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EC number: 205-087-0
CAS number: 133-06-2
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, Kd(ads) and Koc(ads) values were calculated (Table 16), using the direct method. The Koc(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 KF and KFoc 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
Adsorption time (hours)
Speyer 2.1(loamy sand)
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
Refesol 05-G (loam)
Ingleby acid (sand)
(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:
Mean values: arithmetic mean
F = Freundlich adsorption coefficient
KFoc = Freundlich adsorption coefficient related to organic carbon content of soil
KFom = 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 KF(ads), KFoc(ads) and 1/n are presented in the following table:
Soil type (USDA)
Mean: arithemetic mean, SD standard deviation
EFSA quality criteria according to evaluator’s checklist
Mass balance of 14C
Kd * (soil:Vres)
adsKF (95% confidence interval)
ads”1/n” (95% confidence interval)
KfE / Kf
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 KD(ads) value with the soil (dry weight) and its residual water content after phase separation (Vres after adsorption). All these values were far above 0.3 and therefore fulfilled the criteria set according to Boesten ,  and together with the good linear regressions, reliable determinations of the KF value were obtained.
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 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.
Desorption kinetics could not be measured due to the instability of the test item.
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
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
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
Captan solution only
only at pH 7 and 5 were used for t = 0 (15 minutes).
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
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.
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.
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