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EC number: 205-087-0
CAS number: 133-06-2
The objective of the study was to determine the time course of biodegradation of Captan in natural aerobic surface water and to evaluate the major pathway of its degradation. The study was performed with [cyclohexene-14C]Captan under aerobic conditions using natural aerobic surface water taken from a large water body.
Five major metabolites were detected. In test flasks with the low test concentration THPI and THPAM were found at maximal mean amounts of 69.2% AR (at 1 hour) and 77.9% AR (at 14 days), respectively.
Furthermore, the major metabolites THCY (M3) and THPAI (M7) were observed at maximal mean amounts of 88.9% AR (at 1 day) and 35.8% AR (at 60 days), respectively.
Mineralisation and formation of organic volatiles were negligible for both test concentrations. 14CO2 was detected at maximal mean amounts of 1.4% AR at the end of the incubation. The mean amount of organic volatiles formed at the end of the incubation was < 0.1% AR.
Due to the rapid degradation of the test item the performance of kinetic calculations was not possible. Based on the results the DT50-value for [cyclohexene-14C]Captan can be assumed to be < 1 h.
The mean recovery of radioactivity in the sterile samples was 105% AR after two days of incubation. Two major metabolites were detected; THPAM occurred at mean amounts of 28.7% AR and THCY (M3) at 65.9% AR. The same metabolic pattern was observed as in the non-sterile samples showing that the degradation of the parent and its metabolites to the final metabolite THPAI is due to hydrolysis (abiotic as well as microbial).
Resulting from this study a preliminary pathway for aerobic mineralisation of Captan is proposed.
The distribution of radioactivity in the two sediment/water systems is shown in Table 5.2.2-3
The average total recovery for all the systems was 95.1% AR. Recoveries were generally greater than 92%, although day 90 recoveries for both non-sterile systems and day 59 for the Virginia Water non sterile systems were lower at 74.7 - 86.5% AR.
The levels of evolved radiocarbon, as14CO2were seen to increase throughout the 90-day incubation period. The rate of 14CO2evolution was similar for both systems, with approximately 50 % AR mineralised by day 90 of the incubation period. In contrast, < 0.01% AR was mineralised to14CO2 in the same 90 day period in the sterile systems.
Over the first seven days of incubation, the surface waters in the ‘Old Basing’ systems contained approximately 55 % AR, whereas the ‘Virginia Water’ systems contained approximately 80 % AR. The amount of radioactivity in the surface water fell in both systems after day 7 to < 1.2% AR by day 59 in both non-sterile systems. In the sterile systems, the level of radioactivity in the surface water remained high (> 48 % AR) throughout the incubation period.
At day 0, the majority of the radioactivity in the surface water partitioned into ethyl acetate for both sediment phase types. With incubation of the ‘Old Basing’ system, levels of organosoluble material decreased from 14:1 (organic:aqueous phases) at day 0 to 1:1.5 by day 30. With the ‘Virginia Water’ system, the majority of the radiolabel in the surface water was organosoluble throughout the incubation, although the proportion in the ethyl acetate phase decreased to 2.3:1 on day 1, and remained about this level on day 30. By day 59, there were only negligible amounts of radioactivity in the water phases.
In the sterile systems, the majority of the radioactivity in the surface water remained organosoluble throughout the incubation, although by day 90, the ratio in the organic and aqueous phases had fallen to 1.4:1 in the sterile ‘Old Basing’ system and 7.9:1 in the sterile ‘Virginia Water’ systems.
The amount of radioactivity extracted from the sediments fell throughout the incubation period, from approximately 40 % AR at day 0, to 3 % AR at day 90, these values being the sum of the first and second sediment extracts. This trend was accompanied by an increase in the unextractable radiocarbon from the sediment from approximately 2 % AR at day 0 to a maximum of 50 % AR at day 59. In the ‘Virginia Water’ system, the amount of radioactivity extracted from the sediment increased from 9 % AR at day 0 to 16 % AR at day 1, and remained at 16 - 18 % AR until day 30. By day 59 the amount of radioactivity extracted from the sediment had fallen to 9.5 % AR and was similar (10.6 % AR) at day 90. The unextractable radiocarbon in the ‘Virginia Water’ sediments remained low (less than or equal to 9 % AR). Until day 59 when levels rose to a maximum of about 30 % AR.
The amount of radiolabel extracted from the sterile sediments tended to be higher than the corresponding non-sterile sediments. In the sterile ‘Old Basing’ sediments, 53 % AR was extracted at day 0, and declined to 22 % AR at day 90. Correspondingly, levels of unextracted radioactivity in the sterile ‘Old Basing’ sediment increased to approximately 15 % AR over the 90 day incubation.
In the sterile ‘Virginia Water’ sediments, 24 % AR was extracted at day 0, and levels declined over 29 days to 15 % AR, remaining at this level for 90 days. Levels of untextracted radioactivity in the sterile ‘Virginia Water’ sediment remained below 2 % AR.
Even at day 0, directly after application, only 5.6 % AR was found to be captan in the ‘Old Basing’ water sediment extracts. Levels of captan extracted on day 0 from the sterile ‘Old Basing’ systems were also low with only 11 % AR in the extracts. In the ‘Virginia Water’ system, captan comprised a greater proportion of the extracted radioactivity in both sterile and non-sterile systems (61 and 54 % AR, respectively). By day 1, captan was not detected in any of the sample extracts, non-sterile or sterile, with either of the sediment types. Captan has been found to degrade rapidly in soil-water mixtures and the degradation rate is pH dependent.
The major initial metabolite in both sediment/water systems was THPI. In the ‘Old Basing’ system, the decline of THPI followed first-order kinetics over the first 14 days of incubation, with a half-life of about 5 days. Levels of THPI in the ‘Virginia Water’ systems fluctuated over the first 30 days. However, THPI was reduced to undetectable levels (< 0.1 % AR) in both systems by day 60. In the ‘Old Basing’ systems, THPI was distributed approximately 50:50 between the surface water and sediment phases. In the ‘Virginia Water’ systems, THPI was largely associated with the water phase.
THPI was also found in the sterile systems. In the ‘Old Basing’ system, the total amount of extractable THPI was 77 % AR at day 0, which declined to 36 % AR by day 90. In the ‘Virginia Water’ system, the total level of THPI reached a maximum of 80 % AR at day 60, and declined to 64 % AR by day 90.
The second highest level of metabolite was that of THPAM, which was found virtually exclusively in the water extracts for all systems. In the ‘Old Basing’ systems, THPAM was found at maximum levels in the water/sediment extracts on day 7 and 14 (approximately 26 % AR) and declined to undetectable levels (< 0.1 % AR) by day 60. In the ‘Virginia Water’ system, levels of THPAM in the water extracts reached a maximum of 25.5 % AR at day 1, and declined to undetectable levels (< 0.1 % AR) by day 60.
In the sterile ‘Old Basing’ system, THPAM was detected at a maximum level of 25 % AR after 90 days, whilst in the sterile ‘Virginia Water’ systems, THPAM reached only 6.5 % AR at day 60, and declined to < 1 % AR by day 90.
It was shown that under the reflux conditions used for the second sediment extraction, THPAM was converted to THPAI. When in solution, conversion of THPAM to THPAI under reflux conditions was completed in less than 2 hours. It is expected, therefore that levels of THPAM in the second extraction would be reduced as a result of the reflux extraction, and levels of THPAI would be elevated.
THPAI reached a maximum level of 11 % AR on day 14 in the ‘Old Basing’ system, and 7.5 % AR on day 30 in the ‘Virginia Water’ system. THPAI was also detected in sterile systems but at very low levels ( < 2.5 % AR). The majority of THPAI was detected in the second sediment extract for all systems, but as previously discussed some of the THPAI in the fraction may have been produced from THPAM during the reflux extraction.
Low levels of a fourth metabolite, THPI epoxide, were detected in both non-sterile and sterile
systems. THPI epoxide reached a maximum level of 5 % AR in the Old Basing’ non-sterile system on day 1 and declined to undetectable levels (< 0.1 % AR) by day 60. In the ‘Virginia Water’ system, THPI epoxide also reached its maximum level of 10% AR on day 1. In the sterile systems, the highest levels of THPI epoxide were observed later in the incubation period, at day 90 (10 % AR) in the ‘Old Basing’ system and at day 60 (10 % AR) in the ‘Virginia Water’ system.
No unknown metabolites amounting to > 5 % AR were found in any of the sediment/water systems.
The metabolism of 14C-rlng labelled captan (ICIA2845) was studied in natural water-sediment systems under laboratory conditions, at 20°C. The 14C-ring labelled captan was applied to two water-sediment "systems.
One system contained sediment with a high organic matter content and the other sediment with a low organic matter content. Sterile systems were also set up in order to assess the effect of microbial activity on captan degradation.
In this study captan hydrolysed very rapidly to "THPI" (1,2,3,6-tetrahydrophthaIimide) in both the sterile and non-sterile water-sediment systems. After 24 hours incubation, captan was not detected in any of the systems.
THPI was degraded in all non-sterile water-sediment systems to undetectable levels after 59 days incubation. After 24 hours incubation, three other metabolites were identified in all the water-sediment systems. These metabolites were "THPAM" (6-Carbamoyl-3-CycIohexene-1-carboxylic acid) "THPAL" (3-Cyclohexene-6-Dicarboxylic acid) and "THPI" epoxide (7-oxabicyclo(2,2,1]heptane-2,3-dicarboidmide).
Levels of these metabolites were similar in both non-sterile sediment systems. THPAM reached a maximum level of approximately 25% of applied radioactivity while THPAL and THPl epoxide reached maximum levels of 5 to 11% of applied radioactivity. Maximum levels of these metabolites had been reached by day 14 and amounts then declined to undetectable levels by day 59 of the incubation.
After 90 days incubation, approximately 50% of the Initial 14C-captan had been mineralised to 14CO2 in all non-sterile water-sediment systems. The majority of the remaining radioactivity (approximately 26% of applied radioactivity) was very tightly bound and not extractable from the sediment residue.
In the sterile systems, negligible amounts of 14CO2 were evolved throughout the incubation.
After 90 days incubation in the sterile water system, most of-the applied radioactivity was identified as THPl (63.8% of applied radioactivity). After 90 days incubation in the sterile system, THPl levels had fallen to 35.8% of applied radioactivity, THPAM increased to 24.6% of applied radioactivity and levels of the THPl epoxide had reached 9.6% of applied radioactivity.
Results were comparable to mineralisation in aerobic water system (Irmer, 2017).
1. Degradation in sedimentwater systems: Captan hydrolysed rapidly to THPI in both the non-sterile and sterile water-sediment systems. Amounts of THPI decreased in the nonsterile system to undetectable levels after 59 days incubation. Three other metabolites were produced by the degradation of THPI in these water-sediment systems. These metabolites were THPAM (6-carbamoyl-3-cyclohexene-1-carboxylic acid), THPAL (3-cyclohexene-1,6-carboxylic acid) and THPI epoxide (7-oxabicyclo(2,2,1) heptane-2,3-dicarboximide). Approximately 50% of the radiolabelled captan had been mineralised to 14CO2 after 90 days incubation in both the non-sterile water-sediment system.
2. Aerobic Mineralisation in Surface Water – Simulation Biodegradation Test: The objective of the study was to determine the time course of biodegradation of Captan in natural aerobic surface water and to evaluate the major pathway of its degradation. Captan hydrolysed rapidly to THPI in both the non-sterile and sterile water-systems.Mineralisation and formation of organic volatiles were negligible for both test concentrations. 14CO2 was detected at maximal mean amounts of 1.4% AR at the end of the incubation. The mean amount of organic volatiles formed at the end of the incubation was < 0.1% AR.Due to the rapid degradation of the test item the performance of kinetic calculations was not possible. Based on the results the DT50-value for [cyclohexene-14C]Captan can be assumed to be < 1 h at 20°C.
The mean recovery of the applied radioactivity (AR) for the whole system and both concentrations was in the range of 97.1% to 105.2% within 60 days of incubation. Five major metabolites were observed for both test concentrations. They were identified as THPI, THPAM, THCY (M3) and THPAI (M7) with maximum amounts of 69.2% AR, 77.9% AR, 88.9% AR and 35.8% AR, respectively.In sterile samples, after 2 days of incubation, [cyclohexene-14C]Captan was degraded to two major metabolites: THPAM (28.7% AR) and THCY (M3) (65.9% AR). The degradation pattern shows an indication that the parent substance was (partially) degraded by hydrolysis.
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