Radioiodine sorption/desorption and speciation transformation by subsurface sediments from the Hanford Site

Abstract During the last few decades, considerable research efforts have been extended to identify more effective remediation treatment technologies to lower the 129 I concentrations to below federal drinking water standards at the Hanford Site (Richland, USA). Few studies have taken iodate into consideration, though recently iodate, instead of iodide, was identified as the major species in the groundwater of 200-West Area within the Hanford Site. The objective of this study was thus to quantify and understand aqueous radioiodine species transformations and uptake by three sediments collected from the semi-arid, carbonate-rich environment of the Hanford subsurface. All three sediments reduced iodate (IO 3 − ) to iodide (I − ), but the loamy-sand sediment reduced more IO 3 − (100% reduced within 7 days) than the two sand-textured sediments (∼20% reduced after 28 days). No dissolved organo-iodine species were observed in any of these studies. Iodate uptake K d values ([I solid ]/[I aq ]; 0.8–7.6 L/kg) were consistently and appreciably greater than iodide K d values (0–5.6 L/kg). Furthermore, desorption K d values (11.9–29.8 L/kg) for both iodate and iodide were consistently and appreciably greater than uptake K d values (0–7.6 L/kg). Major fractions of iodine associated with the sediments were unexpectedly strongly bound, such that only 0.4–6.6 % of the total sedimentary iodine could be exchanged from the surface with KCl solution, and 0–1.2% was associated with Fe or Mn oxides (weak NH 2 HCl/HNO 3 extractable fraction). Iodine incorporated into calcite accounted for 2.9–39.4% of the total sedimentary iodine, whereas organic carbon (OC) is likely responsible for the residual iodine (57.1–90.6%) in sediments. The OC, even at low concentrations, appeared to be controlling iodine binding to the sediments, as it was found that the greater the OC concentrations in the sediments, the greater the values of uptake K d , desorption K d , and the greater residual iodine concentrations (non-exchangeable, non-calcite-incorporated and non-Mn, Fe-oxide associated). This finding is of particular interest because it suggests that even very low OC concentrations, 3 −, and that IO 3 − sorbs to a greater extent than I − , sheds light into earlier unexplained Hanford field data that demonstrated increases in groundwater 127 I − / 127 IO 3 − ratios and a decrease groundwater 129 IO 3 − concentrations along a transect away from the point sources, where iodine was primarily introduced as IO 3 − . While a majority of the radioiodine does not bind to these alkaline sediments, there is likely a second smaller iodine fraction in the Hanford subsurface that is strongly bound, presumably to the sediment OC (and carbonate) phases. This second fraction may have an impact on establishing remediation goals and performance assessment calculations.