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EC number: 264-885-7
CAS number: 64417-98-7
- 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.
Sneller et al. stated, that the differences between the laboratory and
the field derived data are probably due to disturbance and subsequent
oxidation of the sediments in the laboratory experiments, causing
relatively high concentrations in the pore water. In addition, increased
decay of organic material in the disturbed sediments, involving
reduction-processes, may contribute to release of REEs from sediment.
For these reasons, field derived partition coefficients are preferred
over laboratory derived values for calculation of MACs (maximum
Furthermore, when evaluating the partitioning data one must keep in mind
that pH, the presence of negative counterions and the concentration of
dissolved organic carbon (DOC) in the (pore-) water strongly influence
the concentration of REEs in solution. When pH, DOC concentrations and
negative counterion concentrations are high, a large part of the total
dissolved REE concentrations may not represent ´true´ partitioning.
Assessment of this endpoint and derivation of adsorption coefficients are element-based (i.e., not substance-based). Yttrium zirconium oxide is anticipated to have a limited water solubility. Consequently, only minor amounts of free metal ion (yttrium and zirconium) can be expected to be released to the aquatic part of the environmental compartment under consideration. Therefore, adsorption is considered to be a less important process determining the fate and behaviour of the substance in the environment. Any released yttrium or zirconium will however be subjected to adsorption processes. Individual data on the adsorption capacity of zirconium and yttrium is included in this dossier. For zirconium, a total of five studies was used in a weight of evidence approach to cover the endpoint (Veselý et al., 2001; Gobeil et al., 2005; Roychoudhury and Starke, 2006; Ferrand, 2005; Ferrand et al., 2006). 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 to soil for zirconium. For yttrium, a total of 7 studies was used in a weight of evidence approach (Du et al., 1998; Tao et al., 2000; Wen et al., 2002, 2006; Marcussen et al., 2008; Sneller et al., 2000; Yang et al., 1999). No reliable data were available for suspended matter. A log Kp of 4.78 and 3.59 was retained for sediment-water and soil-water, respectively. Here too, strong adsorption is observed, being however slightly less pronounced in soil.
Information on zirconium
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, five studies were identified containing relevant information
on adsorption of zirconium to particulate matter. 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 (arithmetic 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, free zirconium has to end up in the
aqueous phase of the environmental compartment under consideration
(water column, or pore water in sediment/soil).
Information on yttrium
In total, four studies were identified containing relevant information
on adsorption of yttrium to soil. These studies were considered reliable
and were used in a weight of evidence approach. Wen et al. (2006)
gathered samples of nine Chinese soils and analysed total and water
soluble yttrium concentrations in the laboratory, which resulted in a
range of log Kp values of 2.42 to 3.87. Based on data from Du et al.
(1998), in which adsorption of yttrium was investigated using cultivated
Chinese soil and radiolabeled yttrium, a log Kp of 4.72 could be
obtained for untreated soil. Another batch equilibrium experiment with
Chinese soils yielded log Kp values of 3.61 to 4.47 (Wen et al., 2002).
In the multitracer study of Tao et al. (2000), the adsorption of yttrium
to two Chinese soils, a calcareous soil and a sandy red earth, was
investigated in a batch equilibrium experiment. Log Kp values were 4.67
and 4.76 for the calcareous soil and red earth, respectively. To
determine a final key value for adsorption of yttrium to soil, a single
value (arithmetic mean) was retained for each soil in each study. Based
on those data, the 10th, 50th and 90th percentile of the retained values
was 2.73, 3.59 and 4.70, respectively. The median value of 3.59 was
taken as key log Kp for soil.
For sediment, three studies were included in a weight of evidence
approach. Sneller et al. (2000) reported log Kp values obtained by
Stronkhorst and Yland (1998) of 4.65 to 6.04 for samples taken from the
field and a laboratory study using field samples. Marcussen et al.
(2008) sampled sediment and pore water along two rivers receiving
wastewater from Hanoi, Vietnam. Log Kpsediment-pore water values for
these samples were reported to be between 5.04 and > 6.57. However,
because precipitation processes may have been involved in sediment next
to sorption processes, partitioning coefficients may have been
overestimated. Therefore only the lower boundary of the reported range
was included in the calculation of a key value for partitioning between
sediment and water. Finally, the microcosm study of Yang et al. (1999)
yielded a log Kp sediment of 3.48 when using the data for the 16-d
sampling point. To determine a final key value, a single average
(arithmetic mean) log Kp value was retained for each study. For studies
reporting results for both pore water and surface water, separate
average values were determined for the pore water-based data and the
surface water-based data. Based on the study-specific averages, an
overall average (arithmetic mean) log Kp of 4.78 was obtained.
No data were identified on yttrium adsorption to suspended matter.
Overall, the obtained adsorption coefficients were similar as for many
other metals. Adsorption to sediment appears to be stronger than to soil
Conclusion on yttrium zirconium oxide
Yttrium zirconium oxide has a limited solubility in water. Consequently,
only minor amounts of free metal ion (yttrium and zirconium) can be
expected to be released to the aquatic part of the environmental
compartment under consideration. Therefore, adsorption is considered to
be a less important process determining the fate and behaviour of the
substance in the environment. Individual data on the adsorption capacity
of zirconium and yttrium is given above. Log Kp values for zirconium
were determined to be 5.0, 5.47 and 4.13 for suspended matter, sediment
and soil, respectively, whereas for yttrium, log Kp values of 4.78 and
3.59 were obtained for sediment and soil, respectively (no data for
suspended matter). Yttrium and zirconium both strongly adsorb to
particles, adsorption to soil particles being however somewhat less
strong than adsorption to suspended matter and sediment particles.
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