Study on the Solidification Mechanism of Cr in Ettringite and Friedel’s Salt

Abstract

The solidification of heavy metal Cr has always been a challenge in the treatment of Cr-containing wastes, due to its high mobility in alkaline environments. In addition, the solidification mechanism of Cr has not been fully investigated. In this study, blast furnace slag (BFS)-based cementitious materials were used as binders for the immobilization of heavy metal Cr to investigate the solidification mechanism of Cr in different hydration products. From XRD, FTIR, XPS, and XANES analyses, it could be seen that SO42− in ettringite was replaced by Cr in the form of CrO42−, making SO42− re-dissolve in the liquid phase. The SO42− in the solution would compete with CrO42− ions, leading to the direct influence of SO42− content on the solidification efficiency of Cr. In ettringite, Cr mainly existed in the form of Cr6+, accounting for more than 84% however, Cr was solidified in Friedel’s salt under two coexisting valence states (Cr6+ and Cr3+). This resulted not only from the slow excitation rate of the BFS in the cementitious system that did not contain sulfate, but also from the existence of a certain amount of reducing substances in the BFS, such as Fe2+ and S2−, which could reduce some of Cr6+ to Cr3+. In Friedel’s salt, the residual Cr6+ replaced Cl− in the form of CrO42−, whereas the Cr3+ replaced Al3+. The binding energies of Cr 2p3/1 and Cr 2p3/2 decreased with the addition of Cr, indicating that the coordination numbers of Cr3+ and Cr6+ increased, and that the binding energies of Cr3+ and Cr6+ decreased after entering the structure of Friedel’s salt.