Cobalt(2+) hydrogen citrate

Adsorption / desorption

Description of key information

Partition coefficients for suspended matter, soil, STP, sediments in freshwater and in coastal waters are available. For cobalt, 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 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 ion, released by the parent compound. The result of this assumption is that the ecotoxicology will be similar for all soluble cobalt substances used in the ecotoxicity tests. Therefore, data from soluble cobalt substances are used for the derivation of ecotoxicological and environmental fate endpoints, based on the cobalt ion.With respect to these considerations, data collected on elemental cobalt (e.g. environmental concentrations for Co2+) can also be taken into account.

No adsorption/desorption data are available for cobalt hydrogen citrate, however various reliable data exist for cobalt (measured as environmental concentrations) and different analogue cobalt 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. 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 Kdvalues 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.