Calcium-deficient Hydroxyapatite as a Potential Sorbent for Strontium

A calcium (Ca)-deficient hydroxyapatite was investigated for its potential to remove Sr(2+) from environmentally relevant water. We conducted sorption tests on solutions containing magnesium ion (Mg(2+)) and calcium ion (Ca(2+)) as competing cations at a strontium ion (Sr(2+)) concentration of 0.05 mmol/L. The Ca-deficient hydroxyapatite maintained a high Sr(2+) sorption ratio of above 80% in the presence of Mg(2+) and Ca(2+) at the concentrations between 0.1 and 1.0 mmol/L, whereas the stoichiometric hydroxyapatite showed a lower ratio even in the presence of small amounts of Mg(2+) and Ca(2+) (72% for Mg(2+) and 51% for Ca(2+) at 0.1 mmol/L). For solutions with various Sr(2+) concentrations between 0.01 and 10 mmol/L, Ca-deficient hydroxyapatite exhibited a higher Sr(2+) sorption ratio than stoichiometric hydroxyapatite. The bonding states of Sr(2+) on the Ca-deficient hydroxyapatite were evaluated by extended X-ray absorption fine structure measurements. The results indicated that there are specific sorption sites in Ca-deficient hydroxyapatite where Sr(2+) is stably and preferentially immobilized.