Hydrochar is produced through hydrothermal carbonization, a process in which biomass is heated in water at elevated temperature and pressure. Unlike many thermal conversion technologies, the process can treat wet agricultural and plant residues without energy-intensive drying. This makes hydrochar attractive for converting organic waste into materials that could be used in soil improvement, pollution control, and carbon sequestration. Yet the environmental value of hydrochar depends on whether its carbon remains intact after being placed in soil. Microbial activity, dissolved oxidants, and other chemical reactions can gradually transform carbon-rich materials, releasing part of their carbon back into the environment. Identifying the structural features that slow this degradation is therefore critical for predicting hydrochar performance.

Researchers Jianping Fan, Fangfang Li, and colleagues investigated hydrochar made from four common plant materials: corn leaves, lotus leaves, palm leaves, and pine needles. Their experiments focused on the surface coating that forms during hydrothermal carbonization and on the possibility that this layer originates from the plant cuticle, the natural protective covering found on leaves and needles. The team analyzed the chemical composition and surface properties of the resulting hydrochars, examined their thermal behavior, and tested their resistance to chemical oxidation. They also treated samples with acetone to remove surface compounds and then compared the altered materials with hydrochar that retained its original coating.

The researchers found that the coating reflected the distinctive chemistry of each plant’s cuticle. Lotus leaf hydrochar possessed the most strongly hydrophobic surface and was dominated by nonacosane-4,10-diol, a long-chain compound associated mainly with leaf wax. The coatings formed on corn leaf, palm leaf, and pine needle hydrochars contained greater proportions of palmitic acid or 16-hydroxypalmitic acid, compounds linked to the breakdown of cutin. Cutin is a tough, wax-like polymer embedded in plant surfaces that helps limit water loss and protects tissues from environmental stress. During hydrothermal carbonization, portions of these waxes and cutin-related molecules appear to survive or reorganize, creating an alkyl-rich layer over the newly formed carbon material.

This surface chemistry had important consequences for how the hydrochar interacted with water and how much of its carbon was exposed to chemical attack. Higher hydrophobicity was associated with greater amounts of alkyl carbon, a class of carbon compounds characterized by chains of carbon and hydrogen. When the researchers extracted the coating with acetone, the alkyl carbon content decreased and pores that had previously been covered became accessible. The treatment therefore did more than remove a few molecules from the outside of the hydrochar. It changed the interface between the material and its surroundings, increasing the area available for water, dissolved chemicals, and oxidizing agents to reach reactive carbon sites.

The coating’s protective effect became most apparent during chemical oxidation tests. It appears to operate through two complementary mechanisms. Physically, the hydrophobic layer partially blocks pores and reduces direct contact between oxidants and the carbon underneath. Chemically, its long-chain compounds cover or surround reactive surface groups that would otherwise be vulnerable to oxidation. Once the coating was removed, carbon losses from lotus leaf, palm leaf, and pine needle hydrochars increased by between 10.13 and 16.01 percent. Lotus leaf hydrochar showed the greatest reduction in protection after extraction, consistent with its initially stronger hydrophobic coating and higher concentration of wax-derived alkyl carbon.

The findings also revealed that thermal stability and chemical stability do not necessarily change in parallel. Removing the surface coating produced little overall change in the hydrochars’ resistance to thermal decomposition. The researchers explain that two opposing effects may have balanced each other: the energy required to initiate decomposition increased, but the frequency of molecular reactions also increased. In other words, the altered material may have required more energy for individual decomposition events while simultaneously undergoing those events more readily. This result is important because thermal analysis is often used as a convenient indicator of carbon stability, even though it may not accurately reproduce the chemical conditions hydrochar encounters in soil.

Soil degradation is governed by a complicated mixture of processes, including oxidation, microbial metabolism, moisture movement, pore diffusion, and interactions with minerals. A coating that limits access to reactive carbon could therefore have a major influence even if it represents only a small fraction of the total material. The study suggests that two hydrochars with similar bulk carbon content or aromaticity might behave very differently in the environment if one retains a wax- and cutin-derived surface layer while the other does not. This challenges the assumption that measurements of bulk aromatic carbon alone are sufficient to forecast how long hydrochar will persist. Surface composition, pore accessibility, and water repellency may be equally important indicators.

The results could influence how researchers select feedstocks and design hydrochars for long-term soil carbon storage. Plant residues with naturally waxy or cutin-rich surfaces may produce hydrochar with stronger protective coatings, although the final properties will also depend on processing temperature, pressure, residence time, and the chemistry of the surrounding water. Preserving the coating during washing, transport, and application may become an additional consideration. At the same time, the coating could affect other functions, such as water absorption, nutrient exchange, contaminant binding, and interactions with soil microorganisms. More research will be needed to determine how these properties evolve over months or years in real soils, where physical abrasion and microbial activity may gradually remove or transform the hydrophobic layer.

By showing that plant-derived surface chemistry can persist into hydrothermal carbon products and influence their resistance to oxidation, the study adds a new layer to the science of carbon sequestration. Hydrochar is not simply an inert block of carbon produced from biomass; it is a chemically structured material whose environmental behavior may preserve clues about the plant from which it originated. The authors’ results indicate that alkyl carbon in a hydrophobic coating can act as a protective barrier, helping hydrochar resist chemical degradation and potentially retain more carbon after soil application. If confirmed under field conditions, this overlooked surface effect could help scientists develop more reliable methods for producing stable hydrochar from wet biomass and turning agricultural residues into longer-lasting carbon stores.

Subject of Research: Hydrophobic surface coatings, alkyl carbon, and the chemical stability of hydrochar produced from plant biomass

Article Title: Alkyl carbon in a hydrophobic coating enhances the chemical stability of hydrochar

News Publication Date: 17-Aug-2026

Web References: https://doi.org/10.48130/ebp-0026-0012

References: 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

Image Credits: Jianping Fan, Fangfang Li, Qingkong Chen, Peiwen Zeng, Yanlin Li, Wei Chen, Qiangbin Yang and Hong Yang

Keywords

Hydrochar, hydrothermal carbonization, carbon sequestration, soil carbon, hydrophobic coating, alkyl carbon, plant waxes, cutin, chemical oxidation, environmental stability, biomass conversion, lotus leaves, carbon storage

Kentucky produces an astounding 95% of the world’s bourbon whiskey, a process that generates substantial amounts of stillage—spent grains left after distillation. The volume of this byproduct is staggering; for every barrel of bourbon made, six to ten barrels of stillage remain. Traditionally, this sticky, water-rich waste has been sold as livestock feed or soil fertilizer. However, the challenges of transportation and drying costs have long posed logistical and economic hurdles for distilleries aiming to manage this biomass.

Enter hydrothermal carbonization (HTC), a technique analogous to pressure cooking that converts wet biomass directly into carbon-rich materials. By applying this high-pressure, high-temperature process to stillage, the research team transformed this unwieldy waste into a dry, fine, black carbon powder. This is a critical step, as carbon-based materials are fundamental components in fabricating electrodes for supercapacitors—a class of devices known for rapid energy storage and release.

The conversion process involved subjecting the stillage to HTC in a reactor capable of handling large volumes, ensuring scalability beyond laboratory trials. Following this, the carbon powder was further processed through pyrolysis, heating it to temperatures around 200 degrees Celsius to produce hard carbon. Alternatively, a higher temperature treatment at 800 degrees Celsius with potassium hydroxide (KOH) activation produced activated carbon known for its highly porous structure. These two distinct carbon forms offer complementary electrochemical properties suitable for different supercapacitor designs.

Hard carbon exhibits a disordered layered structure that facilitates lithium-ion intercalation, essential for lithium-ion hybrid supercapacitors. Activated carbon, with its extensive internal surface area due to its porous nature, excels in electric double-layer capacitors (EDLCs). These characteristics make the stillage-derived carbons uniquely suited for developing next-generation energy storage devices that combine high energy density with rapid charge-discharge cycles.

For proof-of-concept, the team constructed coin-sized supercapacitor cells by sandwiching liquid electrolytes between pairs of activated carbon electrodes. Remarkably, these devices demonstrated energy storage capabilities on par with commercial supercapacitors, reaching up to 48 watt-hours per kilogram. This performance metric places the stillage-derived materials as competitive alternatives in the energy storage market, with the added benefit of valorizing industrial waste.

Taking innovation further, the researchers engineered hybrid lithium-ion supercapacitors by pairing a lithium-ion infused hard carbon electrode with an activated carbon electrode. These hybrid devices marry the high power density and durability of capacitors with the superior energy storage of lithium-ion batteries. The stillage-derived hybrid supercapacitors exhibited energy densities up to 25 times greater than conventional counterparts, marking a substantial leap in sustainable energy technology.

Beyond just material development, this research underscores a novel circular economy model where an agricultural byproduct is repurposed for advanced technological applications. The interdisciplinary team collaborated extensively with distillery owners across Kentucky, Illinois, and Canada, ensuring a steady supply of raw material while fostering industry-academic synergies that could facilitate real-world implementation.

Comprehensive physicochemical characterization confirmed the suitability of these carbons for energy storage applications. Techniques such as Raman and Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS), and nitrogen physisorption elucidated the structural and chemical properties critical for electrochemical performance.

Electrochemical testing involved cyclic voltammetry, galvanostatic charge-discharge profiling, and electrochemical impedance spectroscopy, providing in-depth insights into charge storage mechanisms and device efficiency. The activated carbon electrodes exhibited excellent stability, retaining 96% of their capacitance over 15,000 charge-discharge cycles, a testament to their durability and potential longevity in practical applications.

Looking ahead, the research team plans to delve deeper into optimizing the energy storage mechanisms, scaling up device dimensions, and refining electrode fabrication techniques. Such advancements could pave the way for integrating these supercapacitors into electrical grids, particularly to stabilize fluctuating inputs as renewable energy sources become increasingly prevalent.

Economic and life cycle assessments are underway to evaluate the commercial viability and environmental impact of deploying this technology at industrial scales. Early findings suggest that transforming bourbon stillage into high-performance energy storage materials could reduce waste management costs for distilleries while contributing to greener, more sustainable battery and capacitor production.

This innovative project not only addresses a pressing problem at the state level but also signals a wider paradigm shift in how agricultural waste streams are valorized. Collaborations with international partners, including the Friedrich Schiller University Jena in Germany, highlight the global relevance of such sustainable technological solutions.

Funded by the U.S. National Science Foundation and the University of Kentucky, this work was presented at the spring 2026 meeting of the American Chemical Society (ACS), drawing attention from a broad audience of chemists, materials scientists, and energy engineers. The compelling fusion of waste valorization and cutting-edge energy storage underscores the transformative potential of chemistry to enable sustainable advances.

As society increasingly prioritizes circular economy principles and renewable energy integration, the ability to convert industrial residues like bourbon stillage into value-added carbon materials could become a cornerstone of sustainable technology development. The University of Kentucky’s breakthrough exemplifies how regional resources can be leveraged for global impact, turning what was once waste into a powerhouse of energy innovation.

Subject of Research: Bourbon whiskey waste-derived carbons for supercapacitors

Article Title: Bourbon whiskey waste-derived carbons for electric double layer and Lithium-Ion supercapacitors

News Publication Date: March 25, 2026

Web References:
https://acs.digitellinc.com/live/36/page/1271

Image Credits: Josiel Barrios Cossio

Keywords

Bourbon stillage, hydrothermal carbonization, supercapacitors, activated carbon, hard carbon, lithium-ion supercapacitors, energy storage, waste valorization, sustainable materials, electrochemical performance, circular economy, Kentucky bourbon industry

Published in the esteemed open-access journal Carbon Research on September 17, 2025, this groundbreaking research explores how hydrochar, a carbon-rich material derived from hydrothermal carbonization of sewage sludge, can be chemically modified to optimize phosphorus availability to plants. Phosphorus—the critical nutrient underpinning healthy plant growth—remains one of the most challenging elements in agricultural management. Global reserves of phosphate rock, the primary source for conventional fertilizers, are depleting rapidly, while inefficient phosphorus application contributes to environmental degradation via eutrophication. The novel approach presented shifts focus: phosphorus is no longer merely a fertilizer supplement, but a carefully controlled nutrient delivery system engineered at the molecular level.

Hydrothermal carbonization, conducted by heating sewage sludge to 260°C for two hours in an aqueous environment, produces hydrochar—a stable, carbon-dense solid with soil amending properties. The revelation in this research lies in the strategic conditioning of these hydrochars with divalent salts of calcium or magnesium prior to the carbonization process. By incorporating calcium oxide (CaO), calcium chloride (CaCl₂), magnesium oxide (MgO), or magnesium chloride (MgCl₂), researchers have effectively ‘reprogrammed’ the phosphorus forms within the hydrochar, creating two distinct phosphorus profiles tailored for different agricultural needs.

Calcium modification encourages the formation of slow-release, highly crystalline phosphate minerals, predominantly hydroxyapatite and chlorapatite. These mineral phases act as phosphorus reservoirs, releasing nutrients gradually into the soil environment and thereby supporting sustained soil fertility. Quantitative analyses indicated that these minerals increased substantially, by approximately 48.6% to 86.3%, relative to untreated sludge. This slow nutrient release paradigm facilitates long-term soil restoration and carbon sequestration, simultaneously addressing nutrient cycling and climate resilience.

Conversely, magnesium-conditioned hydrochars, particularly those prepared with MgO, show a propensity for generating rapidly soluble phosphorus forms such as Mg₃(PO₄)₂. Though the total increase in phosphorus content ranges from 0 to 50.7%, the bioavailability of this phosphorus markedly enhances, providing plants with a swift nutrient boost. This trait is especially advantageous during initial crop growth phases or in nutrient-depleted soils, where immediate phosphorus accessibility directly translates to improved photosynthetic efficiency and biomass accumulation.

The precision of these phosphorus delivery systems was demonstrated through meticulous pot experiments with mung beans (Vigna radiata). Utilizing the advanced Diffusive Gradients in Thin-films (DGT) technique allowed the real-time assessment of bioavailable phosphorus dynamics in soil-plant interfaces. Hydrochars modified with magnesium salts notably accelerated plant growth metrics, including chlorophyll concentration and photosynthetic rate, underscoring the immediate utility of the soluble phosphorus released.

Intriguingly, the influence of these hydrochar modifications extends beyond nutrient availability to reshape the soil microbial community. Calcium-based hydrochars fostered the enrichment of bacterial taxa such as Skermanella and RB41, genera known for their roles in organic matter degradation and mineral nutrient cycling. These microbial shifts underpin a longer-term enhancement of phosphorus mobilization from soil organic pools. Meanwhile, magnesium hydrochars selectively augmented populations of phosphorus solubilizing bacteria like Pseudomonas and Bacillus, further reinforcing the fast-release nutrient effect through increased biological mediation.

This dual-path strategy for phosphorus management heralds a paradigm shift in sustainable agriculture. Instead of a one-size-fits-all fertilizer product, the nuanced application of calcium or magnesium hydrochars allows precise tailoring of fertilizer regimes to crop developmental stages and soil health status. Employing calcium-based hydrochars aligns with goals of soil ecological restoration and carbon storage, delivering phosphorus gradually for extended fertility. Alternatively, magnesium-enriched hydrochars serve immediate crop nutrient demands, providing a timely and biologically supported phosphorus pulse.

This research exemplifies the transformative potential of interdisciplinary collaboration, bridging environmental engineering, soil chemistry, and microbial ecology. The National Engineering Laboratory for Advanced Municipal Wastewater Treatment and Reuse Technology at Beijing University of Technology, alongside the Key Laboratory of Marine Environment and Ecology at Ocean University of China, synergize expertise to convert waste into a resource of immense agricultural value. This work not only closes the nutrient loop but also creates a blueprint for integrated circular economy strategies in agronomy.

As Dr. Wei Guo of Beijing University of Technology aptly summarizes, “We are not merely recycling phosphorus; we are redesigning its bioavailability and synchronizing it with plant life cycles.” Meanwhile, Dr. Xiaohui Liu from Ocean University of China highlights the soil microbiome’s central role: “This system orchestrates a symbiotic relationship between soil microbes and plants, amplifying the bioavailable phosphorus in a self-sustaining manner.”

Looking beyond the scientific intricacies, the implications for global food security and environmental health are profound. With phosphate rock reserves declining and environmental concerns mounting, transforming sewage sludge into smart fertilizers signifies an ingenious and ecologically responsible solution. It leverages an abundant waste stream to reduce dependency on finite mineral resources and minimizes damaging runoff effects associated with traditional fertilizers.

This novel approach suggests a future where agriculture operates within natural biogeochemical cycles, enhanced by advanced chemical engineering and microbial ecology insights. In this emerging framework, fertilizer production is decentralized, waste valorization becomes standard practice, and nutrient management is adaptive and finely tuned to ecosystem dynamics.

With ongoing advancements, the vision of sustainable, circular agriculture grows more tangible. The pioneering work of these Chinese research teams paves the way for further development and widespread adoption, promising large-scale agricultural productivity gains coupled with responsible environmental stewardship.

In summary, these calcium and magnesium-modified hydrochars redefine phosphorus fertilization. They offer a smart, multifaceted tool for farmers, environmentalists, and scientists seeking a world where agricultural inputs are efficient, sustainable, and integrated within broader ecological cycles. By literally turning sewage into soil gold, this innovation exemplifies how science can propel a greener, more resilient future one pellet at a time.

Subject of Research: Not applicable

Article Title: Soil–plant-microbial evidence for the available phosphorus generation and utilization of Ca/Mg salts conditioned hydrochar from sewage sludge

News Publication Date: 17-Sep-2025

Web References:

• Carbon Research Journal
• Article DOI Link

References:
Zhao, Q., Guo, W., Zhu, Y. et al. Soil–plant-microbial evidence for the available phosphorus generation and utilization of Ca/Mg salts conditioned hydrochar from sewage sludge. Carbon Res. 4, 64 (2025).

Image Credits: Qian Zhao, Wei Guo, Yuhan Zhu, Dongyue Li, Xiaohui Liu, Minda Yu, Dongyang Li, Xiang Gao, Xishi Tai & Jun Li

Keywords

Sewage sludge; Hydrothermal carbonization; Calcium/magnesium salts; Phosphorus species; Plant growth; Microbial community