Biorefinery Uses Microbial Fuel Cells to Upcycle Resistant Plant Waste

A biorefinery uses a microbial fuel cell to digest plant-resistant waste.

Body waste is converted into antioxidant flavonoids for nutrition and medicine.

When nature made lignin – the tough fibrous material that gives plants their tough structure – it didn’t cut corners. Surprisingly, lignin is so strong and durable that it resists bacteria and decay.

So, what happens to all the lignin waste from fields, breweries and paper mills? Most of them are incinerated or landfilled, producing pollution and wasting renewable resources.

Now, Northwestern University researchers have developed a sustainable, low-cost two-step process that can break down carbon waste – including lignin. By treating the waste with a microbe-driven biorefinery, the researchers converted lignin into carbon sources that could be used in high-quality, plant-derived pharmaceuticals and antioxidant nutraceuticals as well as carbon-based nanoparticles for drug or chemical delivery.

The study is featured on the cover of the January issue of the ACS journal Sustainable Chemistry and Engineering.

Kimberly GrayNorthwestern’s, who led the research, said:

Lignin should be of high quality, but is naturally considered waste.

“Lignin makes up 20-30% of biomass but 40% of energy, which is a lot, but it is difficult to find this source of energy. Nature has made lignin so useless that people did not know to use it. Researchers have been trying to solve this problem for decades. Using an oil refinery as an example, we designed a biorefinery that removes waste water and produces of high quality.”

Gray is the Roxelyn and Richard Pepper Family Chair in Civil and Environmental Engineering and a professor of civil and environmental engineering at Northwestern’s McCormick School of Engineering.

Natural building materials

One of the most abundant polymers in the world, lignin is present in all vascular plants. Found between cell walls, lignin provides strong, sturdy plants – such as trees – with structural support. Without lignin, wood and bark would be too weak to support trees. And the wooden houses and furniture would just collapse.

But many industries that use plants – such as the paper and brewing industries – strip away lignin, leaving cellulose, a type of sugar. Instead of using the more resistant natural materials, industrial groups burn lignin as a cheap fuel.

“People want to get rid of lignin to get sugar,” Gray said. “They boil cellulose to make alcohol or grind it to make pulp. And what do they do with lignin? They burn it as cheap fuel. It’s a waste.”

Fuel cell powered by bacteria

To develop a biorefinery for breaking down carbon waste, including lignin, the researchers first created a microbial electrolysis cell (MEC). Similar to a fuel cell, the MEC exchanges energy between the anode and cathode. But instead of a metal-based anode, Northwestern’s bioanode contains exoelectrogens – a type of bacteria that naturally produces electrical energy by eating organic matter.

George WellsA co-author, McCormick associate professor of civil and environmental engineering, said:

Viruses act as a cause.

“Instead of using chemicals, which are often very expensive and require high temperatures, we use biology as the catalyst.”

Kimberly Graycivil and environmental engineer, said:

People want to remove lignin to get sugar.

“They boil cellulose to make alcohol or grind it to make pulp. And what do they do with lignin? They burn it as cheap fuel. It’s a waste.”

The beauty of MEC is that it can handle any type of natural waste – human, agricultural or industrial. The MEC circulates waste water through bacteria, which consumes carbon. Here, they break down organic carbon dioxide into carbon dioxide and naturally inhale electrons.

During this process, the captured electrons flow from the bio-anode to the cathode (made of carbon fiber), where they reduce oxygen to produce water. The process uses protons, moving the pH of the water to turn it into a caustic solution. From there, the caustic solution can be used for any number of applications, including wastewater treatment.

“One advantage of this process is that it effectively treats wastewater to remove harmful carbon,” Wells said. “So, the most important product is clean water.”

But the researchers took the caustic substance and turned their attention to lignin. Lignin compounds last a long time because they contain complex aromatic carbon chains, with a special bonding pattern that forms a ring of six carbon atoms. Each aromatic ring has two different bonds, which are very difficult to separate.

Busting ‘unbreakable’ bonds

When the researchers exposed the lignin to bio-based caustic chemicals, however, the lignin polymers disintegrated in a way that preserved the aromatic rings. About 17% of processed lignin is converted to carbon rings called flavonoids, an antioxidant-rich phytonutrient commonly found in supplements. Typically used in medicinal chemistry, these rings can be used as plant-derived, stable precursors to inexpensive medicines and supplements.

“It breaks the polymer bonds but leaves the ring intact,” Gray said. “If you can keep that ring, you can make quality goods. Chemists have created catalysts that split the whole compound, and then they have to rebuild the ring. But we were able to selectively destroy it to save important buildings.”

The rest of the processed lignin (about 80%) turned into carbon-based nanoparticles, which could be used to incorporate targeted substances into human drug delivery or targeted nutrient delivery. plants. Nanoparticles can also provide a sustainable, plant-based formulation for sunscreens and cosmetics.

Wells said: “It’s exciting to identify and explore a way to recover sustainable resources from many waste streams.” “We have a lot of waste water and lignin streams that are expensive to treat themselves. We’re trying to rethink them as valuable resources.”

Recycling materials without harmful chemicals

Although the researchers could have used a commercially available caustic substance to treat the lignin, their MEC-based method has several advantages. First, a bio-based raw chemical works just fine. Second, it is safe, cheap, environmentally friendly and can produce chemicals where needed.

“There are many caustic substances, such as sodium hydroxide, that are often used in many industrial processes and wastewater treatment,” Wells said. But that involves shipping and storing a lot of toxic chemicals. Not only is it expensive, it is also dangerous for public health. It is much safer and more sustainable to produce chemicals on site with waste products.”

“We avoid having to ship or store a lot of dangerous chemicals and we are not dependent on supply chains or trucks that arrive on time. It gives us the flexibility to produce chemicals right on site when they are needed. ”

The study, “Valorization of lignin under mild conditions: Biorefining flavonoids and lignin nanoparticles,” was supported by the Finite Earth Initiative of the McCormick School of Engineering at Northwestern University.

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