Turning waste lignin into carbon materials that boost biogas production
New study finds lignin-based carbon materials help overcome ammonia inhibition in anaerobic digestion
image: Mitigating ammonia inhibition in anaerobic digestion with lignin-based carbon materials synthesized by hydrothermal carbonization
Credit: Shuaishuai Ma, Hongliang Wang, Xin Gao, Chuanfei Bian & Wanbin Zhu
Researchers in China have discovered an innovative way to improve the stability and efficiency of biogas production by using carbon materials made from lignin, a major byproduct of the paper industry. The study, published in Carbon Research, demonstrates that two lignin-derived materials, lignin hydrochar (LHC) and carbon quantum dots (CQDs), can effectively reduce ammonia inhibition, a long-standing challenge in anaerobic digestion systems.
Anaerobic digestion is a widely used process that converts organic waste into methane-rich biogas, a renewable energy source. However, when the concentration of ammonia in the system becomes too high, it can harm the microorganisms responsible for methane generation, leading to reduced gas yield and system instability. Finding sustainable methods to counter this inhibition is key to advancing waste-to-energy technologies.
The research team, led by Dr. Wanbin Zhu from China Agricultural University, synthesized LHC and CQDs from alkali lignin using a process called hydrothermal carbonization. This technique operates at relatively low temperatures and converts lignin into carbon-rich materials with unique surface and electrical properties. When added to anaerobic digesters, these materials helped microorganisms better tolerate high ammonia levels and restored methane production rates.
The study found that adding 1 gram per liter of carbon quantum dots increased methane output by 24 percent compared to ammonia-stressed systems without additives. Similarly, adding 3 grams per liter of lignin hydrochar raised methane production by over 30 percent. Further experiments showed that increasing the carbonization temperature of LHC to 250 degrees Celsius enhanced its redox capacity and electron transfer ability, leading to even higher methane yields.
“These lignin-based carbon materials acted as electron mediators that improved communication among microbial communities,” said Dr. Zhu. “They helped maintain the balance between different methane-producing pathways, allowing the system to keep running efficiently even under high ammonia conditions.”
Microbial and metabolic analyses confirmed that the carbon additives stimulated beneficial microbial groups, including those involved in breaking down cellulose and producing short-chain fatty acids. They also promoted the growth of methanogens such as Methanosarcina and Methanosaeta, which can tolerate higher ammonia levels. These microorganisms work together more effectively when electron transfer between them is improved, a process known as direct interspecies electron transfer (DIET).
The study highlights two distinct but complementary roles of the lignin-based materials. Lignin hydrochar primarily enhanced the redox environment and supported the formation of humic acid-like substances that facilitate electron transfer, while carbon quantum dots reduced charge transfer resistance and accelerated microbial metabolism. Together, they provided a stable biochemical environment for methane generation.
The findings not only offer a promising approach to solving ammonia inhibition but also open new possibilities for valorizing lignin, an abundant industrial waste. By converting lignin into functional carbon materials, this research supports both circular economy goals and renewable energy development.
“This approach turns a low-value waste product into a powerful tool for clean energy production,” Dr. Zhu added. “It provides a sustainable way to enhance biogas yield while reducing environmental impacts.”
The team believes that lignin-based carbon materials could be further optimized for large-scale biogas plants, helping improve waste treatment efficiency and boost renewable energy output worldwide.
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Journal reference: Ma, S., Wang, H., Gao, X. et al. Mitigating ammonia inhibition in anaerobic digestion with lignin-based carbon materials synthesized by hydrothermal carbonization. Carbon Res. 4, 20 (2025).
https://doi.org/10.1007/s44246-024-00184-3
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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.