Chemical speciation of environmentally significant metals with inorganic ligands: Part 4: The Cd 2+ + OH – , Cl – , CO 3 2– , SO 4 2– , and PO 4 3– systems (IUPAC Technical Report)

The numerical modeling of Cd II speciation amongst the environmental inorganic ligands Cl – , OH – , CO 3 2– , SO 4 2– , and PO 4 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 log 10 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 log 10 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, ∆ r H , 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 °} < 6), in the absence of organic ligands (e.g., humic substances), Cd II speciation is dominated by Cd 2+ (aq), with CdSO 4 (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 °} < 8.6, the speciation of Cd II is still dominated by Cd 2+ (aq), whereas for Cu II [2007PBa] and Pb II [2009PBa] the carbonato- species MCO 3 (aq) dominates. In weakly acidic saline systems (–log 10 {[H + ]/ c ϒ} < 6; –log 10 {[Cl – ]/ c °} < 2.0) the speciation is dominated by CdCl n (2– n )+ complexes, ( n = 1–3), with Cd 2+ (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 CdCO 3 (aq). In contrast, the speciation of Cu II [2007PBa] and Pb II [2009PBa] in seawater is dominated by the respective species MCO 3 (aq). There is scope for additional high-quality measurements in the Cd 2+ + H + + CO 3 2– system as the large uncertainties in the stability constants for the Cd 2+ -carbonato complexes significantly affect the speciation calculations.