Molybdic acid

Adsorption / desorption

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 other databases for the sediment compartment. Moreover, the relevance of the KD-values presented in these other 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 .
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 2,248 L/kg (log Kd of 3.35) is about a factor of 1.3 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.  

Key value for chemical safety assessment

Koc at 20 °C:

1 778

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)

 

Suspended particulate material compartment:

The table below gives an overview of the different relevant Kd,SPM-values that were selected for the derivation of a typical SPM-partition coefficient;

Overview of partition coefficient between the water and suspended particulate material; values in bold were used for the derivation of a typical Kd-value for particulate suspended material

Reference	Kd,SPM-value (L/kg)
RIVM-report (Crommentuyn et al, 1997)	1,122 (log Kd: 3.05)
Popp and Laquer, 1980 – Rio Grande	1,738 (log Kd: 3.24)
Popp and Laquer, 1980 – Rio Puerco	269 (log Kd: 2.43)
Popp and Laquer, 1980 – Rio Salado	155 (log Kd: 2.19)
Magyar,1993	2,000 (log Kd: 3.30) and 2,568 (log Kd: 3.41); Average: 2,284 (log Kd: 3.36)
Neal et al (2000)	7,918 (log Kd: 3.90)
Average:	2,248 (log Kd: 3.35)

As each of these data points represents a specific location, it was decided that the average of these values could be put forward as the typical Kd for suspended particulate material. The derived value of 2,248 L/kg (log Kd of 3.35) is about a factor of 1.3 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 is therefore considered reliable.