Ca-Mg phosphates as an effective sorbents of ⁶⁰Co and ⁸⁵Sr radionuclides

The aim of this work was to study the efficiency of Ca-Mg phosphate sorbents based on dolomite for ⁶⁰Co and ⁸⁵Sr radionuclides removal from aqueous solutions depending on the electrolyte content in the wide range of pH. It was important to determine the influence of competing ions in the solution on the stability of Ca-Mg phosphate sorbents and the effect of the surface changes of sorbents on their selectivity towards ⁶⁰Co and ⁸⁵Sr removal from solutions with high salinity. Dolomite from the Ruba (Belarus, Vitebsk region) deposit with the following chemical composition (wt. %): SiO₂ 1.1, Fe₂O₃ 0.4, Al₂O₃ 0.5, CaO 30.3, MgO 20.0, SO₃ 0.4, K₂O 0.2, Na₂O 0.1 and calcination loss 47.0 was used for the preparation of Ca-Mg phosphates. Sorbents with different chemical and phase composition were obtained in various dolomite phosphating conditions. The process of radionuclides ⁶⁰Co and ⁸⁵Sr uptake from model seawater solutions with salt content 0 35 g/L by phosphate sorbents produced from natural dolomite were studied. It was shown that the efficiency of phosphate sorbents depends on the dolomite phosphating conditions which determine the chemical and phase composition of sorbents, acid-base properties and consequently different selective-sorption properties towards ⁶⁰Co and ⁸⁵Sr radionuclides in aqueous and model sea-water solutions. The influence of pH on the removal efficiency of ⁶⁰Co and ⁸⁵Sr is determined by the value of pHpzc, which depends on the composition of sorbents and processes of protonation-deprotonation on their surface. The efficiency of ⁶⁰So and ⁸⁵Sr sorption from model solutions decreased with salinity increase, due to the increase in ionic strength of the solutions and the competing effect of metal cations in the background electrolyte. The most effective sorbent for ⁶⁰So and ⁸⁵Sr radionuclides showed sample consisting of tertiary phosphates of calcium, magnesium and magnesium ammonium phosphate: Kd ⁶⁰Co from 2.44×10⁴ to 3.32×10³ cm³/g and Kd ⁸⁵Sr from 2.66×103 to 0.14 × 10³ cm³/g in the salinity range 0.0-50.0 g/L, respectively. Thus, high effective Ca-Mg phosphates sorbents towards ⁶⁰So and ⁸⁵Sr were synthesized. Effect of sorbents phase and chemical composition, salinity and pH of solution were studied. The highest sorption efficiency and selectivity was obtained for PD-2 sorbent. The simplicity of Ca-Mg phosphates preparation and use of widely occurring natural raw material dolomite for their production give the scientific basis for industrial implementation of novel sorbents for treatment facilities. The produced materials are promising for application in sea-water and liquid radioactive waste treatment from ⁶⁰So and ⁸⁵Sr radionuclides.