Aluminum cobalt oxide

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.

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):

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.

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.