Tin difluoride

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

no hazard identified

Marine water

Hazard assessment conclusion:

no hazard identified

STP

Hazard assessment conclusion:

no hazard identified

Sediment (freshwater)

Hazard assessment conclusion:

no hazard identified

Sediment (marine water)

Hazard assessment conclusion:

no hazard identified

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

no hazard identified

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

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.

Fluoride:

Read-across to environmental fate and toxicity studies of soluble fluoride salts (predominantly sodium fluoride) is appropriate and scientifically justified. This read-across approach was already applied in the 2001 EU Risk Assessment of hydrogen fluoride.

In solution, fluoride ions form strong complexes with other ions, particularly Ca²⁺, Al³⁺, Fe³⁺, PO₄^3-and B(OH)₄^-. The concentration of fluoride ions in solution is often controlled by the solubility of fluorite; and the concentration inversely proportional to that of Ca²⁺(Salminen et al. 2005 and references therein).

Tin(II):

Read-across to environmental fate and toxicity studies of soluble tin salts, including tin dichloride and tin methane sulfonic acid, is appropriate and scientifically justified

 

Tin is typically regarded as being relatively immobile in the environment. Thus, tin is likely to partition to soils and sediments. In water, inorganic tin may exist as either divalent (Sn2+) or tetravalent (Sn4+) cations under environmental conditions. Dissolved Sn²⁺, a strong reducing agent, is only present in acid and reducing environments and will readily precipitate as tin(II) sulfide or as tin(II) hydroxide in alkaline water. Tin(II) forms SnOH⁺, Sn(OH)₂, and Sn(OH)₃⁻at low concentrations whereas Sn₂(OH)₂²⁺and Sn(OH)₄²⁺polynuclear species predominate at higher concentrations (CICAD 65 - WHO 2005 and references therein). This Sn²⁺- specific behaviour may be a hindrance when conducting tests at low or very low tin concentrations since only highly concentrated and acidified Sn²⁺solutions are stable and tend to precipitate at a low rate. Further, speciation under environmental conditions favours tin oxide compounds, which have low toxicity in organisms largely due to their low solubility, poor absorption, low accumulation in tissues, and rapid excretion.

On release to estuaries, inorganic tin is principally converted to the insoluble hydroxide and is rapidly scavenged by particles, which are the largest sink for the metal. Subsequent release of inorganic tin from benthic sediments is unlikely, except at highly anoxic sites. Weathering of most natural and anthropogenic Sn carriers is intensified under acid, reducing conditions, although SnS₂is insoluble under reducing conditions. In stream sediment, most detrital Sn is held in resistant oxide phases, such as cassiterite, which release Sn very slowly during weathering. Any Sn2+ released oxidises rapidly and is subsequently bound to secondary oxides of Fe or Al such as Sn(OH)₄. Tin forms soluble and insoluble complexes with organic substances. Ambient levels of tin in the environment are typically low. Tin occurs in trace amounts in natural waters, i.e. average concentrations in stream water are assumed to be less than 0.01μg/L (WHO, 2005 and Salminen et al. 2005 and references therein).

In sum, upon release to the environment and dissolution in aqueous media, tin difluoride will dissociate and only be present in its dissociated form, i.e. as tin cation and fluoride anion, and toxicity (if any) will be driven by tin and the fluoride anion. Therefore, data are read-across for the tin cation and for the fluoride anion to assess the ecotoxicity of tin difluoride. Read-across to other soluble fluorides, i.e. potassium fluoride and sodium fluoride, and soluble tin(II) substances, including tin dichloride is fully justified.

Conclusion on classification

For the assessment of the environmental hazard potential of tin difluorate, the assessment entity approach is applied and data for fluoride and soluble tin substances are read-across since only the ions of tin difluoride are available in an aqueous environment and determine the toxicity.

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.

The lowest reported EC50 values are applied in the hazard assessment. 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

 

Aquatic toxicity data of tin and fluoride when expressed as tin difluoride are available for algae, daphnia and fish; respective EC/LC50 values are well above 1 mg/L. Therefore, tin difluoride does not meet classification criteria as short-term hazard to the aquatic environment under Regulation (EC) No 1272/2008 and subsequent adaptations.

 

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. Tin(II) forms SnOH+, Sn(OH)2, and Sn(OH)3−at low concentrations whereas Sn2(OH)22+and Sn(OH)42+polynuclear species predominate at higher concentrations (CICAD 65 - WHO 2005 and references therein). This Sn2+- specific behaviour may be a hindrance when conducting tests at low or very low tin concentrationssince only highly concentrated and acidified Sn2+ solutions are stable and tend to precipitate at a low rate. In the algae toxicity test by Wenzel (2006), dissolved tin concentrations were more than 5-times lower than nominal concentrations and decreased with time due to precipitation. In accordance with the ECHA Guidance on the Application of the CLP Criteria (Version 4.1 – June 2015), “where the algal toxicity ErC50 [ = EC50 (growth rate)] falls more than 100 times below the next most sensitive species and results in a classification based solely on this effect, consideration should be given to whether this toxicity is representative of the toxicity to aquatic plants. Where it can be shown that this is not the case, professional judgment should be used in deciding if classification should be applied”. Since we cannot exclude that essential nutrients co-precipitated with tin and were thus rendered less bioavailable for algae, the effect concentration of algae should be considered with caution. Therefore, the 28-d NOEC for the growth rate of fish, the second lowest long-term endpoint, is applied in the hazard assessment.

 

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.The lowest reported NOEC values are applied in the hazard assessment.

 

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) 3.7 mg F/L 15 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

 

Aquatic long-term toxicity data of tin and fluoride when expressed as tin difluoride are available for algae, daphnia and fish; the lowest (applicable) NOEC/EC10 value corresponds to 0.40 mg SnF₂/L and is thus below 1 mg/L. Based on the classification categories for environmental hazards in Table 4.10. (b) (i) of Regulation (EC) No 1272/2008 and subsequent adaptations, tin difluoride meets classification criteria as long-term hazard to the aquatic environment Category Chronic 2 (H411).