The key effects of polymorphism during PbII uptake by calcite and aragonite

Abstract

Lead can be extracted from contaminated water and fixed as a solid phase by a dissolution–precipitation reaction converting abundant Ca carbonates into sparingly soluble Pb carbonates. Herein, we investigate the mechanism of calcite (CAL) and aragonite (ARG) recrystallization in acidic Pb(NO₃)₂ solutions and demonstrate that the efficiency of this process depends strongly on the crystal structure of Ca carbonate used. The lower reactivity of aragonite is related to surface passivation by a continuous layer of cerussite, which is isostructural with ARG (both orthorhombic). The amount of CaCO₃ converted into PbCO₃ was determined for both CAL and ARG: after 10 days of interaction, 15 ± 2% of calcite and 6 ± 1% of aragonite were transformed into cerussite (maximum theoretical conversion yield of 50%: 2 mmol CaCO_3(s) interacting with 1 mmol Pb_(aq)). In situ atomic force microscopy (AFM) allowed the observation of the mechanism of calcite dissolution in the presence of Pb(NO₃)₂ solutions; we observed a stabilization of the steps along the [010] direction. Pb²⁺ ions were demonstrated to be responsible for the change in the morphology of the dissolving calcite surface; nitrate ions alone did not induce any similar distortion under the same conditions. Two different Pb-bearing phases were formed during the AFM experiments. The most abundant product (cerussite) was made of elongated crystals randomly distributed on the {104} surface of calcite. A minor amount of another Pb-bearing phase was observed; this hexagonal-shaped phase (hydrocerussite) was imaged while growing with a spiral mechanism along the c-axis.