The key effects of polymorphism during Pb II uptake by calcite and aragonite

Di Lorenzo, Fulvio; Ruiz-Agudo, Cristina; Churakov, Sergey V. (2019). The key effects of polymorphism during Pb II uptake by calcite and aragonite. CrystEngComm, 21(41), pp. 6145-6155. Royal Society of Chemistry 10.1039/C9CE01040H

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Lead can be extracted from contaminated water and fixed as a solid phase by a dissolution–precipitationreaction converting abundant Ca carbonates into sparingly soluble Pb carbonates. Herein, we investigatethe mechanism of calcite (CAL) and aragonite (ARG) recrystallization in acidic PbIJNO3)2solutions anddemonstrate that the efficiency of this process depends strongly on the crystal structure of Ca carbonateused. 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 CaCO3converted into PbCO3wasdetermined for both CAL and ARG: after 10 days of interaction, 15±2% of calcite and 6±1% of aragonitewere transformed into cerussite (maximum theoretical conversion yield of 50%: 2 mmol CaCO3IJs)interacting with 1 mmol Pb(aq)).In situatomic force microscopy (AFM) allowed the observation of themechanism of calcite dissolution in the presence of PbIJNO3)2solutions; we observed a stabilization of thesteps along the [010] direction. Pb2+ions were demonstrated to be responsible for the change in themorphology of the dissolving calcite surface; nitrate ions alone did not induce any similar distortion underthe same conditions. Two different Pb-bearing phases were formed during the AFM experiments. The mostabundant product (cerussite) was made of elongated crystals randomly distributed on the {104} surface ofcalcite. A minor amount of another Pb-bearing phase was observed; this hexagonal-shaped phase(hydrocerussite) was imaged while growing with a spiral mechanism along thec-axis

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