The research team, led by Rim Ismail, Sabah Mariyam, Mohammad Alherbawi, and Tareq Al-Ansari, framed their work as a waste-to-waste valorization strategy. Rather than treating sludge, farm residues, and food waste as disposal problems, they converted each into functional materials and then used one to upgrade another. Three feedstocks were compared: dried sewage sludge pellets supplied by Qatar’s Public Works Authority, mixed dried agricultural residues from a local farm, and fresh banana peels from a single purchase batch. Each material was ground, homogenized, and sieved to particles below 120 micrometers to eliminate heat and mass transfer limitations, then characterized through proximate and ultimate analysis, calorimetry, electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy.

The compositional differences among the feedstocks proved decisive. Banana peel stood out with the highest volatile matter content at 87.7 percent and the highest calorific value at 17.7 megajoules per kilogram, alongside a remarkably low ash fraction of just 7.52 percent. Sewage sludge, by contrast, contained 28.79 percent ash and only 31.10 percent carbon, yielding a modest heating value of 15.77 megajoules per kilogram. X-ray diffraction revealed that sludge was dominated by crystalline quartz, calcite, and calcium phosphate phases, while banana peel showed a broad amorphous hump typical of cellulose-rich biomass. Energy-dispersive spectroscopy also detected a notable potassium signal of 3.06 percent on the banana peel surface, a detail that would later prove central to the catalytic story.

Biochars were produced in a fixed-bed quartz reactor at 600 degrees Celsius under two atmospheres: inert nitrogen and reactive carbon dioxide. The carbon dioxide atmosphere enables in-situ activation through the Boudouard reaction, in which carbon reacts with carbon dioxide to form carbon monoxide, enlarging micropores and introducing oxygenated surface groups. Sewage sludge produced the highest biochar yields, reflecting its inorganic content, while agricultural residues produced the lowest, consistent with their readily decomposed cellulose and hemicellulose fractions. Thermogravimetric analysis showed that under carbon dioxide, the lignocellulosic feedstocks suffered sharp additional mass loss above roughly 700 degrees Celsius, whereas the mineral-rich sludge char resisted high-temperature conversion. All single-step biochars, however, exhibited disappointingly low surface areas below 4.2 square meters per gram, underscoring the limits of pyrolysis alone.

To unlock real porosity, the team applied a two-step chemical activation. Each biochar was impregnated with a one molar potassium carbonate solution at a one-to-three biochar-to-activator weight ratio, dried, and then heated to 700 degrees Celsius for two hours under nitrogen. Critically, the researchers skipped the acid-washing step commonly used to demineralize activated carbons, deliberately retaining potassium species within the carbon matrix. The result was a family of K-modified activated biochars rather than fully demineralized carbons. Scanning electron microscopy revealed dramatic transformations: the activated banana peel biochar developed a honeycomb-like network of thin walls and interconnected pores, while sludge-derived samples remained fragmented but mineral-dominated.

The headline result came from gas adsorption measurements. Potassium carbonate activation catapulted the surface area of carbon-dioxide-derived banana peel biochar from 3.17 to 311.9 square meters per gram, a hundredfold increase, with t-plot analysis confirming that micropores accounted for roughly 90.8 percent of that area. X-ray diffraction showed near-complete amorphization of the material, and X-ray photoelectron spectroscopy revealed an enhanced surface potassium signal. Yet the other feedstocks barely responded; sludge and agricultural residue biochars gained only a few square meters per gram, because their mineral contents hindered activator penetration and promoted side reactions. The banana peel material, BPBK-CO2, was selected for catalytic testing alongside its non-activated counterpart, BPB-CO2.

The proof-of-concept application targeted hydrothermal liquefaction of sewage sludge, a process that converts wet biomass into biocrude at 250 to 370 degrees Celsius and elevated pressures without the energy penalty of drying. In a 500-milliliter batch reactor, 30 grams of sludge were processed with 170 milliliters of water at 310 degrees Celsius for 45 minutes, reaching autogenous pressures near 100 bar in subcritical water. Biochar catalysts were added at 5 weight percent of the dry feedstock. The catalyst-free baseline produced a biocrude yield of 25.75 percent. Adding the non-activated banana peel biochar raised this to 37.88 percent, while the activated, high-surface-area version yielded 31.35 percent, a striking inversion that challenged the assumption that porosity governs catalytic performance.

The explanation lay in potassium. Inductively coupled plasma analysis showed the non-activated biochar contained 189.3 milligrams of potassium per gram, more than double the 79.6 milligrams per gram in the activated material. During liquefaction, aqueous-phase potassium rose 3.56-fold over baseline with the non-activated char and 2.03-fold with the activated one, and biocrude yield tracked the aqueous potassium concentration almost perfectly. The researchers propose that dissolved alkali species promote hydrolysis, depolymerization, and decarboxylation of sludge macromolecules while suppressing the repolymerization of reactive intermediates into solids, with surface-associated potassium providing complementary basic sites. Consistent with this mechanism, carbon dioxide in the aqueous phase increased dramatically, from 17.47 percent of the headspace profile at baseline to over 56 percent with either biochar, signaling enhanced decarboxylation.

Biocrude quality improved across the board. The higher heating value rose from 30.15 megajoules per kilogram at baseline to 31.13 with the non-activated char and 31.86 with the activated char, both within the typical range for liquefaction biocrudes. Oxygenated compounds in the oil decreased from 43.18 percent to roughly 36 percent with both catalysts, a shift that could reduce hydrogen demand during downstream hydrodeoxygenation and improve storage stability. Gas chromatography-mass spectrometry also revealed a strong shift toward lighter compounds: the fraction below C8 grew from 41.75 percent at baseline to 57.75 to 64.15 percent with the biochars. Notably, the activated biochar retained a larger share of the biocrude in the C8 to C16 range, the hydrocarbon window most relevant to jet fuel, suggesting activation moderated over-cracking by limiting potassium release.

The study’s authors are careful to frame these results as a paired proof of concept rather than a comprehensive screening, and they acknowledge open questions, including the need for soluble-potassium and potassium-free controls, quantification of potassium leaching, catalyst reuse trials, and full elemental mass balances. Nevertheless, the implications are compelling. The best catalyst for maximizing biocrude yield was not the most porous one but the one richest in a naturally occurring alkali metal, meaning that the cheapest, least-processed waste char may outperform its engineered counterpart depending on the process objective. For a world drowning in sludge and food waste, the idea that banana peels discarded after breakfast could help transform a city’s sewage into liquid fuel is a vivid demonstration of circular bioenergy, where the byproducts of one waste stream become the key that unlocks the value of another.

Subject of Research: Waste-derived potassium-modified activated biochars as catalysts for sewage-sludge hydrothermal liquefaction

Article Title: Comparative preparation of waste-derived K-modified activated biochars and proof-of-concept application in sewage-sludge hydrothermal liquefaction

Article References: Ismail, R., Mariyam, S., Alherbawi, M., & Al-Ansari, T. (2026). Comparative preparation of waste-derived K-modified activated biochars and proof-of-concept application in sewage-sludge hydrothermal liquefaction. Case Studies in Chemical and Environmental Engineering, 14, Article 101493. https://doi.org/10.1016/j.cscee.2026.101493

Image Credits: AI Generated

DOI: 10.1016/j.cscee.2026.101493

Keywords: biochar, hydrothermal liquefaction, sewage sludge, banana peel, potassium carbonate activation, biocrude, pyrolysis, waste valorization, catalysis, renewable energy, BET surface area, circular economy

Hydrothermal liquefaction, or HTL, is an attractive technology because it uses hot, pressurized water to transform wet feedstocks such as algae, manure, and sludge into a liquid fuel precursor. The process, however, leaves behind an aqueous phase that is anything but benign. This wastewater carries a complicated cocktail of dissolved organic compounds, including phenolics, nitrogen heterocycles, carboxylic acids, and sugars, along with abundant nutrients such as nitrogen and phosphorus. That combination makes HTL aqueous phase simultaneously a nuisance and a prize. The nutrients are valuable enough to justify recovery, yet the chemical heterogeneity and toxicity of the stream make detoxification and nutrient reclamation expensive and technically difficult. The researchers note that these challenges have been a persistent barrier to making HTL commercially viable at scale.

Fungi have long intrigued environmental engineers because of their extraordinary metabolic breadth. White-rot basidiomycetes such as Trametes versicolor are famous for decomposing lignin, one of the most recalcitrant plant polymers on Earth, using an arsenal of oxidative enzymes. That same enzymatic versatility makes them promising candidates for degrading or transforming the recalcitrant compounds found in industrial waste streams. But promise is not the same as practice. To harness fungal bioremediation reliably, scientists need to understand which genes actually underpin tolerance to toxic wastewater. Without that genetic map, efforts to breed or engineer more resilient strains have been largely a matter of trial and error.

To build that map, the Illinois team, led by Rachel E. J. Dalke and corresponding author Laurie B. Leonelli, challenged cultures of Trametes versicolor with three distinct HTL aqueous phase samples, each with its own chemical fingerprint. They then performed transcriptomic analysis, sequencing the messenger RNA present in the fungal cells to quantify how thousands of genes changed their expression in response to each wastewater. The design was deliberate: by testing the fungus against multiple, chemically different aqueous phases, the researchers could distinguish genes that respond to one specific compound from genes that form a universal stress program, activated no matter what particular toxins are in the water.

The results revealed a striking pattern of convergent response. Across all three conditions, the fungus upregulated genes encoding NAD(P)-binding proteins, alpha/beta-hydrolases, aldo/keto reductases, and cytochrome P450s. Each of these protein families plays a coherent role in a chemical defense strategy. NAD(P)-binding proteins participate in redox reactions that help cells manage oxidative stress and regenerate reducing power. Aldo/keto reductases chemically reduce reactive aldehydes and ketones, compounds that are notorious for damaging proteins and DNA. Alpha/beta-hydrolases hydrolyze a wide range of esters and related bonds, effectively disassembling foreign molecules. And cytochrome P450s, the celebrated detoxification enzymes found across all kingdoms of life, catalyze oxidative transformations that make hydrophobic toxins more soluble and easier to modify or excrete.

In other words, the fungus does not simply endure the wastewater; it mobilizes an integrated biochemical toolkit that detects, transforms, and neutralizes the offensive chemistry. The transcriptomic data were paired with measurements showing concomitant changes in the biochemical composition of the aqueous phases themselves, evidence that the fungal activity was actually altering the wastewater rather than merely surviving in it. This pairing of gene expression with chemical change is what elevates the study from a catalog of stress genes to a demonstration of active biotransformation. The authors emphasize that these upregulated gene families represent prime candidates for future functional validation, the experimental work needed to confirm exactly what each gene contributes to tolerance and degradation.

The concept of a universal stress response is the study’s most consequential finding. Because the same gene families were induced regardless of which aqueous phase the fungus encountered, Trametes versicolor appears to possess a general-purpose program for coping with chemically heterogeneous toxic streams. For engineers, that is encouraging news. It suggests that a strain optimized for tolerance might perform robustly across the variable feedstocks and operating conditions that any real HTL facility would face, rather than requiring bespoke solutions for every batch of wastewater. It also provides a blueprint for engineering: if researchers can enhance the expression or activity of these core gene families, they may be able to create fungal strains that detoxify and reclaim nutrients from diverse and hostile wastewaters far more efficiently than wild-type organisms.

The implications extend beyond biofuel refineries. Industrial wastewater around the world contains persistent pollutants that are difficult to treat and pose significant risks to ecosystems, and conventional treatment plants struggle with exactly the kind of chemically complex, low-concentration contaminant mixtures that white-rot fungi evolved to dismantle. A validated fungal platform for HTL aqueous phase could therefore serve as a template for treating phenolic wastes, antibiotic residues, and other recalcitrant streams. At the same time, recovering nitrogen and phosphorus from wastewater aligns with growing interest in circular nutrient economies, where wastes from energy production become feedstocks for fertilizer or further bioprocesses rather than disposal liabilities.

The path from gene candidates to engineered strains remains a long one, and the authors are appropriately measured about it. Transcriptomics reveals which genes are switched on; it does not prove what each protein does in vivo, and functional validation through gene knockouts, heterologous expression, and enzymatic assays will be the crucial next step. The study was supported by the U.S. Department of Agriculture’s National Institute of Food and Agriculture through a Hatch Act award, reflecting its relevance to agricultural and biological engineering. But the conceptual advance is clear and timely. As the world seeks to scale up renewable fuels, the waste streams they generate cannot be ignored. What this research shows is that the solution to a very modern industrial problem may be written into the genome of a fungus growing quietly on a fallen log, waiting for the right combination of chemistry to switch on its ancient, adaptable defense machinery.

Subject of Research: Transcriptional stress response gene candidates in Trametes versicolor exposed to hydrothermal liquefaction aqueous phase wastewater

Article Title: Transcriptional reprogramming in Trametes versicolor reveals gene candidates involved in universal stress response to hydrothermal liquefaction aqueous phase

Article References: Dalke, R. E. J., Reynolds, L., Jing, Q., Davidson, P. C., Zhang, Y., & Leonelli, L. B. (2026). Transcriptional reprogramming in Trametes versicolor reveals gene candidates involved in universal stress response to hydrothermal liquefaction aqueous phase. Biotechnology for Biofuels and Bioproducts. https://doi.org/10.1186/s13068-026-02813-y

Image Credits: AI Generated

DOI: 10.1186/s13068-026-02813-y

Keywords: bioremediation, fungi, Trametes versicolor, hydrothermal liquefaction, wastewater treatment, transcriptomics, stress response, nutrient reclamation, cytochrome P450, white-rot fungus, biocrude, gene expression