2,4-D) was purchased from Sigma-Aldrich (St. Louis, MO, USA). The hydrogen peroxide solution (30%), of analytical grade, was purchased from Merck (Darmstadt, Germany). Reagent grade K₂S₂O₈ was provided by VWR (West Chester, PA, USA). Ethanol and tert-butyl alcohols were of ACS reagent grade and were supplied by Merck (Darmstadt, Germany). Sodium chloride and sodium bicarbonate were of reagent grade and were purchased from Zorka Šabac (Šabac, Serbia). HPLC-grade methanol was supplied from J.T. Baker™ (Fisher Scientific, Waltham, MA, USA). Humic acid was purchased from Sigma-Aldrich (St. Louis, MO, USA). All solutions were prepared with ultrapure water from the Smart2Pure system with a conductivity of 0.055 µSm⁻¹ (Thermo Scientific, Waltham, MA, USA).

2.2. Degradation Experiments

The degradation experiments were carried out in a laboratory-scale batch photoreactor equipped with ten low-pressure mercury lamps (Philips, Holland) emitting at 253.7 nm and mounted in parallel on the top of the photoreactor. The distance between the surface of the solution and the UV lamps was kept constant at 220 mm. The light intensity on the surface of the solution was measured with a UV radiometer, the Solarmeter model 8.0 UVC (Solartech, Glenside, PA, USA). The schematic of the photoreactor used for the irradiation experiments can be found in our previous publications [26].

All experiments were conducted using 100 cm³ of working solution; these samples contained the specified initial concentrations of oxidants and the herbicide 2,4-D, and were maintained at their corresponding pH values. The solutions were irradiated in glass Petri dishes. While the initial concentration of 2,4-D examined was maintained at a level higher than those typically observed in water and wastewater matrices, this elevated concentration was selected to facilitate precise analytical evaluation of 2,4-D during the degradation. The evaluation of initial oxidant concentration for the UV-C-activated peroxydisulfate process was carried out at concentrations of 0.5, 1.0, 2.0 and 4.0 mmol dm⁻³, while the concentrations of peroxide for this assessment were 0.1, 0.2, 0.4 and 0.6 mmol dm⁻³. The range for each oxidant applied was determined based on preliminary studies that indicated that, at high initial concentrations of peroxide such as those associated with peroxydisulfate, the rate constant remained unchanged (plateaued). Conversely, at low initial peroxydisulfate concentrations, comparable to those associated with peroxide, the removal rates were negligible. The influence of pH was investigated at initial pH values of 2.0, 3.0, 5.0, 7.0, 9.0 and 10.0 for both oxidation processes. The initial pH value was adjusted using varying concentrations of HCl or NaOH. A series of experiments examining the effects of inorganic water constituents was performed by adding calculated volumes of a 1 mol dm⁻³ solution of NaCl or NaHCO₃. The impact of humic acid (HA) was assessed by incorporating the desired amount of HA stock solution (500 mg dm⁻³) into the working solution. To identify the predominant radical in the UV-C-activated peroxydisulfate, the working 2,4-D solutions (30 mg dm⁻³) with 10 or 100 mmol dm⁻³ of ethanol or tert-butyl alcohol at three initial pH values (3.0, 7.0, and 10.0) were irradiated. In the UV-C-activated hydrogen peroxide, only ^•OH radicals can be generated; this is the reason why the determination of main radical species was not performed for the UV-C-activated peroxide. Samples were collected at specific time intervals (0, 1, 2, 4, 6, 10, 20, 40, 60, 90 and 120 min), and the residual concentration of the herbicide 2,4-D, chemical oxygen demand (COD) and concentration of chloride ions were determined. The removal efficiency (RE) with respect to the herbicide 2,4-D was defined by the following equation, Equation (1):

$$\mathrm{RE}(\%)=\left(1-{\displaystyle \frac{c_{\mathrm{t}}}{c_0}}\right)\times 100$$

(1)

where c₀ is the initial 2,4-D concentration, and c_t is the 2,4-D concentration at irradiation time t. The rate of degradation of the pesticide 2,4-D can be represented by Equation (2) [27]:

$$-{\displaystyle \frac{dc}{dt}}=k_2{c}_{\mathrm{rad}}c$$

(2)

where c is the initial concentration of the pesticide 2,4-D, c_rad is the concentration of sulfate or hydroxyl radicals, k₂ is the rate constant of the second-order reaction and t is the reaction time. The concentration of the generated radicals directly depends on the concentration of the oxidant used. If the rates of formation of ^•OH and SO₄^•− radicals are equal to the rates associated with their consumption, and the applied oxidants (H₂O₂ or S₂O₈²⁻) are present in a large excess in the solution, a steady-state approximation can be considered; thus, integration leads to the following logarithmic dependencies (Equation (3)) [28]:

$$\ln {\displaystyle \frac{c_{\mathrm{t}}}{c_0}}=-k_{\mathrm{app}}t$$

(3)

where c₀ is the initial 2,4-D concentration, c_t is the 2,4-D concentration at time t, k_app is the apparent pseudo-first-order rate constant, and t is the irradiation time. The pseudo-first-order kinetic model, represented by Equation (3), was employed to monitor the kinetics of 2,4-D herbicide degradation in both processes under study. By plotting ln(c_t/c₀) against time, the k_app values were derived from the experimental data. In most cases, the R² values exceeded 0.9, suggesting that the model closely matched the experimental results. Each experiment was conducted three times, and the average of these trials is presented, with error bars indicating the standard deviation.

2.3. Analytical Methods

The concentration of the herbicide 2,4-D during the treatments was monitored using a Dionex UltiMate 3000 HPLC system (Thermo Fisher Scientific, Walthman, MA, USA) equipped with a Zorbax Eclipse Plus C18 column (Agilent Technologies, Santa Clara, CA, USA) and a UV-Vis detector. The mobile phase was a mixture of methanol and 0.1% formic acid in a 60/40 ratio. The flow rate was set at 0.5 mL min⁻¹, with an injection volume of 10 µL, and the column temperature was maintained at 25 °C. The detection wavelength was 228 nm. The pH of the 2,4-D solutions was adjusted using a pH meter (Orion Star A214, Thermo Scientific, Waltham, MA, USA). Chloride concentrations were determined through ion chromatography using the IC Dionex ICS-3000 instrument (Thermo Fisher Scientific, Waltham, MA, USA). For the measurement of chemical oxygen demand, a thermoreactor (RD125, Lovibond, Amesbury, UK), the Lovibond COD Vario tube test designed for the range of 0–150 mg dm⁻³ and the Lovibond Multidirect photometer (Lovibond, Amesbury, UK) were employed. Prior to the COD analysis, samples were treated with catalase enzyme (aqueous suspension from bovine liver, 10,000–40,000 units/mg, Sigma Aldrich, St. Louis, MO, USA) to decompose any remaining peroxide.

3. Results and Discussion

3.1. Degradation of the Herbicide 2,4-D in Different Systems

Considering that the examined pesticide absorbs in the UV region of the spectrum, with absorption maxima at 228 and 284.5 nm, it is hypothesized that its degradation can occur under the influence of UV irradiation (254 nm), even in the absence of oxidants. Consequently, the potential degradation of the pesticide 2,4-D under the influence of UV-C radiation was initially investigated. The results, depicted in Figure 1, demonstrate that an extension in the duration of the UV-C treatment is associated with an enhancement in removal efficiency. Specifically, the removal efficiency following a 15 min UV-C treatment was approximately 14%. Prolonged treatment of the solution with UV-C radiation did not result in a significant change in removal efficiency.

The resistance of 2,4-D to the independent actions of the two oxidants, hydrogen peroxide and peroxydisulfate, was also examined. An additional hypothesis suggests that due to the standard oxidation potential of hydrogen peroxide (1.78 V) and peroxydisulfate (2.01 V), the degradation of pesticides may occur through the independent actions of these oxidants [29]. The results demonstrated that when treatments were performed using only hydrogen peroxide or peroxydisulfate, no change in the initial concentration of 2,4-D was observed over the treatment duration. However, a significant reduction in pollutant concentration was achieved when UV-C radiation was utilized in combination with oxidants. The enhanced efficiency of the process was attributed to the generation of highly reactive hydroxyl or sulfate radicals. Under the UV-C radiation, peroxydisulfate is activated, resulting in the formation of two sulfate radicals (Equation (4)) [30]. The efficiency of pesticide degradation increased due to the higher reactivity of the formed sulfate radicals, compared to peroxydisulfate.

S₂O₈²⁻ + hυ → 2SO₄^•−

(4)

H₂O₂ + hυ → 2^•OH

(5)

The UV-C activation of hydrogen peroxide produces hydroxyl radicals (Equation (5)) [31], which exhibit greater efficiency in pesticide degradation, compared to that of the hydrogen peroxide. Complete pesticide removal was achieved after 15 min of treatment with a combination of UV-C radiation and hydrogen peroxide, while twice as much time was required for complete pesticide removal when using UV-C radiation in combination with peroxydisulfate. The calculated pseudo-first-order rate constants for the UV-activated hydrogen peroxide and peroxydisulfate degradation of the pesticide 2,4-D were 0.212 min⁻¹ and 0.106 min⁻¹, respectively. Hence, the degradation efficiency levels under UV-C-activated hydrogen peroxide and peroxydisulfate were significantly improved, compared to both UV-C irradiation alone and samples tested only with the presence of oxidants; this was consistent with a previous studies [32,33]. Moreover, a significantly shorter irradiation time (15 min) was required for the removal of 2,4-D with UV-C-activated hydrogen peroxide, compared to the applications of the other AOPs [4,10,34].

3.2. Effect of Oxidant Concentration

The concentrations of peroxide and peroxydisulfate constitute an important parameter for evaluation because the degradation efficiency can increase with increasing concentration of oxidants, up to a certain value [30]. Thus, different concentrations of oxidants were evaluated in terms of the efficiency of the degradation of the pesticide 2,4-D (Figure 2).

Pseudo-first-order rate constants for degradation of the pesticide 2,4-D under UV-C-activated hydrogen peroxide were determined for the initial peroxide concentrations 0.1, 0.2, 0.4 and 0.6 mmol dm⁻³ at an initial pH value of 2.0. Additionally, degradation of the pesticide 2,4-D was evaluated in the presence of 0.5, 1, 2 and 4 mmol dm⁻³ od peroxydisulfate activated with UV-C light at an initial pH of 2.0. The pseudo-first-order rate constant increased significantly from 0.124 min⁻¹ at 0.1 mmol dm⁻³ H₂O₂ to 0.419 min⁻¹ at 0.4 mmol dm⁻³ of hydrogen peroxide, and subsequently increased gradually up to a hydrogen peroxide concentration of 0.6 mmol dm⁻³, reaching a value of 0.518 min⁻¹. These results demonstrate that the addition of hydrogen peroxide enhanced the utility of UV photons in degrading the 2,4-D pesticide, as a greater quantity of ^•OH radicals was generated during the UV-C photolysis of hydrogen peroxide as its concentration increased. The well-documented scavenging effect associated with hydrogen peroxide at elevated concentrations likely accounts for the minimal increase in the pseudo-first-order rate constant at the higher investigated concentrations. A comprehensive review of the literature revealed that an increase in oxidant concentration yielded positive effects; however, excessive oxidant levels typically resulted in negative impacts on the photodegradation of organic pollutants [35]. This adverse effect can be attributed to the scavenging action of oxidants on radicals and the self-combination reactions of radicals (Equations (6) and (7)) [36]. Furthermore, the inhibitory effect of the oxidant dose was not observed in numerous studies due to the utilization of oxidant doses below the inhibition threshold [35].

^•OH + H₂O₂ → H₂O₂ + HO₂^•, k = 2.7 × 10⁷ M⁻¹ s⁻¹

(6)

^•OH + HO₂⁻ → OH⁻ + HO₂^•, k = 7.5 × 10⁹ M⁻¹ s⁻¹

(7)

Similar behavior was observed during the degradation of the pesticide 2,4-D under UV-C-activated peroxydisulfate (Figure 2b). As the peroxydisulfate concentration increased from 0.5 mmol dm⁻³ to 2 mmol dm⁻³, the corresponding pseudo-first-order rate constant dramatically increased, and then again slightly increased between the concentrations of 2 mmol dm⁻³ and 4 mmol dm⁻³. The degradation rate of the pesticide 2,4-D in the UV-C-activated peroxydisulfate depends on the amount of reactive radical species generated by oxidant photolysis. The removal rate is limited under conditions of low peroxydisulfate levels, due to reduced generation of reactive radicals, whereas an increase in peroxydisulfate concentration enhances the production of sulfate radicals, thereby improving the observed degradation rate. However, high concentrations of peroxydisulfate might inhibit the reaction, owing to the self-scavenging effect and the self-combination reactions associated with the radicals (Equations (8) and (9)) [37].

SO₄^•− + S₂O₈²⁻ → S₂O₈^•− + SO₄²⁻, k = 1.2 × 10⁶ M⁻¹ s⁻¹

(8)

SO₄^•− + SO₄^•− → S₂O₈²⁻, k = 8.1 × 10⁸ M⁻¹ s⁻¹

(9)

Even though a retardation in the removal rate associated with the pesticide 2,4-D was obtained at higher oxidant concentrations in both UV-C-activated processes investigated, a scavenging effect was not observed, possibly due to the low dosages of peroxide or peroxydisulfate employed. Moreover, a nonlinear dependence of the rate constant on the oxidant concentration was observed at the highest investigated oxidant concentrations; therefore, a decrease in the rate constant can be expected at oxidant concentrations that are higher than those tested. Considering the rate constants obtained, and the removal efficiency obtained with concentrations of 0.2 mM for UV-activated peroxide and 1 mM for UV-activated peroxydisulfate, these concentrations were applied in further investigations. Further, the utilization of UV-activated peroxide is advisable for the degradation of the pesticide 2,4-D, as it facilitates a more rapid degradation with the use of lower concentrations of the oxidant.

3.3. Effect of Initial pH

The initial pH of the system played a crucial role in the UV-C-activated hydrogen peroxide and peroxydisulfate processes, as it could substantially affect several aspects: (1) the conversion of reactive radical species; (2) the redox potential of reactive free radicals; (3) the light-induced breakdown of oxidants; and (4) the durability of contaminants and their susceptibility to radical reactions [38,39]. Thus, the effect of the initial pH on the performance of the UV-C-activated hydrogen peroxide and peroxydisulfate in the degradation of the pesticide 2,4-D was explored. The experiment was performed with an initial concentration of the pesticide 2,4-D of 30 mg dm⁻³; the initial concentrations of H₂O₂ and S₂O₈²⁻ were 0.2 mmol dm⁻³ and 1 mmol dm⁻³, respectively, while the initial pH value of the solution varied from 2.0 to 10.0 (2.0, 3.0, 5.0, 7.0, 9.0 and 10.0) without buffering. The results are shown in Figure 3 as the change in the reaction rate constant as a function of the initial pH value.

Degradation of the 2,4-D pesticide in UV-C-activated hydrogen peroxide and peroxydisulfate followed pseudo-first-order kinetics at all studied pH values. As can be seen, a decrease in the value of the reaction rate constant with an increase in the initial pH value was observed for both investigated processes. With the increase in system pH from 2.0 to 3.0, the rate constant for the pesticide 2,4-D with the UV-C-activated hydrogen peroxide and peroxydisulfate decreased significantly, while, for both processes, the rate constant was not changed significantly when the pH was further increased to 10.0. Particularly, the value of the rate constant for the UV/S₂O₈^2– process decreased significantly, from 0.134 min⁻¹ at pH 2.0 to 0.047 min⁻¹ at pH 3.0, and then again slightly with a further increase in initial pH value, reaching a value of 0.036 min⁻¹ at pH 10.0. The final solution pH value remained constant for the initial pH levels of 2.0 and 3.0. However, a slight decrease was observed for initial pH levels of 5.0, 7.0, 9.0 and 10.0, resulting in pH values of 4.6, 6.6, 8.8 and 9.6, respectively, after 30 min of UV-activated peroxydisulfate treatment. Due to the acid-catalyzed decomposition of peroxydisulfate at lower pH values, a greater quantity of SO₄^•− radicals would be generated in UV/S₂O₈^2– system; thus, the degradation efficiency might be improved in acidic conditions (Equations (10) and (11)) [40]. The acid-catalyzation decreased with an increase in the pH, leading to the reduction in the degradation rate. Moreover, the quantum efficiency of the photodissociation of the persulfate ion was identical at different pH values; thus, the formation of sulfate radicals would not be affected by pH [41].

S₂O₈²⁻ + H⁺ → HS₂O₈⁻

(10)

HS₂O₈⁻ → SO₄^•− + SO₄²⁻ + H⁺

(11)

Similarly, the pseudo-first-order rate constant for the UV-C-activated peroxide was 0.212 min⁻¹ at pH 2.0 and then dropped to 0.112 min⁻¹ with increase in pH to 3.0. A slight decrease in the rate constant was achieved with a further increase in pH, at which point it was 0.087 min⁻¹ at pH 10. In this system, the final solution pH value remained unchanged for initial pH levels of 2.0 and 3.0. However, a slight decrease was observed for initial pH levels of 5.0, 7.0, 9.0, and 10.0 after 30 min of treatment with UV-activated peroxide, resulting in pH values of 4.5, 6.4, 8.7 and 9.4, respectively. In the case of the UV/H₂O₂ system, the decrease in 2,4-D removal with an increase in initial pH can be explained by several reasons. First, it has been reported that the standard redox potential of ^•OH is higher at an acidic pH (2.7 eV) than at alkaline pH values (1.8 eV) [36]. Second, the decrease in value of the pseudo-first order rate constant with increasing pH may be related to the dissociation of hydrogen peroxide under basic pH conditions, producing the hydroperoxide anion (HO₂⁻), which can act as an ^•OH scavenger due to its high reactivity with ^•OH, or react with H₂O₂ (Equations (12)–(14)) [36,42]. Both reactions (13 and 14) generally result in a reduction in the concentration of ^•OH, which can consequently diminish the efficiency of organic pollutant degradation. Given that the initial pH value for the UV-activated peroxide was below the pKa for reaction 12, this scenario was less likely to occur during the oxidative degradation of the pesticide 2,4-D.

H₂O₂ ⇄ HO₂⁻ + H⁺, pKa = 11.6

(12)

HO^• + HO₂⁻ → O₂^•− + H₂O, k = 7.5 × 10⁹ M⁻¹ s⁻¹

(13)

H₂O₂ + HO₂⁻ → H₂O + O₂ + OH⁻, k = 2.7 × 10⁷ M⁻¹ s⁻¹

(14)

Further, at alkaline conditions, ^•OH can convert into its conjugate base ^•O⁻ (Equation 15) [36]. The preferred mechanisms of ^•OH radicals and ^•O⁻ radicals in their reactions with organic compounds may differ. Particularly, in its reactions with organic molecules, ^•OH functions as an electrophile, whereas ^•O⁻ acts as a nucleophile. Consequently, ^•OH readily adds to unsaturated bonds, while ^•O⁻ does not exhibit this behavior; thus, the rate of reaction can be affected by the presence of different forms of radicals.

HO^• + OH⁻ ⇄ H₂O + ^•O⁻, k = 1.2 × 10¹⁰ M⁻¹ s⁻¹

(15)

Furthermore, H₂O₂ undergoes decomposition in the neutral-to-basic pH range (Equation 16), resulting in the production of oxygen molecules and the loss of its oxidative properties; consequently, the generation of ^•OH is diminished [43].

2H₂O₂ → H₂O + O₂

(16)

3.4. COD Removal and Chloride Release

In this experiment, the chemical oxygen demand was determined as a measure of the degree of mineralization of the pesticide 2,4-D during the treatment. The treatment was carried out under the following conditions: UV-C irradiation of two oxidants, H₂O₂ or S₂O₈²⁻; an initial concentration of the pesticide 2,4-D of 30 mg dm⁻³; an initial concentration of peroxydisulfate of 1 mM; and an initial concentration of hydrogen peroxide of 0.2 mmol dm⁻³. Since the mineralization of organic pollutants in water with AOPs is generally a slower process compared to the removal of the pollutants, longer irradiation times were utilized to determine the degree of mineralization. For these experiments, the selected irradiation times ranged up to 120 min, while the degradation studies were performed with up to 30 min of irradiation.

Based on the results presented in Figure 1, the complete degradation of the pesticide 2,4-D under the UV-C-activated hydrogen peroxide was achieved after 15 min of treatment. From Figure 4, we can see that the value associated with chemical oxygen demand, at that moment, decreased by about 28%. A further decrease in the COD value can be attributed to the degradation of the formed intermediates of 2,4-D pesticide. By the end of the UV-C-activated hydrogen peroxide treatment, after 120 min, the total reduction in the COD value was about 74%. In the study of the degradation of the pesticide 2,4-D via UV-C-activated peroxydisulfate, complete degradation was achieved within 30 min, during which the chemical oxygen demand value decreased by only 13%. As the treatment progressed, the COD value exhibited a slight further decrease, ultimately reaching a reduction of 28% by the end of the treatment. This outcome is attributed to the presence of organic intermediates, following the degradation of the 2,4-D pesticide molecules, which are not completely degraded. In the present work, it was determined that a faster reduction in the COD value, as well as a higher percentage of mineralization of the pesticide 2,4-D, were achieved when the pesticide is treated with the UV-C-activated hydrogen peroxide. A study conducted by Chen at al. revealed that about 89.8% of the 2,4-D could be mineralized at 2 h with persulfates activated with FeS (applied conditions: ([2,4-D]₀ = 10 mg/L, [FeS]₀ = 0.15 g/L, [PS]₀ = 1.25 mM, temperature = 30.0 ± 0.2 °C, initial pH = 4.5) [44]. Jaafarzadeh et al. reported that in a system comprising peroxymonosulfate/magnetic copper ferrite nanoparticles/ozone (PMS/MCFNs/O₃), 67.3 % of TOC removal was achieved (conditions: 50.0 mg/L 2,4-D, pH = 6.0, PMS = 2.0 mM, O₃ = 16.0 mg/L, MCFNs = 0.20 g/L, 60 min) [45]. Kermani et al., after 160 min of treatment of 2,4-D by photo-assisted electroperoxone, obtained 85 and 76% COD and TOC removal rates, respectively (conditions: pH=5.6, ozone = 1.8 mg/L. min, DC = 0.9 A, 2,4-D = 58 mg/L) [4]. Rodríguez et al., with modified sulfated CeO₂ materials, achieved TOC removal rates of 25%, 50% and 31% for 0.5SO₄²⁻/CeO₂, 1.0SO₄^2−/CeO₂ and 2.0SO₄²⁻/CeO₂, respectively [46]. Data from the literature indicated that a lesser or greater degree of mineralization of the pesticide 2,4-D can be achieved with other treatments, depending on the applied treatment and conditions. However, the results of this study showed that a satisfactory degree of pesticide mineralization was achieved by applying UV-activated peroxide.

In addition, the extent of mineralization of the pesticide 2,4-D was assessed by measuring the concentration of chloride anion, a product of mineralization. The experimental conditions for determining the concentration of chloride anion were the same as those used for estimating the chemical oxygen demand. As illustrated in Figure 4, the concentration of chloride anion increased with the duration of 2,4-D treatment for both UV-C-activated (hydrogen peroxide and peroxydisulfate) processes. For example, after 15 min of treatment, the concentration of chloride anion was 4.519 mg dm⁻³ for UV-C-activated hydrogen peroxide degradation, whereas it was 6.642 mg dm⁻³ for degradation by UV-C-activated peroxydisulfate. Even though the initial production rate of chloride anion is higher with UV-C-activated peroxydisulfate, the quantity of the produced chloride after 120 min is slightly higher with UV-C-activated hydrogen peroxide. At the end of the investigated treatment time, after 120 min, the chloride anion concentration reached 9.455 mg dm⁻³ for UV-C-activated peroxide, while it was 8.445 mg dm⁻³ for UV-C-activated peroxydisulfate. The increase in chloride anion concentration is attributed to the cleavage of the C-Cl bond in the 2,4-D pesticide molecule, resulting in the conversion of organic chlorine into the Cl⁻ anion. According to stoichiometric calculations, the maximum concentration of chloride anion, assuming complete conversion of organic to inorganic chlorine, is 9.64 mg dm⁻³. The findings indicate that a nearly complete cleavage of the C-Cl bond was achieved with UV-C-activated peroxide, whereas with UV-C-activated peroxydisulfate, some degradation products containing a chlorine atom likely remained in the solution. The results obtained suggest that both investigated systems exhibit good performance for 2,4-D dechlorination. Furthermore, the interaction of reactive radicals with the aromatic part of the molecule, resulting in the formation of the corresponding adducts, which, after the elimination of chloride atom, yield OH-substituted products of 2,4-D, likely represents the initial step in its degradation [12]. Subsequently, dechlorination/hydroxylation or dechlorination of the resulting intermediate likely occurs, leading to the mineralization of 2,4-D [47].

3.5. Effect of the Water Matrix Constituent

Research has shown that water matrices can influence the efficacy of UV-AOPs by altering the impacts of processes. This occurs through the modification of the generation of free radicals, changes in reaction pathways or consumption of the free radicals formed. Among the water matrix components commonly present and studied are dissolved organic matter (DOM), such as humic acid; carbonate/bicarbonate; and chloride anions.

3.5.1. Effects of Bicarbonate

Water and wastewater systems commonly contain carbonate and bicarbonate anions. These anions can interact with the HO^• and SO₄^•− produced during UV-C irradiation of peroxide and peroxydisulfate, resulting in the generation of carbonate or bicarbonate radicals (Equations (17)–(20)) [48,49].

HCO₃⁻ + HO^• → HCO₃^•− + HO⁻ (k = 8.5 × 10⁶ dm³ mol⁻¹ s⁻¹)

(17)

CO₃²⁻ + HO^• → CO₃^•− + HO⁻ (k = 4.2 × 10⁸ M⁻¹ s⁻¹)

(18)

SO₄^•− + HCO₃⁻ → HCO₃^•− + SO₄²⁻ (k = 1.6 ± 0.2 × 10⁶ dm³ mol⁻¹ s⁻¹ at pH 8.4)

(19)

SO₄^•− + CO₃²⁻ → CO₃^•− + SO₄²⁻ (k = 6.4 ± 0.4 × 10⁶ dm³ mol⁻¹ s⁻¹ at pH > 11)

(20)

The existing literature documents both beneficial and detrimental effects of bicarbonate and carbonate anions on various advanced oxidation processes [50,51]. The negative impact is primarily attributed to the anions’ scavenging action relative to key oxidizing agents, while the positive influence is explained by emphasizing the role of carbonate radicals. It is interesting to note that in the technologies which apply peroxymonocarbonate (HCO₄⁻) as oxidant, degradation of investigated pollutants is usually accelerated in the presence of bicarbonate. The main reason for the distinct behavior of bicarbonate in traditional AOPs and PMC-based systems is linked to the differing mechanisms of reactive oxygen species formation. For the HCO₄⁻-based system, the use of different catalysts or thermal conditions is necessary to activate PMC for the effective degradation of organic pollutants. Thus, in the catalyzed HCO₄⁻-based system, the activation of both HCO₄⁻ and H₂O₂ results in an increased generation of reactive oxygen species, specifically OH and CO₃^•− [52]. It is posited that a substantial quantity of ^•OH is produced through the activation of HCO₄⁻. Furthermore, the presence of bicarbonate inhibits the rapid conversion of H₂O₂ to O₂, thereby allowing H₂O₂ oxidants to be utilized at an optimal rate for enhanced decontamination of organic pollutants [53]. In present study, the influence of bicarbonate anion was investigated at concentrations of 5, 10, 20, 50 and 100 mmol dm⁻³ at pH 8.0 for both systems, UV-C-activated peroxide and peroxydisulfate. The results are presented in Figure 5 as the dependence of the pseudo-first-order rate constant on the concentration of bicarbonate.

For all investigated concentrations of bicarbonate, the 2,4-D degradation conformed to pseudo-first-order kinetics. In the presence of bicarbonate anions, pseudo-first-order rate constants decreased from 0.083 min⁻¹ to 0.016 min⁻¹ as bicarbonate concentration increased from 0 mmol dm⁻³ up to 100 mmol dm⁻³ for UV-C-activated peroxide. It is evident that a more significant decrease in the value of k_app was observed at concentrations up to 20 mmol dm⁻³, while the value of the pseudo-first-order rate constant remained almost unchanged at the concentrations of bicarbonate anions of 50 and 100 mmol dm⁻³. In the case of UV-C-activated peroxydisulfate, pseudo-first-order rate constants decreased from 0.032 min⁻¹ to 0.016 min⁻¹ with an increase in bicarbonate anion concentrations from 0 up to 100 mmol dm⁻³. The results show that the inhibitory influence of the bicarbonate anion can be observed in both systems analyzed. This phenomenon is likely attributable to the interaction between the generated hydroxyl of sulfate radicals and the bicarbonate anions, resulting in the formation of less-reactive bicarbonate radicals, as previously mentioned. A more substantial inhibitory effect of bicarbonate was observed in the UV-C-activated peroxide, compared to the UV-C-activated peroxydisulfate, which may be attributed to the higher reaction rate constant associated with reactions between the hydroxyl radicals and the bicarbonate anions. In addition to the more important decrease in degradation rate constants with UV-C-activated peroxide in the presence of bicarbonate, removal rates remained higher than those obtained with UV-C-activated peroxydisulfate, suggesting that this system can be successfully applied to degrade the pesticide 2,4-D in real aqueous matrices.

3.5.2. Effect of Chloride

Chloride anion is another anion with an adverse effect on advanced oxidation processes, as has been reported by many authors [23,39,54]. Complex reactions are involved in the interactions between chloride anions and sulfate or hydroxyl radicals, in which species like Cl^•, Cl₂^•−, ClHO^•− can be generated during the UV-C activation of hydrogen peroxide or peroxydisulfate in the presence of chloride anions (Equations (21)–(28)) [54].

HO^• + Cl⁻ → ClHO^•− (k = 4.3 × 10⁹ M⁻¹ s⁻¹)

(21)

ClHO^•− ⇄ HO^• + Cl⁻ (k = 6.1 × 10⁹ M⁻¹ s⁻¹)

(22)

SO₄^•− + Cl⁻ ⇄ Cl^• + SO₄^2– (k = 3.0 × 10⁸ M⁻¹ s⁻¹)

(23)

Cl^• + H₂O → ClHO^•− + H⁺ (k = 2.5 × 10⁵ M⁻¹ s⁻¹)

(24)

Cl^• + Cl⁻ → Cl₂^•− (k = 8.5 × 10⁹ M⁻¹ s⁻¹)

(25)

Cl₂^•− + H₂O → HClOH + Cl⁻ (k = 1.3 × 10³ M⁻¹ s⁻¹)

(26)

Cl₂^•− + OH⁻ → ClOH^•− + Cl⁻ (k = 4.5 × 10⁷ M⁻¹ s⁻¹)

(27)

Cl₂^•− + Cl₂^•− → Cl₂ + 2Cl⁻ (k = 9.0 × 10⁸ M⁻¹ s⁻¹)

(28)

It has been assumed that chloride anions have, overall, a negative impact on the degradation of organic contaminants within the AOPs, due to the possible involvement of these anions in the scavenging of SO₄^•− and HO^• radicals. In particular, the decline in sulfachloropyridazine removal efficiency in the heat-activated persulfate oxidation process was obtained in the presence of chloride at the molar ratios ([Cl⁻]₀/[PS]₀) of 0:1, 0.1:1, 0.5:1 and 2:1 [55]. Additionally, the redox potentials of all formed reactive chlorine species were much lower than the SO₄^•− and HO^• radicals, which also affects the reduction in process efficiency. On the other hand, different studies have reported the improving effect of chloride anions, which, at higher concentrations, could promote the propagation reactions and production of more SO₄^•−. Xu et al. reported that mineralization of artificial sweetener sucralose by the UV/S₂O₈²⁻ process was enhanced in the presence of 200 mM of chloride by 15% [40]. Contrastingly, the presence of chloride promoted the mineralization rate of the artificial sweetener sucralose within the UV/H₂O₂ system at concentration levels less than 10 mmol dm⁻³ Cl⁻, but inhibited the mineralization of sucralose at Cl⁻ concentrations of 100 and 200 mmol dm⁻³ [40]. Finally, it is possible for the presence of chloride to have little or no effect on the degradation of organic contaminants within various oxidation processes. For instance, pseudo-first order rate constants barely changed when the concentrations of chloride were increased from 0 to 10 mmol dm⁻³ during Kathon removal by UV-C-activated hydrogen peroxide [56].

In order to evaluate the effects of chloride anions on the removal efficiency associated with the investigated pesticide relative to UV-C-activated peroxide and peroxydisulfate, the degradation efficiency relative to the pesticide 2,4-D with chloride concentrations 5, 10, 20, 50 and 100 mmol dm⁻³ was investigated; the results are presented in Figure 6.

For all investigated concentrations of chloride, the 2,4-D degradation conformed to pseudo-first-order kinetics (R² > 0.90). An increase can be noted in the chloride concentration, from 0 up to 100 mmol dm⁻³, leading to a decline in pseudo-first-order rate constants from 0.082 min⁻¹ to 0.032 min⁻¹ for the UV-C-activated peroxide degradation of the pesticide 2,4-D. Comparable results were obtained for the UV-C-activated peroxydisulfate of the pesticide 2,4-D, in which a decrease in the pseudo-first-order rate constant from 0.038 to 0.02 min⁻¹ was observed for chloride anion concentrations of 0 and 100 mmol dm⁻³, respectively. The inhibitory effects in both investigated systems can be attributed to the reactions between SO₄^•− or HO^• and Cl⁻, in which chlorine radicals with lower oxidation potentials, compared to the sulfate and hydroxyl radicals, have been formed. Moreover, the inhibitory effects of the presence of chloride anions on the removal of the pesticide 2,4-D were more pronounced with UV-C-activated peroxide than with UV-C-activated peroxydisulfate, which could be a consequence of the higher rate constant associated with the hydroxyl radical and the chloride anion, compared to the rate constant of the sulfate radical and the chloride anion. Hence, more hydroxyl radicals than sulfate radicals were quenched in the presence of chloride anions, leading to a greater reduction in the 2,4-D degradation rate. The present findings are consistent with those of Liu et al., who observed a more pronounced inhibition of the degradation of the antibiotics levofloxacin and ofloxacin in the UV/H₂O₂ process, compared to the UV/S₂O₈²⁻ process. This indicates that the effect of Cl⁻ in the UV/H₂O₂ process was greater than in the UV/S₂O₈²⁻ process [57]. It is important to emphasize that, in addition to the prominent effect of the presence of chloride on UV-C-activated peroxide and the lower initial concentration of peroxide, the obtained removal rates indicated that this process still provides faster degradation of the pesticide 2,4-D, compared to UV-activated peroxydisulfate.

3.5.3. Effect of HA

Previous studies have clearly demonstrated that the effectiveness of UV-C-based treatment processes in breaking down organic compounds is greatly affected by the presence of DOM in aqueous environments. Studies have shown that DOM can compete with pollutants to deplete the concentrations of generated SO₄^•− or HO^• radicals, thereby reducing the effectiveness of pollutant removal [51,58]. The rate constant for the reaction of DOM with HO^• is (1.6–3.3) × 10⁸ M⁻¹ s⁻¹, while the rate constants for the reaction of SO₄^•− with DOM range from 2.5 × 10⁷ to 8.1 × 10⁷ M⁻¹ s⁻¹ [59,60]. In addition, humic acid, as a representative DOM, can absorb UV light more effectively at wavelengths of less than 400 nm, which can cause light attenuation [61]. Light attenuation due to the presence of humic acid can limit the photolysis of oxidants, affecting further the generation of radicals. However, upon absorption of UV irradiation, some fractions of DOM (such as fulvic acid) become excited and are transformed into the activated triplet state of DOM and various reactive oxygen species, which implies that the DOM can act as a promoter [27]. Hence, it is evident that natural organic matter can influence the photochemical degradation of aqueous contaminants, which may be both beneficial, acting as a photosensitizer, and detrimental, competing for UV photons and sulfate or hydroxyl radicals [62,63].

However, the results of this study indicated that with the addition of humic acid, the rate of pesticide 2,4-D degradation within both UV-C-activated systems decreased; this inhibitory effect is more pronounced at a higher concentration of humic acid (Figure 7).

Particularly, the addition of 1 mg dm⁻³ HA decreases the rate of pesticide 2,4-D degradation from 0.087 to 0.065 min⁻¹ in the UV-C-activated peroxide, and from 0.035 to 0.028 min⁻¹ in the UV-C-activated peroxydisulfate, respectively. The inhibition effect of HA further decreases with increasing concentration and reached values of 0.055 min⁻¹ and 0.022 min⁻¹ with the addition of 10 mg dm⁻³ HA for UV-C-activated peroxide and peroxydisulfate, respectively. As previously discussed, the inhibitory effect of HA on the degradation rate of 2,4-D in both investigated systems is likely attributable to two mechanisms: the scavenging of generated free radicals and the attenuation of UV-C light. The enhanced inhibition of HA in UV-C-activated peroxide can be attributed to the more pronounced scavenging effect resulting from the higher reaction rate with DOM and HO^• radicals. Although the degradation rate constants exhibit a slightly greater decrease with UV-C-activated peroxide in the presence of humic acid (HA), the removal rates exceed those achieved with UV-C-activated peroxydisulfate. This observation suggests that the UV-C-activated peroxide system can be effectively employed to degrade the pesticide 2,4-D in real water matrices, which typically contain high levels of organic matter.

3.6. Identification of Predominant Radical Species in UV-C-Activated Peroxydisulfate

Peroxydisulfate as an oxidant is considered to be a source of both hydroxyl and sulfate radicals, due to the interconversion of radicals in alkaline conditions. It is postulated that in acidic conditions, the dominant active radical species in the activated peroxydisulfate process are sulfate radicals. However, with an increase in the pH value, it is more likely that interconversion of sulfate radicals into hydroxyl radicals occurs due to the rapid reaction between SO₄^•− and OH⁻ [64]. Thus, at pH values above 7.0, both radicals can be present in aqueous solution and participate in the oxidation process. Generally, the detection and differentiation of predominant free radicals in aqueous solutions are possible via direct methods, such as electron paramagnetic resonance, and via indirect methods utilizing probe compounds. The chemical probe method is based on the different reactivities of generated reactive radical species and utilizes chemical probe compounds, which are typically added in excess. Various chemical probe compounds are utilized for simultaneous identification of reactive radicals, such as methanol, ethanol, tert-butyl alcohol, phenol and nitrobenzene [43,65]. Compared to direct detection techniques, the chemical probe method, also known as the free-radicals quenching method, is a readily available and economically viable technique, enabling identification and measurement of the concentration of free radicals for routine analysis [66]. Thus, the differentiation in and impact of the SO₄^•− and HO^• radicals in pollutant breakdown can be assessed by examining the distinct second-order reaction rate constants that these radicals exhibit with specific chemical probe compounds. For example, the range of the reaction rate constant between ethanol and SO₄^•− is (1.6–7.7) × 10⁷ M⁻¹ s⁻¹, whereas for HO^•, it is (1.2–2.8) × 10⁹ M⁻¹ s⁻¹ [67]. This suggests that ethanol effectively scavenges both hydroxyl and sulfate radicals. In contrast, the rate constants for tert-butyl alcohol with SO₄^•− and HO^• radicals are between (4–9.1) × 10⁵ M⁻¹ s⁻¹ and (3.8–7.6) × 10⁸ M⁻¹ s⁻¹, respectively, indicating that tert-butyl alcohol has a scavenging rate for HO^• radicals that is approximately three orders of magnitude higher than that associated with SO₄^•− radicals [67]. Hence, tert-butyl alcohol is generally assumed to be the better probe compound for effective quenching of hydroxyl radicals.

To determine the main radical involved in the UV-C induced peroxydisulfate oxidation of the pesticide 2,4-D, experiments were conducted using two chemical probe compounds, ethanol and tert-butyl alcohol, at concentrations of 10 and 100 mmol dm⁻³ across three initial pH levels: 3.0, 7.0 and 10.0 (Figure 8a–c).

The removal efficiency after 30 min of treatment at an initial pH value of 3.0 without the presence of chemical probe compounds was 88%. The addition of 10 and 100 mmol dm⁻³ of tert-butyl alcohol slightly decreased removal efficiency to 87% and 84%, respectively. Conversely, the addition of ethanol drastically reduced the removal efficiency with respect to the pesticide 2,4-D, resulting, after 30 min of treatment, in values of 59% and 19% for the ethanol concentrations 10 and 100 mmol dm⁻³, respectively. Without radical quenchers, at pH 7.0, the removal efficiency was 88% and moderately dropped with the addition of 10 and 100 mmol dm⁻³ of tert-butyl alcohol to 80% and 74%, respectively. In the presence of 10 and 100 mmol dm⁻³ of ethanol at pH 7.0, removal efficiency was respectively reduced to 54% and 29%. Similar results were obtained at pH 10.0; expressive drops in removal efficiency (88%) were obtained in presence of ethanol (53% for 10 mmol dm⁻³ of ethanol and 29% for 100 mmol dm⁻³ of ethanol) and tert-butyl alcohol (80% and 74% for 10 and 100 mmol dm⁻³, respectively). Considering the high reaction rate constant of ethanol relative to sulfate and hydroxyl radicals, as well as the pronounced inhibitory effect on degradation of the pesticide 2,4-D in their presence, it can be assumed that sulfate and hydroxyl radicals may be generated in the investigated system. Moreover, the presence of ethanol does not completely inhibit degradation of the investigated pesticide, since photolysis of the investigated pesticide occurred to some extent [17]. On the other hand, a slight inhibitory effect of tert-butanol was observed, leading to the conclusion that hydroxyl radicals play a minor role in the process across all tested pH levels. It can be further concluded that the SO₄^•− radical had a more significant impact on the degradation of the pesticide 2,4-D when UV-C-activated peroxydisulfate was applied at all tested pH levels. Similar results were achieved during the decomposition of phenylurea herbicide in the UV/S₂O₈²⁻ system, where it was confirmed that the sulfate radical plays the main role in the removal of the tested herbicides [68]. Moreover, given that sulfate radicals have been identified as the predominant radicals in UV-activated persulfate, and considering that this process exhibits limited efficacy in the degradation of 2,4-D, compared to UV-activated peroxide, it can be concluded that hydroxyl radicals demonstrate greater reactivity towards the pesticide 2,4-D under the investigated conditions.

4. Conclusions

This study investigated the reaction kinetics of the widely used pesticide 2,4-D in UV-C-based advanced oxidation processes. Two oxidants, hydrogen peroxide and peroxydisulfate, were activated with UV-C light and applied to evaluate their efficacy in the removal of the selected pollutant. Direct UV-C irradiation resulted in negligible photolysis of 2,4-D, whereas the addition of both oxidants significantly enhanced the degradation efficiency relative to 2,4-D. Both investigated systems demonstrated the capability for complete degradation of the pesticide 2,4-D, although UV-C-activated hydrogen peroxide exhibited superior efficiency. An increase in oxidant concentration yielded positive effects on the degradation efficiency of both investigated processes. An acidic environment proved more favorable for the UV-C-activated peroxide and peroxydisulfate degradation of the pesticide. The degradation of 2,4-D was adversely affected by the presence of Cl⁻, HCO₃⁻ and HA in both UV-C-activated systems, with the UV-C-activated peroxide process demonstrating greater susceptibility to the water matrix than did the UV-C-activated peroxydisulfate. Although the interconversion of sulfate radicals to hydroxyl radicals can occur in UV-C-activated peroxydisulfate under alkaline conditions, quenching experiments indicated that sulfate radicals were predominantly generated under the applied conditions. The results indicate that UV-activated peroxide exhibits superior oxidative capacity, compared to UV-activated peroxydisulfate, and is effective in degrading the pesticide 2,4-D in real water matrices. Nonetheless, it is crucial to examine the degradation pathway and conduct ecotoxicological assessments to ensure the practical applicability of any water purification technology. Furthermore, the economic considerations associated with implementing such a technology should not be overlooked, potentially including the use of low-energy sources or different combinations of oxidants, or an integration with conventional wastewater treatment techniques.