PNNL

Abstract

Sorption of cesium and strontium during weathering of uncontaminated sediments from the Hanford Site was studied under geochemical conditions chosen to model that of high level radioactive waste leachate at Hanford. Based on prior studies of a range of model clay mineral systems, we postulated that Cs and Sr uptake in the sediments would reflect concurrent and competitive adsorption and co-precipitation effects. Batch experiments were conducted from 0 to 374 d at 298 K to measure long-term kinetics of Cs and Sr uptake to and release from three representative Hanford Site sediments; Ringold silt (RS), Hanford fine sand (HF), and Hanford coarse sand (HC). Sorbent affinity for Cs decreased in the order RS >HF > HC. Similar trends were observed for Sr at early times, but all sediments showed similar Sr sorption capacities after several months reaction time. Strontium uptake by all sediments exceeded that of Cs at nearly all reaction times. After 374 d, the total amount of Cs and Sr sorbed in all systems ranged from 15-37% and 80-93% of the initial concentrations, respectively. Although time-dependent trends for Cs uptake were not clearly evident, the fraction of nonexchangeable Cs increased slightly over time despite fluctuations. Strontium became progressively recalcitrant to desorption after 92 d, suggesting that coprecipitation in neoformed phases may be a significant sequestration mechanism for Sr in sediments of the Hanford vadose zone. SEM-EDS showed formation of Cs and Sr containing secondary solid phases after 183 d. Synchrotron µ-XRF, µ-XRD identified these phases as cancrinite and sodalite. Results indicate that while both Sr and Cs are incorporated into neo-formed precipitates, Sr uptake and lability is more sensitively dependent on mineral transformation reactions in the sediments, whereas Cs uptake is strongly influenced by adsorption, probably associated with high-affiinity and edge sites of native micaceous minerals.