Simulation of FeCO3 ion clusters in aqueous solution: Implications for crystal growth

Abstract

Molecular dynamics simulations are used to investigate the early stages of FeCO3 crystallization from aqueous solution. A model is presented that predicts values for the free energies and enthalpies of hydration of Fe(II) and CO32- ions and water exchange rates about Fe(II) that are in good agreement with available experimental rates obtained from NMR spectroscopy. The predicted free energy of association for the ions is consistent with reported stability constants for the FeCO3° ion pair and Fe(CO3)22- triple ion. The model also predicts that the formation of Fe2CO32+ and other small ion clusters is favored over dissociated ions at equilibrium. At supersaturated conditions accessible by simulation, initial iron carbonate cluster species grow by ion addition and aggregation into linear and branched hydrated chains. Appearance of crystalline order within the clusters may occur upon further collapse of these extended structures into more condensed species.