EQUILIBRIUM AND THERMODYNAMICS OF URANIUM (VI) ADSORPTION BY PARAFFIN PARTICLES MODIFIED WITH TRIALKYLAMINE

Paraffin particles modified with surfactants, in particular the technical product trialkylamine, are affordable, effective adsorbents and flotation carriers of lanthanides and actinides. Such adsorbents are easily regenerated, which is accompanied by the concentration of the solution to be purified. Their use also does not lead to secondary contamination of the treated water. The regularities of uranium (VI) adsorption from sulfate solutions by paraffin particles modified with trialkylamine under equilibrium conditions depending on the pH of the adsorbate solutions and temperature have been established. Solutions similar to those studied are formed during the sulfuric acid stripping of uranium ores, which is the main technological scheme for processing uranium-containing raw materials at the Eastern Mining and Processing Plant (Zhovty Vody, Kirovohrad Oblast, Ukraine). The analysis of the effect of temperature on uranium adsorption from solutions with pH 4 showed that the adsorption mechanism changes with an increase in the equilibrium concentration of the adsorbate – physical adsorption changes to chemisorption and then to a mixed adsorption. Increasing the pH of uranium-containing solutions from 4 to 8 leads to a decrease in the adsorption capacity of the adsorbent. The adsorption capacity of the adsorbent has been proved, mg U/g: 110.9 (pH 4, 298 K), 173.3 (pH 4, 303 K), 196.1 (pH 4, 313 K), 57.4 (pH 6, 298 K), 14.8 (pH 8, 298 K), 132.8 (pH 10, 298 K). A comparative study of the possibility of using a number of models (Langmuir, Freundlich, Dubinin-Radushkevich, and Tеmkin) to describe adsorption isotherms was carried out by determining the parameters characterizing the adsorption process. It was proved that uranium adsorption isotherms from solutions with pH 4 are better described by the Langmuir model, and from solutions with pH 8 – by the Freundlich and Dubinin-Radushkevich models. It was found that the process of uranium adsorption by paraffin particles modified with trialkylamine is spontaneous (ΔGads. < 0, ranges from -21.24 to -32.29 kJ/mol), exothermic (ΔHads. = -36.96 kJ/mol), and is accompanied by the ordering of the system (ΔSads. < 0, ranges from -25.5 to -27.3 J/(mol∙K)). The values of the characteristic energy of adsorption (Dubinin-Radushkevich model) lie in the range of 7.7–16.4 kJ/mol, which is characteristic of the ion exchange mechanism of adsorption.