Pitzer model for the speciation of lead chloride and carbonate complexes in natural waters

Abstract

The speciation of lead in the environment is extremely important in determining its behavior and fate in natural waters. Speciation calculations rely on accurate formation constants, which are often scarce. The constants for Lead Chloride complexes are fairly well known in a wide variety of media, but lead carbonate values are rare and somewhat uncertain. We determined the stoichiometric formation constant (βPbCO3) of lead carbonate in NaCl solutions from 0.05 to 3.0m at 25°C. The thermodynamic formation constant (KPbCO3) was obtained using a Pitzer model (logKPbCO3=6.87±0.09). The known thermodynamic and stoichiometric formation constants of PbCln2-n in HCl, NaCl, NaClO4, MgCl2 and CaCl2, published PbCO3 values in NaClO4, and our new PbCO3 values in NaCl are fit to a Pitzer model to determine the Pitzer coefficients for PbCl+, PbCl2, PbCl3-, and PbCO3. Using this model the speciation of lead can be determined in a wide variety of media relevant to natural waters, including brines and seawater. Calculations of seawater speciation show general agreement with previously published estimates.