[2-[[4-[(2-chloro-4-nitrophenyl)azo]phenyl]ethylamino]ethyl](2-hydroxypropyl)dimethylammonium acetate

Adsorption / desorption

Description of key information

Basic Red 18:1 Acetate is a surface-active substance with a low log Kow (0.3) and high water solubility. Therefore, measured adsorption coefficients at different, environmentally relevant pH values are required for the assessment of the adsorption potential of the substance.

During pre-tests conducted with the structural analogue Basic Red 18:1 Chloride, considerable technical difficulties were encountered. Measurement of the log Koc at pH 7 and 9 is not possible, but an approximation of the log Koc of 5.17 and 5.308 at pH 4 in soil and sewage sludge, respectively, can be provided by estimation.

Supporting information from an OECD 302 B (Zahn-Wellens Test) demonstrates rapid removal of the test substance from the test solution. A value of 63 % elimination within 3 h after start of the test indicates fast removal through adsorption of the test substance.

Based on the experimental results described above and considering the charged moieties within the compound, it can be concluded that Basic Red 18:1 Acetate has a high adsorption potential in soil and sewage sludge.

The predominant pH of surface waters and sewage treatment plants is approximately 7 or sometimes even higher. Soil pH values can vary considerably depending on soil type, spatial (climate) and temporal distribution. Frequency maxima of forest soil and arable land in Germany are in the range of pH 6-7 and 3.6-4.6 (Schwertmann & Fischer, 1982; cited in Scheffer & Schachtschabel, 2002). Although determination of the Koc by the HPLC method at pH 7 and 9 was not possible, the corresponding value of Basic Red 18:1 Acetate can be expected to be significantly higher than the estimates for pH 4 (5.17 and 5.308 for soil and sewage sludge, respectively). In view of the HPLC retention times log Koc values clearly above 6.0 can be anticipated. For modelling distribution in the environment, a high log Koc is truncated at a value of 6.0 by the program EUSES. Therefore, a log Koc of 6 will be used as basis for exposure estimation by means of Chesar/EUSES in a worst-case approach.

Key value for chemical safety assessment

Koc at 20 °C:

1 000 000

Additional information

DESCRIPTION OF RELEVANT SUBSTANCE PROPERTIES
Basic Red 18:1 Acetate is a solid substance which is however only stable in solution in presence of significant concentrations of free acetic acid. It is characterized by a water solubility of 132 (± 19) g/L at 20 °C and pH 6 (OECD 105). The n-octanol/water partition coefficient (log Kow), that was determined in accordance with OECD Guideline 107, is 0.3 at 23 °C and pH 7. Furthermore, Basic Red 18:1 Acetate is a surface-active compound (surface tension < 60 mN/m at 20 °C).

(Reported physicochemical properties (water solubility, log Kow, and surface tension) were experimentally determined for Basic Red 18:1 Chloride and are used in the assessment of the structural analogue Basic Red 18:1 Acetate in a read-across approach.)

Based on the physicochemical properties, a substance can be expected to have a low potential for adsorption if the log Kow value is ≤ 3. However, this may not hold true for substances with good water solubility in combination with a low octanol-water partition coefficient, particularly ionic and/or surface-active compounds. Such substances – like Basic Red 18:1 Acetate – do not necessarily always have a low adsorption potential.

Basic Red 18:1 Acetate is an ionic substance due to negatively and positively charged moieties within the molecule. The basic chemical structure of the substance itself – the acetate disregarded – is cationic. In accordance with the ECHA Guidance R.7a, especially cationic substances tend to adsorb strongly. Therefore, a measured adsorption coefficient is usually needed to verify any pH-dependence of substance adsorption. Besides, as Basic Red 18:1 Acetate is a surface-active substance, the log Kow value is considered a poor predictor of adsorption. Thus, measured adsorption coefficients at different, environmentally relevant pH values are required.

OVERVIEW OF EXPERIMENTAL RESULTS
1. OECD Guideline 121 – estimation of the adsorption coefficient (Koc) by means of HPLC (see Landsberg, Dr. C., 2016)
Due to the substance properties described in section 2, a study in accordance with OECD Guideline 121 was commissioned to further investigate the adsorption potential of the structural analogue Basic Red 18:1 Chloride on soil and sewage sludge. These data are used in a read-across approach in the assessment of Basic Red 18:1 Acetate.

Pre-tests with Basic Red 18:1 Chloride were conducted prior to a main study at pH 4, 7 and 9 in buffered media as the test item is considered to be ionic (Landsberg, 2016). During these pre-tests, considerable technical difficulties were encountered:

The retention time of the test substance Basic Red 18:1 Chloride was much higher compared to those of the reference compounds phenanthrene and 4,4'-DDT. 4,4'-DDT is the reference compound given in the guideline with the highest log Koc value for soil, while phenantrene is the reference compound with the highest log Koc value for soil and sewage sludge. Thus, an estimation of the log Koc value of the test substance Basic Red 18:1 Chloride would only be possible by extrapolation, which provides significant inaccuracy for the study endpoint.

The pH value had a strong influence on the chromatographic behaviour of Basic Red 18:1 Chloride. The retention time of the test substance increased with increasing pH (which is equivalent to decreasing adsorption with increasing pH). At pH 7 and 9 the retention time of the test item was ca. 30 and 40 minutes, respectively. Basic Red 18:1 Chloride considerably exceeded the retention time of the reference substances phenanthrene (pH 7: 4.081 min, pH 9: 4.254 min) and 4,4’-DDT (pH 7: n.d., pH 9: 5.216 min) and was thus outside of the validated range given by OECD 121. At pH 4, the retention time of the test substance (6.356 min) was still above the retention time of the reference compound phenanthrene (4.839 min). However, to provide an approximation of the log Koc for the test item, extrapolated values in soil and sewage sludge at pH 4 were calculated at 5.17 and 5.308, respectively. These values are also outside of the validated range of OECD Guideline 121 (log Koc = 1.5–5.0).

Thus, OECD 121 is considered to not be suitable for the measurement of the log Koc of Basic Red 18:1 Chloride.

2. OECD Guideline 302B – Zahn-Wellens Test (see Müller, 1994)
The inherent biodegradability of the structural analogue Basic Red 18:1 Chloride was investigated in accordance with OECD Guideline 302 B (Zahn-Wellens Test) using non-adapted domestic sludge (Müller, 1994). The initial test substance concentration was 100 mg DOC/L. Elimination of the test substance was followed by measurement of DOC removal from the test vessels. 63% elimination of DOC could be observed after 3 h. After 7 days, 87% DOC had been removed.

Although no adsorption control was included in the experiment, the high elimination within the first 3 h of the experiment indicates fast removal through adsorption of the test substance. As the chloride and the acetate variant of Basic Red 18:1 both consist of the identical cationic moiety which is considered to predominantly determine the fate and behaviour of the substance, the same adsorptive behaviour can be assumed for both Basic Red 18:1 compounds.

(Q)SAR
In accordance with ECHA Guidance R.7 a, the log Koc of organic substances can often be estimated from their octanol-water partition coefficient (Kow), as well as from other properties such as aqueous solubility. Such methods give useful indications on the qualitative/quantitative adsorption coefficient of a substance. Especially for substances that are difficult to test such estimated values are essential.

However, in the case of the ionic Basic Red 18:1 Acetate, the applicability of QSAR models is limited due to the charged moieties within the compound. In addition, the substance is a Quaternary Ammonium Compound (QAC). Adsorption of QACs mainly occurs by an ion-exchange mechanism and depends on cation-exchange capacity of the sorbent and a variety of other parameters (Boethling, 1994). The training set of the estimation program KOCWIN implemented in EPI SuiteTM(US EPA, 2017) for the calculation of the adsorption coefficient does not include any QACs. Therefore, the Koc estimate is outside of the programme's prediction domain.



REFERENCES
Boethling, R. S. 1994. Environmental aspects of cationic surfactants. In J. Cross and E. J. Singer (ed.), Cationic Surfactants: Analytical and Biological Evaluation, vol. 53. Marcel Dekker, Inc., New York, USA.

Müller, G. (1994). Bioabbaubarkeit von Farbstoffen. Testing Laboratory: Bayer AG, Institut für Umweltanalyse und Bewertungen, Geb. W 15, 51368 Leverkusen, Germany. Company owner: DyStar Colours Distribution GmbH. Report Date: 1994-11-08.

Landsberg, C. (2016). Basic Red 18.1 - Chloride Estimation of the Adsorption Coefficient (Koc) on Soil and on Sewage Sludge using High Performance Liquid Chromatography (HPLC) (Statement). Testing laboratory: Noack Laboratorien GmbH, Käthe-Paulus-Straße 1, D-31157 Sarstedt, Germany. Study No. 160803DL / CAH17176 (non-GLP). Company owner: DyStar Colours Distribution GmbH. Report Date: 2016-12-06.

Scheffer & Schachtschabel (2002). Lehrbuch der Bodenkunde. Spektrum Akademischer Verlag, Heidelberg. ISBN: 3-8274-1324-9.

US EPA (2017). Estimation Programs Interface Suite™ for Microsoft® Windows, v 4.11. United States Environmental Protection Agency, Washington, DC, USA.

[LogKoc: 6.0]