Lanthanum(3+) acetate

Toxicity to aquatic algae and cyanobacteria

Description of key information

The key study (Hefner, 2014c) yielded a 72-h NOEC, EC10 and EC50 value (growth rate-based) of 0.2, 0.46 and 0.79 mg La/L for the unicellular green alga Pseudokirchneriella subcapitata (corresponding to 0.45, 1 and 1.8 mg anhydrous lanthanum acetate/L, respectively). However, these values will not be taken forward to PNEC derivation and classification because the effects on growth were observed to be concurrent with phosphate depletion in the test medium due to lanthanum complexing, suggesting that the observed effect on growth is due to phosphate deprivation rather than direct toxicity of the rare earth. This is confirmed by modelling calculations using Visual Minteq v3.0. Further testing is not considered useful because the technical issue of phosphate depletion cannot be overcome (phosphate dosing during the test would result in 100% lanthanum depletion from the test medium).

Additional information

For toxicity to aquatic algae and cyanobacteria, two studies were included in this dossier.

The study of Bringmann and Kühn (1959) reported that the lowest concentration at which lanthanum acetate exerts an adverse effect on biomass (growth) of Scenedesmus algae is 0.15 mg La/L. Results are not considered reliable as lanthanum nor the lanthanum test substance was analytically determined in the test media and it is not clear to what extent the observed effects may have been due to phosphate depletion and deprivation.

The key study (Hefner, 2014c) yielded a 72-h NOEC, EC10 and EC50 value (growth rate-based) of 0.2, 0.46 and 0.79 mg La/L for the unicellular green alga Pseudokirchneriella subcapitata (corresponding to 0.45, 1 and 1.8 mg/L anhydrous lanthanum triacetate, respectively). Due to the known issue with phosphate complexing by rare earth elements in algal growth inhibition tests, phosphate concentrations were monitored during this study. It was observed that at the lower test concentrations, all lanthanum was precipitated shortly after addition to the test medium, because phosphate was in excess. However, at the higher test concentrations, lanthanum was in excess, complexing all phosphate in the test medium. Algal growth was completely impeded at those test concentrations where all phosphate had disappeared from the test medium from the start of the test already. The ErC50 value (0.79 mg La/L) was therefore somewhat lower than the lowest test concentration at which complete phosphate depletion occurred. Therefore, the observed effects are considered to be mostly due to phosphate deprivation instead of direct lanthanum toxicity. Since all algal growth inhibition tests need to be performed in test media containing a phosphate source, testing is considered technically not feasible if reliable results on lanthanum toxicity to algae are to be obtained. Further, the phosphate depletion effect is not considered an environmentally relevant effects, since it would only occur very locally where point source emissions or accidental releases occur, and will never affect an entire ecosystem. Therefore, the effects on algal growth are not taken into account for PNEC derivation and classification.

To further argument the conclusion that the observed toxicity is due to disappearance of phosphate from the test medium as a result of complexing with the rare earth, modelling calculations have been performed using Visual MINTEQ v3.0 using data from the algal growth inhibition study of Hefner (2014c). Modelling of lanthanum speciation was performed as follows:

- All components of the test medium (nominal concentrations) were added to the modelling solution.

- The pH was set to 7.0 (although pH of test solutions was adjusted to 6.5, at the start of testing pH was around 7.0 (or even more) in most treatments).

- Ionic strength was not set to a fixed value, but the model was allowed to calculate it (default).

- Temperature was set to 21°C (average temperature during the test).

- Five modelling problems were added, using set total lanthanum levels (i.e., the measured dissolved La levels at test initiation: 0.2, 0.45, 1.0, 2.1 and 4.4 mg La/L

- The following aqueous La species were modelled by Visual MINTEQ: La3+, La(CH3COO)2+, La(CH3COO)3, La(CH3COO)+2, La(CO3)2-, LaCO3+, LaHCO3+2, La(SO4)2-, LaSO4+, LaCl+2, LaEDTA-, LaHEDTA, LaNO3+2, LaOH+2 and LaH2PO4+2.

- Three possible solid phases were added for La: La(OH)3, La2(CO3)3 and LaPO4. When solubility products are exceeded in the aqueous solution, the model allows precipitation of these phases. Note that the nominally added total phosphate (PO4 3-, total) concentration is 7.67E-06 M, hence no more LaPO4 than that can be formed.

- Under the abovementioned conditions, the model calculations for dissolved versus precipitated La and phosphate can be summarised as follows (measured dissolved La at the end of testing was added for comparison):

La total (initial measured La dissolved) (mg/L)	La dissolved (model calculation) (mg/L)	% La dissolved (model calculation)	La dissolved (measured after 72 h) (mg/L)	% PO4 3- precipitated (model calculation)
0.2	0.00	0.00	0.00369	18.77
0.45	0.00	0.00	0.00212	42.23
1	0.00	0.00	0.0304	93.85
2.1	0.156	7.43	0.0521	100
4.4	0.186	4.23	0.433	100

- La(OH)3 was not calculated to precipitate under the set conditions of testing, only LaPO4 and La2(CO3)3 precipitation occurred. At the three lowest test concentrations, all lanthanum was predicted to be precipitated as LaPO4, whereas no La2(CO3)3 precipitation was predicted. In the media with initially measured concentrations of 2.1 and 4.4 mg La/L, lanthanum was in excess of the present phosphate and therefore all phosphate was precipitated, and the remaining lanthanum in solution was partly precipitated as La2(CO3)3 (respectively 42 and 72% of the initially measured dissolved lanthanum was precipitated as lanthanum carbonate).

- When checking phosphate speciation, it became clear that phosphate precipitation was practically entirely due to precipitation with lanthanum.

- Based on this modelling exercise it is confirmed that under the conditions of the test all lanthanum is precipitated as LaPO4 whenever phosphate is in excess and vice versa. The 72-h EC50 being 0.79 mg La/L (growth rate-based), and phosphate being non-detectable at the start of testing in the treatment with initially measured concentration of 1 mg La/L (as well as in all treatments with higher La concentrations), it is clear that phosphate depletion and growth inhibition are concurrent and hence it can be concluded that the observed effect on growth is rather due to phosphate deprivation than due to a direct toxic effect of the rare earth itself.