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

Abstract

Complex formation between PbII and the common environmental inorganic ligands, Cl–, OH–, CO32–, SO42–, and PO43–, can be significant in natural waters with low concentrations of organic matter. Numerical modeling of the speciation of PbII 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 βp,q,r° valid at Im = 0 mol kg–1 and 25 °C (298.15 K), along with the equations and empirical coefficients required to calculate log10βp,q,r values at higher ionic strengths using the Brønsted–Guggenheim–Scatchard specific ion interaction theory (SIT). Some values for reaction enthalpies, ΔrH, are also reported. In weakly acidic fresh water systems (–log10 {[H+]/c°} < 6), the speciation of PbII is similar to that of CuII. In the absence of organic ligands, PbII 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)22–. In weakly acidic saline systems (–log10 {[H+]/c°} < 6), the speciation is dominated by PbCln(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 CuII 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 PbCln(2–n)+, (n = 0–3) and Pb(CO3)22–. The uncertainty in calculations of PbII 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 PbCl42–, the available value being considered "indicative" only. There is scope for additional detailed high-quality measurements in the Pb2+ + CO32– + Cl– system.