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Environmental fate & pathways

Adsorption / desorption

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Description of key information

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). 

Key value for chemical safety assessment

Other adsorption coefficients

Type:
log Kp (suspended matter-water)
Value in L/kg:
5

Other adsorption coefficients

Type:
log Kp (sediment-water)
Value in L/kg:
5.47

Other adsorption coefficients

Type:
log Kp (soil-water)
Value in L/kg:
4.13

Additional information

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 (arithmetic mean) of 4.13 was taken as key log Kp for soil.

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 information.

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).