Calcium-deficient Hydroxyapatite as a Potential Sorbent for Strontium

Abstract

A calcium (Ca)-deficient hydroxyapatite was investigated for its potential to remove Sr²⁺ from environmentally relevant water. We conducted sorption tests on solutions containing magnesium ion (Mg²⁺) and calcium ion (Ca²⁺) as competing cations at a strontium ion (Sr²⁺) concentration of 0.05 mmol/L. The Ca-deficient hydroxyapatite maintained a high Sr²⁺ sorption ratio of above 80% in the presence of Mg²⁺ and Ca²⁺ 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 Mg²⁺ and Ca²⁺ (72% for Mg²⁺ and 51% for Ca²⁺ at 0.1 mmol/L). For solutions with various Sr²⁺ concentrations between 0.01 and 10 mmol/L, Ca-deficient hydroxyapatite exhibited a higher Sr²⁺ sorption ratio than stoichiometric hydroxyapatite. The bonding states of Sr²⁺ 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 Sr²⁺ is stably and preferentially immobilized.