Comparative analysis of strontium adsorption on bentonite and phyllite under various environmental conditions: implications for radioactive waste repository barriers.

Abstract

In a search for cost-effective buffer materials for radioactive waste disposal, the adsorption capacity of argillaceous phyllite, the removal efficacy of Sr²⁺ (a proxy for ⁹⁰Sr), and the distribution coefficient of phyllites for Sr²⁺ were studied in comparison with bentonite using the batch method at different: pH (3 - 8), ionic strength (IS = 0 - 0.5 M NaCl or CaCl₂), Sr initial concentration (0.01 - 200 mg L^-1), and the adsorbent concentration (2 g/L and 10 g/L). The effect of solution pH on Sr²⁺ uptake strongly depends on different adsorption centres in the rock-forming minerals, i.e., permanent negative surface charge in montmorillonite (major mineral of bentonite) and the pH-dependent charge caused by the proton adsorption/desorption on the amphoteric hydroxyl groups (-Si-OH, -Al₂-OH) at the edge of basal planes in chlorite (major mineral in phyllite). The maximum adsorption capacity of the adsorbent occurred at IS = 0 M, pH = 8, and L:S = 500:1 and followed the Sips isotherm of 34.86-11.32 mg g^-1 for phyllite. The adsorption capacity of bentonite and phyllite decreased with increasing ionic strength. Calcium ions affect Sr²⁺ uptake stronger than Na⁺. Adsorption capacity of bentonite decreased 1.7-fold and 3.9-fold, and of phyllite decreased 2.9-fold and 7.82 fold in the presence of Na⁺ and Ca²⁺, respectively, at IS 0.1 M, C₀ 100 mg L^-1, and liquid to rock ratio of 500:1. Despite phyllites lower adsorption capacity for Sr²⁺ than bentonites, their properties, i.e., low amount of primary Sr²⁺ and lower porosity, make them a suitable buffer material to be used in mixtures with bentonite.