Thorium speciation in synthetic anhydrite: Implications for remediation and recovery of thorium from rare-earth mine tailings

Knowledge about the capacity and mechanisms of thorium uptake in anhydrite is required for not only the safe management of radioactive rare earth element (REE) mine tailings but potential recovery/remediation of this radionuclide from these waste materials for the fourth-generation nuclear power plants. In this study, we investigated Th-doped anhydrite synthesized in both aqueous and sulfuric acid solutions, including samples grown in conditions simulating the Sulfuric Acid Roasting Method (SARM) with monazite-(Ce) ores, at a wide range of temperatures and pH values. Inductively coupled plasma mass spectrometry analyses show samples grown hydrothermally at 353–473 K and pH = 0.3–3 contain 2310–8880 mg/L Th, and 1780 mg/L Th when anhydrite was synthesized by evaporation at 551 K and pH = 0.3. Synchrotron Th LIII-edge X-ray absorption spectroscopy reveals that lattice-bound Th in anhydrite, via the substitution Th4+ + O2− ⇄ Ca2+ with an interstitial O2− ion, can account for at least 1780 mg/L Th, while Th-rich phases such as thorianite and thorite are present in those with still higher Th contents. Therefore, lattice-bound Th in anhydrite must be considered for safe management strategies and future extraction technologies for the recovery/remediation of Th from radioactive REE mine tailings. Synopsis This study suggests that anhydrite is a major carrier of thorium in the tailings of rare earth element mines. Recovery of thorium from tailings not only reduces their radioactivity, but represents a valuable resource for nuclear fuel production.