Heavy metal pollution in farmland creates a problem that does not end when contaminated plants are harvested. Crop residues and plants used to remove metals from polluted soils can themselves contain substantial amounts of mercury, cadmium, lead, nickel and other potentially hazardous elements. If these materials are burned, landfilled or returned directly to farmland, the metals may be released again into the environment.
A new review published in Sustainable Carbon Materials suggests that slow pyrolysis could provide a more sustainable way to manage this contaminated biomass while also turning it into useful carbon materials, energy products and even recoverable metals.
"Heavy metal contaminated biomass should not simply be viewed as waste that needs to be disposed of. With appropriate thermal treatment and careful control of metal behavior, it may become a resource for producing functional carbon materials, recovering metals and generating energy," said corresponding author Yajun Wang.
Slow pyrolysis heats biomass in an oxygen limited environment, producing biochar, bio-oil and gases. The review emphasizes that different metals behave very differently as temperatures rise. Mercury begins to volatilize at relatively low temperatures, followed by cadmium, arsenic, lead and zinc, while metals such as nickel and copper tend to remain more strongly associated with the solid residue.
For example, experiments reviewed by the authors showed that at 550 °C, mercury removal from wheat straw and corn stalk biochar reached 94.0% and 93.2%, respectively. However, much of the released mercury entered the gas phase, highlighting the need for effective gas capture systems rather than assuming that thermal treatment alone eliminates the environmental risk.
The review also examines hydrothermal, or wet pyrolysis, which processes biomass in a heated water based environment. In these systems, metals can be redistributed between hydrochar and the liquid phase rather than simply volatilized. In one reported example, more than 96% of arsenic moved into the aqueous phase, demonstrating the potential for controlled metal separation and recovery.
Importantly, the products left after treatment may have applications far beyond waste disposal. The authors identify potential uses for pyrolysis derived carbon materials in soil amendment, water treatment adsorbents, catalysts, supercapacitor electrodes, metal recovery and energy production. Under controlled conditions, metals already present in contaminated plants may even serve as useful catalytic or electrochemical components rather than unwanted impurities.
The team also analyzed research published from 2001 to 2025 and found rapid growth in the field since 2017. Emerging themes include circular economy strategies, life cycle assessment, carbon capture, multifunctional materials and machine learning, suggesting that research is shifting from basic waste treatment toward integrated resource recovery and sustainable process design.
Despite its promise, the authors caution that pyrolysis is not automatically risk free. Future studies must better determine metal speciation, leaching behavior and long term stability, particularly for less studied contaminants such as arsenic and chromium. Large scale deployment will also require techno-economic analysis, lifecycle assessment, reliable control of metal emissions and clear safety standards for the resulting products.
The review ultimately presents slow pyrolysis as more than a disposal technology. It could become part of a circular strategy that connects contaminated land remediation with carbon material production, metal recovery and renewable energy generation.
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Journal reference: Wang Z, Min Q, He K, Wang R, Shang G, et al. 2026. Slow pyrolysis and product utilization of heavy metal-contaminated biomass from agricultural systems. Sustainable Carbon Materials 2: e028 doi: 10.48130/scm-0026-0024
https://www.maxapress.com/article/doi/10.48130/scm-0026-0024
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About Sustainable Carbon Materials :
Sustainable Carbon Materials (e-ISSN 3070-3557) is a multidisciplinary platform for communicating advances in fundamental and applied research on carbon-based materials. It is dedicated to serving as an innovative, efficient and professional platform for researchers in the field of carbon materials around the world to deliver findings from this rapidly expanding field of science. It is a peer-reviewed, open-access journal that publishes review, original research, invited review, rapid report, perspective, commentary and correspondence papers.
