Silver Nanoparticles Boost Charge-Extraction in Shewanella Microbial Fuel Cells for Biomining Iron



September 26, 2021. By Kolemann Lutz


When cultured in a fuel cell, the bacterium Shewanella oneidensis is well known to use extracellular electron sinks, metal oxides and ions in nature or electrodes, to provide the energy to catabolize metals, organic, and nonorganic materials. On Earth, Shewanella has been developed as a primary microbial candidate for energy production, especially in waste-rich environments, and iron extraction.


In April 2021, seven Delft researchers published findings evaluating the efficiency of Shewanella oneidensis to biologically extract and 3D print structural materials from iron on Moon and Mars. The combination of bacterial treatment with S. oneidensis and magnetic extraction resulted in a 5.8-times higher quantity of iron and 43.6% higher iron concentration compared to solely magnetic extraction. The materials were 3D printed into cylinders and the mechanical properties were tested, resulting in a 400% improvement in compressive strength in the bacterially treated samples. The magnesium perchlorate (Mg(ClO₄)₂) in JSC-2A were also not toxic for Shewanella oneidensis.


By converting chemical energy stored in organic matter to electricity, Microbial fuel cells (MFCs) hold great potential for power generation and wastewater treatment. However, the current MFCs traditionally experience unsatisfactorily low power densities that are primarily limited by the sluggish transmembrane and extracellular electron-transfer processes. Additionally, due to the limited 44% solar irradiance, biocydal factors, and less bioavailable nutrients, bioremediating microbes may have difficult time harvesting enough energy from absorbed photons for metabolism on Mars and Moon.


To address this issue, researchers added silver nanoparticles to electrodes composed of graphene oxide (GO) to the transmembrane and outer-membrane to boost charge extraction efficiency. As nanoparticles release silver ions, bacteria used the electrons during metabolic process to reduce them to silver nanoparticles for uptake or transport into cells. Inside the bacterium, the silver nanoparticles become fine transmission lines that help capture more electrons produced by the bacterium, increasing microbial power harvesting.


“The resulting Shewanella-silver MFCs deliver a maximum current density of 3.85 milliamperes per square centimeter, power density of 0.66 milliwatts per square centimeter, and single-cell turnover frequency of 8.6 × 10^5 per second, which are all considerably higher than those of the best MFCs reported to date”, mentioned in the study by lead author, Bocheng Chao, Professor from the Department of Chemistry and Biochemistry at the University of California, Los Angeles (UCLA).


Additionally, the hybrid MFCs demonstrate a Faraday, coulombic efficiency of 81% with high fuel utilization efficiency. In other words, the film outputs more than 80% of the metabolized electrons to an external circuit, producing 0.66 milliwatts of electricity per square centimeter.

However, the power density (amount of electric current) of microbial fuel cells has been limited because of difficulties connecting the microbes to the anode to pick up electrode.


Cao et al. found that a reduced graphene oxide–silver nanoparticle anode circumvents some of these issues, providing a substantial increase in current and power density. Electron microscopy discovered silver nanoparticles embedded or attached to the outer cell membrane facilitated electron transfer from the internal electron carriers to the anode.


In 2020, researchers from Karlsruhe Institute of Technology (KIT) in Germany created a scaffolding for the Shewanella oneidensis bacteria consisting of a porous hydrogel made up of carbon nanotubes (CNTs) and silica nanoparticles interwoven by DNA strands. Published in the ACS Applied Materials & Interfaces Journal, their research found that this scaffold not only supports the bacteria for several days, it also acts as a conductor under strictly anoxic conditions to extracting the metabolic electrons and produce electrochemical activity with applications extending beyond microbial biosensors, bioreactors, and fuel cell systems.


In 2018, NASA Ames launched the Micro-12 experiment aboard the 15th SpaceX cargo resupply mission to the space station to study Shewanella oneidensis MR-1 biofilms, extracellular electron transport, and performance in microgravity. These biofilms are a thin slime-like substance and are fundamental to the bacteria’s growth. Research found that in anaerobic environments, bacteria colonies growing on rocks use nanowires, small appendages from the biofilm, to identify metal within the rocks and to switch to their backup respiration system.


Around one year later in collaboration with ESA, and NASA, Benjamin Lehner, a PhD student at Delft University of Technology, defended his 2019 thesis proposing multiple 1,400 litre bioreactors filled with self producing Shewanella oneidensis colonies. After being fed micro-algae and absorbing sunlight and CO2, Shewanella could convert raw regolith into 350kgs of magnetite, a magnetic oxide of iron, which can be separated with magnets.


Using a technique called Lithography-based Ceramic Manufacturing (LCM), the 3D printer then converts the raw material into common metal components such as screws, nuts, iron plates and other objects for construction to build a Martian base. Affordable lightweight bacteria can power the bioreactor and reproduce under high doses of radiation and perchlorates. Lehner mentions, “the concept fits nicely for missions on Moon too”.

 

Cao B, Zhao Z, Peng L, Shiu HY, Ding M, Song F, Guan X, Lee CK, Huang J, Zhu D, Fu X, Wong GCL, Liu C, Nealson K, Weiss PS, Duan X, Huang Y. Silver nanoparticles boost charge-extraction efficiency in Shewanella microbial fuel cells. Science. 2021 Sep 17;373(6561):1336-1340. doi:10.1126/science.abf3427. Epub 2021 Sep 16.

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