Aluminum cobalt oxide

Endpoint summary

Additional information

The environmental fate pathways and ecotoxicity effects assessments for cobalt metal and cobalt compounds as well as for aluminium metal and aluminium 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 cobalt and aluminium substances used in the ecotoxicity tests.

As cobalt aluminium oxide has shown to be highly insoluble with regard to the results of the transformation/dissolution test protocol (pH 6, 28 d), 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, if at all. Within this dossier all available data from cobalt and aluminium substances are pooled and used for the derivation of ecotoxicological and environmental fate endpoints, based on the cobalt ion and aluminium ion. For cobalt, only data from soluble substances were available and for aluminium, both soluble and insoluble substance data were available. 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 Co2+ and Al3+ ions released is negligible.

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

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. 

Cobalt

Information taken from WHO CICAD (2006):

Volatility

Cobalt and inorganic cobalt compounds are nonvolatile. Therefore, they are released into the atmosphere in particulate form. Atmospheric transport depends on particle size and density and meteorological conditions. Coarse particles with diameters >2 μm may deposit within 10 km from the point of emission, while smaller particles may travel longer distances. The mass median diameter of atmospheric cobalt was found to be 2.6 μm in one study. Data on the transformations of cobalt in the atmosphere are limited (WHO CICAD, 2006).

Adsorption

Ultimately, the final repository for cobalt is soil and sediment. Released into water, cobalt may sorb to particles and settle into sediment or sorb directly to sediment. Complexation of cobalt to dissolved organic substances can reduce sediment sorption.

Environmental distribution

Interparticle migration of cobalt can affect the transport of metal ions in sediments. In addition, cobalt can be transported in dissolved form or as suspended sediment by rivers and by sea and ocean currents. Concentration profiles of cobalt in deep water suggest that dissolved amounts decrease with increasing depth and that dissolved cobalt is precipitated in the adsorbed state with oxides of iron and manganese and with crystalline sediments such as aluminosilicate and goethite. Humic substances/humic acids are naturally present in aquatic environments and bind strongly to cobalt. Over time, these complexes may transform into stronger complexes where cobalt is less readily disassociated. The distribution coefficient of cobalt in water varies due to pH, redox conditions, ionic strength, and dissolved organic matter concentrations (WHO CICAD, 2006).

References:

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

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.