A research team has found that a naturally formed, water-repellent coating helps hydrochar resist chemical oxidation, revealing how plant leaf chemistry could influence the durability of carbon-rich materials intended for soil applications. The coating contains alkyl carbon derived from leaf waxes and cutin, with its composition varying among plant species. Removing it increased carbon loss during laboratory oxidation tests for hydrochar made from lotus leaves, palm leaves, and pine needles, while leaving thermal stability largely unchanged. These findings could guide biomass selection and processing strategies that preserve protective surface chemistry, potentially improving hydrochar's usefulness for soil carbon storage.
Hydrothermal carbonization converts biomass into hydrochar using heated, pressurized water, offering a way to process wet organic materials without energy-intensive predrying. Hydrochar has attracted interest for soil remediation, pollution control, and carbon sequestration, but its relatively low aromatic carbon content generally makes it less stable than biochar produced through pyrolysis. Previous research showed that compounds from plant cuticles can dissolve during processing and subsequently form hydrophobic surface coatings. However, how differences in these coatings affect hydrochar’s resistance to heat and chemical oxidation remained unclear, limiting understanding of their contribution to carbon persistence.
A study (DOI: 10.48130/ebp-0026-0012 ) published in Environmental and Biogeochemical Processes on 17 August 2026 by Fangfang Li's team, Chongqing University of Arts and Sciences, shows that coatings derived from plant cuticles protect leaf-based hydrochar against chemical oxidation, with protection influenced by coating composition and hydrophobic intensity, while coating removal produces no significant change in overall thermal stability.
The researchers compared hydrochars derived from corn leaves, lotus leaves, palm leaves, and pine needles, using cellulose-derived hydrochar as a control without a hydrophobic coating. Hydrothermal treatment was conducted at 260 °C for five hours, and acetone extraction enabled comparisons between coated materials and their extracted counterparts. Gas chromatography–mass spectrometry identified coating compounds, while spectroscopy, elemental analysis, microscopy, and surface-area measurements characterized the materials. Thermogravimetric analysis assessed thermal stability, and potassium dichromate oxidation measured chemical resistance through carbon loss. The team also tested representative hydrophobic compounds to examine the relationship between molecular water repellency and oxidation resistance. Chemical analysis revealed that the lotus-derived coating was dominated by nonacosane-4,10-diol, a wax-derived compound, whereas coatings on the other leaf hydrochars mainly contained palmitic acid or 16-hydroxypalmitic acid associated with cutin breakdown. An index combining compound abundance and molecular hydrophobicity ranked the lotus coating highest, although this index did not directly predict measured surface wettability. Removing the coatings reduced alkyl carbon content by 15.27%–23.44% and exposed pores, increasing accessible surface area. Thermal measurements showed no significant change in overall stability: increases in the energy barrier to decomposition were offset by increases in a kinetic factor governing reaction frequency. Oxidation tests, however, revealed a clear protective effect. Carbon loss increased by 10.13%–16.01% after extraction for lotus-, palm-, and pine-derived hydrochars, with the largest increase occurring in the lotus material; corn-derived hydrochar showed no significant change. Tests of individual compounds further linked greater molecular hydrophobicity with stronger oxidation resistance. Together, the results indicate that the coating helps shield reactive surfaces and restrict oxidant access, while its alkyl-rich components themselves resist chemical attack. After extraction, differences in carbon loss among the four leaf-derived hydrochars were no longer significant, supporting the coating's role in explaining their contrasting chemical stability under the laboratory conditions used in this comparative experimental study.
The study identifies surface coating chemistry as an important consideration alongside bulk carbon structure when evaluating hydrochar stability. Preserving naturally occurring hydrophobic compounds could offer a route toward more oxidation-resistant materials for soil applications. However, the findings come from laboratory tests across a limited set of feedstocks; long-term soil studies are needed to establish whether this protection translates into greater carbon retention under environmental conditions.
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References
DOI
Original Source URL
https://doi.org/10.48130/ebp-0026-0012
Funding information
This work was supported by the National Natural Science Foundation of China (42267028, 42130711, 42007352, and 32101394), the Chongqing Natural Science Foundation (CSTB2022NSCQ-MSX1107), and the Project of Chongqing University of Arts and Sciences (P2021HH04).
About Environmental and Biogeochemical Processes
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.
Environmental and Biogeochemical Processes
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Alkyl carbon in a hydrophobic coating enhances the chemical stability of hydrochar
17-Aug-2026
The authors declare that they have no competing interests.