Nitrapyrin

Adsorption / desorption

Description of key information

Koc (adsorption) = 326 mL/g (average from 5 soils); Koc (desorption) = 1700 mL/g; Guideline 163-1, Peterson et al. (1987)

Koc (adsorption) = 564 mL/g (average of 8 soils), slurry method,  Hanmaker (1974)

Key value for chemical safety assessment

Koc at 20 °C:

461

Additional information

Available Data

In the key study (Peterson et al. 1987) the adsorption and desorption of the test material was investigated on five soils, at room temperature. The study was conducted under GLP conditions and in accordance with Guideline 163-1 and was assigned a reliability score of 1 in line with the criteria of Klimisch et al. (1997).

The adsorption and desorption of¹⁴C-labelled nitrapyrin were measured using the batch equilibrium method to determine the Kd and Koc values in five US-soils (o.c. content 0.3-5.65%, pH 5.4-7.1). An average adsorption Koc of 326 ml/g was determined for nitrapyrin and it can therefore be classified as having low to medium mobility in soil.

The desorption Koc values for the two intermediate soil metabolites, 2-chloro-6-(dichloromethyl)-pyridine (DCMP) and 6-chloro-2-picolinic acid (6 -CPA), were determined to be 860 mL/g and 190 mL/g, respectively.

Two references are included as supporting information. The first is in the form of a publication, reported by Goring (1961). It appears the study was conducted to sound scientific principles however minimal detail is provided on materials and methods; the study was therefore assigned a reliability score of 4 in line with the criteria of Klimisch et al. (1997).

During the study a 4 ppm solution of radioactive test material was prepared in water. Samples (0.5 g) of finely ground air-dry soils were placed in 10 mL test tubes and 2 mL of the chemical solution added to each soil sample. The tubes were stoppered with rubber stoppers covered with plastic film. The tubes were intermittently shaken for 30 minutes, centrifuged, and 0.25 mL samples of the supernatant placed on lens paper in planchets. The planchets were tightly covered with plastic film and equilibrated for 3 hours, after which counts were made. Check samples without soil were also run through the standardised procedure, and, from the results obtained, the sorption of the test material by the soils was calculated.

Under the conditions of the study there was no consistent relationship between sorption of the test material by the soil and the pH or clay content of the soil. There appeared to be a good relationship between sorption and % organic matter in the soil.

The ratios of the concentrations sorbed by the soils expressed on an organic-matter basis to the concentrations remaining in the soil solution were relatively constant. The test material partitions between the organic matter and the water, the coefficient being about 100 to 200. In conclusion, in moist soil the test material is highly sorbed by organic matter but not appreciably sorbed by the clay fraction of the soil.

The second reference included as supporting information is a publication as reported by Hanmaker (1974). It appears the study was conducted to sound scientific principles however minimal detail is provided on materials and methods. The study pre-dates GLP. The data are included as supporting information and have been assigned a reliability score of 4 in line with the criteria of Klimisch et al. (1997).

During the study the soil adsorption coefficient (Koc) of the test material was determined. No details about the analytical method/test solution preparation are explicitly stated. Adsorption was measured by the slurry method for 8 different soils in a 4:1 water-soil slurry mixture after 4 hours incubation and resulted in an average Koc value of 564 mL/g. Based on the results the test material would be classified as having low to medium mobility.

Organic Partition Coefficient (Koc)

The Kocs for the substance were taken from Hamaker (1974) and Petersen et al (1987). The Kocs and Freundlich coefficients (1/n) are given in the Table below.

Reported Koc values for the substance

Study	Soil	Kf/d	1/n	Koc
Hamaker (1974)	26	0.4	1	491
	110	3.18	1	514
	106	5.39	1	627
	105	4.43	1	457
	93	9.09	1	627
	108	15.85	1	417
	67	44	1	764
Petersen et al. (1987)	M137	0.947	0.8935	287
	M139	2.372	0.8138	377
	M140	19.955	0.8991	352
	M141	2.916	0.8674	360
	M145	5.243	0.8325	254
	Mean		0.94	461