RedOx-controlled sorption of iodine anions by hydrotalcite composites

Abstract

The radioactive contaminant iodine-129 (I-129) is one of the top risk drivers at radiological waste disposal and contaminated groundwater sites where nuclear material fabrication or reprocessing has occurred. However, currently there are very few options available to treat I-129 in the groundwater, partially related to the complex biogeochemical behavior of iodine in the subsurface and occurrence of I-129 in the multiple chemical forms. We hypothesize that layered hydrotalcite materials containing redox active transition metal ions offer a potential solution, benefiting from the simultaneous adsorption of iodate (IO₃⁻), and iodide (I⁻) anions, which exhibit different electronic and structural properties and therefore may require dissimilar hosts. To test this hypothesis, Cr³⁺-based materials were selected based on the rationale that Cr³⁺ readily reduces IO₃⁻ in solution. It was combined with either redox-active Co²⁺ or redox-inactive Ni²⁺ so that two model materials were prepared by hydrothermal synthesis including Co²⁺–Cr³⁺ or Ni²⁺–Cr³⁺ (M–Cr). Obtained M–Cr materials composed of Co²⁺–Cr³⁺ or Ni²⁺–Cr³⁺ layered hydrotalcite and small fractions of Co₃O₄ spinel or Ni(OH)₂ theophrastite phases were structurally characterized before and after uptake of periodate (IO₄⁻), IO₃⁻, and I⁻ anions. It was found that the IO₃⁻ uptake is driven by its chemical reduction to I₂ and I⁻. Interestingly, in the Co²⁺–Cr³⁺ hydrotalcite, Co²⁺ and not Cr³⁺ serves as a reductant while in the Ni²⁺–Cr³⁺ hydrotalcite Cr³⁺ is responsible for the reduction of IO₃⁻. A different uptake mechanism was identified for the IO₄⁻ anion. The Co²⁺–Cr³⁺ hydrotalcite phase efficiently uptakes IO₄⁻ by a diffusion-limited ion exchange mechanism and is not accompanied by the redox process, while Cr³⁺ in the Ni²⁺–Cr³⁺ hydrotalcite reduces IO₄⁻ to IO₃⁻, I₂ and I⁻. Iodide exhibited high affinity only to the Co–Cr material. The Co–Cr material performed remarkably well for the removal of IO₃⁻, I⁻ and total iodine from the groundwater collected from the US DOE Hanford site, WA, USA, outperforming non-redox active hydrotalcites (e.g., Mg²⁺–Al³⁺) reported previously. This work demonstrates that redox-controlled sorption can be a highly effective method for the treatment of anions based on elements with mobile oxidation states. Further, multiple anions of interest could be simultaneously removed through a combination of approaches.