(0.8 V, 1.0 V, and 1.2 V) compared to the open circuit control (Daghio et al.
2018a). Similarly, also phenol (Friman et al. 2013; Huang et al. 2011; Palma et al.
2018; Zhang et al. 2017) and recalcitrant polycyclic aromatic hydrocarbons—
PAHs (Adelaja et al. 2014, 2015; Hamdan et al. 2017; Yan et al. 2012; Yu et al.
2017; Zhang et al. 2010) were successfully removed in BESs. Anodic oxidation of
the halogenated hydrocarbon 1,2-dichloroethane (1,2-DCA) was also shown
(Pham et al. 2009).
During the stimulation of the bioremediation by anodic oxidation of the contaminant, the electrode is the final electron acceptor for the microbial respiration. It is
thus possible to hypothesize that with higher anodic potentials the energy gain for
the microorganisms is higher, and, therefore, the biodegradation rate can be
increased. Several studies have tried to correlate the anodic potential with the
microbial activity using easily degradable substrates such as acetate. The data,
however, gave controversial results since a positive correlation was observed in
some cases, while in other studies the opposite was observed (Aelterman et al. 2008;
Wagner et al. 2010). The data on the influence of the anodic potential when
hydrocarbons are the electron donors are lacking. During the bioelectrochemical
toluene removal in BES reactors inoculated with a marine sediment, and with
artificial seawater as growth medium, two anodic potentials were tested (i.e.,
+200 mV and +500 mV) without any appreciable effect on the biodegradation rate
(Daghio et al. 2016). When the anode was polarized at +500 mV, an unidentified
redox active moiety with a midpoint potential of about +400 mV was detected and its
involvement in the EET was hypothesized (Daghio et al. 2016). More information
on the influence of the anode potential is, therefore, necessary, but it is reasonable
that the optimal anodic potential is highly dependent on specific in situ conditions
and on the microbial communities involved in the biodegradation.
Oxygen evolution using high-potential electrodes is an alternative strategy to use
electrodes for the stimulation of the oxidation of petroleum hydrocarbons in contaminated environments (Daghio et al. 2017). Indeed, oxygen is not only an important electron acceptor for the microbial respiration, but also a key reagent involved in
the initial attack of hydrocarbons in aerobic environments through the addition of
hydroxyl groups catalyzed by oxygenases (Gkorezis et al. 2016; Weelink et al.
2010). A small amount of oxygen can then be useful to trigger the degradation of
hydrocarbon contaminants. Examples of stimulation of the biodegradation by oxygen evolution with electrodes have been reported for cis-dichloroethene (cis-DCE)
removal, with an electrode polarized at +1500 mV (Aulenta et al. 2013), and for TPH
removal by applying an external voltage (2 V) on dimensionally stable anodes—
DSA; i.e., Ti mesh covered with mixed metal oxides, primarily consisting of Ir and
Ru (Bellagamba et al. 2017).
When the reduction of the contaminant is the reaction that has to be stimulated
(e.g., for the dehalogenation of halogenated hydrocarbons) the cathodic process can
be exploited. Several studies have successfully showed dehalogenation of
2,6-dichlorophenol (2,6-DCP) (Skadberg et al. 1999), trichloroethene (TCE)
(Aulenta et al. 2008b), and tetrachloroethene (PCE) (Lohner and Tiehm 2009)
through H 2 production using electrodes. However, a direct electron uptake from
378
M. Daghio and A. Franzetti
2018a). Similarly, also phenol (Friman et al. 2013; Huang et al. 2011; Palma et al.
2018; Zhang et al. 2017) and recalcitrant polycyclic aromatic hydrocarbons—
PAHs (Adelaja et al. 2014, 2015; Hamdan et al. 2017; Yan et al. 2012; Yu et al.
2017; Zhang et al. 2010) were successfully removed in BESs. Anodic oxidation of
the halogenated hydrocarbon 1,2-dichloroethane (1,2-DCA) was also shown
(Pham et al. 2009).
During the stimulation of the bioremediation by anodic oxidation of the contaminant, the electrode is the final electron acceptor for the microbial respiration. It is
thus possible to hypothesize that with higher anodic potentials the energy gain for
the microorganisms is higher, and, therefore, the biodegradation rate can be
increased. Several studies have tried to correlate the anodic potential with the
microbial activity using easily degradable substrates such as acetate. The data,
however, gave controversial results since a positive correlation was observed in
some cases, while in other studies the opposite was observed (Aelterman et al. 2008;
Wagner et al. 2010). The data on the influence of the anodic potential when
hydrocarbons are the electron donors are lacking. During the bioelectrochemical
toluene removal in BES reactors inoculated with a marine sediment, and with
artificial seawater as growth medium, two anodic potentials were tested (i.e.,
+200 mV and +500 mV) without any appreciable effect on the biodegradation rate
(Daghio et al. 2016). When the anode was polarized at +500 mV, an unidentified
redox active moiety with a midpoint potential of about +400 mV was detected and its
involvement in the EET was hypothesized (Daghio et al. 2016). More information
on the influence of the anode potential is, therefore, necessary, but it is reasonable
that the optimal anodic potential is highly dependent on specific in situ conditions
and on the microbial communities involved in the biodegradation.
Oxygen evolution using high-potential electrodes is an alternative strategy to use
electrodes for the stimulation of the oxidation of petroleum hydrocarbons in contaminated environments (Daghio et al. 2017). Indeed, oxygen is not only an important electron acceptor for the microbial respiration, but also a key reagent involved in
the initial attack of hydrocarbons in aerobic environments through the addition of
hydroxyl groups catalyzed by oxygenases (Gkorezis et al. 2016; Weelink et al.
2010). A small amount of oxygen can then be useful to trigger the degradation of
hydrocarbon contaminants. Examples of stimulation of the biodegradation by oxygen evolution with electrodes have been reported for cis-dichloroethene (cis-DCE)
removal, with an electrode polarized at +1500 mV (Aulenta et al. 2013), and for TPH
removal by applying an external voltage (2 V) on dimensionally stable anodes—
DSA; i.e., Ti mesh covered with mixed metal oxides, primarily consisting of Ir and
Ru (Bellagamba et al. 2017).
When the reduction of the contaminant is the reaction that has to be stimulated
(e.g., for the dehalogenation of halogenated hydrocarbons) the cathodic process can
be exploited. Several studies have successfully showed dehalogenation of
2,6-dichlorophenol (2,6-DCP) (Skadberg et al. 1999), trichloroethene (TCE)
(Aulenta et al. 2008b), and tetrachloroethene (PCE) (Lohner and Tiehm 2009)
through H 2 production using electrodes. However, a direct electron uptake from
378
M. Daghio and A. Franzetti