Date of Award

January 2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

First Advisor

Yun Ji

Abstract

Lignin is the second most abundant biopolymer, and the first most abundant source of phenolic structures, on Earth. Recent research has focused on lignin monomers as replacements for phenolic monomers derived from nonrenewable resources. Biological decomposition and conversion may be one way to accomplish this objective. In this study, the effects of two basidiomycetous fungi (Coriolus versicolor and Trametes gallica) and two actinobacteria strains (Microbacterium sp. and Streptomyces sp.) and their combination on lignocellulose (kenaf) decomposition was evaluated.

The results showed that after 8 weeks of incubation up to 34 wt. % of the kenaf biomass was degraded, and the combination of fungi and bacteria was the most efficient. Lignin decomposition accounted for ~ 20 wt. % of the observed biomass reduction, regardless of the culture used. Most of this lignin was present as solubilized oligomers rather than monomers. Only after the monosaccharides were utilized by the microorganisms was the production of laccase, manganese-dependent peroxidase and lignin peroxidase enzymes induced, allowing lignin degradation to commence. The presence of carbohydrates was found to be detrimental for lignin degradation.

In a subsequent series of experiments, we targeted the degradation/modification of isolated industrial-kraft lignin while trying to reduce the process time by solubilizing lignin with DMSO to increase lignin availability for enzymes. The addition of 2 vol% DMSO to nutrient free aqueous media increased the lignin solubility up to 70% while the quasi-immobilized fungi (pre-grown on agar) maintained their ability to produce lignolytic enzymes. While biological treatment was done for 6 days, significant modification was already observed in less than 24 hours. The resulting product showed the removal of phenolic monomers and/or their immediate precursors and a significant intramolecular cross-linking among the reaction products. Thus a new path for lignin biotreatment and further utilization was observed leading to the formation of polymers rather than monomers.

Our interest therefore shifted to lignin utilization as a biochemically modified macromolecule. The biologically modified lignin was isolated via two different paths: 1) precipitation by acidification followed by washing with water or alcohols, or 2) vacuum evaporation followed by drying. The results of novel detailed chemical analysis of the modified lignin polymers showed that each of the washing steps can be used as a modification process, since each of them produced a slightly different polymer, with varied thermal stability, swelling and buffer capacities.

The resulting lignin based polymers turned out to be insoluble in either organic solvents such as DMSO, DMF, NMP, dioxane etc., or in water. However, under alkaline conditions (1M NaOH) all of these new polymers were converted into pH sensitive anionic-hydrogels showing remarkable thermal stability and varied sulfur content, which, like other properties, could also be controlled by precursor polymer washing with solvents. One of these hydrogels was further converted into a nonporous membrane and tested as a filter for water desalination. The preliminary results of 78% salt rejection under 270 kPa obtained with 1.35 Ö­ thick, ligninbased membranes are promising for further exploration.

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