Post-substitution of magnesium at CaI of nano-hydroxyapatite surface for highly efficient and selective removal of radioactive 90Sr from groundwater

Abstract

We have modified the ion-exchange affinity of nano-Hydroxyapatite (Ca₅(PO₄)₃OH, HAP) surface for the rapid and selective adsorption of ⁹⁰Sr from groundwater. The modification was achieved by the post-substitution of cations (Na⁺, Mg²⁺, Cu²⁺, Ba²⁺, Fe³⁺, and Al³⁺) for parent Ca²⁺ within surface structure of HAP. The diffraction patterns of modified HAP showed a slight shift of the (002) peak between 25° and 27° 2θ depending the ionic radius of the substituted cation. Magnesium substituted HAP, Mg-HAP, exhibited the highest removal efficiency (>95%) for 10 ppm of Sr²⁺, which is attributable to the higher ion-exchange affinity of substituted Mg²⁺ than parent Ca²⁺ toward Sr²⁺. The results of various analyses revealed that Mg substitution dominantly occurred at the Ca^I site of HAP, which enabled the Mg-HAP to adsorb Sr²⁺ at both of Ca^I and Ca^II sites whereas bare HAP could adsorb Sr²⁺ mainly at Ca^II site. Adsorption isotherms and the kinetics of Mg-HAP for Sr²⁺ were evaluated using a bi-Langmuir isotherm and a pseudo-second-order kinetic model, which demonstrated the Mg-HAP exhibited the highest adsorption capacity (64.69 mg/g) and fastest adsorption kinetics (0.161-1.714 g/(mg·min)) than previously modified HAPs. In the presence of competing cations at circumneutral pHs, the enhanced performance of the Mg-HAP led to a greater than 97% reduction of ⁹⁰Sr (initial radioactivity = 9500 Bq/L) within 1 h. The distribution coefficient of Mg-HAP was 1.3-6.6 × 10³ mL/g while that of bare HAP was 1.2-6.6 × 10² mL/g. The findings in the present study highlight that the ion-exchange affinity of Ca^I and Ca^II sites on HAP surface plays a key-role in ⁹⁰Sr uptake. The proposed modification method can simply increase the affinity of HAP surface, therefore, this work can further improve the deployment of an in situ remediation technology for ⁹⁰Sr contaminated groundwater, i.e., Mg-HAP-based permeable reactive barrier.