Enzyme-Enhanced ECF Bleaching: Improving Pulp Bleachability and Reducing Its Environmental Footprint

Date of Award

8-2024

Degree Name

Doctor of Philosophy

Department

Chemical and Paper Engineering

First Advisor

Kecheng Li, Ph.D.

Second Advisor

Zhihua Jiang, Ph.D.

Third Advisor

Mert Atilhan, Ph.D.

Keywords

Enzyme-assisted bleaching, hemicellulase pretreatment, hexenuronic acid, kraft pulp, lignin-carbohydrate complexes

Abstract

Enzymes have emerged as effective biocatalysts and environmentally friendly agents in pulp bleaching processes. Despite their potential, the fundamental chemistry underlying enzyme action on pulp fibers during bleaching remains poorly understood, limiting their widespread industrial application. This dissertation aims to investigate how enzymes interact with the chemical components of pulp fibers and their impact on elemental chlorine-free (ECF) bleaching in terms of pulp brightness, chemical consumption, and AOX generation.

Enzymatic pretreatment prior to ECF bleaching sequences was conducted on hardwood and softwood kraft pulp. Xylanase pretreatment significantly reduced chlorine dioxide consumption and enhanced pulp brightness in hardwood kraft pulp. Optimal xylanase formulation achieved a 40% reduction in chlorine dioxide charge with a low enzyme dosage (<2.0 IU/g odp). For softwood kraft pulp, xylanase also improved brightness and reduced chemical usage, although to a lesser extent. However, combining xylanase and mannanase did not yield many additional benefits.

The impact of xylanase on hexenuronic acid (HexA) removal and subsequent ECF bleaching efficiency was investigated and compared with hot acid treatment. While both methods effectively removed HexA, xylanase pretreatment demonstrated superior bleaching efficiency. This enhancement was attributed not only to HexA removal but also to the breakdown of the hemicellulose barrier on fiber surfaces, which increased fiber accessibility to bleaching chemicals, thereby improving the effectiveness of bleaching chemicals in reacting with lignin in fiber walls.

Furthermore, this study investigated changes in Lignin-Carbohydrate Complexes (LCCs) in pulp fibers after enzyme treatment and after ECF bleaching of softwood kraft pulp. Xylanase and xylanase/mannanase treatments released more lignin and reduced total lignin content in treated pulp. Lignin content in the glucan-rich LCC1 fraction was lower in xylanase-treated samples, while the glucomannan-rich LCC2 fraction exhibited higher lignin content. Additionally, lignin-carbohydrate linkages detected in mannanase-treated samples were minimal in xylanase and xylanase/mannanase-treated samples. These findings reveal that hemicellulose polysaccharides and lignin-carbohydrate bonds were effectively hydrolyzed with xylanase-involved treatments. In the subsequent ECF bleaching, 23% less chlorine dioxide was used to achieve the same brightness of 89.0 %ISO. Xylanase alone resulted in similar brightness improvement and bleaching chemical reduction as the combined xylanase/mannanase, indicating that mannanase is less effective.

In conclusion, the findings of this dissertation highlight the significant potential of enzyme-enhanced ECF bleaching to improve pulp bleachability and reduce environmental impact. Xylanase treatments, in particular, proved effective in enhancing pulp brightness and reducing chemical consumption in both hardwood and softwood kraft pulps. The study elucidated that beyond hexenuronic acid removal, xylanase's ability to modify lignin-carbohydrate complexes and degrade hemicellulose barriers on fiber surfaces plays a crucial role in enhancing the accessibility of fibers to bleaching chemicals. These insights contribute to advancing the understanding of enzymatic mechanisms in pulp bleaching processes, paving the way for more sustainable and efficient industrial practices in the pulp and paper industry.

Access Setting

Dissertation-Abstract Only

Restricted to Campus until

8-1-2026

This document is currently not available here.

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