Dodecaaluminium trimolybdenum dodecaoxide

Adsorption / desorption

Description of key information

Partition coefficients for suspended matter, soil, STP, sediments in freshwater and in coastal waters are available. 
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 aluminium metal and aluminium compounds as well as for 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 ion, released by the parent compound. The result of this assumption is that the ecotoxicological behaviour will be similar for all soluble aluminium and molybdenum substances used in the presented ecotoxicity tests. As aluminium molybdenum oxide has shown to be only slightly soluble in water (pH 4.5, 7d) and poorly soluble in ecotoxicity test media (pH 7.5-8.5, 96h), it can be assumed that under environmental conditions in aqueous media, the components of the substance will be present in a bioavailable form only in minor amounts (Mo) or hardly, if at all (Al). Within this dossier all available data from soluble and insoluble aluminium and molybdenum substances are taken into account and used for the derivation of ecotoxicological and environmental fate endpoints, based on the aluminium ion and molybdenum ion. All data were pooled and considered as a worst-case assumption for the environment. However, it should be noted that this represents an unrealistic worst-case scenario, as under environmental conditions the concentration of soluble Al³⁺and MoO₄^2-ions released from aluminium molybdenum oxide is negligible (Al) or low (Mo), respectively.

No adsorption/desorption data are available for aluminium molybdenum oxide, however various reliable data exist for aluminium and molybdenum (measured as environmental concentrations) and different analogue aluminium and molybdenum substances showing statistical or conservative partition coefficients for suspended matter, soil, STP, sediments in freshwater and in coastal waters.

The amount of aluminium associated with suspended particles is dependent on the chemical conditions. Factors that are known to affect aluminium speciation, such as pH and DOC, are also known to affect adsorption and desorption from particle surfaces. The amount of aluminium bound to particles as a result of surface complexation (i.e. adsorption) was shown to be pH dependent, but was typically less than 8% of the total aluminium at pH 6, and was further reduced to below 1% at pH values above 7. The corresponding Log Kd values for this distribution ranged between 3 and 5. 

Aluminium

A number of chemical factors can alter the speciation of aluminium, thereby affecting the extent of adsorption and desorption of aluminium on suspended particles, as a result aluminium speciation is complex and changes significantly with changes in pH.   In the absence of organic matter, Al³⁺is the predominant aluminium species at low pH (less than 5.5). As pH increases above 5.5, aluminium-hydroxide complexes formed by hydrolysis become increasingly important and dominate aqueous aluminium speciation (Figure 4.2.1-1). The presence of a moderate amount of organic matter in soft water (2 mg/L as dissolved organic carbon or DOC is used here) results in organically complexed aluminium being the dominant aluminium form when the pH is between 4 and 7. Above pH 7, anionic aluminium hydroxide predominates, although organically complexed aluminium remains the second most important form of dissolved aluminium. 

 

Aluminium speciation can also include the formation of insoluble polymeric aluminium-hydroxide species.  Polymeric aluminium hydroxides tend to exist as amorphous colloids and solid phases. The kinetics of this transformation to polymeric species, including aqueous colloids and amorphous precipitates, depends on many factors but typically occurs over a time scale of minutes to hours. Subsequent formation of more crystalline solid phases may take additional time, as much as a few days. As a result of these relatively slow transformations from dissolved to crystalline forms of aluminium, there is a considerable range of solubilities that have been reported for aluminium hydroxide solid phases (Lindsay and Walthall, 1996).

As a result of this dynamic chemistry, the amount of aluminium associated with suspended particles is dependent on the chemical conditions. Factors that are known to affect aluminium speciation, such as pH and DOC, are also known to affect adsorption and desorption from particle surfaces. To illustrate this further, the amount of aluminium associated with suspended particles was estimated by chemical simulation that included aqueous aluminium speciation (inorganic and organic), aluminium solubility, and complexation by NOM. For these simulations a NOM concentration of 4 mg/L (2 mg/L as DOC) and a total suspended solids (TSS) concentration of 1 mg/L were chosen to represent a reasonable lower bound for the range of values of these substances that would be expected in the environment. Suspended particles were assumed to be composed primarily of silica (80%) with a small amount of clay (10%) and particulate organic matter (10%). Aluminium concentrations were set to the maximum allowable by solubility with amorphous gibbsite at a temperature of 20⁰C. Under these conditions, the amount of aluminium bound to particles as a result of surface complexation (i.e. adsorption) was pH dependent, but was typically less than 8% of the total aluminium at pH 6, and was further reduced to below 1% at pH values above 7 (Figure 4.2.1.-2A). This distribution was similar in both soft and hard waters. The corresponding Log Kd values for this distribution are shown in Figure 4.2.1.-2B, with values between 3 and 5.  Very similar results were obtained with higher DOC concentrations of 4 mg/L.

References:

Lindsay W. L. and Walthall P. M. (1996). The solubility of aluminium in soils. In: The environmental chemistry of aluminium. (G. Sposito, ed.), pp. 333–361. USA: Lewis Publishers, Boca Raton.

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.