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Environmental fate & pathways

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Additional information

Calcium fluoride is a simple inorganic salt. It will slowly dissociate in water at environmentally relevant pH, giving rise to the formation of calcium and fluoride ions.

The fate and behaviour of fluoride in the environment is discussed below. The information is primarily taken from the EU RAR for HF, EU RAR for CaF2, the Dutch ICD fluorides document (Sloofet al,1989) and the WHO Environmental Health Criteria n° 227, fluorides.

Fluoride is present in all environmental compartments. Sources of environmental fluoride are anthropogenic (industrial, application of phosphate fertiliser) as well as natural (volcanic, weathering, marine aerosols). The environmental behaviour of fluoride is essentially independent of source.

Environmental partitioning

Air

Fluorides in the atmosphere may be in gaseous or particulate form. Atmospheric fluorides can be transported over large distances or can be removed from the atmosphere via wet and dry deposition. Seasonal climatic conditions are expected to influence the rate at which and mode by which atmospheric fluorides are deposited. Fluoride compounds, with the exception of sulfur hexafluoride, are not expected to remain in the troposphere for long periods or to migrate to the stratosphere.

 

Water

In surface water at environmentally relevant pH, calcium fluoride will dissociate to a very limited extent (as a consequence of its low water solubility) to form calcium and fluoride ions.

The concentration of free fluoride ions is also strongly dependent on the presence of other inorganic mineral species. In the presence of phosphate and calcium, insoluble fluoride salts are formed, a large part of which are transferred to sediment. Under water conditions where phosphate and calcium levels are relatively high, there will be virtually no free fluoride in the water.

The transport and transformation of inorganic fluorides in water are influenced by pH, water hardness and the presence of ion-exchange materials such as clays. Fluoride is usually transported through the water cycle complexed with aluminum.

 

Sloof (1987) reports mean fluoride concentrations in the Netherlands of 0.2 -1.7 mg/l, with seasonal variations. In waters in the Dutch province Zeeland, concentrations vary between 1.0 and 9.5 mg/L. Background levels of fluoride of 4.7 mg/L are reported in the Black Forest and levels higher than 20 mg/L have also been reported in other European countries, in areas with fluoride-containing rocks. The background fluoride concentrations in surface water will depend on geological, physical and chemical characteristics. In seawater, fluoride is present as free fluoride (51%), magnesium fluoride (47%), calcium fluoride (2%) and traces of HF. Total fluoride concentrations in seawater are reported to be generally higher than those in freshwater, with an average concentration of 1.4 mg/L.

 

Sediment

The main form of fluorine in sediment is as insoluble fluorides that are formed when free, dissolved fluorides form complexes with calcium carbonates and phosphates that are present in the water phase. For example, insoluble fluorapatite (calcium fluorophosphate) and other insoluble complexes are formed locally and may accumulate as sediment.

Reported are values of up to 200 mg/kg for marine sediment and up to 450 mg/kg for river sediments on a dry matter basis.

Soil

In soil (pH<6), fluoride is predominantly found in as complexes such as fluorspar, cryolite and apatite and clay minerals.

At pH values of above 6, the fluoride ion is the dominant species. The fluoride ion has strong complexation properties and therefore upon increasing fluoride concentration there is also an increase in the Al and Fe concentrations in the soil. In addition, a positive correlation has been noted between the concentration of fluoride and that of organic carbon in the soil solution which may indicate that fluoride also forms complexes with carbon.

The binding of fluorides to soil material can take place by one of several mechanisms. Below pH 5.5, adsorption is low as fluoride exists as AlF complexes. At pH values of above 5.5, adsorption is lower due to the reduced electrostatic potential.  The adsorption of fluorine in soil can be described by a Freundlich isotherm, up to a concentration of 20 mg F/L in acidic soil and up to 10 mg F/L in alkaline soils.  At higher concentrations, precipitation tends to occur. Fluoride precipitates in the presence of excess calcium ions. As a result of this precipitation the concentration of free fluoride in calcareous soils is very low.

Fluoride is extremely immobile in the soil as a result of precipitation and adsorption.  Little leaching is observed; 5% leaching has been reported in soil with fluoride concentrations of up to 80 mg/dm3.  However, some leaching to the B-horizon is possible in soils with low clay content.

Fluoride concentrations in clay soil in the Netherlands are reported to range from 330 -660 mg/kg, with an average value of over 500 mg/kg. The concentration of total fluoride in Dutch agricultural soils is correlated with the clay content. Samples of greenhouse soil may have slightly higher fluoride contents as a result of the use of with fluorine-containing phosphate fertiliser. A correlation was also found between soil fluoride content and pH; as the pH increased, the concentration of soluble fluoride also increased.

 

Stability and biodegradation

Calcium fluoride is a simple inorganic salt and (due to its low water solubility) will slowly dissociate in water at environmentally relevant pH to form calcium and fluoride ions. Subsequently, the Ca2+ions will remain in the aqueous solution, whereas a (small) part of the F- ions will form HF due to the fact that the latter is a weak acid. This is the theoretical hydrolysis of the inorganic salt CaF2. The liberated ions will be indistinguishable from those naturally present in the environment and will be further transformed in water, sediment and soil to a variety of other calcium and fluorine-containing compounds.

Biodegradation is not a relevant concept for inorganic substances such as CaF2.

 

Bioaccumulation

The key study for the bioaccumulation endpoint is a 28 -days bioaccumulation study on the read-across source substance sodium fluoride (NaF) conducted according to OECD 305, a flow-through fish test. Cyprinus carpio was exposed to 5 and 0.5 mg NaF/L. The bioconcentration factors measured at steady state are ≤ 0.66 L/kg when exposed to 5 mg NaF/L and < 6.4 L/kg when exposed to 0.5 mg NaF/L.

Conversion of the BCF value to reflect the difference in molecular weight between sodium fluoride and calcium difluoride is not relevant, as the bioaccumulated species is fluoride, not calcium difluoride.

A number of additional bioaccumulation studies are mentioned in RIVM's Integrated criteria document on fluorides. Even though the original study reports are not available today, the information in the review report is considered relevant and adequate to serve as supporting information. The BCF values reported for fluoride in the publication range from <1 to 7.5 in aquatic plants and from 50 to 150 in fish and crustacea.

Overall, the available data indicate that fluoride has low BCF values, and is considered not bioaccumulative in accordance with the REACH criteria.

Via the EU RAR on HF and the Dutch ICD fluorides document (Sloofet al,1989), the following additional information is available on bioaccumulation in terrestrial species:

A correlation between fluoride levels in earthworms and elevated soil fluoride levels from polluted sites has been demonstrated, however levels were due to the soil content of the worm gut.

Elevated fluoride content in woodlice collected from the vicinity of an Al-reduction plant has been demonstrated (Janssen et al, 1989).

Sloof et al(1989) note that uptake of fluoride into plants from soil is low as a consequence of the low bioavailability of fluoride in the soil and that atmospheric uptake is generally the most important route of exposure. A relatively high rate of fluoride uptake is noted for grass species, and the consumption of fluoride containing plants may lead to elevated fluoride levels in animals and humans.

In the terrestrial environment, fluoride accumulates in the skeleton of vertebrates and invertebrates. Lowest fluoride levels are found in herbivores, with higher levels in omnivores and highest levels in predators, scavengers and pollinators; the findings indicate a moderate degree of biomagnification. Vertebrate species store most of the fluoride in the bones and (to a lesser extent) the teeth; elevated levels of fluoride in the bones and teeth have been shown in animals from polluted areas.