Recently, I posted about how biochar made from rice husk waste can capture green dyes from wastewater. The current post explores research from September 2025 that showed that lignin from pulp mills can remove azo dyes, Congo red, and methyl orange from wastewater.
Azo dyes are a large class of
synthetic anionic dyes having a specific organic chemistry signature. They are
often toxic and can be carcinogenic. They are used in 60-70% of commercial
textile production. They dissolve in water and resist biodegradation. They are
often found in wastewater effluent near textile plants, and these dyes also
enter municipal wastewater through washing clothes.
According to Phys. org:
“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.”
“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.”
The aminated phenolated
lignin removed 96% of the Congo red dye and 81% of the methyl orange dye in the
lab tests. That dye can be recovered and reused. The lignin is biodegradable.
"The process is really scalable. It's a relatively
green process. And it is highly effective," Chem said.
The aminated phenolated
lignin is considered to be a bio-adsorbent.
The figure below shows the
different ways lignin can be chemically modified to enhance its capacity for
bioadsorption and metal ion removal. The aminated lignin captures anionic dyes
due to its positively charged amine groups under acidic conditions.
The amination and phenolation
reactions are shown below.
As the figure below shows, a
basic pH of 10-11 was found to give the highest adsorption capacities and
removal efficiencies.
The paper notes that the
phenolation produced a higher amine content and surface area in the resulting
lignin structure, resulting in higher dye adsorption/capture rates. pH and
contact times were found to have the most effect on those rates.
“In addition to higher amine content, SEM showed a more
porous structure for Am-PL {aminated phenolated lignin} with the higher
specific surface area expected to support increased dye adsorption. UV-vis
spectroscopy was used to measure the removal of anionic dyes from water
solution as a function of pH and contact time.”
References:
Pulp
mill waste becomes green solution to remove toxic dyes. Science X staff.
Phys.org, September 26, 2025. Pulp
mill waste becomes green solution to remove toxic dyes
Aminated
Phenolated Lignin for Effective Anionic Dye Removal for Water Remediation. David
Chem, Samantha Glidewell, Fatema Tarannum & Keisha B. Walters. Published:
11 August 2025. Journal of Polymers and the Environment. Volume 33, pages
4430–4445. Aminated
Phenolated Lignin for Effective Anionic Dye Removal for Water Remediation |
Journal of Polymers and the Environment | Springer Nature Link
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