Sudarsu V. Ramanaiah

The increased demand for alternative sustainable energy sources has boosted research in the field of fuel cells (FC). Among these, microbial fuel cells (MFC), based on microbial anodes and different types of cathodes, have been the subject of renewed interest due to their ability to simultaneously perform wastewater treatment and bioelectricity generation. Several different MFCs have been proposed in this work using different conditions and configurations, namely cathode materials, membranes, external resistances, and microbial composition, among other factors. This work reports the design and optimization of MFC performance and evaluates a hydrogel (Ion Jelly^®) modified air-breathing cathode, with and without an immobilized laccase enzyme. This MFC configuration was also compared with other MFC configuration performances, namely abiotic and biocathodes, concerning wastewater treatment and electricity generation. Similar efficiencies in COD reduction, voltage (375 mV), PD (48 mW/m²), CD (130 mA/m²), and OCP (534 mV) were obtained. The results point out the important role of Ion Jelly^® in improving the MFC air-breathing cathode performance as it has the advantage that its electroconductivity properties can be designed before modifying the cathode electrodes. The biofilm on MFC anodic electrodes presented a lower microbial diversity than the wastewater treatment effluent used as inocula, and inclusively Geobacteracea was also identified due to the high microbial selective niches constituted by MFC systems.

Bacteria make a huge contribution to the purification of the environment from toxic stable pollutants of anthropogenic and natural origin due to the diversity of their enzyme systems. For example, the ability to decompose 3-chlorobenzoate (3CBA) by the four representative genera of Actinobacteria, such as Rhodococcus, Gordonia, Microbacterium, and Arthrobacter, was studied. In most cases, the formation of 4-chlorocatechol as the only key intermediate during the decomposition of 3CBA was observed. However, Rhodococcus opacus strain 1CP was an exception, whose cells decomposed 3CBA via both 3-chloro- and 4-chlorocatechol. The enzyme 3-Chlorobenzoate 1,2-dioxygenase (3CBDO) induced during the growth of these bacteria in the presence of 3CBA differed significantly in substrate specificity from the benzoate dioxygenases induced upon growth in the presence of benzoate. The R. opacus 6a strain was found to contain genes encoding chlorocatechol 1,2-dioxygenase, chloromuconate cycloisomerase, and dienelactone hydrolase, whose nucleotide sequence was 100% consistent with the sequences of the corresponding genes encoding the enzymes of the modified 4-chlorocatechol ortho-cleavage pathway of the strain R. opacus 1CP. However, the gene encoding chloromuconolactone dehalogenase (clcF) was not found in the representatives of the actinomycete genera, including Gordonia and Arthrobacter. A linear mega-plasmid carrying 3-chlorocatechol degradation genes remained stable after maintaining the R. opacus 1CP strain on an agar-rich medium for 25 years. In general, a similar plasmid was absent in actinobacteria of other genera, as well as in closely related species of R. opacus 6a.

Synthetic plastics derived from fossil fuels—such as polyethylene, polypropylene, polyvinyl chloride, and polystyrene—are non-degradable. A large amount of plastic waste enters landfills and pollutes the environment. Hence, there is an urgent need to produce biodegradable plastics such as polyhydroxyalkanoates (PHAs). PHAs have garnered increasing interest as replaceable materials to conventional plastics due to their broad applicability in various purposes such as food packaging, agriculture, tissue-engineering scaffolds, and drug delivery. Based on the chain length of 3-hydroxyalkanoate repeat units, there are three types PHAs, i.e., short-chain-length (scl-PHAs, 4 to 5 carbon atoms), medium-chain-length (mcl-PHAs, 6 to 14 carbon atoms), and long-chain-length (lcl-PHAs, more than 14 carbon atoms). Previous reviews discussed the recent developments in scl-PHAs, but there are limited reviews specifically focused on the developments of mcl-PHAs. Hence, this review focused on the mcl-PHA production, using various carbon (organic/inorganic) sources and at different operation modes (continuous, batch, fed-batch, and high-cell density). This review also focused on recent developments on extraction methods of mcl-PHAs (solvent, non-solvent, enzymatic, ultrasound); physical/thermal properties (Mw, Mn, PDI, Tm, Tg, and crystallinity); applications in various fields; and their production at pilot and industrial scales in Asia, Europe, North America, and South America.

The electrochemical features of microbial fuel cells’ biocathodes, running on wastewater, were evaluated by cyclic voltammetry. Ex situ and in situ electrochemical assays were performed and the redox processes associated with the presence of microorganisms and/or biofilms were attained. Different controls using sterile media (abiotic cathode microbial fuel cell) and membranes covering the electrodes were performed to evaluate the source of the electrochemistry response (surface biofilms vs. biotic electrolyte). The bacteria presence, in particular when biofilms are allowed to develop, was related with the enhanced active redox processes associated with an improved catalytic activity, namely for oxygen reduction, when compared with the results attained for an abiotic microbial fuel cell cathode. The microbial main composition was also attained and is in agreement with other reported studies. The current study aims contributing to the establishment of the advantages of using biocathodes rather than abiotic, whose conditions are frequently harder to control and to contribute to a better understanding of the bioelectrochemical processes occurring on the biotic chambers and the electrode surfaces.