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Environmental fate & pathways

Adsorption / desorption

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Description of key information

Two reliable OECD 106 Guideline studies investigated the adsorption behaviour of benoxacor onto soil. Spare (1987) conducted the experiment with four sterilised soils and reported Koc values in the range 42.3-176.5 L/kg. Reischmann (1997) reported Koc values of for five soils between 210.9 and 340.4 L/kg. It is proposed to use the overall mean value of 218 L/kg for the chemical safety report.

Key value for chemical safety assessment

Koc at 20 °C:
218

Additional information

Two studies investigated the adsorption behaviour of benoxacor onto soil. Spare (1987) performed a GLP-compliant study with four soils equivalent to the OECD Guideline 106 (Adsorption - Desorption Using a Batch Equilibrium Method). The resulting Koc values ranged (42.3-176.5 L/kg) with a mean value of 128.5 L/kg.

In a second study, Reischmann (1997) investigated the adsorption/desorption behaviour of 14C-benoxacor in five different soil types. The mean adsorption constant based on the organic carbon content (OC) of all soils was determined to Koc = 290 L/kg (range 210.9 to 340.4 L/kg). The study is considered to be reliable.

The soil adsorption values reported by both studies are presented in the Table below.

Soil

Organic carbon

Adsorption [mL/g]

Reference

Type

[%]

Kd

Koc

 

Maryland clay

2.8

4

140

Spare, 1987

Maryland sand

0.5

0.2

42.3

Mississippi loam

0.7

1.2

176.5

Iowa sandy loam

2.9

4.6

155

Mean K-values

 

2.5

128.5

Speyer 2.1

0.4

1.2

340.4

Reischmann, 1997

Collombey

0.8

1.6

210.9

Les Evouettes

2.1

7.1

335.6

Vetroz

4.4

13

296.3

Illarsaz

19.3

51.5

266.4

Mean K-values

 

14.9

290

Overall Mean K-values

 

9.4

218.1556

 

The study by Spare (1987) indicates that benoxacor is adsorbed relatively weakly to sterile soil, whilst the more recent study performed by Reischmann (1997) indicates greater adsorption potential for non-sterilised soil. As a worst-case, it is proposed to use the overall mean value of 218 L/kg in the Chemical Safety Assessment. This means that the substance falls within the classification of moderately mobile, as defined by PSD (2000). This designation does not take account of the fast soil degradation rate of benoxacor.