Abstract
Adsorption of different oxidation species of selenium (Se), selenate (SeO4 2-) and selenite (SeO32-), with varying pHs (2-10) and ionic strengths (I=0.01 M, 0.1 M and 1.0 M NaNO3) was measured on quartz, aluminum oxide, and synthetic iron oxide (ferrihydrite) using batch reactors to obtain a more detailed understanding of the adsorption mechanisms (e.g., inner- and outer-sphere complex). In addition to the batch experiments with single minerals contained in native Hanford Site sediment, additional batch adsorption studies were conducted with native Hanford Site sediment and groundwater as a function of 1) total Se concentration (from 0.01 to 10 mg L-1) and 2) soil to solution ratios (1:20 and 1:2 grams per mL). Results from these batch studies were compared to a set of saturated column experiments that were conducted with natural Hanford sediment and groundwater spiked with either selenite or selenate to observe the transport behavior of these species. Both batch and column results indicated that selenite adsorption was consistently higher than that of selenate in all experimental conditions used. These different adsorption mechanisms between selenite and selenate result in the varying mobility of Se in the subsurface environment and explain the dependence on the oxidation species.
Highlights
Selenium (Se) is required for adequate nutrition, but at high concentrations Se can be toxic to humans and animals
The sediment was collected from borehole C3177 (299-E24-21) located in the center of the 200 East Area, northeast corner of the immobilized low-activity waste (ILAW) disposal site, at the Hanford Site Nuclear Reservation located in southeastern Washington state [21]
X-ray diffraction (XRD) analysis revealed that the Hanford sediment is dominated by quartz and feldspars with lesser amounts of chlorite, mica and amphibole, which is typical of Hanford Site sediments (Table 1)
Summary
Selenium (Se) is required for adequate nutrition, but at high concentrations Se can be toxic to humans and animals. Animals have a narrow range between deficient and toxic concentrations of Se. Contamination of Se, caused by irrigation drainage, was discovered in 1983 at Kesterson Reservoir in California and was linked to deformities in waterfowl [2]. The planned disposal of immobilized low-activity waste (ILAW) glass containing 79Se (t1/2=2.9x105 years) [4], located in the 200 East area of the Hanford Site, southeastern Washington State, may pose a significant risk for radioactive Se release [5]. Dissolution of the vitrified radioactive waste matrix containing 79Se, a fission product of 235U, could result in Se release into the environment. To prevent spreading of radioactive Se contamination, a more in depth understanding of the migration of Se in the environment is required [5,6,7]
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