Turning brown coal into a sustainable water clean‑up and recycling tool
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
Researchers from the University of Melbourne have discovered a sustainable new use for lignite, also known as brown coal, showing how it can clean up toxic heavy metals from wastewater and then be reused as a light‑driven catalyst for breaking down pollutants.
The study, published in Carbon Research, reveals a two‑step process that transforms this low‑grade coal from a pollutant source into a tool for environmental restoration. The team led by Jing Hu, Deli Chen, and collaborators in Australia and China developed a rapid “sulfonation” method that uses sulfuric acid at moderate temperature to modify the surface chemistry of lignite. In only half an hour, the treatment doubled the material’s capacity to capture cadmium—a highly toxic metal that contaminates industrial and agricultural water sources.
Detailed analyses showed that sulfonation introduced new oxygen‑ and sulfur‑based functional groups and created ultrafine pores that trapped cadmium ions through both chemical bonding and ion exchange. Under optimized conditions, the modified lignite removed more than 30 milligrams of cadmium per gram, outperforming many conventional adsorbents while requiring less energy to produce.
Rather than treating the used materials as hazardous waste, the researchers repurposed the cadmium‑laden adsorbents into photocatalysts by converting the captured metal into cadmium sulfide (CdS) nanoparticles on the lignite surface. When exposed to light, the recycled material degraded over 80 percent of methylene blue dye—a common test pollutant—within two hours and retained more than 82 percent of its activity after three reuse cycles.
Microscopic and spectroscopic tests revealed that its high efficiency came from the uniform distribution of CdS particles and the generation of reactive oxygen species that broke down the dye molecules. This “waste‑to‑catalyst” innovation not only prevents cadmium from leaking back into the environment but also turns an industrial liability into a source of clean‑up power.
“Our approach creates a circular pathway where a low‑value fossil resource becomes part of the solution to water contamination,” said lead author Jing Hu. “It offers dual benefits—removing heavy metals from wastewater and reusing the material for further pollution control.”
The findings suggest that lignite, often viewed as an environmental challenge, could play a surprising role in sustainable remediation technologies, helping industries move closer to both cleaner water and zero‑waste goals.
===
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://doi.org/10.1007/s44246-025-00204-w
===
About Carbon Research
The journal Carbon Research is an international multidisciplinary platform for communicating advances in fundamental and applied research on natural and engineered carbonaceous materials that are associated with ecological and environmental functions, energy generation, and global change. It is a fully Open Access (OA) journal and the Article Publishing Charges (APC) are waived until Dec 31, 2025. It is dedicated to serving as an innovative, efficient and professional platform for researchers in the field of carbon functions around the world to deliver findings from this rapidly expanding field of science. The journal is currently indexed by Scopus and Ei Compendex, and as of June 2025, the dynamic CiteScore value is 15.4.