The synergistic effects of high nitrate concentrations on sediment 1 bioreduction

16 Groundwaters at nuclear sites are often characterised by low pH and high nitrate 17 concentrations (10-100 mM). These conditions are challenging for bioremediation, often 18 inhibiting microbial Fe(III)-reduction that can limit radionuclide migration. Here, sediment 19 microcosms representative of the UK Sellafield site were used to study the influence of 20 variable pH and nitrate concentrations on microbially-mediated TEAP (terminal electron 21 accepting processes) progression. The rate of bioreduction at low pH (~ 5.5) was slower than 22 that in bicarbonate-amended systems (pH ~ 7.0), but in the low pH systems, denitrification 23 and associated pH buffering resulted in conditioning of the sediments for subsequent Fe(III) 24 and sulfate reduction. Under very high nitrate conditions (100 mM), bicarbonate amendment 25 (pH ~ 7.0) was necessary for TEAP progression beyond denitrification and the reduction of 26 100 mM nitrate created alkaline conditions (pH 9.5). 16S rRNA gene analysis showed that 27 close relatives of known nitrate reducers Bacillus niacini and Ochrobactrum grignonense 28 dominated the microbial communities. In the 100 mM nitrate system, close relatives of the 29 Fe(III)-reducing species Alkaliphilus crotonatoxidans and Serratia liquifaciens were 30 observed. These results highlight that denitrification can support bioreduction via pH 31 conditioning for optimal metal reduction and immobilization. 32 33