Enhancing the purification performance of layered mica for thallium-containing wastewater by mechanochemical activation – An ingenious insight into the pathway of lattice state immobilization

Abstract

The deep treatment of monovalent thallium (Tl+) and the study of Tl+ incorporation mechanism are of great value for water safety. However, current research objectives have focused on how to oxidize or precipitate Tl+, while neglecting to investigate the underlying mechanisms of the direct immobilization and stable storage state of Tl+. Therefore, the present study examines in detail how Tl+ is directly incorporated and stably stored by silicate minerals. In this study, an idea is proposed to enhance the purification of Tl-containing wastewater by using mechanically activated mica to stabilize Tl+ in the silicate structure in the form of lattice residue state. Elucidation of the removal mechanism was first performed with focus on Tl+ exchange with interlayer K+ and H(H2O)+ inside the silicate mineral based on the quantitative data and a series of characterizations such as XRD, XPS, FTIR, SEM-EDS and TG. As adsorbents for Tl-containing wastewater, both activated phlogopite and biotite for the purification of target wastewater demonstrated excellent performances at a dose of 2 g/L and a reaction time of 240 min, reaching quantitatively down to 23.88 μg/L and nearly 0 μg/L respectively, from an initial Tl+ concentration of 1203 μg/L, so that the effluent concentrations would meet the requirements of US EPA and Chinese discharge standards for industrial wastewater and drinking water. In addition, density functional theory (DFT) calculation was used to quantify the adsorption energy of Tl+ being solidified in the interlayer structure to further give insight into the mechanism of Tl+ immobilization by silicate mineral.