Separation of lattice-incorporated Cr(vi) from calcium carbonate by converting microcrystals into nanocrystals via the carbonation pathway based on the density functional theory study of incorporation energy

Abstract

Calcium carbonate easily allows foreign ions, such as Cr(VI), to incorporate into its lattice, leading to the long-term slow release of heavy metals and consequent environmental risk. Although CaCO₃ is one of the major minerals in Cr-containing solid waste, little is known regarding the chemical speciation, potential for release and methods of separation of Cr(VI) from CaCO₃. In this study, density functional theory (DFT) calculations were performed to figure out the Cr(VI) incorporation energy in different polymorphs of CaCO₃. It was revealed that when co-occurring with calcite and vaterite, CrO₄²⁻ is more easily incorporated into vaterite than in the calcite lattice due to lower doping resistance, while changing the Cr(VI) species from CrO₄²⁻ to HCrO₄⁻ and Cr₂O₇²⁻ can significantly enhance the doping resistance. Based on these theoretical results, a CO₂-assisted phase transformation method is proposed to separate lattice-incorporated Cr(VI) from CaCO₃. Characterization results show that during carbonation (with 3 MPa CO₂ and a liquid : solid ratio of 10 : 1) under acidic conditions generated by CO₂ and H₂O, CaCO₃ microcrystals were converted to nanocrystals through a dissolution-reprecipitation process, and the incorporated Cr(VI) was released. Furthermore, increased doping resistance was observed, as Cr(VI) could not be re-doped onto nano-CaCO₃ due to the ionic species changing from CrO₄²⁻ to HCrO₄⁻ and Cr₂O₇²⁻. As a result, 77.1% of incorporated Cr(VI) was separated from CaCO₃ and the Cr(VI) toxicity of the residue decreased from 19.5 mg L⁻¹ to 0.27 mg L⁻¹. Furthermore, predictive experiments of the Cr(VI) leaching characteristics indicated that the stability of the residue was notably enhanced after carbonation treatment. This CO₂-assisted phase transformation system provides a promising method for the separation of lattice-incorporated heavy metal pollutants, using a simple and low-cost method to mitigate the risk posed by the slow release of heavy metals from solid waste.