The synergistic effects of high nitrate concentrations on sediment bioreduction

Groundwaters at nuclear sites are often characterised by low pH and high nitrate concentrations (10-100 mM). These conditions are challenging for bioremediation, often inhibiting microbial Fe(III)-reduction that can limit radionuclide migration. Here, sediment microcosms representative of the UK Sellafield site were used to study the influence of variable pH and nitrate concentrations on microbially-mediated TEAP (terminal electron accepting processes) progression. The rate of bioreduction at low pH (~ 5.5) was slower than that in bicarbonate-amended systems (pH ~ 7.0), but in the low pH systems, denitrification and associated pH buffering resulted in conditioning of the sediments for subsequent Fe(III) and sulfate reduction. Under very high nitrate conditions (100 mM), bicarbonate amendment (pH ~ 7.0) was necessary for TEAP progression beyond denitrification and the reduction of 100 mM nitrate created alkaline conditions (pH 9.5). 16S rRNA gene analysis showed that close relatives of known nitrate reducers Bacillus niacini and Ochrobactrum grignonense dominated the microbial communities. In the 100 mM nitrate system, close relatives of the Fe(III)-reducing species Alkaliphilus crotonatoxidans and Serratia liquifaciens were observed. These results highlight that denitrification can support bioreduction via pH conditioning for optimal metal reduction and immobilization.