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EC number: 231-717-9
CAS number: 7699-43-6
- Soil B (the calcareous soil) had more affinity for Zr than soil A (the
acidic soil) and sorption also occurred faster in soil B. This may be
explained by the fact that the H+ ions present in the acidic soil enter
in competition with Zr ions for adsorption to available sites on the
- The very low soil:solution ratio used in this study was necessary
because at higher ratios the concentrations of Zr in solution would be
below the detection limit of the available method of analysis. However,
such low soil:solution ratios favor adsorption and therefore the Kp
values may have been affected by these experimental conditions.
- The method of Kp determination does not allow to distinguish between
the different solid forms of Zr in the experiment (adsorbed to iron
oxides, adsorbed to organic matter, precipitated as hydroxydes or
carbonates). According to formerly obtained results, the authors mention
that adsorption to iron oxides may be the predominant process in soil.
- The desorption experiments indicate that the concentrations of Zr in
soil remain largely unaffected, suggesting that non-reversible processes
are involved such as inner sphere complexation or surface precipitation.
Two other studies from Thibault et al. (1990) and Drndarski and
Golobocanin (1995) were also available. The first study predicted Kp
values for 4 different types of soil based on a previously reported
plant to soil concentration factor. The log Kp values for sandy, loamy,
clay and organic soil were calculated to be 2.76, 3.31, 3.49 and 3.86,
respectively. Since this plant to soil concentration factor was also a
predicted value and no information was reported on the applicability and
accuracy of the regression equation used for prediction of Kp values,
the reliability of the study is considered not high enough. This study
was not used for the derivation of the final Kp value for soil but was
considered a supporting study.
The other study from Drndarski and Golobocanin (1995) investigated a
series of sampling sites along the Sava River. Analyses of zirconium
concentrations in both filtered water and sediment yielded a log Kp of
approximately 3.1, which was only reported in a figure, and therefore
cannot be considered reliable. The result was used as supporting
Assessment of this endpoint and derivation of adsorption coefficients are element-based (i.e., not substance-based). A total of five studies was used in a weight of evidence approach to cover the endpoint. Two studies were added as supportive, but the data were not used to derive the key adsorption coefficients. Reliable data were available for soil, suspended matter, and sediment. The following final key values were retained: a log Kp of 5.00 for suspended matter-water, a log Kp of 5.47 for sediment-water, and a log Kp of 4.13 for soil-water. Adsorption to sediment and suspended matter appears to be slightly more pronounced than for soil for zirconium. Based on these Kp values, zirconium clearly has a strong potential for adsorption to particulate matter. For adsorption to occur however, zirconium has to end up in the aqueous phase of the environmental compartment under consideration (water column, or pore water in sediment/soil).
Adsorption of zirconium compounds (as such) to particles of suspended
matter, sediment, or soil, is not expected to occur. It is rather the
zirconium cation (or potentially other cationic zirconium species) that
will adsorb to particulate matter. Therefore, the assessment of
adsorption capacity and the derivation of adsorption coefficients is
element-based (and not substance-based).
In total, seven studies were identified containing relevant information
on adsorption of zirconium to particulate matter. Five of these studies
were considered reliable and were used in a weight of evidence approach.
Data were available for soil, sediment, and suspended matter and will be
further discussed below.
For suspended matter, two studies were identified as useful. Veselý et
al. (2001) reported a median log Kp of 3.23 for a series of samplings
along Czech rivers. Gobeil et al. (2005) analysed samples from several
locations along the St. Lawrence river, at one location river water was
sampled and at the other location effluent of the Montreal waste water
treatment plant was sampled. Based on average concentrations of
zirconium in filtered water and suspended particulate matter, log Kp
values of 6.26 and 5.51 were calculated for these locations. Because
there is a limited amount of values available, the average log Kp
(arithmetic mean) of 5.00 for these two studies is selected as key value
for characterising distribution between suspended matter and water.
For sediment, only one reliable study is available (Klimisch score of
2). In this study, zirconium concentrations were determined in paired
samples of filtered water and sediment from 20 sites along the
Blesbokspruit, South Africa. Based on data from this study (Roychoudhury
and Starke, 2006) an average log Kp value (arithmetic mean) of 5.47 was
calculated, the range being 5.12-5.92.
For soil, two reliable studies were retained for the determination of
the key value. Ferrand (2005) (see also Ferrand et al., 2006) conducted
batch equilibrium experiments with ZrOCl2 solutions and two different
soils (acidic sandy clayey loamy soil and a clayey calcareous soil). The
Kp values resulting from this study were 6,000 L/kg (dw) (or log Kp of
3.78) for the acidic soil and 30,000 L/kg (dw) for the calcareous soil
(or log Kp of 4.48). The average log Kp value (artihmetic mean) of 4.13
was taken as key log Kp for soil.
Overall, strong adsorption of zirconium to particulate matter is
observed, whether soil, sediment, or suspended matter. For adsorption to
occur however, zirconium has to end up in the aqueous phase of the
environmental compartment under consideration (water column, or pore
water in sediment/soil).
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