Calcium zirconium oxide

Adsorption / desorption

Description of key information

Assessment of this endpoint and derivation of adsorption coefficients are element-based (i.e., not substance-based). A total of eight studies was used in a weight of evidence approach to cover the endpoint: five for zirconium, and three for calcium. 
For zirconium, reliable data (Klimisch 2) are 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 partitioning. 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 calcium, three studies (Klimisch 2) were included in the weight of evidence approach. The log Kp values derived based on the results of these studies range from 0.72 to 1.69, indicating that calcium does not partition to particulate matter to the same extent as zirconium.
For adsorption to occur however, zirconium and calcium have to end up in the aqueous phase of the environmental compartment under consideration (water column, or pore water in sediment/soil). 

Additional information

1. Information on zirconium (dioxide)

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 considered sufficiently reliable to be included in the 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 and reliable (Klimisch score of 2). 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 of zirconium 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 (Klimisch score of 2) were retained for the determination of the key value for zirconium. 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).

2. Information on calcium (oxide)

Results of three studies (all scored Klimisch 2) are included in the weight of evidence approach, whereby very low Kp values were obtained. In a first study (Gustafsson et al., 2000), Kp values could be calculated for adsorption of calcium to soil particles from the B horizon of a podzolized soil. In a first set of experiments exchangeable Ca2+ was measured in centrifugates of 4 samples of the soil under consideration. These experiments yielded Kp values of 5.3 to 49.1 L/kg. The second set of experiments were batch equilibrium experiments with the B2 horizon of the same soil and different solutions (with or without added CaCl2 and/or HCl). From the results of the experiment with added CaCl2 and without added HCl, a Kp of 38.0 could be calculated, which agrees very well with those from the first set of experiments.

In the study of Suárez et al. (2001), batch equilibrium experiments were performed with agricultural soil and a leachate obtained by simulating the interaction of rainfall with a lime-stabilized sludge (20% CaO) applied to the soil. During the 6-h batch equilibrium experiment, Ca2+ concentration decreased due to adsorption to soil particles. The Kp for Ca2+ was calculated to be 11.3 L/kg. Further, although the pH of the leachate was quite high, the high buffer capacity of the soil maintained the pH, hence there would be no problem with the deposition of the sludge under consideration.

Based on these studies, Kp (soil-water) values for calcium range from 5.3 L/kg to 49.1 L/kg (log Kp = 0.72-1.69).

3. Conclusion on calcium zirconium oxide

Calcium zirconium oxide is a substance of low water solubility. The limited amounts of calcium and zirconium that will be released from the substance upon its release to the environment will be subject to speciation and distribution. Any released zirconium can be expected to be stongly adsorbed to particulate matter in the environmental compartment under consideration, whereas adsorption to particulate matter is expected to be much less strong for calcium, based on the available data.