Dissolution and solubility of Pb-substituted calcite, Ca-substituted cerussite and their mixtures at 25 °C

Abstract

A series of Pb-substituted calcite, Ca-substituted cerussite and their mixtures with the bulk solid Pb/(Ca + Pb) atom fractions (XPbCO3) of 0.00–1.00 were synthesized and their dissolution in water for one year was experimentally studied at 25 °C. During dissolution of the solids (XPbCO3 = 0.01–0.90) in the N2-degassed water and the air-saturated water, the aqueous Pb concentrations increased rapidly to 0.136–2.095 μmol/L in 1 h and 0.140–3.431 μmol/L in 1–6 h, which were followed by a slight decrease-increase fluctuation to a steady state of 0.235–0.961 μmol/L and 0.177–0.483 μmol/L after 300–360 d, respectively. During dissolution in the CO2-saturated water, the aqueous Pb concentrations increased quickly up to the peak value of 1.351–89.107 μmol/L in <12 h, and subsequently declined slowly to a steady state of 0.194–0.515 μmol/L after 300–360 d. The aqueous Pb/(Ca + Pb) molar ratios (XPb2+,aq) declined with time and were considerably lower than the solid XPbCO3. During dissolution in the N2-degassed water, the air-saturated water and the CO2-saturated water, the mean values of log IAP at the steady state (≈log Ksp) were calculated to be −8.55 ± 0.16 ∼ −8.54 ± 0.11, −8.35 ± 0.21 ∼ −8.35 ± 0.17 and − 8.34 ± 0.07 ∼ −8.31 ± 0.13 for calcite [CaCO3]; −13.07 ± 0.53 ∼ −12.97 ± 0.52, −13.23 ± 0.34 ∼ −13.11 ± 0.20 and − 13.56 ± 0.36 ∼ −13.29 ± 0.26 for cerussite [PbCO3], respectively. In terms of the bulk composition, the calcite-type crystals and the cerussite-type crystals, the log IAP values of the solids at the steady state declined almost linearly with the increasing XPbCO3. The highest XPbCO3 value in the calcite-type crystals and the lowest XPbCO3 value in the cerussite-type crystals were measured to be 0.226 and 0.648, respectively. Thereout, the Guggenheim parameters were estimated to be a0 = 0.405 and a1 = −1.49 for the calcite side, and a0 = 1.77 and a1 = −0.474 for the cerussite side of the calcite-cerussite solid solutions. In the Lippmann diagrams constructed for the Calcite–PbCO3 (R3¯c) system, the Aragonite–Cerussite (Pmcn) system and the Calcite–Cerussite system with the estimated Guggenheim coefficients, the solids dissolved non-stoichiometrically and moved gradually up to the quasi-equilibrium curve for the solids with lower XPbCO3, and then along this curve from right to left, approached and crossed the solutus curve and the saturation curve for calcite, and sequentially moved up along the quasi-equilibrium curve for the solids with higher XPbCO3. Finally, the considerably Pb-poor water was simultaneously in equilibrium with both the Pb-richer cerussite-type crystals and the Pb-poorer calcite-type crystals. The result can provide a deeper insight into the environmental and geochemical cycle of lead.