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Environmental fate & pathways

Adsorption / desorption

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Description of key information

The overall Kd range was 1.38 x 10³ to at least 6.10 x 10³, the overall Koc range was 1.33 x 10⁴ to at least 2.39 x 10⁵ and the overall log10 Koc range was 4.12 to at least 5.38 at 25 ± 2 °C.

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Additional information

The adsorption characteristics of the test material were determined using a batch equilibrium method in accordance with the standardised guidelines OECD 106 and EU Method C.18 under GLP conditions.

The distribution coefficient (Kd) and the organic carbon normalised adsorption coefficient (Koc) for the neodymium content of the test material were determined for five soil types at 25 ± 2 °C. The soils were: soil type 2, loam (pH 7.3); soil type 3, silt loam (pH 6.2); soil type 4, silt loam (pH 5.1); soil type 5, loamy sand (pH 5.8); and soil type 7, loamy sand (pH 3).

Due to limited aqueous solubility of the test material and the irrelevance of organic co-solvents to inorganic test materials, stock solutions were prepared by shaking saturated solutions of test material in 0.01 M calcium chloride solution overnight, at ambient laboratory temperature.

With the exception of the acidic soil type 7, definitive results were calculated from the initial sampling time-point, employing an adsorption equilibration period of 2 hours, due to an inherent instability of the dissolved neodymium content of the soil-less control solutions as a function of time. For the soil type 7, potentially due to the acidic nature of the soil/solution mixture promoting aqueous solubility, equilibrium was not evident at 2 hours. However consistent replicates were obtained following 5 hours equilibration and therefore this data were used for calculation of the distribution coefficient (Kd) and the organic carbon normalised adsorption coefficient (Koc) values.

 

The adsorption properties (as distribution coefficients) observed for the dissolved neodymium content of the sample solutions showed no significant correlation to the organic carbon content of the soils evaluated.

From the soil type 7 results obtained, pH was evidently a very significant soil/solution criterion for the behaviour of this inorganic ion in soil. Even though soil type 7 presented both the highest organic carbon content and cation exchange capacity of the five soils investigated (22.7 % and 49.8 meq/100 g, respectively), the resultant distribution coefficient (Kd) value was the second lowest of the five soils investigated. Such a result was inversely proportional to the expectations of the methodology, predominantly aimed at organic compounds, as the acidic nature of this soil promoted aqueous solubility of the metal cation and therefore reduced the proportion partitioning on to the soil phase. Only soil type 5 resulted in a lower distribution coefficient (Kd) value. Soil type 5 presented both the lowest organic carbon content and cation exchange capacity of the five soils investigated (1.01 % and 9.2 meq/100 g, respectively). Therefore on comparison of soil type 5 and soil type 7, it can be observed that both the influence of an approximately 20 fold increase in organic carbon content and an approximately 5 fold in cation exchange capacity were negated by the decrease in the aqueous phase solution pH.

 

Under the conditions of this study, the overall Kd range was 1.38 x 10³ to at least 6.10 x 10³, the overall Koc range was 1.33 x 10to at least 2.39 x 10and the overall log10 Koc range was 4.12 to at least 5.38 at 25 ± 2 °C.