Radioiodine is a major component of nuclear waste inventory as a by-product of nuclear fission. Specifically, I-129 is a risk driving factor due to its long half-life (16 million years). Past poor waste management has led to accidental releases of radioiodine into the environment, causing adverse effects in the quality of water and the biosphere. In aqueous systems, iodine exists as iodide (I-), a highly mobile species that interacts minimally with natural substrates, and as iodate (IO3-) which is known to sorb and even incorporate into minerals, leading to natural iodine attenuation. Iodate is the prevalent species under oxidizing conditions, with remediation efforts focusing on the oxidation of iodide to iodate as a possible remediation pathway. Manganese oxides are ubiquitous in nature, with their oxidation potential being disproportionate to their concentration, as small amounts of mineral can significantly affect redox properties of dissolved contaminants. This geochemical control over the fate and transport of redox sensitive risk driving contaminants is attributed to the high reduction potential of the Mn(IV or III)/Mn(II) redox couple and the redox cycling of manganese by microorganisms. Though manganese oxides are commonplace oxidizing agents, the oxidation of iodide by manganese oxides has not been thoroughly studied. In this work, we investigated the oxidation of iodide by four different manganese oxides under a variety of environmentally relevant conditions, to determine if they provide potential immobilization pathways for iodine in the environment by turning iodide into iodate. It was observed that the reaction kinetics are sensitive to mineral oxidation state, pH, solid:liquid ratio, ionic strength, and initial iodide concentration. Even though the oxidation of iodide to iodate (-1 to +5 oxidation state) is thermodynamically favorable, our results show that redox reaction stops at the molecular iodine intermediate, providing alternative reaction pathways for the fate and transport in the environment.

Radio-iodine transformations and speciation in the environment as a result of oxidation by manganese minerals: Challenges in iodine fate and transport prediction

ACS Spring 2022

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