Chemical Speciation of Environmentally Significant Metals with Inorganic Ligands. Part 4: The Cd2+ + OH–, Cl–, CO32–, SO42–, and PO43– systems (IUPAC Technical Report)

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chemical speciation, cadmium, environmental systems, IUPAC Analytical Chemistry Division, ligands, stability constants

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The numerical modeling of CdIIspeciation amongst the environmental inorganicligands 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 constantsand related thermodynamic data. It recommends values of log10βp,q,r°valid at Im=0 mol kg–1and 25 °C (298.15 K), along with the equations and empirical reaction ion inter-action coefficients, Δε, required to calculate log10βp,q,rvalues at higher ionic strengths usingthe Brønsted–Guggenheim–Scatchard specific ion interaction theory (SIT). Values for thecorresponding reaction enthalpies, ΔrH, are reported where available. Unfortunately, with theexception of the CdII-chlorido system and (at low ionic strengths) the CdII-sulfato system, theequilibrium reactions for the title systems are relatively poorly characterized.In weakly acidic fresh water systems (–log10{[H+]/c°} < 6), in the absence of organicligands (e.g., humic substances), CdIIspeciation is dominated by Cd2+(aq), with CdSO4(aq)as a minor species. In this respect, CdIIis similar to CuII[2007PBa] and PbII[2009PBa].However, in weakly alkaline fresh water solutions, 7.5 < –log10{[H+]/c°} < 8.6, the specia-tion of CdIIis still dominated by Cd2+(aq), whereas for CuII[2007PBa] and PbII[2009PBa]the carbonato- species MCO3(aq) dominates. In weakly acidic saline systems (–log10{[H+]/c°} < 6; –log10{[Cl–]/c°} < 2.0) the speciation is dominated by CdCln(2–n)+com-plexes, (n= 1–3), with Cd2+(aq) as a minor species. This is qualitatively similar to the situ-ation for CuIIand PbII. However, in weakly alkaline saline solutions, including seawater, thechlorido- complexes still dominate the speciation of CdIIbecause of the relatively low sta-bility of CdCO3(aq). In contrast, the speciation of CuII[2007PBa] and PbII[2009PBa] in sea-water is dominated by the respective species MCO3(aq). There is scope for additional high-quality measurements in the Cd2++ H++ CO32–sys-tem as the large uncertainties in the stability constants for the Cd2+-carbonato complexes sig-nificantly affect the speciation calculations.

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Pure and Applied Chemistry, v. 83, issue 5, p. 1163-1214