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 AIT, 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.
ASJC Scopus subject areas
- Environmental Chemistry