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

We have modified the ion-exchange affinity of nano-Hydroxyapatite (Ca(PO)OH, HAP) surface for the rapid and selective adsorption of Sr from groundwater. The modification was achieved by the post-substitution of cations (Na, Mg, Cu, Ba, Fe, and Al) for parent Ca 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 Sr, which is attributable to the higher ion-exchange affinity of substituted Mg than parent Ca toward Sr. The results of various analyses revealed that Mg substitution dominantly occurred at the Ca site of HAP, which enabled the Mg-HAP to adsorb Sr at both of Ca and Ca sites whereas bare HAP could adsorb Sr mainly at Ca site. Adsorption isotherms and the kinetics of Mg-HAP for Sr 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 Sr (initial radioactivity = 9500 Bq/L) within 1 h. The distribution coefficient of Mg-HAP was 1.3-6.6 × 10 mL/g while that of bare HAP was 1.2-6.6 × 10 mL/g. The findings in the present study highlight that the ion-exchange affinity of Ca and Ca sites on HAP surface plays a key-role in Sr 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 Sr contaminated groundwater, i.e., Mg-HAP-based permeable reactive barrier.