Transforming lignite from polluting fuel to powerful tool for water cleanup
image: Rapid sulfonation of lignite for cadmium removal in wastewater and subsequent recycle into photocatalysts
Credit: Jing Hu, Bing Han, Clayton Butterly, Wei Zhang, Ji-Zheng He & Deli Chen
A pioneering study reveals that low-grade coal, known as lignite, can be effectively converted into a resource for environmental protection—offering solutions for wastewater cleanup and resource recycling. Researchers from the University of Melbourne, in collaboration with Deakin University and Yancheng Institute of Technology, have developed a fast and energy-efficient method to modify lignite, greatly boosting its ability to strip toxic cadmium from polluted water.
The team used a process called sulfonation, treating lignite with sulfuric acid at relatively low temperatures. This approach led to more than double the performance in cadmium removal compared to untreated lignite. By introducing sulfonic acid and carboxyl groups, the process increased the number of sites on lignite that bind cadmium, while also creating ultra-micropores for maximum adsorption. Tests confirmed that the modified lignite performs well even in acidic conditions, which is critical for treating water from mines and industrial facilities.
What makes this approach particularly sustainable is that the spent adsorbent, the lignite loaded with captured cadmium, does not become a hazardous waste. Instead, it is transformed into a valuable photocatalyst by converting the cadmium into cadmium sulfide nanoparticles directly on the lignite’s surface. This new material is capable of breaking down the dye methylene blue, a common water pollutant, with an 83 percent degradation rate under light exposure. Even after three rounds of use, the catalyst retained over 82 percent of its original efficiency.
Further analysis revealed exactly how the modified lignite captures cadmium from water: through a combination of surface complexation, ion exchange, and the introduction of highly acidic functional groups. The researchers also found that the new photocatalyst works mainly by generating powerful reactive oxygen species, such as superoxide and hydroxyl radicals, that break down pollutants rapidly under light.
The process developed by Hu and colleagues is not only technically effective but also cost-efficient. It uses simpler equipment and less energy than conventional approaches, potentially making lignite an attractive option for large-scale applications in developing regions and for industries seeking to manage both heavy metal pollution and carbon emissions.
Importantly, this method aligns with circular economy principles by transforming what was once a polluting fuel into both a cleanup agent and a recyclable resource for new products. The authors recommend further research into the long-term stability of the modified lignite and its real-world effectiveness in treating various wastewaters. They also call for studies on scaling up the process and ensuring its safety before widespread adoption.
This work demonstrates how creative chemical engineering and sustainable thinking can turn environmental problems into practical solutions for industry and society.
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Journal reference: Hu, J., Han, B., Butterly, C. et al. Rapid sulfonation of lignite for cadmium removal in wastewater and subsequent recycle into photocatalysts. Carbon Res. 4, 42 (2025). https://link.springer.com/article/10.1007/s44246-025-00204-w
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