Dodecaaluminium trimolybdenum dodecaoxide

Endpoint summary

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. 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. 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. This distribution was similar in both soft and hard waters. The corresponding Log Kd values for this distribution were between 3 and 5.

Molybdenum

The amount of (high quality) data for the sediment compartment that are represented by the FOREGS-datase is large.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 2,793 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).

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.