Potassium trifluorozincate

Adsorption / desorption

Description of key information

Adsorption of potassium trifluorozincate to soil is not to be expected as the substance instantly dissociates into various ions. The following information is available on bioaccumulation of zinc and fluoride ions. For zinc a Kp value of 110000 L/kg is determined for suspended matter-water, a Kp value of 73000 L/kg for sediment-water and a Kp value of 158.5 L/kg for soil-water. 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). 

Key value for chemical safety assessment

Other adsorption coefficients

Type:

other: Kp (suspended matter-water) for zinc

Value in L/kg:

110 000

Other adsorption coefficients

Type:

other: Kp (sediment-water) for zinc

Value in L/kg:

73 000

Other adsorption coefficients

Type:

other: Kp (soil-water) for zinc

Value in L/kg:

158.5

Other adsorption coefficients

Type:

other: Koc for fluoride

Value in L/kg:

3.16

Additional information

Adsorption of potassium trifluorozincate 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 zinc and fluoride ions:

Zinc:

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. Partition coefficients for zinc in freshwater has been reviewed in the EU risk assessment report on zinc (ECB 2010). Based on the extensive experimental evidence, a partition coefficient for the distribution between solid particulate matter and water (Kpsusp) of 110.000 L/kg (log value 5.04) has been defined for EU waters and is used throughout the RAR.

The Kp for the distribution between sediment and water (Kpsed) was estimated in the RAR from that for particulate matter, as follows: Kpsed = Kpsusp/ 1.5, based on the average difference in concentrations of zinc and other metals in both media. For zinc this results in a Kpsed of 73,000 L/kg (ECB 2010). For soil, a solids-water partitioning coefficient of 158.5 L/kg (log value 2.2) was determined experimentally on 11 American soils. This value is based on the part of the metals that can actually exchange and may therefore be assumed to be in equilibrium with the water phase. This value was used in the EU-RAR for Zinc. A marine Kd was derived based on data from several marine waters. The geomean value for zinc in seawater is reported to be 6010 L/kg.

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.