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

Powell, K.J., Brown, P.L., Byrne, R.H., Gajda, T., Hefter, G., Leuz, A-K, Sjöberg, S. and Wanner, H. (2011) Chemical speciation of environmentally significant metals with inorganic ligands. Part 4: The Cd2+ + OH–, Cl–, CO32–, SO42–, and PO43– systems (IUPAC Technical Report). Pure and Applied Chemistry, 83 (5). pp. 1163-1214.

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The numerical modeling of Cd-II speciation amongst the environmental inorganic ligands Cl-, OH-, CO32-, SO42-, and PO43- 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 log(10) beta(p,q,r) valid at I-m = 0 mol kg(-1) and 25 degrees C (298.15 K), along with the equations and empirical reaction ion interaction coefficients, Delta epsilon, required to calculate log(10)beta(p),(,q,r) values at higher ionic strengths using the Bronsted-Guggenheim-Scatchard specific ion interaction theory (SIT). Values for the corresponding reaction enthalpies, Delta H-r, 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 (-log(10){[H+]/c degrees} < 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 < -log(10) {[H+]/c degrees} < 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 (-log(10) {[H+]/c degrees} < 6; -log(10) {[Cl-]/c degrees} < 2.0) the speciation is dominated by CdCln(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+ + CO32- system as the large uncertainties in the stability constants for the Cd2+-carbonato complexes significantly affect the speciation calculations.

Item Type: Journal Article
Murdoch Affiliation(s): School of Chemical and Mathematical Science
Publisher: International Union of Pure and Applied Chemistry
Copyright: © 2011 IUPAC
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