Effect of reducing groundwater on the retardation of redox-sensitive radionuclides

• Main Authors: Rose TP, Zavarin M, Hu QH
• Format: Article
• Language: English
• Published: BMC 2008-12-01
• Series: Geochemical Transactions
• Online Access: http://www.geochemicaltransactions.com/content/9/1/12

<p>Abstract</p> <p>Laboratory batch sorption experiments were used to investigate variations in the retardation behavior of redox-sensitive radionuclides. Water-rock compositions were designed to simulate subsurface conditions at the Nevada Test Site (NTS), where a suite of radionuclides were deposited as a result of underground nuclear testing. Experimental redox conditions were controlled by varying the oxygen content inside an enclosed glove box and by adding reductants into the testing solutions.</p> <p>Under atmospheric (oxidizing) conditions, radionuclide distribution coefficients varied with the mineralogic composition of the sorbent and the water chemistry. Under reducing conditions, distribution coefficients showed marked increases for ⁹⁹Tc (from 1.22 at oxidizing to 378 mL/g at mildly reducing conditions) and ²³⁷Np (an increase from 4.6 to 930 mL/g) in devitrified tuff, but much smaller variations in alluvium, carbonate rock, and zeolitic tuff. This effect was particularly important for ⁹⁹Tc, which tends to be mobile under oxidizing conditions. A review of the literature suggests that iodine sorption should <it>decrease </it>under reducing conditions when I^-is the predominant species; this was not consistently observed in batch tests. Overall, sorption of U to alluvium, devitrified tuff, and zeolitic tuff under atmospheric conditions was less than in the glove-box tests. However, the mildly reducing conditions achieved here were not likely to result in substantial U(VI) reduction to U(IV). Sorption of Pu was not affected by the decreasing Eh conditions achieved in this study, as the predominant sorbed Pu species in all conditions was expected to be the low-solubility and strongly sorbing Pu(OH)₄.</p> <p>Depending on the aquifer lithology, the occurrence of reducing conditions along a groundwater flowpath could potentially contribute to the retardation of redox-sensitive radionuclides ⁹⁹Tc and ²³⁷Np, which are commonly identified as long-term dose contributors in the risk assessment in various radionuclide environmental contamination scenarios. The implications for increased sorption of ⁹⁹Tc and ²³⁷Np to devitrified tuff under reducing conditions are significant as the fractured devitrified tuff serves as important water flow path at the NTS and the horizon for a proposed repository to store high-level nuclear waste at Yucca Mountain.</p>