Theoretical and Experimental Study of the Stability of Thiamethoxam Under Different Environmental Conditions

Abstract

The neonicotinoid insecticide thiamethoxam (TMX) is widely applied in agriculture, owing to its high spectrum of target pests. Its frequent use contributed to its accumulation in the environment, mainly in water; therefore, its natural degradation mechanisms are relevant to understand the physicochemical factors that can accelerate its decomposition. So, this study evaluated the stability of TMX against variations in pH, temperature, and exposure time to solar radiation, with the purpose of assessing the natural mechanisms of its degradation in water. Further, simulations of the reaction mechanisms at the molecular level were performed. It was observed that the degradation of TMX in the environment is favored by its exposure to solar radiation for several days and in more acidic pH conditions. However, TMX degradation did not result in reduced ecotoxicity. Basic pH values also help in the degradation of TMX, but by a lower percentage than that in an acid medium. Although exposure of TMX to solar radiation promotes heating of the compound, the isolated effect of thermal energy (temperature) is not sufficient for its degradation. The computer simulations showed the regions with higher electron densities and that the TMX structure is stable, preventing the bonds from breaking with increasing temperature, up to 60 °C. The HO⁻ and H₃O⁺ ions do not interact significantly with the molecule to the point of modifying its structure. With solar radiation, an electron can change to the excited state, contributing to TMX degradation due to a triplet configuration that allows it to react with the ions in the solution. In this way, the present work contributed to jointly present a theoretical and experimental study of the forms of natural degradation of the TMX contaminant.

1. Introduction

Faced with a scenario of constant population growth, the need to produce food on a large scale became inevitable, which can be achieved using genetically modified seeds, machines/equipment that facilitate and optimize agricultural productivity, and different types of inputs [1,2].

Among the inputs used on a large scale in agriculture are pesticides, which, according to Law No. 7802 of 11 July 1989, are chemical, physical, or biological substances used as defoliants, desiccants, growth stimulators and inhibitors, applied with the purpose of controlling pests [3]. A pesticide is a compound used to repel, kill, or prevent any pest that may compromise the increase in crop productivity [4].

However, pesticides do not only reach target organisms, which results in damage to aquatic and terrestrial ecosystems and to humans. According to the International Labor Organization/World Health Organization (ILO/WHO), 70,000 acute and chronic poisonings per year are caused by pesticides in developing countries [5]. These data are of greatest concern for Brazil, which is the second largest consumer of pesticides in the world [6,7,8].

Neonicotinoids are among the most used classes of insecticides, which are derived from nicotine and are neurotoxic to insects. Thiamethoxam (TMX) is a second-generation neonicotinoid that acts as a nicotinic acetylcholine receptor agonist. This class of insecticide promotes the interruption of nervous stimuli in insects and is associated with the global decline of bees [9]. It also targets natural enemies of insects, such as ladybugs and praying mantises [6]. TMX is available in over 130 countries in formulations used worldwide [10]. It has high solubility in water, which facilitates its entry into the aquatic environment through surface runoff and soil infiltration, negatively impacting the environment and compromising aquatic invertebrate populations and human health [11].

For humans, the forms of direct exposure to pesticides are through inhalation, contact with the skin, and involuntary ingestion during handling of the chemical additive [12]. Other forms of exposure are through aerial spraying and consumption of contaminated food and water [13]. Incorrect disposal and overdoses of pesticides facilitate the transport of these products through rainwater, thus contaminating surface and groundwater [14]. There are reports of TMX concentrations in tap water ranging from 0.24 to 57.3 ng/L, and reaching 0.28 μg/L in drinking water, with 0.1 μg/L being the maximum concentration of this pesticide permitted in water for human consumption in several countries [11].

Sustainable methods and strategies were addressed for the degradation of insecticides, aiming to avoid persistence in the environment and reduce environmental impact. Although these contaminants are increasingly accumulating in the environment, the natural self-purification mechanisms of water bodies cannot be ignored, as they contribute to the degradation of these pollutants. Among these mechanisms, the photodegradation processes caused by solar radiation are significant. Relevant evidence for stability studies of pesticides exposed to solar radiation is the efficient use of photochemical advanced oxidative processes (AOPs) in water treatment systems contaminated by persistent organic compounds [15,16].

This study aims to evaluate the stability of the insecticide TMX in the face of variations in pH, temperature, and time of sun exposure, with the purpose of assessing the natural mechanisms of degradation of this pesticide in water. In conjunction with the experimental work, computational simulations were carried out to understand in a molecular way the stereoelectronic and structural properties responsible for the natural degradation mechanisms of TMX.

2. Materials and Methods

2.1. Experimental Part

2.1.1. Preparation of Thiamethoxam Solution and Quantification

A TMX solution of 16.5 mg·L⁻¹ was prepared for the tests using commercial TMX (CRUISER^® 350 FS, Syngenta (Basel, Switzerland), 35 g·L⁻¹ 3-(2-chloro-1,3-thiazol-5-ylmethyl) -5-methyl-1,3,5-oxadiazinan-4-ylidene(nitro)amine (thiamethoxam) and 83 g·L⁻¹ other ingredients not described in the technical sheet by the manufacturer) with pH 6. The commercial TMX can be diluted with water with values from 300 mL/100 kg to 1.5 L/100 kg of seed. For our experiment, an approximate absorbance value of 1 was used, in accordance with the Lambert–Beer law, so that it was possible to read the concentration on the spectrophotometer. The concentration of the samples was determined using UV-Vis spectrophotometry at 248 nm [17].

2.1.2. Assessment of Thiamethoxam Stability at Different Temperatures

To evaluate the stability of TMX at different temperatures, the solutions were subjected to temperatures of 24 °C, 40 °C, and 60 °C. The tests were carried out in triplicate with TMX quantification every 30 min using UV-Vis spectrophotometry at a wavelength of 248 nm. The evaluated temperatures were selected based on conditions that could be achieved naturally. The temperature of 24 °C corresponds to that of the laboratory environment where the analyses were carried out, whereas 40 °C is an approximation of the ambient temperature in the State of Tocantins–Brazil, and 60 °C is maximum temperature found in the literature [18]. The total exposure time of the tests was 3 h. To control the temperature, a water bath was used and constant temperature monitoring was carried out with the aid of a digital thermometer.

2.1.3. Assessment of the Stability of Thiamethoxam Exposed to Solar Radiation

At this stage, tests were carried out with TMX solutions at different pH values (3.0, 5.0, 6.0, 7.0, and 9.0) and the stability of the compound was measured without and with exposure to natural solar radiation. To adjust the pH, sodium hydroxide (NaOH) 0.1 mol·L⁻¹ and sulfuric acid (H₂SO₄) 0.05 mol·L⁻¹ were used. The tests were conducted with a total exposure time of 12 days (288 h) and TMX quantification was performed daily using UV-Vis spectrophotometry at 248 nm. Natural solar radiation monitoring was performed using a portable radiometer (Instrutherm Mês-100, Sao Paulo, Brazil), which reads radiation between 400 and 1000 nm. Temperature monitoring was carried out with the aid of a digital thermometer.

A kinetic degradation assay of TMX by solar radiation was performed using pseudo-first-order and pseudo-second-order models.

For the test at a pH value with greater degradation, a chemical oxygen demand (COD) analysis was performed applying the spectrophotometric method with a reading at 620 nm, using as digestion reagents potassium dichromate (K₂Cr₂O₇) in an acidic medium, acidic sulfur dioxide (H₂SO₄), and concentrated with a silver sulfate catalyst (Ag₂SO₄) at 148 °C for 2 h [19].

2.1.4. Ecotoxicological Evaluation Using the Vibrio fischeri Bioindicator

This test was performed for the TMX solution at pH 5 after 12 days, with and without exposure to solar radiation, with the sample at pH 5 being adopted, as it presented greater degradation of TMX. The ecotoxicological test was carried out with cuvettes containing a lyophilized formulation of bacterial biomass from Vibrio fischeri NRRL-B 11177, vacuum-packed by Monitox^®, with the experimental procedure conducted as indicated by the manufacturer. For the test, the pH of the samples was adjusted to pH 7 using a 0.1 mol·L⁻¹ sodium hydroxide solution (ideal pH for the test: 6–8.5). The salinity of the samples was adjusted using 1 mL of osmotic adjustment solution (10% NaCl) for each 10 mL of sample (ideal salinity for the test: 20 to 50 g·L⁻¹). Initially, a reusable ice pack was removed from the freezer and its temperature was set for 30 min on the laboratory bench. Then, a metal thermoblock was placed on the reusable ice block with the cuvettes containing the lyophilized bacterial biomass formulation. The bacterial biomass was reactivated using 1 mL of ice-cold reactivation buffer. After mixing the biomass with the reactivation buffer, two 0.5 mL aliquots of the mixture were placed in two empty cuvettes, one for the sample and the other for the control. After 15 min, the two cuvettes were read in the luminometer (I₀). Immediately afterwards, 0.5 mL of 2% NaCl solution was added to the control cuvette, and 0.5 mL of the sample was added to the other cuvette. After 30 min, the cuvettes were read in the luminometer (I₃₀).

To calculate the % of inhibition, the following Equations (1)–(3) were used:

$$\mathrm{F}\mathrm{C}={\displaystyle \frac{{\mathrm{I}}_{30}}{{\mathrm{I}}_0}},$$

(1)

$${\mathrm{I}}_{0\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}=\mathrm{F}\mathrm{C}·{\mathrm{I}}_0,$$

(2)

$$\%\mathrm{i}\mathrm{n}\mathrm{h}\mathrm{i}\mathrm{b}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=100·\left(1-{\displaystyle \frac{{\mathrm{I}}_{30}}{{\mathrm{I}}_{0\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}}}\right)$$

(3)

where FC is the light correction factor, I₀ is the initial measurement of the samples in the luminometer, and I₃₀ is the measurement of the samples in the luminometer after 30 min. According to the method, a percentage of inhibition greater than 20% indicates that the sample presents toxicity for the concentration tested, and if the percentage of inhibition is less than 20%, the sample does not present toxicity for the concentration tested.

2.2. Theoretical Calculations

Theoretical calculations were performed using density functional theory (DFT) with the functional M06-2X [20,21] and basis set 6-31+G(d,p) [22,23]. The structures were optimized to the minimum energy, and the vibrational frequencies were determined. No imaginary frequencies were found, which proved that the structures were at the energy minimum. The effect of the water solvent was calculated using the implicit solvation model density (SMD) [24].

The bond lengths, molecular electrostatic potential (MEP), and frontier molecular orbitals (FMO) were determined at the M06-2X/6-31+G(d,p)/SMD level with the optimized molecules. The frontier molecular orbitals (singly occupied molecular orbitals, SOMO) were generated with a 0.02 au of isovalue. The methodology of natural bond orbitals (NBO) [25] was used to evaluate bond orders of molecules. All calculations were performed using the Gaussian 09 program [26], and some structures were visualized with the GaussView program [27].

3. Results and Discussion

Experimental Part

Initially, the stability of TMX at different temperatures (24 °C, 40 °C, and 60 °C) was evaluated and the results are presented in

Table 1. Evaluation TMX degradation at different time (min) and temperatures (°C).

	TMX Concentration (mg·L−1)
Time (min)	24 °C	40 °C	60 °C
0	16.5 ± 0.05	16.4 ± 0.1	16.5 ± 0.1
30	16.5 ± 0.1	16.5 ± 0.1	16.4 ± 0.4
60	16.6 ± 0.02	16.6 ± 0.1	16.5 ± 0.2
90	16.6 ± 0.04	16.7 ± 0.1	16.6 ± 0.1
120	16.6 ± 0.1	16.6 ± 0.1	16.7 ± 0.6
150	16.6 ± 0.1	16.6 ± 0.1	16.7 ± 0.2
180	16.6 ± 0.04	16.7 ± 0.1	16.7 ± 0.2

. TMX concentration is expressed in mg·L⁻¹.

According to Table 1, there was no degradation of the pesticide even after heating the sample at 60 °C for 3 h. The variations presented are not very significant given the concentration of the compound used in the experiment. These results show that even if natural waters reach a temperature of 60 °C, the TMX will not suffer degradation. The temperatures analyzed represent the conditions that could be reached naturally.

In conjunction with the experimental results, computational simulations were carried out, and Figure 1 represents the structure of the TMX molecule with its respective bond lengths (Figure 1a) and bond order (Figure 1b).

From the results in Figure 1, it is possible to observe that the TMX molecule presents a stable structure. Analyzing the bond lengths in Figure 1a, the greatest distances are those bonded to chlorine and sulfur, more specifically in the bonds Cl–C (1.722 Å), C–S (1.738 Å), and S–C (1.730 Å). These greater bond lengths occur because the S and Cl atoms are larger. For the carbon that forms a bond between the two rings, of 5 and 6 atoms, the distances are 1.463 and 1.498 Å, respectively. The distances between the H–C bonds are all approximately 1.0 Å. The distances between the C–O bonds are close to 1.4 Å. The bonds of the functional group have the following lengths: N=N (1.314 Å), N–C (1.375 Å), and N=O (1.244 and 1.248 Å).

The bond order reflects the chemical bonds, single, double, triple, or in resonance, as well as the energy and strength of each bond. For almost the entire molecule, the bond orders varied between 0.872 (C–H bond) and 1.643 (C=C bond). As demonstrated, the TMX molecule is very stable and the increase in temperature close to 60 °C is not sufficient to break even the weakest bonds, which does not allow the degradation of the pesticide.

After the temperature study, tests with different pH values in the absence of solar radiation were carried out and the results are presented in

Table 2. Evaluation of the stability of TMX with pH variation in the absence of solar radiation.

pH	Day 1	Day 12	Degradation
3	0.9987	1.0135	±1%
5	0.9990	0.9903	±1%
6	0.9827	0.9706	±1%
7	0.9798	0.9610	±2%
9	0.9780	0.9580	±2%

. Table 2 shows TMX absorbance results.

As presented in Table 2, TMX degradation varied between 1% and 2%, percentages that were insignificant given the total concentration of the compound used in the test. The results in Table 1 and Table 2 indicate that temperature (up to 60 °C) and pH variation do not alter the degradation of TMX and therefore, after these analyses, tests were carried out with solar radiation.

To understand how the effect of pH did not alter the degradation of TMX, electrostatic potential maps for hydroxyl (HO⁻), hydronium (H₃O⁺), and TMX were generated and the results are presented in Figure 2. The molecular electrostatic potential (MEP) is a tool used to identify reactive sites [28], where the MEP surface is useful for determining possible electrophilic and nucleophilic reaction sites. The negative regions (reddish) of the MEP are related to the reactivity of an electrophilic attack, whereas the positive regions (blue) are related to the reactivity to the nucleophilic attack; the neutral region is represented by the green color.

Analyzing the results for TMX, it is possible to observe that in the –NO₂ group, the electron density is high, resulting from the lone electron pairs of the oxygens. The other regions of the molecule have a bluish/green color, which indicates partially positive charges. It can be observed from the MEP that the hydronium ion, shown in Figure 2b, has a predominantly blue color; that is, its charges are partially positive and can interact with nucleophiles; hence, it has a greater affinity for attacking the predominantly red region of the TMX molecule. The MEP result for hydroxyl, shown in Figure 2c, is predominantly red/orange when located close to oxygen, and it has a more yellow tone when located closer to hydrogen. The regions in red represent a greater density of electrons.

Based on the MEP results, it is possible to infer that regions in bluish tones can interact with nucleophiles and regions in reddish tones can interact with electrophiles, but the interaction between the species does not allow the TMX to be broken down by the ions.

In the test using solar radiation, the maximum temperature of the samples during exposure for 14 h was 46.3 °C. The results obtained in tests with different pH values in the presence of solar radiation over a period of 288 h are presented in Figure 3. The average daily radiation intensity received by the system was 893 W·day m⁻².

In the experiment using solar radiation, the degradation of TMX was notable compared with that in the test without solar radiation. The best result was achieved at pH 5, with a degradation of 82.12% of the compound. The second highest degradation was observed at pH 3, with a degradation percentage of 77.41%, followed by those at pH 6, with 75.55%, and pH 7 and 9, with 71.95% and 67.17%, respectively. The pH values influenced the stability of TMX exposed to solar radiation. It was found that pH 3, 5, and 6 resulted in higher degradation, whereas pH 7, which is neutral, and the basic pH 9 showed lower degradation percentages, respectively. It is inferred that a more acidic pH leads to greater degradation of the pesticide when exposed to ultraviolet (UV) radiation.

The kinetics of the TMX degradation were determined by applying pseudo-first-order and pseudo-second-order kinetic models, with the results presented in

Table 3. Kinetic parameters k and R² for TMX degradation.

	Pseudo-First-Order		Pseudo-Second-Order
	k (Day−1)	R2	k (dm3 mol−1 Day−1)	R2
pH 3 + Solar radiation	0.1367	0.9972	0.0197	0.9558
pH 5 + Solar radiation	0.1596	0.9907	0.0262	0.9427
pH 6 + Solar radiation	0.1337	0.9957	0.0184	0.9422
pH 7 + Solar radiation	0.1183	0.9955	0.0148	0.9412
pH 9 + Solar radiation	0.1031	0.9933	0.0115	0.9588

.

According to the reaction kinetic study, the TMX degradation process is best described by the pseudo-first-order kinetic model according to the higher value of R². In relation to reaction speed, degradation process occurs with greater speed at pH 5 + solar radiation, as it has a higher k value.

The efficiency of contaminant degradation when using solar radiation combined or not with different agents can be explained by changes in the electronic state of the organic molecules present in the solution (³CDOM*) [29], which could be represented by the pesticide. As shown by Neves et al. [30], radiation is necessary to excite the electron in organic matter to an excited triplet state ³CDOM* to subsequently react with the molecules of the reaction medium. Thus, the frontier molecular orbitals for TMX in the triplet state were generated and are represented in Figure 4.

The results indicate that the reactivity of the molecule increases due to a decrease in the gap energy, where the E_Gap is 7.57 eV for the TMX molecule in the singlet state and E_Gap = 4.34 eV for the molecule in the triplet state. Analyzing the orbitals, it is possible to observe that the lower SOMO presents delocalization of pi (π) orbitals on the 6-atom ring and a low electronic density on the thiazole ring. The upper SOMO presents delocalization of π orbitals on all of the thiazole ring. The thiamethoxam molecule in the triplet state undergoes changes in the formation of the double bonds of the thiazole ring, which allows a greater reaction with the hydronium ion, which justifies its better degradation in acidic pH.

It is important to highlight that Nelson P.N. [31] evaluated the TMX hydrolysis mechanism via computational simulations at neutral pH and high humidity. The results found by the author suggest the possibility of hydrolysis of TMX in its natural state and with radiation. It also indicates a highly stable product that requires ecotoxicological evaluation [31]. The data from the present work contributed to the results already reported in the literature and evaluated the eco-toxicity of the systems. In this context, the ecotoxicological test was conducted with TMX at pH 5, both with and without exposure to solar radiation for 12 days. As a result, it was observed that both samples exhibited toxicity, with a percentage of inhibition for the sample exposed to solar radiation of 27.4% and for the sample without exposure of 23.7%. Given the noticeable difference, it can be concluded that even after the degradation of TMX, the toxicity of the sample remained.

Regarding the COD of the samples, the solution exposed to solar radiation had a COD of 7 mg·L⁻¹, whereas the sample without sun exposure had a COD of 13 mg·L⁻¹. Although oxidation of organic matter was observed, it is important to note that this result refers to TMX and the dye used in the commercial formulation. Nevertheless, even with a degradation exceeding 75%, the complete oxidation of organic matter was 46%, indicating that radiation altered the structure of the contaminant but did not necessarily lead to its complete mineralization.

4. Conclusions

It is concluded that the degradation of TMX in the environment is favored by exposing the compound to solar radiation for several days and in more acidic pH conditions, but the degradation does not result in a reduction in the product’s toxicity. Basic pH values also help in the degradation of TMX, but with a lower percentage than that in an acidic environment. Although the exposure of TMX to solar radiation promotes the heating of the compound, the isolated effect of thermal energy is not sufficient for its degradation. The computational simulations show the regions rich and deficient in electrons throughout the MEP. The redder regions, close to the oxygens, presented a greater density of electrons, whereas the bluer regions, close to the thiazole ring, exhibited a lower density of electrons. The bond lengths and orders indicated the sites with weaker and stronger bonds. It was also shown that an increase in temperature did not break the molecule. When solar radiation interacts with TMX, a change in the electronic state of the molecule can occur, resulting in a triplet configuration that allows it to react with the ions in the solution, degrading the pesticide. Finally, this study provides new insights into the degradation of TMX in a natural way, contributing to its behavior in the environment.