Tin difluoride

Endpoint summary

Description of key information

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

Abiotic degradation: Physico-chemical processes other than dissolution reactions are not considered relevant for tin difluoride since the chemical safety assessment of inorganic substances is typically based on total dissolved elemental concentrations without considering the (pH-dependent) speciation in the environment. Thus, (abiotic) degradation is not a relevant process for tin and fluoride and the chemical safety assessment is based on total elemental concentrations.

Biodegradation: For an inorganic substance such as tin difluoride for which the chemical assessment is based on the elemental concentration (i.e., pooling all inorganic speciation forms together), biotic degradation is an irrelevant process: biotic processes may alter the speciation form of an element, but it will not eliminate the element from the terrestrial compartment by degradation or transformation. This elemental-based assessment (pooling all speciation forms together) can be considered as a worst-case assumption for the chemical assessment.

Transport and distribution: Transport and transformation of fluoride in soil are influenced by pH and the formation of predominantly aluminium and calcium complexes. Adsorption to the soil solid phase is stronger at slightly acidic pH values (5.5–6.5). Fluoride is not readily leached from soils. From the information available, Tin is generally regarded as being relatively immobile in the environment and it appears likely that inorganic tin will partition to soils and sediments.

Bioaccumulation: Based on bioaccumulation data of its moieties, i.e. tin and fluoride, tin difluoride appears to have a low potential for bioaccumulation to aquatic organisms. Whereas bioaccumulation of tin in soil organisms seems to be low, data are not available for fluoride. However, based on the low aquatic bioaccumulation potential, a similar low potential may be assumed for terrestrial organisms.

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 environmental fate and toxicity 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.