The effect of aging and pH on Pb(II) sorption processes at the calcite-water interface

The effect of aging on Pb(II) retention in 1 μM Pb, calcite suspensions at pH 7.3, 8.2, and 9.4, under room-temperature conditions, was explored via a combination of batch sorption-desorption experiments and X-ray absorption spectroscopy (XAS). Short-term experiments, up to 12 days, reveal the predominance of an adsorption mechanism at pH 8.2, as confirmed by XAS analysis. Linear-combination fitting of XANES spectra indicates a dual sorption mechanism, with ∼95% adsorbed and ∼5% coprecipitated, and ∼75% adsorbed and ∼25% coprecipitated Pb at pH 7.3 and 9.4, respectively. For long-term sorption, 60-270 days, slow continuous uptake occurs at pH 7.3 and 8.2, determined by EXAFS to be due to an adsorption mechanism. At pH 9.4, no further uptake occurs with aging, and the solid-phase distribution of Pb is commensurate with that for short-term experiments, suggesting that coprecipitated metal may alterthe calcite surface precluding further Pb sorption. Desorption experiments indicate that at pH 7.3 and 8.2 long-term sorption products-constituted primarily of Pb inner-sphere adsorption complexes-are reversibly bound. For aged pH 9.4 samples, significant sorption irreversibility indicates that the coprecipitated component is not readily exchangeable with the aqueous phase, and thus coprecipitation may be effective for long-term metal sequestration.