D-gluconic acid, compound with N,N''-bis(4-chlorophenyl)-3,12-diimino-2,4,11,13-tetraazatetradecanediamidine (2:1)

Endpoint summary

Description of key information

The available screening tests on Chlorhexidine biodegradability indicate poor biodegradability. The test results of the standard screening studies are largely influenced by its high toxicity to microorganisms (EC₂₀of 11 mg/l, sludge respiration) and by adsorption. 

A simulation study according to OECD308 confirmed the poor biodegradability. Transfer of chlorhexidine digluconate from the water into the sediment was rapid with DT50 values of 1.4 and 4.1 days in the two systems tested. The rate of degradation in the total water/sediment systems was however slow, with DT50 values of >1000 and 277 days for the two examined water sediment systems, respectively.

Additional information

For the dissociation product gluconic acid it can be assumed that it is readily biodegradable, as it is part of the metabolism of natural carbohydrates.

For Chlorhexidine digluconate, in two OECD standard tests on ready biodegradability the test substance was applied at concentrations below the levels proposed by the Guidelines. Both results indicate that Chlorhexidine digluconate can be considered as readily biodegradable. However, the substance is probably only removed by adsorption or the precision of the analyses is insufficient. Therefore, these results should be used with care.

For the evaluation it has to be considered that at higher concentrations Chlorhexidine digluconate failed to degrade in standard screening studies, probably due to toxic effects on the inoculum. This can be explained by the biocidal activity, which is the intended effect of Chlorhexidine digluconate in biocidal products.

Therefore, the mineralization and primary degradation of 14C-labelled Chlorhexidine digluconate was investigated in a modified CO2 Evolution test at a concentration of 0.20 mg/L 14C-labelled Chlorhexidine digluconate. Only minor amounts of 2.3% were found mineralized in the adsorption solutions after 60 days of incubation. In the aqueous supernatant 12.9% of the applied radioactivity (AR) were found. Further 38.0% could be extracted from filtered particulate matter. Not-extractable residues were determined by combustion of the particulate matter after extraction to be 21.1% AR.

However, it has been demonstrated that some bacterial strains can grow in a medium containing Chlorhexidine. Several intermediates were identified as degradation metabolites. As most of the available studies used isolated bacteria strains and the tests were run under laboratory conditions, the results do not allow a classification of biodegradability or a conclusion about degradation rates in biological treatment plants and in environmental compartments. Furthermore, it remains unresolved if those metabolites of chlorhexidine biodegradation that are observed in laboratory tests are also formed in relevant amounts under more realistic STP conditions. However, no evidence exist that biodegradation under laboratory conditions will rely on different pathways than under STP conditions.

The route and rate of degradation of chlorhexidine digluconate was further investigated under aerobic conditions at 20 ± 2ºC in two contrasting water sediment systems in the dark (according to OECD308). Chlorhexidine digluconate was shown to degrade to unextractable sediment bound residues and minor metabolites only, with only 4-Chloroaniline being identified.Transfer of chlorhexidine digluconate from the water into the sediment was rapid with DT50 values of 1.4 and 4.1 days in the two systems tested. The rate of degradation in the total water/sediment systems was however slow, with DT50 values of
>1000 and 277 days for Calwich Abbey and Swiss Lake systems, respectively.

In conclusion, Chlorhexidine digluconate was proven to be poorly biodegradable and is considered “not biodegradable” in the present risk assessment.