A new study shows that coal mine waste, often seen as an environmental problem, can actually help create stronger, longer-lasting biochar that locks carbon in soil and supports cleaner environments. The research, published in Carbon Research, reveals that adding low-carbon coal gangue to crop residues during biochar production can significantly increase the carbon stability of the resulting material while reducing the release of dissolved organic matter (DOM), which influences greenhouse gas emissions and pollutant movement in soil.

Biochar, a charcoal-like product made by heating plant matter under low-oxygen conditions, is widely recognized for improving soil health and storing carbon for centuries. However, its long-term stability and chemical behavior depend strongly on how it is made. The new study led by Min Chen and colleagues from Anhui University of Science and Technology and Zhejiang University explored how mixing agricultural straw with coal gangue during pyrolysis affects the resulting biochar’s carbon structure and chemical resilience.

The researchers produced biochar and coal gangue–straw co-pyrolysis biochar at temperatures of 300, 450, and 600 degrees Celsius. They discovered that the addition of coal gangue enhanced the thermal and chemical stability of the biochar by up to 68 percent and reduced the release of DOM by as much as 55 percent compared with biochar made from straw alone. “Coal gangue acts as a mineral shield that protects unstable carbon from decomposing,” said lead author Min Chen. “It helps form more condensed and graphitic carbon structures even at lower temperatures.”

The study found that co-pyrolysis at 450 degrees Celsius produced a biochar with similar aromatic and graphitic carbon characteristics as the one produced at 600 degrees, effectively lowering the temperature required for stable biochar production. This temperature reduction can save energy and make large-scale biochar applications more sustainable. The team also discovered that the silica, alumina, and iron oxides naturally present in coal gangue work together to strengthen the biochar’s structure. Among them, iron oxide showed the strongest effect in stabilizing dissolved organic matter, while alumina and silica contributed to the formation of stable organo-mineral complexes.

In addition to its role in carbon storage, reducing DOM release is important for environmental protection. DOM from unstable biochar can promote microbial activity that increases greenhouse gas emissions and helps move heavy metals through soil and water. By suppressing this release, coal gangue, modified biochar could make carbon sequestration strategies safer and more effective. “This approach not only recycles coal mine waste but also creates a high-value material that contributes to both waste management and climate mitigation,” explained co-author Shengyong Lu of Zhejiang University.

The findings suggest a promising pathway for integrating industrial byproducts into sustainable carbon technologies. Coal gangue, typically used for low-value applications such as road construction, can be transformed into a useful additive that enhances the quality of biochar for agricultural and environmental use. The study also offers insights into designing future co-pyrolysis systems using silica- and alumina-rich waste materials to optimize carbon retention.

“This research provides essential guidance for turning solid waste into a climate-friendly resource,” said senior author Xiaoyang Chen. “By understanding the synergy between minerals and carbon during pyrolysis, we can develop more stable biochars for soil improvement, carbon sequestration, and pollution control.”