Bacterial mobilization of polonium

Polonium has been observed as the sole a-emitting nuclide in groundwaters of central Florida, in the absence of its radiogenic parents, at levels of 1,000 dpm/l or more. Because of the chemical similarity of Po to S (both occupy the same column in the periodic table), studies were begun to determine whether bacteria, particularly those species active in sulfur cycling, could account for the selective solubilization and mobilization of Po. Possible sources of Po are the U-rich phosphate rock and phosphogypsum (gypsum), a byproduct in the manufacture of phosphoric acid. This paper reports on a series of experiments involving the interaction of bacteria with this waste gypsum that resulted in the solubilization of Po. Bacteria were isolated from gypsum that were capable of mediating Po release in column experiments when fed a growth medium. Sulfate-reducing bacteria were particularly effective at mediating Po release provided the sulfide levels did not rise above 10 μM, in which case Po was apparently coprecipitated as a metal sulfide. Conversely, the ability of sulfate-reducing bacteria to effctively remove dissolved Po when sulfide levels are high suggests that these bacteria may be used as an effective bioremediation tool at reducing groundwater Po levels.