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The amount of (high quality) data that are represented by the FOREGS-database is much larger than the information given in the two other databases for the sediment compartment. Moreover, the relevance of the KD-values presented in those two studies is questionable. Therefore, the typical KD-sediment of 1,778 L/kg derived using the FOREGS-data is considered as a reliable value for this compartment.

For the particulate suspended matter compartment, it was decided that the average of all relevant values could be put forward as the typical KD for suspended particulate material. The derived value of 2,793 L/kg (log KD of 3.45) is about a factor of 1.6 higher than the KD that was found for the sediment compartment. This finding is in line with the observations for other metals where the KD,SPM was also somewhat higher compared to the KD for sediment.

For the terrestrial compartment, it was decided to put forward the typical value of 871 L/kg (Log KD: 2.94) as reported by Crommentuyn et al (1997). This value has been used by the Dutch authorities (RIVM) for setting environmental quality criteria.

More detailed information can be found in the section "Additional information" and in the Background Document "Environmental fate properties of molybdenum", which is attached in the technical dossier in IUCLID Section 13.

Additional information

Sediment compartment:

The lowest value of 850 L/kg (log KD: 2.93) was published in RIVM-report and is based on data taken from Stortelder et al (1989). This value should not be considered as it is an estimate based on the KD for suspended particulate matter, using a roughly estimated correction factor of 1.5 which may be irrelevant for anionic metal forms. Cook (2000) reported a KD-value that was 3.5 times higher (3,020 L/kg), and this for lakes that were adjacent to the sub-economic porphyry Mo-prospects in the Endako region of Central British Columbia (Canada). The fact that this typical KD-value was determined in an area where Mo-enriched rocks and soils naturally occur, may have influenced the outcome of this analysis and may therefore be less relevant for other areas. The third source was the FOREGS-monitoring survey which produced a reliable dataset on relevant molybdenum baseline concentrations in water and sediment (n>800). Taking the quality and quantity of FOREGS-data into account, the value of 1,778 L/kg (Log KD of 3.25) is put forward as a typical partition coefficient for molybdenum between the water and sediment compartment.

Suspended particulate material compartment:

Each individual data point for binding to suspended particulate material represents a specific location; the average of these seven values is put forward as the typical KD for suspended particulate material. The derived value of 2,793 L/kg (log KD of 3.45) is about a factor of 1.6 higher than the KD that was found for the sediment compartment. This finding is in line with the observations for other metals where the KD,SPM was also somewhat higher compared to the KD for sediment.

Soil compartment:

Crommentuyn et al (1997) proposed a typical log KD of 2.95 for the soil compartment, based on literature data (e. g., Buchter et al, 1989). This value was used by RIVM for setting environmental quality criteria, and can be seen as a reasonable worse case for molybdenum, i. e., it more or less represents the 90th percentile of the CSIRO data set (90th percentiles of 2.721).