A naturally formed water repellent coating on hydrochar could play an overlooked role in protecting carbon from chemical degradation, according to a new study examining hydrochar made from several common plant leaves. The findings suggest that hydrophobic alkyl carbon derived from plant waxes and cutin can act as a protective surface layer, potentially improving the ability of hydrochar to retain carbon in soil.
"The hydrophobic alkyl carbon coating may strengthen hydrochar stability in soil, which is expected to promote its capacity for soil carbon sequestration," the authors concluded.
Hydrochar is a carbon rich material produced by heating biomass in water under elevated temperature and pressure. Because this hydrothermal carbonization process can handle wet biomass without energy intensive drying, hydrochar is increasingly being explored for soil remediation, pollution control, and carbon sequestration. However, its long term environmental value depends strongly on how resistant its carbon is to degradation.
Researchers led by Jianping Fan and Fangfang Li investigated an often overlooked feature of hydrochar: a hydrophobic coating that can form on its surface during hydrothermal carbonization. They produced hydrochar from corn leaves, lotus leaves, palm leaves, and pine needles, then examined the chemical composition, surface characteristics, thermal behavior, and resistance to chemical oxidation before and after removing the coating.
The researchers found that the coating was closely linked to the natural cuticle that covers plant leaves. Lotus leaf hydrochar had the most hydrophobic coating, dominated by nonacosane-4,10-diol originating mainly from leaf wax. In contrast, coatings on hydrochar made from corn leaves, palm leaves, and pine needles were dominated by palmitic acid or 16-hydroxypalmitic acid, compounds associated with the breakdown of cutin.
Importantly, greater coating hydrophobicity was associated with higher levels of alkyl carbon in the hydrochar. When the coating was removed with acetone, alkyl carbon content declined while previously covered pores became exposed, increasing the material's accessible surface area.
Removing the coating produced a surprising contrast between thermal and chemical stability. Overall thermal stability changed little, because increases in the energy required for thermal decomposition were counterbalanced by increases in molecular reaction frequency.
Chemical stability told a different story. The coating acted as a physical and chemical barrier, covering reactive sites, blocking pores, and limiting contact between oxidizing agents and the underlying carbon. After the coating was removed, the carbon loss of lotus leaf, palm leaf, and pine needle hydrochars during chemical oxidation increased by 10.13% to 16.01%. The lotus leaf hydrochar, which had the strongest hydrophobic coating, showed the greatest loss of protection after coating removal.
The results also challenge the idea that bulk properties such as aromatic carbon content alone are sufficient to predict hydrochar stability. Surface coatings and their chemical composition can substantially influence how hydrochar responds to environmental oxidation.
The study provides new insight into how the original biological structures of plant materials can continue to influence carbon stability even after hydrothermal processing. Understanding these surface effects could help researchers select suitable biomass feedstocks and design more stable hydrochars for long term soil carbon storage and environmental applications.
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Journal reference: Fan J, Li F, Chen Q, Zeng P, Li Y, et al. 2026. Alkyl carbon in a hydrophobic coating enhances the chemical stability of hydrochar. Environmental and Biogeochemical Processes 2: e016 doi: 10.48130/ebp-0026-0012
https://www.maxapress.com/article/doi/10.48130/ebp-0026-0012
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About the Journal:
Environmental and Biogeochemical Processes (e-ISSN 3070-1708) is a multidisciplinary platform for communicating advances in fundamental and applied research on the interactions and processes involving the cycling of elements and compounds between the biological, geological, and chemical components of the environment.
