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Adsorption / desorption

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Description of key information

It is not technically feasible to perform a study on the substance.  Based on the organic acid ligand only the estimated Log Koc for the free acid is > 4.

Key value for chemical safety assessment

Koc at 20 °C:
24 080

Additional information

In accordance with the column 2 specific rules for adaptation from column 1, a study for determination of the adsorption/desorption coefficent (Log Koc) is technically unfeasible. The partition coefficient could not be determined using a procedure designed to be compatible with EC Method C19 and OECD Guideline 121. The substance is a zinc-organic acid which dissociates, i.e. is unstable, in polar or protiotic solvents such as methanol and water. Even though the HLPC screening method uses a methanol and water mobile phase, it was decided to attempt the test. The test item was dissolved in tetrahydrofuran at a nominal 1 g/L for the test as this is one of the least polar and non-protiotic solvents that would be compatible with the test system and in which the test item was soluble.

Initially method development was performed to check the test item could be detected adequately and check the system was performing as required. The test system incoporated an Agilant 1200 HPLC with a variable wavelength detector (VWD) at 210 nm and connected to a Polymer Laboratories evaporative light scattering detector (ELSD). The purpose of the VWD was to detect the reference standards and dead time, while the ELSD was to detect the test item. The column was an XSelect HSS cyano 5 µ (150 x 4.6 mm) at 30 °C and 10 µL of the solutions were injected. The mobile phase was unadjusted 55:45 v/v methanol:water at approximately neutral pH and a flow rate of 1.0 mL/min; for the sample injections a gradient from 30 minutes (after elution of DDT) and over minutes to 100% tetrahydrofuran was used with the intention of it eluting the expected highly retained test item.

After the test runs were completed the sample chromatograms were observed and contained no test item peak on the gradient. A fresh sample was prepared and injected soon after completion of the analysis to assess if the test item had decomposed while in the vial waiting for analysis. Running of the fresh solution suggested that it was not solely down to instability in solution. A further fresh sample was injected using a shorter run time and the peak returned. By using shorter run times, a larger peak was obtained.

It was considered the longer retention time on the column allowed decomposition of the test item. The reduction in peak area from the 10 to 15 minute retention time also suggests decomposition. If compared to an environmental situation, the hydrophobic test item would be expected to absorb to the organic matter in soil and become immobile (absorption of positive charged metal ions to organic matter in soil is well known). The substance would then dissociate to zinc ions and the organic acid ligand. It is not possible to assess the timeframe for total dissociation from the test due to the potential interference of methanol on the rate. Long term exposure to the environment should therefore be assessed in regard to the metal ions and organic acid ligands rather than the test item.

Comparison of the small multiple peaks against run times showed no consistency in retention time, number or profile. Dissociation was therefore a function of the operational conditions in place for each run. However, based on the organic acid ligand only the estimated Log Koc is > 4 (KOCWIN v2.00).

[LogKoc: 4.4]