Methane Production and Conductive Materials: A Critical Review

Abstract

Conductive materials (CM) have been extensively reported to enhance methane production in anaerobic digestion processes. The occurrence of direct interspecies electron transfer (DIET) in microbial communities, as an alternative or complementary to indirect electron transfer (via hydrogen or formate), is the main explanation given to justify the improvement of methane production. Not disregarding that DIET can be promoted in the presence of certain CM, it surely does not explain all the reported observations. In fact, in methanogenic environments DIET was only unequivocally demonstrated in cocultures of Geobacter metallireducens with Methanosaeta harundinacea or Methanosarcina barkeri and frequently Geobacter sp. are not detected in improved methane production driven systems. Furthermore, conductive carbon nanotubes were shown to accelerate the activity of methanogens growing in pure cultures, where DIET is not expected to occur, and hydrogenotrophic activity is ubiquitous in full-scale anaerobic digesters treating for example brewery wastewaters, indicating that interspecies hydrogen transfer is an important electron transfer mechanism in those systems. This paper presents an overview of the effect of several iron-based and carbon-based CM in bioengineered systems, focusing on the improvement in methane production and in microbial communities' changes. Control assays, as fundamental elements to support major conclusions in reported experiments, are critically revised and discussed.

Cold oxygen plasma induces changes on the surface of carbon materials enhancing methanogenesis and N2O reduction in anaerobic sludge incubations

“…Recently, there has been a great interest in applying conductive materials for enhancing CH 4 production in anaerobic digestion processes (for a recent overview, see Martins et al). These materials, which include GAC, carbon cloth and biochar, among others, can stimulate DIET yielding improved CH 4 production rates .…”

“…In this context, it is very important to note that as hydrogen is produced by fermentative bacteria and utilized by hydrogenotrophic methanogens, this coupling requires an interspecies electron transfer (IET) process . In fact, recent works have reported that supplementing anaerobic consortia with conductive materials promotes IET enhancing CH 4 production . The electron transfer among microorganisms without mediating diffusive electron carriers is known as direct interspecies electron transport (DIET) and, according to the literature, there are two types of DIET mechanisms: via conductive pili with cytochromes involved, and via abiotic transfer with external conductive materials .…”

“…The electron transfer among microorganisms without mediating diffusive electron carriers is known as direct interspecies electron transport (DIET) and, according to the literature, there are two types of DIET mechanisms: via conductive pili with cytochromes involved, and via abiotic transfer with external conductive materials . DIET promoted by conductive materials has several advantages as these materials trigger enhanced CH 4 production rates, high CH 4 yields and improve the biodegradation of recalcitrant contaminants. For instance, Colunga et al reported the enhancement on the redox conversion of different toxic contaminants by graphene oxide .…”

“…Conductive materials applied in anaerobic digesters to increase CH 4 production include graphene, graphite, biochar, activated carbon (AC), carbon cloth, iron‐oxide minerals (e.g. magnetite) and others . Granular activated carbon (GAC) is the most studied conductive material and, according to the literature, it has increased CH 4 production and minimized sludge production in anaerobic digesters .…”

“…3 In fact, recent works have reported that supplementing anaerobic consortia with conductive materials promotes IET enhancing CH 4 production. 4 The electron transfer among microorganisms without mediating diffusive electron carriers is known as direct interspecies electron transport (DIET) and, according to the literature, there are two types of DIET mechanisms: via conductive pili with cytochromes involved, and via abiotic transfer with external conductive materials. 2 DIET promoted by conductive materials has several advantages as these materials trigger enhanced CH 4 production rates, high CH 4 yields 2,4 and improve the biodegradation of recalcitrant contaminants.…”