Abstract
Unlike previous lignin biodegradation studies, white rot fungi were used to produce functional biopolymers from Kraft lignin. Lignin-based polymers (hydrogel precursors) partially soluble in both aqueous and organic solvents were produced employing a relatively fast (6 days) enzymation of Kraft lignin with basidiomycetes, primarily Coriolus versicolor, pre-grown on kenaf/lignin agar followed by either vacuum evaporation or acid precipitation. After drying followed by a treatment with alkaline water, this intermediate polymer became a pH-sensitive anionic hydrogel insoluble in either aqueous or organic solvents. The yield of this polymer increased from 20 to 72 wt% with the addition of 2% dimethylsulfoxide to distilled water used as a medium. The mechanical stability and buffering capacity of this hydrogel can be adjusted by washing the intermediate polymer/hydrogel precursor prior to drying with solvents of different polarity (water, methanol or ethanol). Any of these polymers featured a significant thermal resilience assessed as a high thermostable “coked” fraction in thermal carbon analysis, apparently resulting from significant covalent cross-linking that occurs during the treatment of their intermediate precursors.
Highlights
Lignin, a large scale by-product of pulp and paper industry[1] and bioethanol production[2], is considered the most abundant source of renewable aromatics on Earth[3,4]
An insoluble polymer/hydrogel (IP-H) insoluble in both aqueous and organic solvents was produced by drying this intermediate product followed by its treatment in a highly alkaline solution and the final drying
A water-soluble polymeric product obtained by a 6-day lignin treatment in 2% DMSO with quasi-immobilized Coriolus versicolor, was converted into another polymer product featuring a limited solubility in both aqueous and non-aqueous solvents
Summary
A large scale by-product of pulp and paper industry[1] and bioethanol production[2], is considered the most abundant source of renewable aromatics on Earth[3,4]. The current research efforts focus on production of phenolic monomers (e.g., vanillin, guaiacol) and other low-MW chemicals[5], with their application in polymer industry[6]. The monomer production from lignin is usually low (5–35 wt%), with the rest being undesired phenolic oligomers or polymers[7,8]. In physical hydrogels the crosslinking involves hydrogen bonding, polyelectrolyte complexation, molecular entanglement, hydrophobic association and ionic bridging. All of these forces are relatively weak and these hydrogels have a tendency to disintegrate over time[9,10]. We described the production of a water-soluble lignin-based polymer[19]. We discovered that this polymer can be converted into less www.nature.com/scientificreports/
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