Iodine and microbial interactions in an organic soil

Abstract

Iodine-129 in groundwater discharging from a geologic disposal vault could accumulate in wetlands by chemical sorption onto low pH, highly organic solid surfaces or by direct or indirect microbial processes. Previous work indicated that saturation of anion sorption sites, microbial toxicity, or swamping of the I reduction/oxidation reaction decreased the retention of a wetland sphagnum for iodine with increased iodine porewater concentrations. Bog water and peat of an iodine-rich bog were studied to elucidate the role of micro-organisms in the retention and accumulation of iodine in a temperate wetland. Agar plate culture of a wide spectrum of microbes, including yeasts and moulds, with bog groundwater showed anaerobes to be more sensitive to high iodine concentrations than aerobes. Toxicity to anaerobes may occur at 110 mg I litre −1 groundwater. Although iron-related and slime-forming bacteria were not affected at 2000 mg I litre −1, iodine was toxic to sulphate-related bacteria (SRB), as indicated by Biological Activity Reaction Tests. A 50% reduction in the SRB population occurred at groundwater concentrations of 75 mg I litre −1. Microbe enumeration in iodine-rich bog groundwater, using acridine orange staining, showed that the native wetland microbe population was sensitive to concentrations in excess of 200 mg I litre −1 groundwater. Three independent experiments confirm the toxicity threshold concentration for anaerobic and native microbes is ~100mg I litre −1 groundwater. Iodine sorption to fresh peat was slightly slower and more limited under anoxic conditions. Autoclaving the peat, reinoculation following autoclaving and a sucrose addition all inhibited iodine sorption. These results suggest that microbes may only play a minor and indirect role in iodine sorption through the decomposition of organic matter.