Decyldimethylamine

Short-term toxicity to aquatic invertebrates

Description of key information

For four category members six reliable studies (reliability category 1 or 2) are available with EC50 (48 h) values between 4.99 µM (C10 DMA; RL 1) and 0.253 µM (C12-14 DMA; RL 1). No obvious relationship between chain length and toxicity exists.

Key value for chemical safety assessment

Fresh water invertebrates

Fresh water invertebrates

Dose descriptor:

EC50

Effect concentration:

56 µg/L

Additional information

Dimethyl Alkyl Amines (DMA), which are cationic surfactants at pH relevant in the environment, exhibit strong sorption to test organisms and walls of test vessels due to a combination of ionic and hydrophobic interaction. The sorption coefficient was found to be concentration dependent. Due to these properties, the test items are difficult to test in synthetic water and results from such tests depend on the test settings applied. In river water, which contains particulate as well as dissolved organic carbon, Dimethyl Alkyl Amines (DMA) are either dissolved in water or adsorbed to dissolved and particulate matter. This reduces the difficulties encountered in tests with synthetic water caused by the high adsorption potential (adsorption losses due to settling on surfaces). In general, the adsorbed fraction of DMA is difficult to extract from the test system, which normally leads to low analytical recoveries especially in the old media, while initially measured concentrations (fresh media) are generally within +/- 20% as recommended by the guidelines. Due to the short exposure periods applied in these tests (semi-static design) these low recoveries cannot be explained by biodegradation. No, or negligible sorption to glassware occurs under these conditions, which was confirmed by measurements. This ensures reliable as well as reproducible results and means that the test substance is present in the test system and therefore available for exposure (dissolved in water and adsorbed, also called bulk). This so-called Bulk Approach is described by ECETOC (2003). Consequently, nominal concentrations were used for these tests instead of measured ones.

Therefore, reliable (reliability category 1) tests with river water as dilution water were newly performed (NOACK, 2012) for four category members with different chain lengths (C10 DMA, C12-14 DMA, C16 DMA and C16-18 DMA). These tests were of semi-static test design (renewal after 24 hours) and involved analytical determination of test item adsorbed to glass walls as well as initial and final test item concentration in test water and are regarded to be of higher reliability and relevance than the tests performed with synthetic dilution water. Natural river water from river “Innerste” (Lower Saxony) was used as dilution water in these tests. This river has been chosen due to its properties representing typical conditions of a German medium-sized river. The concentration of suspended matter measured in the river water was in a range of 14.0 to 15.6 mg/L, the non-purgable organic carbon concentration was between 3.2 and 3.3 mg/L.

Sometimes mitigating effects are observed for river water tests compared to tests involving synthetic water. This was not the case for results on acute invertebrate toxicity of DMA. Where reliable studies for both test types are available for comparison (C10, C12-14) results are very close to one another and EC50 (48 h) observed in the river water test was even lower than the one involving synthetic water for C12-14-DMA (0.253 µM and 0.377 µM, respectively).
While in the supporting study with C10-DMA and artificial water no vehicle was used, for longer chain-length DMAs this becomes more and more indispensable. Accordingly, for C12-14-DMA, Tween 80 was used to stabilize the test solutions as a substitute for the natural organics fraction present in river water.
Finally, the most recently performed study on acute Daphnia toxicity with C16-18-DMA reliably demonstrates that artificial water (Elendt M4) alone is inappropriate for the difficult substance properties (surface-active; adsorbing; low solubility especially for the long-chain DMAs) of DMA category members. The absence of any stabilizing organic constituents (DOC, suspended matter) normally present in environmental waters caused an instable system, producing erratic immobilization results. Possible reasons are the much lower water solubility found in the Elendt M4 test medium compared to the physico-chemical test result, combined with the specific property of DMAs to adsorb to organic and inorganic materials by different mechanisms, including ionic interaction. The sorption processes are mostly non-linear, means are concentration dependent. Due to these properties, the test item is difficult to test in artificial water (e.g. sorption to the test organism and walls of the test vessel). As a consequence, in this study, three different preparation methods for the loading rates ("WAF") were elaborated to cope with these problems (three range finding tests carried out). The different WAF preparation methods produced very different measured concentrations and partly conflicting immobilization results. Due to their surface-active properties, the DMAs are prone to form micelles or possibly micellar aggregates, depending on the respective experimental procedure applied. As mentioned above, they are known to adsorb to glass surfaces. Therefore, glass test-vessels were pre-saturated in the definitive test. A side effect could be that this introduced additional test material into the system (e.g., some surfactants are prone to form multiple layers on surfaces). In the definitive test, actually no WAFs or saturated solutions were prepared, but a stock dispersion at a nominal concentration of 10 mg/L, far above even the solubility in pure water (4 mg/L). This stock dispersion was diluted to the test concentrations applied. This procedure seems to have caused major inhomogeneities with local concentration peaks, as indicate measured concentrations up to 279% of nominal (analytically confirmed). Inhomogeneous and locally high concentrations have probably caused immobilization of Daphnia at low loading rates (an 48 h-EC50 of 0.052 µM was determined). In conclusion, this study reliably demonstrates the inappropriateness of aquatic test systems using artificial water for members of the DMA category. The study is therefore disregarded due to major methodological deficiencies. While reliable, it is not relevant for environmental hazard and risk assessment.
For the same category member (C16-18-DMA) a reliable study using natural river water is available (key study). Because sometimes, river water tests are criticized for underestimating the true toxicity of the respective test item, the adsorbed and the truly dissolved fraction of the (bulk) EC50 was estimated using EUSES methodology. In consequence, the combination of total organic carbon (3.25 mg/L) and suspended matter (14 mg/L) from the specific river water used in this study leads to a calculated adsorbed fraction of only 3.8%. I.e., the bulk EC50 of 0.653 µM corresponds to an EC50 (truly dissolved) of 0.628 µM. This difference is marginal and accounted for in the exposure and risk assessment, because (according to the ECETOC (2003) "bulk approach") bulk concentrations in water are used to calculate RCR values.

Concluding from the reliable key studies using river-water, the determined EC50 (48 h, immobility) is highest for C10 DMA (4.99 µM; river water; RL 1), intermediate for C16-18 DMA (0.653 µM; river water; RL 1) and lowest for C12-14 and C16 DMA (0.253 µM and 0.247 µM, respectively; river water; RL 1). Thus, judging from reliable experimental test results, while there is no evident relationship between chain length and toxicity, members of the DMA category are to be regarded as acutely toxic to aquatic invertebrates.

Based on the highest toxicity observed for C12-14-DMA in the reliable river water test, this EC50 (48 h; nominal; analytical verification) of 0.253 µM is used as the key value for the category. However, the original value obtained with C12-14-DMA of 56 µg/L C12-14-DMA is used as key value. This is justified because a) molar concentration cannot be used for environmental hazard and risk assessment and b) correction for MW would theoretically result in somewhat lower values for shorter chain DMAs, but the available reliable river water study for C10-DMA demonstrates a much lower toxicity (4.99 µM).