Captan

Adsorption / desorption

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

adsorption / desorption: screening

Type of information:

experimental study

Adequacy of study:

key study

Study period:

22 March 2021 to

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 106 (Adsorption - Desorption Using a Batch Equilibrium Method)

Version / remarks:

January 21, 2000

Deviations:

no

GLP compliance:

yes (incl. QA statement)

Type of method:

batch equilibrium method

Media:

soil

Specific details on test material used for the study:

1) Unlabelled test item
Supplier of Test Item: Adama
Common Name: Captan
Chemical Name: N-(trichloromethylthio)cyclohex-4-ene-1,2-dicarboximide
Batch No.: 625-054-00
IES Code: 12188
Physical Form: solid
Purity: 98.8%
Expiration Date: May 16, 2022

2) 14C-labelled test item - [14C]Captan
Supplier of Test Item: Izotop
Product Name: Captan, [cyclohexene 1,2-14C]
Batch No.: XXVI/69/A/3-7
IES Code: 12491
Specific Radioactivity: 7.398 MBq/mg
Radiochemical Purity: 99.16%
Storage Conditions: Below -18°C
Expiration Date: Not needed since the purity was determined before use.

Radiolabelling:

yes

Test temperature:

All experiments included controls and blanks were performed at a constant temperature (20.4 °C ± 0.3 °C).

Analytical monitoring:

yes

Matrix no.:

#5

Matrix type:

loamy sand

% Clay:

4.1

% Silt:

9.3

% Sand:

86.6

% Org. carbon:

0.63

pH:

4.7

CEC:

3.7 meq/100 g soil d.w.

Matrix no.:

#4

Matrix type:

loamy sand

% Clay:

8

% Silt:

4

% Sand:

88

% Org. carbon:

0.8

pH:

4.1

CEC:

5 meq/100 g soil d.w.

Matrix no.:

#3

Matrix type:

clay

% Clay:

64

% Silt:

6

% Sand:

30

% Org. carbon:

2.1

pH:

6.5

CEC:

30.9 meq/100 g soil d.w.

Matrix no.:

#2

Matrix type:

sand

% Clay:

8

% Silt:

0

% Sand:

92

% Org. carbon:

2.8

pH:

4

CEC:

8.5 meq/100 g soil d.w.

Matrix no.:

#1

Matrix type:

loam

% Clay:

20.3

% Silt:

46.7

% Sand:

33

% Org. carbon:

1.74

pH:

4.8

CEC:

44.5 other: mmol/kg

Details on matrix:

Five acidic soils (Refesol 05-G (Germany; loam), Ingleby acid (UK; sand), Stenson (UK; clay), Warsop (UK, loamy sand) and Speyer 2.1 (Germany; loamy sand)) differing in particle size, organic matter content, cation exchange capacity and pH were used in the study.

Sample No.:

#5

Duration:

4 h

Initial conc. measured:

148 other: µg/mL

pH:

5

Temp.:

ca. 20.4 °C

Sample No.:

#5

Duration:

1

Initial conc. measured:

148 other: µg/mL

pH:

5

Temp.:

ca. 20.4 °C

Sample No.:

#5

Duration:

24 h

Initial conc. measured:

128.3 other: µg/mL

pH:

5

Temp.:

ca. 20.4 °C

Sample No.:

#5

Duration:

8 h

Initial conc. measured:

136 other: µg/mL

pH:

5

Temp.:

ca. 20.4 °C

Sample No.:

#5

Duration:

2 h

Initial conc. measured:

126.5 other: µg/mL

pH:

5

Temp.:

ca. 20.4 °C

Sample No.:

#5

Duration:

4 h

Initial conc. measured:

117.33 other: µg/mL

pH:

5

Temp.:

ca. 20.4 °C

Sample No.:

#5

Duration:

1 h

Initial conc. measured:

117.33 other: µg/mL

pH:

5

Temp.:

ca. 20.4 °C

Sample No.:

#4

Duration:

4 h

Initial conc. measured:

148 other: µg/mL

pH:

4.7

Temp.:

ca. 20.4 °C

Sample No.:

#4

Duration:

1

Initial conc. measured:

148 other: µg/mL

pH:

4.7

Temp.:

ca. 20.4 °C

Sample No.:

#4

Duration:

24 h

Initial conc. measured:

128.3 other: µg/mL

pH:

4.7

Temp.:

ca. 20.4 °C

Sample No.:

#4

Duration:

8 h

Initial conc. measured:

136 other: µg/mL

pH:

4.7

Temp.:

ca. 20.4 °C

Sample No.:

#4

Duration:

2 h

Initial conc. measured:

126.5 other: µg/mL

pH:

4.7

Temp.:

ca. 20.4 °C

Sample No.:

#4

Duration:

4

Initial conc. measured:

117.33 other: µg/mL

pH:

4.7

Temp.:

20.4 °C

Sample No.:

#4

Duration:

1 h

Initial conc. measured:

117.33 other: µg/mL

pH:

4.7

Temp.:

ca. 20.4 °C

Sample No.:

#3

Duration:

4 h

Initial conc. measured:

148 other: µg/mL

pH:

6.5

Temp.:

ca. 20.4 °C

Sample No.:

#3

Duration:

1 h

Initial conc. measured:

148 other: µg/mL

pH:

6.5

Temp.:

ca. 20.4 °C

Sample No.:

#3

Duration:

24 h

Initial conc. measured:

128.3 other: µg/mL

pH:

6.5

Temp.:

ca. 20.4 °C

Sample No.:

#3

Duration:

8 h

Initial conc. measured:

136 other: µg/mL

pH:

6.5

Temp.:

ca. 20.4 °C

Sample No.:

#3

Duration:

2

Initial conc. measured:

126.5 other: µg/mL

pH:

6.5

Temp.:

ca. 20.4 °C

Sample No.:

#3

Duration:

4

Initial conc. measured:

117.33 other: µg/mL

pH:

6.5

Temp.:

ca. 20.4 °C

Sample No.:

#3

Duration:

1 h

Initial conc. measured:

117.33 other: µg/mL

pH:

6.5

Temp.:

ca. 20.4 °C

Sample No.:

#2

Duration:

4 h

Initial conc. measured:

148 other: µg/mL

pH:

4.4

Temp.:

ca. 20.4 °C

Sample No.:

#2

Duration:

1 h

Initial conc. measured:

148 other: µg/mL

pH:

4.4

Temp.:

ca. 20.4 °C

Sample No.:

#2

Duration:

24 h

Initial conc. measured:

128.3 other: µg/mL

pH:

4.4

Temp.:

ca. 20.4 °C

Sample No.:

#2

Duration:

8 h

Initial conc. measured:

136 other: µg/mL

pH:

4.4

Temp.:

ca. 20.4 °C

Sample No.:

#2

Duration:

2

Initial conc. measured:

126.5 other: µg/mL

pH:

4.4

Temp.:

ca. 20.4 °C

Sample No.:

#2

Duration:

4 h

Initial conc. measured:

117.33

pH:

4.4

Temp.:

ca. 20.4 °C

Sample No.:

#2

Duration:

1 h

Initial conc. measured:

117.33 other: µg/mL

pH:

4.4

Temp.:

ca. 20.4 °C

Sample No.:

#1

Duration:

4 h

Initial conc. measured:

148 other: µg/mL

pH:

5.3

Temp.:

ca. 20.4 °C

Sample No.:

#1

Duration:

1 h

Initial conc. measured:

148 other: µg/mL

pH:

5.3

Temp.:

ca. 20.4 °C

Sample No.:

#1

Duration:

24 h

Initial conc. measured:

128.3 other: µg/mL

pH:

5.3

Temp.:

ca. 20.4 °C

Sample No.:

#1

Duration:

8 h

Initial conc. measured:

136 other: µg/mL

pH:

5.3

Temp.:

ca. 20.4

Sample No.:

#1

Duration:

2 h

Initial conc. measured:

126.5 other: µg/mL

pH:

5.3

Temp.:

ca. 20.4 °C

Remarks:

Screening Adsorption

Sample No.:

#1

Duration:

4 h

Initial conc. measured:

117.33 other: µg/mL

pH:

5.3

Temp.:

ca. 20.4 °C

Remarks:

Screening Adsorption

Sample No.:

#1

Duration:

1 h

Initial conc. measured:

117.33 other: µg/mL

pH:

5.3

Temp.:

ca. 20.4 °C

Remarks:

Screening Adsorption

Remarks:

Desorption of [14C]Captan was not investigated, since the test item was found to be too unstable

Computational methods:

Calculations were performed with a commercially available computer program (Excel in Microsoft Office Professional 2013). The results given in the tables were rounded to the last digit given. For further calculations, unrounded values were used. Thus, hand calculations may differ slightly from those presented.

Key result

Type:

Koc

Value:

155 other: mL/g

Temp.:

20.4

Sample No.:

#5

Type:

Koc

Value:

133

pH:

5

Temp.:

20.4 °C

Matrix:

loamy sand (Speyer 2.1)

% Org. carbon:

0.63

Sample No.:

#4

Type:

Koc

Value:

132 other: mL/g

pH:

4.4

Temp.:

204 °C

Matrix:

loamy sand (Warsop)

% Org. carbon:

0.8

Sample No.:

#3

Type:

Koc

Value:

83 other: ml/g

pH:

6.5

Temp.:

20.4 °C

Matrix:

Clay (Stenson)

% Org. carbon:

2.1

Sample No.:

#2

Type:

Koc

Value:

161 other: mL/g

pH:

4.4

Temp.:

20.4 °C

Matrix:

Sand (Ingleby acid)

% Org. carbon:

2.8

Sample No.:

#1

Type:

Koc

Value:

265 other: ml/g

pH:

5.3

Temp.:

20.4 °C

Matrix:

Loam (RefeSol 05-G)

% Org. carbon:

1.74

The first preliminary ratio test was performed using two soils at one test item concentration and four different soil-to-solution ratios (1:5, 1:20, 1:30 and 1:50). The aim was to determine the optimal soil-to-solution ratio after 4 hours of adsorption.

The amount of radioactivity adsorbed to the soil was calculated (using the indirect method) by subtracting the amount of radioactivity determined in the aqueous phase from the initial amount applied. The amount adsorbed was found to increase with an increasing soil-to-solution ratio, reaching maximum mean amounts of 47.5% and 15.1% AR in soils 3 and 5, respectively, at the highest soil-to-solution ratio of 1:5.

Since the results showed discrepancy between Soil Stenson and Speyer 2.1, a second ratio test was performed with all five soils at one test item concentration and two different soil-to-solution ratios (1:1, 1:2) to find the optimal soil-to-solution ratio for every soil.

Degradation of the test item was observed initially in the stock solutions and in the supernatant solutions, therefore stability tests were performed. The entire amount of radioactivity could be recovered in soilless control samples, with a lower parental mass balance than in soil supernatants. The soil-to-solution ratio of 1:1 (10 g soil/10 mL CaCl₂) for all soils was selected as the optimal ratio to obtain sufficient adsorption of radioactivity to soil.

The adsorption kinetics, first part of the screening test, was performed to investigate the time taken to reach equilibrium in all five soils at one test item concentration and a soil-to-solution ratio of 1:1. The adsorption was determined by measuring the radioactivity in the aqueous phase after 1, 2, 4, 8 and 24 hours. An equilibrium was reached after 2 hours for soils 1, 2, 4 and 5. For Soil Stenson, an equilibrium was reached after 1 hour. HPLC analyses of the supernatant solutions revealed degradation. The stability of Captan adsorbed to soil was further investigated in the mass balance part of the test. One interval for each soil (Soil Stenson: 1h, Soils RefSol 05-G, Ingleby acid, Warsop, Speyer 2.1: 2 h) was considered to be adequate for the subsequent tests.

 

For the mass balance part of the test, extraction methodology had to be developed. A complete mass balance of the radioactivity applied was obtained after the respective hours of adsorption resulting in total recoveries ranging from 92.3% to 94.7% of applied radioactivity (mean, Table 15) with non-extractables ranging from 1.9% to 17.0% AR. The chromatograms of the mass balance samples showed instability of the test item in the aqueous phase (HPLC) and soil extracts (TLC). For the TLC analysis, a suitable method had to be developed. Based on the radioactivity determined in supernatants and soil extracts, K_d(ads) and K_oc(ads) values were calculated (Table 16), using the direct method. The K_oc(ads) values amounted to 226, 168, 101, 154 and 138 mL/g for soils 1, 2, 3, 4 and 5, respectively. The “Kd*ratio” values for all soils were above 0.3, suggesting to use the indirect method for further tests, however since decomposition of the test item was observed, the advanced test was performed using the direct method.

 

The desorption kinetics were not performed due to instability of the test item.

The advanced test was performed using all five soils at a soil-to-solution ratio of 1:1 for all soils. Five test concentrations covering two orders of magnitude (2.381, 1.015, 0.259, 0.104 and 0.025 µg/mL) were used to investigate the adsorption isotherms of [¹⁴C]Captan after 2 hours of adsorption for soil RefeSol 05-G, Ingleby acid, Warsop and Speyer 2.1 and 1 hour of adsorption for Soil Stenson. A solubility test was performed to ensure viability of the chosen test concentrations. The direct method was employed, i.e. soil samples were extracted after the adsorption step and both phases (extract and aqueous phase) were analysed by HPLC (aqueous phase) or TLC (extracts). The results obtained were evaluated by applying the linear Freundlich equation and soil adsorption coefficients, including the Freundlich adsorption constants K_F and K_Foc and the regression constant 1/n, were determined. For Soil Stenson, evaluation was performed, excluding the outlier sample B of the highest test item concentration.

 

Table 14 Screening test: Mass balance of the radioactivity applied at different adsorption times and for soil-to-solution ratio of 1:1.

Values are given in percent of initial applied radioactivity (AR).

% of initial	Soil-to-solution ratio		Adsorption time (hours)
soil		Replicate	1	2	4	8	24
RefeSol 05-G (loam)	1:1	A	94.1	88.8	84.1	83.6	91.1
		B	80.5	89.3	89.5	87.1	89.7
		Mean	87.3	89.0	86.8	85.3	90.4
Ingleby acid (sand)	1:1	A	94.8	92.2	86.6	84.5	88.4
		B	97.0	91.2	89.9	89.0	84.8
		Mean	95.9	91.7	88.3	86.8	86.6
Stenson (clay)	1:1	A	77.1	77.3	77.5	74.4	77.1
		B	77.7	82.5	75.4	77.4	76.9
		Mean	77.4	79.9	76.4	75.9	77.0
Warsop (loamy sand)	1:1	A	91.2	87.8	100.4	86.2	86.0
		B	97.5	93.1	99.9	89.1	85.5
		Mean	94.3	90.4	100.2	87.7	85.7
Speyer 2.1 (loamy sand)	1:1	A	99.7	88.2	100.9	84.6	88.1
		B	102.3	92.8	102.4	85.1	91.5
		Mean	101.0	90.5	101.6	84.9	89.8

 

Table 15 Screening test: Mass balance of the radioactivity applied after 1 (soil Stenson) or 2 (soils RefeSol 05-G, Ingleby acid, Warsop and Speyer 2.1) hours of adsorption at a soil-to-solution ratio of 1:1.

Values are given in percent of initial applied radioactivity (AR)

Test concentration: 0.12 to 0.15 µg/mL in 0.01 M CaCl₂

Sample	mass applied	(ads)aq	(ads)extract	(ads)res	recovery
	[µg]	[%]	[%]	[%]	[%]
Refesol 05-G (loam)
Sample A	0.63	22.1	66.6	4.2	93.0
Sample B	0.63	23.2	66.2	4.2	93.5
Mean	0.63	22.7	66.4	4.2	93.2
Ingleby acid (sand)
Sample A	0.63	19.5	72.7	2.9	95.1
Sample B	0.63	18.5	72.7	3.1	94.3
Mean	0.63	19.0	72.7	3.0	94.7
Stenson (clay)
Sample A	0.74	31.9	45.3	18.9	96.0
Sample B	0.74	32.2	45.5	15.2	92.8
Mean	0.74	32.0	45.4	17.0	94.4
Warsop (loamy sand)
Sample A	0.63	43.5	44.3	2.5	90.3
Sample B	0.63	47.1	45.9	2.5	95.5
Mean	0.63	45.3	45.1	2.5	92.9
Speyer 2.1 (loamy sand)
Sample A	0.63	52.4	35.8	1.8	90.0
Sample B	0.63	56.7	36.1	1.9	94.7
Mean	0.63	54.5	35.9	1.9	92.3

(ads)aq          =     % of the amount applied remaining in the aqueous phase

(ads)extract    =     % of the amount applied extracted from soil using acetone/0.05 M ammonium carbonate (v/v, 9:1) three times

(ads)res         =     % of the amount applied irreversibly bound to the soil matrix, determined by combustion

 

Table 29      Advanced test: Alternative Freundlich isotherm parameters after 1 (Soil  Stenson) or 2 hours (Soils 1, 2, 4 and 5) of adsorption.

Outlier sample B of the highest test concentration of soil Stenson was not used for evaluation:

 

Parameter	Refesol 05-G (loam)	Ingleby acid (sand)	Stenson   (clay)	Warsop (loamy sand)	Speyer 2.1 (loamy sand)	Mean values	Standard deviation	Geomean
log(KF)	0.665	0.653	0.242	0.023	-0.077
KF	4.619	4.498	1.747	1.056	0.837
KFoc	265	161	83	132	133	155	68	144
KFom	154	93	48	77	77	90	39	84
1/n	1.05	0.99	0.89	1.00	1.08	1.00	0.07	1.00
r^2	0.9914	0.9982	0.9835	0.9950	0.9951

Mean values: arithmetic mean

_F           = Freundlich adsorption coefficient

K_Foc       = Freundlich adsorption coefficient related to organic carbon content of soil

K_Fom      = Freundlich adsorption coefficient related to organic matter content of soil

1/n        = Regression constant

r²                    = Regression coefficient

 

The obtained results, using the direct method, were evaluated by applying the linear Freundlich equation.

The resulting constants K_F(ads), K_Foc(ads)  and 1/n are presented in the following table:

Adsorption parameters:

Parameter		Soil
		RefeSol 05-G	Ingleby acid	Stenson	Warsop	Speyer 2.1	Mean ±SD	Geo- mean
Soil type (USDA)		Loam	Sand	Clay	Loamy sand	Loamy sand
KF(ads)	[mL/g]	4.619	4.498	1.747	1.056	0.837
KFoc(ads)	[mL/g]	265	161	83	132	133	155±68	144
KFom(ads)	[mL/g]	154	93	49	77	77	90±39	84
1/n	-	1.05	0.99	0.89	1.00	1.08	1.00±0.07	1.00
r^2	-	0.9914	0.9982	0.9835	0.9950	0.9951

Mean: arithemetic mean, SD standard deviation

EFSA quality criteria according to evaluator’s checklist

 

Soil	Units	RefeSol 05-G	Ingleby acid	Stenson	Warsop	Speyer 2.1
Adsorption method	-	direct	direct	direct	direct	direct
Soil:solution ratio	(g dw/mL)	5:5	5:5	5:5	5:5	5:5
Mass balance of ^14C	%	91.5-100.6	91.5-101.4	71.6- 82.3	93.9-106.2	93.4-106.8
Adsorbed percentage	%	50.5-68.2	60.7-74.3	29.3-36.5	36.1-48.5	28.8-36.9
Kd * (soil:Vres)		6.22- 9.23	9.11-10.15	3.16- 6.31	2.65- 3.27	1.88- 2.67
^adsKF (95% confidence interval)	L/kg dw	3.483/ 6.123	3.973/ 5.092	1.152/ 2.649	0.900/ 1.238	0.712/ 0.984
^ads”1/n” (95% confidence interval)		0.9699/ 1.1298	0.9552/ 1.0236	0.7908/ 0.9977	0.9379/ 1.0533	1.0181/ 1.1412
^adsR^2		0.9914	0.9982	0.9835	0.9950	0.9951
^adsKF,OC		265	161	83	132	133
KfE / Kf		1.04	1.00	1.32	1.00	1.06

 

The coefficients of determination (r²) for the linear regression of the Freundlich Adsorption Isotherms were equal to or greater than 0.98 across all soils.

The visual fit of both, the standard regression and the residual plots were therefore acceptable.

Furthermore, the p values (= Kd * (soil:Vres)) were calculated by multiplying the K_D(ads) value with the soil (dry weight) and its residual water content after phase separation (V_res after adsorption). All these values were far above 0.3 and therefore fulfilled the criteria set according to Boesten [4], [6] and together with the good linear regressions, reliable determinations of the K_F value were obtained.

 

Validity criteria fulfilled:

yes

Conclusions:

The adsorption characteristics of the test item [14C]Captan was investigated in five different acidic European soil types, covering a range of soil properties: Soil RefeSol 05-G (loam), Soil Ingleby acid (sand), Soil Stenson (clay), Soil Warsop (loamy sand) and Soil Speyer 2.1 (loamy sand). The test item was not stable during the adsorption process to soil therefore the direct method was used for the advanced test.

In the advanced test, five test concentrations were used, covering two orders of magnitude. The adsorption parameters resulting from the Freundlich isotherms are summarised in the following tables. For Soil Stenson, evaluation was performed, excluding the outlier sample B of the highest test item concentration.
The arithmetic mean Freundlich isotherm coefficient for adsorption KFoc(ads) was 155 and the corresponding geometric mean 144. The regression constants (1/n) for adsorption ranged between 0.89 and 1.08 with an arithmetic mean of 1.00, indicating a slight dependence of adsorption on the test item concentration for individual soils.

According to quality check parameters from EFSA, 2017, OECD 106 evaluators checklist, systematic errors could be excluded and the obtained adsorption parameters are considered fully significant, reliable and valid.

Executive summary:

Captan hydrolyzes rapidly in the five soil solution mixtures at pH 4.4 to pH 6.5.

The arithmetic mean Freundlich isotherm coefficient for adsorption K_Foc(ads) was 155 and the corresponding geometric mean 144. The regression constants (1/n) for adsorption ranged between 0.89 and 1.08 with an arithmetic mean of 1.00, indicating a slight dependence of adsorption on the test item concentration for individual soils.

Desorption kinetics could not be measured due to the instability of the test item.

Reference 2

Endpoint:

adsorption / desorption: screening

Remarks:

other: stability in soil/water

Type of information:

experimental study

Adequacy of study:

supporting study

Study period:

1988

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

test procedure in accordance with national standard methods with acceptable restrictions

Qualifier:

according to guideline

Guideline:

other: OPPTS 835.1230 Adsorption/desorption (batch equilibrium): EPA 163-1

GLP compliance:

no

Remarks:

no protocol prepared and study was not audited

Type of method:

batch equilibrium method

Media:

soil

Specific details on test material used for the study:

RADIOLABELLED TEST SUBSTANCE
- Position of radiolabel: Radiolabelled position: in the phenyl system
- connecting atoms beween both ring systems
- Specific activity: 20.8 mCi/mmole, 126 mCi/g
- Source: Chevron Chemical Company, Richmond, CA
- Radiochemical purity: 95%

NON RADIOLABELLED TEST SUBSTANCE
Captan:
- Batch number: M1053C
- Purity: 99%
Tetrahydrophthalimide THPI
- Batch number: 792-C
- Purity: 98%

Radiolabelling:

yes

Test temperature:

21°C

Analytical monitoring:

yes

Details on sampling:

Captan was shaken with Visalia sandy loam and samples were taken 1, 2, 3, and 4 hours at pH 5 and 7

Matrix no.:

#1

Matrix type:

sandy loam

% Clay:

7.9

% Silt:

32.4

% Sand:

60.1

% Org. carbon:

0.7

pH:

7.6

CEC:

8.3 meq/100 g soil d.w.

Bulk density (g/cm³):

1.02

Key result

Sample No.:

#1

Type:

Koc

Remarks on result:

not determinable

Remarks:

KOC was not determined as equilibrium state was not reached to determine Freundlich isotherme. The results suggest that it is not possible to determine the Freundlich adsorption constants of captan at equilibrium In soil/water mixtures because of the decomposition which occurs at pH 5 and 7.

Key result

Sample No.:

#1

Phase system:

solids-water in soil

Type:

Kp

Remarks on result:

not determinable

Remarks:

Kp was not determined as equilibrium state was not reached to determine Freundlich isotherme.The results suggest that it is not possible to determine the Freundlich adsorption constants of captan at equilibrium In soil/water mixtures because of the decomposition which occurs at pH 5 and 7.

Prior work demonstrates that the rate of captan hydrolysis is pH dependent (Reference: Wolfe, N.L., Zepp, R.G., Porter, J.C., and Hollis, R.C. (1970) Captan Hydrolysis, J. Ag. Food Chem., 24, 1041), such that slower hydrolysis occurs at lower pH. Therefore, the stability of captan in soil/solution mixtures was determined at the soil/solution ambient pH of 7 and at pH adjusted to 5. The purified water used in these experiments yielded a 0.01 M CaCl₂ solution of pH 6. Prior measurement indicated that 0.01 M CaCl₂ pH 3 combined with 2.5 g Visalia sandy loam resulted in soil/solution mixtures at pH 5. Therefore, solutions with and without soil containing approximately 2 ppm [¹⁴C]Captan were prepared at pH 5, 6 or 7. The experiments were done at 21°C, the lower end of the acceptable 18-30°C range, to also improve the stability of captan.

Results in table 5.4.1-2 show the percentage of radioactivity recovered in each of the fractions analysed from every test mixture. Overall accountability of radioactivity was good, averaging 98 ± 6% (n = 18) for all test mixtures.

In the time-0 solutions essentially complete extraction (98%) of the radioactivity was achieved with toluene. This was determined to consist of 90% and 94% captan in the pH 7 and 5 solutions, respectively. Thus, parent captan is completely extracted from aqueous solutions with toluene. Any radioactivity not extracted from aqueous solution with toluene must, therefore, represent break down products of captan. In solutions where the least degradation was expected (time-0 and pH 5 solutions) the extractability of radioactivity with toluene was greatest. Generally, extractability of radioactivity with toluene decreased as time from preparation increased, corresponding to the hydrolysis of captan In the solutions. Chromatography analyses showed that in the aqueous solutions all of the captan along with some THPI was partitioned into the toluene extract.

Only traces of captan occurred in some of the methylene chloride extracts and no captan occurred in the extracted aqueous fractions. THPI was the only degradate observed.

 

Adsorption of radioactivity by the Visalia soil at pH 7 was 3-6% (C labelled tubes), and at pH 5 (D labeled tubes) adsorption was 5-10%. Thus, greater adsorption occurred at the lower pH. Nearly complete extraction of radioactivity from the soils was achieved with the combination of toluene and acidic methanol solvents. Toluene extracts removed most of the captan (Identified by HPLC) from the soil, although small amounts of captan did occur in the methanol extracts. The proportion of captan in the toluene extracts from soils at pH 5 was greater than that from soils at pH 7.

The percent of applied captan remaining in the soils and solution is shown in

table 5.4.1-2. Degradation is pH dependent and is most rapid at pH 7 and least rapid

at pH 5- Captan apparently also degrades rapidly in the soil. Furthermore, the ratio of captan in soils and solution does not remain constant making reliable estimation of a valid Freundlich constant impossible.

Table 5.4.1-2 Captan (% distribution) remaining in solution and soil samples in soil/water stability study

Time (Hours)	Captan solution only (pH 6)			Captan solution only (pH 5)			Captan soil/solution (pH 7)			Captan soil/solution (pH 5)
	Soiln.	Soil	Total	Soiln.	Soil	Total	Soiln.	Soil	Total	Soiln.	Soil	Total
0	98	-	98	105		105	98		98	105		105
1	75	-	75	95		95	45		49	77		83
2	70	-	70	74		74	26		29	66		73
3	67	-	67	80		80	14		15	64		68
4	44	-	44	73		73	12		13	59		62

¹Solutions only at pH 7 and 5 were used for t = 0 (15 minutes).

Conclusions:

Captan hydrolyses rapidly in soil solutions at both pH 5 (t 1/2 = 5 hours) and pH 7 (t 1/2 = 1 hour), although hydrolysis is slower at pH 5. Because of the steady decomposition of captan, a true equilibrium of captan is never reached, and therefore a reliable Freundlich adsorption constant cannot be determined. Hence batch equilibrium studies cannot be used to investigate the adsorption/desorption characteristics of the captan parent molecule.

Executive summary:

The feasibility of conducting batch equilibrium studies on captan was evaluated by shaking solutions of 2 ppm captan in 0.01 MCa Cl₂ with Visalia sandy loam at pH 7 and pH 5. The decomposition of captan in the solution and soil was determined after 1, 2, 3 and 4 hour of shaking.

Captan shaken with Visalla sandy loam showed rapid decomposition after 1, 2, 3 and 4 hour respectively at pH 7 (t 1/2 = 1 hour) and significant decomposition after the same intervals at pH 5 (t ½ = 5 hours). The only degradate was tetrahydrophthalimide.

These results suggest that it is not possible to determine the Freundlich adsorption constants of captan at equilibrium In soil/water mixtures because of the decomposition which occurs at pH 5 and 7.

The Freundlich adsorption constant is typically used as a measure of the ability of a chemical to adsorb to soil.

It is implicitedly assumed by guidelines that the compound being studied is stable over a period of time necessary to obtain equilibrium between the soil and solution phases. If a true equilibrium state cannot be reached, a valid Freundlich adsorption constant cannot be measured.

 

Captan is well known (Lee, K.S. 1989a) to undergo pH dependent hydrolysis in water with half-lifes of 18.8 hours to 8 minutes in the pH range 5 - 9, respectively.

Description of key information

In a new study (Völkel, 2022) on different soil types an adsorption coefficient of Koc(ads) 155 was determined. Desorption kinetics were not measured due to the instability of the test item.

In earlier studies ist was not possible to determine a reliable Freundlich absorption constant due to fast hydrolysis of Captan.

Key value for chemical safety assessment

Koc at 20 °C:

155