Sweet Flag Biochar Supercharges Persulfate to Destroy Antibiotic Pollution in Water

Tetracycline is one of the most heavily produced antibiotics on the planet, manufactured in thousands of tons every year, and much of it ends up where it should not be: in rivers, groundwater, and agricultural soils. Because the molecule is chemically robust, conventional biological and physicochemical treatment plants struggle to break it down, leaving residual antibiotic in effluents where it can fuel the spread of antibiotic resistance genes and resistant bacteria. A research team led by Haichen Cui of Nanjing Forestry University, working with collaborators at the China Institute of Water Resources and Hydropower Research and Nanjing Water Group, has now reported a strikingly simple answer. Writing in the Journal of the Saudi Chemical Society, they describe a manganese and nitrogen co-doped biochar, made from fresh sweet flag (Acorus calamus), that activates peroxydisulfate to degrade more than 93 percent of tetracycline in water within two hours at neutral pH.

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