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The paper describes how the conditions of preparing ferrocyanides on hydrated titanium dioxide support affect the surface texture of the resulting materials, their elemental and phase composition, and their ability to sorb cesium. The sorbents prepared under the optimal conditions exhibited increases specificity to Cs (K d = 10 5.6±1.0 ml g -1 ) and high capacity (no less than 270 mg g -1 ).Many-year research and the world practice show that mixed transition metal ferrocyanides are the most specific sorbents for cesium radionuclides. Various procedures for preparing ferrocyanides have been described in the literature: precipitation followed by freeze granulation or drying [1], deposition of thin ferrocyanide films onto the surface of inert (glass, polyethylene terephthalate, Millipore polypropylene membranes) [2][3][4] or sorption-active (clinoptilolite, glauconite) [4-6] supports, preparation of composite sorbents consisting of transition metal ferrocyanides and silica gel [7], alumina gel [8], ion-exchange resins [9], or Taunit carbon nanostructured materials [10], and sol-gel method [11].It is noted in practically all the published papers that each synthesis step strongly affects the sorptionkinetic and service characteristics of the sorbents. The properties of the sorbents prepared are determined by the chemical composition, pore structure, and degree of perfection of the sorbent crystal lattice. By varying the pore structure of the material or the chemical nature of the sorbent surface, it is possible to control and improve the sorption properties.It was shown in [2-6] that thin-layer sorbents exhibit the maximally developed surface, the sorption activity that is maximum possible for granulated materials, and good kinetic characteristics; their sorption capacity is utilized to the largest extent. To solve practical problems in radiochemistry and related fields, it is always possible to choose a support for the subsequent preparation of a sorption material exhibiting a set of the required properties. Apparently, the most promising line in the synthesis of new sorption materials is preparation of sorbents by surface modification of sorption-active supports exhibiting highly developed surface. Our studies showed that in this case it is possible to prepare highly specific polyfunctional and complex sorbents. The relationships in formation of the new sorption-active phase, observed in the course of the modification, resemble those observed not only in the growth of thin films on inert supports, but also in the preparation of sorption reagent systems [12].

The influence of surface modification of hydrated titanium dioxide (HTD) by mixed nickel-potassium ferrocyanide (NPF) phase on its selectivity for strontium and mechanism of strontium sorption was studied. It was shown that NPF-HTD sorbent possesses the same selectivity for strontium as the HTD sorbent, but higher static exchange capacity (SEC). The Henry distribution coefficient of strontium from a tap water with pH 7.8 ± 0.1 was 10 (3.0 ± 0.3) L kg -1 ; the SEC, calculated according to Langmuir's isotherm, was 140 mg g -1 . Kinetic studies have shown that strontium sorption is limited by both internal and external diffusion as well as by the rate of chemical transformation of the NPF-HTD sorbent.

Sorption isotherm of caesium from tap water by mixed nickel-potassium ferrocyanide based on hydrated titanium dioxide is obtained for a wide range of concentrations of caesium. It is shown that there are three types of specificity to caesium sorption sites in this sorbent. Sorption chemisms of caesium are studied, factors conditioned high sorption capacity of the sorbent are revealed. It is shown that occupation of sorption sites I and II is well approximated by Langmuir equilibrium and this process can be described within the bounds of theory of ion exchange. The expected sorption chemism of caesium by sorption sites III at high concentrations of caesium ([50 mg L -1 ) is precipitation of mixed nickel-caesium ferrocyanide in pore space of the sorbent.