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

We have modified the ion-exchange affinity of nano-Hydroxyapatite (Ca-5(PO4)(3)OH, HAP) surface for the rapid and selective adsorption of Sr-90 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 degrees and 27 degrees 2 theta 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 CaI 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 Sr-90 (initial radioactivity = 9500 Bq/L) within 1 h. The distribution coefficient of Mg-HAP was 1.3-6.6 x 10(3) mL/g while that of bare HAP was 1.2-6.6 x 10(2) 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 Sr-90 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-90 contaminated groundwater, i.e., Mg-HAP-based permeable reactive barrier.