Chemical speciation of environmentally significant metals with inorganic ligands. Part 3: The Pb2+ + 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-09-03-05

Pure Appl. Chem., 2009, Vol. 81, No. 12, pp. 2425-2476

http://dx.doi.org/10.1351/PAC-REP-09-03-05

Published online 2009-10-29

Chemical speciation of environmentally significant metals with inorganic ligands. Part 3: The Pb²⁺ + OH^–, Cl^–, CO₃^2–, SO₄^2–, and PO₄^3– systems (IUPAC Technical Report)

Kipton J. Powell¹, Paul L. Brown², Robert H. Byrne³, Tamas 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-5232 Villigen, Switzerland
⁷ Department of Chemistry, Umeå University, S-901 87 Umeå, Sweden

Abstract: Complex formation between Pb^II and the common environmental inorganic ligands, Cl^–, OH^–, CO₃^2–, SO₄^2–, and PO₄^3–, can be significant in natural waters with low concentrations of organic matter. Numerical modeling of the speciation of Pb^II 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 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 coefficients required to calculate log₁₀ β_p,q,r values at higher ionic strengths using the Brønsted–Guggenheim–Scatchard specific ion interaction theory (SIT). Some values for reaction enthalpies, Δ_rH, are also reported. In weakly acidic fresh water systems (–log₁₀ {[H⁺]/c°} < 6), the speciation of Pb^II is similar to that of Cu^II. In the absence of organic ligands, Pb^II speciation is dominated by Pb²⁺(aq), with PbSO₄(aq) as a minor species. In weakly alkaline solutions, 8.0 < –log₁₀ {[H⁺]/c°} < 9.0, the speciation is dominated by the carbonato species PbCO₃(aq) and Pb(CO₃)₂^2–. In weakly acidic saline systems (–log₁₀ {[H⁺]/c°} < 6), the speciation is dominated by PbCl_n^(2–ⁿ⁾⁺ complexes, (n = 0–3), with Pb²⁺(aq) as a minor species. In this medium (and in seawater), the speciation contrasts with that of Cu^II because of the higher stability of the Pb²⁺-chlorido- complexes. In seawater at –log₁₀ {[H⁺]/c°} = 8.2, the calculated speciation is less well defined, although it is clearly dominated by the uncharged species PbCO₃(aq) (41 % of [Pb]_T) with a significant contribution (16 %) from Pb(CO₃)Cl^– and minor contributions (5–10 %) from PbCl_n^(2–ⁿ⁾⁺, (n = 0–3) and Pb(CO₃)₂^2–. The uncertainty in calculations of Pb^II speciation in seawater arises from (a) the large uncertainty in the stability constant for the apparently dominant species PbCO₃(aq), (b) the reliance on statistical predictions for stability constants of the ternary species Pb(CO₃)Cl^– and Pb(CO₃)OH^–, and (c) the uncertainty in the stability constant for PbCl₄^2–, the available value being considered "indicative" only. There is scope for additional detailed high-quality measurements in the Pb²⁺ + CO₃^2– + Cl^– system.