Anodic process of SMFCs
As pointed out above, research on SMFCs has indicated that several reactions contribute to the generation of electrical current: (i) the chemical oxidation of microbially produced reductants, such as humic acids, Fe2+, but especially sulfur compounds at the anode, (ii) the microbial oxidation of organics such as acetate and (iii) the microbial oxidation of S0 to SO42− (Reimers et al., 2001; Bond et al., 2002; Tender et al., 2002; Holmes et al., 2004a; Ryckelynck et al., 2005; Lowy et al., 2006). Through the release of electrons during these processes, captured by the anode, a current is generated in an electrical circuit. The responsible anodic microbial communities have been investigated by several authors and in a number of different aquatic sediments – marine, salt-marsh and freshwater. A clear enrichment of δ-Proteobacteria, more specifically of the family Geobacteraceae, was most often observed on anodes. The most closely related organism for anodes residing from marine sediments was Desulfuromonas acetoxidans. For freshwater sediments, the most related species were Geobacter species, of which Geobacter sulfurreducens is the most studied organism (Bond et al., 2002; Tender et al., 2002; Holmes et al., 2004b; Lovley, 2006a). Desulfuromonas acetoxidans and G. sulfurreducens were found to be able to oxidize acetate with an anode as sole electron acceptor (Bond et al., 2002). In the study of Holmes and colleagues (2004b), Desulfobulbus/Desulfocapsa species appeared to be especially enriched on anodes from estuarine sediments, while the Fe(III)-reducing Geothrix species were only found to be enriched on freshwater anodes. Moreover, Reimers and colleagues (2006) discovered that the anodic community diversity changed with anode depth: the top section of a rod shaped anode, positioned vertically in the sediment, was less diverse than the bottom section. Recently, marine sediment was used to enrich a thermophilic and anodophilic consortium, capable to produce 10-fold more current at 60°C than at 22°C. This consortium mainly consisted of the Gram-positive Thermincola carboxydophila (Mathis et al., 2008).
Energy generation with SMFCs
The production of energy in the form of readily usable electrical power is the most direct application of SMFCs. The energy can be generated for a long period in remote areas and from a sustainable source. They allow an in situ electricity generation for small electrical apparatuses. As such, SMFCs could be used to power sensors and data transmitters, for example, in the middle of the ocean. To overcome low currents and voltages, capacitors and converters can be added (Shantaram et al., 2005). This principle was recently demonstrated with an SMFC with a bio-anode which could power a meteorological buoy and data transmitter with an average power consumption of 18 mW, by applying a power conditioner and a capacitor (Tender et al., 2008).
The average sustainable power which has been obtained from SMFCs is in the order of 10 to 20 mW m−2 of anodic electrode footprint area (EFA) (Reimers et al., 2001; Tender et al., 2002), with a sustainable maximum of 28 mW m−2 EFA or 104 mA m−2 EFA (Tender et al., 2002; Ryckelynck et al., 2005). The SMFCs described in literature thus far were mostly positioned in marine waters (Reimers et al., 2001; 2006; Bond et al., 2002; Tender et al., 2002; Holmes et al., 2004b; Ryckelynck et al., 2005; Lowy et al., 2006; Rezaei et al., 2007). They have the advantage of high salt concentrations in the electrolyte and thus a high ion conductivity and low cathode passivation. Nonetheless, the output of SFMCs in general is restricted because of a number of limitations:
Kinetics at the anode. Lowy and colleagues (2006
) could temporarily increase the current densities at the anode by increasing the kinetics of microbial reduction of the anode. This was done by applying microbial oxidants onto or into anode graphite disks as charge transfer mediators. An anode with adsorbed anthraquinone-1,6-disulfonic acid delivered a maximum, non-sustainable power density of 98 mW m−2
EFA, while a maximum, non-sustainable power density of 105 mW m−2
EFA was achieved using Mn2+
incorporated in the anode. Current densities decreased with the square root of time, resulting from mass transfer limitations (Lowy et al., 2006
). In the context of kinetics, temperature should be mentioned as a significant factor, as Reimers and colleagues (2001
) noticed, an abrupt drop in power resulting from a temperature decrease of 4.5°C.
Mass transfer limitations for electron donors to reach the anode form a major limitation for power production from SMFCs. Lowy and colleagues (2006
) stated that a sustained power generation of 100 mW m−2
EFA should be possible in environments with high concentrations of anode reactants. Such a test was performed in a cold ocean sulfide seep (Reimers et al., 2006
), where the advective flow of sulfide- and methane-rich fluids could indeed – initially – support a higher power production. A maximum power output, sustainable during 24 h, of 1100 mW m−2
EFA was obtained. The anode used in this study was a long graphite rod, having a high ratio of electrode surface over EFA. A deposition of elemental sulfur on the anode, resulting in a passivation and hence deactivation of the anode surface, became apparent as an additional limitation of the SMFC system (Reimers et al., 2006
). Nielsen and colleagues (2007
) installed benthic chambers, housing the anode, on top of the sediment. In order to increase the power production, advective flows of reductant-rich porewater were led through an anode with high surface area. These flows were either caused by natural processes or created through active pumping. Continuous power densities of 233 mW m−2
EFA were reached.
The organic matter content of sediments as such is a limiting factor as well. Rezaei and colleagues (2007
) tried to increase the power generation through the addition of particulate organic matter (chitin and cellulose) in the anodic matrix, an approach which is also not yet sustainable.
Cathode catalysis. Oxygen availability, as a limiting factor for cathode performance of SMFCs, was handled in a labscale SMFC by using a rotating cathode, increasing the power production with a factor 1.7 as compared with a non-rotating cathode. Sustainable outputs of 25 mW m−2
EFA were reached with a rotating cathode and with sucrose fed at the anode (He et al., 2007
Although research on sediment fuel cells has focussed on marine environments, freshwater environments can also sustain electrical current production. Lower output values have been reported, likely due to the decreased conductivity and the fact that salt water – as opposed to freshwater – enhances ‘virtual’ corrosion at the cathode, which is beneficial for the cathode performance (Bergel et al., 2005). Holmes and colleagues (2004b) obtained a sustainable current production of 20 mA m−2 total anode surface for marine SMFCs versus 9 mA m−2 total anode surface for freshwater environments. In laboratory freshwater SMFCs, the mass transfer limitation was attenuated by growing rice plants in the sediment (De Schamphelaire et al., 2008), which could continuously deliver fresh substrates at the anode, under the form of rhizodeposits, hence increasing the sustainable power production with a factor 7. A maximum sustainable output of 33 mW m−2 EFA could be attributed to an oxidation of rhizodeposits. Kaku and colleagues (2008) also reported the application of an SMFC in a freshwater rice paddy field. However, a low sustainable output (3 mW m−2 EFA) was reached with their system.
In order to allow a better comparison of the results discussed above, an overview of power and electrical currents obtained with SMFCs is presented in Table 1.
Table 1. Overview of current and power resultsa obtained with SMFCs.
|Operating characteristics of the SMFCb||Power density (mW m-2 EFA)||Power density (mW m-2 ES)||Current density (mA m-2 EFA)||Current density (mA m-2 ES)||Reference|
|• Marine sediment in laboratory|| || || || ||Reimers et al. (2001)|
|• Platinum mesh or carbon fibres||10|| || || |
|• Marine environments|| || || || ||Tender et al. (2002); Ryckelynck et al. (2005)|
| Estuarine (constant voltage)||28||9||104||35|
| Salt marsh (constant current)||26||9||100||34|
|• Graphite disk with holes|| || || || |
|• Marine sediment in laboratory|| || || || ||Bond et al. (2002)|
|• Graphite disk|| ||16|| || |
|• Laboratory incubations|| || || || ||Holmes et al. (2004b)|
| Marine|| || || ||20|
| Salt marsh|| || || ||7|
| Freshwater|| || || ||9|
|• Graphite disk|| || || || |
|• Coastal site|| || || || ||Lowy et al. (2006)|
|• Graphite disk with AQDS||98||47||560||266|
| with Mn2+ and Ni2+ (maximum non-sustainable resultsa)||105||47||350||158|
|• Ocean cold seep|| || || || ||Reimers et al. (2006)|
|• Vertical graphite rod||1100||34||2647||82|
|• Estuarine environment|| || || || ||Nielsen et al. (2007)|
| Forced advection||233||0.18||466||0.35|
| Natural advection||140||0.71||350||1.77|
|• Carbon brush|| || || || |
|• Laboratory seawater incubation|| || || || ||Rezaei et al. (2007)|
|• Pillow-shaped carbon cloth filled with Chitin 80||51||8||184||31|
|• Laboratory incubation of river sediment amended with sucrose solution and a rotating cathode||25||12.5||5||2.5||He et al. (2007)|
|• Carbon cloth|| || || || |
|• Freshwater matrix planted with rice plants|| || || || ||De Schamphelaire et al. (2008)|
|• Graphite felt||33||16||55||26|
|• Freshwater rice paddy field|| || || || ||Kaku et al. (2008)|
|• Graphite felt||3||1.5||15||7|
|• Field deployment in salt marsh, powering a meteorological buoy||387||16||1105||46||Tender et al. (2008)|
|• Array of vertical graphite plates|| || || || |
SMFCs for the oxidation of organic compounds in sediments
Organic overloading of submerged soils through anthropogenic and natural processes may result in negative consequences for the aquatic environment as well as the area immediately surrounding it. Such accumulation may affect the aquatic community, for example by leading to oxygen deficiency, and cause an increase in greenhouse gas (GHG) emissions. Methane emissions have medians between 0.3 and 300 mg CH4 m−2 day−1, depending on the type of underwater soil and the possible cultivation type. A maximum of 2.9 g CH4 m−2 day−1 was noted for rice paddies (Le Mer and Roger, 2001). Organic matter accumulation may also impede navigation in waterways. Apart from the general accumulation of sediment organic matter, sediments may also be contaminated with specific organic compounds, such as hydrocarbons, of which the removal is desired.
Human intervention, for example by active aeration, may enhance the oxidative breakdown of these various types of organics. However, the applicable techniques for such an intervention have several limitations. Recently, an alternative technique for sediment oxidation was proposed (De Schamphelaire et al., 2007), encompassing a microbial manganese cycle in the water body (Fig. 2B). Oxidized Mn can act as electron acceptor at the sediment, while the reduced Mn diffuses upwards and is re-oxidized in the oxidized parts of the water body. Manganese hence represents an electron shuttle between the sediment organic matter and the final electron acceptor oxygen, thereby increasing the oxidation rate in the sediment. Here, we bring forth an SMFC as alternative, but related technique.
By constructing an MFC in the sediment, the anoxic oxidation of sediment organic matter can be stimulated and possibly accelerated. The basis hereof lies within the spatial separation of the oxidative, electron-generating half-reaction at the anode and the electron-consuming half-reaction at the cathode. As a result, the oxidation of the reduced compounds within the sediment is no longer limited by the availability of electron acceptors within the sediment, but rather relies on the vicinity of an anode.
Before quantitatively discussing the attainable anodic sediment oxidation rate, we will go deeper into the processes involved. During biodegradation in sediments, the complex assemblage of organic matter in sediments is hydrolysed to long-chain fatty acids, aromatic compounds and fermentables. The latter, such as sugars and amino acids, are subsequently fermented to acetate and minor fermentation products, such as ethanol and H2. Finally, these fermentation products, as well as the long-chain fatty acids and aromatic compounds, can be further microbially oxidized. Microorganisms such as Geobacteraceae can be responsible for this oxidation, using Fe3+, Mn4+ or humic acids as electron acceptor in natural conditions. If an MFC is present, Geobacteraceae are able to use the solid anode as an alternative electron acceptor, hence generating an electrical current. The fact that Geobacteraceae are 10-fold more enriched on anodes generating current than on control anodes suggests that direct oxidation of organic matter is an important current generating process in SMFCs. The process can be mediated through a direct contact of the microorganisms with the electrode, involving outer-membrane c-type cytochromes (Lovley, 2006a,b). Other microorganisms enriched on sedimentary anodes, such as the freshwater bacterium Geothrix fermentans, use electron shuttles to oxidize organic compounds at the anode (Bond and Lovley, 2005).
In sulfide-rich sediments, sulfur compounds can be a major electron donor for the anode, although these processes are related to organic matter oxidation as well. The sulfide present in these sediments results from the microbial sulfate reduction coupled to organic matter oxidation. Part of the sulfide formed reacts with Fe2+ to form reduced sulfur minerals, FeS and FeS2. Dissolved sulfide, as well as sulfides trapped in iron minerals, can abiotically be oxidized to S0 at an anode (Ryckelynck et al., 2005). The sulphate-reducing Desulfocapsa and Desulfobulbus genera were found to be enriched at an active anode in sulfide-rich sediments (Holmes et al., 2004b), while it was demonstrated that Desulfobulbus propionicus can oxidize S0 to sulfate (SO42−) with an electrode as sole electron acceptor (Holmes et al., 2004a). Hence, the further oxidation of S0 to SO42− (microbially mediated or abiotic) is most likely one of the current generating processes in this type of sediments. Alternatively, Desulfobulbus/Desulfocapsa genera can disproportionate S0 to SO42− and S2−, while the latter can be re-oxidized (Ryckelynck et al., 2005). Overall, these processes result in the regeneration of sulfate as an electron acceptor for the oxidation of organic matter. Moreover, elemental sulfur can also act as an electron acceptor for the microbial degradation of organic matter (Ho et al., 2004). Sulfur compounds can thus serve as electron shuttles for the oxidation of organic matter in the anodic half-cell and the amplification of a sulfur cycle at the anode can increase the oxidation rate of sediment organic matter (see also Fig. 2A).
Further evidence for the stimulating effect of an active anode can be found in the comparison of chemical profiles around active and control anodes in sediments. Tender and colleagues (2002) described a linear sulfide gradient above and below the active anode in a salt marsh in Tuckerton, New Jersey, with a sulfide depletion at the anode surface. An enrichment of dissolved iron and a modest enrichment of sulfate could also be observed near the active anode. These results indicate the regeneration of sulfur compounds acting as electron acceptors. In Yaquina Bay estuary, Ryckelynck and colleagues (2005) found a sulfate gradient near the active anode – sulfate concentrations decreased when approaching the active anode and increased again in a 3-cm-thick zone around this anode – but not near the control anode. Moreover, smaller increases in ammonia and phosphate were observed near the control anode. This implies that higher dissimilatory sulfate reduction rates occurred near the active anode. Second, although the organic carbon content was similar immediately above the active and control anodes, the organic matter content was higher near the sediment–water interface in case of the active anode. The steeper organic matter gradient around the active anode also suggests a higher oxidation rate of organic matter in the presence of a current generating anode. Reimers and colleagues (2007) applied marine plankton, representing the biofuel for SMFCs, in two-chambered reactor-type MFCs. For about 2 months, they operated six cells in closed circuit and one in open circuit as control. Although there was no statistical difference in the final level of total organic carbon between the active and control cells, the active cells did demonstrate significantly lower dissolved organic carbon levels. Recently, experiments were performed with model SMFCs, where a mixture of sand, starch and inoculum was applied around the anode (L. De Schamphelaire and W. Verstraete, unpubl. results). Six cells were actively operated as SMFCs by closing the electrical circuit, while six other cells were held in open circuit and acted as control cells. After approximately 1 month, the cells were dismantled. The actively operated SMFCs demonstrated levels of soluble chemical oxygen demand (COD) and volatile fatty acids which were significantly (P < 0.01) lower than in the control cells (respectively 20% and 50% lower).
The elements discussed above all argue for an increased rate of organic matter oxidation, stimulated by an active anode. The effect of the anode can mainly be noticed on dissolved organic matter oxidation, for which the availability of oxygen or sulfate is the limiting factor. The degradation of complex organic matter also involves hydrolysis and fermentation steps, which can be rate-limiting, and are, as far as known, not directly mediated by an active anode. The anodic process hence actually stimulates the final steps in the breakdown of complex organic matter, with acetate presumably as the most important electron donor because of its pivotal role in the degradation of organic matter by anaerobic microbial consortia (Lovley, 2006a).
At this moment, the process can be observed from a more quantitative point of view. Based on the values represented in Table 1, one can envisage an SMFC able to generate a sustainable electrical current of 100 mA m−2 of EFA. The current density is highly dependent on the anode design, as shown in Table 1. In the research of Tender and colleagues (2002), 40% of SMFC electron transfer in a salt marsh resulted from dissolved sulfide oxidation to elemental sulfur S0. As only modest oxidation of this S0 occurred, 60% of the electron transfer was attributed to other processes, such as acetate oxidation or more generally, to carbon oxidation. In the work of Ryckelynck and colleagues (2005), 90% of the total electron transfer was due to a regeneration of SO42−, which was originally consumed/reduced during the oxidation of organic matter. The regeneration of sulfate creates a new electron acceptor for organic matter oxidation. The observations from these studies indicate that at least 60–90% of SMFC electron transfer can be attributed to a direct or indirect oxidation of organic carbon. If the number of 100 mA m−2 EFA is related to sediment oxidation rates, one obtains a charge production of 8640 Coulomb per day, which equals a transfer of 89.5 mmol electrons per day. This correlates with a direct or indirect sediment oxidation rate of 13.4 mmol C (60%) or 20.1 mmol C (90%) m−2 sediment surface day−1.
The values obtained can be compared with carbon accumulation rates: an organic carbon accumulation of 0.01–9.1 mmol C m−2 day−1 in highly productive marine areas was reported in Ryckelynck and colleagues (2005), while an excess oxidation demand for carbon oxidation of 0.21–2.4 mmol C m−2 day−1 at continental margin sediments was reported by Hartnett and Devol, (2003). A carbon burial rate of 1.8 mmol C m−2 day−1 in lake sediment was mentioned by Thomsen and colleagues (2004). These numbers show that the present performance of SMFCs should suffice to oxidize all deposited organic carbon per m2, assuming that the organics or their fermentation intermediates can be oxidized by anodophiles.
Besides the oxidation of organic matter, naturally residing in sediment layers, the anode reaction can also be of interest for the oxidation of organic contaminants in sediment layers, such as petroleum compounds. Adding oxygen to the subsurface to stimulate the aerobic microbial community is a typical strategy for accelerating contaminant degradation, but adding anaerobic electron acceptors can be a good strategy as well (Finneran and Lovley, 2001). The insertion of an anode as electron acceptor might have the same effect. Indeed, Geobacter metallireducens was shown to be capable to oxidize the aromatic contaminant benzoate at an electrode (Bond et al., 2002).
Organic contaminants are a concern both in freshwater and marine systems. As for the oxidation of excess organic matter, this is often more crucial in freshwater than in marine environments. Due to the lower water depths in fresh waters, oxygen deficiency sooner affects the aquatic community and impediment of the waterway can sooner set in. Because of the proximity to rural areas, odour is also a more important issue. As more studies are performed on the better-performing marine SMFCs than on freshwater SMFCs, an expansion of the research focus is needed: the SMFCs should be engineered to perform as well in freshwater – with low salt concentrations – and with little or no water movement, which implies that an effective cathodic reaction should be ensured as well.
SMFCs for the oxidation of organic loading in constructed wetlands – an application
The aforementioned concept could be applied to enhance organic matter oxidation in the bottom layer of constructed wetlands. The latter, which are often reed beds, are used as a means of purifying pre-settled effluent from rural housing and agricultural waste streams before discharge into watercourses (McGechan et al., 2005). Microorganisms living in the rhizosphere of the reed plants perform both anaerobic and aerobic degradation processes. Aerobic conditions are created as oxygen enters the rhizosphere through diffusion, convection and to a major extent through the aerenchyma of the stems and roots of the reed plants. The total oxygen transfer capacity lies in the range of 160–1400 mmol O2 m−2 day−1 (McGechan et al., 2005). In temperate climates, the reed plants are dormant during winter, which diminishes the oxygen supply through the aerenchyma to the root zone (Ouellet-Plamondon et al., 2006). A general issue with wetlands in temperate and tropical regions is the fact that they produce an effluent of limited quality due to background levels of organic matter, solids and nutrients generated within the wetland (Greenway and Woolley, 1999). To aid a wetland in meeting good effluent standards, an SMFC could be installed in the root zone of the constructed wetland (Fig. 2C). The electrode matrix of the SMFC anode, which will act as an additional electron acceptor, might thus compensate for the insufficient aeration of the root zone by the aerenchyma, dormant periods in plant growth, lower oxygen fluxes during the night (Armstrong and Armstrong, 1990) or lower oxygen dissolution at high temperatures. The resulting enhanced oxidation in the wetland might furthermore be accompanied by a reduced emission of methane from the wetland, as will be explained in the next section. A prerequisite for the installation of an independent SMFC system is the possibility for a sustainable operation of the cathode. Operation during winter might hence be hindered as low temperatures with possible ice formation can severely hamper or even harm the cathodic reaction. Water movement, through an aeration of the cathode, might be applied to prevent freezing of the cathode. An alternative option – instead of the SMFC – to overcome the lower performance of the wetlands is the artificial aeration of the entire root zone, which requires a substantial energy input and additional costs (Ouellet-Plamondon et al., 2006).
Kern and Idler (1999) and Puigagut and colleagues (2007) reviewed the performances of several constructed wetlands throughout literature and obtained carbon removal rates of 137–1726 mmol C m−2 day−1. If artificial aeration would be required to sustain for example a carbon removal of 500 mmol C m−2 day−1, an aeration energy requirement of 16 Wh m−2 day−1 would be needed, assuming an oxygen transfer capacity of 2 kg O2 kWh−1 and an oxygenation capacity per load of 2. For an average household of four persons and 3 m2 of reed bed per person, a daily energy requirement of 0.19 kWh would be needed. On the other hand, if an SMFC could be installed inside the wetland, it would require no net energy input but rather produce energy. Alternatively, a reactor-type MFC could be positioned at the outlet of the constructed wetland, in order to treat the wetland effluent before discharge in a watercourse.
Based on the values discussed in the last section, an SMFC could be envisaged which can oxidize approximately 17 mmol C m−2 day−1 in a sustainable way using a plain graphite disk as anode, during the production of 100 mA of current m−2 EFA. This current density can be increased with at least a factor 25 (Table 1) through the combination of an increased ratio of electrode surface versus EFA and a high mass transfer. An affordable, granular – with high specific surface and thus high electrode surface over EFA ratio – and conductive material could be used to replace part of the support layer for growth of the reed plants, as this will allow sufficient COD oxidation rates. Alternatively, several parallel layers of electrode material with high specific surface area (e.g. carbon mats) could be installed in the root zone. Additionally, the flux of wastewater through the anode matrix could be increased, for example by installing a recirculation system, in order to increase the mass transfer. With electrical current densities a factor 25 higher, i.e. of the order of 2.5 A m−2, 425 mmol C m−2 day−1 could be oxidized, which is fair within the range of COD removal rates in constructed wetlands. The total power which could be obtained from the system (with for example 0.5 V per SMFC electrode pair) is 1.25 W m−2 constructed wetland. This means that this SMFC could daily produce 0.36 kWh instead of the 0.19 kWh energy requirement for artificial aeration.
SMFCs to control redox-dependent processes in sediments
The anode of an SMFC, buried in the sediment layer, is characterized by a specific potential. The latter is influenced by a number of factors (Rabaey and Verstraete, 2005a; Logan et al., 2006; Rabaey et al., 2007), such as: (i) the activation overpotential: this is a current-dependent potential drop between the electrode surface and the surrounding environment. The extent of catalysis of the anode, its surface area and its geometry are determining factors for this potential drop; (ii) the supply of electron donor, the efflux of breakdown products and both their respective redox potentials. This is mainly a diffusional problem, also influenced by the charge of the molecules; (iii) the internal resistance of the SMFC, which causes a potential loss between the anode and the cathode due to diffusional limitations for the cations; and (iv) the strength of the cathode. A well-catalysed cathode enables to increase the anode potential in conjunction with an adequate external resistance. Consequently, an anodic redox potential of a relatively high value can be obtained, which potentially turns this anode into an attractive electron acceptor in the sediment. This offers a way to manipulate redox-dependent metabolic processes in the sediment. An overview of such redox-dependent processes is given in Table 2. Notably the formation of nuisance compounds could hence be alleviated.
Methanogenesis and sulfate reduction, resulting in the formation of respectively the GHG CH4 and the toxic and odorous H2S, are commonly known anaerobic processes. They can be diminished at higher redox potentials (Devai and DeLaune, 1995a; Singh, 2001), such as by the presence of an anode with a redox potential sufficiently higher than the potential of the corresponding redox couples. The standard redox potential of the CO2/CH4 couple is −0.244 V versus SHE (Stumm and Morgan, 1996). The corresponding methanogenesis process is generally found not to occur at soil redox potentials above −0.150 V versus SHE (Singh, 2001). Some studies however demonstrate that a positive soil redox potential cannot inhibit the methanogenesis process as long as H2 is sufficiently available (Conrad, 2002). As H2 can however also be oxidized at an anode, with the use of a proper (bio)catalyst (Rosenbaum et al., 2005), methanogenesis can even be decreased by an SMFC in this case. Ishii and colleagues (2008) successfully demonstrated that an electron-capturing anode can suppress methanogenesis. They showed this in reactor-type MFCs fed with cellulose and inoculated with rice paddy field soil. A comparison of electron fluxes to CH4 and current generation suggested that the suppression of methanogenesis was not merely ascribable to simple competitive inhibition.
In case of sulfate reduction, the standard redox potential for the SO42−/H2S couple is −0.214 V versus SHE (Stumm and Morgan, 1996). Devai and DeLaune, (1995b) found that in a range of relative redox potentials varying from −0.240 V to +0.220 V versus SHE, representing the anaerobic range of redox potentials in flooded soils, H2S formation was significantly decreased from −0.100 V versus SHE and higher.
Mercury is a metal pollutant which can be transformed into methyl mercury in reducing conditions. The latter is highly neurotoxic and is prone to biomagnification (Compeau and Bartha, 1984). The majority of CH3Hg results from biomethylation of mercury (Compeau and Bartha, 1984), performed by sulfate reducers and as recently discovered, also by iron-reducing bacteria (Fleming et al., 2006). Several researchers have reported a decrease in methylation (and an increase in demethylation), following an increase in redox potential from −0.22 V to +0.15 V versus SHE (Compeau and Bartha, 1984; Matilainen et al., 1991; DeLaune et al., 2004). An oxidized sediment layer could serve as a barrier for the efflux of CH3Hg from the sediment (Gagnon et al., 1996; Mason et al., 2006).
The anode of an SMFC can further be applied to influence redox reactions involved with other metals, in order to decrease the toxicity of sediments. The solubility and mobilization of arsenic are for instance decreased under conditions with high redox potential (Signes-Pastor et al., 2007).
Another anaerobic process leading to unwanted emissions is the production of phosphine. The emission of this highly toxic gas has been detected from sediments and landfills among others. The redox couples involved in the reduction of phosphate to phosphine (PH3) are extremely low (between −1.22 V and −0.48 V versus SHE at pH 7), which makes it highly unlikely that a dissimilatory energy metabolism is involved (Devai and DeLaune, 1995b; Roels and Verstraete, 2001). The presence of an anode could however still decrease the likelihood of anaerobic phosphine formation due to an increased redox potential in the anode environment.
Contrarily, reductive processes can lead to desired bioconversions such as reductive dehalogenation. For example, the R-Cl/R-H couple has a high redox potential in general: between +0.25 V and +0.6 V versus SHE (Holliger et al., 1998), which turns halogenated compounds into interesting electron acceptors from a thermodynamic point of view. However, kinetics forces the processes to occur at lower redox potentials. For example, electron donors with a redox potential lower than −0.36 V versus SHE are required in the catalytic cycle of the Desulfitobacterium dehalogenase (Holliger et al., 1998). In a reversed SMFC, the buried electrode could function as cathode, delivering electrons in order to stimulate the reductive dehalogenation and hence promote the breakdown of contaminants. This has been demonstrated in a reactor-type MFC, although in the presence of redox mediators (Aulenta et al., 2007).
To achieve a reversed SMFC system, the redox potential of the cathode would have to be decreased by supplying electrons to the sediment through an external power supply. Gregory and Lovley (2005) demonstrated the bioremediation of uranium with such a system, by reducing the soluble and thus mobile U(VI) to the relatively insoluble U(IV). This reduction could be accomplished by delivering electrons to microorganisms through a cathode buried in a soil layer and poised at −0.3 V versus SHE. The biologically reduced uranium was relatively stable and only re-oxidized and dissolved in the presence of oxygen. Therefore, this process could allow for example removal of uranium from groundwater.
Generally, it can be stated that the electrodes of an SMFC can be applied to influence redox-dependent processes, in order to decrease the adverse and/or harmful characteristics of sediments. Ryckelynck and colleagues (2005) demonstrated that the chemical profile of sediments was shifted in a zone of 3 cm around an active anode. The influence of the anode is thus limited in space, but is substantial, certainly in sediments with a forced water flow. An oxidized electrode grid could for instance be placed in the upper sediment layer, in order to form a protective barrier for nuisance or hazardous compounds. A well-catalysed and effective cathodic reaction would be required to pull the electrons through the electrical circuit and generate the required oxidizing force at the anode. In case the severity of a spill or the nature of the toxicant would require this, small amounts of energy could be applied to the system in order to poise the anode potential or to reverse the polarity of the system.