Cobalt molybdate

Adsorption / desorption

Description of key information

Partition coefficients for suspended matter, soil, STP, sediments in freshwater and in coastal waters are available. For Co, log Kd values for all types ranged from 0.41 to 5.83. For Mo, log Kd values for all types ranged from 2.94 to 3.45.

Additional information

The environmental fate pathways and ecotoxicity effects assessments for cobalt metal and cobalt compounds as well as for the molybdenum metal and molybdenum compounds is based on the observation that adverse effects to aquatic, soil- and sediment-dwelling organisms are a consequence of exposure to the bioavailable cobalt ion and molybdenum ion, released by the parent compound. The result of this assumption is that the ecotoxicology will be similar for all soluble cobalt and molybdenum substances used in the ecotoxicity tests. Therefore, data from soluble cobalt and molybdenum substances are used for the derivation of ecotoxicological and environmental fate endpoints, based on the cobalt ion and molybdenum ion, respectively.With respect to these considerations, data collected on elemental cobalt (e.g. environmental concentrations for Co2+) and elemental molybdenum (e.g. environmental concentrations for Mo2 +) can also be taken into account.

No adsorption/desorption data are available for cobalt molybdenum oxide, however various reliable data exist for cobalt and molybdenum (measured as environmental concentrations) and different analogue cobalt and molybdenum substances showing statistical or conservative partition coefficients for suspended matter, soil, STP, sediments in freshwater and in coastal waters. For Co, log Kd values for all types ranged from 0.41 to 5.83. For Mo, log Kd values for all types ranged from 2.94 to 3.45.

Cobalt

Information taken from WHO CICAD (2006):

Soil mobility of cobalt is inversely related to the strength of adsorption by soil constituents. The adsorption of cobalt to soil occurs rapidly, within 1–2 h. Mineral oxides such as iron and manganese oxide, crystalline materials such as aluminosilicate and goethite, and organic substances can retain cobalt. Soil oxides adsorb larger levels of cobalt than do other materials. Clay minerals adsorb relatively smaller amounts of cobalt. Desorption of cobalt from soil oxides is low, although humic acids and montmorillonite desorb substantial amounts. Adsorption in clay soils is most likely due to ion exchange at cationic sites of clay with simple ionic cobalt or hydrolysed ionic species such as CoOH+. Adsorption of cobalt with iron or manganese increases with pH. As pH increases, insoluble hydroxides and carbonates may form that also reduce cobalt mobility. In contrast, adsorption to mobile colloids would enhance cobalt mobility. Typically, cobalt is more mobile than other metals, such as lead, chromium(II), zinc, and nickel, in soil, but less mobile than cadmium. The partition coefficient, Kd, of cobalt ranged from 0.2 to 3800 L/kg in a wide variety of soils. In 36 Japanese agricultural soils, the mean Kd was 1840 L/kg (minimum 130 L/kg, maximum 104,000 L/kg, median 1735 L/kg). Soil properties that exhibited the highest correlation with Kd were exchangeable calcium, pH, water content, and cation exchange capacity. The mean Freundlich adsorption constant, Kf, and isotherm exponent, n, values in 11 soils in the United States were 37 L/kg and 0.754, respectively. The Kf values ranged from 2.6 to 363 L/kg and correlated with soil pH and cation exchange capacity. In another study, 13 soils from the southeastern United States had soil pH values that ranged from 3.9 to 6.5, and cobalt sorption ranged from 15% to 93%. Soil pH accounted for 84–95% of sorption variation (WHO CICAD, 2006).

Information taken from Environment Canada (2011):
Available partition coefficients for soil-water range from 0.41 to 3.49 (log Kd). This large variability is due in part to differences in soil and soil solution properties (pH, soil organic matter and other sorbent phases, DOC, ionic strength) and the nature of the metal added (Environment Canada, 2011).
Available partition coefficients for suspended sediment-water (Kd) are high for cobalt with log Kd values ranging from 4.18 to 5.83 (Environment Canada, 2011). The partition coefficients for sediment-water, log Kd, are less than those for suspended matter, ranging from 2.92 to 3.48, which suggests that cobalt will remain for the most part in bottom sediments after having entered this compartment.

Thus, a median log Kd of 2.99 can be derived for soil, 5.33 for suspended sediment and 3.20 for sediment, with equivalent Kd values of 977 for soil, 213800 for suspended sediment and 1585 for sediment, respectively (Environment Canada, 2011).

                                                                                                             

References:

World Health Organization (2006). Concise International Chemical Assessment Document 69. COBALT AND INORGANIC COBALT COMPOUNDS.

Environment Canada. Health Canada (2011). Screening Assessment for the Challenge. Cobalt, cobalt chloride, cobalt sulfate.

Molybdenum

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)
Hoede et al. 1987 – Solo River	5,623 (log Kd: 3.75)
Hoede et al. 1987 – Wono Kromo River	4,898 (log Kd: 3.69)
Hoede et al. 1987 – Porong River	2,291 (log Kd: 3.36)
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:	2,793 (log Kd: 3.446)

As each of these data points represents a specific location, it was decided that the average of these seven 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.

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.

CSIRO (Janik et al. 2010) reported apparent log Kd-values in more than 500 soil samples, ranging between 0.23 and 3.92, and with 50th and 90th percentiles of 1.631 and 2.721, respectively.

The RIVM value of 2.94 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. It should be noted that the apparent Kds of CSIRO

a ) does not take into account the presence of exchangeable molybdenum that was already present in the soil samples, and

b) does not take into account other Mo-fractions in the soil samples (which may become available un der some conditions)

Consequently, the use of the CSIRO-Kd values for modeling purposes may result in PECs that underestimate the Mo-levels as they only take into account the added fraction (PECadded).

It is therefore decided to use the log Kd of 2.94 - as proposed by Crommentuyn et al. (1997) - as a typical Kd for the terrestrial compartment.

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 considere 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.

References:

Crommentuijn, T., Polder, M.D. and van de Plassche, E.J. (1997). Maximum Permissible Concentrations and Negligible Concentrations for metals, taking background concentrations into account. National Institute of Public Health and Environmental Protection, Bilthoven, The Netherlands. Report N° 601501 001.

Lijzen, J.P.A., Baars, A.J., Otte, P.F., Rikken, M.G.J., Swartjes, F.A.,Verbruggen, E.M.J. and Van Wezel, A.P., 2001. Technical evaluation of the Intervention Values for soil/sediment and groundwater. RIVM Report 711701023. Bilthoven, The Netherlands: Dutch National Institute for Public Health and the Environment.

Stortelder, P.B.M., Van der Gaag, M.A., Van der Kooy, L.A. (1989). Perspectives for water organisms (Part 1 and 2°. DBW/RIZA Nota No. 89.016a+b, Lelystad, The Netherlands.