Efficient immobilization of Sr(II) ions in ceramic matrices using thermal transformation of Ti–Ca–Mg phosphate adsorbents

Keeping safety and sustainability in view with managing radioactive aqueous wastes and other wastewaters containing Sr(II) ions, the current work was designed and performed to immobilize this ion in ceramic matrices. The adsorption properties of Ti–Ca–Mg composite phosphates with different ratios of Ti/(Ca + Mg) towards Sr(II) ions have been studied. Chemical structures and phase transformations of phosphate adsorbents and Sr(II)-loaded products after heat treatment in the range of 20–1000 °C were scrutinized by XRD, DTA-DTG, FTIR and elemental analyses. Hardly soluble calcium, magnesium pyrophosphates, Ca–Mg and Ca–Ti phosphates with a NaZr2(PO4)3 or CaTi4(PO4)6 skeleton structure (abbreviated as NZP or CTP) were obtained through heat treatment of composite phosphates at 800–1000 °C. The effect of the Ti/(Ca + Mg) ratio existing in the initial adsorbents on the phase and chemical composition of heat-treated products was studied, and the mechanism of their formation was proposed. Heat treatment of Sr-saturated Ti–Ca–Mg phosphates at 1000 °C led to the formation of Sr-substituted hydroxyapatite and whitlockite, which caused a high immobilization strength for Sr(II) ions. The degree of desorption of Sr(II) ions after heat-treatment of Sr-saturated adsorbents decreased by 10–120 times, declining to the amounts of 0.31–0.54% in seawater and 0.03–0.12% in artesian. The obtained Ti–Ca–Mg composite phosphates are of interest for the adsorption and immobilization of Sr(II) ions from liquid radioactive wastes (LRW) into ceramic matrices and can be used in full-scale plants.