Role of Nano-Fe3O4 for enhancing nitrate removal in microbial electrolytic cells: Characterizations and microbial patterns of cathodic biofilm

Conductive magnetite nanoparticle (Nano-Fe3O4) can facilitate numerous biological reduction reactions as an outstanding electron mediator for electron transfer. The positive role of Nano-Fe3O4 for nitrate removal has gradually gained attention recent years, however, it has not been clarified for the persistence of the promoting effect under different concentrations addition. Performance of nitrogen removal and characteristics of cathodic biofilm were evaluated in this study after Nano-Fe3O4 addition with gradient concentration of 100∼500 mg L-1 in microbial electrolytic cells (MEC). Our study illustrated that the optimal concentration was 200 mg L-1 as the removal rate of nitrate increased by 24.76% and the removal rate of total dissolved nitrogen by 29.72%. At the optimal concentration, Nano-Fe3O4 increased cathodic biofilm DNA concentration by 61.04%, enhanced electron transport system activity, enriched iron redox bacteria, denitrifying bacteria and genes, as well as increased extracellular polymeric substances (EPS) amount, especially the protein content of soluble-EPS. However, promoting effect on nitrate removal was not visible in high concentration (500 mg L-1) addition, its electron transport system activity and EPS content were even declined. XPS results indicated that high concentration of Nano-Fe3O4 may reduce the availability of electrons to cathodic biofilm by competing for electrons, which inhibit nitrate removal.

Production of short-chain fatty acids (SCFAs) as chemicals or substrates for microbes to obtain biochemicals

Carboxylic acids have become interesting platform molecules in the last years due to their versatility to act as carbon sources for different microorganisms or as precursors for the chemical industry. Among carboxylic acids, short-chain fatty acids (SCFAs) such as acetic, propionic, butyric, valeric, and caproic acids can be biotechnologically produced in an anaerobic fermentation process from lignocellulose or other organic wastes of agricultural, industrial, or municipal origin. The biosynthesis of SCFAs is advantageous compared to chemical synthesis, since the latter relies on fossil-derived raw materials, expensive and toxic catalysts and harsh process conditions. This review article gives an overview on biosynthesis of SCFAs from complex waste products. Different applications of SCFAs are explored and how these acids can be considered as a source of bioproducts, aiming at the development of a circular economy. The use of SCFAs as platform molecules requires adequate concentration and separation processes that are also addressed in this review. Various microorganisms such as bacteria or oleaginous yeasts can efficiently use SCFA mixtures derived from anaerobic fermentation, an attribute that can be exploited in microbial electrolytic cells or to produce biopolymers such as microbial oils or polyhydroxyalkanoates. Promising technologies for the microbial conversion of SCFAs into bioproducts are outlined with recent examples, highlighting SCFAs as interesting platform molecules for the development of future bioeconomy.

Biological hydrogen production: molecular and electrolytic perspectives

Exploration of renewable energy sources is an imperative task in order to replace fossil fuels and to diminish atmospheric pollution. Hydrogen is considered one of the most promising fuels for the future and implores further investigation to find eco-friendly ways toward viable production. Expansive processes like electrolysis and fossil fuels are currently being used to produce hydrogen. Biological hydrogen production (BHP) displays recyclable and economical traits, and is thus imperative for hydrogen economy. Three basic modes of BHP were investigated, including bio photolysis, photo fermentation and dark fermentation. Photosynthetic microorganisms could readily serve as powerhouses to successively produce this type of energy. Cyanobacteria, blue green algae (bio photolysis) and some purple non-sulfur bacteria (Photo fermentation) utilize solar energy and produce hydrogen during their metabolic processes. Ionic species, including hydrogen (H⁺) and electrons (e^-) are combined into hydrogen gas (H₂), with the use of special enzymes called hydrogenases in the case of bio photolysis, and nitrogenases catalyze the formation of hydrogen in the case of photo fermentation. Nevertheless, oxygen sensitivity of these enzymes is a drawback for bio photolysis and photo fermentation, whereas, the amount of hydrogen per unit substrate produced appears insufficient for dark fermentation. This review focuses on innovative advances in the bioprocess research, genetic engineering and bioprocess technologies such as microbial fuel cell technology, in developing bio hydrogen production.