Graduation Year

2023

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Chemistry

Major Professor

George P. Philippidis, Ph.D.

Co-Major Professor

Randy Larsen, Ph.D.

Committee Member

Theresa Evans-Nguyen, Ph.D.

Committee Member

Ioannis Gelis, Ph.D.

Committee Member

Maya Trotz, Ph.D.

Keywords

Cellulosic biomass, Fermentation, Hydrochar, Hydrolysate, Mixotrophy

Abstract

Rapid population growth and global industrialization have substantially heightened the demand for fossil-based fuels and products in various sectors of the global economy, including energy production, transportation fuels, and as raw materials for petrochemicals. The intense consumption of fossil fuels has caused immense environmental impacts, especially pertaining to carbon dioxide emissions. Shifting to renewable feedstocks (raw materials) is expected to reduce these emissions by lowering the carbon footprint of fuels and products compared to traditional fossil-derived alternatives. This transition aligns with the goal of creating a sustainable and circular economy, emphasizing efficient resource use, and reducing waste generation through recycling and reuse. By converting agricultural residues (currently considered waste materials) into feedstocks, renewable biofuels and bioproducts significantly reduce carbon emissions because of continuous carbon sequestration through photosynthesis, as achieved with plants and algae.

Cellulosic biomass, which encompasses a wide range of abundant, mostly agricultural residues, is investigated for potential use as an inexpensive and renewable feedstock for synthesizing high-value products through thermochemical pretreatment and enzymatic hydrolysis. Those processes break the cellulose in the biomass down to glucose-rich hydrolysate for fermentation to a wide range of industrially important fuels and products, such as ethanol and organic acids. Carinata-derived and sweet sorghum derived-biomass is successfully hydrolyzed to glucose-rich hydrolysate, which is readily fermented by microorganisms to organic acids, such as propionic acid, which find applications in food and other industries, replacing fossil-based organic acids.

Following a parallel route, cellulosic biomass is subjected to hydrothermal carbonization to generate value-added hydrochar and a nutrient-rich aqueous phase that serves as a renewable source of nutrients for both microbial fermentation and algal cultivation. Physicochemical analysis of the hydrochar indicates its suitability as a solid fuel, boasting high fixed-carbon content and minimal ash content, which highlights its potential as a valuable source of solid fuel. Additionally, hydrochar is further converted via chemical activation to activated carbon with applications as a biobased adsorbent in wastewater treatment processes and heavy metal remediation.

However, environmental sustainability also requires cost-competitiveness to incentivize society to move towards a greener future. In an effort to reduce the cost of biofuels and bioproducts, there is a need for high-productivity organisms and low-cost cultivation media. Chlorella vulgaris growth on organic carbon (glucose) derived from cellulosic biomass, such as Brassica carinata, proves to be faster than growth solely via photosynthesis resulting in higher productivity of both algal biomass and high-value bioproducts, such as lutein (essential for healthy vision) and lipids. Carinata-based glucose and other nutrients are not only more sustainable, but also cheaper than synthetic media, while promoting a circular economy. In order to gain insights into changes to cellular components associated with such a switch to faster growth, biochemical alterations are monitored between algae cells grown on biomass-derived media and algae cells grown on photosynthesis. In addition, chemical mutagenesis is successfully employed to generate mutants of C. vulgaris that demonstrate even higher productivity in hydrolysate than the parent (wild type) strain. These findings can play a crucial role in optimizing the combination of biomass utilization and algae cultivation to promote efficient and economically viable production of biofuels and bioproducts.

Overall, the knowledge gained from the integration of biomass conversion and algal growth processes is expected to facilitate societal advancement towards a lower carbon bioeconomy that encompasses environmental, economic, and social sustainability.

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