Chemical speciation of environmentally significant metals with inorganic ligands. Part 4: The Cd2+ + OH&ndash;, Cl&ndash;, CO32&ndash;, SO42&ndash;, and PO43&ndash; systems (IUPAC Technical Report)

Powell, Kipton J.; Brown, Paul L.; Byrne, Robert H.; Gajda, Tamás; Hefter, Glenn; Leuz, Ann-Kathrin; Sjöberg, Staffan; Wanner, Hans

doi:10.1351/PAC-REP-10-08-09

Pure Appl. Chem., 2011, Vol. 83, No. 5, pp. 1163-1214

http://dx.doi.org/10.1351/PAC-REP-10-08-09

Published online 2011-03-29

Chemical speciation of environmentally significant metals with inorganic ligands. Part 4: The Cd²⁺ + OH^–, Cl^–, CO₃^2–, SO₄^2–, and PO₄^3– systems (IUPAC Technical Report)

Kipton J. Powell¹*, Paul L. Brown², Robert H. Byrne³, Tamás Gajda⁴, Glenn Hefter⁵, Ann-Kathrin Leuz⁶, Staffan Sjöberg⁷ and Hans Wanner⁶

¹ Department of Chemistry, University of Canterbury, Christchurch, New Zealand
² Rio Tinto Technology and Innovation, 1 Research Avenue, Bundoora VIC 3083, Australia
³ College of Marine Science, University of South Florida, 140 Seventh Avenue South, St. Petersburg, FL 33701-5016, USA
⁴ Department of Inorganic and Analytical Chemistry, A. József University, P.O. Box 440, Szeged 6701, Hungary
⁵ School of Chemical and Mathematical Sciences, Murdoch University, Murdoch, WA 6150, Australia
⁶ Swiss Federal Nuclear Safety Inspectorate, CH-5200 Brugg, Switzerland
⁷ Department of Chemistry, Umeå University, S-901 87 Umeå, Sweden

Abstract: The numerical modeling of Cd^II speciation amongst the environmental inorganic ligands Cl^–, OH^–, CO₃^2–, SO₄^2–, and PO₄^3– requires reliable values for the relevant stability (formation) constants. This paper compiles and provides a critical review of these constants and related thermodynamic data. It recommends values of log₁₀ β_p,q,r° valid at I_m = 0 mol kg^–1 and 25 °C (298.15 K), along with the equations and empirical reaction ion interaction coefficients, ∆ε , required to calculate log₁₀ β_p,q,r values at higher ionic strengths using the Brønsted–Guggenheim–Scatchard specific ion interaction theory (SIT). Values for the corresponding reaction enthalpies, ∆_rH, are reported where available. Unfortunately, with the exception of the Cd^II-chlorido system and (at low ionic strengths) the Cd^II-sulfato system, the equilibrium reactions for the title systems are relatively poorly characterized. In weakly acidic fresh water systems (–log₁₀ {[H⁺]/c°} < 6), in the absence of organic ligands (e.g., humic substances), Cd^II speciation is dominated by Cd²⁺(aq), with CdSO₄(aq) as a minor species. In this respect, Cd^II is similar to Cu^II [2007PBa] and Pb^II [2009PBa]. However, in weakly alkaline fresh water solutions, 7.5 < –log₁₀ {[H⁺]/c°} < 8.6, the speciation of Cd^II is still dominated by Cd²⁺(aq), whereas for Cu^II [2007PBa] and Pb^II [2009PBa] the carbonato- species MCO₃(aq) dominates. In weakly acidic saline systems (–log₁₀ {[H⁺]/cϒ} < 6; –log₁₀ {[Cl^–]/c°} < 2.0) the speciation is dominated by CdCl_n^(2–ⁿ⁾⁺ complexes, (n = 1–3), with Cd²⁺(aq) as a minor species. This is qualitatively similar to the situation for Cu^II and Pb^II. However, in weakly alkaline saline solutions, including seawater, the chlorido- complexes still dominate the speciation of Cd^II because of the relatively low stability of CdCO₃(aq). In contrast, the speciation of Cu^II [2007PBa] and Pb^II [2009PBa] in seawater is dominated by the respective species MCO₃(aq). There is scope for additional high-quality measurements in the Cd²⁺ + H⁺ + CO₃^2– system as the large uncertainties in the stability constants for the Cd²⁺-carbonato complexes significantly affect the speciation calculations.