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Environmental fate & pathways

Adsorption / desorption

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Description of key information

The adsorption coefficient of the test item was determined at acidic and approximately neutral pH using the HPLC screening method according to EC Method C19 and OECD TG 121. The resulting adsorption coefficients were:
pH 3.4: Koc = 3820 and > 427000; Log Koc = 3.58 and > 5.63
pH 6.5: Koc = 239, 647 and > 427000; Log Koc = 2.38, 2.81 and > 5.63

Key value for chemical safety assessment

Additional information

The determination of the adsorption coefficient was carried out using the HPLC screening method, designed to be compatible with Method C19 Adsorption Coefficient of Commission Regulation (EC) No 440/2008 and OECD TG 121.

The test utilized a high performance liquid chromatograph. A commercially available cyanopropyl reverse phase HPLC column containing lipophilic and polar moeties was used.

It was known that the test item was a complex mixture (UVCB). Some of those components contained a carboxylic acid group with an estimated pKa of 4.2. During method development, it was noted that the group of peaks eluting isocratically were inconsistent in retention time and peak profile. This was considered to be partially due to the carboxylic acid group. Using a mobile phase at approximately pH 3 gave improved peak consistency and profile. The test was therefore performed with acidic and neutral mobile phases.

As the pKa was 4.2, the ideal mobile phase pH would be two units lower to give pH 2.2. This was considered too low for the purposes of the test as it was outside the environmentally relevant pH range as few soils would have a pH this low. Consequently, it was decided to adjust the mobile phase to approximately one pH unit below the pKa; this was still outside the limits stipulated by the test guidelines, but some acidic soils are around this pH.

The adsorption coefficient of the test item has been determined as follows:

pH 3.4: Koc = 3820 and > 427000; Log Koc = 3.58 and > 5.63

pH 6.5: Koc = 239, 647 and > 427000; Log Koc = 2.38, 2.81 and > 5.63

The results suggested that a greater amount of the test item would retain on to soil in acidic conditions while lesser retained components would have a higher retention in acidic conditions compared to neutral conditions. There appears to be a lower percentage of highly retained components in neutral conditions and correspondingly a higher percentage of slightly retained components.