posted on 2020-08-20, 01:29authored byFulvio Di Lorenzo, Georgia Cametti, Dimitri Vanhecke, Sergey V. Churakov
The
possibility to develop a process for lead removal from wastewater,
based on calcium carbonate minerals, depends on the overall efficiency
of the uptake process. Aqueous Pb tends to form cerussite (CER) via
a dissolution–precipitation reaction when interacting with
aragonite (ARG) and calcite (CAL). From a thermodynamic perspective,
the two processes have a similar driving force, because the solubility
of CAL and ARG is approximatively the same (Ks ≈ 1 × 10–8). Experimentally,
the macroscopic yield of reaction was found to be very different.
Using ex situ electron microscopy, diffraction, and in situ atomic
force microscopy, we demonstrate that the Pb uptake mechanism by the
two most abundant CaCO3 polymorphs is controlled by the
kinetics of processes at the solid–solid and solid–liquid
interfaces. Aragonite is isostructural with the product phase (CER)
that easily precipitates taking advantage of the template effect offered
by the surfaces of the substrate. The reaction proceeds through an
interface-coupled dissolution–precipitation that leads to a
mineral replacement. Because of a crystallographic mismatch, the reaction
between CAL and CER mainly occurs as a simple solvent-mediated transformation.
Our study unveiled the mechanistic reasons behind the different reaction
yields shown by CAL and ARG toward Pb uptake. Similar conclusions
can be extended to other contaminant(aq)-CaCO3(s) systems, thus increasing the predictability of limestone efficiency
toward the uptake of heavy metals.