Cryolite, tripotassium

Adsorption / desorption

Description of key information

Adsorption of potassium cryolite to soil is not to be expected as the substance instantly dissociates into various ions. The following information is available on the sorption behaviour of potassium, fluoride and aluminium ions. For fluoride a Koc value of 3.16 is calculated based on a log Kow of -1 in EUSES (in the EU-RAR for hydrogen fluoride a log Kow of -1.4 is suggested). From the data available for aluminium and potassium, no actual Kd and/or Koc values can be determined.

Additional information

Adsorption and precipitation properties of potassium cryolite in dissolved form depend on the species formed. Therefore the properties are very dependent on i.a., pH, redox and contents of ions in the matrix. Due to the absence of data on potassium cryolite, data compilations of other potassium, fluoride and aluminium species have been reviewed.

Potassium

Potassium is usually the most abundant of the major nutrient elements for plants and animals in soil (the total potassium content of soils are reported to vary from <0.01% to about 4% with a typical content of about 1%. Total potassium content in sediments are about 2%). Therefore possible potassium emissions originating from uses of potassium cryolite are unlikely to contribute significantly to background concentrations in pore water or on the soil solid phase.

Fluoride

For the sorption characteristics of fluoride only qualitative data are available from the EU-RAR for hydrogen fluoride (ECB, 2001). Fluoride in soil is mainly bound in complexes with aluminium, iron or calcium dependent on the pH and the availability of these counter ions. Fluoride binds to clay by displacing hydroxide from the surface of the clay. The adsorption follows Langmuir adsorption equations and is strongly dependent upon pH and fluoride concentration. It is most significant at pH 3–4, and it decreases above pH 6.5. Low affinity of fluorides for organic material results in leaching from the more acidic surface horizon and increased retention by clay minerals and silts in the more alkaline, deeper horizons. Increased amounts of fluoride are released from fluoride salts and fluoride-rich wastes in soils with high cation exchange capacity. This effect is greatest when there were more exchange sites available and when the fluoride compound cation had greater affinity for the exchange material. Fluoride is also shown to be extremely immobile in soil as determined by lysimeter experiments: 75.8–99.6% of added fluoride was retained by loam soil for 4 years and was correlated with the soil aluminium oxides/hydroxides content. Soil phosphate levels may also contribute to the mobility of inorganic fluoride. In sandy acidic soils, fluoride tends to be present in water-soluble forms.

From the data available for fluoride no actual Kd and/or Koc values can be determined. At neutral pH the major part of fluoride retention in soil appears to be a result of formation of complexes. True adsorption of fluoride and consequential formation of equilibrium between soil/sediment and porewater is not expected based on the anionic character of fluoride. Therefore, fluoride is assumed to have low solids-water partitioning coefficients in the different environmental compartments. For pragmatic reasons, for environmental exposure assessment a Koc is calculated based on a log Kow of -1 in EUSES (in the EU-RAR for hydrogen fluoride a log Kow of -1.4 is suggested). When using the QSAR for non-hydrophobics, a Koc of 3.16 is determined.

Aluminium

Aluminium is a very common element in the natural environment, and its content in the earth's crust amounts to about 8%, which makes it the third most abundant element after oxygen and silicon. Aluminium is one of the most abundant elements in soil and concentrations vary widely. A range of 700 to 100 000 mg/kg was quoted by the U.S. Geological Survey. Therefore, potentially natural processes far outweigh the contribution of anthropogenic sources, with regard to overall environmental exposure. 

From the data available for aluminium, no actual Kd and/or Koc values can be determined. The adsorption – desorption process of aluminium has been reviewed by EPRI (1984). The activity of aluminium in soil, sediment and ground waters depends upon its chemistry and the characteristics of the local environmental system. At a pH greater than 5.5, aluminium compounds exist predominantly in an undissolved form, the exception to this is the presence of high amounts of dissolved organic material or humic acid, which can bind with aluminium and cause increased aluminium concentrations in streams and lakes.

References:

- ECB (2001). European Union Risk Assessment Report; Hydrogen Fluoride. European Commission Joint Research Centre, Institute for Health and Consumer Protection. Existing Substances 1st priority list, Volume: 8. Existing Substances. EUR 19729. ISBN 92-894-0485-X.

- EPRI (1984). Chemical attenuation rates, coefficients and constants in leachate migration, vol.1: A critical review. Electric power research institute EPRI EA-3356.