2017 Volume 42 Issue 2 Pages 17-24
The behavior of cyphenothrin (1) [(RS)-α-cyano-3-phenoxybenzyl (1RS)-cis-trans-2,2-dimethyl-3-(2-methylprop-1-enyl)cyclopropanecarboxylate] in an aquatic environment was investigated by using the 14C-labeled trans and cis isomers. In parallel with the rapid partition from water phase to bottom sediment, 1 was degraded with the first-order half-lives of 2.0 (trans-1) and 7.3 days (cis-1) in the water-sediment system under dark conditions. 1 underwent extensive microbial degradation via ester cleavage to form 3-phenoxybenzoic acid, finally forming bound residues and mineralizing to CO2. Aqueous photolysis significantly accelerated the degradation of 1 with a half-life of <1 day, mainly via photo-induced oxidation at the 2-methylprop-1-enyl group and ester cleavage without cis-trans isomerization. These results strongly suggest that 1 is unlikely to persist in the actual aquatic environment due to its rapid photolysis and extensive microbial degradation.
The potential toxicological impacts of synthetic pyrethroids on aquatic organisms, judging from their insecticidal mode of action, necessitate their risk assessment even for public hygiene uses causing their limited contamination of water bodies. For this purpose, information on the fate and behavior of synthetic pyrethroids is indispensable to estimate their residues in each compartment of the aquatic environment. After entering the water phase, synthetic pyrethroids generally show a rapid partition from water to bottom sediment due to their highly hydrophobic nature, and they are microbially degraded in the aerobic sediment mainly via hydrolytic cleavage of an ester linkage with its susceptibility dependent on their geometrical and/or optical isomerism.1–4) Hydrolysis is one of their abiotic transformation processes, and more accelerated alkaline hydrolysis is known for the type-II pyrethroids possessing an α-cyano benzyl moiety.5) Photodegradation is another key process, and direct photolysis via absorption of light energy results in the homolytic bond cleavage of an excited moiety to form intermediate reactive radicals and/or ester hydrolysis.6–8) Additionally, the photo-induced oxidation at an electron-rich C=C bond in the chrysanthemic acid moiety is known to proceed as indirect photolysis of phenothrin and tetramethrin.9,10)
Cyphenothrin (1) [(RS)-α-cyano-3-phenoxybenzyl (1RS)-cis-trans-2,2-dimethyl-3-(2-methylprop-1-enyl)cyclopropanecarboxylate] is one of the early generation pyrethroids and exhibits a high insecticidal potency.11) The geometric isomer ratio in 1 is 4 : 1 (trans:cis) with the 1R-trans-αS isomer being most biologically active among eight isomers to control flies, mosquitoes, cockroaches, etc.12) Although the abiotic and microbial degradation profiles in an aquatic environment are necessary for risk assessment, sufficient information on 1 in a water-sediment system and aqueous photolysis is not available. The objective of this study is to determine the dissipation and degradation profiles of 1 through aerobic metabolism in a water-sediment system and aqueous photolysis by using the 14C-labeled trans and/or cis isomers, each of which consists of four stereoisomers (1RS-αRS). The relative degradability of each stereoisomer was further examined in the case of the water-sediment study. The soil adsorption studies of each isomer were also conducted to determine their partition profiles.
Two 1RS-αRS isomeric mixtures of 1, abbreviated as cis-1 and trans-1, uniformly labeled with 14C at the phenoxyphenyl ring (11.64 and 11.72 MBq/mg; radiochemical purity, 98.5%) were synthesized in our laboratory.11) The four isomeric mixtures of 1, 1R-trans-αRS, 1S-trans-αRS, 1R-cis-αRS and 1S-cis-αRS, were prepared in our laboratory. 3-Phenoxybenzyl alcohol (2), 3-phenoxybenzoic acid (3) and 3-phenoxybenzaldehyde (4) were purchased from Sigma-Aldrich (USA). (RS)-α-Cyano-3-phenoxybenzyl (1R)-trans-2,2-dimethyl-3-carboxycyclopropanecarboxylate (5) and (RS)-α-cyano-3-phenoxybenzyl (1R)-trans-2,2-dimethyl-3-formyl-cyclopropanecarboxylate (6) were prepared by treating 1R-trans-1 (6 mg in 3 mL of acetonitrile) with ozone generated using a microozonizer (ON-1-2, NIPPON OZONE, Japan) at room temperature for 10 min and 20 sec, respectively. Their chemical structures were confirmed by LC-MS (Waters ZQ2000 mass spectrometer equipped with an ESI interface) and 1H NMR (Varian Unity 400 spectrometer at 400 MHz) analyses. 5: MS m/z 366 [M+H]+, 388 [M+Na]+, 731 [2M+H]+; 1H NMR δH (CDCl3): 7.02–7.42 (9H, m, aromatic H), 6.36 (1H, s, CN-CH), 2.31–2.34 (2H, m, CO-CH, COOH-CH), 2.01 (1H, s, COOH), 1.27–1.33 (6H, m, -CH3). 6: MS m/z 350 [M+H]+, 367 [M+NH4]+, 372 [M+Na]+; 1H NMR δH (CDCl3): 9.63 (1H, s, CHO), 7.02–7.43 (9H, m, aromatic H), 6.35 (1H, s, CN-CH), 2.50–2.59 (2H, m, CO-CH, CHO-CH), 1.28–1.39 (6H, m, -CH3). Pure water with an electrical resistivity of more than 18 MΩ cm was supplied with a Puric-MX IIAN (Organo, Japan) and used for the preparation of buffer solutions and for chromatographic analyses.
2. RadioassayThe radioactivity in water, organic extracts, and trapping media was individually determined by liquid scintillation counting (LSC) with a Tri-Carb 3110TR (PerkinElmer, USA) liquid scintillation spectrometer equipped with an automatic external standard by adding 10 mL of Emulsifier Scintillator Plus™ (PerkinElmer). The amount of 14C collected in the NaOH trap was identified as 14CO2 by adding BaCl2, resulting in quantitative precipitation as Ba14CO3. The background level of radioactivity in LSC samples was 30 dpm (0.5 Bq). Unextractable soil and sediment residues were powdered after drying, and a portion was subjected to combustion analysis using a sample oxidizer model 307 (PerkinElmer) under 14C recovery of >95%.
3. ChromatographyThe organic extracts of water and sediment were individually analyzed by reversed-phase high-performance liquid chromatography (HPLC) for either the quantification of 1 and its degradates and/or their chemical identification. A Shimadzu LC-20AT pump equipped with a SUMIPAX ODS A-212 column (5 µm, 6-mm i.d.×150 mm; Sumika Chemical Analysis Service (SCAS), Japan) was operated at a flow rate of 1 mL/min using a mobile phase with stepwise changing as follows: 0 min, %A (acetonitrile): %B (0.05% formic acid), 10 : 90; 0 to 10 min, linear, 55 : 45 at 10 min; 10 to 40 min, linear, 95 : 5 at 40 min; 40 to 45 min, isocratic, 95 : 5 at 45 min. The isomeric contents of 1 and its stereoisomers were determined by a normal-phase chiral HPLC using three SUMICHIRAL OA-2000 columns (5 µm, 4-mm i.d.×250 mm; SCAS) connected in a series under the isocratic solvent system of n-hexane/n-butanol=300 : 1 (v/v) at a flow rate of 0.5 mL/min. The radioactivity in the column effluent was monitored with a flow scintillation radio detector, Radiomatic 150TR (PerkinElmer), with a 500-µL liquid cell using Ultima-Flo AP® (PerkinElmer) as the scintillator. Each 14C peak was identified by HPLC co-chromatography by comparing its retention time with those of non-radiolabeled references monitored at 254 and/or 278 nm with a Shimadzu SPD-20A UV/Vis detector. For thin-layer chromatography (TLC), silica gel 60F254 thin-layer plates (200×200 mm, 0.25-mm layer thickness; E. Merck, Germany) were used with the solvent systems of A (chloroform/methanol; 9/1, v/v) and B (n-hexane/ethyl acetate/acetic acid; 40/10/1, v/v/v). Autoradiograms were prepared by exposing the TLC plate to a BAS-IIIs Fuji imaging plate (Fuji Photo Film, Japan), and the radioactivity in each spot was quantified using a Typhoon 9200 Variable Mode Imager (GE Healthcare UK, UK). Non-radiolabeled reference standards were detected by ultraviolet light at 254 nm. Typical HPLC retention times (tR) and TLC Rf values of 1 and related degradates are listed in Table 1.
| Designation | HPLC tR (min) a) | TLC Rf b) | ||
|---|---|---|---|---|
| Reversed-phase | Chiral | A | B | |
| cis-1 | ||||
| 1R-cis-αS | 41.5 | 100.6 | 0.76 | 0.74 |
| 1R-cis-αR | 102.9 | |||
| 1S-cis-αR | 104.3 | |||
| 1S-cis-αS | 105.7 | |||
| trans-1 | ||||
| 1R-trans-αR | 42.3 | 107.6 | 0.76 | 0.74 |
| 1S-trans-αS | 110.7 | |||
| 1R-trans-αS | 113.6 | |||
| 1S-trans-αR | 119.7 | |||
| 2 | 20.4 | na | 0.60 | 0.57 |
| 3 | 21.0 | na | 0.30 | 0.61 |
| 4 | 25.2 | na | 0.74 | 0.69 |
| 5 | 25.6 | na | 0.26 | 0.61 |
| 6 | 29.9 | na | 0.74 | 0.70 |
na: not analyzed. a) Typical HPLC retention time. b) TLC Rf values with indicated solvent systems. A, chloroform/methanol (9/1, v/v); B, n-hexane/ethyl acetate/acetic acid (40/10/1, v/v/v).
The soil adsorption coefficients (Kd) of [14C]cis-1 and [14C]trans-1 in five soils (Table 2) were examined using the batch equilibrium method.13) Each soil (1.0 g) was added to a glass centrifuge tube with a Teflon-lined screw cap containing 150 mL of 0.01 M CaCl2, resulting in a soil-to-solution ratio of 1 : 150 (w/v). After pre-equilibration at 25±2°C in darkness overnight in an incubator (Bio-shaker BR-300LF, TAITEC, Japan), the appropriate volume of acetonitrile solution of cis- or trans-1 (28.0 µg/mL or 26.0 µg/mL, respectively) was added in duplicate to give a nominal concentration of 0.005 mg/L. The glass tubes were shaken at 25±2°C in darkness and centrifuged at 2500 rpm for 10 min (Himac CR20G, Hitachi, Japan). A suitable equilibration time was determined by shaking for 4, 6, 8, and 24 hr. A 1-mL aliquot of the supernatant separated by decantation was radioassayed in duplicate to quantify 14C in the aqueous phase, and the adsorbed 14C on each soil was conveniently estimated by subtraction of the quantified values in an aqueous phase from the applied 14C. The decanted aqueous solution and remaining soil were further extracted by ethyl acetate and acetone, respectively, followed by HPLC analysis of each extract to quantify 1. To confirm 14C adsorbed onto the vessel wall, the soil just after decantation was scraped off the test tube, and then the vessel wall was rinsed with acetonitrile. The Kd values were estimated using the following equation by assuming the linear adsorption isotherm:
 |
| Soil | Soil classification (USDA) | OC (%) | pH (H2O) | cis-1 | trans-1 | ||
|---|---|---|---|---|---|---|---|
| Kd | Koc | Kd | Koc | ||||
| Atwater (US) | Loamy sand | 0.3 | 6.3 | 633 | 204280 | 606 | 195590 |
| Williams (US) | Loam | 1.8 | 6.9 | 1281 | 71160 | 1443 | 80160 |
| Speyer 5M (Germany) | Sandy loam | 1.0 | 7.3 a) | 6577 | 657720 | 1894 | 189430 |
| Speyer 2.2 (Germany) | Loamy sand | 1.8 | 5.5 a) | 9580 | 541230 | 1278 | 72230 |
| Saitama (Japan) | Loam | 3.0 | 5.9 | 1159 | 39150 | 1057 | 35700 |
OC: organic carbon. a) pH values measured with 0.01 M CaCl2
The behavior of [14C]cis-1 and [14C]trans-1 was examined in one water-sediment system collected from Turano Pool (Turano, Provincia di Brescia, Italy). The sediment and water were respectively filtered through 2-mm and 250-µm sieves prior to use to remove stones and plant debris. The sediment was characterized as silt loam (USDA), with 3.7% organic carbon and pH 7.5 (measured with CaCl2), and the associated water pH was 7.8. A portion of the sediment was taken into a cylindrical glass vessel (4.4 cm in diameter) to a depth of 2 cm (14 g/dry weight basis). The associated water was gently added to each vessel to a depth of 6 cm (90 mL) above the sediment surface in accordance with OECD 308.14) The water-sediment system was pre-incubated in darkness at 20±1°C for four weeks prior to application.
The application rate was adjusted to 0.46 µg/vessel by conveniently assuming the field application rate of 3 g a.i./ha,15) which provided sufficient analytical sensitivity to detect degradation products under the aqueous concentration less than its water solubility of 0.009 mg/L at 25°C.16) A 900-µL aliquot of acetonitrile solution of each [14C] was dropwise fortified to the water surface in each vessel using a pipet, followed by gentle mixing. This organic solvent volume, not exceeding 1% of the overlying water (v/v), was necessary to prevent the adsorption of 1 onto the glass surface of the vessel. The treated glass vessels were placed on an orbital shaker (SCS-20N, Sanki Seiki, Japan) at 60 rpm to moderately mix the water phase and incubated in the dark at 20±1°C. Humidified air was continuously passed over the water surface in sequence to one gas-washing bottle containing 100 mL of ethylene glycol and another containing 200 mL of 0.5 M NaOH solution to trap volatile 14C.
At appropriate intervals, the overlying water, sediment, and trapping media were individually sampled and analyzed immediately in the same manner as previously reported.17) The sediment was successively extracted three times with acetone and twice with acetone/0.5 M HCl (8/2, v/v), and then the concentrated extracts were analyzed by HPLC co-chromatography with the reference standards. The bound residues at the final sampling were exhaustively extracted using a Soxhlet apparatus with 90 mL of acetone/0.1 M HCl (8/2, v/v) at 100°C for 5 hr, and the bound 14C was further fractionated into humin, humic acid, and fulvic acid using the published methods.18) Ethyl alcohol (100 mL) was added to the water layer sample, concentrated in vacuo, and then analyzed using HPLC.
In order to compare the degradation rate constants of eight isomers of 1, the water-sediment study at a ten-times exaggerated application rate was also conducted similarly to that above, except for the application of [14C] to the bottom sediment. The HPLC fraction corresponding to cis-1 or trans-1 eluted at 41–42 or 42–43 min, isolated from the combined water and sediment extracts, was further subjected to chiral-HPLC analysis.
6. Photodegradation studyThe photodegradation profile of 1 was conveniently examined by using a trans isomer, since it is a major component. 1R-Trans-1 is stable at pH 4 and 25°C but hydrolyzed with the half-lives of 112 (pH 7) and 4.6 days (pH 9),19) similar to most synthetic pyrethroids.5) Therefore, the aqueous photolysis of [14C]trans-1 was investigated in 50 mM acetate buffer at pH 4 in the presence of 1% acetonitrile, according to OECD 316.20) The buffer solution sterilized by autoclaving for 15 min at 1.5 kg/cm2 and 121°C was used to prepare the 500-mL aqueous solution of 1 at the nominal concentration of 0.002 mg/L, less than its water solubility.16) The effect of co-solvent on the photolytic profiles was examined using 10% acetonitrile to prepare the homogeneous solution. Each aqueous solution was aseptically transferred into a 1-L cylindrical reaction vessel with the top covered by a Pyrex glass plate to cut off UV light below 290 nm and continuously irradiated under gentle stirring for 26 hr at 25±2°C. In order to examine the formation and decline profiles of photodegradates, the test solution including a 1% co-solvent was further irradiated for 21 days with continuous passing of CO2-free and humidified air to correct volatile 14C using the same trap systems as in the water-sediment study. The light source was a 2-kW xenon arc lamp (UXL-25SC, Ushio, Japan) with its spectral energy distribution similar to that of natural sunlight, as evidenced by measurement with a USR-40 spectroradiometer (Ushio). Integrated irradiance at 300 to 400 nm was almost constantly 28.0 W/m2 throughout the experiments. A dark control experiment was conducted separately without any volatile traps.
An aliquot of each test solution was periodically measured by LSC to determine the concentration of 14C and confirm a solution homogeneity. After each test solution was decanted, the original reaction vessel was rinsed with 10 mL of acetonitrile and analyzed to quantify the compounds adsorbed onto the glass surface. The decanted solution was extracted twice with 200 mL of n-hexane, and the remaining aqueous layer for 7- and 21-day samples was acidified to ca. pH 2 with conc. HCl followed by two further extractions with 200 mL of ethyl acetate. After confirming that little 14C (<7% of applied radioactivity, %AR) remained in the aqueous layer, the combined organic layer was concentrated and subjected to HPLC/TLC co-chromatography with reference standards. The isolated fraction of trans-1 eluted at 42–43 min in reversed-phase HPLC was further subjected to chiral-HPLC analysis to examine the possible isomerization.
7. Degradation kineticsThe degradation kinetics for the water-sediment study was obtained by non-linear regression analysis. The following single first-order kinetics was applied in accordance with FOCUS DEGKIN21): C=C0 e−kt, where C0 and C are the quantified values of cis- or trans-1 (%AR) at time zero and incubation time t (day), respectively, and k is the first-order rate constant (1/day). The first-order rate constants for the photodegradation study (k in 1/hr) were estimated using the following equation: ln (C/C0)=−kt + b, where C0 is the summed trans-1 detected in the extracted solution and rinsate of the reaction vessel at time zero, C is the summed trans-1 at irradiation time t (hr), and b is a constant. The half-lives (DT50) and period of 90% degradation (DT90) of cis- and/or trans-1 were estimated using the following equations: DT50=0.693/k and DT90=2.303/k, respectively. The photodegradation rate of trans-1 in the actual environment was estimated by using the reported midday sunlight intensity in summer months at 30–50°N latitude.22)
By the periodical monitoring of 14C adsorbed to each soil up to 24 hr, the test system was found to be equilibrated after 4 hr (data not shown). The percentages of 14C adsorbed to five soils after 4 hr were 81–91% and 79–90%AR for cis-1 and trans-1, respectively. The sufficient stability of 1 (>97%) in the adsorbed soils was confirmed by HPLC, whereas significant degradation was observed in the supernatant of some soil suspensions even after 4 hr, representing 11–96% and 33–95% for cis-1 and trans-1, respectively. To establish the equilibrium and minimize potential degradation at the same time, the quantified values of 1 in aqueous and soil phases for 4-hr shaking samples were conveniently used for the calculation of Kd. The amounts of 1 remaining on the glass surface were negligible (<0.5%AR). LSC and HPLC analyses of the extracts showed that cis-1 and trans-1 in aqueous phases amounted to 1.8–19% and 7–20%AR, respectively, and the corresponding adsorbed ratio to each soil was 78–91% (cis-1) and 79–88%AR (trans-1). The Kd and Koc values were calculated to be 633–9580 mL/g and 39150–657720 mL/g o.c. for cis-1, and 606–1894 mL/g and 35700–195590 mL/g o.c. for trans-1, as summarized in Table 2. The high adsorption nature of 1 failed to estimate the corresponding Freundlich adsorption coefficients. No pH dependency of the adsorption onto the soil was observed. Nor was there any relation between clay content and Kd values.
2. Behavior of 1 in the water-sediment systemThe redox potential ranged from 134 to 264 mV in the overlying water and from −105 to −161 mV in the bottom sediment, indicating that the water column was kept aerobic by continuous air flow, but the sediment was moderately anaerobic. As summarized in Table 3, the pH value in each phase was maintained at a constant, and good 14C recovery was obtained (88.4–105.1%AR) throughout the study. 14C in the water phase gradually decreased to 23.9 and 6.8%AR after 30 days for cis-1 and trans-1, respectively, whereas the corresponding values in sediment increased to 55.7 and 46.5%AR. The concomitant increase of bound 14C was observed, amounting to 19.3%AR (cis-1) and 33.2%AR (trans-1). An insignificant amount of 14C was collected by the acidic Soxhlet extraction (<1%AR at the end of the study), and the bound residues were fractionated into humin (6.2–12.8%AR), humic acid (5.6–9.4%AR), and fulvic acid (10.3–10.4%AR). Both cis-1 and trans-1 were rapidly dissipated, with DT50 values of 2.5 and 1.0 days (water layer) and 7.3 and 2.0 days (total system), respectively, as summarized in Table 4.
| % of the applied 14C | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Incubation period (days) | ||||||||||||
| cis-1 | trans-1 | |||||||||||
| 0 | 1 | 3 | 7 | 14 | 30 | 0 | 1 | 3 | 7 | 14 | 30 | |
| Water Phase | 90.6 | 72.9 | 67.5 | 39.3 | 32.9 | 23.9 | 70.7 | 61.9 | 75.2 | 42.5 | 20.8 | 6.8 |
| cis-1 | 90.3 | 57.5 | 39.6 | 13.0 | 6.7 | 8.7 | nd | nd | nd | nd | nd | nd |
| trans-1 | 0.4 | 0.3 | nd | nd | nd | nd | 70.1 | 27.2 | 15.6 | 4.2 | nd | nd |
| 3 | nd | 13.8 | 24.0 | 26.0 | 24.3 | 14.3 | nd | 28.5 | 54.5 | 36.1 | 19.8 | 6.8 |
| Others | nd | 1.3 | 3.8 | 0.3 | 1.9 a) | 1.0 | 0.6 | 6.2 | 5.1 | 2.2 | 1.0 | nd |
| Sediment Phase | 14.3 | 29.0 | 36.8 | 55.3 | 55.7 | 39.8 | 31.3 | 38.1 | 27.6 | 39.9 | 46.5 | 40.4 |
| Extract | 14.3 | 28.9 | 36.2 | 46.5 | 41.5 | 20.5 | 31.3 | 37.8 | 23.7 | 16.3 | 14.9 | 7.3 |
| cis-1 | 14.0 | 28.5 | 35.2 | 35.6 | 18.9 | 6.1 | nd | nd | nd | nd | nd | nd |
| trans-1 | nd | 0.3 | nd | nd | nd | nd | 31.3 | 37.4 | 20.1 | 9.7 | 2.6 | nd |
| 3 | 0.3 | nd | 0.3 | 5.4 | 14.8 | 8.9 | nd | 0.4 | 3.0 | 4.4 | 8.0 | nd |
| Others | nd | nd | 0.7 | 5.5 | 7.8 a) | 5.5 | nd | nd | 0.5 | 2.2 | 4.3 | 7.3 |
| Bound residue | <0.1 | 0.1 | 0.6 | 8.8 | 14.2 | 19.3 | <0.1 | 0.3 | 3.9 | 23.7 | 31.5 | 33.2 |
| Volatile | — | 0.3 | 0.9 | 4.0 | 7.4 | 28.0 | — | 0.7 | 1.7 | 9.4 | 21.9 | 41.1 |
| 14CO2 | — | 0.1 | 0.3 | 3.1 | 6.9 | 27.5 | — | 0.4 | 1.0 | 8.2 | 21.5 | 39.9 |
| Others | — | 0.2 | 0.6 | 0.9 | 0.5 | 0.5 | — | 0.3 | 0.6 | 1.2 | 0.4 | 1.2 |
| Mass balance | 105.0 | 102.2 | 105.1 | 98.7 | 96.0 | 91.8 | 102.1 | 100.7 | 104.5 | 91.8 | 89.2 | 88.4 |
| Water Phase | ||||||||||||
| pH | 8.6 | 8.6 | 8.7 | 8.8 | 8.7 | 8.7 | 8.6 | 8.6 | 8.6 | 8.5 | 8.7 | 8.8 |
| DO (mg/L) | 7.2 | 7.8 | 7.5 | 7.5 | 7.7 | 7.9 | 6.5 | 7.2 | 7.1 | 7.3 | 7.8 | 7.4 |
| ORP (mV) | 246 | 223 | 237 | 227 | 258 | 170 | 180 | 153 | 134 | 251 | 264 | 198 |
| Sediment Phase | ||||||||||||
| pH | 8.3 | 8.4 | 8.3 | 8.3 | 8.5 | 8.4 | 8.3 | 8.3 | 8.3 | 8.3 | 8.5 | 8.6 |
| ORP (mV) | −116 | −109 | −124 | −120 | −161 | −105 | −126 | −135 | −131 | −112 | −128 | −114 |
nd: not detected. −: not applicable. DO: dissolved oxygen. ORP: oxygen redox potential. a) multiple components (each, <6.0%AR, Total system).
| Water-sediment | Rate constant | half-lives and 90%-degradation (days) | r2 | ||
|---|---|---|---|---|---|
| k (1/day) | DT50 | DT90 | |||
| cis-1 | water layer | 0.280 | 2.5 | 8.2 | 0.952 |
| total system | 0.095 | 7.3 | 24.3 | 0.870 | |
| trans-1 | water layer | 0.725 | 1.0 | 3.2 | 0.946 |
| total system | 0.341 | 2.0 | 6.8 | 0.973 | |
| Photodegradation | Rate constant | half-lives (hrs) | r2 | ||
| co-solvent | k (1/hr) | Xenon arc lamp | Natural sunlight30–50°N | ||
| trans-1 | 1% | 0.0843 | 8.2 | 9.2 | 0.820 |
| 10% | 0.0614 | 11.3 | 12.7 | 0.763 | |
At the end of the study, cis-1 decreased to 8.7 and 6.1%AR in water and sediment phases, respectively, while trans-1 completely disappeared from the total system. The HPLC analysis of aqueous and sediment extracts showed that cis-trans isomerization did not proceed, and 3 was detected as a major metabolite. The amount of 3, produced from cis-1 and trans-1, increased to 26.0 and 54.5%AR after 7 and 3 days in the overlying water but decreased to 14.3 and 6.8%AR after 30 days, respectively. In the sediment, 3 concomitantly increased to 14.8 (cis-1) and 8.0%AR (trans-1) after 14 days and then decreased to <8.9%AR at the end of the study. None of other degradates exceeded 6.0%AR throughout the study. 14CO2, the major component in the volatile fraction, finally amounted to 27.5–39.9%AR.
As a result of exaggerated application, the degradation half-lives of both isomers in the total system increased by a factor of 3–4 to ca. 19 days for cis-1 and 9 days for trans-1. This slower degradation was at least partly caused by the direct application of test substances to the sediment layer with a high concentration, but the same metabolite 3 was significantly detected with the maximal amount of 20.3%AR (cis-1) and 34.8%AR (trans-1) after 15 days. Similarly, 14CO2 was significant in the volatiles with 16.5–21.2%AR after 30 days. Chiral-HPLC analysis of the isolated parent fraction showed that the degradation rate of each isomer in the sediment decreased in the order of 1S-trans-αS>1S-trans-αR>1R-trans-αS>1R-trans-αR>1R-cis-αR>1R-cis-αS>1S-cis-αR>1S-cis-αS, as shown in Table 5.
| 1-isomers | DT50 (days) | r2 |
|---|---|---|
| 1R-cis-αR | 14.2 | 0.806 |
| 1R-cis-αS | 16.1 | 0.756 |
| 1S-cis-αR | 17.4 | 0.752 |
| 1S-cis-αS | 20.5 | 0.758 |
| 1R-trans-αR | 13.1 | 0.844 |
| 1R-trans-αS | 9.0 | 0.942 |
| 1S-trans-αR | 8.2 | 0.916 |
| 1S-trans-αS | 5.7 | 0.929 |
The distribution of radioactivity originating from trans-1 is summarized in Table 6, with satisfactory recovery (96.1–102.0%AR) during 26-hr irradiation or incubation. Most 14C in the test solution was extractable as >71%AR with the unextracted aqueous 14C being less than 4.1%AR. The increased volume of co-solvent did not improve 14C adsorption onto the inner vessel wall. The adsorbed 14C slightly decreased under irradiation to 3.4–6.2%AR after 26 hr, whereas higher adsorption was observed for the dark samples (24.4–26.2%AR at 26 hr). Trans-1 was the main component of the adsorbed 14C according to the HPLC analysis of rinsates, with the oxidized product 6 detected as a minor one (<2.2%AR). Thus, the dissolution of adsorbed trans-1 via re-equilibrium caused by its reduced concentration in water under irradiation likely resulted in the gradual decrease of adsorbed 14C.
| % of the applied 14C | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Period of exposure (hrs/days) | ||||||||||||
| 10% co-solvent | 1% co-solvent | |||||||||||
| Light | Dark | Light | Dark | |||||||||
| (hr) | (hr) | (hr) | (day) | (hr) | ||||||||
| 0 | 4 | 20 | 26 | 26 | 0 | 4 | 20 | 26 | 7b) | 21b) | 26 | |
| Extracted 14C a) | 89.3 | 84.7 | 91.0 | 91.4 | 73.2 | 90.8 | 89.4 | 94.4 | 88.6 | 87.8 | 80.4 | 71.1 |
| trans-1 | 79.0 | 64.0 | 37.3 | 12.1 | 67.5 | 82.9 | 24.0 | 7.2 | 6.1 | nd | nd | 68.5 |
| cis-1 | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd |
| 2 | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd |
| 3 | nd | nd | nd | nd | nd | nd | nd | nd | nd | nd | 2.0 | nd |
| 4 | nd | 1.6 | 7.9 | 15.0 | nd | nd | 2.7 | 10.5 | 11.8 | 38.4 | 35.0 | nd |
| 5 | nd | 0.4 | 3.5 | 1.5 | nd | nd | nd | 2.0 | 4.8 | nd | 4.2 | nd |
| 6 | 10.3 | 16.6 | 18.1 | 32.4 | 5.7 | 7.9 | 47.6 | 49.4 | 37.3 | 40.9 | 16.9 | 2.6 |
| Others | nd | 2.1 | 24.2 | 30.4 | nd | nd | 15.1 | 25.3 | 28.6 c) | 8.5 | 22.3 | nd |
| Adsorbed 14C | 7.4 | 13.2 | 8.2 | 3.4 | 26.2 | 7.3 | 8.4 | 2.6 | 6.2 | 2.6 | 0.4 | 24.4 |
| trans-1 | 5.7 | 11.0 | 6.0 | na | 22.4 | 7.3 | 7.2 | na | 5.5 | na | na | 23.8 |
| 6 | 1.6 | 2.2 | 2.1 | na | 3.3 | nd | 1.2 | na | 0.7 | na | na | 0.6 |
| Others | 0.1 | nd | 0.1 | na | 0.5 | nd | nd | na | nd | na | na | nd |
| Aqueous 14C | 0.2 | 0.9 | 2.8 | 4.1 | 0.1 | 1.0 | 1.4 | 1.8 | 2.3 | 6.9 | 2.4 | 0.6 |
| Volatiles 14C | — | — | — | — | — | — | — | — | — | 1.7 | 5.8 | — |
| 14CO2 | — | — | — | — | — | — | — | — | — | 1.1 | 4.4 | — |
| Others | — | — | — | — | — | — | — | — | — | 0.6 | 1.4 | — |
| Mass balance | 96.9 | 98.8 | 102.0 | 98.9 | 99.5 | 99.1 | 99.2 | 98.8 | 97.1 | 99.0 | 88.9 | 96.1 |
nd: not detected. na: not analyzed. —: not applicable. a) with n-hexane. b) further extraction with ethyl acetate at pH 2. c) multiple components (each,<7.0% AR).
Total amount of trans-1 in dissolved and adsorbed 14C rapidly decreased to ca. 12%AR after 26-hr irradiation, while more than 90%AR was recovered in darkness, indicating the accelerated degradation of trans-1 by irradiation. Almost the same ratio of four stereoisomers (data not shown) without the appearance of cis isomer (Table 6) in the HPLC analyses indicates that neither homolytic cleavage at the cyclopropane ring nor epimerization at the α-cyanobenzyl carbon proceeds in the photodegradation of trans-1 with the half-life of 8.2–11.3 hr independent of a co-solvent volume, as summarized in Table 4. The rapid photolysis made it difficult to clarify any differences in degradation rates among four stereoisomers, but the almost constant isomeric ratio implied their comparable degradation.
Because of the insignificant effect of a co-solvent volume on the photodegradation profiles (Table 6), the behavior of photodegradates was further examined in the presence of a 1% co-solvent up to 21 days with the volatile traps being equipped. 6 showed the maximal formation (49.4%AR) after 20 hr and finally decreased to 16.9%AR. Similar profiles were observed for 4 but were less pronounced. Further oxidation of formyl groups in 4 and 6 led to the respective formation of 3 and 5, each finally amounting to 2.0 and 4.2%AR. None of the unknown degradates exceeded 7.0%AR throughout the study. The volatile 14C gradually increased with irradiation, amounting to 1.7–5.8%AR after 7–21 days, the majority of which was 14CO2.
After the water application of 1 to the water-sediment system, it was rapidly dissipated from water by translocation to sediment and degradation to 3 with a gradual mineralization. The favorable partition of 1 to sediment can be accounted for by its extremely high Koc values (>35700 mL/g o.c.), like other synthetic pyrethroids (>105 mL/g o.c.).1) Alkaline hydrolysis is most probable in the water-sediment system with an aqueous pH of 8.5–8.8, and 4 should be the main product via ester cleavage and the release of HCN from the corresponding cyanohydrin.19) However, the further oxidation product 3 was the main metabolite and no trace amount of 4 was detected. 3 was also the main degradate of many pyrethroids in sediment,2) and its formation was greatly reduced by sterilization for esfenvalerate applied to an alkaline natural water in darkness.3) Furthermore, a faster degradation of trans-1 than cis-1 was observed, as reported for other pyrethroids possessing the phenoxyphenyl ring.1,2) This is accounted for by the more favorable hydrolysis of the pyrethroid trans isomer by carboxylesterases than the cis one.4,23,24) Although the degradation profile of 1 under the sterile condition is not available, the formation of 3 from 1 is most likely via microbial processes, and 4, if formed, could be immediately oxidized to 3 by involving some bacteria.25) The gradual increase of 3 in the water phase can be explained by its favorable desorption from the alkaline sediment due to its ionization (pKa=3.95)26) and low Koc value estimated by EPI Suite™ even for the unionized form (236.8 mL/g o.c).27) The subsequent mineralization of the phenoxyphenyl ring may occur at the aerobic water-sediment interface, as reported for esfenvalerate.3,28)
As demonstrated in the water-sediment study, sediments could be an important sink for 1 in the aquatic environment. Even with exaggerated application to the sediment, faster degradation of the trans isomer was observed with almost half of the mineralization, as compared with the lower-rate application, indicating the involvement of sediment microbes in degrading 1. The chiral HPLC analysis of trans-1 showed more preferential degradation of 1S- and αS-isomers than 1R- and αR-ones (Table 5), which is consistent with previous observations in the soil and sediment metabolism of some pyrethroids.4,29,30) Meanwhile, the opposite tendency was observed for cis-1. Such discrepancies in enantioselective degradation may arise from the variety of microbial populations, which are dependent on sampling locations and experimental conditions such as the moisture content and aerobicity of sediments.31–33) Although no cis-trans isomerization has occurred at the cyclopropyl moiety, the epimerization at the α-cyanobenzyl carbon via abiotic base-catalyzed reaction probably proceeded along with degradation processes33) in the tested water-sediment system at pH >8. These factors, acting in a complicated manner, may cause the different enantioselectivity in the degradation of 1. However, the evaluated half-lives of 5.7–20.5 days clearly show the non-persistency of 1 and its stereoisomers in sediment.
In the aquatic photolysis study, some adsorption of trans-1 onto a glass surface was observed due to its high hydrophobicity (trans-1: log Kow=6.29).16) The aqueous solution was irradiated vertically from the top of each cylindrical vessel and photolysis on the glass surface would be of minor importance, as evidenced that the adsorbed 14C was mainly comprised of unchanged trans-1. Based on the proportion of light intensities of the artificial and typical natural sunlight, the converted photodegradation half-life of trans-1 was estimated to be 9.2–12.7 hr at 30–50°N latitude in summer.22) Therefore, trans-1 is clearly susceptible to photolysis and the photodegradation plays an important role in its dissipation process in surface water under natural sunlight.
Different from the dichlorinated analogue cypermethrin, trans-1 did not undergo cis-trans isomerization, although the UV absorption profiles are very similar between them.8) Since 4 and 6 were mainly formed instead, the oxidation and hydrolysis reactions were kinetically more favorable than the photo-induced homolytic cleavage of the C1-C3 bond in the cyclopropane ring. A similar photodegradation profile, no isomerization with oxidation and ester cleavage, was reported for aqueous photolysis of trans-resmethrin.34) The oxidative cleavage of the C=C bond at the 2-methylprop-l-enyl group predominantly proceeded to form the aldehyde derivative (6) subsequently oxidized to carboxylic acid (5). Since the ozonolysis of olefins forms the corresponding aldehydes,35) the atmospheric ozone is the most likely reactant for trans-1. It was also reported in the photolysis of metofluthrin that ozonolysis was greatly enhanced by light irradiation.36) The gradual increase of 4 even after the disappearance of trans-1 indicated that it was formed from either trans-1 or its derivatives, 5 and 6 (Table 6). A large number of minor degradates with volatiles showed further degradation of these photoproducts. 4 appeared to be the final photoproduct identified with partial oxidation to 3 in water. However, 4 could be mostly partitioned to sediment in an aquatic environment, judging from its Koc value of 418.1 mL/g o.c. estimated by EPI Suite™,27) and it is considered to be rapidly oxidized to 3 via microbial degradation. In addition, 3 released again to water could be photodegraded to smaller molecules such as 3-hydroxybenzoic acid and 2,5-dihydroxybenzoic acid via hydroxylation and ether cleavage, followed by the opening of the benzene ring and mineralization to CO2.26)
The photodegradation profile of cis-1, a minor component of 1, was not studied here but is considered very similar to that of trans-1. The UV absorption of a pyrethroid relevant to sunlight photodegradation at >290 nm is generally very low, and the effect of a geometrical isomerism in the cyclopropyl ring is not significant as reported in the photolysis of permethrin37) and cypermethrin.38) The photoproduct distribution is also similar between cis and trans isomers through the photolysis of permethrin37) and cypermethrin38) in water and that of resmethrin on a silica gel surface.34) Therefore, the experimental results for trans-1 can be reasonably translated to 1.
Based on our water-sediment and photolysis studies, the degradation pathways of 1 in an aquatic environment are proposed in Figure 1. 1 is considered to rapidly dissipate in an aquatic environment to form 3 and 4 directly from 1 and/or via the oxidized esters 5 and 6, and these degradates are also unlikely to persist by further degradation to form bound residues and CO2.
