Northwestern team uses microbial fuel cell to upcycle resistant plant waste

Northwestern University researchers have developed a sustainable, two-step process that can upcycle organic carbon waste such as lignin, which could allow it to be used in pharmaceuticals for drug or chemical delivery.

The team's work was featured in the January issue of the journal ACS Sustainable Chemistry. By processing waste through a microbe-driven biorefinery, the researchers turned lignin into carbon sources that could be used in high-value, plant-derived pharmaceuticals and antioxidant nutraceuticals, as well as carbon-based nanoparticles for drug delivery, the report stated.

"Lignin should have tremendous value, but it's intrinsically regarded as waste," said Kimberly Gray of Northwestern's McCormick School of Engineering in the report. "Lignin makes up 20-30% of biomass but 40% of the energy, which is a lot, but it's difficult to tap this energy source. Nature made lignin so recalcitrant to processing that people haven't figured out how to use it." 

"Researchers have been trying to solve this problem for decades," she added. "Using an oil refinery as a template, we developed a biorefinery that takes in waste streams and produces high-value products."

Lignin is present in all vascular plants and gives strong, sturdy plants, like trees, structural support, the release stated. Without lignin, wood and bark would be too weak to support trees. 

Most industries that use plants, however, such as the paper manufacturing and brewing industries, strip out lignin, leaving behind cellulose. Instead of making use of nature's ultra-resistant material, industrial teams burn lignin as a cheap fuel.

To develop a biorefinery for breaking down carbon waste, the researchers first engineered a microbial electrolysis cell (MEC), which exchanges energy between an anode and a cathode. Northwestern's bio-anode comprises exoelectrogens, a type of bacteria that naturally generates electrical energy by eating organic matter, the researchers said.

"The microbes act as the catalyst," said study co-author George Wells, associate professor of civil and environmental engineering at McCormick. "Instead of using chemical catalysts, which are often very expensive and require high temperatures, we're using biology as the catalyst." 

The MEC cycles waste-filled water through the bacteria, which eat up the carbon. The extracted electrons then flow from the bio-anode to the cathode (made of a carbon cloth), where they reduce oxygen to generate water. 

The process consumes protons, driving up the water's pH to turn it into a caustic solution, which can be used in several ways, including wastewater treatment. When the researchers exposed lignin to the bio-based caustic chemical, lignin's polymers broke apart in a way that preserved the aromatic rings, including rings of carbon called flavonoids, an antioxidant-rich phytonutrient often found in supplements. 

The rest of the processed lignin (about 80%) became carbon-based nanoparticles, which could be used to encompass substances for targeted drug delivery. The study demonstrates a promising strategy for transforming lignin waste into valuable carbon, the researchers said.