Microbial Fuel Cell Based on Nitrogen-Fixing Rhizobium anhuiense Bacteria.

In this study, the nitrogen-fixing, Gram-negative soil bacteria Rhizobium anhuiense was successfully utilized as the main biocatalyst in a bacteria-based microbial fuel cell (MFC) device. This research investigates the double-chambered, H-type R. anhuiense-based MFC that was operated in modified Norris medium (pH = 7) under ambient conditions using potassium ferricyanide as an electron acceptor in the cathodic compartment. The designed MFC exhibited an open-circuit voltage (OCV) of 635 mV and a power output of 1.07 mW m-2 with its maximum power registered at 245 mV. These values were further enhanced by re-feeding the anode bath with 25 mM glucose, which has been utilized herein as the main carbon source. This substrate addition led to better performance of the constructed MFC with a power output of 2.59 mW m-2 estimated at an operating voltage of 281 mV. The R. anhuiense-based MFC was further developed by improving the charge transfer through the bacterial cell membrane by applying 2-methyl-1,4-naphthoquinone (menadione, MD) as a soluble redox mediator. The MD-mediated MFC device showed better performance, resulting in a slightly higher OCV value of 683 mV and an almost five-fold increase in power density to 4.93 mW cm-2. The influence of different concentrations of MD on the viability of R. anhuiense bacteria was investigated by estimating the optical density at 600 nm (OD600) and comparing the obtained results with the control aliquot. The results show that lower concentrations of MD, ranging from 1 to 10 µM, can be successfully used in an anode compartment in which R. anhuiense bacteria cells remain viable and act as a main biocatalyst for MFC applications.

Assessment of Rhizobium anhuiense Bacteria as a Potential Biocatalyst for Microbial Biofuel Cell Design.

The development of microbial fuel cells based on electro-catalytic processes is among the novel topics, which are recently emerging in the sustainable development of energetic systems. Microbial fuel cells have emerged as unique biocatalytic systems, which transform the chemical energy accumulated in renewable organic fuels and at the same time reduce pollution from hazardous organic compounds. However, not all microorganisms involved in metabolic/catalytic processes generate sufficient redox potential. In this research, we have assessed the applicability of the microorganism Rhizobium anhuiense as a catalyst suitable for the design of microbial fuel cells. To improve the charge transfer, several redox mediators were tested, namely menadione, riboflavin, and 9,10-phenanthrenequinone (PQ). The best performance was determined for a Rhizobium anhuiense-based bio-anode mediated by menadione with a 0.385 mV open circuit potential and 5.5 µW/cm2 maximal power density at 0.35 mV, which generated 50 µA/cm2 anode current at the same potential.

Vertical distribution of polycyclic aromatic hydrocarbons in the brackish sea water column: ex situ experiment.

BACKGROUND: Oil spills can cause severe damage within a marine ecosystem. Following a spill, the soluble fraction of polycyclic aromatic hydrocarbons is rapidly released into the water column. These remain dissolved in seawater over an extended period of time, even should the insoluble fraction be removed. The vertical distribution of the aromatic hydrocarbon component and how these become transferred is poorly understood in brackish waters. This study examines the vertical distribution of polycyclic aromatic hydrocarbons having been released from a controlled film of spilled oil onto the surface of brackish water. METHODS: The study was undertaken under controlled conditions so as to minimize the variability of environmental factors such as temperature and hydrodynamics. The distribution of polycyclic aromatic hydrocarbons was measured in the dissolved and suspended phases throughout the 1 m water column with different intensity of water sampling: 1, 2, 4, 7, 72, 120, 336, 504 and 984 h. RESULTS: The total concentration of polycyclic aromatic hydrocarbons ranged from 19.01 to 214.85 ng L-1 in the dissolved phase and from 5.14 to 63.92 ng L-1 in the suspended phase. These hydrocarbons were released immediately following a controlled spill attaining 214.9 ng L-1 in the dissolved phase and 54.4 ng L-1 in the suspended phase near the cylinder bottom after 1-2 h. The 2-3 ring polycyclic aromatic hydrocarbons dominated in the dissolved phase (60-80%), whereas the greater amount of 4-6 ring polycyclic aromatic hydrocarbons (55-90%) occurred in the suspended phase. A relatively low negative correlation (rS = -0.41) was determined between the concentration of phenanthrene and suspended matter, whereas a high negative correlation (r =  - 0.79) was found between the concentration of pyrene and suspended matter. Despite the differences in the relationships between the concentration ratio and amount of suspended matter the obtained regressions allow roughly to predict the concentration of polycyclic aromatic hydrocarbons.