Pulp Mill Waste Becomes Green Solution to Remove Toxic Dyes

Dyes like Congo red and methyl orange create brightly hued shirts, sweaters and dresses. But these commonly used azo dyes can be toxic, carcinogenic and are hard to remove from wastewater. 

David Chem, a University of Arkansas chemical engineering Ph.D. candidate, developed an environmentally friendly solution to remove these dyes using a common byproduct of the pulp and paper industry. 

Azo dyes are used in 60%-70% of commercial textile production. The dyes dissolve easily in water and resist biodegradation, which makes them an environmental hazard. The runoff from garment plants has the highest concentration of azo dyes, but they also end up in municipal wastewater from washing clothes. 

To remove azo dyes from water, Chem started with lignin, a low-cost, widely available biopolymer derived from plant cell walls. Each year, 50 to 70 million tons of lignin are produced by the pulping industry. Most of it ends up in landfills. 

"Lignin extraction is hard to process. It has a complex structure," Chem said. "It is underutilized as a biopolymer." 

The researchers first added phenol to powdered lignin, making its surface more reactive. Then amino groups were added to give the lignin a positive charge so it would bond with the negatively charged azo dyes. 

This two step, dual-functionalization modification of lignin has been previously tested as a way to remove heavy metal ions, but the U of A researchers were the first to apply this approach to harmful dyes. 

In the lab, the modified lignin removed 96% of the Congo red dye and 81% of the methyl orange dye. With this method, both the dyes and the lignin can be reused. 

"The process is really scalable. It's a relatively green process. And it is highly effective," Chem said. 

Chem published his results in the Journal of Polymers and the Environment. The other authors, all from the U of A, are professor Keisha Bishop Walters, Chem's dissertation director and chair of the Ralph E. Martin Department of Chemical Engineering; post-doctoral fellow Fatema Tarannum; and Samantha Glidewell, an undergraduate at the time of the research.