Microbial Methylation of Iodide in Unconfined Aquifer Sediments at the Hanford Site, USA

Abstract

Incomplete knowledge of environmental transformation reactions limits our ability to accurately inventory and predictably model the fate of radioiodine. The most prevalent chemical species of iodine include iodate (IO3−), iodide (I−), and organo-iodine. The emission of gaseous species could be a loss or flux term but these processes have not previously been investigated at radioiodine-impacted sites. We examined iodide methylation and volatilization for Hanford Site sediments from three different locations under native and organic substrate amended conditions at three iodide concentrations. Aqueous and gaseous sampling revealed methyl-iodide to be the only iodinated compound produced under biotic conditions. No abiotic transformations of iodide were measured. Methyl-iodide was produced by 52 out of 54 microcosms, regardless of prior exposure to iodine contamination or the experimental concentration. Interestingly, iodide volatilization activity was consistently higher under native (oligotrophic) Hanford sediment conditions. Carbon and nutrients were not only unnecessary for microbial activation, but supplementation resulted in >three-fold reduction in methyl-iodide formation. This investigation not only demonstrates the potential for iodine volatilization in deep, oligotrophic subsurface sediments at a nuclear waste site, but also emphasizes an important role for biotic methylation pathways to the long-term management and monitoring of radioiodine in the environment.

Iodate Reduction by Shewanella oneidensis Requires Genes Encoding an Extracellular Dimethylsulfoxide Reductase

“…An unstable radioisotope of iodine, 129 I, is a nuclear waste product produced during uranium and plutonium fission reactions and displays a long half-life of 16 million years ( Timar et al, 2014 ). 129 I is found in contaminated groundwater at the U.S. Department of Energy Savannah River and Hanford Superfund sites from a long history of nuclear weapons testing ( Emerson et al, 2014 ; Timar et al, 2014 ; Bagwell et al, 2019 ). Following the 2011 Fukushima nuclear reactor catastrophe, westerly winds deposited a large portion of 129 I in the Pacific Ocean, where radioactive IO 3 – and I – are the predominant 129 I forms ( Hou et al, 2007 , 2013 ; Bluhm et al, 2011 ; Kenyon et al, 2020 ).…”

Production of Methyl-Iodide in the Environment

“…Some soil organic compounds with large carbonyl moieties, or alkyl-chlorides can enhance the formation of volatile iodine compounds, while aromatic compounds can stabilize the iodine in soil ( Taghipour and Evans, 2001 ). Biotic transformation of iodine into the methyl-iodides plays also prominent role in the oligotrophic sediments at nuclear waste sites where the isolated microcosm possessed iodine methylation abilities ( Bagwell et al, 2019 ).…”

Experimental and Numerical Study of Radioiodine Sorption and Transport in Hanford Sediments

“…Groundwater monitoring results indicate that 129 I groundwater plumes at Hanford have been slowly but steadily shrinking since the early 1990s, , but large areas still exist with concentrations that exceed 1 pCi/L, the maximum concentration level allowed by federal and state regulations. Volatilization is likely a relatively minor mechanism responsible for the observed groundwater plume attenuation, but this process would be more significant in the vadose zone owing to the presence of an interconnected gas phase . The exact mechanisms for the attenuation of radioiodine groundwater plumes over time at Hanford are unknown, but sorption has been assumed to be the dominant process.…”

“…Volatilization is likely a relatively minor mechanism responsible for the observed groundwater plume attenuation, but this process would be more significant in the vadose zone owing to the presence of an interconnected gas phase. 14 The exact mechanisms for the attenuation of radioiodine groundwater plumes over time at Hanford are unknown, but sorption has been assumed to be the dominant process.…”