Sustainable freshwater recovery from radioactive wastewater by gas hydrate formation

Abstract

Freshwater production from radioactive wastewater is a key issue not only to minimize environmental contamination by nuclear power plants but also to develop new types of nuclear technology such as small modular reactors. Here, we present a sustainable hydrate-based desalination (HBD) technology for freshwater recovery from radioactive wastewater, specifically from Cs, Sr, and I-containing wastewater. CHClF2 and SF6 were selected as a hydrate former for HBD processes, which can be operated under relatively mild conditions. X-ray diffraction and Raman spectroscopy measurements indicated that the crystal structure of the CHClF2 and SF6 hydrates formed with radioactive chemicals was identical to that of pure CHClF2 and SF6 hydrates, respectively. Formation kinetics experiments were performed to investigate the inhibition effect of Cs+, Sr2+, and I− on CHClF2 and SF6 hydrates, indicating that the apparent rate constant, initial reaction rate, and pressure drop caused by the formation of gas hydrate significantly depend on the sub-cooling temperature and concentrations of the radioactive chemicals. A close examination using scanning electron microscopy with energy dispersive X-ray spectroscopy and in situ temperature-dependent solid-state NMR spectroscopy found evidence of the exclusion of radioactive chemicals from solid gas hydrates, signaling the potential of HBD technology for use in freshwater production from radioactive wastewater. More importantly, the proposed HBD process is applicable to wastewater in the very large concentration ranges of Cs+, Sr2+, and I− covering from the percent to hundreds of parts per million (ppm) and even the sub-ppm ranges, revealing equivalent recovery and separation efficiency rates for all wastewater solutions. We conclusively present a clear-cut picture of the dual-functional HBD process of freshwater recovery and radioactive chemical separation from wastewater. © 2023 Elsevier B.V.