| Pb(II) is a key contaminant which can be deleterious to humans. Due to the ubiquity of calcite in geological formations, it may prove to be an important sink for this metal. The interaction of Pb(II) with calcite at ambient pressure and temperature, and under various aqueous conditions has been explored to elucidate dominant sorption mechanisms. Macroscopic sorption and desorption experiments implementing 210Pb radiotracers were employed to quantify sorption and reversibility, alluding to occurrent mechanisms, confirmed via the use of X-ray absorption spectroscopy. This combination of macroscopic and molecular-scale techniques provides a thorough assessment of sorption mechanisms relevant to the system, and the conditions under which they occur.; Varying electrolyte type and ionic strength at pH 8.2, and the correlation between the Pb sorption edge and PbCO30(aq) speciation from pH 7.3--9.4 reveal that PbCO3 0(aq) complexes are the primary Pb aqueous species interacting with the calcite surface. At pH 8.2, a continuum from adsorption to precipitation is evident for 1 to 60 muM Pb concentrations, with the dominance of adsorption and precipitation at 1 muM and ≥20 muM Pb, respectively, and combined adsorption and precipitation at intermediate concentrations. At 1 muM Pb at pH 7.3 and 9.4, Pb retention occurs as a combination of adsorption and coprecipitation with enhanced incorporation at pH 9.4. Aged pH 7.3, 8.2 and 9.4 samples indicate increased metal lability at pH ≤ 8.2, attributed to additional Pb uptake over time via formation of exchangeable surface adsorption complexes. No change in uptake, speciation, and lability of sorbed Pb was observed at pH 9.4. High adsorption and coprecipitation Kd values exemplify the affinity of Pb for the calcite surface and the link between these distinct but inter-related mechanisms.; The findings of this work emphasize the importance of aqueous metal speciation and solution conditions in the extent of uptake, the mechanisms via which retention occurs, and the effect of aging on metal sequestration, in a single-metal mineral system. Overall, this study is the first step in understanding the processes via which Pb-calcite interactions occur and provides insight into Pb sequestration, and ultimately its mobility, biovailability and toxicity in calcitic systems. |
