The ionic strength dependence of lead (II) carbonate complexation in perchlorate media

Lead speciation in many aqueous geochemical systems is dominated by carbonate complexation. However, direct observations of Pb2+ complexation by carbonate ions are few in number. This work represents the first investigation of the equilibrium “equation 1” over a range of ionic strength. Through spectrophotometric observations of formation at 25 °C in NaHCO3–NaClO4 solutions, “equation 2” formation constants of the form “equation 3” were determined between 0.001 and 5.0 molal ionic strength. Formation constant results were well represented by the equation: “equation 4”.

This result, combined with previous critical assessments of formation constants for the equilibrium “equation 5”, was used to estimate the ionic strength dependence for the equilibrium “equation 6”: “equation 7”.

where “equation 8”. The carbonate complexation constants produced in this study, combined with previous complexation constants for formation of PbII chloride and hydroxide species, were used to predict formation constants for mixed-ligand species Pb(CO3)Cl- , Pb(OH)Cl0 , and Pb(CO3)OH- Formation constant estimates for the system “equation 9” were then used to assess PbII speciation in seawater. In the absence of complexation by organics, approximately 1.9% of the total lead in surface seawater (S = 35, t =25 °C, pH not, vert, similar 8.2 (free H+ concentration scale)) is present as free hydrated Pb2+. Carbonate complexes, “equation 10” and Pb(CO)3Cl–, are predominant forms of PbII in seawater at high pH, and lead chloride complexes are predominant species at low pH. For pH > 7.7 the sum concentration of “equation 11”, Pb(CO3)Cl-, PbOH+, and Pb(OH)Cl0 in seawater exceeds the sum concentration of Pb2+, PbCl+, “equation 12”, and “equation 13”

Easley R. A., & Byrne R. H., in press. The ionic strength dependence of lead (II) carbonate complexation in perchlorate media. Geochimica et Cosmochimica Acta. doi:10.1016/j.gca.2011.07.007. Article (subscription required).

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