Pb (II) sorption with calcite: A radiotracer study

A ²¹⁰Pb radiotracer was used to monitor Pb solid-aqueous phase partitioning in calcite sorption experiments at ambient temperature, pH = 8.2, and atmospheric PCO₂ in 0.15 M NaNO₃ solutions. A 24 h isotherm is linear up to Pb concentrations of 4 × 10^-6 M, above which an increase in slope suggests precipitation. The effect of Pb concentration, calcite loading, and ionic strength on Pb sorption with time was monitored. Sorption kinetics are rapid, followed by a slower sorption step. At a constant calcite loading of 500 mgL^-1, fractional sorption decreases with increasing initial Pb concentration. The reverse is observed for surface coverage, with 0.6, 5.6 and 40.2% of available Ca²⁺ sites occupied at 10^-8, 10^-7 and 10^-6 M Pb after 96 h. At a constant Pb concentration of 10^-6 M, fractional sorption increases with increasing particle loading, however surface coverage decrease with 72.5 and 22.1% Ca²⁺ sites occupied at 100 and 200 mgL^-1 calcite after 96 h. The adsorption coefficient (K_d) is approximately 10³, increases with initial Pb concentration, but remains unaffected by variable calcite loading. Absence of an ionic strength effect on Pb sorption is interpreted as the dominance of inner-sphere complexation. For desorption experiments conducted over a range of initial sorption times, an average desorption index > 0.8 but < 1 indicates that sorption is largely reversible, but is accompanied by slight incorporation. Solid-solution formation increases with time, as observed by slower initial desorption rates for samples with longer sorption times. These findings indicate that Pb may be effectively sequestered by calcite; however re-release via desorption is likely as Pb does not become significantly incorporated into the mineral structure.