Strontium sorption and precipitation behaviour during bioreduction in nitrate impacted sediments

The behaviour of strontium (Sr2 +) during microbial reduction in nitrate impacted sediments was investigated in sediment microcosm experiments relevant to nuclear sites. Although Sr2 + is not expected to be influenced directly by redox state, bioreduction of nitrate caused reduced Sr2 + solubility due to an increase in pH during bioreduction and denitrification. Sr2 + removal was greatest in systems with the highest initial nitrate loading and consequently more alkaline conditions at the end of denitrification. After denitrification, a limited re-release of Sr2 + back into solution occurred coincident with the onset of metal (Mn(IV) and Fe(III)) reduction which caused minor pH changes in all microcosms with the exception of the bicarbonate buffered system with initial nitrate of 100 mM and final pH > 9. In this system ~ 95% of Sr2 + remained associated with the sediment throughout the progression of bioreduction. Analysis of this pH 9 system using X-ray absorption spectroscopy (XAS) and electron microscopy coupled to thermodynamic modelling showed that Sr2 + became partially incorporated within carbonate phases which were formed at higher pH. This is in contrast to all other systems where final pH was < 9, here XAS analysis showed that outer sphere Sr2 + sorption predominated. These results provide novel insight into the likely environmental fate of the significant radioactive contaminant, 90Sr, during changes in sediment biogeochemistry induced by bioreduction in nitrate impacted nuclear contaminated environments.