Hydration Effects on the Stability of Calcium Carbonate Pre-Nucleation Species

Carlos Rodriguez-Navarro,Christine Putnis,Encarnacion Ruiz-Agudo,Alejandro Burgos-Cara

doi:10.3390/min7070126

Carlos Rodriguez-Navarro, Christine Putnis + Show 2 more

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https://doi.org/10.3390/min7070126

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Abstract

Recent experimental evidence and computer modeling have shown that the crystallization of a range of minerals does not necessarily follow classical models and theories. In several systems, liquid precursors, stable pre-nucleation clusters and amorphous phases precede the nucleation and growth of stable mineral phases. However, little is known on the effect of background ionic species on the formation and stability of pre-nucleation species formed in aqueous solutions. Here, we present a systematic study on the effect of a range of background ions on the crystallization of solid phases in the CaCO3-H2O system, which has been thoroughly studied due to its technical and mineralogical importance, and is known to undergo non-classical crystallization pathways. The induction time for the onset of calcium carbonate nucleation and effective critical supersaturation are systematically higher in the presence of background ions with decreasing ionic radii. We propose that the stabilization of water molecules in the pre-nucleation clusters by background ions can explain these results. The stabilization of solvation water hinders cluster dehydration, which is an essential step for precipitation. This hypothesis is corroborated by the observed correlation between parameters such as the macroscopic equilibrium constant for the formation of calcium/carbonate ion associates, the induction time, and the ionic radius of the background ions in the solution. Overall, these results provide new evidence supporting the hypothesis that pre-nucleation cluster dehydration is the rate-controlling step for calcium carbonate precipitation.

Highlights

Calcium carbonate precipitation has been widely studied due to the extensive distribution of carbonates, predominantly calcium carbonate, in surface rocks of the earth and scale formation in industrial processes
The latter frequently leads to the stabilization of more soluble metastable phases, such as amorphous phases, that are known to play a key role in biomineralization processes [9,10,11,12]
It should be considered that water molecules in the solvation shell of a solute ion are stabilized by the presence of background counterions in the solution [65,68]

Summary

Introduction

Calcium carbonate precipitation has been widely studied due to the extensive distribution of carbonates, predominantly calcium carbonate, in surface rocks of the earth and scale formation in industrial processes It presents a relatively simple model system to work with, and its wide occurrence in many biominerals provides interdisciplinary significance [1,2]. Nucleation is a precondition for mineral growth, and is interpreted as an energetic event in which a system tends towards a reduction in its total free energy once activation energy barriers are overcome [52,53] In this energetic scenario, several different interactions have to be considered (i.e., water-water, ion-water, and ion-ion interactions) [54], which will affect both the pre-nucleation and nucleation regimes

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