Cerium tetranitrate

Toxicity to aquatic algae and cyanobacteria

Description of key information

The key study (Hefner, 2014c) yielded a 72-h EC50 value (growth rate-based) of > 36 mg Ce/L, i.e. > 100 mg/L cerium tetranitrate, for the unicellular green alga Pseudokirchneriella subcapitata. Dissolved cerium could not be detected in any of the treatments at the start and end of testing. Therefore, nominal concentrations were used for expressing the EC50 values. Due to the absence of exposure, it was not deemed necessary to perform a definitive test (i.e., only a range finding test was performed). Nevertheless, significant inhibition of growth rate was observed in the undiluted filtrate (nominal loading rate 100 mg Ce(NO3)4/L) (-35.4%) as well as in the 1:10 dilution (-5.8%). Phosphate monitoring however indicated that in the undiluted filtrate, phosphate was already depleted from the start of testing, whereas in the 1:10 dilution accelerated phosphate depletion was observed throughout the test compared to the control treatment. This phosphate depletion can be ascribed to heavy complexation of cerium with phosphates. Based on these observations, the observed adverse effects on growth were considered to be due to phosphate deprivation and not due to primary toxicity of the test substance. Therefore, the effect concentrations will not be used for classification purposes (anyhow the data for fish are driving classification for environmental hazards). 

Additional information

One study on the toxicity of cerium tetranitrate to algae was included in this dossier.

The key study from Hefner (2014c) is a 72-h algal growth inhibition test with Pseudokirchneriella subcapitata in which cerium tetranitrate was used as test item. The growth rate-based 72-h EC50 was > 36 mg Ce/L (corresponding to > 100 mg cerium tetranitrate/L). At the start and end of the preliminary (range finding) test, no dissolved cerium was detected in any of the treatments. Therefore, EC50 values were related to the theoretical concentrations of the test item based on the loading rate. Further no definitive test was performed. Although no dissolved cerium was detected throughout the test, significant reduction of growth rate was observed in the undiluted filtrate of the treatment with loading rate of 100 mg cerium tetranitrate/L (-35.4%) as well as in the 1:10 dilution (-5.8%). Phosphate monitoring throughout the test however indicated that no phosphate was present in the undiluted filtrate from the start of testing, and that accelerated phosphate depletion compared to the control treatment occured in the 1:10 dilution. Therefore, the observed effects on growth can be ascribed to phosphate deprivation due to heavy complexation of cerium with phosphates instead of a direct toxic effect of the test substance.

Consequently, the (unbound) EC50 values will not be taken forward to classification. Similar observations were made for cerium ammonium nitrate (another tetravalent cerium compound) and for trivalent cerium (Ce+III) compounds such as CeCl3 and Ce(NO3)3 (see respective dossiers). For the trivalent cerium compounds it was confirmed by modelling calculations using Visual MINTEQ v3.0 that under the conditions of the test all cerium is precipitated as CePO4 whenever phosphate is in excess and vice versa. The good agreement between measured dissolved cerium concentrations at the end of testing and the modelled dissolved cerium concentrations further increase the credibility of these model calculations. Visual MINTEQ calculations can however not be performed for tetravalent cerium (Ce+IV) because Visual MINTEQ focuses on solution chemistry, whereas Ce+IV - according to the Pourbaix diagram (see environmental fate section) - is not stable in solution. However, depending on the conditions of the test medium (pH, redox potential of solution, see Pourbaix diagram), Ce+IV could partly be reduced to Ce+III, which can stay in its turn partly in solution depending on speciation conditions (pH, test medium composition, etc.). The addition of tetravalent cerium compounds to aqueous test media can indeed result in measurable dissolved cerium (Ce+III). This has been demonstrated in the acute toxicity experiments with daphnids with cerium tetranitrate (Hefner, 2014b) and cerium ammonium nitrate (Hefner, 2014d) as well as in an acute toxicity experiment with fish with cerium ammonium nitrate (Hefner, 2014a). In the fish experiment, a 100 mg/L solution of anhydrous cerium ammonium nitrate at pH 6 (up to maximally 6.4) resulted in a mean measured dissolved Ce concentration of 0.22 mg Ce/L, whereas in the daphnid experiments, similar solutions in daphnid test medium gave rise mean measured dissolved Ce concentrations of 3.1 µg Ce/L in the test with cerium ammonium nitrate (pH 6.2 up to maximally 7.2) and 47 µg Ce/L in the test with cerium tetranitrate (pH 5.9 up to maximally 7.0). This is respectively 0.86, 0.01, and 0.1% of the nominal Ce loading. However, should there be a comparable reduction of Ce+IV in the algal growth medium, any Ce+III formed would most likely heavily complex with phosphates in the algal test medium and consequently precipitate from the test medium (note that no phosphates are present in the standard fish and daphnid test media). With nominally 1.18E-05 M PO4^{3 -}present in the test medium, 1.65 mg Ce/L (Ce+III) could be complexed. Based on the observations in fish and daphnid test medium, it is not expected that so much Ce+III would be formed. Any Ce+III formed can therefore be assumed to precipitate with phosphates in the test medium, explaining why no dissolved cerium could be detected in any of the treatments. The fact that all phosphates were depleted already from the start of the test in the treatment with the nominal loading rate of 100 mg/L anhydrous cerium tetranitrate suggests that, on a molar basis, a similar amount of Ce+III and phosphates was present in the test medium, however, part of the phosphate complexation could also be due to Ce+IV (which does not occur in dissolved state). Unfortunately no data are available on the potential contribution of undissolved Ce+IV to phosphate complexation and hence no predictions can be made on this.

Overall, based on abovementioned argumentation, it is considered justified to consider the observed effect on algal growth as a phosphate deprivation effect. This effect occurs due to the limitations of the test system and is not expected to be environmentally relevant. Phosphate deprivation effects may only occur extremely locally at discharge points but are not considered to adversely affect entire algal populations/communities in the receiving (local) ecosystem.

References not included in this dossier:

- Hefner N, 2014d. Cerium ammonium nitrate: Acute toxicity to Daphnia magna in a 48-hour immobilization test. Harlan Laboratories Ltd., Zelgliweg 1, 4452 Itingen, Switzerland. Report no. D60755. Owner company: Treibacher Industrie AG.