Nanoscale Modification of Lignocellulosic Biomass Components for Value-Added Applications

Abstract

The primary goal of this dissertation was to understand how modification of lignocellulosic nanomaterials enhances their properties for value-added applications. This is essential for meeting the current demand for sustainable alternatives. To offset the impacts of climate change while supporting a growing world population, “green” alternatives are required for many traditional, synthetic materials. This dissertation describes two key applications utilizing biomass waste to support sustainability. First, this research investigated the production of tailored activated carbons from fundamental biomass components. This effort resulted in models which allow for predictable, yet tunable activated carbon properties. The bio-derived carbons were then used in water treatment applications for PFAS removal to understand which activated carbon properties and feedstock component affect contaminant adsorption. Second, this research explored the surface functionalization of cellulose nanocrystals with functional biomolecules to develop efficient nanocarriers for agricultural applications.

In recent decades, there has been significant interest in using biomass to produce activated carbon for environmental remediation and energy storage applications. Previous studies have concentrated on individual biomass sources, neglecting the interactions among lignocellulosic components crucial to determine activated carbons’ structural properties and suitability for a variety of applications. This chapter sought to address this knowledge gap by exploring how the relative compositions of cellulose nanofibers, cellulose nanocrystals, xylan, and lignin affected the resulting properties of activated carbons produced through a two-step chemical activation process. Simplex-lattice mixture designs were used to understand how the composition affected activated carbon specific surface area and micropore fraction. Mixture regression models showed that specific surface area was highest when the activated carbon was composed of 17% cellulose nanofibers and 17% cellulose nanocrystals sourced from wood pulp and 67% alkali kraft lignin by mass. In addition, the micropore fraction was largest with a precursor mixture of 50% cellulose nanocrystals and 50% lignin by mass. The presence of xylan in the feedstock mixtures did not impact the resulting activated carbon properties in a predictable manner. Equations produced by this work were experimentally validated to within 10% of the model values. The results of this investigation provide a foundation for future efforts to tune activated carbon properties based on feedstock biomass composition.

In response to strict regulations by the United States Environmental Protection Agency regarding per- and polyfluoroalkyl substance (PFAS) concentrations in drinking water, the application aspect of this work investigated the efficacy of activated carbons derived from agricultural waste components for water treatment. By varying the proportions of lignin, cellulose nanocrystals, and cellulose nanofibers in the feedstock mixtures, specific PFAS adsorption efficiencies ranged from 31 to 98%, indicating that biomass structure influences contaminant adsorption. Activated carbon prepared from a mixture of 50% lignin, 35% cellulose nanocrystals, and 15% cellulose nanofibers achieved over 98% removal of three regulated PFAS compounds without requiring additives or surface functionalization. In addition, this mixture effectively removed four unregulated short and long-chain PFAS. Specific surface area was shown to be the most critical factor in determining PFAS adsorption, and both positive and negative surface charges are beneficial to harness hydrophobic and electrostatic interactions. Conversely, cellulose nanofibers in the precursor led to lower adsorption. The combination of these two investigations aids in developing a structure-process-property relationship for the adsorption of contaminants from groundwater using biomass-derived activated carbons.

More sustainable agricultural practices are also critical to meeting humanity's food needs while minimizing adverse environmental impacts. Engineered nanomaterials used as nanocarriers promise to reduce the volume of agrochemicals required for efficient crop production but concerns about contamination of agricultural products motivate the use of naturally occurring alternatives. In the second portion of this research, plant-derived cellulose nanocrystals were investigated as “green” alternatives to synthetic nanomaterials for the delivery of agrochemicals to plants. The production of fluorescently labeled cellulose nanocrystals demonstrated that these nanocrystals could penetrate the plant cell wall and enter cells without causing any adverse effects on the overall plant phenotype, genome, and metabolome. The efficiency of these nanocarriers to deliver chemicals was demonstrated by the covalent conjugation of cellulose nanocrystals with the common herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). Plant cell culture experiments confirmed that cellulose nanocrystals can be used as a 2,4-D nanocarrier and reduce the volume of plant growth regulators needed to cultivate plant cells. To further probe the use of cellulose nanocrystals as a carrier for a variety of agricultural biomolecules, plasmid DNA was electrostatically bound to cellulose nanocrystals to carry genetic information to the plant cell. Results showed that cellulose nanocrystals are effective at carrying genetic material to the target site in the plant cell and can do so at rates better than other biomolecules. The schemes described are the first reports of herbicide and DNA-conjugated cellulose nanocrystals. Additionally, this work creates a new platform for cyclical agricultural practices where cellulose nanocrystals extracted from agricultural waste can be used for the direct delivery of agrochemicals to crops, thereby reducing the environmental burden created by both agricultural waste and excess herbicides.

Overall, this dissertation advanced understanding and utilization of lignocellulosic nanomaterials through microscale modifications, which is crucial for enhancing sustainability and moving towards a greener economy. This work bridges fundamental research with practical applications and provides insight into the structure-property-processing relationships essential for advancing the use of biomass-derived nanomaterials in large-scale operations. By repurposing waste biomass into functional nanomaterials, humans can create innovative and sustainable solutions from materials that are often viewed as waste.