Biofilters cut old landfill carbon footprint

Researchers in the US are testing biofilter systems as a viable alternative to releasing methane from passive landfill vents into the atmosphere. The technology could reduce the overall impact of old landfills on global warming. Details are reported in the current issue of the International Journal of Environmental Engineering.

Organic matter rotting in smaller, old landfill sites generates a slow trickle of the potent greenhouse gas, methane, into the atmosphere, amounting to just 2 or 3 kilograms per day per vent. In contrast to controlled methane generate for biofuel from modern, managed landfills, tapping this slow stream of the gas is not viable technologically or economically. However, methane has an infrared activity 21 times greater than carbon dioxide and so represents an important anthropogenic source of this greenhouse gas when attempting to balance the climate change books. Indeed, landfills contribute 12% of worldwide anthropogenic methane emissions due to the decomposition of organic waste.

Old landfills typically have passive gas vents. Methane is simply released into the atmosphere from these vents, or if the rate of emission is high enough it can be burned, or flared. According to Tarek Abichou and Jeffery Chanton of the Florida State University, Jose Morales of Environmental and Geotechnical Specialists, Inc., Tallahassee, Florida and Lei Yuan of Geosyntec Consultants in Columbia, Maryland, methane oxidation has recently been viewed as a more benign alternative to venting or flaring of landfill methane.

The researchers tested two biofilter designs capable of oxidizing methane gas to carbon dioxide and water. Both are packed with so-called methanotrophic bacteria, microbes that digest methane. They found that the radial biofilter design gave a much higher methane oxidation rate than a vertical biofilter. The higher surface area exposed to methane flow led to greater oxygen penetration into the biofilters, essential for microbial digestion. The radial biofilter has a surface area of well over 1.2 square meters whereas the vertical biofilter amounts to just 0.3 square meters area.

The team also found that the average percent oxidation rate of 20% and higher for the radial biofilter was possible when the air temperature was 20 to 36 Celsius, indicating the optimal soil temperature for methanotrophic bacteria to oxidize methane. Vertical biofilters averaged a little over 12% oxidation.

Abichou, T., Yuan, L., Chanton, J., & Morales, J. (2011). Mitigating methane emissions from passive landfill vents: a viable option for older closed landfills International Journal of Environmental Engineering, 3 (3/4) DOI: 10.1504/IJEE.2011.041354

Microbial power

New insights into the workings of a metal-munching bacteria and how it exploits semiconducting nanominerals could provide a new approach to making biological fuel cells for an almost all-natural power supply for electronic gadgets and medical devices.

Fuel cells are essentially electrical batteries that have a fuel supply rather than relying on built in chemical energy. Researchers have been developing microbial fuel cells that use a positive electrode, anode, coated with a bacterial film. These cells use a fuel comprising a substrate that the bacteria can break down to release electrons. Tapping into this electron release process to draw a current requires transferring the electrons to the anode.

Prof. Dr. Kazuhito Hashimoto

Prof. Dr. Kazuhito Hashimoto

Now, a team led by Kazuhito Hashimoto of the University of Tokyo, Japan, has investigated how this transfer is carried out in the subterranean microbe Shewanella loihica. Instead of breathing air, this microbe respires by reducing the iron(III) ions in iron oxides from the sediments in which it lives to the iron(II) state. This releases electrons which are used to release energy for it to live, grow, and multiply.

The team added microbial cells to a solution containing very finely divided nanoscopic iron(III) oxide particles and poured the solution into a chamber containing electrodes. A layer of bacteria and iron oxide particles was gradually deposited on to the electrodes at the bottom of the chamber. When the cells were “fed” lactate ions, a current was detected. Electrons from the metabolism of the lactate are thus transferred from the bacteria to the electrode.

SEM image of iron oxide and microbial cells on electrode after more than a day of current generation (Credit: Wiley/VCH)

SEM image of iron oxide and microbial cells on electrode after more than a day of current generation (Credit: Wiley/VCH)

The Tokyo team has now demonstrated using scanning electron microscopy how, in the presence of iron(III) oxide nanoparticles, the metal-reducing bacteria form aggregates that can conduct electricity. The SEM images show a thick layer of cells and nanoparticles on the electrode; the surfaces of the cells are completely coated with iron oxide particles. The researchers were able to show that the semiconducting properties of the iron oxide nanoparticles, which are linked to each other by the cells, contribute to the surprisingly high current recorded.

The cells act as an electrical connection between the individual iron oxide particles. Cytochromes, enzymes in the outer cell membrane of these bacteria, transfer electrons between the cells and the iron oxide particles without having to overcome much of an energy barrier. The result is a conducting network that even allows cells located far from the electrode to participate in the generation of current, the researchers explain.

We are now succeeding in the direct real-time observation of the layer formation using a special optical microscope, Hashimoto told Spotlight, This will be reported shortly.

Further reading

Angew. Chem. Int. Edn 2009, 48, 508-511

Prof. Dr. Kazuhito Hashimoto laboratory

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Nano hotrods go turbo

Adding a silver-gold alloy to nanorods fuelled by hydrogen peroxide gives them a significant speed boost, according to US chemists.

The idea of nanobots that can be jettisoned into the blood stream and will race around the body scraping off cholesterol plaques from the inside of arteries, zapping tumours, and spiking invading microbes, remains firmly in the realm of science fiction…at least for now. However, scientists are taking the first tentative steps towards building the sub-microscopic components that these nanobots will need to work.

Professor Joseph Wang

Professor Joseph Wang

One essential component is a propulsion system for the nanomachines. Now, Joseph Wang, and his team at the University of California, San Diego, USA, and colleagues at Arizona State University, in Tempe, have developed a tiny motor based on nanorods, which they say can swim extremely fast. The new devices are almost as fast moving as the best biological nanomachines, bacteria.

These nanorods travel about 75 times their own length in one second, explains Wang, We are approaching the speed of the most efficient biological nanomotors, including flagellated bacteria.

Nanorods of precious metal alloys propel themselves forward with catalytic power (Credit: Wang et al/Wiley-VCH)

Nanorods of precious metal alloys propel themselves forward with catalytic power (Credit: Wang et al/Wiley-VCH)

Rather than acting as the driving force for a nanobot, the first simple application of a nanomotor might be to quickly transport a payload pharmaceutical or other therapeutic agent to a specific target site in the body. Similar devices might also be used to drive diagnostic or environmental sample molecules through the microscopic channels of lab-on-a-chip (LoC) or microelectromechanical systems (MEMS) devices.

The team points out that, generating forward propulsion on this scale and fighting, molecular diffusion effects and viscosity is not as trivial as it seems. With this in mind, the team created their nanomotors from catalytic rods with a different metal at each end. The researches used platinum at one end of the nanorods and a silver-gold alloy at the other. With hydrogen peroxide added as a fuel to the system they were able to propel the nanorods to speeds of 150 micrometres per second (0.54 metres per hour).

The platinum catalyses the splitting of hydrogen peroxide into oxygen gas and hydrogen ions, and absorbs the electrons released in the process. These are absorbed by the silver-gold segment, which then accelerates a kind of reverse reaction in which hydrogen peroxide and protons combine to form water. The concomitant production of oxygen and water generates an electrical current that drives the nanorod through the fluid, platinum head first.

The silver-gold alloy causes the electrons to be transferred more quickly [than with gold alone], explains Wang, This increases the fuel decomposition rate and the nanorod is accelerated faster. The team explains that by fine-tuning the amount of silver in the alloy they can control the speed of the nanorods. Fuel additives or variations of the platinum segment will make these rods even faster, predicts Wang. The team also intends to find a way to generate hydrogen peroxide in situ from glucose to fuel the nanorods for applications in the human body.

U. Korcan Demirok, Rawiwan Laocharoensuk, Kalayil Manian Manesh, Joseph Wang (2008). Ultrafast Catalytic Alloy Nanomotors Angewandte Chemie International Edition DOI: 10.1002/anie.200803841

Further reading

J. Am. Chem. Soc., 2008, 130, 8164-8165

Professor Joseph Wang homepage