Solubility-product constant and thermodynamic properties for synthetic otavite, CdCO[sub 3(s)], and aqueous association constants for the Cd(II)-CO[sub 2]-H[sub 2]O system

Journal Article · Tue Jun 01 04:00:00 EDT 1993 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(93)90384-9· OSTI ID:6457122

Stipp, S L.S.; Leckie, J O; Parks, G A ^[1]; Nordstrom, D K ^[2]

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1. Stanford Univ., CA (United States)
2. Geological Survey, Lakewood, CO (United States)

Normally, cadmium is found only in trace quantities at the Earth's surface. However, during recent decades, anthropogenic activity has increased environmental cadmium concentrations to dangerous levels in some localities. Crystalline otavite precipitation may play a role in controlling cadmium concentration in contaminated areas as well. Considerable disparity exists in the published thermodynamic data for selected species in the Cd(II)-CO[sub 2]-H[sub 2]O system near 25[degrees]C and 1 atm pressure. Evaluation of published experimental and estimated data for aqueous cadmium-carbonate species suggests an association constant, pK, of [minus]3.0 [+-] 0.4 for CdCO[sup 0][sub 3], about [minus]1.5 for CdHCO[sup +][sub 3], and [minus]6.4 [+-] 0.1 for Cd(CO[sub 3])[sup 2[minus]][sub 2] (T = 298.15 K; P = 1 atm; I = 0). Examination of all available data for cadmium-hydrolysis species and [beta]-Cd(OH)[sub 2(s)] confirms that the consistent set of constants presented by Baes and Mesmer (Hydrolysis of Cations, 1976) is the best available. The solubility of synthetic octavite, CdCO[sub 3(s)], has been measured in KClO[sub 4] solutions where I [le] 0.1 M. The authors calculated pK[sub sp] = 12.1 [+-] 0.1 (T = 25.0[degrees]C; P = 1 atm; I = 0) from measured concentrations of Cd[sup 2+], measured P[sub CO2] and pH, a selected set of equilibrium constants, and activity corrections estimated using the Davies equation. Values at 5 and 50[degrees]C were 12.4 [+-] 0.1 and 12.2 [+-] 0.1, respectively. Based on the new solubility data and the CODATA key values for Cd[sup 2+] and CO[sup 2[minus]][sub 3], a new set of thermodynamic properties is recommended for otavite: [delta]G[sup 0][sub f] = [minus]674.7 [+-] 0.6 kJ/mol; [delta]H[sup 0][sub f] = [minus]751.9 [+-] 10 kJ/mol; S[sup 0] = 106 [+-] 30 J/mol K; and [delta]G[sup 0][sub r] for the reaction Cd[sup 2+] + CO[sup 2[minus]][sub 3] [r equilibrium] CdCO[sub 3(s)] is [minus]69.08 [+-] 0.57 kJ/mol.

• OSTI ID: 6457122
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Journal Name: Geochimica et Cosmochimica Acta; (United States) Vol. 57:12; ISSN GCACAK; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
540220* -- Environment
Terrestrial-- Chemicals Monitoring & Transport-- (1990-)
CADMIUM CARBONATES
CADMIUM COMPOUNDS
CARBON COMPOUNDS
CARBONATES
CHEMISTRY
GROUND WATER
HYDROGEN COMPOUNDS
MATERIALS
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
POLLUTION
SOIL CHEMISTRY
SYNTHETIC MATERIALS
THERMODYNAMIC PROPERTIES
WATER
WATER POLLUTION