The Influence of Aqueous Se(IV) on the Stability of Different CaCO₃ Polymorphs Precipitated under Ambient Conditions

Abstract

Selenium is an essential bio-element, but because of its bioaccumulation potential, it can become toxic and is an important pollutant. The ubiquitous mineral calcite (CaCO3) has the ability to immobilize anions as SeO32− by different sorption or coprecipitation processes. Experimental studies have found that SeO32− can incorporate in the crystal structure of calcite by substituting CO32−. The presence of foreign ions in aqueous solution strongly affects CaCO3 precipitation, helping stabilize less stable polymorphs such as vaterite and aragonite or hydrated phases. In this work, we studied the aging process of calcium carbonates precipitated from aqueous solutions highly supersaturated with respect to CaCO3 and slightly supersaturated with respect to CaSeO3·H2O under ambient conditions, for times up to 30 days in which solids were kept in the remaining aqueous solution. Under these conditions, CaCO3 precipitated mainly as low crystallinity vaterite aggregates that hosted up to 16% atomic ratio Se:C. Vaterite purified and increased its crystallinity with aging time, but the vaterite–calcite transformation was strongly inhibited. The incorporation of Se(IV) in vaterite did not significantly affect the cell parameters or the external morphology of the aggregates. The precipitation of selenite as CaSeO3·H2O was conditioned by the availability of free Ca2+ and SeO32− that was not previously incorporated into precipitated carbonates.

Keywords

• calcium carbonate polymorphism
• selenite
• calcium carbonate precipitation
• vaterite