Power/Performance Bits: Nov. 14

Bacteria power wastewater cleanup; perovskites for terahertz communications; spider silk microphones.


Bacteria power wastewater cleanup
Researchers at the King Abdullah University of Science and Technology (KAUST) are exploring ways to detoxify warm, salty industrial wastewater while simultaneously generating electricity. They are using bacteria with remarkable properties: the ability to transfer electrons outside their cells (exoelectrogenes) and the capacity to withstand extremes of temperature and salinity (extremophiles).

The researchers used water collected from three deep-water brine pools in the Red Sea to fill prototype microbial electrolysis cells (MECs), also known as microbial fuel cells. “The Red Sea brine pools are good places to find extremophilic bacteria because they are among the world’s most extreme natural environments, with salinity up to 25% and temperatures higher than 46°C,” explains Noura Shehab, chief technologist at RPD Innovations. “However, their potential for electricity generation has not previously been explored.”

Typically, MECs take the form of glass bottles, which are filled with wastewater, and contain sterile graphite and stainless-steel electrodes. “Any exoelectrogenic bacteria present can oxidize organic matter in the wastewater, generating electrons that are transported to the positive electrode. When a small voltage is applied to the system, the electrons combine with protons in the water at the negative electrode, producing hydrogen gas,” explains Krishna Katuri, a scientist at KAUST. Thus, they simultaneously clean the water and generate hydrogen, a clean, transportable fuel.

An outline of the MEC system being tested at KAUST. Bacteria from briny pools may be able to help clean up wastewater from industry as well as generate energy.  (Source: Srikanth Pedireddy/KAUST)

In this study, MECs with water from one brine pool, Valdivia, generated a stable electric current for almost two months, withstanding temperatures of 70°C and 25% salinity. Genetic analysis of the biofilm of microorganisms colonizing the anode showed a high proportion of the genus Bacterioides and significantly more of these bacteria than were found in the original samples, showing that members of this genus have strong potential to thrive under extreme conditions, while generating electric current.

The researchers are now exploring ways to enrich MECs with other bacteria known as electrotrophs, which are able to consume electrons, also from the Red Sea. According to Pascal Saikaly, associate professor of environmental science and engineering at KAUST, “these have the potential to convert waste products, such as carbon dioxide, into valuable chemical products, such as methane and acetate.”

Perovskites for terahertz communications
Researchers at the University of Utah discovered that a combination of an organic and inorganic perovskites can be layered on a silicon wafer to boost terahertz communications systems.

The terahertz range is a band between infrared light and radio waves and utilizes frequencies that cover the range from 100 gigahertz to 10,000 gigahertz. By depositing a special form of multilayer perovskite onto a silicon wafer, the researchers were able to modulate terahertz waves passing through it using a simple halogen lamp. Modulating the amplitude of terahertz radiation is important because it is how data in such a communications system would be transmitted.

Previous attempts to do this have usually required the use of an expensive, high-power laser. What makes this demonstration different is that it is not only the lamp power that allows for this modulation but also the specific color of the light. Consequently, they can put different perovskites on the same silicon substrate, where each region could be controlled by different colors from the lamp. This is not easily possible when using conventional semiconductors like silicon.

“Think of it as the difference between something that is binary versus something that has 10 steps,” said Ajay Nahata, electrical and computer engineering professor at the University of Utah. “Silicon responds only to the power in the optical beam but not to the color. It gives you more capabilities to actually do something, say for information processing or whatever the case may be.”

Additionally, the process of layering perovskites on silicon is simple and inexpensive by using a method called “spin casting,” in which the material is deposited on the silicon wafer by spinning the wafer and allowing centrifugal force to spread the perovskite evenly.

Nahata says it’s probably at least another 10 years before terahertz technology for communications and computing is used in commercial products.

“This basic capability is an important step towards getting a full-fledged communications system,” Nahata said. “If you want to go from what you’re doing today using a modem and standard wireless communications, and then go to a thousand times faster, you’re going to have to change the technology dramatically.”

Spider silk microphones
Researchers from Binghamton University argue that the sensing properties of spider silk could lead to better microphones for hearing aids than traditional, pressure-based systems.

Ron Miles, a professor at Binghamton, argues that there’s much to be learned from insects regarding hearing. “We use our eardrums that pick up the direction of sound based on pressure, but most insects actually hear with their hairs,” he explained. The spider silk is able to pick up the velocity of the air instead of the pressure of the air.

Many terrestrial arthropods, including mosquitos, flies and spiders, have fine hairs on their bodies that move with the sound waves traveling through the air, creating the most sensitive biological sensors known. Miles wanted to recreate this type of hearing, known as flow sensing, inside a microphone.

The microphone improves the directional sensing across a wide variety of frequencies that are often too quiet for microphones to pick up on. For someone with a hearing aid, that means being able to cancel out background noise when having a conversation in a crowded area. The same concept could be applied to the microphone inside cell phones, the researchers say.

Spider silk is thin enough that it also can move with the air when hit by sound waves. “This can even happen with infrasound at frequencies as low as 3 hertz,” said Miles, below the range of human hearing.

The study used spider silk, but the team says any fiber that is thin enough could be used in the same way.

While the spider silk picks up the direction of airflow with great accuracy, that information has to be translated into an electronic signal to be of use. “We coated the spider silk with gold and put it in a magnetic field to obtain an electronic signal,” said Miles. “It’s actually a fairly simple way to make an extremely effective microphone that has better directional capabilities across a wide range of frequencies.”

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