Batch and flow-through continuous stirred reactor experiments of Sr2+-adsorption onto smectite: influence of pH, concentration and ionic strength

Document available in extended abstract form only. Strontium-90 (t1/2 = 29.1 years) resulting from the nuclear fission process is one of the main constituents connected with nuclear waste fuel. Concerning the physical properties and sorption behaviour one of the suitable buffer materials used as a backfill in the geological disposal systems for high-level radioactive wastes is smectite. The 2 μm clay fractions of di-octahedral smectite were used for adsorption experiments in batch and flow-through experiments. Flow-through experiments were carried out at different pH (4 and 8) and concentrations (8.00x10-2 mmolSr2+/L, 2.0x10-1 mmolSr2+/L, 3.3x10-1 mmolSr2+/L and 4.1 x10-1 mmol Sr2+/L). Batch experiments were carried out at different ionic strength ([KNO3]=10-2 M and [KNO3]=10-3 M), pH (4 and 8), whereas the concentration ranged between 0.19 mmolSr2+/L and 9.60 mmolSr2+/L. The adsorption strontium rate in flow-through experiments was found higher at pH 4 than at pH 8, where less of 20% amount of strontium was adsorbed on clay surface after 34 hours. This is explained by the surface charge of smectite layers with a permanent negative charge on the basal planes due essentially to isomorphic substitution. Also additional polar sites are conditionally charged by direct protonation of outer edge surfaces. Therefore, more negative sites become available for the strontium ions sorption, as the pH increasing. After adsorption, the clays were submitted to a desorption process. The rate of desorption at pH 4 is initially too fast due to the elevated amount of strontium released by smectite. After 255 min, the strontium amount desorbed is very low and the rate of desorption approached to zero. By contrast at pH 8 the rate of desorption is practically constant, and after 255 min there is a significant amount of strontium released by clay. Due to these different behaviors in different pH conditions, after 10 hours of desorption, the amount of strontium that left the clay in acidic conditions 432 was about 15% of the amount that was adsorbed, and less half of the strontium quantity that was desorbed at pH 8. Thus, after the desorption process, the strontium quantity that was retained on clay surface was slightly higher at pH 4, with tendency to increase as the Sr2+ concentration increases in solution. Batch experiments were performed under extreme conditions of pH and ionic strength in a wide range of Sr2+ concentration. The results obtained suggest that the sorption of Sr2+ on smectite obeys Langmuir isotherms. Two distinct cases were observed: i) The amount of Sr2+ adsorbed by smectite is close the same (from 0.39 mmol Sr2+/L -pH ∼ 8 to 0.38 mmolSr2+/L -pH ∼4) at higher ionic strength ([KNO3]=10-2 M) and different pH (4 and 8); ii) otherwise, there is a decrease in the amount of metal adsorbed at pH 4 (from 0.47 mmolSr2+/L -pH ∼ 8 to 0.42 mmolSr2+/L - pH ∼4), at lower ionic strength ([KNO3]=10-3 M), which means that the influence of pH is less significant with the increase of ionic strength. (authors)