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.

Keywords: iodine cycling; methylation; radioiodine; vadose zone; volatilization.

Figure 1

Iodomethane production from Hanford subsurface sediments. Bar graphs show averaged (± standard deviation) iodomethane production among experimental triplicates. Black bars indicate sediment microcosms that only received synthetic groundwater. Gray bars indicate sediment microcosms that received synthetic groundwater, glucose (1 mM), and yeast extract (20 g/L). Student t-tests were used to determine statistical significance (p < 0.05, indicated by “*”) in iodomethane production between treatments.