Calcium fluoride

Endpoint summary

Description of key 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 and 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.