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Micrometric cerium dioxide

 

Short-term toxicity to fish:

Two studies of reliability 2 according to Klimisch were available (Bazzon, 2000 and Lambert, 2007). The first of them was selected as a key study.

The 96hr-acute toxicity of cerium dioxide to Brachydanio rerio was studied under static conditions according to OECD Guideline 203. Brachydanio rerio were exposed to control and test chemical at a nominal concentration of 100 mg/L. Mortality/immobilization were observed daily. No mortality was observed during the test, neither in the control nor in the group exposed to the test item. The 96 hour LL50 and NOELR were thus > 100 mg/L.

Based on the result of this study, cerium dioxide is considered as not harmful to fish.

The second study was flagged as a supporting study. It was conducted according to an American guideline under semi-static conditions. Fathead minnows (Pimephales promelas) were exposed to control and cerium dioxide at concentrations of 12500, 25000 and 50000 mg/L. As no significant mortality was observed, the 96h-LC50 was set equal to > 50000mg/L; confirming that cerium dioxide is not harmful to the fish species tested.

 

Short-term toxicity to aquatic invertebrates:

Five studies were available. One of them (RCC, 2007), quoted as reliability 1 according to Klimisch, was selected as a key study; the other ones of reliability 2 (Park et al, 2007 and Lambert, 2007) and 4 (Van Hoecke et al., 2009 and Gaiser and al., 2009) were selected as supporting studies.

In the studies of RCC (2007) and Park et al. (2007), daphnids were exposed in static conditions according to OECD Guideline 202 to control and a cerium dioxide saturated solution for 48 hr. Mortality/ immobilization were observed daily. In the control and the saturated solution, no immobilized test organisms were determined during the test period of 48 h. Thus, the 48 hour EL50 and the 48 hour NOEC were > 100 mg/L (RCC, 2007) or > 100% saturated solution (Park et al, 2007). As no adverse effect was observed at the highest tested concentrations, which corresponded to the maximum concentration of dissolved test item, cerium dioxide appears as not acutely toxic for daphnids up to its solubility limit into water. The study of Lambert (2007) was conducted according to an American guideline under semi-static conditions. Daphnids (Ceriodaphnia dubia) were exposed to controls and cerium dioxide at concentrations of 12500, 25000 and 50000 mg/L. As no significant mortality was observed, the 48h-EC50 was set equal to > 50000 mg/L. The publication of Van Hoecke et al. (2009) primarily deals with nanoparticulate cerium dioxide. However, some results are given concerning the bulk form. A 48-hour NOEC superior to 1000 mg/L was reported. The article of Gaiser et al. (2009) does not contain a sufficient level of details. However, it also concludes on the absence of mortality under the tested conditions.

Based on the result of these studies, cerium dioxide is considered as not harmful to daphnids.

 

Long-term toxicity to aquatic invertebrates:

One study of reliability 1 according to Klimisch was available (Harlan, 2009) and was selected as a key study.

The effect of cerium dioxide on the survival and reproduction of Daphnia magna was investigated in a semi-static test over 22 days following the OECD Guideline 211. Daphnids were exposed to control, and test chemical at nominal concentration of 100 mg of dry substance /L (loading rate) and the dilutions 1:3.2,1:10,1:32 and 1:100 of the saturated solution. The mortality and reproduction of the daphnids were compared with the corresponding parameters in the control and symptoms of toxicity were recorded.

The test item Cerium dioxide had no toxic effect on survival and reproduction of Daphnia magna after the exposure period of 22 days up to the loading rate of 100 mg/L. Thus, the NOELR of the test item was determined to be at least the loading rate of 100 mg/L. The LOELR was above the loading rate of 100 mg/L. As no adverse effect, neither in terms of survival, nor in terms of reproduction, was observed at the highest loading rate tested (i.e. 100 mg/L), which corresponded to the maximum concentration of dissolved test item, cerium dioxide does not exhibit any chronic toxicity to daphnids up to its solubility limit into water.

Another study of reliability 4 according to Klimisch was available (Van Hoecke et al., 2009) giving same results in terms of survival (21- or 22-day NOEC 100 mg/L), but not in terms of reproduction (21-d NOEC = 32 mg/L in Van Hoecke et al. 2009, versus, 22-d NOEC 100 mg/L in the Harlan's study). However, due to the low level of experimental details provided, it is not possible to conclude on the reliability of the Van Hoecke et al. study.

 

Toxicity to aquatic algae:

Three studies were available. One of them (RCC, 2007), quoted as reliability 1 according to Klimisch, was selected as a key study; the other one of reliability 2 (Lambert, 2007) and 4 (Van Hoecke et al., 2009) were flagged as supporting studies.

In the RCC study (2007), the influence of cerium dioxide on the growth of the green algal species Scenedesmus subspicatus was investigated in a 72 -hour static test according to OECD Guideline 201 and GLP. Due to the low water solubility of the test item, a supersaturated dispersion with the loading rate of 100 mg/L was prepared. Then the dispersion was filtered and the undiluted filtrate and the dilutions 1:2, 1:4, 1:8, 1:16 and 1:32 were used as test media. Additionally, a control was tested in parallel. A statistically significant inhibitory effect was reported on the growth of Scenedesmus subspicatus after the test period of 72 h first at the loading rate of 100 mg/L. Thus, this loading rate was determined as the 72h-LOELR. The 72h-NOELR was determined to be the loading rate of 50 mg/L. The 72h- EL50 was set superior to 100 mg/L. The inhibition of algal growth in the highest test concentration was presumably caused by a secondary effect, the complexation of the essential algal nutrient phosphate by the test item. The measured concentration of phosphate in the undiluted filtrate of the dispersion stirred for seven days was much lower than in the test water. A statistically significant decrease of the phosphate concentration was determined at the loading rate of 100 mg/L. Thus, the growth inhibition determined at this loading rate, which corresponded to the maximum concentration of dissolved test item, may have been caused by depletion of phosphate in the test medium, rather than by a toxic effect of cerium dioxide. As a consequence, it seems that cerium dioxide should not have toxic effect on the algae up to its solubility limit into water.

The study of Lambert (2007) was conducted according to an American guideline under static conditions. Selenastrum capricornutum were exposed to control and test chemical at 2500, 5000, 12500, 25000 and 50000 mg/L for 96 hr. Growth were observed during the test and 96-hour IC25 of 33631 mg/L (test A) and 36607 mg/L (test B) were calculated; implying a low toxicity of cerium oxide to algae.

The publication of Van Hoecke et al. (2009) primarily deals with nanoparticulate cerium dioxide. However, some results are given concerning the bulk form. A 72-hour NOEC superior to 1000 mg/L was reported.

Based on the result of these studies, cerium dioxide is considered as not harmful to algae.

 

Toxicity to microorganisms:

Two studies were available. One of them (RCC, 2006) quoted as reliability 1 according to Klimisch, was selected as a key study; the other one of reliability 2 (Park et al, 2007) was selected as a supporting study.

In both studies, the inhibitory effect of cerium dioxide on the respiration rate of aerobic wastewater microorganisms of activated sludge was investigated in a 3-hour respiration inhibition test according to OECD 209 and GLP.

The 3-hour NOEC of cerium dioxide to activated sludge microorganisms was 1003.8 mg/L (RCC, 2006) and 1000 mg/L (Park et al, 2007).

The 3-hour EC20, EC50 and EC80 were clearly higher than 1003.8 mg/L (RCC, 2006) and 1000 mg/L (Park et al, 2007).

Based on the result of these studies, cerium dioxide has no toxic effect on the respiration rate of activated sludge microorganisms.

 

Nanometric cerium dioxide

 

Short-term toxicity to fish:

One study of reliability 2 according to Klimisch was available (Van Hoecke et al., 2009) and selected as a key study.

The 72hr-acute toxicity of nanometric cerium dioxide to Brachydanio rerio embryo was studied according to an OECD draft Guideline. Brachydanio rerio embryo were exposed to control and test chemical at nominal concentration of 13, 25, 50, 100 and 200 mg /L. Lethal and sub-lethal endpoints were observed daily, and the hatched embryos were counted at the end of the exposure. No acute toxicity was observed during the test up to 200 mg/L. The 72 hour LL50 and NOELR were thus > 200 mg/L.

Based on the result of this study, nanometric cerium dioxide is considered as not harmful to fish embryos.

 

Short-term toxicity to aquatic invertebrates:

Four studies were available. One of them (Van Hoecke et al., 2009), quoted as reliability 2 according to Klimisch, was selected as a key study; the other ones of reliability 2 (Park et al, 2007 and Van Hoecke et al., 2009) and 4 (Gaiser et al., 2009), were selected as supporting studies.

In the study of Van Hoecke et al. (2009), daphnids were exposed according to OECD guideline 202 to control and test chemical concentrations ranging from 10 to 1000 mg/L for 48 hours. By observing immobilization during the test; no acute effect of nanometric cerium dioxide was reported. The 48h-EC50 and -NOEC were thus found to be > 1000 mg/L. Van Hoecke et al. (2009) also tested another aquatic invertebrate: Thamnocephalus platyurus, in a 24-hour toxicity test. They did not observed toxicity sign up to 5000 mg/L; resulting in 24-hour EC50 and -NOEC > 5000 mg/L.

In the study of Park et al. (2007), daphnids were exposed in static conditions according to OECD Guideline 202 to control and a cerium dioxide saturated solution for 48 hr. Mortality/immobilization were observed daily. In the control and the saturated solution, no immobilized test organisms were determined during the test period of 48 h. Thus, the 48 hour EL50 and the 48 hour NOEC were > 100% saturated solution. The article of Gaiser et al. (2009) does not contain a sufficient level of details. However, it also concludes on the absence of mortality under the tested conditions.

Based on the result of these studies, nanometric cerium dioxide is considered as not harmful to daphnids.

 

Long-term toxicity to aquatic invertebrates:

One study of reliability 2 according to Klimisch was available (Van Hoecke et al., 2009) and was selected as a key study.

The effect of cerium dioxide on the survival and reproduction of Daphnia magna was investigated over 21 days following the OECD Guideline 211. Daphnids were exposed to control, and test chemical at nominal concentrations of 18, 32, 56 and 100 mg/L. Parent mortality and offsprings numbers were recorded. Significant adverse effects on survival and reproduction were observed in the range 18 - 100 mg/L. However, due to the complexation of nutritive algal cells with nanoparticles, it cannot be excluded these effects were linked to food deprivation rather than impacts of nanoparticulate cerium dioxide. Several observations supported this hypothesis:

- The cerium dioxide nanoparticles aggregates clustered together with algal cells.

- The daphnids exposed to cerium dioxide nanoparticles were smaller than control individuals.

- Algae were absent from the gut of cerium dioxide nanoparticles-exposed organisms.

These observations suggest that decreased reproduction and eventual mortality was due to the inability to take up sufficient food. Such hypothesis is supported by the results of the algal cell density measurements which showed that when agglomerates of nanoparticulate cerium dioxide were mixed with the algal cells, both clustered together and sedimented.

 

Toxicity to aquatic algae:

One study was available (Van Hoecke et al., 2009). It was quoted as reliability 2 according to Klimisch and selected as a key study.

The influence of nanometric cerium dioxide on the growth of the green algal species Pseudokirchneriella subcapitata was investigated in a 72-hour test according to OECD Guideline 201. Algae were exposed to control and test chemical concentrations of 3.2, 5.6, 10, 18 and 32 mg/L. The cell density was measured daily and the average specific growth rate was calculated. Significant effects of nanometric cerium dioxide were observed, with 72-hour EC50 and -NOEC in the ranges 10 - 100 mg/L and 1 - 10 mg/L, respectively. To explain the observed toxicity, the authors tested several hypotheses: toxicity was due to: (1) an artefact of measurement, (2) adsorption to the algal cell walls, (3) dissolved cerium, (4) nutriment depletion, (5) shading. However, none of these hypotheses can be unambiguously considered as valid. Nevertheless, it should be kept in mind that clustering between algal cells and nanoparticles and indirect effect of phosphate depletion cannot be completely ruled out due to weight of evidence brought by other studies.

Based on the result of this study, nanometric cerium dioxide is considered as harmful to algae.

 

Toxicity to microorganisms:

One study was available (Park et al., 2007). It was quoted as reliability 1 according to Klimisch and selected as a key study.

The inhibitory effect of nanometric cerium dioxide on the respiration rate of aerobic wastewater microorganisms of activated sludge was investigated in a 3-hour respiration inhibition test according to OECD 209.

The 3-hour NOEC of cerium dioxide to activated sludge microorganisms was 1000 mg/L. The 3-hour EC20, EC50 and EC80 were clearly higher than 1000 mg/L.

Based on the result of these studies, nanometric cerium dioxide has no toxic effect on the respiration rate of activated sludge microorganisms.