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

During the last few decades, considerable research efforts have been extended to identify more effective remediation treatment technologies to lower the ¹²⁹I 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 (IO₃⁻) to iodide (I⁻), but the loamy-sand sediment reduced more IO₃⁻ (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 NH₂HCl/HNO₃ 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, <0.2%, may have an impact on iodine geochemistry. The findings that these sediments can readily reduce IO₃^−, and that IO₃⁻ sorbs to a greater extent than I⁻, sheds light into earlier unexplained Hanford field data that demonstrated increases in groundwater ¹²⁷I⁻/¹²⁷IO₃⁻ ratios and a decrease groundwater ¹²⁹IO₃⁻ concentrations along a transect away from the point sources, where iodine was primarily introduced as IO₃⁻. 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. - Highlights: • I⁻ and IO₃⁻ uptake K_d and desorption K_d′ values increased with very low OC. • Hanford sediments reduce significant portion of groundwater IO₃⁻ to I⁻. • Major portions of sedimentary I strongly bind to the carbonate and OC fractions