Effective Removal of Strontium from Seawater by Coprecipitation with Barite(BaSO4)
K. Tokunaga1, 2) and N. Kozai1)
1): Interfacial Reaction Field Chemistry Gr., JAEA, 2): Ningyo-toge Center, JAEA

Strontium-90 (90Sr: half-life 28 years) is a fission
product of uranium, which has been released to theenvironment from nuclear plant accidents, such as thosethat occurred in Chernobyl and Fukushima.
TheFukushima Daiichi Nuclear Power Plant accident (2011)
has led to serious 90Sr contamination in seawater, butthere has been a lack of effective removal techniques forremoving Sr from seawater because of the strong
inhibition of Sr uptake by the high concentration of
sodium and other competitive ions in seawater. In thepresent study, we examined the removal efficiency of Sr
by using barite (BaSO4) under various experimental
conditions to develop techniques for the direct removal ofSr from seawater [1]. Barite is a commonly found innature and it can be used to remove toxic and/or
radioactive elements from aqueous solutions. However, it
has not been widely used in environmental studies despite
its useful properties [2],[3]. In this work, barite wasprecipitated from a mixture of (i) Na2SO4 aqueous
solution and (ii) BaCl2 aqueous solution. The effects ofpH, ionic strength, coexistent Ca2+ and Mg2+ ions, and[Ba2+]/[SO42-] ratio in the initial solution wereinvestigated for determining the optimum condition.
Figure 1 shows the removal efficiency of Sr bycoprecipitation with barite according to the experimental
parameters. It is found that Sr uptake by barite depends
on the pH and [Ba2+]/[SO42-] ratio of the initial aqueous
solution, but it seems relatively independent of ionicstrength, [Ca2+] and [Mg2+] in the initial solution. The
most effective parameter on the removal efficiency was
the [Ba2+]/[SO42-] ratio in the initial aqueous solution
because Sr was able to be removed completely from theaqueous solution under the condition that the[Ba2+]/[SO42-] ratio was low. In addition, the negligible
effects of ionic strength and competitive [Ca2+] and[Mg2+] ions in the initial solution were observed,
suggesting the effectiveness of its application to removalof Sr from seawater. From this observation, it is expected
to provide a good estimate of its ability to effectively
remove Sr from the aqueous solution by using theoptimized experimental design (Figure 2). It would bepossible to remove Sr from the real seawater by 93 %within 70 hours regardless of small solid concentration(0.15 g/L).
Based on the results, it is found that Sr is efficiently
and selectively incorporated into barite compared to othercations and minerals. This high efficiency may be due tothe dependence of ionic radii on the partition behavior oftrace elements. Previous studies have shown that the traceelements having similar radii to the host ion are strongly
incorporated into the crystal structure. The distribution

coefficient (Kd) values of Mg2+, Ca2+, Sr2+, Rb+, and Cs+
are 2.8×100, 7.9×102, 5.8×106, 2.6×101, 2.6×100 L/kg,
respectively, showing the dependence as suggested above
between Kd and ionic radius. These findings revealed thatSr can be incorporated into barite significantly more thanother ions because of its similar ionic size of Sr2+ (1.21 Å)
to Ba2+ (1.44 Å). Thus, barite is considered to be aneffective material and to show a better performance inseawater.

References[1] K. Tokunaga et al., J. Hazard. Mater. 359, 307-315 (2018).
[2] K. Tokunaga et al., Environ. Sci. Technol. 51, 9194-9201(2017).
[3] K. Tokunaga et al., Chemical Geology 447, 59-69 (2016).