Chemical speciation of environmentally significant metals with inorganic ligands. Part 3: The Pb2+ + OH–, Cl–, CO32–, SO42–, and PO43– systems (IUPAC Technical Report)

• Published in: Pure and applied chemistry 2009-12, Vol.81 (12), p.2425-2476
• Main Authors: Powell, Kipton J., Brown, Paul L., Byrne, Robert H., Gajda, Tamás, Hefter, Glenn, Leuz, Ann-Kathrin, Sjöberg, Staffan, Wanner, Hans
• Format: Article
• Language: English
• Subjects: chemical speciation; Complex formation; Empirical equations; Enthalpy; environmental; lead; Ligands; Low concentrations; Organic chemistry; Organic matter; Seawater; Speciation; Stability; Stability constants; Uncertainty
• ISSN: 0033-4545; 1365-3075
• DOI: 10.1351/PAC-REP-09-03-05

Complex formation between Pb and the common environmental inorganic ligands, Cl , OH , CO , SO , and PO , can be significant in natural waters with low concentrations of organic matter. Numerical modeling of the speciation of Pb amongst these inorganic ligands requires reliable values for the relevant stability (formation) constants. This paper provides a critical review of such constants and related thermodynamic data. It recommends values of log ° valid at = 0 mol kg and 25 °C (298.15 K), along with the equations and empirical coefficients required to calculate log values at higher ionic strengths using the Brønsted–Guggenheim–Scatchard specific ion interaction theory (SIT). Some values for reaction enthalpies, Δ , are also reported. In weakly acidic fresh water systems (–log {[H ]/ } < 6), the speciation of Pb is similar to that of Cu . In the absence of organic ligands, Pb speciation is dominated by Pb (aq), with PbSO (aq) as a minor species. In weakly alkaline solutions, 8.0 < –log {[H ]/ } < 9.0, the speciation is dominated by the carbonato species PbCO (aq) and Pb(CO . In weakly acidic saline systems (–log {[H ]/ } < 6), the speciation is dominated by PbCl complexes, ( = 0–3), with Pb (aq) as a minor species. In this medium (and in seawater), the speciation contrasts with that of Cu because of the higher stability of the Pb -chlorido- complexes. In seawater at –log {[H ]/ } = 8.2, the calculated speciation is less well defined, although it is clearly dominated by the uncharged species PbCO (aq) (41 % of [Pb] ) with a significant contribution (16 %) from Pb(CO )Cl and minor contributions (5–10 %) from PbCl , ( = 0–3) and Pb(CO . The uncertainty in calculations of Pb speciation in seawater arises from (a) the large uncertainty in the stability constant for the apparently dominant species PbCO (aq), (b) the reliance on statistical predictions for stability constants of the ternary species Pb(CO )Cl and Pb(CO )OH , and (c) the uncertainty in the stability constant for PbCl , the available value being considered "indicative" only. There is scope for additional detailed high-quality measurements in the Pb + CO + Cl system.