The kinetics of sorption of uranium(VI) compounds from sulfate and carbonate solutions using four samples of mesoporous zirconium-silica nanosorbents obtained by bitemplate (solubilization) synthesis was studied. The sorption equilibrium set in time and the kinetic characteristics of sorption were shown to depend on the sorbent (its composition, specific surface area, dispersity, and pore size), the temperature, and the composition and pH of the solution from which uranium compounds are sorbed. The sorption kinetics was described by a first order equation. The limiting stage of the process was found to be the external diffusion of uranium containing particles to the sorbent surface.

Organic-inorganic ion-exchangers were obtained by incorporation of zirconium hydrophosphate into gel-like strongly acidic polymer matrix by means of precipitation from the solution of zirconium oxychloride with phosphoric acid. The approach for purposeful control of a size of the incorporated particles has been developed based on Ostwald-Freundich equation. This equation has been adapted for precipitation in ion exchange materials. Both single nanoparticles (2–20 nm) and their aggregates were found in the polymer. Regulation of salt or acid concentration allows us to decrease size of the aggregates approximately in 10 times. Smaller particles are formed in the resin, which possess lower exchange capacity. Sorption of U(VI) cations from the solution containing also hydrochloride acid was studied. Exchange capacity of the composites is ≈2 times higher in comparison with the pristine resin. The organic-inorganic sorbents show higher sorption rate despite chemical interaction of sorbed ions with functional groups of the inorganic constituent: the models of reaction of pseudo-first or pseudo-second order can be applied. In general, decreasing in size of incorporated particles provides acceleration of ion exchange. The composites can be regenerated completely, this gives a possibility of their multiple use.

Bitemplate (solubilization) synthesis was used to obtain new mesoporous zirconium-silica nanosorbents that can be successfully used to recover uranium compounds from sulfate and carbonate solutions.Studies concerned with sorption methods for recovery of uranium compounds from aqueous solutions, seawater, and wastewater pay primary attention to synthesis of new, or modifi cation of already known, sorbents in order to raise their sorption capacity for uranium, improve their selectivity, and elucidate the sorption mechanism in relation to the pH of the medium and forms in which the sorbate is present in solution.The following sorbents have been used: natural minerals (quartz, muscovite, labradonite [1], pozzolan [2]); zeolites [3], including organozeolites [4] produced by treatment of clinoptilolite-containing tuffs with polyhexamethylene guanidine; sorbents based on wood and lignin [5] with high content of phosphorus; polymers: polymeric chelate sorbents [6]; biopolymer chitosan [7]; Al, Si, Ti oxides, sulfi des, and phosphates, including hydrogels based on the SiO 2 -TiO 2 formulation (in hydrogen form) [8]; crystalline TiO 2 modifi ed with amorphous aqueous SiO 2 [9], titanium phosphates and phosphosilicates [10]; ion-exchange resins: phosphoric acid resins and sulfocation-exchange resins [11]; anionexchange resins: macroporous [12], low-basic [13], high-basic [14], and resins of gel and porous structure [15]; activated carbons: oxidized [16] and modifi ed with arsenazo reagents [17].The wide assortment of sorbents enables their choice for recovery of uranium compounds from aqueous media of various compositions. At the same time, there hardly is any evidence about purposeful synthesis of sorbents with prescribed pore sizes that would match the size of molecules or ions of uranium compounds being sorbed. However, the number of studies in this area has noticeably increased recently (2007)(2008)(2009)(2010) [18][19][20][21][22].The goal of our study was to fi nd whether recovery of uranium(VI) compounds from aqueous solutions by sorption with zirconium-silica nanosorbents is possible and can be effi cient. EXPERIMENTALZirconium-silica nanosorbents were synthesized by bitemplate (solubilization) method from inexpensive reagent: sodium silicate, zirconyl chloride, and industrial fraction of a surfactant [23,24]. Freshly synthesized starting samples were subjected to hydrothermal (980°C) and thermal (600°C) treatments. The specifi c surface of the samples (Table 1) was determined from the argon sorption by the BET method [25] (sample nos. 1-10) and from isotherms of low-temperature sorption-desorption of nitrogen (Fig. 1, sample nos. 1 and 9), measured with Quantachrome NovaWin2 instrument. The pore size distribution was found from the desorption isotherms by using the BJH model.As objects of study served sulfate and carbonate solutions of uranium(VI), which contained 2 × 10 -4 mol (50 mg) of the metal per liter. The concentration of

The adsorption behaviour of tributyl phosphate (TBP) on silica gel was studied. It was found that the adsorption isotherm shapes were complicated, being considered as S- and L-type isotherms according to the Giles classification. Adsorption itself was polymolecular and of a physical nature, the first adsorptive layer involving hydrogen bonding between the protons of surface silanol groups and the electron-donating oxygen atom of the adsorbate molecule. Subsequent adsorption layers were formed via van der Waals interaction. The free energy of adsorption of the system lay between −22.1 kJ/mol and −23.8 kJ/mol. The enthalpy change was negative and very small, i.e. −6.3 kJ/mol, while the entropy change was positive and in the range 53.9 J/(mol K) to 55.4 J/(mol K). The increase in entropy was explained in terms of the mobility of the TBP molecules in the adsorptive layer arising from their replacement on the silica gel surface by water molecules derived from the aqueous medium.

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