Chemical Speciation of Environmentally Significant Metals with Inorganic Ligands. Part 3: The Pb2+ + OH–, Cl–, CO32–, SO42–, and PO43– systems (IUPAC Technical Report)

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chemical speciation, environmental, lead, ligands, stability constants

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Complex formation between PbIIand the common environmental inorganic lig-ands, Cl–, OH–, CO32–, SO42–, and PO43–, can be significant in natural waters with low con-centrations of organic matter. Numerical modeling of the speciation of PbIIamongst these in-organic ligands requires reliable values for the relevant stability (formation) constants. Thispaper provides a critical review of such constants and related thermodynamic data. It recom-mends values of log10βp,q,r°valid at Im= 0 mol kg–1and 25 °C (298.15 K), along with theequations and empirical coefficients required to calculate log10βp,q,rvalues at higher ionicstrengths 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 PbIIissimilar to that of CuII. In the absence of organic ligands, PbIIspeciation is dominated byPb2+(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) andPb(CO3)22–. In weakly acidic saline systems (–log10{[H+]/c°} < 6), the speciation is domi-nated 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 CuIIbecause of the higher stability ofthe Pb2+-chlorido- complexes. In seawater at –log10{[H+]/c°} = 8.2, the calculated specia-tion is less well defined, although it is clearly dominated by the uncharged speciesPbCO3(aq) (41 % of [Pb]T) with a significant contribution (16 %) from Pb(CO3)Cl–andminor contributions (5–10 %) from PbCln(2–n)+, (n= 0–3) and Pb(CO3)22–. The uncertaintyin calculations of PbIIspeciation in seawater arises from (a) the large uncertainty in the sta-bility constant for the apparently dominant species PbCO3(aq), (b) the reliance on statisticalpredictions 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 thePb2++ CO32–+Cl–system.

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Pure and Applied Chemistry, v. 81, issue 12, p. 2425-2476