Cobalt di(acetate)

Adsorption / desorption

Description of key information

Cobalt compounds generally exhibit a high adsorption potential and little desorption in soil. The Kd range for cobalt is 0.2-3800 l/kg in wide variety of soils. Soil and sediment mobility of cobalt compounds are strongly dependent on several factors such as: organic ligands, anions, pH and redox potential.

Additional information

Cobalt di(acetate) readily dissociates in aqueous solution or with soil moisture into free cobalt ions and acetic acid. The pKa of 7.75 indicates that at pH values below, cobalt(di)acetate is almost completely dissociated, wherease the percentage decreases at higher pH values. Therefore an assessment on the bioaccumulative potential of cobalt di(acetate) has been performed by means of its dissociation products. The main conclusions are:

a) The inorganic component (cobalt) is adsorptive (Kd range for cobalt: 0.2-3800 l/kg)

b) In contrast, the organic component (acetic acid CAS: 64-19-7) is not adsorptive (log Koc = 0).

Summary according the adsorption/desorption potential of cobalt and cobalt compounds, as given in CICAD 69 (WHO 2006)

The distribution coefficient of cobalt in water varies due to pH, redox conditions, ionic strength, and dissolved organic matter concentrations (Mahara & Kudo, 1981). As pH is increased from 5 to 7.5, the uptake of ⁶⁰Co from the water to sediment increased rapidly (Benes et al., 1989a, 1989b). Liquid-to-solids ratio and ionic strength did not affect ⁶⁰Co uptake by sediment. ⁶⁰Co has also been found to be more mobile in anaerobic aquatic environments than in aerobic freshwater environments (Mahara & Kudo, 1981). For example, in anaerobic seawater–sediment systems, ⁶⁰Co was 250 times more mobile than in aerobic freshwater– sediment systems. In anaerobic conditions, 30% of ⁶⁰Co added to a freshwater–sediment system was mobile, whereas in aerobic conditions, 98% was permanently fixed. In anaerobic seawater systems, mobile ⁶⁰Co consisted of non-ionic forms associated with low molecular weight organic substances that were stable as pH changed. Mobile ⁶⁰Co was mostly ionic.

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 (McLaren et al., 1986). 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 (Brooks et al., 1998). 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 (Mahara & Kudo, 1981; Smith & Carson, 1981; Baes & Sharp, 1983; King, 1988). 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) (Yasuda et al., 1995). 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 (Buchter et al., 1989). 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% (King, 1988). Soil pH accounted for 84–95% of sorption variation.

References:

WHO (2006). Concise International Chemical Assessment (CICAD) Document 69, Cobalt and inorganic cobalt compounds.

Mahara Y, Kudo A (1981) Interaction and mobility of cobalt-60 between water and sediments in marine environments possible effects by acid rain. Water Research, 15(4):413–419.

Benes P, Jurak M, Crenik M (1989a) Factors affecting interaction of radiocobalt with river sediments: II. Composition and concentration of sediment temperature. Journal of Radioanalytical and Nuclear Chemistry Letters, 132(2):225–239.

Benes P, Jurak M, Kunkova M (1989b) Factors affecting interaction of radiocobalt with river sediments: I. pH and composition of water and contact time. Journal of Radioanalytical and Nuclear Chemistry Letters, 132(2):209–223.

McLaren RG, Lawson DM, Swift RS (1986) Sorption and desorption of cobalt by soils and soil components. Soil Sciences, 37:413–426.

Smith IC, Carson BL (1981) Trace metals in the environment. Ann Arbor, MI, Ann Arbor Science Publishers.

Baes CF, Sharp RD (1983) A proposal for estimation of soil leaching and leaching constants for use in assessment models. Journal of Environmental Quality, 12(1):17–28.

King LD (1988) Retention of metals by several soils of the southeastern United States. Journal of Environmental Quality, 17(2):239–246

Yasuda H, Uchida S, Muramatsu Y, Yoshida S (1995) Sorption of manganese, cobalt, zinc, strontium, and cesium onto agricultural soils: Statistical analysis on effects of soil properties. Water, Air, and Soil Pollution, 83:85–96.

Buchter B, Davidoff B, Amacher MC, Hinz C, Iskandar IK, Selim HM (1989) Correlation of Freundlich Kd and n retention parameters with soils and elements. Soil Science, 148(5):370–379.