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Una tecnología nueva y económica puede limitar la acumulación de algas en las paredes de los fotobiorreactores que pueden ayudar a convertir el dióxido de carbono en productos útiles. La reducción de este ensuciamiento evita limpiezas costosas y permite que ocurra más fotosíntesis en los embalses. Crédito: José-Luis Olivares, MIT
Aplicar un poco de tensión a las paredes de los tanques de crecimiento de algas puede prevenir la acumulación de nubes y permitir más[{» attribute=»»>photosynthesis to happen.
Algae grown in transparent tanks or tubes supplied with carbon dioxide can convert the greenhouse gas into other compounds, such as food supplements or fuels. But the process leads to a buildup of algae on the surfaces that clouds them and reduces efficiency, requiring laborious cleanout procedures every couple of weeks.
MIT researchers have come up with a simple and inexpensive technology that could substantially limit this fouling, potentially allowing for a much more efficient and economical way of converting the unwanted greenhouse gas into useful products.
The key is to coat the transparent containers with a material that can hold an electrostatic charge, and then applying a very small voltage to that layer. The system has worked well in lab-scale tests, and with further development might be applied to commercial production within a few years.
The findings were reported on April 13, 2023, in the journal Advanced Functional Materials, in a paper by recent MIT graduate Victor Leon PhD ’23, professor of mechanical engineering Kripa Varanasi, former postdoc Baptiste Blanc, and undergraduate student Sophia Sonnert.
Because the algae cells naturally carry a small negative electric charge on their membrane surface, the team figured that electrostatic repulsion could be used to push them away. Credit: Courtesy of the researchers
No matter how successful efforts to reduce or eliminate carbon emissions may be, there will still be excess greenhouse gases that will remain in the atmosphere for centuries to come, continuing to affect global climate, Varanasi points out. “There’s already a lot of carbon dioxide there, so we have to look at negative emissions technologies as well,” he says, referring to ways of removing the greenhouse gas from the air or oceans, or from their sources before they get released into the air in the first place.
When people think of biological approaches to carbon dioxide reduction, the first thought is usually of planting or protecting trees, which are indeed a crucial “sink” for atmospheric carbon. But there are others. “Marine algae account for about 50 percent of global carbon dioxide absorbed today on Earth,” Varanasi says. These algae grow anywhere from 10 to 50 times more quickly than land-based plants, and they can be grown in ponds or tanks that take up only a tenth of the land footprint of terrestrial plants.
What’s more, the algae themselves can then be a useful product. “These algae are rich in proteins, vitamins and other nutrients,” Varanasi says, noting they could produce far more nutritional output per unit of land used than some traditional agricultural crops.
If attached to the flue gas output of a coal or gas power plant, algae could not only thrive on the carbon dioxide as a nutrient source, but some of the microalgae species could also consume the associated nitrogen and sulfur oxides present in these emissions. “For every two or three kilograms of CO2, a kilogram of algae could be produced, and these could be used as biofuels, or for Omega-3, or food,” Varanasi says.
Omega-3 fatty acids are a widely used food supplement, as they are an essential part of cell membranes and other tissues but cannot be made by the body and must be obtained from food. “Omega 3 is particularly attractive because it’s also a much higher-value product,” Varanasi says.
Most algae grown commercially are cultivated in shallow ponds, while others are grown in transparent tubes called photobioreactors. The tubes can produce seven to 10 times greater yields than ponds for a given amount of land, but they face a major problem: The algae tend to build up on the transparent surfaces, requiring frequent shutdowns of the whole production system for cleaning, which can take as long as the productive part of the cycle, thus cutting overall output in half and adding to operational costs.
The fouling also limits the design of the system. The tubes can’t be too small because the fouling would begin to block the flow of water through the bioreactor and require higher pumping rates.
Varanasi and his team decided to try to use a natural characteristic of the algae cells to defend against fouling. Because the cells naturally carry a small negative electric charge on their membrane surface, the team figured that electrostatic repulsion could be used to push them away.
The idea was to create a negative charge on the vessel walls, such that the electric field forces the algae cells away from the walls. To create such an electric field requires a high-performance dielectric material, which is an electrical insulator with a high “permittivity” that can produce a large change in surface charge with a smaller voltage.
“What people have done before with applying voltage [to bioreactors] ha sido con superficies conductoras”, explica Leon, “pero lo que estamos haciendo aquí es específicamente con superficies no conductoras.
Agrega: “Si es conductor, entonces pasa corriente y descarga las células. Lo que estamos tratando de hacer es repulsión electrostática pura, por lo que la superficie sería negativa y la celda sería negativa, por lo que obtendría repulsión. Otra forma de describirlo es como un campo de fuerza, donde antes las células tocaban la superficie y recibían descargas.
El equipo trabajó con dos materiales dieléctricos diferentes, dióxido de silicio, esencialmente vidrio, y hafnia (óxido de hafnio), los cuales resultaron ser mucho más efectivos para minimizar el ensuciamiento que los plásticos convencionales utilizados para fabricar fotobiorreactores. El material se puede aplicar en una capa extremadamente delgada, de solo 10 a 20 nanómetros (milmillonésimas de metro) de espesor, por lo que se necesitaría muy poco para recubrir un sistema de fotobiorreactor completo.
«Lo que nos emociona aquí es que podemos demostrar que, únicamente a partir de las interacciones electrostáticas, podemos controlar la adhesión celular», dice Varanasi. «Es casi como un interruptor de encendido y apagado, poder hacer eso».
Además, dice Leon, «dado que estamos usando esta fuerza electrostática, realmente no esperamos que sea específica de una célula, y creemos que es posible aplicarla a otras células además de las algas». En trabajos futuros, nos gustaría intentar usarlo con células de mamíferos, bacterias, levaduras, etc. También podría usarse con otros tipos de algas valiosas, como la espirulina, que se usan ampliamente como suplementos dietéticos.
El mismo sistema podría usarse para repeler o atraer células simplemente invirtiendo el voltaje, según la aplicación particular. En lugar de algas, se podría usar una configuración similar con células humanas para producir órganos artificiales mediante la producción de un andamio que podría tener la tarea de atraer células a la configuración correcta, sugiere Varanasi.
«Nuestro estudio esencialmente resuelve este importante problema de la bioincrustación, que ha sido un cuello de botella para los fotobiorreactores», dice. «Con esta tecnología, ahora podemos realmente explotar todo el potencial» de estos sistemas, aunque se requerirá un mayor desarrollo para pasar a sistemas comerciales prácticos.
En cuanto a qué tan pronto podría estar listo para su implementación a gran escala, dice: «No veo por qué no dentro de tres años, si obtenemos los recursos adecuados para poder hacer avanzar este trabajo».
Referencia: «Control electrostático de baja potencia ajustable externamente de la adhesión celular con películas dieléctricas nanométricas de alta k» por Victor J. Leon, Baptiste Blanc, Sophia D. Sonnert y Kripa K. Varanasi, 13 de abril de 2023, Materiales funcionales avanzados.
DOI: 10.1002/adfm.202300732
El estudio fue apoyado por la empresa de energía Eni SpA, a través de MIT Energy Initiative.