Recovery of rare-earth and radioactive elements from contaminated water through precipitation: A review

Recovery of rare-earth elements (REEs) and radioactive elements have become an important issue because they are precious materials for economic growth, environmental protection, and national security. This study provides a comprehensive review of precipitation processes for the recovery of REEs and radioactive elements according to precipitant type and precipitation mechanism. The optimal recovery performance of these approaches varies with several water quality factors, namely, solution pH, dissolved oxygen, temperature, the presence of co-ions, ionic strengths, the initial concentration of target elements, and the presence of organic substances (e.g., humic acids). For instance, an interesting phenomenon was observed when U(VI) was reductively precipitated by ferrous ion under high-O2 conditions, since precipitated U(IV) on iron surfaces reoxidized during groundwater remediation when it is exposed to O2. These optimal recovery performance and its precipitation mechanisms are clarified via three main characterization approaches: (i) optical and electron microscopy approaches, (ii) X-ray diffraction–based approaches, and (iii) spectroscopy approaches. These characterization methods are complementary and can enhance the reliability of precipitation mechanisms. Lastly, a concise overview is provided regarding the complex systems and potential research challenges of the precipitation process for the recovery of REEs and radioactive elements, based on the fact that complex recovery systems could be optimized by consideration of mass-balance calculations, economical assessment, timescale, and redox sources from an engineering perspective.