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Environmental fate & pathways

Adsorption / desorption

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Description of key information

Key value for chemical safety assessment

Koc at 20 °C:
72 200

Additional information

The adsorption coefficient of Chlorhexidine digluconate on soil was determined by the HPLC method following the OECD Guideline 121 and 92/69/EEC, part 19. With two different mobile phases the test substance was not eluted. Therefore, it has to be stated that the Kocof Chlorhexidine is higher than of the highest calibration substance (trifluralin) with a log Kocof 3.9 (Koc> 7944). In the HPLC column, separation of Chlorhexidine digluconate into the base chlorhexidine and gluconate is expected. Therefore, the Kocof the free chlorhexidine was probably determined and not the Koc of Chlorhexidine digluconate (2001-0234-DGO).

The adsorption of Chlorhexidine digluconate onto sewage sludge was assessed in a non Guideline test. Ring-U-14C labelled chlorhexidine in a concentration of 50 µg/L was exposed to activated sludge from a municipal treatment plant for 5 days. Based on radioactivity measurements, a Kocof 72200 L/kg was calculated. These values are probably also related to Chlorhexidine base, due to dissociation of the gluconate (Freitag et al. 1982, 1985).

In addition, Sugio & Kojima (1992) investigated the adsorptivity on activated sludge for Chlorhexidine digluconate. The result showed that the adsorption complies with the Freundlich isotherm equation and the amount of Chlorhexidine digluconate adsorbed on 1 g activated sludge (dry state) in equilibrium concentration of 0.5 ppm Chlorhexidine digluconate was about 8 mg. This results in a Kpvalue of 16000 L/kg which is equal to Kocof 43200 L/kg using the organic carbon content of activated sludge recommended by EUSES.

In conclusion, the adsorption on sludge, soil and sediment of Chlorhexidine is expected to be relatively high. As the substance is supposed to be not readily degradable, adsorption is expected to be the major removal mechanism in biological sewage treatment plants. The high sorption indicates that the substance is nearly immobile in soils.