The material itself is a study in green chemistry. The researchers cleaned and dried sweet flag plants, ground the biomass through a 100-mesh sieve, and pyrolyzed it in a limited-oxygen environment inside a muffle furnace, ramping the temperature to 600 degrees Celsius at 5 degrees per minute and holding it there for 90 minutes. To introduce the dopants, they soaked the powdered biomass in solutions of manganese chloride and urea, sonicated the mixture, stirred it for 24 hours, and then repeated the calcination step. By varying the relative amounts of biochar precursor, urea, and manganese salt, the team produced a family of catalysts with different doping ratios, ultimately identifying a 1:1:1 ratio of biochar to nitrogen to manganese as the optimum. The approach recycles waste biomass, sequesters carbon, and avoids the energy-hungry activation methods, such as ultraviolet lamps, heating, or electrochemical cells, that many persulfate systems demand.

Characterization revealed why the co-doped material works so well. Scanning electron microscopy showed that the original biochar has a rough, sheet-like surface, and that manganese and nitrogen modification roughens it further, decorating the layered carbon with stacked spherical Mn-N nanoparticles that multiply the available active sites. Energy-dispersive X-ray spectroscopy confirmed the presence of carbon, oxygen, manganese, and nitrogen, with manganese accounting for a remarkable 35.35 percent of the sample mass. X-ray diffraction identified manganese oxides in multiple oxidation states, including MnO2, MnO, and Mn3O4, while Fourier-transform infrared spectroscopy detected characteristic Mn-O bonds and C-NH-C linkages, confirming that both dopants were chemically incorporated rather than merely deposited on the surface.

Not every physical change was an improvement. Brunauer-Emmett-Teller measurements showed that the specific surface area of the biochar collapsed from 117.534 square meters per gram in the pristine material to just 7.232 square meters per gram after doping, a consequence of nitrogen species and manganese particles partially filling the micropores and mesopores. Nitrogen adsorption-desorption isotherms, showing combined type I and type IV behavior with H4 hysteresis loops, indicated that both micropores smaller than 2 nanometers and mesopores between 2 and 50 nanometers coexist in the material. The trade-off proved worthwhile: although adsorption performance declined, the doping introduced a wealth of catalytically active sites and structural defects, including pyridine nitrogen, graphitic nitrogen, and manganese oxide species, that more than compensated by dramatically accelerating persulfate activation.

X-ray photoelectron spectroscopy provided the most detailed picture of the catalytic machinery. The nitrogen 1s spectra revealed pyridine N, pyrrole N, and graphite N at binding energies of 398.10, 399.29, and 400.20 electronvolts, with pyridine nitrogen dominating at 52.58 percent before reaction. Manganese appeared as Mn(II), Mn(III), and Mn(IV), and the valence distribution shifted as the reaction proceeded: the fractions of Mn(II) and Mn(III) fell from 44.31 and 30.75 percent to 39.75 and 23.72 percent, while Mn(IV) rose to 36.52 percent. This valence climb is the fingerprint of electron transfer, in which the lower-valence manganese ions donate electrons to persulfate, cleaving its O-O bond and generating sulfate radicals as the metal centers oxidize toward higher states.

Performance testing under optimized conditions, 2 millimolar peroxydisulfate, 1.0 gram per liter of catalyst, and 20 milligrams per liter of tetracycline at pH 7, delivered a removal rate of 93.76 percent in 120 minutes, with a pseudo-first-order rate constant of 0.0199 per minute. The comparisons were telling. Peroxydisulfate alone did essentially nothing, plain biochar adsorbed 48.52 percent of the antibiotic, and singly doped catalysts paired with persulfate managed 68.00 percent for manganese-only and 60.65 percent for nitrogen-only materials. The co-doped system outperformed a comparable delta-MnO2/biochar persulfate system reported previously, which achieved 85.5 percent removal, and it did so through simple catalyst addition rather than the energy-intensive three-dimensional electrochemical activation used in some rival designs.

The team systematically probed the operational window. Increasing the peroxydisulfate concentration from 0.5 to 2 millimolar raised removal from 72.82 to 93.76 percent, but further additions plateaued because excess oxidant self-quenches the very sulfate radicals it generates. Raising the catalyst dose from 0.3 to 1.0 grams per liter improved degradation, though pushing to 2.0 grams per liter brought no further gain, likely because superfluous doped species compete with pollutants for radicals. Degradation efficiency exceeded 93 percent at low initial tetracycline concentrations and declined gradually as pollutant loading rose. Encouragingly, the system proved remarkably pH-tolerant, holding removal near 93 percent across the range from pH 3 to 9, with only strongly alkaline conditions at pH 11 dropping performance to 74.89 percent, a flexibility that matters for real wastewater streams of variable composition.

Electron paramagnetic resonance spectroscopy and radical quenching experiments untangled the chemistry driving the degradation. The EPR spectra captured three reactive species: sulfate radicals, hydroxyl radicals, and singlet oxygen, the latter a non-radical oxidant generated at structural defects and carbonyl groups on the nitrogen-doped carbon. Kinetic calculations from quenching tests using methanol, tert-butanol, L-histidine, and p-benzoquinone assigned contribution rates of 73.7 percent for superoxide radicals, 51.5 percent for singlet oxygen, 39.2 percent for hydroxyl radicals, and 13.4 percent for sulfate radicals, revealing that the non-radical and superoxide pathways, long underappreciated in persulfate chemistry, actually dominate. Liquid chromatography-mass spectrometry identified twelve intermediates and allowed the researchers to propose three degradation routes involving deamination, demethylation, dehydroxylation, ring cleavage, and ring opening of the tetracycline skeleton.

Durability and practicality rounded out the assessment. Across three consecutive reuse cycles the catalyst degraded 93.8, 86.0, and 80.3 percent of the antibiotic, retaining more than 80 percent of its initial activity, with the gradual decline attributed to leaching of manganese active centers, a known challenge for metal-carbon composites. In a proof-of-concept flow experiment, catalyst-loaded polyurethane sponges packed into a reaction column fed by a peristaltic pump removed 83.1 percent of tetracycline initially and stabilized around 69 percent after adsorption saturation. Computational toxicity prediction with the TEST software suggested that acute toxicity, measured as LC50 values against fathead minnow and the water flea Daphnia magna, generally decreased as tetracycline was converted into smaller intermediates, although the authors caution that these are model predictions rather than bioassays and that some fragments retain significant toxicity.

The study, published open access on 29 September 2026 and supported by China’s National Key Research and Development Program, arrives amid growing alarm over antibiotic residues in the environment and the resistance crisis they accelerate. Its significance lies less in any single number than in the demonstration that a cheap, plant-derived carbon, doped with two abundant elements and paired with a common oxidant, can rival far more elaborate treatment trains without external energy input. The authors are candid that the flow-through results and cycling stability represent early-stage proof of concept rather than an engineered solution, and that manganese leaching and residual toxic intermediates must be addressed. Even so, the work charts a credible path from laboratory beaker toward continuous water treatment, and it adds sweet flag, a wetland plant once prized in traditional medicine, to the growing roster of biomass feedstocks being reborn as environmental catalysts.

Subject of Research: Mn/N co-doped biochar-activated persulfate degradation of tetracycline in water

Article Title: Mn/N co-doped biochar-activated persulfate degradation of tetracycline in water

Article References: Cui, H., Li, L., Jia, Y., Xu, N., Wang, Z., Liu, Z., Zhang, J., Zhang, C., Gong, W., Feng, W., Shan, Y., & Xue, H. (2026). Mn/N co-doped biochar-activated persulfate degradation of tetracycline in water. Journal of Saudi Chemical Society, 30(5), Article 74. https://doi.org/10.1007/s44442-026-00113-6

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00113-6

Keywords: biochar, persulfate activation, tetracycline, advanced oxidation processes, manganese doping, nitrogen doping, Acorus calamus, antibiotic pollution, water treatment, singlet oxygen, sulfate radicals, catalyst reusability

The team set out to answer a question that matters enormously to growers of medicinal and aromatic plants: which drying strategy preserves the valuable chemistry of A. calamus while keeping energy bills and carbon footprints manageable? To do so, they cut rhizomes into slices of two distinct thicknesses, five millimetres and eight millimetres, and subjected each batch to four different drying treatments: shade drying, solar drying, conventional hot-air drying, and microwave drying. This factorial design allowed the researchers to disentangle the effects of slice geometry from the effects of heat delivery, a distinction that most earlier studies had blurred by testing only one size class at a time.

The energy results were striking. Microwave drying outperformed hot-air drying on every efficiency metric the team measured. The specific moisture extraction rate, which describes how many kilograms of water can be removed per kilowatt-hour of energy input, reached values between 0.559 and 0.746 kilograms per kilowatt-hour under microwave treatment. Correspondingly, the specific energy consumption of microwave drying fell to between 1.34 and 1.79 kilowatt-hours per kilogram of moisture removed, and the total electrical energy consumed per batch ranged from just 0.114 to 0.152 kilowatt-hours. Perhaps most importantly for commercial operations, microwave drying slashed the drying duration to a fraction of the time required by conventional methods, which rely on slow convective heat transfer from the surface inward.

Size mattered in ways that cut against intuition. In microwave drying, the smaller five-millimetre slices delivered better energy performance than their thicker counterparts, simply because they lost moisture faster and therefore spent less time drawing power. Yet the article highlights note a subtler pattern: larger rhizome slices exhibited higher effective moisture diffusivity and higher efficiency in certain analyses, reflecting the complex interplay between internal water migration, heat penetration depth, and the geometry of the plant tissue. This size sensitivity means that processors cannot simply scale recipes from one slice thickness to another; the drying kinetics, and the energy demand that follows from them, shift measurably with every change in cutting protocol.

To make sense of the drying curves, the researchers fitted their data to mathematical thin-layer drying models, and the Midilli–Küçük model emerged as the best description of the moisture-loss behaviour across treatments. Such models are more than academic exercises. They allow engineers to predict drying times under different conditions, optimize dryer design, and scale laboratory findings to industrial operations without repeating every experiment at full scale. The finding that a single model describes the behaviour across methods and sizes gives the industry a practical tool for planning microwave and conventional drying schedules for calamus rhizomes and, by extension, for similar dense rhizomatous materials.

Energy efficiency alone would be meaningless if the dried product lost the compounds that make it medicinally valuable. The team therefore turned to gas chromatography–mass spectrometry, or GC–MS, to profile the volatile and semi-volatile constituents of the dried rhizomes. The analysis identified 23 such compounds, and the drying treatments significantly reshaped their relative abundances. This is where the study delivers its most intriguing twist: gentle drying methods tended to preserve the monoterpenoid fraction of the volatile profile, while more intense heat treatments raised the relative levels of phenylpropanoids, the class of aromatic compounds that includes beta-asarone, the signature constituent of calamus oil.

This divergence has real pharmacological consequences. Monoterpenoids and phenylpropanoids contribute different aromatic and biological properties, and the balance between them determines the character and potency of the final product. A processor choosing shade drying will obtain a rhizome chemically closer to the fresh plant, rich in delicate monoterpenoid notes, whereas a processor using hot-air or microwave drying will steer the chemistry toward the phenylpropanoid-dominated profile. Neither outcome is inherently superior; the right choice depends on whether the target market values fresh-like aroma or the heavier, longer-lasting constituents. What the study makes clear is that drying is not a neutral preservation step but an active chemical intervention.

The mechanism behind these shifts lies in the volatility and thermal stability of the compounds themselves. Monoterpenoids are small, highly volatile molecules that evaporate readily during prolonged exposure to air and moderate heat, which explains why slow methods such as shade and solar drying, despite their long durations, can retain them better than expected, and why intense heat can strip them away. Phenylpropanoids, by contrast, are bulkier and less volatile, and some may even form or accumulate as heat drives off lighter neighbours or triggers mild oxidative transformations within the tissue. Similar drying-induced metabolomic shifts have been documented in other medicinal plants by the same research group, including ashwagandha and the Himalayan rhododendron Rhododendron anthopogon, suggesting a general principle at work across aromatic species.

For the medicinal plant industry, the practical message is twofold. First, microwave drying offers a compelling route to sustainability: it cuts energy consumption dramatically, shortens processing from hours or days to minutes, and requires only electrical power that can increasingly come from renewable sources. Second, the choice of slice thickness is a controllable lever that processors can use to fine-tune both energy demand and product chemistry. Combined with the growing toolkit of cultivation optimizations, the same group has previously published life-cycle and techno-economic analyses of hydroponic, aeroponic, and geoponic cultivation of A. calamus, the new findings support an integrated approach in which every step from field to finished rhizome is designed with both efficiency and chemistry in mind.

As global demand for plant-based medicines and natural aroma ingredients continues to climb, studies of this kind fill a critical gap between agronomy and pharmacology. They remind producers that the quality of a herbal product is decided not only in the field but in the dryer, and that seemingly mundane engineering choices, the thickness of a slice, the hum of a microwave magnetron, the angle of a solar collector, reverberate all the way into the molecular composition of the medicine. For Acorus calamus, a plant whose fragrant rhizomes have travelled through millennia of human use, the future of its post-harvest processing now looks faster, leaner, and considerably more precise.

Subject of Research: Effects of drying method and rhizome slice size on drying kinetics, energy consumption, and volatile compound profile of Acorus calamus

Article Title: Size-sensitive post-harvest drying behavior, energy demand, and volatile profile in Acorus calamus rhizomes

Article References: Nautiyal, P., Trivedi, V. L., Negi, A., Landi, M., & Nautiyal, M. C. (2026). Size-sensitive post-harvest drying behavior, energy demand, and volatile profile in Acorus calamus rhizomes. Food Science and Biotechnology. https://doi.org/10.1007/s10068-026-02309-3

Image Credits: AI Generated

DOI: 10.1007/s10068-026-02309-3

Keywords: Acorus calamus, microwave drying, hot-air drying, solar drying, shade drying, energy efficiency, post-harvest processing, volatile compounds, monoterpenoids, phenylpropanoids, GC-MS, medicinal plants