All-Inorganic Open-Framework Chalcogenides, A₃Ga₅S₉·xH₂O (A = Rb and Cs), Exhibiting Ultrafast Uranyl Remediation and Illustrating a Novel Post-Synthetic Preparation of Open-Framework Oxychalcogenides

Fast and effective uranyl sequestration is of interest to the nuclear industry. Recently, layered chalcogenide materials have demonstrated fast, selective, and efficient sorption properties toward uranyl cations, and the development and investigation of new types of chalcogenide materials continues to be of interest and represents an intriguing option for uranyl remediation. Three new all-inorganic A3Ga5S9·xH2O (A = Rb, Rb/Cs, and Cs) open-framework chalcogenides were obtained via an in situ alkali carbonate to alkali sulfide conversion process achieved under mild hydrothermal conditions. The structures of the all-inorganic open-framework chalcogenides consist of a twofold interpenetrated diamond-like 3D framework containing pseudo-T3 [Ga10S20]10– supertetrahedra. Forty-eight percent of the structural volume is occupied by A+ cations and water species, as established by single-crystal X-ray diffraction (SCXRD), infrared (IR) spectroscopy, and energy-dispersive spectroscopy (EDS). The dynamic nature of the A+ cations and water molecules within the pores was investigated via SCXRD as well as by IR spectroscopy-monitored H2O-to-D2O exchange experiments. Framework stability was probed with post-synthetic treatment of A3Ga5S9·xH2O (A = Rb and Cs) samples in acidic solutions that resulted in the formation of the oxysulfide (A/H)3Ga5S9–yOy·xH2O (A = Rb and Cs; y = 0–1), as shown by SCXRD and IR. Ion-exchange studies on A3Ga5S9·xH2O (A = Rb and Cs) samples were carried out utilizing a uranyl acetate solution. The presence of the UO22+ species in the ion-exchange product was supported by IR spectroscopy and EDS. Batch method ion-exchange experiments on Cs3Ga5S9·xH2O powder demonstrated fast kinetics with 95% uranyl removal from the uranyl acetate solution during the first minute, a maximum uranyl uptake capacity of 15 mg/g, and the subsequent elution of uranyl species with KCl solution. The porous and dynamic nature of the A3Ga5S9·xH2O framework coupled with effective UO22+···S2– bonding interactions makes it a good potential sorbent for uranyl remediation from aqueous media.