SREL Reprint #3227

Strontium and Cesium Release Mechanisms during Unsaturated Flow through Waste-Weathered Hanford Sediments

Hyun-shik Chang1,3, Wooyong Um1,5, Kenton Rod1, R. Jeff Serne1, Aaron Thompson2, Nicolas Perdrial3,
Carl I. Steefel4, and Jon Chorover3

1Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99354, United States
2University of Georgia, Athens, Georgia 30602, United States
3University of Arizona, Tucson, Arizona 85721, United States
4Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
5Pohang University of Science and Technology (POSTECH), South Korea

Abstract: Leaching behavior of Sr and Cs in the vadose zone of Hanford site (Washington) was studied with laboratory-weathered sediments mimicking realistic conditions beneath the leaking radioactive waste storage tanks. Unsaturated column leaching experiments were conducted using background Hanford pore water focused on first 200 pore volumes. The weathered sediments were prepared by 6 months reaction with a synthetic Hanford tank waste leachate containing Sr and Cs (10-5 and 10-3 molal representative of LO- and HI-sediment, respectively) as surrogates for 90Sr and 137Cs. The mineral composition of the weathered sediments showed that zeolite (chabazite-type) and feldspathoid (sodalite-type) were the major byproducts but different contents depending on the weathering conditions. Reactive transport modeling indicated that Cs leaching was controlled by ion-exchange, while Sr release was affected primarily by dissolution of the secondary minerals. The later release of K, Al, and Si from the HI-column indicated the additional dissolution of a more crystalline mineral (cancrinite-type). A two-site ion-exchange model successfully simulated the Cs release from the LO-column. However, a three-site ion-exchange model was needed for the HI-column. The study implied that the weathering conditions greatly impact the speciation of the secondary minerals and leaching behavior of sequestrated Sr and Cs.

SREL Reprint #3227

Chang, Hyun-shik., W. Um, K. Rod, R. J. Serne, A. Thompson, N. Perdrial, C. I. Steefel, and J. Chorover. 2011. Strontium and Cesium Release Mechanisms during Unsaturated Flow through Waste-Weathered Hanford Sediments. Environmental Science and Technology 45(19): 8313-8320.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).