Effect of organic carbon substrates on denitrification rates in sediment
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Nitrate (NO3-) is a ubiquitous groundwater contaminant in agricultural and wastewater discharge areas. The prediction of microbial mediated NO3- removal in subsurface environments requires an understanding of the rates at which electron donors are utilized by denitrifying microbes. This study focuses specifically on the following organic carbon compounds as electron donors: glucose, acetate, adenine, cysteine and fulvic acid. Six triplicate series of flow through reactors (FTRs) containing 35 cm3 of natural, organic-poor sediment were supplied for 10 weeks with solutions containing nitrate and the individual carbon compounds, along with a no-carbon added control. The organic carbon compounds were selected to yield a range of different types of organic carbon (sugars, amino acids etc.) as well as a range of Gibbs Free Energy (∆G) values when their oxidation is coupled to denitrification. The initial flow rate of the FTRs was 1 ml h-1. Once steady NO3- concentrations were reached in the outflow, the flow rate was increased to 2 ml h-1 and, subsequently, 4 ml h-1. Potential denitrification rates (RD) measured for the different carbon substrates spanned a range of 0 to 114 nmol cm-3 h-1. Fulvic acid did not induce denitrification, while acetate yielded the highest rate. The outflow solutions for FTRs supplied with adenine and cysteine contained ammonia and sulfate, respectively. These results are consistent with the molecular structure of adenine, which contains an amine group, and of cysteine, containing an amine and thiol group. The results show that the addition of C-substrates to the sediment promotes denitrification, and the rate at which it occurs are dependant on which C-substrate is provided. RD results were used to determine if the denitrification rates imposed by the different carbon substrates could be predicted using theoretical approaches such as ∆GR or the nominal oxidation state of carbon (NOSC). However, predictions determined by thermodynamics alone were not significantly correlated with the observed trends in denitrification rates.