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EC number: 701-200-7
CAS number: -
As cryolite dissociates in water and the risks are assumed to be determined by fluoride, it is more appropriate to assess the adsorption behaviour of fluoride. In a column leaching test, Koc values varying from 603 - 6502 were found for fluoride in 4 different soils, tested at cryolite concentrations varying from 5 - 50 ppm (Dykeman, 1985). As the percentage of fluoride in the molar weight in both substances is practically identical, similar Koc values can be expected for reaction mass of potassium aluminium tetrafluoride and tripotassium hexafluoroaluminate. The geometric mean of 1498 (log Koc 3.18) will be used in the assessment.
As no information is available on
adsorption/desorption of reaction mass of potassium aluminium tetrafluoride
and tripotassium hexafluoroaluminate, data from its structural analogue
cryolite is taken into account. In a column leaching study using four
different soils with cryolite at an equivalent application rate of 16
lb/acre, fluoride (the only species monitored) showed little mobility. A
fluoride ion specific electrode was used for quantitation. Background
fluoride concentrations from the control soils, which varied from about
2 to 14 ppm, were subtracted from the treated soils. Most fluoride
remained within the top 24 cm of the columns. Some extraneous leaching
did occur to a maximum depth interval of 36-42 cm, but was probably an
artefact of method limitations and/or natural soil variation. No
fluoride was detected in the leachate of the 42 cm columns. For
comparison, sodium fluoride, which was run through equivalent soil
columns at equivalent fluoride concentration, showed virtually the same
leaching profile for fluoride as cryolite (Dykeman, 1985). Koc values
are given in the Table below.
Table: Measured Kd values and calculated approximate Koc value
based on the assumption that % organic carbon ~ % organic matter / 1.724
% organic matter (measured)
% organic carbon (calculated)
Measured Kd range
Calculated Koc range
1.4 – 6.6
Ramona Sandy Loam
8.1 – 15.1
Aguila Clay Loam
973 – 1.318
Calhoun Silt Loam
19.3 – 52.8
2,377 – 6,502
The data show two striking features.
First, there are large soil-dependent differences in sorption that were
mentioned before, with "apparent" simple Kd values ranging from 1 to 53
in standard units (calculated range of apparent Koc values is from
approximately 600 to 6500, see Table). Second, there is a pronounced,
regular concentration-dependent spread of Kd values within three of the
four individual soils. For these three soils the higher the
concentration, the lower the Kd. For example, in sand with 5 ppm of
cryolite, the Kd is 6.6, but at 50 ppm of cryolite, the Kd is only 1.4.
These features prompted the U.S. EPA
to conduct a Freundlich analysis using the reported data. Results
yielded exponents (1/n values) of approximately 1/2 for three soils
(exponents of 0.56, 0.49, and 0.69 for sand, sandy loam, and silt loam
soils, respectively) (U.S. EPA, 1996). The exponent of about 1/2, the
seeming approach to "saturation" of fluoride, and the apparent lack of
correlation with organic matter in these soils suggested that the
mineral precipitation with a divalent cation is responsible for the
As calcium is usually a dominant
exchangeable cation in soils, and also forms insoluble calcium fluoride,
the U.S. EPA tested the precipitation hypothesis using the adsorption
data, the solubility product constant for calcium fluoride (the mineral
fluorite), and the assumptions that approximately half of the fluorine
in cryolite is available as fluoride and that exchangeable calcium ion
in many soils usually accounts for about 0.1 to 0.2 of the maximum CEC
(individual exchangeable cations were not reported). Calculations using
the various measured Kd's and water to soil ratios showed fluoride
concentrations consistent with those predicted.
Unlike the other three soils, the
fourth soil (Aguila clay loam) had uniform sorption coefficients for all
four of the tested concentrations. Kd values averaged approximately 8.9
± 1.3 in standard units, the pH is 8.0, and its CEC is given as 43.6
meq/100 g. These high values are typical of a calcareous soil and
require special interpretation. With a large reserve of calcium, small
changes in its equilibrium concentration due to precipitation with
fluoride are offset, and the soil is far from being saturated with
fluoride. Additional calcium ion available from equilibrium with
abundant solid carbonate opposes any shifts in dissolved calcium
concentration. Thus, the observed sorption behaviour is again
explainable if calcium fluoride precipitation occurs. As the percentage
of fluoride in the molar weight in both substances is practically
identical, similar Koc values can be expected for the reaction mass.
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