Efficient extraction of slowly-released Cr(vi) from nano-sized ion channels in Cr(vi)–ettringite from reduced chromite ore processing residue

Abstract

Residual unreacted Cr(VI) incorporated in ferrous-sulfate-reduced chromite ore processing residue (rCOPR) can be slowly released during deposition, posing a significant threat to the environment. Recent studies indicate that such Cr(VI) mainly exists within the nano-sized ion channels of ettringite, forming Cr(VI)–ettringite. Therefore, efficient extraction and recovery of the incorporated Cr(VI) from rCOPR, especially from Cr(VI)–ettringite, are urgently needed in order to eliminate the long-term hazardous effects of rCOPR. In this work, NaHCO₃ was used to extract Cr(VI) from rCOPR with or without hydrothermal treatment, and the process under hydrothermal conditions (up to 120 °C) was able to nearly completely (>98%) extract Cr(VI). The results of X-ray diffraction, aberration-corrected scanning transmission electron microscopy, and X-ray absorption fine structure spectroscopy analyses were combined to elucidate the underlying extraction mechanisms. First, CO₃²⁻ exchanged with CrO₄²⁻ in the ion channels of Cr(VI)–ettringite, transforming ettringite into nano-CaCO₃. This transformation effectively destroyed the ion-channel structure of Cr(VI)–ettringite, causing the release of incorporated CrO₄²⁻. However, in this stage, some CrO₄²⁻ ions were still strongly adsorbed onto the nano-CaCO₃, leading to the incomplete extraction of Cr(VI). Furthermore, the hydrothermal treatment was demonstrated to be essential to the acceleration of the nano-CaCO₃ crystal growth to achieve the full Cr(VI) extraction. This novel strategy provides a viable approach to eliminate the pollution associated with the slow release of Cr(VI) from rCOPR in a fast and cost-effective manner.