A team of environmental chemists has developed a new catalyst made from discarded coffee grounds that can efficiently remove hydrogen sulfide, a highly toxic industrial gas, from waste streams while producing useful elemental sulfur. The research demonstrates how agricultural waste can be transformed into high-value materials for pollution control and sustainable industry.
Hydrogen sulfide is widely generated in petroleum refining, wastewater treatment, and metal processing. Even at very low concentrations it can harm human health and corrode industrial equipment, while higher levels can be immediately dangerous. Traditional removal technologies often rely on metal catalysts or chemical absorption systems that generate secondary waste or require costly regeneration.
In the new study, researchers converted spent coffee grounds into a nitrogen-rich porous carbon material that functions as a metal-free catalyst. The material was produced using a two-step process involving hydrothermal treatment followed by controlled heating, which created a structure rich in pores, defects, and nitrogen functional groups. These features play a key role in activating oxygen molecules and breaking down hydrogen sulfide.
The optimized catalyst showed outstanding performance during laboratory testing. At moderate temperatures around 180 degrees Celsius, it achieved complete hydrogen sulfide conversion while maintaining almost perfect selectivity toward elemental sulfur rather than undesirable by-products such as sulfur dioxide. The material also remained stable for more than 100 hours of continuous operation and performed well even under humid conditions and in carbon dioxide-rich gas streams, environments that often degrade catalyst efficiency.
"This work shows that waste coffee grounds can be transformed into a powerful catalyst for environmental protection," said one of the study's lead researchers. "We demonstrated that a sustainable biomass material can rival or even outperform conventional metal catalysts while avoiding heavy metals and reducing costs."
Advanced spectroscopy, microscopy, and theoretical simulations helped the team uncover why the catalyst performs so well. Nitrogen atoms embedded in the carbon structure create highly reactive sites that promote adsorption of hydrogen sulfide and activation of oxygen. Computer modeling further showed that carbon atoms adjacent to these nitrogen sites serve as key reaction centers, enabling efficient conversion of the toxic gas into solid sulfur.
Beyond gas purification, the findings suggest broader opportunities for turning agricultural residues into advanced functional materials. Coffee is one of the most widely consumed beverages in the world, producing millions of tons of spent grounds each year that are often landfilled or incinerated. Converting this waste into catalysts not only addresses disposal challenges but also supports circular economy strategies.
The researchers note that the catalyst can be regenerated through simple washing and heating procedures, allowing repeated reuse and reducing long-term operational costs. This recyclability enhances its potential for industrial implementation.
"Our results demonstrate a pathway for coupling waste valorization with pollution control," the authors said. "By combining green chemistry principles with practical engineering, we can design materials that benefit both industry and the environment."
The team hopes future work will explore scaling up production and adapting similar biomass-derived catalysts for other environmental applications, including air purification, water treatment, and energy conversion technologies.
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Journal Reference: Zhao, F., Pan, Z., Wang, F. et al. Coffee grounds derived porous nitrogen-rich biochar as a metal-free catalyst for efficient selective oxidation of hydrogen sulfide to sulfur. Biochar 8, 20 (2026).
https://doi.org/10.1007/s42773-025-00541-4
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About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.
