Iodine pentafluoride

Adsorption / desorption

Description of key information

Koc values for iodate and iodide are determined at 377 and 74 L/kg, respectively. For fluoride a Koc of 3.16 L/kg is determined

Key value for chemical safety assessment

Other adsorption coefficients

Type:

other: Koc for iodate

Value in L/kg:

377

Other adsorption coefficients

Type:

other: Koc for iodide

Value in L/kg:

74

Other adsorption coefficients

Type:

other: Koc for fluoride

Value in L/kg:

3.16

Additional information

No studies on the transport and distribution of iodine pentafluoride are available or can be performed as in contact with water iodine pentafluoride reacts instantly and violently under formation of hydrogen fluoride and iodate. Hydrogen fluoride will further react to fluoride and iodate in water forms an equilibrium with iodide. Therefore available data from studies with fluoride, iodate and iodide are given as indication of the sorption properties of iodine pentafluoride.

Iodate/iodide

A series of experiments with iodate and iodide were performed in a variety of soils with distinct properties as to pH, cation exchange capacity (CEC), soil organic matter (SOM) content and ferric/aluminium oxides content (Dai et al., 2004, 2009). It was found that iodate and iodide adsorption isotherms could be well fitted with both the Langmuir and the Freundlich equations. Soils rich in organic matter and with high cation exchange capacity had high affinities for iodide. The adsorption capacity of iodate is greater than that of iodide, and iodate is more readily sorbed by soils than iodide. Furthermore, soil organic matter has significantly negative effects on iodate adsorption in soils, but it has significantly positive effects on iodide adsorption. Therefore organic matter in soil environments plays an important role in controlling iodine availability and iodide is easily retained in soil by the application of organic matter. From the available data sorption distribution coefficients (Kd) were determined. Kd values ranged from 0.2 to 1.97 L/kg for iodide and from 0.85 to 53.75 L/kg for iodate.

For purpose of risk assessment, Koc values for iodide and iodate are calculated from the data reported by Dai et al.. First, fractions organic carbon were calculated for the different soils using the following equation: % organic carbon = % organic matter/ 1.724. From these fractions organic carbon, Kd values per soil were recalculated to Koc values, subsequently to log Koc values and finally the geometric mean of these log Koc values was calculated. The geomean log Koc was converted back to a (non-logarithmic) Koc expressed as L/kg. In this manner, for iodide and iodate geometric mean Koc values of 74 and 377 L/kg are determined.

Fluoride

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.