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For the assessment of the environmental fate and toxicity of tin difluorate and the corresponding derivation of PNEC values, the assessment entity approach is applied and data available for the assessment entities tin and fluoride are read-across.

Based on aquatic toxicity data of its moieties, i.e. tin and fluoride, tin difluoride appears to have a low potential for short-term toxicity to aquatic organisms.

Short-term toxicity data of tin are available for freshwater organisms covering three trophic levels. The respective reliable LC/EC50 values are unbound, i.e. a 50% inhibitory/toxic effect was not observed at the highest test concentration.

Based on the EU RAR "hydrogen fluoride" (2001), EC50-values of fluoride toxicity for algae range from 43 - 122 mg/L, for daphnids from 97 - 352 mg/L, and for fish from 51 - 340 mg/L.

Thus, short-term toxicity data covering three trophic levels are available for tin and fluoride and are summarized as follows:

Table: Short-term toxicity data of tin and fluoride for freshwater organisms

Trophic level

lowest EC value for tin

lowest EC value for fluoride

Algae

72-h ErC50

> 0.179 mg Sn/L

> 0.236 mg SnF2/L

96-h EbC50

43 mg F/L

177 mg SnF2/L

Daphnia

7-d EC50 (mobility)

> 38.4 mg Sn/L

> 50.7 mg SnF2/L

48-h EC50

97 mg/L

400 mg SnF2/L

Fish

96-h LC50

> 38.4 mg Sn/L

> 50.7 mg SnF2/L

96-h LC50

51 mg/L

210 mg SnF2/L

 

Based on aquatic toxicity data of its moieties, i.e. tin and fluoride, tin difluoride appears to have a medium potential for long-term toxicity to aquatic organisms.

Long-term toxicity data of tin are available for freshwater organisms covering three trophic levels. Based on the EU RAR "hydrogen fluoride" (2001), a long-term NOEC-value is available for fish; i.e. 21-d LC5 of 4 mg/L. For daphnia, NOEC-values of fluoride toxicity range from 3.7 to 14.1 mg/L and for algae from 50 - 249 mg/L.

Thus, long-term toxicity data covering three trophic levels are available for tin and fluoride and are summarized as follows:

Table: Long-term toxicity data of tin and fluoride for freshwater organisms

Trophic level

lowest EC value for tin

lowest EC value for fluoride

Algae

72-h ErC10

0.009 mg Sn/L (dissolved)

0.012 mg SnF2/L

7-d NOEbC

50 mg F/L

206 mg SnF2/L

Daphnia

21-d NOEC (reproduction)

4.8 mg Sn/L

6.3 mg SnF2/L

21-d NOEC (reproduction)

8.9 mg F/L (arithmetic mean)

37 mg SnF2/L

Fish

28-d NOEC (growth rstr)

0.30 mg Sn/L

0.40 mg SnF2/L

21-d LC5

4.0 mg F/L

16 mg SnF2/L

 

Additional information

Read-across justification

Tin difluoride is an inorganic solid at room temperature and consists of the tin cation and fluoride anions. Based on the solubility of tin difluoride in water (300-428 g/L according to handbook data (Merck, 2006; Gestis, 2015)), a complete dissociation of tin difluoride resulting in tin and fluoride ions may be assumed under environmental conditions. The respective dissociation is reversible and the ratio of the salt /dissociated ions is dependent on the metal-ligand dissociation constant of the salt, the composition of the solution and its pH. The metal-ligand equilibrium constant for the formation of tin difluoride is reported as follows (Japan Nuclear Cycle Development Institute, 1999):

Sn2++ 2F- <=>SnF20(log K =7.74)

Thus, it may reasonably be assumed that based on the tin-difluoride formation constant, the respective behaviour of the dissociated tin cations and fluoride anions in the environment determine the fate of tin difluoride upon dissolution with regard to (bio)degradation, bioaccumulation, partitioning resulting in a different relative distribution in environmental compartments (water, air, sediment and soil) and subsequently determine its ecotoxicological potential.

Therefore, in the assessment of the ecotoxicity of tin difluoride, read-across to data for fluoride and soluble tin substances is applied since only the ions of tin difluoride are available in an aqueous environment and determine the environmental fate and toxicity. Read-across to environmental fate and toxicity studies of soluble fluoride salts (predominantly sodium fluoride) is therefore appropriate and scientifically justified. This read-across approach was already applied in the 2001 EU Risk Assessment of hydrogen fluoride.Read-across to environmental fate and toxicity studies of soluble tin salts, including tin dichloride and tin methane sulfonic acid, is further appropriate and scientifically justified.