Reaction mass of Limestone and calcium fluoride and calcium sulfate

Endpoint summary

Description of key information

As the substance is inorganic, no biodegradation studies are required. Based on knowledge of each constituent, the substance is expected to have a low potential of adsorption and a low potential of bioaccumulation.

Additional information

Overview

The substance is a reaction mass of calcium fluoride and calcium sulfate and calcium carbonate. The main constituent is calcium fluoride (47.5%), then calcium sulfate (22.3%) and calcium carbonate (12.1%). Sources of environmental fluoride are anthropogenic (industrial, application of phosphate fertiliser) and natural (volcanic, weathering, marine aerosols). The environmental behaviour of fluoride is essentially independent of source. Fluorides are removed rapidly from the environment by wet and dry deposition; wet and dry deposition rates for total fluorides in the Netherlands are reported to be ~30 mg/m2 and ~17 mg/m2, respectively. Calcium sulfate and calcium carbonate are dissolved in calcium, sulfate and carbonate ions in the environment : they are ubiquitous ions and they are essential to all living organisms.

 

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 and will not be subject to biodegradation. 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. 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 Zeelans, 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.

In the environment, the substance will undergo dissociation into calcium, fluoride, sulfate and carbonate ions; they are all ubiquitous in the environment. The ions will dissociate; Calcium fluoride will hydrolise under a limited extent due to its low solubility (15 mg/L). Calcium will be assimilated by species in the water and is necessary to maintain a good chemical balance in soils, water and plants. Sulfur will become part of the sulfur cycle or be assimilated by microorganisms and other species that require sulfate as an essential substance in their biological systems/ processes; and carbonate will become part of the carbon cycle.  

 

Sediment

The main form of fluorine in sediment is as insoluble complexes.  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. Information gathered on the behaviour of fluoride ions in water indicate that insoluble fluorapatite and other insoluble complexes are formed locally, which may accumulate as sediment. 

Studies in soil and sediment can not be performed due to the absence of an analytical test method that could distinguish between contributions to the analysed solution calcium concentration originating from the test material and that originating from the required soil/ sediment/ solution matrix. Even if the method guidelines were amended/deviated from and pure water was used as the aqueous phase for the soil/ sediment/ solution systems, a significant contribution of dissolved calcium would be expected in all samples due to the presence of the substance and other calcium salts as a naturally occurring mineral fraction in soils and sediments. Therefore, it would not be possible to distinguish between contributions to the analysed solution calcium concentration originating from the test material and that originating from the soils or sediments themselves.

Calcium fluoride and calcium carbonate present limited solubility in water and solubility characteristics are known to be sensitive to both solution pH and also the presence/partial pressure of carbon dioxide. Therefore, changes with respect to aqueous phase pH and dissolved carbon dioxide concentrations on exposure to soils and sediments may lead to shifts in the relative solubility of the test material and potential precipitation and sedimentation on centrifugation of the samples; a process which can not be separated analytically from any true adsorption onto the soil or sediment phases.

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. Once released into the environment, calcium fluoride will ionise to a limited extent to form calcium and fluoride ions which will combine with various minerals to form a variety of other fluoride compounds with a limited mobility

The accumulation and mobility of calcium sulfate in soils has been documented in the available literature. Calcium sulfate has a low potential for adsorption to soil. The sulfate ion is sufficiently mobile to penetrate subsoil readily. Calcium accompanies sulfate into the subsoil (Shainberg et al. 1989). The rate at which soluble sulfates are gradually leached away is dependent upon the water supply. Calcium sulfate also accumulates in humid regions, the upper layers of soil and rock are kept thoroughly leached, and as fast as they are formed the soluble products are removed in the drainage water. In semi-arid regions, the soils are not fully leached and soluble salts tend to accumulate. The organic carbon content of the soils is not anticipated to play a significant role in the mobility of simple inorganic salts such as calcium sulfate and therefore the actual test endpoint, that of an organic carbon normalised adsorption coefficient (Koc) is probably not actually valid/relevant for this type of substance. For the calcium content at least, pH, water potential and carbon dioxide partial pressure amongst others will be controlling factors.

The accumulation and mobility of calcium carbonate in soils has been documented in soil chemistry as the process of “calcification”. Calcification is the general process by which naturally occurring calcium carbonate or the product of dissolved calcium ions and either bicarbonate or carbonate ions (from dissolved carbon dioxide, the dominant form of the dissolved product being dependant on the solution pH) accumulates in soils. Most commonly, calcium carbonate accumulates in subsurface horizons of soils in subhumid, semiarid, or arid regions. Calcium carbonate has a propensity to leach through soil if water is applied, i.e. it does have some mobility through soil, providing sufficient water is present. As it moves downwards into layers where the water content is low, the leaching will stop. On this basis, calcium carbonate does not have a high potential for adsorption to soil. Likewise, the potential for adsorption of calcium carbonate to sediment is also expected to be low.

 

Biodegradation

The substance is a reaction mass of calcium fluoride, calcium sulfate and calcium carbonate : it is an inorganic substance and therefore does not undergo hydrolysis or biodegradation. In the environment, the susbtance will dissociate into calcium, fluoride, sulfate and carbonate ions. These ions are naturally ubiquitous in the environment; fluoride ions which will combine with various minerals to form a variety of other fluoride compounds, calcium will be assimilated by species present in the water, soil or sediment and is necessary to maintain a good chemical balance in the environment, sulfate and carbonate will become part of the sulfur and carbon cycle.

Accumulation

The main constituent of the substance is the calcium fluoride : 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. Sloof et al(1989) conclude that the limited data indicate that fluoride biomagnification in the aquatic environment is of little significance. Fluoride accumulates in aquatic organisms predominantly in the exoskeleton of crustacea and in the skeleton of fish; no accumulation was reported for edible tissues. 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. The bioaccumulation potential of fluoride from the substance will therefore be limited by its low water solubility and its tendency to adsorb to sediment.

Regarding the 2 others constituents of the substance, calcium sulfate and calcium carbonate, calcium, sulfate and carbonate ions are essential to all living organisms (flora and fauna) and their intracellular and extra-cellular concentrations are actively regulated. Therefore, bioaccumulation is not expected.

References:

EU RAR for HF

Dutch ICD fluorides document (Sloof et al, 1989).

Shainberg I, Sumner ME, Miller WP, Farina MPW, Pavan MA and Fey MV (1989) Use of gypsum on soils. Advanced Soil Sci., 9: 1-111