Waste Cotton Hulls Become Powerful Catalyst for Cleaner Water

A team of researchers has developed a green catalyst from cotton hulls that can dramatically improve the ability of ozone to remove stubborn organic pollutants from water. The study, published in Biochar, shows that a nitrogen-doped biochar catalyst called N-BC-800 can efficiently degrade N,N-diethyl-meta-toluamide, better known as DEET, a widely used insect repellent that is increasingly detected in rivers, wastewater, and other aquatic environments.

DEET is valued for its broad-spectrum protection against mosquitoes and other insects, but once released into wastewater, it can persist in the environment and resist conventional treatment. Although ozone is already used in water purification, ozone alone can be selective and may not fully mineralize some pollutants. The new study shows that modifying biochar with nitrogen can turn ozone into a much more powerful treatment tool.

Using cotton hulls as the raw material and urea as the nitrogen source, the researchers prepared N-BC-800 through a two-step pyrolysis process. In catalytic ozonation tests, the material achieved 94% removal of DEET, far outperforming ozone alone and unmodified biochar. The apparent second-order rate constant reached 2538 M⁻¹ s⁻¹, representing a 106-fold increase compared with ozone alone and a 25-fold increase compared with ozone combined with ordinary biochar.

“This work shows that agricultural waste can be transformed into a high-value catalyst for advanced water treatment,” said corresponding author Prof. Yonghui Song. “By tailoring the surface chemistry of biochar, we can make ozone work faster and more effectively against pollutants that are difficult to remove.”

The researchers found that the catalyst’s strong performance came from a combination of structural and chemical changes. Nitrogen doping increased the surface area, introduced defects into the carbon framework, and improved electron transfer. More importantly, detailed experiments and density functional theory calculations identified pyridinic nitrogen and surface C=O groups as the key active sites. These sites work together to adsorb and activate ozone, promoting the formation of reactive oxygen species, especially superoxide radicals and hydroxyl radicals, which drive DEET degradation.

“The most exciting finding is the synergy between pyridinic nitrogen and C=O groups,” said Prof. Zhiwei Song. “These two surface sites do not simply act alone. Together, they enhance electron transfer to ozone and accelerate the generation of reactive oxygen species.”