Biorefining Lignin into High-Value Products: Coupling a Microbial Electrolysis Cell to Lignin Depolymerization

Obrzut, Natalia

doi:https://doi.org/10.21985/n2-md81-3s85

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Biorefining Lignin into High-Value Products: Coupling a Microbial Electrolysis Cell to Lignin Depolymerization

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Lignin is the largest store of renewable aromatic carbon. Due to its refractory nature, however, its chemical potential is not fully realized, rather, most lignin is treated as a waste, burned for low-value energy. Here, we propose a biorefinery where the treatment of wastewater by a microbial electrolysis cell (MEC) produces “clean” water and a caustic catholyte that can depolymerize lignin under mild conditions (i.e. ambient temperature and pressure) into two high value product streams. The specific goal of this research is to characterize the range of products that can be biorefined from lignin as a function of its source, method of extraction, and catholyte characteristics. We determined the MEC operating conditions to produce a depolymerization solvent and characterize and quantify solution and colloidal phase products using an array of analytical techniques. We characterized bulk features such as soluble lignin, phenolic content and flavonoid content using UV vis spectroscopy. We found greater than 80% soluble lignin after an hour, and nearly complete solubilization over 7 days. We also determined bulk flavonoids at 20% and bulk phenolics at 50%. Using high resolution liquid chromatography and tandem mass spectrometry, we obtained and identified 11% of discrete aromatics (monomers and flavonoids). This contrasts with previous reports in the literature that obtain lower yields of low value products (BTX) with base catalyzed depolymerization. We closed mass balance by simultaneously producing lignin nanoparticles. We characterized the lignin nanoparticles through nanoparticle tracking analysis and dynamic light scattering for size, concentration, and polydispersity, zetasizer for zeta potential (surface charge that indicates colloidal stability), and scanning electron microscopy to verify the shape and size. In part, our higher product yield is due to selective repolymerization to form flavonoids and nanoparticles. Furthermore, we explored the efficacy of depolymerization in the conditions of our biorefinery when we change the lignin source. The biomass source (i.e. herbaceous, softwood, and hardwood) influences the amount of lignin available, as well as the ratio of monomers (S/G) and content of breakable bonds (-O-4 linkages). The structure, specifically the number of -O-4 linkages, is further affected by the extraction method (i.e. Milled Wood (mild), Organosolv (medium), and Klason (harsh)). We found that herbaceous lignin extracted via the Organosolv process is best suited for depolymerization in our biorefinery. Herbaceous Organosolv lignin, solubilizes to the greatest extent (~100% over 7 days), produces the greatest amount of phenolics and flavonoids, and has the most and smallest lignin nanoparticles. Additionally, this depolymerized mixture has the highest antioxidant capacity. Only the harsh, Klason method successful extracts lignin from all three sources, but the herbaceous and hardwood sources are depolymerized to a lesser extent than the Organosolv method. With low density of labile bonds, softwood lignin cannot be depolymerized under the conditions of our biorefinery. Milled Wood lignin does not efficiently extract lignin from any of the biomass, and the extracted lignin is low purity, which does not allow it to be processed in our biorefinery. Finally, we explored the extent to which we can tune the lignin nanoparticles. We varied the salt concentration, salt type, and pH and monitored the concentration, size, and shape of the nanoparticles, as well as the antioxidant capacity of the mixture. We found that the pH can be used to produce a higher concentration of lignin nanoparticles quickly, albeit at a larger size. We also found that the salt type influences the shape of the lignin nanoparticles – ranging from spherical (phosphate), rod-like (nitrate and chloride), and flower-like (carbonate). Additionally, the shape and the size influence the antioxidant capacity due to changes in the surface area to volume ratio. The flower-like nanoparticles have a high surface area to volume ratio and the highest antioxidant capacity, the same antioxidant capacity to the industrial antioxidant Trolox. The antioxidant capacity of both discrete products and nanoparticles illustrates their high potential value in the pharmaceutical, nutraceutical, personal care, and agricultural industries.

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