Forming electrochemically active biofilms from garden compost under chronoamperometry

Dimensionally stable anodes (DSA) were polarized at different constant potential values for several days in garden compost. After an initial lag period ranging from 1 to 10.5 days, the current increased fast and then stabilized for days. Current densities higher than 100 mA m2 and up to 385 mA m2 were obtained with the sole organic matter contained in compost as substrate. Control experiments performed with sterilized compost, oscillations of the current with the temperature, kinetics of the exponential phase of current increase and observations of the surface of electrodes by epifluorescence microscopy showed that the current was controlled by the colonization of the electrode surface by a biofilm which originated the indigenous flora of compost. Three individually addressed electrodes polarized at different potentials in the same reactor led to identical current evolutions on each electrode, which underlined the key role of the microbial flora of the compost in the discrepancy observed in the other experiments. Chronoamperometry revealed a promising technique to check natural environments for new electrochemically active microbial species

Acetate to enhance electrochemical activity of biofilms from garden compost

Dimensionally Stable Anodes embedded in garden compost and maintained under constant polarization at 0.50 V/SCE for several days progressively became covered by a microbial biofilm that gave them the capability to oxidize the organic matter contained in the compost. The effect of acetate supply on the electrochemical activity of biofilms was investigated either by adding acetate after biofilm formation or mixing it into the compost initially. Addition of acetate allowed the current density values to increase up to 545 mA/m2. Six individually monitored electrodes set up in the same reactor showed very good reproducibility, indicating that discrepancies observed between the different experiments were mainly due the different batches of compost. A numerical treatment of the epifluorescent microscopy pictures allowed the biofilm coverage ratios to be assessed. Comparing the variations of current density during chronoamperometry with the biofilm surface coverage ratios and with the current obtained by cyclic voltammetry led us to propose a mechanism based on two different steps that corresponded to different time-scales: slow acetate oxidation through the cell metabolism and fast electron transfer between the cell and the electrode surface

Effect of surface roughness, biofilm coverage and biofilm structure on the electrochemical efficiency of microbial cathodes

Biofilms of Geobacter sulfurreducens were formed under chronoamperometry at −0.5 V and −0.6 V vs. Ag/AgCl on stainless steel cathodes and tested for fumarate reduction. Increasing the surface roughness Ra from 2.0 μm to 4.0 μm increased currents by a factor of 1.6. The overall current density increased with biofilm coverage. When the current density was calculated with respect to the biofilm-coated area only, values up to 280 A/m2 were derived. These values decreased with biofilm coverage and indicated that isolated cells or small colonies locally provide higher current density than dense colonies. Steel composition affected the current values because of differences in biofilm structure and electron transfer rates. Biofilms formed under polarisation revealed better electrochemical characteristics than biofilm developed at open circuit. This work opens up new guidelines for the design of microbial cathodes: a uniform carpet of isolated bacteria or small colonies should be targeted, avoiding the formation of large colonies

Testing various food-industry wastes for electricity production in microbial fuel cell

Three food-industrywastes: fermented apple juice (FAJ), wine lees and yogurt waste (YW) were evaluated in combination with two sources of inoculum, anaerobic sludge and garden compost, to produce electricity in microbialfuelcells. Preliminary potentiostatic studies suggested that YW was the best candidate, able to provide up to 250 mA/m2 at poised potential +0.3 V/SCE. Experiments conducted with two-chamber MFCs confirmed that wine lees were definitely not suitable. FAJ was not able to start an MFC by means of its endogenous microflora, while YW was. Both FAJ and YW were suitable fuels when anaerobic sludge or compost leachate was used as inoculum source. Sludge-MFCs had better performance using YW (54 mW/m2 at 232 mA/m2). In contrast, compost-leachate MFCs showed higher power density with FAJ (78 mW/m2 at 209 mA/m2) than with YW (37 mW/m2 at 144 mA/m2) but YW gave more stable production. Under optimized operating conditions, compost-leachate MFCs fueled with YW gave up to 92 mW/m2 at 404 mA/m2 and 44 mW/m2 in stable conditions

Electrochemical micro-structuring of graphite felt electrodes for accelerated formation of electroactive biofilms on microbial anodes.

Electroactive biofilms were formed on graphite felt anodes under constant potential polarisation using compost leachate as the inoculum and dairy waste as the substrate. A new surface treatment is presented that consists of creating micro-cavities on the electrode surface by strong anodisation. This surface treatment, combined with pre-adsorption of the dairy waste, proved very efficient in accelerating the formation of electroactive biofilms and increasing the current values up to 1600 mA/m2 for dairy waste oxidation

Treatment of dairy wastes with a microbial anode formed from garden compost

Garden compost has already been identified as a source of efficient electro-active (EA) biofilms. The work described here consisted of lixiviating the compost and then using the leachate as a microbial source. This procedure gave promising results for the treatment of yogurt waste (YW) in a microbial fuel cell (MFC). Experiments performed in MFC set-ups were compared with electrochemical cells under polarization at +0.1 V versus SCE. Different parameters were tested to optimize the microbial anode. Preliminary acclimation of the compost microbial flora to YW was revealed to be unnecessary. Forming biofilms firstly in pure leachate before exposing them to YW showed that high concentrations of this type of substrate were detrimental to current generation. Pre-treatment of the electrode by pre-adsorbing YW led to a 10-fold increase in the current density. The highest current densities were obtained at 40 and 60 °C, revealing the diversity of electro-active microorganisms coming from soils. Values up to 1,450 mA m−2 were reached at 40 °C

Possible role of Geobacter sulfurreducens in anaerobic corrosion of steels

Geobacteraceae are the most widespread microorganisms in soils and sediments in which microbial reduction of Fe(III) is an important process, either in the natural degradation of organic compounds or in their bioremediation. Geobacter species have been shown to be predominant microorganisms on electrodes harvesting electricity from the sediments. They have the capability to oxidize organic electron donor to carbon dioxide transferring the electron directly to electrodes [1]. On the other side, the ability of Geobacter sulfurreducens to reduce nitrate to nitrite or fumarate to succinate with a graphite electrode serving as electron donor has also been demonstrated [2]. Direct electron transfer to solid electrodes is achieved through periplasmic and outer membrane c-type cytochromes [3]. Outer membranes proteins and even some kind of conductive pili that serve as biological nanowires are also involved in the electron transfer chains, mainly to Fe(III) and Mn(IV) oxides [4]. The aim of this study was to assess the possible influence of G. sulfurreducens on the occurrence of corrosion of steels. Experiments were performed with pure cultures of G. sulfurreducens on mild steel (XC45) and three different kinds of stainless steels (ferritic steel, 304L, 316L). In each case the free potential increased by 200 to 300 mV after the injection of the bacteria. On the contrary, control experiments performed with the injection of the sterile medium or the bacteria suspension after filtration on a 0.2 µm filter did not induce any variation in the free potential. The presence of the cell was consequently directly responsible for the potential increase of the coupons. The occurrence or not of corrosion was discussed with respect to this potential increase and the nature of the medium. Besides, preliminary results allow assessing the possibility to use G. sulfurreducens to design protective biofilms

Comparison of four methods for quantification of biofilms in biphasic cultures

Three methods for determining the total biofilm amount in biphasic cultures have been compared: dry weight by filtration after solvent treatment, optical density with a biomass probe and protein content. The activity of the biofilm was estimated through mineral nitrogen consumption. Calculation of the coefficients of variation shows that these parameters could be used to characterise such a biofilm. The optical density by biomass probe was the most reliable one (repeatability <0.5%) to quantify total biofilm and a linear relation was verified against dry weight

Role of direct microbial electron transfer in corrosion of steels

It has recently been discovered that many microbial species have the capacity to connect their metabolism to solid electrodes, directly exchanging electrons with them through membrane-bound redox compounds,nevertheless such a direct electron transfer pathway has been evoked rarely in the domain of microbial corrosion. Here was evidenced for the first time that the bacterium Geobacter sulfurreducens is able to increase the free potential of 304 L stainless steel up to 443 mV in only a few hours, which represents a drastic increase in the corrosion risk. In contrast, when the bacterial cells form a locally wellestablished biofilm, pitting potentials were delayed towards positive values. The microscopy pictures confirmed an intimate correlation between the zones where pitting occurred and the local settlement of cells. Geobacter species must now be considered as key players in the mechanisms of corrosion

Effect of the semi-conductive properties of the passive layer on the current provided by stainless steel microbial cathodes

Geobacter sulfur reducens biofilms were formed under constant polarisation at −0.6 V vs. Ag/AgCl on stainless steel cathodes to catalyse the reduction of fumarate. The time-evolution of the current strongly depended on the quality of the inoculum. Inoculating with young cells significantly shortened the initial lag-phase and using the same inoculum improved the reproducibility of the current–time curves. The whole set of experiments showed that 254SMO stainless steel provided higher current densities (on average 14.1 A/m2) than biofilms formed on 316L stainless steel (on average 4.5 A/m2). Biofilm coverage assessed by epifluorescent microscopy showed that coverage ratios were generally higher for 316L than for 254SMO. It must be concluded that 254SMO is more efficient in transferring electrons to bacterial cells than 316L. Mott–Schottky diagrams recorded on both materials under conditions of electrolysis in the absence of microorganisms showed that the surface oxide layers had similar n-type semi-conductive behaviour for potential values higher than the flat band potential. In contrast, 316L exhibited slight p-type behaviour at potential lower than the flat band potential, while 254SMO did not. The higher electrochemical performances of biocathodes formed on 254SMO are explained by semi-conductive properties of its passive layer, which prevented the p-type behaviour occurring in cathodic electrolysis conditions