| Abstract
| - Weathering behavior of kaolinite was studied in batchsystems under geochemical conditions characteristic oftank waste released to the vadose zone at the Hanford Site,WA (0.05 M AlT, 2 M Na+, 1 M NO3-, pH ∼14, Cs+ andSr2+ present as co-contaminants). Time series experimentswere conducted from 0 to 369 d, with initial Cs+ andSr2+ concentrations ranging from 10-5 to 10-3 M. Dissolutionof kaolinite increased soluble Si and Al to maximumlevels at 7 d (Cs and Sr concentrations of 10-5 and 10-4M) or 33 d (Cs and Sr concentrations of 10-3 M). Subsequentprecipitation of Si and Al was coupled to the formationof oxalate-extractable solids that incorporated Cs and Sr.Strontium sorption was nearly complete within 24 h forinitial Sr concentrations (Sr0) ≤ 10-4 whereas Cs uptakeincreased over the full year of the experiment for all initialCs concentrations. Spectroscopic analyses revealedneoformed solids including the zeolite Na−Al silicate (Al-chabazite), and feldspathoids sodium aluminum nitratesilicate (NO3-sodalite), and sodium aluminum nitrate silicatehydrate (NO3-cancrinite), which can incorporate Cs. Single-pulse 27Al solid-state nuclear magnetic resonance (NMR)spectroscopy yielded first-order rate constants (k) for mineraltransformation that decreased from 3.5 × 10-3 to 2 ×10-3 d-1 as Cs and Sr concentrations were increased from10-5 to 10-3 M. Discrete strontium silicate solids werealso observed. The incongruent dissolution of kaolinitepromoted the sequestration of contaminants into increasinglyrecalcitrant solid phases over the 1-yr time period.
|
