Fluorine

Adsorption / desorption

Description of key information

For fluoride a Koc of 3.16 L/kg is determined

Key value for chemical safety assessment

Other adsorption coefficients

Type:

other: Koc for fluoride

Value in L/kg:

3.16

Additional information

No studies on the transport and distribution of fluorine are available or can be performed as in contact with water fluorine reacts instantly and violently under formation of hydrogen fluoride and oxygen. Hydrogen fluoride will further react to fluoride. Therefore available data from studies with fluoride are given as indication of the sorption properties of fluorine.

For the sorption characteristics of fluoride only qualitative data are available. 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.