A method for studying the photochemical transformation kinetics of thiamethoxam in environmental water

Abstract

The present invention provides a method for studying the photochemical transformation kinetics of thiamethoxam in environmental water. The method comprises the following steps: S1, using a chemical luminometer method to determine the quantum efficiency of direct photolysis of thiamethoxam in a multi-position photochemical reaction chamber; S2, using a pulsed laser flash photolysis method to respectively determine the quantum efficiency of thiamethoxam and hydroxyl free radicals ( . OH) and carbonate radicals (CO3 .‑ ) of the secondary reaction rate; S3 uses the relative rate method to determine the reaction rate of thiamethoxam and singlet oxygen ( 1 O2) secondary reaction rate; S4 used CDOM model compounds to simulate dissolved organic matter (CDOM) in natural water environment, and indirectly determined the reaction rate of thiamethoxam and triplet dissolved organic matter ( 3 CDOM * ) of the secondary reaction rate; S5 uses the kinetic parameters obtained in steps S1-S4 to simulate and predict the photodegradation kinetics of thiamethoxam in real environmental water bodies using the kinetic model software APEX.

Description

Technical Field

[0001] The invention relates to a method for studying the photochemical transformation kinetics of thiamethoxam in environmental water, and belongs to the field of chemical reaction kinetics. Background Art

[0002] Thiamethoxam (TMX) is a second-generation, highly effective, low-toxic neonicotinoid insecticide that has good stomach-killing, contact-killing and systemic activity against pests, and exhibits low toxicity to mammals such as humans. Therefore, thiamethoxam is widely used in agriculture and forestry, and is also used in urban health protection. When thiamethoxam is applied, only about 5% can be absorbed by crops (Environmental fate and exposure; neonicotinoids and fipronil. Environmental Science and Pollution Research, 22(1), 35–67. doi:10.1007 / s11356-014-3332-7). Due to its good water solubility, most of the rest enters the environmental water body through infiltration, leaching, etc. However, many studies have pointed out that thiamethoxam shows potential safety risks to many non-target aquatic organisms and humans (Chronic Toxicities of Neonicotinoids to Nymphs of the Common New Zealand Mayfly Deleatidium spp. Environmental Toxicology and Chemistry 2019, 38, (11), 2459-2471; Physiological Effects of Neonicotinoid Insecticides on Non-Target Aquatic Animals-An Updated Review. International Journal of Molecular Sciences 2021, 22, (17), 35; Neonicotinoid contamination of global surface waters and associated risk to aquatic invertebrates: A review. Environment International 2015, 74, 291-303), therefore, the transformation process of large amounts of thiamethoxam entering environmental waters should be systematically studied.

[0003] Photochemical transformation is an important degradation pathway for organic pollutants in environmental waters, including direct photolysis of pollutants by absorbing sunlight and the reaction with nitrate ions NO3 in environmental waters. - 、Nitrite ion NO2 - Hydroxyl radicals generated by direct or indirect reactions of photosensitive substances such as soluble organic matter (CDOM) under sunlight . OH, carbonate radical CO3 .- , Singlet oxygen 1 O2, triplet CDOM ( 3 CDOM * ) and other active transient species reactions (Bodrato, M.; Vione, D., APEX (Aqueous Photochemistry of Environmentally occurring Xenobiotics): a free software tool to predict the kinetics of photochemical processes in surface waters. Environmental Science-Processes&Impacts 2014, 16, 732-740).

[0004] At present, most of the research on thiamethoxam in water bodies is limited to the investigation and research on its detection in environmental water bodies and the development of thiamethoxam degradation methods in water bodies. The research on thiamethoxam degradation mainly focuses on optimizing conditions to accelerate degradation. However, studying specific degradation methods is not as meaningful as studying its degradation kinetics in environmental water bodies for thiamethoxam that exists in large quantities in environmental water bodies.

[0005] Studying the photochemical transformation kinetics of pollutants in the environment is of great significance for assessing the persistence of pollutants in the environment. However, there is currently no systematic research method on the photodegradation kinetics of thiamethoxam in environmental water bodies, and there are also many gaps in the kinetic parameter data of the reaction between thiamethoxam and various free radicals in water bodies. Therefore, it is particularly important to design a scheme that can predict its photolysis behavior in environmental water bodies. Summary of the invention

[0006] The purpose of the present invention is to provide a research method for the lack of a systematic research scheme for the photochemical transformation kinetics of thiamethoxam in environmental water bodies.

[0007] The above-mentioned purpose of the present invention is implemented by the following technical solutions:

[0008] A method for studying the photochemical transformation kinetics of thiamethoxam in environmental water bodies comprises the following steps:

[0009] Step S1: In a multi-position photochemical reaction chamber, the quantum efficiency of direct photolysis of thiamethoxam is measured by a chemical irradiometer method. The specific operation steps are:

[0010] The PNA / pyr binary chemical exposimeters and aqueous solutions of thiamethoxam were exposed to light in a multi-position photochemical reaction chamber under the same environmental conditions. Samples were taken after different reaction times to measure the photolysis rates of the chemical exposimeters and thiamethoxam, and the quantum efficiency of direct photolysis of thiamethoxam was calculated using the formula.

[0011] Furthermore, the structural formula of thiamethoxam is as follows:

[0012]

[0013] Furthermore, in step S1, the light source used in the multi-position photochemical reaction box is a 500W high-pressure mercury lamp equipped with a cut-off filter below 290nm.

[0014] Preferably, the light source needs to be cooled by circulating water to avoid excessively high temperature of the reaction solution.

[0015] Preferably, since the photolysis rate of PNA in the PNA / pyr binary exposure meter is related to the concentration ratio of PNA / pyr, and this method is more accurate only when the photolysis rates of PNA and thiamethoxam are roughly similar, it is necessary to adjust the appropriate concentration ratio of PNA / pyr so that the pseudo-first-order rates of photodegradation of PNA and thiamethoxam are roughly similar.

[0016] Further, in step S1, the quantum efficiency of the PNA / pyr binary chemical exposure meter is Φ PNA =0.29[Pyr]+0.00029 (see the literature p-Nitroanisole / Pyridine and p-Nitroacetophenone / PyridineActinometers Revisited:Quantum Yield in Comparison to Ferrioxalate.Environmental Science&Technology Letters 2017,4,11-14). Since the light source of the irradiation light meter is the same as that of thiamethoxam, the direct photolysis quantum efficiency of thiamethoxam can be obtained by formula Eq.1:

[0017]

[0018] In Eq.1, Φ TMX and Φ PNA are the quantum efficiencies of thiamethoxam and PNA in the light meter, respectively; k TMX and k PNAare the first-order rates of direct photolysis of thiamethoxam and PNA in the light meter, respectively; I λ is the light intensity of the light source at wavelength λ; ε PNA(λ) and ε TMX(λ) are the molar absorption coefficients of PNA and thiamethoxam at wavelength λ, which can be determined by ultraviolet absorption spectroscopy; b is the optical path length during the reaction; c is the wavelength of thiamethoxam at wavelength λ, which can be determined by ultraviolet absorption spectroscopy; PNA and c TMX are the concentrations of PNA and thiamethoxam, respectively.

[0019] Step S2: Determination of the reaction between thiamethoxam and carbonate radical (CO3 .- ) and thiamethoxam and hydroxyl radicals ( . OH) of the secondary reaction rate. The specific steps are:

[0020] (1) preparing a free radical precursor solution and placing it in a beaker, and using a circulating water pump to pump the free radical precursor solution in the beaker into a quartz reaction tube;

[0021] (2) using pulsed excitation light emitted by a laser to irradiate the free radical precursor solution in the quartz tube to generate free radicals, and then adding the concentrated mother solution of thiamethoxam to the beaker of the external reaction solution in portions;

[0022] (3) A continuous diode laser is used as the detection light, and the spectral absorption method is used to monitor the change in free radical concentration caused by the reaction of different concentrations of thiamethoxam with free radicals online. The free radical concentration change signal is then converted by an oscilloscope and averaged multiple times before being transmitted to a computer for data collection.

[0023] Further, in step S2(1), when determining the concentration of thiamethoxam and CO3 .- When the secondary reaction rate is , the free radical precursor solution is HCO3 - or CO3 2- Mixed solution with H2O2; HCO3 - or CO3 2- Contains HCO3 - or CO3 2- Inorganic salt solutions such as NaHCO3 or Na2CO3 can be used.

[0024] Preferably, CO3 .- HCO3 in free radical precursor solution - or CO3 2- The concentration of should be much greater than the concentration of H2O2 (at least 100 times).

[0025] Further, in step S2(1), when determining the . The secondary reaction rate of OH is .The OH radical absorption wavelength is around 230nm, and almost all organic matter absorbs in this short-wave ultraviolet region, and cannot absorb it like CO3 .- Direct monitoring by absorption spectroscopy . OH free radicals, so the present invention uses commonly used . SCN with known OH radical secondary reaction rate - As a reference (see Tropospheric Aqueous-Phase Free-Radical Chemistry: Radical Sources, Spectra, Reaction Kinetics and Prediction Tools. Chemphyschem 2010, 11, 3796-3822), a certain amount of SCN was added. - Added into H2O2 aqueous solution as a free radical precursor solution, the generated . OH will quickly contact SCN - The reaction is converted into (SCN)2 .- Free radicals; among them SCN - SCN - Inorganic salts such as KSCN and NaSCN can be used.

[0026] Preferably, (SCN)2 .- SCN in free radical precursor solution - The concentration is much higher than that of H2O2 (at least 20 times).

[0027] Furthermore, in step S2(2), the wavelength of the pulsed excitation light is 266 nm, the energy range of the excitation light is 20-40 mJ / pulse, and the pulse width is 2 ns.

[0028] Furthermore, in step S2(2), the thiamethoxam concentrated mother solution should be prepared using the free radical precursor solution corresponding to the experiment as a solvent, with a concentration of 10 -3 M level; the total volume of the thiamethoxam concentrated mother solution added should be controlled within 5% of the total reaction free radical precursor solution.

[0029] Furthermore, in step S2(3), the detection light needs to pass through a dichroic spectrophotometer so that it coincides with the excitation light and irradiates into the quartz reaction tube on the same light path; in the determination of thiamethoxam and CO3 .- When the secondary reaction rate is .- A continuous diode laser with a wavelength of 635 nm near the maximum absorption peak is used as the detection light; in the determination of thiamethoxam and . When measuring the secondary reaction rate of OH, a continuous diode laser with a wavelength of 460 nm was used as the detection light.

[0030] Furthermore, the relationship between thiamethoxam and CO3 .- and . When the secondary reaction rate of OH is 1, the reactions occurring in step S2 are respectively as follows R1-R3 and R4-R8:

[0031]

[0032]

[0033]

[0034]

[0035]

[0036]

[0037]

[0038] OH+TMX→Products R8

[0039] After the 266nm laser is triggered, free radicals are generated instantly and absorb the detection light. The intensity of the detection light drops sharply from the original I0 to I t＝0 The free radicals generated subsequently react with different concentrations of thiamethoxam added to the solution and are consumed. At this time, the intensity of the detection light detected is I t At this time, the first-order decay rate k of free radicals at different thiamethoxam concentrations can be obtained by detecting the change in light intensity from formula Eq.2: 1st :

[0040]

[0041] In Eq.2, I0 is the initial light intensity of the detection light, I t＝0 I is the intensity of the detection light after the excitation light has just been triggered and passes through the reaction tube. t It is the intensity of the detection light after the free radical passes through the reaction tube during the gradual decay process.

[0042] Furthermore, the relationship between thiamethoxam and CO3 .- When the reaction rate is the second order, the concentration of thiamethoxam [TMX] on CO3 .- The decay rate k is pseudo-first order 1st The slope of the graph can be used to obtain the relationship between thiamethoxam and CO3 .- The rate of the second-order reaction.

[0043] Furthermore, the determination of thiamethoxam and . The secondary reaction rate of OH is generated instantly after the 266nm laser is triggered. . OH rapidly and SCN- The reaction is converted into (SCN)2 .- , at this time (SCN)2 .- Absorbance of 460nm detection light Recorded as Thiamethoxam and SCN were added continuously - Competition and . The reaction of OH, thus (SCN)2 .- The absorbance of the detection light at 460 nm is reduced to Recorded as Thiamethoxam and SCN - right . The competitive reaction of OH can obtain the kinetic relationship described in Eq.3, from which the reaction of thiamethoxam and . Secondary reaction rate of OH:

[0044]

[0045] In Eq.3, and The (SCN)2 produced at the moment of excitation light triggering without and with thiamethoxam .- Absorbance of 460nm detection light; k OH,TMX Thiamethoxam and . The secondary reaction rate constant of OH; For SCN - and . The secondary reaction rate constant of OH is 7.8×10 9 M –1 ·s –1 (See the literature Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (.OH / .O-) in aqueous solution [J]. Journal of Physical and Chemical Reference Data (USA), 1988, 17 (2): 513-886); [TMX] and [SCN - ] are thiamethoxam and SCN in the solution - concentration.

[0046] However, the secondary reaction rate determination for pollutants that absorb at 266 nm is slightly different. The addition of such pollutants will compete with H2O2 for the absorption of 266 nm excitation light photons, resulting in a further reduction in the production of OH (SCN)2.- , which ultimately causes errors in the results of the secondary reaction rate. Therefore, when pollutants with absorption at 266nm of the excitation light are added, the final secondary reaction rate calculation formula Eq.3 needs to be Multiply by an absorbance correction factor Corrected to The corrected calculation formula is Eq.4

[0047]

[0048] In Eq.4, and are the absorbance at 266 nm of the free radical precursor solution without thiamethoxam, and the absorbance at 266 nm of the solution after adding the concentrated mother solution of thiamethoxam during the reaction; the meanings of the other characters are consistent with those in Eq. 3.

[0049] Step S3: Determine the relative rate of thiamethoxam and singlet oxygen ( 1 O2) of the secondary reaction rate. The specific steps are:

[0050] Preparation containing thiamethoxam, 1 The mixed solution of O2 sensitizer and reference substance was placed in a quartz tube and stirred magnetically to fully dissolve oxygen. The reaction tube was placed in a light box for illumination. Samples were taken at regular intervals to measure the photodegradation rates of thiamethoxam and reference substance. 1 The competitive reaction relationship between thiamethoxam and O2 1 The secondary reaction rate of O2.

[0051] Furthermore, in step S3, the light source in the illumination box is a light source with an emission spectrum of 550-700nm, which can be a high-pressure mercury lamp with a 290nm cut-off filter to filter out light below 290nm, or an LED light source with an emission spectrum of 550-700nm.

[0052] Further, in step S3, the 1 O2 sensitizers are those that can generate 1 O2 substances, including methylene blue and Bengal rose red; the reference substance is acetaminophen (APAP), which 1 The secondary reaction rate of O2 is (3.68±0.73)×10 7 M –1 ·s –1(See the literature Assessing the photochemical transformation pathways of acetaminophen relevant to surface waters: Transformation kinetics, intermediates, and modeling. Water Research 2014, 53, 235-248).

[0053] Furthermore, due to the differences between thiamethoxam and the reference substance in the same solution environment, 1 O2 produced a competitive reaction, and both thiamethoxam and the reference substance APAP only reacted with 1 O2 reaction and degradation, the reference 1 The secondary reaction rate of O2 is known, and the competitive kinetic relationship can be obtained from Eq.5: 1 The secondary reaction rate of O2 is:

[0054]

[0055] In Eq.5, Thiamethoxam and 1 Secondary rate constant of O2 reaction; [TMX]0, [TMX] t 、[APAP]0、[APAP] t are the concentrations of thiamethoxam and reference substance APAP at reaction time zero and after reaction time t, respectively; APAP is the reference substance. 1 The secondary reaction rate constant of O2 is (3.68±0.73)×10 7 M –1 ·s –1 .

[0056] Step S4: Using CDOM model compounds to simulate dissolved organic matter (CDOM) in natural water environments, indirectly determining the dissolved organic matter ( 3 CDOM * ) of the secondary reaction rate. The specific steps are:

[0057] (1) Prepare a certain concentration of CDOM model compounds, such as SRFA purchased by the International Humic Substances Association, to simulate and replace CDOM in natural environmental water bodies;

[0058] (2) Benzene, furfuryl alcohol and 2,4,6-trimethylphenol were used as . OH, 1 O2, 3 CDOM* The probe compound was prepared by preparing ①TMX aqueous solution, ②(SRFA+benzene) solution, ③furfuryl alcohol (FFA) solution, ④(SRFA+FFA+quencher) solution, ⑤2,4,6-trimethylphenol (TMP) aqueous solution, ⑥(SRFA+TMP+quencher+N2) solution, and ⑦(SRFA+TMX) solution respectively;

[0059] (3) The series of reaction solutions prepared in the above steps are placed together in a multi-position photochemical reaction box and uniformly irradiated with a lamp having an emission spectrum above 290 nm as a light source. Samples are taken after different illumination times, and the direct photolysis pseudo-first-order rate k of TMX is obtained from solutions ① to ⑦, respectively. d , the formation rate of phenol, the first-order rate of direct photolysis of FFA k3, the pseudo-first-order rate of photolysis of FFA in SRFA solution k4, the first-order rate of direct photolysis of TMP k5, the pseudo-first-order rate of photolysis of TMP in SRFA solution k6, and the photolysis rate of TMX in SRFA solution k7.

[0060] Furthermore, in step S4(2), the SRFA used is SRFA purchased from the International Humic Substances Association, and more preferably, its concentration range is 1-20 mgC / L.

[0061] Furthermore, in step S4(2), the concentration of benzene is in the μM range, and the concentrations of FFA and TMP are 10 -5 M level.

[0062] Furthermore, in step S4(2), methanol, ethanol, isopropanol or other quenchers need to be added to the ④(SRFA+FFA+quencher) solution and the ⑥(SRFA+TMP+quencher+N2) solution, so that the FFA and TMP in the solution are only due to the reaction with 1 O2 and 3 CDOM * and degraded by reaction, and the ⑥(SRFA+TMP+quencher+N2) solution needs to be continuously purged with N2 to ensure an anaerobic environment during illumination.

[0063] Furthermore, in step S4(3), the light sources in the multi-position photochemical reaction box should be subjected to circulating water cooling to prevent the emitted light from being overheated and causing the reactants to degrade through thermal decomposition.

[0064] Furthermore, it is known that .The conversion efficiency of OH and benzene to phenol is 0.85 (see Optical and Photochemical Characterization of Chromophoric Dissolved Organic Matter from Lakes in Terra Nova Bay, Antarctica. Evidence of Considerable Photoreactivity in an Extreme Environment. Environmental Science & Technology 2013, 47, 14089-14098). . The secondary reaction rate of OH and benzene is 7.8×10 9 M –1 ·s –1 (See Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (.OH / .O-) in aqueous solution [J]. J Phys Chem Ref Data (USA), 1988, 17 (2): 513-886.), so SRFA is generated by light exposure. . The rate of OH can be obtained from the generation rate of phenol in the solution ②(SRFA+benzene) in step S4(2) above. By assuming a steady-state kinetics for the solution, the rate of SRFA light generation can be obtained. . Steady-state concentration of OH:

[0065]

[0066]

[0067] In Eq.6 and Eq.7, [phenol], [benzene], [·OH] ss are phenol, benzene and . OH concentration; Generated for SRFA . The rate of OH, in M·s -1 ;k benzene,·OH Benzene and . The secondary reaction rate constant of OH is 7.8×10 9 M –1 ·s –1 .

[0068] Further, in step S4(3), the difference in pseudo-first-order rate of FFA degradation can be obtained from the obtained first-order rate k3 of direct photolysis of FFA and the pseudo-first-order rate k4 of photolysis of FFA in SRFA solution, which is the difference between the pseudo-first-order rate of FFA degradation and the first-order rate k4 of FFA degradation. 1 The part degraded by O2 reaction can be obtained from Eq.8 as follows: 1 The generation rate of O2 can be obtained by assuming a steady state for the reaction solution from Eq.9: 1 O2 steady state concentration:

[0069]

[0070]

[0071] In Eq.8 and Eq.9, k4 and k3 are the pseudo-first-order degradation rates of FFA in SRFA solution and pure water solution, respectively; [ 1 O2] ss For solution 1 The steady-state concentration of O2; For FFA and 1 The secondary reaction rate constant of O2 is 1.2×10 8 M -1 ·s -1 (See the document Rate constants for the decay and reactions of the lowest electronically excited singlet state of molecular oxygen in solution [J]. JPhys Chem Ref Data (USA), 1981, 10 (4): 809-999.). k′ is 1 The rate constant of physical quenching of O2 in solution is 5.5×10 5 s –1 (See the document Optical and Photochemical Characterization of Chromophoric Dissolved Organic Matter from Lakes in Terra Nova Bay, Antarctica. Evidence of Considerable Photoreactivity in an Extreme Environment [J]. Environ Sci Technol, 2013, 47 (14089-14098).).

[0072] Further, in step S4(3), the difference in the pseudo-first-order rate of TMP degradation can be obtained from the obtained TMP direct photolysis first-order rate k5 and the TMP photolysis pseudo-first-order rate k6 in the SRFA solution, which is the TMP degradation factor and 3 CDOM * The part degraded by the reaction can be expressed as the kinetic equation shown in Eq.10 below:

[0073]

[0074] In Eq.10, k6 and k5 are the pseudo-first-order rates of photolysis of TMP in SRFA solution and pure water, respectively; 3 CDOM * ] ss For SRFA solution 3 CDOM * The steady-state concentration of Generated for SRFA illumination 3 CDOM * The rate, in M·s -1 ; TMP and real environment water 3 CDOM * The average value of the second-order reaction rate constant is 8.1×10 8 M –1 ·s –1 (See the document Singlet Oxygen Phosphorescence as a Probe for Triplet-State Dissolved Organic Matter Reactivity. Environ. Sci. Technol. 52, 9170-9178.); k″ is 3 CDOM * The physical quenching rate is 5.5×10 5 s –1 (See the document Optical and Photochemical Characterization of Chromophoric Dissolved Organic Matter from Lakes in TerraNova Bay, Antarctica. Evidence of Considerable Photoreactivity in an Extreme Environment. Environ. Sci. Technol. 47, 14089-14098.)

[0075] Further, in step S4(3), when the solution ⑦(SRFA+TMX) is illuminated, the pseudo-first-order rate k7 of photolysis of TMX in the system includes the direct photolysis part and the . OH, 1 O2, 3 CDOM * The indirect photolysis part of the reaction can be expressed by the kinetic equation shown in Eq.11:

[0076]

[0077] In Eq.11, k7 is the first-order rate of photodegradation of thiamethoxam in SRFA solution (i.e., solution ⑦), S λ is the light shielding effect factor of SRFA on thiamethoxam in the solution, and its calculation method is shown in the following Eq.12; ∑ λ S λ k is the total light shielding effect of SRFA on thiamethoxam in the spectral range after the filter reaches the solution, and the calculation method is shown in the following Eq.13; d k is the first-order rate constant of direct photolysis of thiamethoxam in pure water, i.e. solution ①; TMX,·OH , Thiamethoxam and . OH, 1 The secondary reaction rate constant of O2; [·OH] ss 、[ 1 O2] ss In solution . OH, 1 The steady-state concentration of O2; Because thiamethoxam in solution ⑦ 3 CDOM * The first-order rate constant for the degradation reaction.

[0078]

[0079]

[0080] In Eq.12 and Eq.13, S λ and∑ λ S λ are the light shielding effect factors of SRFA on thiamethoxam at wavelength λ and in the entire spectrum of the light source reaching the solution at the experimental concentration; α λ is the light extinction coefficient of SRFA at the experimental concentration, in cm -1 ; ε λ is the absorption coefficient of thiamethoxam at wavelength λ; [TMX] is the concentration of thiamethoxam; b is the optical path length, in cm; I λ is the light intensity of the light source at wavelength λ.

[0081] Furthermore, TMX is 3 CDOM * Degradation part of the reaction There is the following relationship in Eq.14:

[0082]

[0083] In Eq.14, Thiamethoxam and 3 CDOM * The second-order reaction rate constant of is the SRFA production at the reaction concentration 3 CDOM * The rate of k″ is 3 CDOM * The physical quenching rate of [TMX] is the concentration of thiamethoxam.

[0084] Combining Eq.11 and Eq.14, we can get TMX and 3 CDOM * The second-order rate of the reaction.

[0085] Step S5: The photochemical kinetic parameters determined in the above steps S1-S4, including the quantum efficiency of direct photolysis of thiamethoxam, the reaction of thiamethoxam with CO3 .- , . OH, 1 O2 and 3 CDOM * The secondary reaction rate is input into the APEX software (the full name of the software is: Aqueous Photochemistry of Environmentally-Occurring Xenobiotics, the Octave visual operation interface and APEX software package can be obtained from the ZIP-Document), and other natural water parameters, solar spectrum, etc. can be changed arbitrarily. The laboratory results are extrapolated to different environmental conditions to simulate and predict the photochemical transformation kinetics of thiamethoxam in environmental water, including the changes in the photolysis half-life of thiamethoxam with the physical environment, the proportion of direct photolysis and indirect photolysis contributed by each free radical during thiamethoxam photodegradation, etc.

[0086] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0087] (1) The present invention is the first to measure the relationship between thiamethoxam and . OH, CO3 .- The second-order reaction rate constant of thiamethoxam was measured in a photochemical reaction chamber and the direct photolysis quantum efficiency of thiamethoxam was obtained. 1 O2,3 CDOM * The second-order reaction rate constant was calculated and combined with the APEX software model to simulate and predict the photodegradation kinetics of thiamethoxam in real environmental water bodies, filling the current gap in the photochemical transformation kinetics of thiamethoxam in the aquatic environment and providing a basis for the environmental risk assessment of thiamethoxam.

[0088] (2) The method for determining the . OH, CO3 .- The method for determining the secondary reaction rate constant is simple in process, has few interfering factors, can quickly obtain data, has high measurement accuracy, and accurate test results. This method is also applicable to other organic pollutants and . OH, CO3 .- The determination of the second-order reaction rate constant of pollutants in water environment . OH, CO3 .- The study of reaction kinetics is of great value.

[0089] (3) The method for determining the 3 CDOM * The second-order reaction rate constant method uses model compounds to simulate dissolved organic matter in natural water environments for the first time, and uses humic acid isolates from actual entities as CDOM substitutes. This is closer to the actual situation than the traditional method of selecting single substances such as anthraquinone-2-sulfonic acid sodium salt, riboflavin, 4-carboxybenzophenone, etc. to replace CDOM in actual water bodies. BRIEF DESCRIPTION OF THE DRAWINGS

[0090] Figure 1 = ln([TMX]0 / [TMX] t )~ln([PNA]0 / [PNA] t ) logarithmic plot.

[0091] Figure 2 To add different concentrations of thiamethoxam and CO3 .- The change in the intensity of the 635nm detection light caused by the reaction.

[0092] Figure 3 Thiamethoxam~CO3 .- Logarithmic plot of the first-order rate of decay.

[0093] Figure 4 To add different concentrations of thiamethoxam and SCN - compete . OH reacts to make (SCN)2 .- The change in the intensity of the 460nm detection light caused by the reduction.

[0094] Figure 5 To determine the. The OH reaction rate picture.

[0095] Figure 6 (a) Figure 6 (c) Figure 6 (d) are the degradation rates of TMX, FFA, and TMX in each parallel reverse solution, Figure 6 (b) is the generation rate of phenol.

[0096] Figure 7 The water depth is 0.5m, and other water conditions are [NO2 - ]=1.0μM,[CO3 2- ]=10μM,[HCO3 - ]=1×10 – 3 M,[Br - ]=1×10 –11 The degradation kinetic rate of thiamethoxam in environmental water bodies at M in sunny summer weather and the contribution of each degradation pathway.

[0097] Figure 8 The DOC concentration is 1 mgC / L, and other water conditions are [NO2 - ]=1.0μM,[CO3 2- ]=10μM,[HCO3 - ]=1×10 –3 M,[Br - ]=1×10 –11 The degradation kinetic rate of thiamethoxam in environmental water bodies at M in sunny summer weather and the contribution of each degradation pathway.

[0098] Fig. 9 For thiamethoxam in [NO2 - ]=1.0μM,[CO3 2- ]=10μM,[HCO3 - ]=1×10 –3 M,[Br - ]=1×10 –11 Prediction results of degradation kinetic half-life of M in ambient water bodies in summer sunny weather with water depth and DOC concentration. DETAILED DESCRIPTION

[0099] The present invention is further described below in conjunction with the examples, which are only used to explain the present invention and are not used to limit the scope of the present invention. The reagents used in the following examples can all be obtained from commercial channels.

[0100] Example 1

[0101] In a multi-position photochemical reaction chamber, the direct photolysis quantum efficiency of thiamethoxam (TMX) was determined using the chemical irradiometer method. The specific implementation process is as follows:

[0102] (1) In a multi-position photochemical reaction chamber, a 500W high-pressure mercury lamp with a 290-420nm cutoff filter was used as the light source, and a PNA / pyr mixed solution was used as a binary chemical exposure meter;

[0103] (2) preparing a mixed solution containing a PNA concentration of 31 μM and pyr concentrations of 0.3 mM, 1.6 mM, 3.1 mM, 6.2 mM, 15.5 mM, and 24.8 mM, respectively, and placing it together with a 31 μM thiamethoxam aqueous solution in a multi-position photochemical reaction chamber for irradiation;

[0104] (3) 2 mL samples were collected 10, 20, 30, and 40 minutes after illumination for HPLC analysis to determine the pseudo-first-order rates of direct photolysis of PNA and thiamethoxam.

[0105] Select the data of PNA concentration of 31μM and PNA / pyr concentration ratio of 1:200, and use Φ PNA =0.29[Pyr]+0.00029, the PNA quantum efficiency is calculated to be 0.002; ln([TMX]0 / [TMX] t )~ln([PNA]0 / [PNA] t ) to draw a graph, such as Figure 1 As shown, the ratio of the first-order degradation rates of PNA and TMX was obtained from the slope Then by the formula The quantum efficiency of TMX was obtained to be 0.023±0.001, which is close to the 0.019±0.001 reported by Lu et al. (see Quantum Yields for Direct Photolysis of Neonicotinoid Insecticides in Water: Implications for Exposure to Nontarget Aquatic Organisms. Environmental Science & Technology Letters 2015, 2, 188-192).

[0106] Example 2

[0107] Determination of carbonate free radicals (CO3 .- ) and thiamethoxam (TMX) secondary reaction rate. The specific implementation process is:

[0108] (1) The laser energy of the 266nm pulsed excitation light was set to 20mJ, the wavelength of the pulsed excitation light was 266nm, and the pulse width was 2ns. The operating temperature was room temperature 25°C, and a 0.017M H2O2+0.1M NaHCO3 mixed solution was prepared as a reaction free radical precursor solution, and this solution was used to prepare a 0.0041M thiamethoxam concentrated mother solution;

[0109] (2) 400 mL of a 0.017 M H2O2 + 0.1 M NaHCO3 mixed solution was pumped into a quartz reaction tube using a circulating water pump, and irradiated with a 266 nm pulsed laser using a flash photolysis instrument. 1 mL, 0.5 mL, 0.5 mL, and 0.5 mL of TMX concentrated mother solution were added to the reaction solution in portions, and the CO3 .- Changes in the intensity of the 635nm detection light detected by the photodetector caused by changes in concentration.

[0110] Signal changes such as Figure 2 As shown in the figure, it can be seen that the intensity of the detection light drops from the initial value I0 to the minimum value I0 immediately before and after the excitation light is triggered. t＝0 The reason is that a large amount of CO3 is produced in the reaction tube at this time. .- , and then the detected light intensity signal gradually recovered, because the CO3 generated at this time .- Gradually physically quenched or consumed by reacting with different concentrations of thiamethoxam; the slope of the curve in the figure is CO3 .- First-order reaction rates with different concentrations of thiamethoxam.

[0111] Use this data TMX was processed to obtain CO3 at different concentrations .- The pseudo-first-order decay rate is plotted against TMX, as shown in Figure 3 As shown, the slope of TMX and CO3 .- The secondary reaction rate is (1.62±0.05)×10 8 M -1 ·s -1 .

[0112] Example 3

[0113] Determination of hydroxyl radicals ( . The secondary reaction rate of OH) and thiamethoxam (TMX). The specific implementation process is:

[0114] (1) Parameters such as laser energy were adjusted as described in (1) of Example 2, the operating temperature was room temperature 25° C., a 0.05 mM KSCN + 0.017 M H2O2 solution was prepared as a free radical precursor solution, and a 0.0034 M thiamethoxam concentrated mother solution was prepared using the solution as a solvent;

[0115] (2) 350 mL of 0.05 mM KSCN + 0.017 M H2O2 solution was pumped into a quartz reaction tube using a circulating water pump, and irradiated with a 266 nm pulsed laser using a flash photolysis instrument. The TMX concentrated mother solution was added to the reaction solution in 5 portions, 1 mL each time, and the reaction was monitored by a 460 nm detection light. . Changes in the intensity of the 460nm detection light detected by the photodetector caused by changes in OH concentration.

[0116] Signal changes such as Figure 4 As shown in the figure, it can be seen that the intensity of the detection light signal drops sharply from the initial maximum value I0 to the minimum value I around the zero moment, that is, around the moment of the excitation light triggering. t＝0 When thiamethoxam is not added, the difference can be converted into . All OH is converted into (SCN)2 .- Hours, (SCN)2 .- The absorbance It can be seen from the figure that after gradually adding thiamethoxam, I t＝0 gradually increases, that is, due to the consumption of the added thiamethoxam . OH makes (SCN)2 .- reduce.

[0117] (3) The absorbance of 0.05 mM KSCN + 0.017 M H2O2 free radical precursor solution at 266 nm was measured by UV absorption spectroscopy. A series of solutions with different TMX concentrations were prepared using the free radical precursor solution as solvent and their absorbance at 266 nm was measured to make a standard curve. The absorbance at 266 nm of the solution after each addition of thiamethoxam concentrate was obtained from the standard curve.

[0118] (4) The signal detected in step (2) conforms to the relationship: Therefore Figure, such as Figure 5 As shown, its slope is in 1.29×10 10 Ms -1 , thus obtaining thiamethoxam and . The secondary reaction rate of OH is (4.15±0.14)×10 10 M -1 ·s-1 This value is consistent with the result of 3.9×10 9 M –1 ·s –1 The difference is one order of magnitude (see Kinetics and mechanism of thiamethoxam abatement by ozonation and ozone-based advanced oxidation processes. Journal of Hazardous Materials 2020, 390, 122180), but due to the ozone generation used by Wang et al. . The method of OH does not take into account the side reactions between thiamethoxam and HO2, and the data obtained are not as good as those obtained by the pulsed laser flash photolysis method for determining hydroxyl radicals ( . The results of the secondary reaction rate of OH) and thiamethoxam are reliable.

[0119] Example 4

[0120] The CDOM model compounds were used to simulate the dissolved organic matter (CDOM) in natural water environments, and a kinetic model was constructed to indirectly determine the dissolved organic matter ( 3 CDOM * ) of the secondary reaction rate. The specific implementation process is:

[0121] (1) Prepare 10 mg / L of SRFA purchased from the International Humic Substances Association to simulate and replace CDOM in natural environmental water bodies;

[0122] (2) Prepare ① 0.1 mM TMX aqueous solution, ② (10 mg / L SRFA + 3 mM benzene) solution, ③ 58 μM furfuryl alcohol (FFA), ④ (10 mg / L SRFA + 58 μM FFA + 1% isopropanol) solution, ⑤ 80 μM 2,4,6-trimethylphenol (TMP) aqueous solution, ⑥ (10 mg / L SRFA + 80 μM TMP + 1% isopropanol + N2) solution, and ⑦ (10 mg / L SRFA + 0.1 mM TMX) solution respectively;

[0123] (3) The series of reaction solutions prepared in step (2) were placed together in a multi-position photochemical reaction box and uniformly irradiated with a 500W medium-pressure mercury light source with a 290-420nm bandpass filter. After different illumination times, 2 mL of samples were taken for HPLC analysis and detection. The sampling interval was as follows: Figure 5 As shown in the corresponding abscissa, the direct photolysis pseudo-first-order rate k of TMX obtained from solutions ①-⑦ is d, the generation rate of phenol, the first-order rate of direct photolysis of FFA k3, the pseudo-first-order rate of photolysis of FFA in SRFA solution k4, the first-order rate of direct photolysis of TMP k5, the pseudo-first-order rate of photolysis of TMP in SRFA solution k6, and the photolysis rate of TMX in the SRFA solution k7; the results are shown in Figure 6 As shown, Figure 6 (a) is the first-order rate k of photodegradation of thiamethoxam in pure water (solution ①) and SRFA solution (solution ⑦) d and k7; Figure 6 (b) is the benzene in solution ② and . The rate at which OH reacts to form phenol Figure 6 (c) is the photolysis rate k3 and k4 of FFA in pure water (solution ③) and SRFA solution (solution ④), respectively; Figure 6 (d) is the degradation rates k5 and k6 of TMP in pure water (solution ⑤) and SRFA solution (solution ⑥), respectively.

[0124] The phenol generation rate in the reaction solution ② is based on and The formula is used to obtain the [·OH] generated by 10 mg / L SRFA solution under light. ss (1.65±0.03)×10 -17 M; the degradation rate in solution ③④ is based on and The calculated results show that the light-induced generation of 10 mg / L SRFA solution is 1 O2] ss (1.53±0.09)×10 -13 M; the degradation rate of TMP in solution ⑤⑥ is based on Obtain 10mg / L SRFA solution light production 3 CDOM * Rate (6.22±1.04)×10 -8 M.s -1 ; Finally, according to and Get thiamethoxam and 3 CDOM * The secondary reaction rate is (2.82±0.35)×10 9 M –1 ·s –1 .

[0125] Example 5

[0126] The photochemical kinetic parameters obtained in Examples 1-4 include the direct photolysis quantum efficiency of thiamethoxam, the reaction of thiamethoxam with CO3 .- , . OH, 1 O2 and 3 CDOM * The secondary reaction rate is transmitted to APEX software. Other parameters such as solar spectrum data are taken from the data of sunny summer weather at 45° north latitude. The natural water parameters use the default parameters, i.e. [NO2 - ]=1.0μM,[CO3 2- ]=10μM,[HCO3 - ]=1×10 –3 M,[Br - ]=1×10 –11 M, water depth range is 0-5m, DOC concentration range is 0-20mgC / L, and the outdoor photodegradation kinetics of thiamethoxam under these conditions are simulated and predicted.

[0127] like Figure 7 The figure shows the effect of thiamethoxam in water body conditions of [NO2 - ]=1.0μM,[CO3 2- ]=10μM,[HCO3 - ]=1×10 –3 M,[Br - ]=1×10 –11 The degradation kinetics rate and contribution of each degradation pathway in the surface water of the environmental water body of M (water depth 0.5m) change with the DOC content. From the main figure, it can be seen that the total degradation rate of thiamethoxam in the surface water under this condition will decrease with the increase of DOC content, but when the DOC content increases to 6mgC / L, the decrease in degradation rate is gradually less obvious. As can be seen from the upper right figure, 1 O2 had little contribution to the degradation of thiamethoxam. When the DOC concentration was low, the degradation of thiamethoxam was mainly contributed by direct photolysis, followed by . OH and CO3 .- , with the increase of DOC, direct photolysis . OH and CO3 .- The contribution of 3 CDOM * The contribution of DOC content gradually increases. The reason for this phenomenon is that the increase in DOC content can produce more 3 CDOM * , increase 3 CDOM *However, the degradation of thiamethoxam at low concentrations shows that direct photolysis is very important to it, and the increased DOC will produce a light shielding effect on thiamethoxam, slowing down the direct photolysis, and the increased DOC will also affect the CO3 .- and . OH produces a quenching effect, that is, the increase in DOC causes 3 CDOM * The degradation promotion effect is smaller than the direct photolysis slowdown caused by the increase of DOC, CO3 .- and . The reduction of OH slows down the degradation, but when the DOC content increases to a certain level, this effect gradually tends to balance, making the degradation rate stable.

[0128] Figure 8 The results are as follows: thiamethoxam at 45° north latitude in summer, clear weather, DOC concentration of 1 mgC / L, and other water conditions of [NO2 - ]=1.0μM,[CO3 2- ]=10μM,[HCO3 - ]=1×10 –3 M,[Br - ]=1×10 –11 The degradation kinetics rate of M in environmental water and the contribution of each degradation pathway vary with water depth. Figure 8 It can be seen that at low DOC concentration (1 mgC / L), the photodegradation rate of thiamethoxam decreases with the increase of water depth, and direct photolysis, . OH, CO3 .- and 3 CDOM * The contribution of OH and CO3 also decreased. The reason for this phenomenon is that the increase in water depth gradually reduces the amount of sunlight that thiamethoxam can receive, reducing direct photolysis, and further increasing the amount of OH and CO3 .- and 3 CDOM * The amount generated also gradually decreases.

[0129] Fig. 9 Thiamethoxam in [NO2 - ]=1.0μM,[CO3 2- ]=10μM,[HCO3 - ]=1×10 –3 M,[Br - ]=1×10 –11The prediction results of the degradation half-life of M in environmental water bodies with water depth and DOC concentration. From the prediction results, it can be seen that the photolysis half-life of thiamethoxam in environmental water bodies will gradually increase with the increase of water depth and DOC content. In the range of water depth of 0-5 meters and DOC content of 0-20 mgC / L, the half-life of thiamethoxam is 0.3-12.3 days.

[0130] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. For ordinary technicians in this field, other different forms of changes or modifications can be made based on the above descriptions and ideas. For example, after obtaining the basic kinetic parameters according to Examples 1-4, other kinetic results can be simulated in the APEX software. It is not necessary and impossible to list all implementation methods here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for studying the photochemical transformation kinetics of thiamethoxam in environmental water, characterized in that: The following steps are involved: S1 The quantum efficiency of direct photolysis of thiamethoxam was determined by chemical irradiometer method in a multi-position photochemical reaction chamber; S2 Determination of Thiamethoxam and Carbonate Radical CO3 by Pulsed Laser Flash Photolysis .- Thiamethoxam and hydroxyl radicals . The secondary reaction rate of OH; S3 Determination of the Relative Rate of Thiamethoxam and Singlet Oxygen 1 The secondary reaction rate of O2; S4 Use CDOM model compounds to simulate dissolved organic matter CDOM in natural water environments and indirectly determine thiamethoxam and triplet dissolved organic matter by constructing a kinetic model 3 CDOM * The secondary reaction rate of S5 The photochemical kinetic parameters obtained in steps S1-S4, including the direct photolysis quantum efficiency of thiamethoxam, the reaction of thiamethoxam with CO3 .- , . OH, 1 O2 and 3 CDOM * The secondary reaction rate of the reaction was input into APEX software, and the laboratory results were extrapolated to the real environmental water conditions to simulate and predict the photochemical transformation kinetics of thiamethoxam in environmental water bodies; In step S1, the specific steps of determining the quantum efficiency of direct photolysis of thiamethoxam are as follows: irradiating a chemical irradiator and an aqueous solution of thiamethoxam in a multi-position photochemical reaction chamber under the same environmental conditions, sampling and determining the photolysis rates of the chemical irradiator and thiamethoxam after different reaction times, and obtaining the quantum efficiency of direct photolysis of thiamethoxam; the chemical irradiator is a p-nitroanisole / pyridine binary irradiator, i.e., a PNA / pyr binary irradiator; wherein the concentration ratio of PNA to pyr needs to be adjusted to a similar pseudo-first-order rate of photodegradation of PNA and thiamethoxam; Since the light source of the irradiation light meter is the same as that of thiamethoxam, the direct photolysis quantum efficiency of thiamethoxam can be obtained by formula Eq.1: In Eq.1, Φ TMX and Φ PNA are the quantum efficiencies of thiamethoxam and PNA in the light meter, respectively; k TMX and k PNA are the first-order rates of direct photolysis of thiamethoxam and PNA in the light meter, respectively; I λ is the light intensity of the light source at wavelength λ; ε PNA(λ) and ε TMX(λ) are the molar absorption coefficients of PNA and thiamethoxam at wavelength λ, which can be determined by ultraviolet absorption spectroscopy; b is the optical path length during the reaction; c is the wavelength of thiamethoxam at wavelength λ, which can be determined by ultraviolet absorption spectroscopy; PNA and c TMX are the concentrations of PNA and thiamethoxam, respectively; In step S5, APEX software is used to integrate the experimental results of steps S1-S4 to perform simulation and prediction to obtain the photochemical transformation kinetics of thiamethoxam in the natural water environment, including the change of the photolysis half-life of thiamethoxam with the physical environment, and the proportion of direct photolysis and indirect photolysis contributed by each free radical during thiamethoxam photodegradation.

2. The method for studying the photochemical conversion kinetics of thiamethoxam in environmental water according to claim 1, characterized in that: The structural formula of thiamethoxam is as follows: 。 3. The method for studying the photochemical conversion kinetics of thiamethoxam in environmental water according to claim 1, characterized in that: In step S2, the concentration of thiamethoxam and CO3 is determined. .- Thiamethoxam and . The specific steps of the secondary reaction rate of OH are: using a circulating water pump to pump the free radical precursor solution contained in the external beaker into the quartz reaction tube, using the pulsed excitation light emitted by the laser to irradiate the free radical precursor solution in the quartz tube to generate free radicals, then adding the concentrated mother solution of thiamethoxam to the beaker of the external reaction solution in batches, using the detection light to monitor the change of free radical concentration after the reaction of different concentrations of thiamethoxam with free radicals online by spectral absorption method, and then converting the free radical concentration change signal by an oscilloscope and averaging it multiple times before transmitting it to a computer for data collection.

4. The method for studying the photochemical conversion kinetics of thiamethoxam in environmental water according to claim 1, characterized in that: In step S3, the reaction between thiamethoxam and singlet oxygen is determined. 1 The specific steps of the secondary reaction rate of O2 are: prepare a mixture containing thiamethoxam, 1 The mixed solution of O2 sensitizer and reference substance was placed in a quartz tube and stirred magnetically to fully dissolve oxygen. It was then placed in a light box for illumination. After different reaction times, samples were taken to determine the photodegradation rates of thiamethoxam and reference substance. 1 The competitive reaction relationship between thiamethoxam and O2 1 The secondary reaction rate of O2.

5. The method for studying the photochemical conversion kinetics of thiamethoxam in environmental water according to claim 4, characterized in that: Said 1 O2 sensitizers are those that can generate 1 The substance that reduces O2 includes methylene blue or rose Bengal; the reference substance is acetaminophen APAP.

6. The method for studying the photochemical conversion kinetics of thiamethoxam in environmental water according to claim 1, characterized in that: In step S4, the dissolved organic matter of thiamethoxam and triplet is determined. 3 CDOM * The specific steps of the secondary reaction rate are as follows: using benzene, furfuryl alcohol and 2,4,6-trimethylphenol as . OH, 1 O2, 3 CDOM * The probe compounds of thiamethoxam, furfuryl alcohol, and 2,4,6-trimethylphenol in pure water, as well as thiamethoxam and each probe compound in CDOM solution were placed in a multi-position photochemical reaction chamber for illumination, and a chemical kinetic model of thiamethoxam was established to distinguish the thiamethoxam in CDOM solution. . OH, 1 O2, 3 CDOM * After the contribution of 3 CDOM * The rate of the second-order reaction.

7. The method for studying the photochemical conversion kinetics of thiamethoxam in environmental water according to claim 6, characterized in that: Said 1 O2 and 3 CDOM * The probe compound needs to be added with a quencher in a solution containing CDOM; and 3 CDOM * The solution containing the probe compound and the quencher and containing CDOM needs to be continuously aerated with N2 during illumination to ensure an anaerobic environment.

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