Strontium adsorption and penetration in kaolinite at low Sr²⁺concentration

Behavior of radioactive strontium (Sr2+) in contaminated soils is an important issue in relation to nuclear power plant accidents. The Sr2+ adsorption on kaolinite and its migration in a kaolinite soil were investigated because toxic effects of radioactive Sr2+ have been found to be very severe for living organisms at low Sr2+ concentrations. Adsorption isotherms of Sr2+ on kaolinite at different salt (NaCl) concentration and pH were obtained by the batch method. The calculated distribution coefficients (KD) ranged between 600 and 40,000 L kg−1, which showed a strong preference for the adsorbed phase. The results were used to evaluate the ratio (r) of penetration length of Sr2+ relative to that of water in a model kaolinite soil. When the Sr2+ solution was percolated constantly into the kaolinite soil, the penetration of Sr2+ was delayed strongly at low Sr2+ concentration due to adsorption. The Sr2+ penetration length was only 0.001–0.056 of the water penetration length at pH 6.5 (0.1–10 mmol L−1 NaCl). At pH 4.1 (1 mmol L−1 NaCl) the effect was about 17 times less than at pH 6.5 (1 mmol L−1 NaCl). Under all conditions, the Sr2+ penetration increased when the Sr2+ concentration increased due to the KD decrease. The Sr2+ isotherms could be fitted well to the Langmuir adsorption equation, which indicates that only one site type is involved in the Sr2+ adsorption.