Aluminate(1-), tetrafluoro-, cesium (1:1), (T-4)-

Adsorption / desorption

Description of key information

Adsorption of cesium tetrafluoroaluminate 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 cesium, fluoride and aluminium ions. For cesium the lowest Kd value was 0.6 L/kg for a measurement made on a system containing a soil consisting primarily of quartz, kaolinite, and dolomite and an aqueous phase consisting of groundwater with a relatively high ionic strength (I. 0.1 M). The largest cesium Kd values was 52,000 L/kg for a measurement made on a pure vermiculite solid phase. The average cesium Kd value was 2635 ± 530 L/kg. 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, no actual Kd and/or Koc values can be determined.

Key value for chemical safety assessment

Other adsorption coefficients

Type:

other: Kd (soil-water) for cesium

Value in L/kg:

2 635

Other adsorption coefficients

Type:

other: Koc for fluoride

Value in L/kg:

3.16

Additional information

Adsorption of cesium tetrafluoroaluminate to soil is not to be expected as the substance instantly dissociates into various ions at environmental relevant pH. The following information is available on the sorption behaviour of cesium, fluoride and aluminium ions:

Cesium:

For metals, adsorption/desorption translates in the distribution of the metals between the different fractions of the environmental compartment, e. g. water (dissolved fraction, fraction bound to suspended matter) or soil (fraction bound or complexed to the soil particles, fraction in the soil pore water). This distribution between the different compartments is translated in the partition coefficients between these different fractions. Cesium exists in soil solution as hydrated ions. The adsorption of cesium is concentration dependent and the value of Kd (soil/water adsorption coefficient) decreases with an increase in the cesium ion concentration. At trace concentrations the Cs+ adsorption is significantly influenced by soil pH due to competitive exchange reactions. The mineral composition of the soil also affects the mobility of cesium. It has been reported that certain clay minerals (e.g. micas and hydrobiotite) adsorb Cs+ irreversibly, whilst others (e.g. vermiculite and montmorillite) hold it much less strongly. It seems also that the presence of soil organic matter decreases the adsorption of Cs+, thus making it more bioavailable. Cesium adsorption has been determined to be maximal at pH 7-8. Cesium does not form any complexes or precipitates or colloids.

Partition coefficients for Cesium in soil have been reviewed in by the US EPA (US EPA 1999). Cesium Kd values and some important ancillary parameters that influence cation exchange were collected from the literature and tabulated. The lowest cesium Kd value was 0.6 ml/g for a measurement made on a system containing a soil consisting primarily of quartz, kaolinite, and dolomite and an aqueous phase consisting of groundwater with a relatively high ionic strength (I. 0.1 M). The largest cesium Kd value was 52,000 ml/g for a measurement made on a pure vermiculite solid phase. The average cesium Kd value was 2635 ± 530 ml/g.

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.

- US EPA (1999). Understanding Variation in Partition Coefficient, Kd, values. Volume II:Review of Geochemistry and Available Kd Values for Cadmium, Cesium, Chromium, Lead, Plutonium, Radon, Strontium, Thorium, Tritium (3H), and Uranium.EPA 402-R-99-004B.