Captan

Biodegradation in water and sediment: simulation tests

Link to relevant study record(s)

Reference

Endpoint:

biodegradation in water: sediment simulation testing

Type of information:

experimental study

Adequacy of study:

key study

Study period:

April 1992 to March 1993

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

test procedure in accordance with national standard methods with acceptable restrictions

Remarks:

Guideline not specified, but complies with the SETAC guideline. Deviations: None

Qualifier:

according to guideline

Guideline:

other: SETAC guideline

Deviations:

no

GLP compliance:

yes

Radiolabelling:

yes

Remarks:

14C ring-labelled captan

Oxygen conditions:

aerobic

Inoculum or test system:

natural water / sediment

Details on source and properties of surface water:

Approximately 6 litres of water and 10 litres of sediment were collected from each site and transported in polypropylene containers. The physical state of the sediment and water in situ were noted.

Details on source and properties of sediment:

The metabolism of 14C-rlng labelled- captan (ICIA2845) was studied in natural water-sediment systems under laboratory conditions, at 20°C in 2 water/sediment systems.
One system contained sediment with a high organic matter content ('Old Basing', 21.5% organic matter) and the other sediment with a low organic matter content ('Virginia Water', 5.4% organic matter).
Sterile systems were also set up in order to assess the effect of microbial activity on captan.degradation.. Each.water-sediment system contained 10% sediment dry matter in stream water.

Duration of test (contact time):

90 d

Initial conc.:

1.2 mg/L

Based on:

test mat.

Parameter followed for biodegradation estimation:

CO2 evolution

radiochem. meas.

Compartment:

natural water / sediment: freshwater

% Recovery:

95.1

Compartment:

sediment

DT50:

ca. 1 - < 1.5 h

Type:

(pseudo-)first order (= half-life)

Temp.:

20 °C

Remarks on result:

other: After 24 hours, captan was not detected in any of the systems, and a DT50 could not be determined. worst-case half-life for captan may be estimated by making the worst-case calculation using zero order hydrolysis kinetics between time 0 and 24 hours.

Transformation products:

yes

No.:

#1

No.:

#2

No.:

#3

No.:

#4

No.:

#5

Evaporation of parent compound:

no

Volatile metabolites:

yes

Remarks:

carbon dioxide

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 of¹⁴CO₂evolution 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.

Conclusions:

Captan was hydrolysed very rapidly to THPI in both sterile and non-sterile water/sediment systems. After 24 hours incubation, captan was not detected in any of the systems.In the non-sterile systems, the radioactivity in surface water decreased during the incubation period to levels lower than 2.5 % of the applied radioactivity, although the radioactivity increased in the sediment as bound residues, reaching approximately 25% AR. In the sterile systems, radioactivity remained in the surface water at levels of 63-76% AR after 90 days incubation; in the sediment bound residues were lower than 15%. THPI was observed as main metabolite in all the systems from day 1. It exhibited a DT50 of approximately 5 days in a non-sterile system containing a high organic matter content, high pH sediment, in which it was evenly distributed between sediment and water. In a non-sterile system containing a low organic matter, low pH sediment, THPI was predominantly in the water phase, and a DT50 could not accurately be determined. However, THPI had completely degraded to undetectable levels in both non-sterile systems in 59 days. In sterile systems, THPI degraded more slowly than in non-sterile systems, suggesting that microorganisms enhanced the degradation.Other metabolites detected were THPAM, THPAI and THPI epoxide. In nonsterile systems, maximum levels had been reached by day 14; THPAI and THPI epoxide did not exceed 11% AR, although THPAM reached 22-27% AR. Amounts then declined to undetectable levels after 59 days of incubation for these 3 metabolites.In the sterile systems, THPAM, THPAI and THPI epoxide were also observed after 30 days of incubation, suggesting that the degradation of THPI occurs in the absence of microbial activity, although more slowly. In the non-sterile systems, approximately 50% of the captan applied had been mineralised to 14CO2 after 90 days incubation, whereas in the sterile systems, negligible amounts of 14CO2were evolved throughout the incubation, implying that microbial activity is required for mineralisation.

Executive summary:

Captan was hydrolysed very rapidly to THPI in both sterile and non-sterile water/sediment systems. After 24 hours incubation, captan was not detected in any of the systems.

However, a worst-case half-life for captan may be estimated by making the worst-case calculation using zero order hydrolysis kinetics between time 0 and 24 hours; starting with the initial concentration of 1.2 mg/L degrading to below the limit of determination, at <0.1 mg/L. As such, the zero order half-life (or DT50) for captan under sterile or non-sterile conditions is <1.5 hours, under the conditions of the study. Clearly, the DT90 of captan is also less than 24 hours, in both sterile and non-sterile systems.

In the non-sterile systems, the radioactivity in surface water decreased during the incubation period to levels lower than 2.5 % of the applied radioactivity, although the radioactivity increased in the sediment as bound residues, reaching approximately 25% AR. In the sterile systems, radioactivity remained in the surface water at levels of 63-76% AR after 90 days incubation; in the sediment bound residues were lower than 15%. THPI was observed as main metabolite in all the systems from day 1. It exhibited a DT50 of approximately 5 days in a non-sterile system containing a high organic matter content, high pH sediment, in which it was evenly distributed between sediment and water. In a non-sterile system containing a low organic matter, low pH sediment, THPI was predominantly in the water phase, and a DT50 could not accurately be determined. However, THPI had completely degraded to undetectable levels in both non-sterile systems in 59 days. In sterile systems, THPI degraded more slowly than in non-sterile systems, suggesting that microorganisms enhanced the degradation.

Other metabolites detected were THPAM, THPAI and THPI epoxide. In nonsterile systems, maximum levels had been reached by day 14; THPAI and THPI epoxide did not exceed 11% AR, although THPAM reached 22-27% AR. Amounts then declined to undetectable levels after 59 days of incubation for these 3 metabolites.

In the sterile systems, THPAM, THPAI and THPI epoxide were also observed after 30 days of incubation, suggesting that the degradation of THPI occurs in the absence of microbial activity, although more slowly. In the non-sterile systems, approximately 50% of the captan applied had been mineralised to 14CO₂after 90 days incubation, whereas in the sterile systems, negligible amounts of 14CO₂were evolved throughout the incubation, implying that microbial activity is required for mineralisation.

Description of key information

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.