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Environmental fate & pathways

Adsorption / desorption

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Description of key information

A total of twelve studies was used in a weight of evidence approach to cover the endpoint. Data were available for soil, suspended matter, and sediment. The following final key values were retained: a log Kp of 4.4 for suspended matter-water, a log Kp of 5.26 for sediment-water, and a log Kp of 3.19 for soil-water. Adsorption to sediment and suspended matter appears to be more pronounced than adsorption to soil for lanthanum.

Key value for chemical safety assessment

Additional information

In total, 12 studies were selected as useful for covering the adsorption/desorption endpoint using a weight of evidence approach. Data were available for soil, sediment, and suspended matter and will be further discussed below.

For suspended matter only two studies were identified as useful. Veselý et al. (2001) reported a median log Kp of 5.4 for a series of samplings along Czech rivers. Moermond et al. (2001) analysed samples from several locations along the Rhine-Meuse estuary. Based on lanthanum concentrations measured in suspended matter and water, a log Kp of 3.37 was calculated. Because there is a limited amount of values available, the average log Kp of 4.4 for these two studies is selected as key value for characterising distribution between suspended matter and water.

For sediment five studies were included in the weight of evidence approach. Based on data from the study of Moermond et al. (2001) for sediment, surface water and pore water, log Kp values could be calculated ranging from 1.91 to 3.11. Sneller et al. (2000) reported values obtained by Stronkhorst and Yland (1998) of 4.85 to 6.37 for samples taken from the field and a laboratory study using field samples. Veado et al. (2006) sampled water and sediment along Das Velhas River Basin, Brazil, and based on their data, a log Kp sediment of 1.65 could be calculated for the unpolluted sampling site. The polluted sites were not taken into account because lanthanum may have been present in the metallic form (e.g., through mining activities and discharge of mining waste). Weltje et al. (2002) investigated lanthanum distribution between sediment and surface water or pore water in samples taken along the Rhine-Meuse catchment (the Netherlands) and reported log Kp values of 5.40 to 6.30 for sediment-surface water and 5.48 to 5.70 for sediment-pore water. Finally, the microcosm study of Yang et al. (1999) yielded a log Kp sediment of 2.85 when using the data for the 16-d sampling point. To determine a final key value, a single average log Kp value was retained for each study. Pore water-based and surface water-based data were however not lumped, individual average values were retained for this. The 10th, 50th and 90th percentile of the retained values was 2.18, 5.26 and 5.97, respectively. The median of 5.26 was taken as key log Kp.

For soil, six studies were included in the weight of evidence approach. Five of these studies investigated adsorption of lanthanum in Chinese soils. The sixth study was on Australian soils. Wen et al. (2006) gathered samples of nine Chinese soils and analysed total and water soluble lanthanum concentrations in the laboratory, which resulted in a range of log Kp values of 2.20 to 4.20. Based on data from Du et al. (1998), in which adsorption of lanthanum was investigated using cultivated Chinese soil and radiolabeled lanthanum, a log Kp of 3.61 could be obtained. This study also indicated that lanthanum sorption is rather determined by the presence of oxides and silicate clays than by CaCO3 and organic matter. Three other batch equilibrium experiments with Chinese soils yielded similar log Kp values of 2.12 to 3.02 (Shan et al., 2002), 3.61 and 3.72 (Tao et al., 2000) and 3.41 to 4.43 (Wen et al., 2002). Finally, batch equilibrium experiments with Australian soils (Stokes et al., 1999) yielded log Kp values between 2.3 and 2.8. To determine a final key value for adsorption of lanthanum to soil, a single value was retained for each soil in each study. The 10th, 50th and 90th percentile of the retained values was 2.44, 3.19 and 4.10, respectively. The median value of 3.19 was taken as key log Kp for soil.

Overall, the obtained adsorption coefficients were similar as for many other metals. Adsorption to soil appears to be mild, however a stronger adsorption of lanthanum to suspended matter and sediment seems to occur.

Other adsorption coefficient indicated as dimensionless:
- log Kp (suspended matter-water) ,4.4
- log Kp (sediment-water) ,5.26
- log Kp (soil-water) ,3.19