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+Pb2+ complexation by carbonate ions are few in number. This work represents the first investigation of the equilibrium Pb2++CO32-⇌PbCO30 over a range of ionic strength. Through spectrophotometric observations of PbCO30 formation at 25 °C in NaHCO3–NaClO4 solutions, PbCO30 formation constants of the form CO3β1=[PbCO30]/[Pb2+][CO32-] were determined between 0.001 and 5.0 molal ionic strength. Formation constant results were well represented by the equation: logCO3β1=(6.789±0.022)-4.088·I0.51+1.5I0.5+(0.244±0.012)I. This result, combined with previous critical assessments of formation constants for the equilibrium PbCO30+CO32-⇌Pb(CO3)22-, was used to estimate the ionic strength dependence for the equilibrium Pb2++2CO32-⇌Pb(CO3)22-: logCO3β2=(10.41±0.18)-4.088·I0.51+1.5I0.5-(0.31±0.33)I where CO3β2=[Pb(CO3)22-]/[Pb2+][CO32-]2. The carbonate complexation constants produced in this study, combined with previous complexation constants for formation of PbIIPbII chloride and hydroxide species, were used to predict formation constants for mixed-ligand species Pb(CO3)Cl-Pb(CO3)Cl-, Pb(OH)Cl0Pb(OH)Cl0, and Pb(CO3)OH-Pb(CO3)OH-. Formation constant estimates for the system Pb2+-HCO3--Cl--H+ were then used to assess PbIIPbII 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 ∼8.2 (free H+H+ concentration scale)) is present as free hydrated Pb2+Pb2+. Carbonate complexes, PbCO30 and Pb(CO3)Cl-Pb(CO3)Cl-, are predominant forms of PbIIPbII in seawater at high pH, and lead chloride complexes are predominant species at low pH. For pH >7.7 the sum concentration of PbCO30, Pb(CO3)Cl-Pb(CO3)Cl-, PbOH+PbOH+, and Pb(OH)Cl0Pb(OH)Cl0 in seawater exceeds the sum concentration of Pb2+Pb2+, PbCl+PbCl+, PbCl20, and PbCl3-.