2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-pyridine carboxylate; imazapyr (ISO)

Adsorption / desorption

Description of key information

In a key study the adsorption and desorption behavior of the radiolabeled test substance and its transformation products, pyridine 2,3-dicarboxylic acid (metabolite #1) and furo[3,4 -b]pyridin-5(7H)-one,7 -hydroxy- (metabolite #2), were determined on two sediments. The sediments used were a sand obtained from a pond in Florida (pH 7.7) and a silt loam from a pond in Missouri (pH 6.6). For the test substance a Koc values of 30.6 and 99.8 were determined on sand and silt loam, respectively with a mean value of 65.2. These results demonstrated that the test substance will have a low binding to the two sediments. The Koc value of 217 for metabolite #1 indicates that it will be moderately bound to the sandy sediment, while the value of 6053 indicates that this compound will be tightly bound to the silt loam sediment. The results for metabolite #2 (sand: Koc=134; silt loam. Koc=1020) of demonstrates that this compound has moderate binding to the two sediments studied.

Key value for chemical safety assessment

Koc at 20 °C:

65.2

Additional information

Key study (American Cyanamid Company, ENV 98-25, 2000)

The adsorption/desorpton of the test substance and two metabolites were estimated with the batch-equilibrium method equivalent to OECD guideline 106. The test was performed with the test substance and its transformation products, pyridine 2,3-dicarboxylic acid (metabolite #1) and furo[3,4 -b]pyridin-5(7H)-one,7 -hydroxy- (metabolite #2). The radiolabeled test substancs were used with a C14-label at the 6-position of the pyridine ring for all three compounds. The sedments used were a sand obtained from a pond in Florida (pH 7.7) and a silt loam from a pond in Missouri (pH 6.6).

A kinetics test was conducted prior to the definitive test. An adsorption solution containing 1 ppm of the test substance was sampled at 0, 4 or 5, 20.5 or 21.5 and 24 hour intervals. The results of the kinetics test showed that adsorption equilibrium was archieved within 24 hours of incubation for all three test compounds. Based on these results, a 24 -hour equilibrium period was chosen for the definitive test.

Approximately 10 mL of a 0.01 M calcium chloride solution containing the test substance was added to 5 g of sediment and shaken for 24 hours. The concentration of the test substances used in this study were 1.12, 0.56, 0.11 and 0.55 ppm for the test substance, 1.09, 0.54, 0.11 and 0.054 ppm for metabolite #1 and 0.754, 0.372, 0.0744 and 0.0370 ppm for metabolite #2. Each concentration was tested in triplicate. Once equilibrated, the phases were separated and the aqueous adsorption solution was removed.

Approximately 10 mL of 0.01 M calcium chloride solution was then added to the sediments from the adsorption phase and shaken for 27.5, 24, or 44 hours, and the aqueous desorption solution was removed. The sediments were extracted multiple times with 0.1 M or 1 M sodium hydroxide. Following the extraction process, the sediments were air dried, and the remaining radioactivity was determined by combustion analysis.

The stability of the test substances in the adsorption and desorption solutions and in the extracts was determined by reverse phase HPLC. All three compounds were stable in the adsorption and desorption solutions. The radioactive mass balance was 98.2 to 103.2 % for the test substanc, 95.7 to 104.9% for metabolite #1 and 97.2 to 104.1% for metabolite #2.

The results of the adsorption/desorption test ist summarized below:

Substance	Sediment	Kd ads	Koc	Kf	1/n	R²	Kd des
Test substance	Sand Silt Loam	0.144 0.639	30.6 99.8	0.091 0.523	0.733 0.887	0.976 0.999	0.338 1.02
Metabolite #1	Sand Silt Loam	1.02 38.7	217 6053	0.699 31.6	0.811 0.955	0.998 0.999	1.60 140
Metabolite #2	Sand Silt Loam	0.63 6.53	134 1020	0.551 5.77	0.939 0.962	0.998 1.000	1.89 16.9

In conclusion, the study demonstrated that the test substance will have a low binding to the two sediments tested (sand, silt loam), that metabolite #1 will be moderately bound to the sandy sediment from Florida and tightly bound to the silt loam sediment in Missouri, and that metabolite #2 will be moderately bound to both sediments.

[LogKoc: 1.81]