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Read-across concept (environment) for tin bis(tetrafluoroborate):

Tin bis(tetrafluoroborate) is an inorganic substance which will dissociate into tin and tetrafluoroborate ions upon dissolution in the environment (water solubility >50 % w/w).

 

In the environment, 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. Whereas tin(II) dominates in reduced (oxygen-poor) water and will readily precipitate as tin(II) sulfide or as tin(II) hydroxide in alkaline water, tin(IV) readily hydrolyses and can precipitate as tin(IV) hydroxide. In general, tin(IV) would be expected to be the only stable ionic species in the weathering cycle. Tin(II) can be hydrolysed into SnOH+, Sn(OH)2, and Sn(OH)3at low concentrations, and thisbehaviour can be described by the following equation:

 

SnX2+ H2O <--> "SnXOH"(s) + HX

 

This Sn2+specific behaviour may be a hindrance when conducting tests at low or very low tin concentrations.Since the Sn2(OH)22+and Sn(OH)42+polynuclear species predominate at higher concentrations, highly concentrated and acidified Sn2+solutions are stable and only tend to precipitate at a very low rate.

 

On release to estuaries, inorganic tin is principally converted to the insoluble hydroxide and is rapidly scavenged by particles, which are the largest sinkfor the metal. Subsequent release of inorganic tin from benthic sediments is unlikely, except at highly anoxic sites. Since the mobility of Sn is highly pH dependent, Sn2+is only present in acid and reducing environments. 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 as Sn(OH)4or SnO(OH)3-. Tin forms soluble and insoluble complexes with organic substances. Tin is generally regarded as being relatively immobile in the environment. Ambient levels of tin in the environment are generally quite 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 (summarised in WHO, 2005 and athttp://www.gtk.fi/publ/foregsatlas, accessed on 12.03.2013).

 

The environmental behaviour of the tetrafluoroborate anion is expected to be different, and in a conservative approach it is assumed that the tetrafluoroborate anion remains stable and mobile under environmental conditions.

 

Upon release to the environment and dissolution in aqueous media, tin bis(tetrafluoroborate) will dissociate and only be present in its dissociated form, i.e. as tin cation and tetrafluoroborate anion, andtoxicity (if any) will be driven by tin and the tetrafluoroborate anion. Therefore,data are read-across for the tin cation and for the tetrafluoroborate anionto assess theecotoxicity of tin bis(tetrafluoroborate).Read-across to other soluble tetrafluoroborates, i.e. potassium tetrafluoroborate (CAS# 14075-53-7) and sodium tetrafluoroborate (CAS# 13755-29-8), and soluble tin substances, including tin bis(methanesulfonate) (CAS# 53408-94-9) and tin dichloride (CAS# 7772-99-8) is fully justified.

Due to the salt-character and physico-chemical properties (negligible vapour pressure, very high solubility), the Henry constant is near to zero, and therefore the test item as well as its dissociation products are not volatile from aqueous solutions.

Tin in water may partition to soils and sediments. Cations such as Sn2+ and Sn4+ will generally be adsorbed by soils to some extent, which reduces their mobility. Tin is generally regarded as being relatively immobile in the environment (Gerritse et al. 1982; WHO 1980). However, tin may be transported in water if it partitions to suspended sediments (Cooney 1988), but the significance of this mechanism has not been studied in detail. Transfer coefficients for tin in a soil-plant system were reported to be 0.01–0.1 (Senesi et al. 1999).

A bioconcentration factor (BCF) relates the concentration of a chemical in plants and animals to the concentration of the chemical in the medium in which they live. It was estimated that the BCFs of inorganic tin were 100, 1,000, and 3,000 for marine and freshwater plants, invertebrates, and fish, respectively (Thompson et al. 1972). Marine algae can bioconcentrate tin(IV) ion by a factor of 1,900 (Seidel et al. 1980).