Radioiodine is a major risk driving contaminant in nuclear waste inventory, possessing a complex chemistry. Past poor waste management practices have led to accidental releases of radioiodine into natural waters. Radioiodine has a long half-life (16 million years), has a high biological affinity, and is a known carcinogen in humans. Though radioiodine possesses complex speciation, under environmental conditions, it can be predominantly found in three forms: iodide (I-), molecular iodine (I2), and iodate (IO3-). Iodide is a highly mobile anionic species with few interactions with natural substrates, whereas iodate has extensive sorptive capabilities. However, molecular iodine has different pathways it can undergo, such as volatilization into the atmosphere, sorption, or formation of organo-iodine complexes. Iodine’s mobility is determined by speciation, thus understanding what factors determine speciation is key to understanding its fate and transport in the environment. In nature, the speciation of radioiodine is dependent on the presence of redox active agents. One such prolific group of strong oxidizing agents are manganese oxides. Manganese oxides are naturally occurring minerals that exert strong geochemical controls on redox sensitive contaminants, such as radioiodine, due to the high reduction potential of the Mn(IV or III)/Mn(II) redox couples and the redox cycling of manganese by microorganisms. Though manganese oxides are environmentally abundant, the oxidation of radioiodine by manganese oxides has yet to be thoroughly studied. In this work, we examine the effects of the most common manganese oxide, birnessite, on the oxidation and sorption of radioiodine. This is under a variety of environmentally relevant conditions to determine if there is a potential attenuation pathway for radioiodine in the environment. It was observed that the oxidation and sorptive capacities of birnessite minerals are dependent on crystal structure, pH, and initial iodide concentration. Upon reaching equilibrium, a portion of the total iodine concentration is lost due to iodate sorption, indicating an attenuation pathway, while the rest was mostly found as molecular iodine, providing alternative reaction pathways (e.g., iodinated organic compounds) for the fate and transport of radioiodine in the environment.

Manganese oxides as geochemical regulators of the fate and transport of iodine in the environment

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