Gel formation of lignin and biodegradation of the lignin gels by microorganisms

Abstract

The multifunctional, polyphenolic nature of isolated lignins invites practical applications in which lignin serves as the principal component of cross-linked network polymers. Unmodified kraft lignin was first condensated with phenols and then further condensated with resorcinol under alkaline conditions, to give lignin–phenol–resorcinol resin (LPR resin). Lignin gels were prepared from the methanol-diluted LPR resin with the use of organic cross-linking agent glutaraldehyde (GA) via reacting for 3 days at 25°C. The gel formation depended on the amounts of methanol, and not the amounts of GA. The lignin gel obtained exhibited a reversible expansion–contraction behavior when it was alternatively immersed in ethanol and in water. Degradation of cross-linked LPR–GA hydrogels by some microorganisms has been investigated. From the experimental results, the following findings were obtained. The lignin–GA gels are degraded by the four Basidiomycotia fungi examined. The most effective degradation of the lignin–GA gels was obtained with the fungus Flammulina velutipes. The present biodegradation results by wood-rotting fungi might give some clues as to future biotechnology of cross-linked lignin and lignin hydrogels.