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Environmental fate & pathways

Adsorption / desorption

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Description of key information

The amount of (high quality) data that are represented by the FOREGS-datase 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 these two studies is questionable. Therefore, the typical Kd-sediment of  1,778 L/kg that was derived with the FOREGS-data is considered as a reliable value for this compartment (log-value of 3.25).

Key value for chemical safety assessment

Other adsorption coefficients

Type:
log Kp (solids-water in sediment)
Value in L/kg:
3.25

Additional information

Sediment compartment:

The overview table below presents the three references that can be used for the determination of a typical Mo-Kd-value for the sediment compartment:

  1. The lowest value (850 L/kg) was published in RIVM-report and is based on data taken from Stortelder et al (1989). As stated before, 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.
  2. Cook (2000), on the other hand, 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 were Mo-enriched rocks and soils naturally occur, may have influenced the outcome of this analysis and may therefore be less relevant for other areas.
  3. 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.

Using the average of the three values for the determination of a typical Kd-value was avoided since such an approach would give each of these values a similar weight in the derivation of the final Kd: the amount of (high quality) data that are represented by the FOREGS-dataset, however, is much larger that the information given in the two other studies. Moreover, the relevance of the Kd-values presented in these two studies is questionable.

  

Overview of partition coefficient between the water and sediment compartment

Reference

 

Kd-value

RIVM-reports (Crommentuyn et al, 1997; Lijzen et al, 2001) 

850 (log Kd: 2.93)

Cook, 2002

3,020 (log Kd: 3.48)

FOREGS-dataset

1,778 (log Kd: 3.25)