Heterogeneous photodegradation of mesotrione in nano α-Fe₂O₃/oxalate system under UV light irradiation

Abstract

The aim of this study is to investigate the behavior of mesotrione herbicide photodegradation under UV light irradiation with co-existence of iron oxides and oxalic acid. α-Fe₂O₃ and oxalate can set up a novel photo-Fenton-like system under UV irradiation in nature environment without H₂O₂ additional. The adsorption capacity of mesotrione was investigated in the dark by batch experiment, and the results were well fitted by Langmuir model. The effects of the iron oxide dosage, initial concentration of oxalic acid (C⁰_ox), mesotrione, and initial pH on the mesotrione photodegradation were investigated. The photoproduction of hydroxyl radicals (˙OH) during the photochemical process was also examined in diverse catalytic systems. The results indicated that mesotrione photodegradation follows pseudo-first-order kinetics. The mesotrione photodegradation moves slowly in the presence of α-Fe₂O₃ or oxalic acid. Interestingly, we found out that mesotrione photodegradation was distinctly enhanced when α-Fe₂O₃ and oxalic acid co-existed under UV irradiation. We learned that the combination of α-Fe₂O₃ and an oxalate system is an excellent agent to accelerate mesotrione herbicide decomposition. Results from this study could be further applied in the natural environment to facilitate environmental protection.

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1. Introduction

Herbicides are used worldwide to improve yields of agriculture crops. Owing to extensive production and use, herbicides have been detected throughout the world in soils, groundwater and surface water, resulting in high risks to human and ecosystem health. Much attention has been focused on the mineralization process in soils and aquatic environments to determine the short- and long-term effects of continued contamination by herbicides.¹

Iron oxides (including oxy-hydroxides) comprise 5.1% of the mass ratio of the earth's crust. Oxalic acid, mainly secreted by plant roots, is ubiquitous in water and soil.² Iron oxides coexist with oxalic acid in nature and can set up a so-called photo-Fenton-like system under light irradiation.^2,3 In fact, this photo-Fenton-like system can generate a series of strong oxidant species, such as O₂˙⁻, H₂O₂, HO₂˙and ˙OH, which have high efficiency for degradation of aqueous organic compounds, especially at very low concentrations.⁴ Thus, such degradation eventually influences the environmental fate and ecological risk of herbicides in the environment. Many studies show that herbicides could be degraded in the presence of Fe(III)-oxalate complexes under solar light irradiation.^5–7

Mesotrione (2-[4-(methylsulfonyl)-2-nitrobenzoyl]-1,3-cyclohexanedione;⁸ see Fig. 1 for molecular structure), a selective herbicide⁹ in the triketone group, was developed as a substitute for atrazine by Syngenta Crop Protection, and registered in Europe in 2000.¹⁰ It was marketed in 2001 under the commercial name Callisto,¹¹ and consumption has increased significantly since then. Because of its extensive use, several new and advanced technologies were developed for mesotrione degradation, such as the Fenton process,¹² electro-Fenton,¹³ photochemical degradation,¹⁴ ozonization,¹⁵ dielectric barrier discharge (DBD reactor),¹⁵ and biodegradation,^16–20 but little is known about the photochemical behavior of herbicides in α-Fe₂O₃ and oxalate systems.

Fig. 1 Chemical structure of mesotrione.

The goal of this study was to investigate the photodegradation behavior of mesotrione alongside iron oxides and oxalic acid. The mechanisms of its degradation in heterogeneous systems are also discussed here. We focused on the α-Fe₂O₃/oxalate system due to its abundance in soil and surface water in the natural environment. Photochemical degradation of mesotrione was examined in an α-Fe₂O₃/oxalate system under UV irradiation under various conditions that may coexist with mesotrione in the environment, including diverse α-Fe₂O₃ dosages, initial concentrations of oxalate, and initial pH values. To investigate the mechanism of heterogeneous mesotrione photodegradation, the concentration of ˙OH in the reaction process was detected. Additional mechanistic tests were performed by adding phenol to scavenge hydroxyl radical in irradiation experiments. There are no current reports on the photodegradation of mesotrione in an α-Fe₂O₃/oxalate system under UV irradiation.

2. Materials and methods

2.1. Reagents

Mesotrione was obtained from the Jiangsu Academy of Agricultural Sciences, China (technical grade, 98%). α-Fe₂O₃ (99.5%, 30 nm) was purchased from Shanghai Ziyi Reagent Co. China. Stock solution of mesotrione 10 g L⁻¹ was prepared in acetonitrile and stored in the dark at 4 °C. Oxalate (Fluka, 99.0%) and other analytical-grade chemicals were purchased from Sinopharm Chemical Reagent Co., China. For extraction and HPLC analysis, methylene dichloride (AR), acetonitrile (HPLC grade) and phosphoric acid (GR) were used. Chromatographic-grade methyl alcohol and acetonitrile were purchased from Tedia Company, USA. Analytical-grade benzene (Sinopharm Chemical Reagent Co., China) was used as a probe to determine the photoproduction of hydroxyl radical (˙OH) in aqueous solution. All chemicals were used without further purification and all solutions were prepared using double-distilled water.

2.2. Adsorption isotherm experiment

Adsorption of mesotrione in α-Fe₂O₃ was determined by using batch experiments in darkness. A fixed amount of α-Fe₂O₃ (0.10 g) was added to 10 mL of mesotrione solution with varying concentrations in quartz tubes, which were sealed and agitated for 24 h at 300 rpm and 20 ± 1 °C. Suspensions were filtered and then centrifuged (5000 rpm for 8 min) for determination of mesotrione concentration in aqueous solution by liquid chromatography.

2.3. Experiments of mesotrione photodegradation

After adsorption, the photodegradation of mesotrione was carried out in an XPA-7 photochemical reactor (Xujiang electromechanical plant, Nanjing, China). Before irradiation, the suspension was sealed and agitated for 30 min to reach adsorption equilibrium. The temperatures of the reaction solutions were maintained at 20 ± 1 °C by cooling water circulation. A 500 W medium Hg lamp with a maximum light intensity output at 365 nm was the irradiation source. The lamp was placed into a hollow quartz trap located at the center of the reactor. The light intensity and illumination at quartz tube positions were measured to be 8.96 × 10² mW cm⁻² by a UV irradiation meter (UV-A, Beijing Normal University, China), and 7.9 × 10⁴ LUX by a LUX meter (AS-813, Smart Sensor, China), respectively. The initial pH of reaction solutions was adjusted with dilute hydrochloric acid solution and sodium hydroxide solution, and final volume of the solution was adjusted to 50 mL with double-distilled water. Then, the solution was placed into the photochemical reactor and stirred with magnetic stirrers. At given irradiation time intervals, the analytical sample was withdrawn from the suspension with a pipette and immediately filtered for further analysis in order to prevent further reaction.

2.4. Analysis

Concentrations of mesotrione in irradiated and non-irradiated samples were obtained by the following method. Samples from adsorption and photodegradation experiments were collected and filtered using a syringe equipped with a 0.45 μm membrane filter. The water samples were first extracted twice by methylene chloride under 20 °C for 15 min. The extracted liquor was then evaporated by rotary evaporator to nearly dry and dissolved in acetonitrile in constant volume. Mesotrione was quantified using a PerkinElmer HPLC equipped with a SPHERI-5 RP-18 column (4.6 mm × 150 mm, 5 μm). Acetonitrile-phosphoric acid aqueous solution (pH = 4.0) was employed as mobile effluent. The flow rate was 0.5 mL min⁻¹, and the ultraviolet detector was set at 220 nm.

Aromatic hydroxylation has proved to be one of typical ˙OH reactions and used for ˙OH reactions in the case of Fenton/Fenton-like reactions.^21–23 Scavenging of ˙OH by excessive benzene was introduced into different reaction systems to determine the ˙OH quantum yield under irradiation of a 500 W Hg lamp. Formation of phenol from the reaction of benzene and ˙OH was detected at 254 nm by HPLC (PerkinElmer Flexar with XDB-C18; 5 μm, 4.6 × 250 mm). A 25% (v/v) acetonitrile was used as a mobile phase at a flowing rate of 1.0 mL min⁻¹ under isocratic conditions at 25 °C. Samples of 10 μL were injected into the column through the sample loop for analysis.

3. Results and discussion

3.1. Adsorption behavior of α-Fe₂O₃

The adsorption isotherm of mesotrione on α-Fe₂O₃ by plotting the equilibrium concentration (C_e) versus the amount of mesotrione adsorption (Q_e) is shown in Fig. 2, which was well fitted by the Langmuir adsorption model,where C_e is the equilibrium concentration in the solution in mM, K is the adsorption equilibrium constant in L mol⁻¹, and Q_emax is the saturated adsorption capacity in mol g⁻¹. The saturated adsorption amount (Q_emax) of mesotrione on α-Fe₂O₃ was 0.447 mmol g⁻¹ and adsorption equilibrium constant (K) was 52.44 L mol⁻¹ with the correlative coefficient R² of 0.9965.

Fig. 2 Adsorption isotherm of mesotrione on α-Fe₂O₃ obtained by plotting C_e versus Q_e. Inset graph shows dependence of C_e/Q_e on C_e.

3.2. Adsorption behaviour of mesotrione by α-Fe₂O₃

Mesotrione photodegradation under different conditions is presented in Fig. 3. The rate of mesotrione photodegradation was at 6.8% under UV light (500 W Hg) irradiation with the absence of oxalic acid and α-Fe₂O₃ (curve a).

Fig. 3 Photodegradation of 10 mg L⁻¹ mesotrione under UV irradiation (500 W Hg lamp) in 50 mL solutions with (a) UV only; (b) UV + 2.0 mM oxalic acid; (c) UV + 0.2 g L⁻¹ α-Fe₂O₃; and (d) UV + 2.0 mM oxalic acid + 0.2 g L⁻¹ α-Fe₂O₃.

When 0.2 g L⁻¹ α-Fe₂O₃ was present with the same conditions as curve a, the rate of mesotrione degradation was nearly same as curve a; removal of mesotrione was 9.7% after 60 min (curve c). Mesotrione removal significantly increased up to 21.5% level at 60 min under UV light with 2.0 mM oxalic acid (curve b). When both 2.0 mM oxalic acid and α-Fe₂O₃ with 0.2 g L⁻¹ dosage were added to the reaction suspension under UV irradiation (curve d), removal of mesotrione rose to 85.9%.

The photocatalytic degradation of mesotrione followed first-order reaction kinetics; the first-order kinetics constants (k) for mesotrione degradation were 0.51 × 10⁻², 0.53 × 10⁻², 1.20 × 10⁻², 31.49 × 10⁻² under different conditions (Fig. 3). From curves b to d, it can be seen that α-Fe₂O₃ had low photocatalytic activity with the absence of oxalic acid, and mesotrione can be efficiently degraded by α-Fe₂O₃/oxaliate system under UV light irradiation. The results showed that iron oxides, oxalate and UV light play important roles in mesotrione degradation, and mesotrione photodegradation should be greatly enhanced with the cooperation of iron oxide and oxalate.

In order to understand the photoreaction process of mesotrione degradation in an α-Fe₂O₃/oxalate system, the interaction of α-Fe₂O₃ and oxalate under UV light irradiation was examined. On the surface of α-Fe₂O₃, oxalic acid is first adsorbed by α-Fe₂O₃ particles to form α-Fe₂O₃/oxalate complexes with high photochemical activity as described in eqn (2). Fe(II) and CO₂˙⁻ under UV excitation can be generated both on the surface or in solutions, as described by eqn (3) and (4). Obviously, the higher oxalate concentration leads to greater Fe(II) concentration. Then CO₂˙⁻ reacts with oxygen to produce superoxide ions O₂˙⁻ as described in eqn (5) and (6), and Fe(II) reacts with O₂˙⁻ to form H₂O₂ in acid solution as described by eqn (7). After H₂O₂ was formed, ˙OH could be generated by reaction of H₂O₂ with Fe(II) as described by eqn (8). Finally, mesotrione was oxidized by ˙OH as described by eqn (9), which has strong oxidation potential. Note that the photochemical reactions happened both on the surface of α-Fe₂O₃ as a heterogeneous photo-Fenton process and in the solution as a homogeneous one.²⁴

Iron oxide + nH₂C₂O₄ → [Fe^III(C₂O₄)_n]³⁻²ⁿ/[≡Fe^III(C₂O₄)_n]³⁻²ⁿ + nH₂O (2)

[≡Fe^III(C₂O₄)_n]^(2n−3)− + hν → [Fe^II(C₂O₄)_n]²⁻/[≡Fe^III(C₂O₄)_n]²⁻ + (CO₂)˙⁻ (3)

[Fe^III(C₂O₄)_n]^(2n−3)− + hν → [Fe^II(C₂O₄)_(n−1)]^(2n−4)− + (C₂O₄)˙⁻ (4)

(C₂O₄)˙⁻ → CO₂ + CO₂˙⁻ (5)

CO₂˙⁻ + O₂ → CO₂ + O₂˙⁻ (6)

O₂˙⁻/˙OOH + nH⁺ + Fe²⁺ → Fe³⁺ + H₂O₂ (7)

Fe²⁺ + H₂O₂ → Fe³⁺ + OH + ˙OH (8)

˙OH + mesotrione → ⋯ → CO₂ + H₂O (9)

3.3. Effect of α-Fe₂O₃ dosage on mesotrione photodegradation

The dosage of α-Fe₂O₃ on mesotrione degradation in the presence of oxalic acid with an initial concentration of 2.0 mM is shown in Fig. 5. The effect of α-Fe₂O₃ dosage on the photodegradation of mesotrione with 2 mM oxalic acid under irradiation of 500 W medium-pressure Hg lamp was investigated; results illustrated that it was a very slow process with oxalate added. Approximately 20% of mesotrione was consumed after 60 min of irradiation without α-Fe₂O₃.

However, addition of α-Fe₂O₃ markedly accelerated the degradation of mesotrione. Mesotrione with an initial concentration of 10 mg L⁻¹ nearly reached equilibrium in 5 min in the presence of 0.2 g L⁻¹ α-Fe₂O₃, suggesting that α-Fe₂O₃ was an excellent photocatalyst for mesotrione degradation assisted by oxalic acid. Mesotrione removal increased from 72.0% to 85.9% when α-Fe₂O₃ was increased from 0.1 to 0.2 g L⁻¹, but decreased slightly when α-Fe₂O₃ increased from 0.2 to 0.6 g L⁻¹. Excessive dosage of α-Fe₂O₃ may block the scattering of UVA light in the reaction suspension and decrease formation of ˙OH.

Mesotrione photodegradation in an α-Fe₂O₃/oxalate system was followed by first-order kinetics. The first-order kinetic constants (k) versus dosage of α-Fe₂O₃ are plotted in Fig. 4: 1.2 × 10⁻², 7.44 × 10⁻², 31.49 × 10⁻², 15.63 × 10⁻², 16.0 × 10⁻² min⁻¹ with 0, 0.10, 0.20, 0.4 and 0.6 g L⁻¹ α-Fe₂O₃, respectively. This suggests that the dosage of 0.2 g L⁻¹ was an optimal concentration of α-Fe₂O₃ for mesotrione photodegradation with oxalate present. α-Fe₂O₃ as a heterogeneous photocatalyst can significantly accelerate formation of [≡Fe^III(C₂O₄)_n]³⁻²ⁿ. Under UV irradiation, more ˙OH can be produced during the photochemical reaction with greater [≡Fe^III(C₂O₄)_n]³⁻²ⁿ formed. In fact, excessive dosage of α-Fe₂O₃ limits penetration of UV light in the solution and leads to quick decay of UV light intensity. A similar phenomenon was observed while investigating the schwertmannite (γ-Fe₂O₃) photocatalytic degradation of methyl orange by oxalate under UV irradiation.²³

Fig. 4 Effect of α-Fe₂O₃ dosage on photodegradation of 10 mg L⁻¹ mesotrione under UV irradiation (500 W Hg lamp) in presence of 2.0 mM oxalic acid. Inset shows dependence of k on iron oxide dosage.

3.4. Effect of initial concentration of oxalate (C⁰_ox) on mesotrione photodegradation

To study the effect of initial C⁰_ox on mesotrione photodegradation, a set of experiments with initial mesotrione of 10 mg L⁻¹ and α-Fe₂O₃ dosage of 0.2 g L⁻¹ were carried out under UV irradiation (500 W Hg lamp), followed by experiments on different initial C⁰_ox without pH control. The initial concentration of oxalate also played an important role in mesotrione degradation as shown in Fig. 5. In the absence of oxalate, degradation was extremely slow, and mesotrione concentration almost remained constant under irradiation for 60 min (curve 0.0 mM). However, mesotrione degradation could be efficiently enhanced in the presence of oxalate. The increase of oxalate in α-Fe₂O₃/oxalate suspension significantly shortened degradation time. However, a higher oxalate concentration did not result in more rapid mesotrione degradation. Excessive oxalate led to lower pH in the photochemical reaction system, and also to formation of Fe³⁺, which may reduce formation of ˙OH,²⁰ as described in eqn (7) and (8).

Fig. 5 Effect of initial concentration of oxalic acid on photodegradation of 10 mg L⁻¹ mesotrione under UV irradiation by 0.2 g L⁻¹ Fe₂O₃. Inset shows dependence of k on C⁰_ox.

Mesotrione photodegradation in α-Fe₂O₃/oxalate system was followed by first-order kinetics. First-order kinetic constants (k) versus C⁰_ox are plotted in Fig. 5. The k values of mesotrione degradation were 0.53 × 10⁻², 10.24 × 10⁻², 31.49 × 10⁻², 30.35 × 10⁻² and 32.89 × 10⁻² when the initial concentration of oxalic acid was 0.0, 1.0, 2.0, 3.0 and 4.0 mM, respectively. The possible reason is that excessive oxalic acid occupied the adsorbed sites on the α-Fe₂O₃ surface, and could also react competitively with generated ˙OH together with mesotrione. Thus, controlling optimal concentrations of α-Fe₂O₃ and oxalate for mesotrione photodegradation is necessary.

3.5. Effect of initial concentration of mesotrione on mesotrione photodegradation

The effect of initial concentration of mesotrione ranging from 2.0 to 20.0 mg L⁻¹ on photodegradation was investigated in the presence of 0.2 g L⁻¹ α-Fe₂O₃ and 2.0 mM oxalic acid under UV irradiation (500 W Hg lamp). Results are presented in Fig. 6. The same tendency for mesotrione degradation can be observed in the curves. The k values of mesotrione degradation were 37.44 × 10⁻², 32.69 × 10⁻², 31.49 × 10⁻² and 15.15 × 10⁻² when the initial concentration of mesotrione was 2.0, 5.0, 10.0 and 20.0 mM, respectively. The rate of mesotrione photodegradation decreased almost linearly with the increase of initial concentration of mesotrione.

Fig. 6 Effect of initial concentration of mesotrione on photodegradation in presence of 0.2 g L⁻¹ α-Fe₂O₃ and 2.0 mM oxalic acid under UV irradiation. Inset shows dependence of k on C⁰_me.

3.6. Effect of initial pH value on photodegradation of mesotrione

To investigate the effect of initial pH value on mesotrione photodegradation, a series of experiments were carried out at different initial pH's, which were adjusted by NaOH or HCl before reaction, with initial concentration of mesotrione of 10 mg L⁻¹ in the presence of 0.2 g L⁻¹ α-Fe₂O₃ and 2.0 mM oxalic acid under UV irradiation (500 W Hg lamp). Results showed that the optimal initial pH value should be around 4.0, at which the photoreaction was almost completed in 20 min (see Fig. 7). The first-order kinetic constants (k) were 5.36 × 10⁻², 30.2 × 10⁻², 0.92 × 10⁻², 0.12 × 10⁻² and 0.17 × 10⁻² when the initial pH values were 2.0, 4.0, 6.0, 8.0 and 10.0 g L⁻¹, respectively.

Fig. 7 Effect of initial pH value on photodegradation of mesotrione.

Several studies had reported that the main Fe(III)-oxalate species were Fe^III(C₂O₄)²⁻ and Fe^III(C₂O₄)₃³⁻, which are highly photoactive when the pH was around 4.^25–27 The degradation of mesotrione would be inhibited considerably when the initial pH value interval ranges from 4 to 6. Furthermore, the degradation of mesotrione was almost neglected with the initial pH of 8.0 and 10.0, respectively. The α-Fe₂O₃/oxalate system at a lower initial pH value might have a higher concentration of [≡Fe^III(C₂O₄)_n]³⁻²ⁿ. In addition, the H₂O₂ produced at a high rate in the α-Fe₂O₃/oxalate system under irradiation with pH value ranging from 1.5 to 4.0, but decreased when pH was over 4.0. When pH was over 6.0, the Fe(III) species led to formation of Fe(OH)₂ and Fe(OH)₃ precipitate in the solution, which inhibited the photochemical reaction. On the other hand, the dissolution of α-Fe₂O₃ by H⁺ was excessive at the lower initial pH 2.0. Therefore, formation of the α-Fe₂O₃/oxalate complex would be hindered, and less [≡Fe^III(C₂O₄)_n]³⁻²ⁿ formed. Thus, the yields of ˙OH decreased, leading to a lower rate of mesotrione degradation.

3.7. Production of hydroxyl radicals (˙OH) in different reaction systems

Organic pollutants could be degraded by the hydroxyl radicals (˙OH) with high oxidation potential produced in photochemical reactions. Therefore, the ˙OH could be an indicator for photochemical degradation in the abovementioned α-Fe₂O₃/oxalate system. To investigate the mechanism of mesotrione photodegradation in α-Fe₂O₃/oxalate system, the concentration of ˙OH was detected during photochemical processes. Results are shown in Fig. 8.

Fig. 8 Production of hydroxyl radicals (˙OH) in different reaction systems under UV irradiation (500 W Hg lamp).

˙OH concentration in the system depends on both generation and consumption rates. As illustrated in Fig. 8, both α-Fe₂O₃ and oxalate alone showed low yields of ˙OH under UV irradiation via a 500 W Hg lamp. However, in the presence of both α-Fe₂O₃ and oxalate, a considerable number of ˙OH were detected. Hydroxyl radicals were quickly produced in the initial 10 min, and then the amount of ˙OH in the reaction system decreased with ˙OH consumption by degradation of certain organic compounds. The maximum concentration of ˙OH observed in 10 min was approximately 60 μM.

Benzene was selected as the hydroxyl radical scavenger due to its fast reaction with ˙OH. In contrast, photodegradation of mesotrione was significantly inhibited when benzene was added. Fig. 9 shows that with 0.32 mM and 0.64 mM benzene added, mesotrione degradation rates after 30 min were 15.17% and 5.65%, respectively, indicating that the higher benzene concentration would scavenge most of the ˙OH generated in the system, and block mesotrione photodegradation.

Fig. 9 Effect of hydroxyl radical scavenger on mesotrione-degradation performance under UV irradiation (500 W Hg lamp) with (a) blank; (b) 0.64 mM benzene; (c) 0.32 mM benzene; and (d) control.

4. Conclusion

The photodegradation of herbicide mesotrione in the heterogeneous α-Fe₂O₃/oxalate system, as a photo-Fenton-like system without additional H₂O₂ under UV irradiation, was investigated in this study. α-Fe₂O₃ could be an efficient photocatalyst for the degradation of organic matter in the natural environment. It can significantly accelerate photodegradation of herbicide mesotrione by the presence of oxalate. There are several steps in mesotrione photodegradation that can be described as (1) oxalate is adsorbed onto the surface of α-Fe₂O₃ to form [≡Fe^III(C₂O₄)_n]³⁻²ⁿ; (2) Fe(II) are generated and H₂O₂ are formed under UV irradiation; (3) ˙OH are generated in the Fenton-like system; and (4) mesotrione are decomposed by ˙OH. The photodegradation of the herbicide mesotrione depended strongly on initial pH value, doses of α-Fe₂O₃, and initial concentration of oxalate and mesotrione. The optimal α-Fe₂O₃ dosage was 0.2 g L⁻¹ and the optimal C⁰_ox was 2.0 mM with UV light irradiation (500 W Hg lamp), respectively. The results obtained in this study are helpful for understanding the fate of mesotrione in the environment and to assess the risk of herbicide mesotrione used on crops, and also provide a viable technology for removal of mesotrione from water.

Acknowledgements

This study was supported by a private sector---university research institute cooperation project in Jiangsu Province (grant BY2014108-15), the National Natural Science Foundation of China (21277115), and Six Talents Peak Project, Jiangsu Province (agricultural lands, 2011).

Notes and references

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