Salinity dependence of Ra-226 adsorption on montmorillonite and kaolinite

The effect of NaCl concentration (10.0-1,000 mM) on Ra-226 adsorption was investigated in the presence of montmorillonite and kaolinite. A positive correlation was observed between the dissolved Ra-226 and NaCl concentrations in the presence of these adsorbents. Distribution coefficients decreased from the order of 10(4) to 10(0) (mL g(-1)) with an increase in NaCl concentration. Although the coefficients were higher for montmorillonite than kaolinite at lower NaCl concentrations, the trend was reversed at higher NaCl concentrations (a parts per thousand yen500 mM) owing to the sharper reduction of the coefficient for montmorillonite with the increase in NaCl concentration. The rapid reduction was ascribed to higher negative charge density of montmorillonite, which leads the Ra2+ adsorption mechanism to approach charge-compensating ion exchange.