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 story begins with one of the most abundant and cheapest starting materials imaginable. Fresh Bauhinia variegata flowers were gathered from the campus of P P Savani University in Surat, washed thoroughly with tap water, double-distilled water and ethanol, and dried first in the sun and then overnight at about 100 degrees Celsius. The dried petals were ground into a fine powder and mixed with zinc chloride in a one-to-two weight ratio, then left for a full month to allow deep impregnation. Zinc chloride activation is a well-established chemical route to porous carbon: during subsequent high-temperature treatment it promotes dehydration, aromatization and the development of an extensive pore network throughout the carbon matrix.

Carbonization itself took place in a tubular furnace under a continuous flow of nitrogen gas. The temperature was ramped from room temperature to approximately 900 degrees Celsius at a controlled 5 degrees Celsius per minute and held there for thirty minutes, driving off oxygen-containing surface groups and locking in the porous architecture. After natural cooling, the blackened product was crushed, washed with concentrated hydrochloric acid to strip out residual activating agent and inorganic impurities, and rinsed repeatedly with hot deionized water until the filtrate reached a neutral pH. X-ray photoelectron spectroscopy confirmed the thoroughness of this purification: no zinc signal appears in the spectra, because at 900 degrees Celsius zinc species largely volatilize thanks to the element’s high vapor pressure, and any survivors were dissolved away during the acid wash.

The innovation that elevates this work beyond routine biomass carbonization is the manganese doping step. Rather than relying on simple soaking, the team dispersed the activated carbon in ethanol and added manganese(III) oxide corresponding to a 2 weight percent manganese loading, then subjected the suspension to 70 minutes of high-intensity probe sonication at 750 watts and 20 kilohertz in pulsed mode. Acoustic cavitation, the rapid formation and collapse of microscopic bubbles in the liquid, generates intense local temperature and pressure spikes that drive precursor particles into the carbon pores and anchor them to the surface with unusual uniformity. Ethanol was replenished periodically during treatment to compensate for evaporation, and the dried powder was subsequently annealed at 450 degrees Celsius under nitrogen for three hours to strengthen the metal-carbon interactions and stabilize the manganese species.

The spectroscopic fingerprints of the resulting Mn-doped activated carbon, abbreviated Mn-AC, tell a convincing story. High-resolution XPS of the C 1s region shows peak broadening relative to the pristine material, evidence of increased surface functionalization after manganese loading. The O 1s spectrum deconvolutes into components assigned to Mn-O-C bonding near 530.0 electron-volts, surface hydroxyl groups near 531.6 electron-volts, and highly oxygenated species near 533.5 electron-volts, reflecting the strong metal-carbon interactions forged during sonication. The Mn 2p region displays the characteristic 2p3/2 and 2p1/2 doublet, confirming coexisting Mn3+ and Mn4+ oxidation states on the carbon surface. Scanning electron microscopy revealed the complementary morphological shift: smooth, compact, plate-like structures in the pristine carbon give way to a roughened, granular, nodular texture after doping, with energy-dispersive X-ray mapping confirming that carbon, oxygen and manganese are distributed evenly across the framework without agglomeration.

With the material characterized, the team fabricated two symmetric electrochemical double-layer capacitor cells using a gel polymer electrolyte composed of poly(vinylidene fluoride-co-hexafluoropropylene) and the ionic liquid 1-ethyl-3-methylimidazolium tricyanomethanide. Electrode slurries containing 90 weight percent active material and 10 weight percent binder were drop-cast onto graphite sheets to a thickness of 50 to 80 micrometers, with a mass loading of roughly 1 milligram per square centimeter. Electrochemical impedance spectroscopy measured from 100 kilohertz down to 10 millihertz delivered the first clear verdict: the Mn-AC cell achieved a specific capacitance of 20 farads per gram at low frequency, compared with 15 farads per gram for the undoped carbon. The authors attribute the gain to increased density of polar surface functionalities and adsorption sites, improved electrolyte wetting and ion accessibility within the pores, and additional fast surface redox contributions from the manganese species.

Cyclic voltammetry reinforced the picture. After identifying an optimized window of minus one to plus one volt, the researchers swept scan rates from 5 to 100 millivolts per second. The Mn-AC cell produced CV profiles that were noticeably more rectangular than those of the pristine device, a hallmark of superior double-layer formation, and the calculated specific capacitances at 5 millivolts per second were 13.20 and 8.59 farads per gram respectively. Galvanostatic charge-discharge testing added the durability data: the pristine carbon delivered about 2 farads per gram at 1 milliampere with an energy density of 0.38 watt-hours per kilogram and a power density of 1160 watts per kilogram, while Mn-AC reached approximately 9.80 farads per gram, an energy density of 1.42 watt-hours per kilogram and a power density of 1020 watts per kilogram. Crucially, both cells remained roughly stable over 10,000 charge-discharge cycles.

The second act of the study is arguably the more striking. Counter electrodes are a critical and often expensive component of dye-sensitized solar cells, which conventionally rely on platinum. Here the researchers coated layers of AC and Mn-AC onto fluorine-doped tin oxide glass, paired them with titanium dioxide working electrodes sensitized with N719 dye, and sandwiched a previously reported PEO-KI/I2-EMImSCN polymer electrolyte between them. Under one-sun illumination from a solar simulator, the Mn-AC device nearly doubled the power conversion efficiency of the pristine-carbon cell, driven primarily by a substantial increase in short-circuit current density while open-circuit voltage and fill factor stayed comparable. The improvement points to enhanced electrocatalytic activity toward the iodide-triiodide redox reaction and faster charge transfer at the electrode-electrolyte interface, both direct consequences of manganese incorporation.

The broader context makes the result notable. Recent literature has seen explosive interest in biomass-derived carbons, from machine-learning-guided designs predicting surface areas near 2822 square meters per gram and capacitances around 322 farads per gram, to nitrogen- and phosphorus-doped porous carbons achieving energy densities approaching 49 watt-hours per kilogram. Flower-derived carbons in particular have proven versatile, serving as dye adsorbents, carbon dioxide capture media and aptasensor substrates. What this study adds is a demonstration that a modest, sonochemically delivered transition-metal dose can simultaneously upgrade a carbon’s capacitive behavior and its electrocatalytic performance, without sacrificing the porous framework or requiring scarce, costly noble metals. The authors caution that excessive manganese loading could block pores and hinder ion transport, an effect not specifically investigated here, and that no systematic optimization of manganese content was performed.

The ionic liquid electrolyte deserves its own mention. Unlike volatile, flammable organic solvents, ionic liquids are essentially non-volatile salts that remain liquid below 100 degrees Celsius, offering wide operating voltage windows and a highly ionized environment. The team’s earlier work identified EMImTCM as particularly well suited to both supercapacitor and dye-sensitized solar cell platforms, and pairing it with a PVdF-HFP gel matrix here produced a mechanically robust, safer electrolyte suited to flexible device concepts. Looking forward, the authors suggest that polymer-based electrolytes, tuned through further doping and composite formation, could push device performance well beyond what aqueous-electrolyte systems reported in most prior literature achieve. For now, the takeaway is simple and compelling: a tree that dots the streets and gardens of South Asia sheds flowers that, with a zinc chloride bath, a blast of nitrogen and seventy minutes of ultrasound, become a genuine dual-purpose energy material.

Subject of Research: Sonochemically manganese-doped activated carbon derived from Bauhinia variegata flowers for supercapacitor and dye-sensitized solar cell electrodes

Article Title: Sonochemically manganese doped activated carbon from Bauhinia variegata for dual energy application

Article References: Punetha, V. D., Dhapola, P. S., Singh, P. K., Matiyani, M., Singh, R., Kumar, S., & Pathak, V. (2026). Sonochemically manganese doped activated carbon from Bauhinia variegata for dual energy application. Discover Electrochemistry, 3(1), Article 71. https://doi.org/10.1007/s44373-026-00159-6

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00159-6

Keywords: activated carbon, Bauhinia variegata, manganese doping, sonochemical synthesis, supercapacitor, dye-sensitized solar cell, biomass-derived carbon, ionic liquid electrolyte, energy storage, electrocatalysis, XPS, gel polymer electrolyte