(636c) Developing Lignin-Based Polyelectrolytes for Applications As Eco-Friendly Flocculants

In the 21^st century, the forefront of chemical research focuses on supplanting dependence on fossil resources by advancing the development of sustainable materials from renewable chemical feedstocks, such as, lignocellulosic biomass. Lignocellulosic biomass is a composite material and is comprised of three major biochemical components: cellulose, hemicellulose, and lignin. The carbohydrate components of lignocellulose (cellulose, hemicellulose) have received industrial attention for producing cellulosic fibers¹ and bio-ethanol via fermentation.² The remaining biochemical component, lignin, is an amorphous biopolymer consisting of phenolic subunits and is generally considered as a waste byproduct and an inexpensive fuel for heat. However, the economic viability of biorefineries can drastically improve by valorizing lignin into value-added products.^3,4 As such, intense research interests have emerged for developing chemical routes for depolymerizing lignin into phenolic subunits and subsequently, using these subunits as chemical building blocks for synthesizing sustainable materials with industrial applications.

A promising catalytic thermochemical process for depolymerizing lignin is reductive catalytic fractionation (RCF). RCF involves tandem solvolysis and reductive depolymerization of lignin, facilitated by a reducing agent and heterogeneous redox catalyst, into a mixture of phenolic monomer and oligomer units.⁴ The produced phenolic monomers have found applications as components for resins,⁵ adhesives,⁶ and flavorants.⁷ However, the resultant phenolic oligomers have received little interest as chemical building blocks even though they comprise over 40 wt% of the fractionated lignin material.⁴ In order to improve the viability and potential of lignin, the entire fractionated material needs to be integrated into attractive chemical products.

In the present study, phenolic oligomers produced from RCF of lignocellulosic biomass (corn stover, poplar, pine) are used as chemical building blocks for synthesizing polyelectrolytes. Polyelectrolytes are polymers containing charged functional groups and have applications for flocculating colloidal suspensions of clay minerals in wastewaters produced by the mineral processing, papermaking, pharmaceutical, and cosmetic industries.^8,9 Synthetic polyelectrolytes flocculate clay dispersions with high efficiency, however, their lack of biodegradability motivates the development of environmentally benign alternatives.^9,10 In the present investigation, eco-friendly polyelectrolytes are synthesized by the Mannich reaction using bio-based chemical building blocks, specifically, lignin oligomers and the amino acid, lysine. The material properties and molecular structure of the lignin-based polyelectrolytes are characterized by NMR, dynamic light scattering, and LC/MS. The performance of the lignin-based polyelectrolytes as flocculants is investigated by measuring the turbidity of kaolin suspensions before and after treatment with polyelectrolytes. The influence of solution pH and polyelectrolyte dosage are further investigated for identifying optimum conditions for flocculation. The biodegradability of the lignin-based polyelectrolytes is assessed by measuring the biochemical oxygen demand (BOD).

References

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