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Ecotoxicological information

Toxicity to aquatic algae and cyanobacteria

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Description of key information

The 72-h ErC50 for cesium potassium fluoroaluminate in freshwater algae (Pseudokirchneriella subcapitata) was 13.5 mg/L. The 72-h NOEC for yield is 2.3 mg/L. 

Key value for chemical safety assessment

EC50 for freshwater algae:
13.5 mg/L
EC10 or NOEC for freshwater algae:
2.3 mg/L

Additional information

One reliable study is available and was selected as key study. The study was performed according to OECD 201 and under GLP conditions. As cesium potassium fluoroaluminate is not completely soluble in the test medium due to formation of precipitates with salts in the medium, solutions from a loading rate at 100 mg/L were prepared. Algae were exposed to 0, 1.0, 3.0, 10, 30 and 100% of the filtrated stock solution under static conditions for 72 hours. The test solutions were analytically verified by measuring the fluoride content. The biological endpoints of the test are based on the average of the measured concentrations being 0.79, 2.3, 7.5, 24 and 75 mg/L. The 72-h EC50 for growth rate and yield are determined at 13.5 mg/L and 9.8 mg/L, respectively. The 72-h NOEC for yield is 2.3 mg/L.

A previous study (Bouwman, 2004c) performed by NOTOX BV according to OECD 201 has been disregarded because of significant methodological deficiencies. In this study a 72-h EC50 for growth rate of 1.1 mg/L was observed, the 72-h NOEC for yield and growth rate was 0.60 mg/L. The results of this study are difficult to interpret. The dose-response shows a higher inhibition at higher concentrations. However, under influence of the test substance the algal cells exposed to all but the lowest test concentration clotted together, which inhibited growth. Due to the clotting, cell densities could not be determined at these concentrations. During the exposure period all but the highest test concentration were observed to recover from this effect and started growing.

Test substance particles were observed in all concentrations. It is unknown what the precipitate consisted of since only the concentration cesium was measured during the study. The algal medium that was used during the study contains a relatively high amount of nutrient salts, necessary to the growth of the algae and recommended in the OECD 201 guideline. It may this be that cesium potassium fluoroaluminate precipitated partly due to the presence of these nutrient salts. It may also reduce the availability in the test medium of salts and trace elements which are essential for the growth of algae. The inhibition of algae growth is in that case a secondary effect, which cannot be attributed to the substance specific inherent toxic properties. Due to the complexation of the test substance in the test medium, the EC50 values are not reliable.

The quantification of the test substance is based on the analysis of cesium which is present at 5.64% w/w in the test substance. The measured concentrations of the test substance were actually calculated by the measured concentrations of Cesium. Analyses of a sample taken at the start of the final algae test from the undiluted filtrate prepared at a loading rate of 100 mg/L showed a cesium concentration of 902 µg/L, corresponding to a test substance concentration of 16 mg/L. Overall, the concentrations of cesium in the test solutions are significantly lower than expected based on the percentage of cesium in the test substance. No explanation was given for this difference. Since the analytical method used to quantify the test substance in the test solution is not considered sufficiently robust, the effect concentrations found in this study are disregarded for assessment. In addition, the test substance consists of four elements (Cs, K, Al and F) and the biological effects are expected to be mostly governed by the presence of fluoride and aluminium ions formed upon dissociation of the fluoroaluminate moieties. Therefore in order to reflect the toxicity of the test substance, it is not correct to express the results as the mean measured cesium concentrations.