(434e) Fractionation of Kraft Lignin By Solvent Extraction and Exploration for Their Value-Added Applications

Authors: 
Li, C., Hebei University of Technology
Chang, L., Hebei University of Technology
Kraft lignin (KL), the byproduct from the kraft process in the paper industry, is an abundant renewable resource. During the widely used industrial kraft pulping process, native lignin structures can be altered significantly by fragmentation and condensation. Thus, there are significant challenges associated with the utilization of KL as high value-added materials due to their polydispersity, heterogeneity and nature variable. In a manner similar to crude oil, KL need refining in order to actualize their potential commercial value.

The sequential organic solvent extraction is considered as one of the most frequent fractionation approaches. Many solvents, such as including hexane, diethyl ether, ethyl acetate, acetone, or dioxane, have been used. Lignin fractions obtained by sequential solvent extractions have been tested for various applications, including the elaboration of polymer blends, the preparation of polyurethanes or adhesives, the use as antioxidants or dispersants for carbon nanotubes.

In our recent researches, we give a series of exploration for the use of KL fractions obtained by different solvents. For most of organic solvents, KL could by be partially dissolved. Thus, we can obtain two kinds of fractions when KL was added into one solvent: the soluble fractions and insoluble fractions. We found that the soluble fractions could easiyl form a series of nonospheres with different characteristics depending on the solvents, while, the insoluble fractions exhibit a significantly improved adsorptive property.

For the soluble fractions, how to exploit the advantages of their solubility is more meaningful than recovering them from the solutions. KL can be considered an amphiphilic biopolymer because of the presence of both hydrophilic and hydrophobic groups. Similar to amphiphilic block copolymers, we found that KL could easily form a series of nanospheres via self-assembly in a mixture of solvent and water. The most exciting aspect of these studies is that these hollow nanospheres are obtained by a completely physicochemical process, without the need for any chemical modification. Moreover, it was also found that both the size and the shell thickness of these nanospheres can be easily controlled, which would allow the permeability of the nanospheres to be adapted to the desired application.

The size and shell thichness mainly depended on the solvent and the preparation conditions. For example, KL could form hollow nanocapsules via self-assembly induced by adding water to an ethanol solution of KL, while, it could form solid nanosphere via self-assembly induced by adding water to a KL/dioxane solution. Obviously, the thicker shell contributes to stabilization of the particles and their performance in some applications. Meanwile, the sizes of these KL nanocapsules can vary in the range of tens to hundreds of nanometers, depending on the preparation conditions, which enhance their flexibility to adapt to the potential applications in various fields.

The nanospheres formed by self-assembly had be characterized by transmission electron microscopy (TEM), scanning elec-tron microscopy (SEM), atomic force microscopy (AFM), dynamic light scattering (dynLS) and static light scattering (statLS). From the results of various imaging techniques and laser light scattering techniques, the structural characters of these products and their aggregation mechanism were revealed and confirmed.

For the insoluble fractions, it was found that their adsorption capacity for some hydrophobic matter had improved significantly. For example, when the ethanol insoluble fractions of KL were used as the adsorbing material for methylene blue, the adsorption capacity of insoluble fractions of KL is five times than the unfractioned KL. Similarly, the adsorption capacity of some other insoluble fractions obtained by different solvents also shown a significant improved for other adsorbate, such as butanol, lead, and phenol.

The adsorbent obtained in this method based on lignin has many obvious advantages. The whole preparation process is very simple and environment-friendly. It doesn’t require any chemical modifications and uses only the easily separated and recycled solvent. And the product is inexpensive which makes it is competitive with many other raw materials used for absorbent.

The utility of insoluble KL as adsorbents for the adsorptive removal or recovery of these adsorbates from aqueous solution had been studied systematacially. Effects of such parameters as adsorbent dose, initial concentration, contact time and initial pH of the solution on the adsorption process have also been investigated. Equilibrium adsorption data have been fitted to various isotherm equations to determine the best isotherm model to represent the experimental sorption data.

Overall, selective extraction with organic solvents may now offer different lignin fractions with significant commercial ramifications.