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Environmental fate & pathways

Biodegradation in soil

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Reference
Endpoint:
biodegradation in soil
Type of information:
experimental study
Adequacy of study:
key study
Study period:
9 October 1989 to 1 November 1990
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to guideline
Guideline:
other: USEPA 162-2
Deviations:
no
GLP compliance:
yes
Test type:
laboratory
Radiolabelling:
yes
Oxygen conditions:
anaerobic
Soil classification:
USDA (US Department of Agriculture)
Soil no.:
#1
Soil type:
sandy loam
% Clay:
11.8
% Silt:
33.4
% Sand:
54.8
% Org. C:
0.41
pH:
7.7
CEC:
9.1
Soil No.:
#1
Duration:
90 d
Soil No.:
#1
Initial conc.:
8.76 mg/kg soil d.w.
Based on:
act. ingr.
Parameter followed for biodegradation estimation:
CO2 evolution
radiochem. meas.
Soil No.:
#1
Temp.:
25 ± 2°C
Humidity:
moisture of the soil was adjusted to 75% of field capacity (11-25% by weight).
Soil No.:
#1
Remarks on result:
not measured/tested
Key result
Soil No.:
#1
% Degr.:
ca. 97.46
Parameter:
CO2 evolution
Sampling time:
3 d
Remarks on result:
other: Standard deviation not measured
Key result
Soil No.:
#1
DT50:
0.91 d
Type:
(pseudo-)first order (= half-life)
Temp.:
> 25 - < 27 °C
Remarks on result:
other: Standard deviation
Remarks:
Standard deviation was not measured in the study
Transformation products:
yes
Remarks:
The other major metabolite, thiocarbonic acid was also formed, but not exact identified in the study (Cas no: 26555-35-1 or Cas no: 26555-40-8) was not measured
No.:
#1
Evaporation of parent compound:
no
Volatile metabolites:
no
Residues:
no
Conclusions:
Captan was rapidly degraded in soil under aerobic conditions: 99% degradation being observed after 7 days. Carbon dioxide was the major degradate, and mineralisation increased during the study: 82% mineralisation occurred within 37 days. Other intermediate metabolites were observed produced by hydrolysis reactions, notably tetrahydrophthalimide (THPI) and tetrahydrophthalamic acid (THPAM), but levels of these compounds decreased towards the end of the study as mineralisation increased.
Executive summary:

Captan was used in a soil metabolism study under anaerobic conditions. Sampling times were at 0, 6 and 24 hours and 3, 7, 14, 30 and 90 days after initiation of anaerobic conditions.

Captan was readily degraded into various metabolites under anaerobic conditions. One of the major degradates was14CO2 (35% after 3 days; all of the radioactivity after 14 days). The other major metabolite, thiocarbonic acid was also formed rapidly and was aqueous extractable. The highest amount of thiocarbonic acid was 6.32% under anaerobic conditions.

The14C bound to soil averaged 21.4% in the study.

Captan was rapidly degraded in the soil under anaerobic conditions. Its experimentally observed half-life was approximately 0.91 d.

Description of key information

Anaerobic metabolism of [trichloromethyl - 14C] captan in soil, 25°C ± 2°C, sandy loam in the dark, incubation 90 days: DT50 = 0.91 days, USEPA 162-2.

Key value for chemical safety assessment

Half-life in soil:
0.91 d
at the temperature of:
25 °C

Additional information

Soil degradation of captan is very rapid under both aerobic and anaerobic conditions, with a DT50 of less than 1 day. Both the tetrahydrophthalimid ring and the trichloromethyl sidechain moieties of captan are mineralised to carbon dioxide. The principal initial degradate is THPI which undergoes cleavage of the imide ring to give THPAM. Other metabolites are generally at levels below 5% of the applied dose, apart from THCY (max. 20%) which is an important degradate under anaerobic conditions, although these conditions would not normally be encountered during the use of captan. THCY has been demonstrated to rapidly degrade to THPAM under aerobic conditions. THPAM is further degraded to CO2, which is incorporated into natural constituents of the soil.