Uranium Removal from Aqueous Solutions by Aerogel-Based Adsorbents—A Critical Review

Abstract

Aerogels are a class of lightweight, nanoporous, and nanostructured materials with diverse chemical compositions and a huge potential for applications in a broad spectrum of fields. This has led the IUPAC to include them in the top ten emerging technologies in chemistry for 2022. This review provides an overview of aerogel-based adsorbents that have been used for the removal and recovery of uranium from aqueous environments, as well as an insight into the physicochemical parameters affecting the adsorption efficiency and mechanism. Uranium removal is of particular interest regarding uranium analysis and recovery, to cover the present and future uranium needs for nuclear power energy production. Among the methods used, such as ion exchange, precipitation, and solvent extraction, adsorption-based technologies are very attractive due to their easy and low-cost implementation, as well as the wide spectrum of adsorbents available. Aerogel-based adsorbents present an extraordinary sorption capacity for hexavalent uranium that can be as high as 8.8 mol kg–1 (2088 g kg–1). The adsorption data generally follow the Langmuir isotherm model, and the kinetic data are in most cases better described by the pseudo-second-order kinetic model. An evaluation of the thermodynamic data reveals that the adsorption is generally an endothermic, entropy-driven process (ΔH0, ΔS0 > 0). Spectroscopic studies (e.g., FTIR and XPS) indicate that the adsorption is based on the formation of inner-sphere complexes between surface active moieties and the uranyl cation. Regeneration and uranium recovery by acidification and complexation using carbonate or chelating ligands (e.g., EDTA) have been found to be successful. The application of aerogel-based adsorbents to uranium removal from industrial processes and uranium-contaminated waste waters was also successful, assuming that these materials could be very attractive as adsorbents in water treatment and uranium recovery technologies. However, the selectivity of the studied materials towards hexavalent uranium is limited, suggesting further developments of aerogel materials that could be modified by surface derivatization with chelating agents (e.g., salophen and iminodiacetate) presenting high selectivity for uranyl moieties.

Radionuclide Removal from Aqueous Solutions Using Oxidized Carbon Fabrics

“…Furthermore, because the mol-equivalent concentration of the carboxylic groups on the adsorbents surface was several orders of magnitude higher than the metal ion (radionuclide) concentration, the binding of the radionuclides by the carboxylic surface groups was expected to be the predominant adsorption mechanism. Moreover, in the near-neutral and alkaline pH region, hydrolysis and carbonate complexation of the actinide ions becomes predominant, resulting in the formation of ternary surface complexes at pH 7 and at pH 9 in the stabilization of the actinide ions as carbonate complexes, because the carbonate anion is the predominant ligand in the alkaline region under ambient conditions [10,11].…”

“…Uranium, under ambient conditions, basically exists in its hexavalent form as a uranyl cation (UO 2 2+ ) and in environmental waters can undergo hydrolysis [6,9] and carbonate complexation [10,11], and can interact with inorganic and organic colloids and mineral/rock surfaces [12,13]. In seawater, the U(VI)-tricarbonato complex (UO 2 (CO 3 ) 3 4− ) is the prevailing U(VI) species and stabilizes uranium in the aqueous phase.…”

Radioactivity/Radionuclide (U-232 and Am-241) Removal from Waters by Polyurea-Crosslinked Alginate Aerogels in the Sub-Picomolar Concentration Range

“…These materials include organic, inorganic oxide, chalcogen, carbon and composite aerogels. A detailed presentation of their sorption capacity and the critical parameters that affect their performance is presented in recent publications [ 9 , 10 ]. Among them, hydroxyapatite [ 11 , 12 ], polyurea-crosslinked calcium alginate (X-alginate) [ 9 ] and rGO/ZIF-67 (rGO: reduced graphene oxide; ZIF-67: cobalt-based zeolitic imidazole framework) [ 13 ] aerogels have sorption capacities close to or above 2000 g/kg (i.e., 2088, 2023 and 1888 g/kg, respectively).…”

Removal of 241Am from Aqueous Solutions by Adsorption on Sponge Gourd Biochar

“…A wide spectrum of physical, chemical, and biological processes has been developed and applied for the removal of radionuclides from the nuclear process and contaminated wastewaters including membrane and osmosis technologies, sorption and ion exchange, chemical precipitation, electroremediation, and bioremediation [ 8 ]. Adsorption technologies are of particular interest since effective adsorbents such as metal organic frameworks [ 9 ], aerogels [ 10 , 11 , 12 ], silica-based materials [ 13 ], polymers [ 14 , 15 ], biomasses [ 16 , 17 , 18 , 19 ], and carbon-based composites [ 20 , 21 , 22 , 23 , 24 , 25 ] can be prepared easily and at low-cost. One such material is biochar, prepared by the pyrolysis of residual dry biomass and agricultural waste.…”