Microbial ureolysis
offers the potential to remove metals including
Sr2+ as carbonate minerals via the generation of alkalinity
coupled to NH4
+ and HCO3
– production. Here, we investigated the potential for bacteria, indigenous
to sediments representative of the U.K. Sellafield nuclear site where 90Sr is present as a groundwater contaminant, to utilize urea
in order to target Sr2+-associated (Ca)CO3 formation
in sediment microcosm studies. Strontium removal was enhanced in most
sediments in the presence of urea only, coinciding with a significant
pH increase. Adding the biostimulation agents acetate/lactate, Fe(III),
and yeast extract to further enhance microbial metabolism, including
ureolysis, enhanced ureolysis and increased Sr and Ca removal. Environmental
scanning electron microscopy analyses suggested that coprecipitation
of Ca and Sr occurred, with evidence of Sr associated with calcium
carbonate polymorphs. Sr K-edge X-ray absorption
spectroscopy analysis was conducted on authentic Sellafield sediments
stimulated with Fe(III) and quarry outcrop sediments amended with
yeast extract. Spectra from the treated Sellafield and quarry sediments
showed Sr2+ local coordination environments indicative
of incorporation into calcite and vaterite crystal structures, respectively.
16S rRNA gene analysis identified ureolytic bacteria of the genus Sporosarcina in these incubations, suggesting they have
a key role in enhancing strontium removal. The onset of ureolysis
also appeared to enhance the microbial reduction of Fe(III), potentially
via a tight coupling between Fe(III) and NH4
+ as an electron donor for metal reduction. This suggests ureolysis
may support the immobilization of 90Sr via coprecipitation
with insoluble calcium carbonate and cofacilitate reductive precipitation
of certain redox active radionuclides, e.g., uranium.