The efficiency of Pb uptake by CaCO3 is different for calcite and aragonite due to surface passivation.

No abstract

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 (K s ≈ 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 CaCO 3 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) -CaCO 3(s) systems, thus increasing the predictability of limestone efficiency toward the uptake of heavy metals.

One of the most promising strategies for the safe and permanent disposal of anthropogenic CO2 is its conversion into carbonate minerals via the carbonation of calcium and magnesium silicates. However, the mechanism of such a reaction is not well constrained, and its slow kinetics is a handicap for the implementation of silicate mineral carbonation as an effective method for CO2 capture and storage (CCS). Here, we studied the different steps of wollastonite (CaSiO3) carbonation (silicate dissolution → carbonate precipitation) as a model CCS system for the screening of natural and biomimetic catalysts for this reaction. Tested catalysts included carbonic anhydrase (CA), a natural enzyme that catalyzes the reversible hydration of CO2(aq), and biomimetic metal-organic frameworks (MOFs). Our results show that dissolution is the rate-limiting step for wollastonite carbonation. The overall reaction progresses anisotropically along different [hkl] directions via a pseudomorphic interface-coupled dissolution–precipitation mechanism, leading to partial passivation via secondary surface precipitation of amorphous silica and calcite, which in both cases is anisotropic (i.e., (hkl)-specific). CA accelerates the final carbonate precipitation step but hinders the overall carbonation of wollastonite. Remarkably, one of the tested Zr-based MOFs accelerates the dissolution of the silicate. The use of MOFs for enhanced silicate dissolution alone or in combination with other natural or biomimetic catalysts for accelerated carbonation could represent a potentially effective strategy for enhanced mineral CCS.