THE FLORIDA STATE UNIVERSITY ARTS AND SCIENCES ...

nitrate concentration in a system it does not serve as a removal mechanism and hence
can’t be considered a nitrate attenuation mechanism.
Denitrification is the reduction of
−
NO 3 to nitrogen gas (N2) and it considered by most to
be the only acceptable nitrate attenuation method that can completely remove nitrates
from the system. Denitrification refers to the dissimilatory reduction, by essentially
anaerobic bacteria (Cavigelli and Robertson, 2000), of one or both of the ionic nitrogen
oxides (nitrate, ( NO ), and nitrite, ( NO )) to the gaseous oxides (nitric oxide, (NO), and
−
3
−
2
nitrous oxide, (N2O)), which may then be further reduced to di-nitrogen (N2). The
nitrogen oxides act as terminal electron acceptors in the absence of oxygen (Knowles,
1982). It may also be simply referred to as a microbial respiratory process where nitrate is
used as the terminal electron acceptor and is reduced to N2 gas (Puckett & Cowdery,
2002).
The reaction of denitrification may be represented as a half reaction by:
−
+ −
2 NO3 + 12H
+ 10e
→ N 2 + 6H
2O
3
---- Equation 1.1
The bacteria responsible for denitrification in an aquifer, depending on the species, obtain
energy from the oxidation of organic or inorganic compounds (electron donor). In order
to complete this oxidation reaction, a reduction environment (anaerobic) and an electron
-
acceptor (O2, NO3 , Mn4 + , Fe3 + and SO4 - ) are also required (Korom, 1992; Rivett et al.,
2008). Based on Gibbs free energy, bacteria use the electron acceptors in the following
-
order: O2, NO3 , Mn4 + , Fe3 + and SO4 - and CH4 (Figure 1). This means that when O2
becomes limited in the saturated zone, bacteria start using nitrate as an electron acceptor.
Organic carbon is the most common electron donor and tends to be oxidized
preferentially by acceptors that yield the most energy to bacteria (Starr & Gillham, 1993).