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Environmental fate & pathways

Adsorption / desorption

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Description of key information

The adsorption/desorption behaviour of the test item, Lanthanum oxide, on soil was determined using three soils and the batch equilibrium method according to OECD Guideline N°106. The following adsorption/desorption distribution coefficients were obtained for the soils at the selected ratio of 1/50:
- Mean Koc value: 5480283
- Mean Koc, des value: 358213

Key value for chemical safety assessment

Koc at 20 °C:
548 023

Additional information

The adsorption/desorption behaviour of the test item, Lanthanum oxide, on soil was determined using three soils and the batch equilibrium method according to OECD Guideline N°106.

Initially, a preliminary test was performed in order to investigate the adsorption behaviour of lanthanum oxide . For this purpose, three soils, a test concentration of 0.64 mg/L and three soil (dry weight) to aqueous phase ratios: 1/50 (1 g/50 mL), 1/25 (1 g/25 mL) and 1/5 (5 g/25 mL) were used. Very high adsorption was observed for all three ratios. After 48 hours of agitation, virtually all of the initial amount of test item was adsorbed (>=99%).

Therefore, for the subsequent screening test, the maximum soil-to-solution ratio of 1/50 was selected together with a test item concentration of 1.29 mg/L and the same three soils. The adsorption/desorption kinetics of lanthanum oxide were determined after 2, 4 or 5, 24 and 48 hours of agitation. After 2 hours of agitation, adsorption represented 96%, 100% and 100% for soils I to III, respectively. Only low desorption was observed during the 48-hour desorption period (maximum of 3% of test item desorbed for all three soils).

The mass balance was performed during the screening test at the soil to solution ratio of 1/50. The amount of applied test item recovered from the samples represented 62%, 79% and 76% for soils I to III, respectively. Most of the applied amount was extracted from the soils, with only =< 0.5% of applied detected in the supernatants. The mass balance results confirmed the strong adsorption of lanthanum oxide to soil.

The following adsorption/desorption distribution coefficients were obtained for the soils at the selected ratio of 1/50:

- Mean Koc value: 5480283

- Mean Koc, des value: 358213

Sediment partition coefficients were derived for Lanthanum ions using paired field-measured concentrations in sediment and water from locations throughout Europe.

Stream sediment and water concentrations in sediment and water from locations throughout Europe were obtained from the FOREGS Geochemical Database (Salminen R. Ed., 2005, http://www.gsf.fi/foegs/geochem). The sediment and water samples were collected simultaneously, allowing the measurments to be paired by the location identifiers in the database. A total of 808 paired sediment and water concentrations were obtained.

Sediment partitioning coefficients were calculated for each data pair as follows:

Kd = Cs/Caq

Cs: total concentration of the test substance in the solid phase (mg/kg)

Caq: concentration of the test substance in the aqueous phase (mg/L)

The detection limit for lanthanum in water was recorded to be 0.002 micro-g/L. All concentrations below the detection limit were recorded as half the detection limit (0.001 micro-g/L). 8 of the values included non-detected water concentrations.

The detection limit in sediment was 0.02 mg La/kg.

Empirical percentiles were derived from those values.

Sediment-water partition coefficients for Lanthanum compounds

N

Kd (L/kg)

Minimum

Maximum

Median

10thpercentile

90thpercentile

95% confidence level

808

-29100

74400000

904594

54340

6887500

411686

log Kd 90th percentile: 6.83

log Kd median: 5.96

log Kd 10th percentile: 4.74

Sediment/pore water partition coefficients for the Lanthanum ion under environmental conditions have been reported in the literature.

Moermond et al. (2001) derived Koc values for Lanthanum from laboratory assays with natural sediment and pore water taken from the Rhine-Meuse estuary, The Netherlands. The Koc values were calculated in a range of log 5.67 – log 6.92 depending mainly on alkalinity, pH and chlorine concentrations as well as the presence of complexation agents. The authors developed a biologic ligand model using mutiple regression analysis that yielded the follwing equation:

Kd Sediment-porwater [L/kg] = 1.29 x 10exp6 +2.47 x 10exp4 x Alksurf - 3.3 0 x 10exp4 x Alkpore - 4.65 x Clsurf - 1.25 x 10exp5 pHpore - 1.97 x 10exp4 x % < 16 micro-m + 1.38 x 10exp5 x %Corg

Alk: alklainity in mEq/L, Cl in mg/L, Corg = % organic carbon in the sediment. The equation is only valid within the experimental conditions used:

Alk surf: 1.22 -4.5, Alk pore: 5.58 -13.78, Clsurf: 5.8 -18.8, Clpore: 3.5 - 18.5, pHsurf7.3 - 8.4, pHpore: 7.3 -7.8, %Corg: Sed: 0.88 - 2.81.

Sneller et al. (2001) supported this scale of Koc values (log Kp 4.85 - 6.37), pointing out, that laboratory derived data are about one order of magnitude lower than field derived data. The authors stated that this difference was probably due to disturbance and subsequent oxidation of the sediment in the laboratory experiments, causing relatively high Lanthanum concentrations in the pore water. Generally,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 Lanthanum in solution.

From this studies it can be concluded that Lanthanum will be highly adsorbed to soil and sediment.