A study of iodine, caesium, and strontium sorption on alkali-earth hydrophosphates from the aqueous phase

This work studied the sorption of 131I-, 131IO3-137Cs+ and 85Sr2+ ions on various samples of Ca, Sr, and Ba hydrophosphates. The samples of alkali-earth hydrophosphates (AEHP) were synthesized by the following reactions: MCl2 + Na3PO4 + NaOH → M5(PO4)3OH; MCl2 + NH4OH + (NH4)2HPO4 → M5(PO4)3OH; MCl2 + NaOH + (NH4)2HPO4 → M5(PO4)3OH (where M is Ca, Sr, or Ba). The experiments on a study of the sorption of 131I-, 131IO3-137Cs+, and 85Sr2+ ions on AEHP from aqueous solutions of KI, KIO3, CsNO3, or Sr(NO3)2 were carried out. It was established that all AEHP samples adsorbed hardly any ionic species of radioactive iodine from aqueous solutions (Kd was not higher than 5.5 ml/g). The degree of separation of 137Cs and 85Sr by AEHP precipitates after 120 min of contact between the solid and liquid phases was about 5 to 25 % and about 15 to 80 % of their initial amount in the solution, respectively. The distribution coefficient Kd for 137Cs was not higher than 40 ml/g, whereas Kd for 85Sr had relatively higher values (100 ml/g and higher). The sorption of 85Sr and 137Cs by AEHP precipitates decreased in the sequence Ca>Sr>Ba. It is necessary to note that for 85Sr from water solution Sr(NO3)2 on Ca hydrophosphate solid phase decreased with contact time between the solid and liquid phases from 1150 ml/g after 5 min of contact to 683 ml/g after 120 min of contact. At the same time, pH value in system increased with 6.7 in an initial solution up to 8.19 after 120 min of contact between the solid and liquid phases. Our data disagree with those presented by Smiciklas et al. (2000), from which it follows that Sr2+ sorption from Sr(NO3)2 solution at a concentration of 5x10-4 M over the pH 5-10 range on crystal Ca hydrophosphate does not depend on pH of the initial solution. However, Sr2+ sorption sharply increases for pH > 10 of the initial solution. In our studies of 85Sr sorption the pH of solution during all experiment did not exceed 10. Observable decrease of Kd for 85Sr in the course of contact of the liquid and solid phases is connected, apparently, with the structural reorganization of Ca hydrophosphate solid phase. In conclusion it is necessary to note that in a natural environment minerals of apatite with the Ca5(PO4)3(OH,Cl,F) structure are capable to accumulate 90Sr and 134,137Cs radionuclides by sorption of their ions from water solutions. (author)