Linking cesium and strontium uptake to kaolinite weathering in simulated tank waste leachate

Weathering behavior of kaolinite was studied in batch systems under geochemical conditions characteristic of tank waste released to the vadose zone at the Hanford Site, WA (0.05 M AI_T, 2 M Na⁺, 1 M NO₃^-, pH ∼14, Cs⁺ and Sr²⁺ present as co-contaminants). Time series experiments were conducted from 0 to 369 d, with initial Cs⁺ and Sr²⁺ concentrations ranging from 10^-5 to 10^-3 M. Dissolution of kaolinite increased soluble Si and Al to maximum levels at 7 d (Cs and Sr concentrations of 10^-5 and 10^-4 M) or 33 d (Cs and Sr concentrations of 10^-3 M). Subsequent precipitation of Si and Al was coupled to the formation of oxalate-extractable solids that incorporated Cs and Sr. Strontium sorption was nearly complete within 24 h for initial Sr concentrations (Sr₀) ≤ 10^-4 whereas Cs uptake increased over the full year of the experiment for all initial Cs concentrations. Spectroscopic analyses revealed neoformed solids including the zeolite Na-Al silicate (Alchabazite), and feldspathoids sodium aluminum nitrate silicate (NO₃-sodalite), and sodium aluminum nitrate silicate hydrate (NO₃-cancrinite), which can incorporate Cs. Singlepulse ²⁷Al solid-state nuclear magnetic resonance (NMR) spectroscopy yielded first-order rate constants (k)for mineral transformation that decreased from 3.5 × 10^-3 to 2 × 10^-3 d^-1 as Cs and Sr concentrations were increased from 10^-5 to 10^-3 M. Discrete strontium silicate solids were also observed. The incongruent dissolution of kaolinite promoted the sequestration of contaminants into increasingly recalcitrant solid phases over the 1-yr time period.