A comprehensive new review published in Case Studies in Chemical and Environmental Engineering argues that the answer to this problem may lie in an element long overshadowed by its more famous rival. Advanced oxidation processes, which generate highly reactive chemical species capable of shredding organic molecules apart, have traditionally relied on iron, following the classic Fenton chemistry discovered in the late nineteenth century. But iron-based systems carry a punishing constraint: they work well only in acidic conditions, around pH 2.5 to 4, requiring costly acidification and neutralization steps and producing iron-laden sludge that must be disposed of. Copper, the review’s authors contend, offers a fundamentally more flexible alternative.

The chemistry behind this advantage is striking. Iron(III), the oxidized form that must be recycled back to iron(II) to keep the catalytic cycle turning, precipitates out of solution above pH 4 because of an extraordinarily low solubility product, roughly 4 × 10⁻³⁸. Copper(II), by contrast, remains soluble at neutral pH, with a solubility product about eighteen orders of magnitude higher. More importantly, the rate-limiting reduction step is dramatically faster for copper: copper(II) reacts with hydrogen peroxide at a rate constant of 4.6 × 10² M⁻¹s⁻¹, while the equivalent iron reaction crawls along at 9.1 × 10⁻⁷ M⁻¹s⁻¹. The result is a catalyst that maintains removal efficiencies above 80 percent across the entire pH range, where iron systems collapse once conditions turn neutral or alkaline.

Copper’s multivalent nature adds another dimension. Unlike iron, which cycles mainly between two oxidation states, copper can shuttle among three: Cu(I), Cu(II), and Cu(III). The Cu(I) reaction with hydrogen peroxide runs roughly four orders of magnitude faster than the iron equivalent, and Cu(III) itself is a formidable oxidant, with a redox potential of 1.7 to 2.4 volts, far exceeding the iron(III)/iron(II) couple. This triple-state flexibility allows copper catalysts to activate a whole family of oxidants, including peroxymonosulfate, peroxydisulfate, hydrogen peroxide, and peracetic acid, through both radical pathways involving hydroxyl and sulfate radicals and non-radical pathways dominated by singlet oxygen or direct electron transfer. Iron systems, by comparison, are overwhelmingly radical-driven.

That dual pathway capability is both a strength and a scientific puzzle. In some copper systems, radical scavengers quench nearly all degradation; in others, radicals contribute almost nothing. The dominant mechanism can shift with pH, moving from radical-dominated oxidation under acidic conditions to Cu(III)-mediated non-radical oxidation at neutral and alkaline values. Part of the confusion stems from detection limits: conventional electron paramagnetic resonance cannot distinguish Cu(III) from hydroxyl or sulfate radicals because their spin-trap signals overlap. The review highlights emerging diagnostic tools, including Raman spectroscopy of the Cu(III)-OH stretching band at 614 cm⁻¹, selective copper chelators, and ultraviolet-visible detection of Cu(III)-periodate complexes, and urges researchers to combine multiple lines of evidence rather than relying on any single method.

Catalyst design is advancing rapidly along several fronts. Simple copper salts are abundant and cheap, with municipal wastewater containing 5 to 10 milligrams per liter of soluble copper, but Cu(II) alone is sluggish at activating oxidants. Adding reductants or ligands helps: gallic acid boosted tetrabromobisphenol A removal from 40 to 80 percent in a Cu(II)/peroxymonosulfate system, while thiosulfate accelerated benzoic acid degradation nearly sixfold. Heterogeneous catalysts perform far better still. Copper oxide activated peroxymonosulfate achieved complete bisphenol A removal where soluble copper managed only 15 percent, and lower-valence Cu₂O outperformed CuO dramatically on similar targets. Bimetallic combinations amplify the effect further; adding iron(III) to zero-valent copper more than doubled acetaminophen degradation beyond the sum of the individual metals, a synergy attributed to denser active sites and faster interfacial electron transfer.

The most transformative platform, however, may be copper integrated with conductive carbon frameworks, particularly single-atom catalysts derived from metal-organic frameworks. When individual copper atoms are anchored within nitrogen-doped carbon matrices, they gain both electronic stabilization and exceptional catalytic versatility. Carbon-coated copper oxide achieved complete tetracycline removal in 40 minutes, double the performance of bare CuO, while a CuBTC metal-organic framework annealed at 300 degrees Celsius degraded bisphenol A completely within 30 minutes with copper leaching below 0.02 percent. Even the coordination geometry matters: Cu-N₄ sites tend to promote radical pathways, whereas unsaturated Cu-N₂ sites drove complete removal of 2,4-dichlorophenol sustained over 14 days. Computational studies using density functional theory now allow researchers to predict how heteroatom doping tunes copper’s electronic structure and oxidant adsorption, guiding rational design rather than trial and error.

Real-world performance, though, depends on messy details. Oxidant choice matters, with an empirical activity ranking of peroxymonosulfate and peracetic acid above peroxydisulfate and hydrogen peroxide, though the authors caution this trend is not universal. Combining oxidants can be surprisingly powerful: adding hydrogen peroxide to a Cu(II)/peroxymonosulfate system raised naproxen degradation rates 28-fold, because hydrogen peroxide’s lower redox potential thermodynamically favors the copper redox cycle. Dosage follows a Goldilocks pattern, since excessive oxidant triggers radical self-quenching and excess catalyst increases copper leaching. Water matrix constituents complicate matters further. Anions often scavenge radicals into less reactive secondary species, yet chloride can sometimes enhance degradation by forming copper-chloride complexes. Dissolved organic matter can inhibit oxidation by competing for reactive species, but at low concentrations it may actually stabilize copper redox cycling and act as an electron shuttle.

Safety remains the field’s central tension. Copper is a more potent inducer of oxidative stress in microorganisms than iron, and World Health Organization guidelines cap copper in surface water at 2 milligrams per liter, well below the 5 milligram-per-liter limit for iron. The review notes that most reported copper leaching falls below regulatory thresholds, with some catalysts releasing as little as 0.003 milligrams per liter over repeated cycles, but it insists compliance alone is insufficient evidence of safety. Chronic low-level release, cumulative accumulation, and shifts in copper speciation during long-term operation all demand scrutiny. Minimizing leaching, the authors argue, should be treated as a primary design criterion rather than an afterthought.

The path from laboratory to treatment plant is still long. Most copper-based systems have been tested only in short-term batch experiments with suspended powder catalysts, and the technology’s readiness level remains low to medium compared with fully proven iron Fenton processes. The review calls for integrated catalyst-reactor designs, including packed-bed, flow-through, and catalytic-membrane configurations that enable catalyst recovery and continuous operation, alongside techno-economic and life-cycle assessments to confirm genuine environmental benefit. It also urges researchers to look beyond parent-compound removal toward full mineralization, transformation-product identification, and toxicity testing, since partially oxidized byproducts can sometimes be as harmful as the original pollutants. If those challenges can be met, copper’s pH resilience, oxidant versatility, and rapidly maturing single-atom catalyst designs could position it as the backbone of next-generation water purification, turning an abundant, versatile metal into a decisive weapon against the micropollutants threatening global water security.

Subject of Research: Copper-based advanced oxidation processes for degrading organic micropollutants in water treatment

Article Title: Copper-based advanced oxidation processes for micropollutant degradation: A review of reaction mechanisms, modulation strategies, and future applications

Article References: Ly, Q. V., Nguyen, D. V., Quang, D. V., Kim, T.-H., Hwang, Y., Nguyen, T., Wu, D., & Hur, J. (2026). Copper-based advanced oxidation processes for micropollutant degradation: A review of reaction mechanisms, modulation strategies, and future applications. Case Studies in Chemical and Environmental Engineering, 14, Article 101495. https://doi.org/10.1016/j.cscee.2026.101495

Image Credits: AI Generated

DOI: 10.1016/j.cscee.2026.101495

Keywords: copper catalysts, advanced oxidation processes, micropollutants, water treatment, Fenton reaction, single-atom catalysts, sulfate radicals, Cu(III) oxidation, metal-organic frameworks, pharmaceuticals in water, antimicrobial resistance, wastewater reuse

The heart of the new work is graphitic carbon nitride, usually abbreviated g-C₃N₄, a metal-free polymeric semiconductor that has attracted intense interest since researchers discovered that simple organic precursors such as urea or melamine can be cooked into it by heating. The material is cheap, chemically robust, and absorbs enough visible light to run photocatalytic reactions under sunlight-like illumination. When photons strike the catalyst, electrons are promoted from the valence band to the conduction band, leaving positively charged holes behind. Both the electrons and the holes can, in principle, drive chemical reactions that shred organic pollutants. The catch is that pristine g-C₃N₄ is a poor conductor, and its light-excited electrons and holes recombine with each other far too quickly, releasing energy as heat before they can do useful chemistry. That recombination problem has long been the bottleneck keeping carbon nitride photocatalysts out of real-world water treatment.

The Chinese team, led by Zemin Zhu and Guodong Zheng, with Tiechui Yuan providing the concept and funding support, attacked the problem with a two-pronged strategy: doping and homojunction formation. First, they prepared two subtly different versions of carbon nitride. One, labeled CN, was made from urea. The other, labeled BCN, was derived from melamine and doped with boron atoms through a two-step calcination process. Boron is a clever choice of dopant because its electronic structure differs from that of carbon and nitrogen in the carbon nitride lattice. Substituting boron into the framework narrows the material’s band gap, which means less energy is needed to kick an electron into the conduction band. The practical consequence is a red-shifted light absorption edge: the doped material harvests a wider slice of the visible spectrum than its undoped counterpart, capturing photons that would otherwise go to waste.

Second, the researchers combined the two carbon nitride variants into a single composite, creating what materials scientists call a homojunction. Unlike a heterojunction, which joins two chemically different semiconductors, a homojunction interfaces two materials of the same chemical family but with slightly different electronic structures. Because the two sides are chemically compatible, they form intimate, well-matched interfaces without the lattice strain and recombination centers that can plague mismatched heterojunctions. The band alignment between CN and BCN gives photoexcited electrons and holes a built-in energetic incentive to separate, with one partner material preferentially hosting electrons and the other hosting holes. Separated charges live longer, travel further, and are far more likely to reach the catalyst surface and participate in pollutant-degrading reactions rather than simply annihilating each other.

The electrochemical measurements reported in the study confirm that the design works as intended. The CN/BCN homojunction exhibits low electrochemical impedance, meaning charges move through it with little resistance, and an optimal photocurrent response, a direct sign that light is being converted into separated, mobile charge carriers rather than being lost to recombination. The composite also shows an appropriate capability for capturing charge carriers, further extending the lifetime of the reactive electrons and holes. Each of these properties, on its own, would represent a modest improvement; together they compound into a catalyst that is substantially more photoactive than either of its parent materials.

The performance numbers are striking. When the CN/BCN catalyst was combined with potassium persulfate, a common and inexpensive oxidizing salt, and illuminated with visible light, tetracycline removal reached 98.6 percent within 60 minutes. The corresponding kinetic rate constant was 0.0748 per minute, a measure of how quickly the degradation reaction proceeds. Persulfate chemistry is central here. Persulfate ions are themselves fairly sluggish oxidants, but when they accept electrons from photoexcited catalysts they can be cleaved into sulfate radicals, extremely aggressive species that attack organic molecules indiscriminately. A photocatalyst that efficiently supplies electrons to persulfate therefore acts as an amplifier, converting a mild bulk oxidant into a torrent of destructive radicals right at the pollutant’s doorstep.

Using a combination of radical scavenging experiments, the team identified which reactive species actually do the work of degrading tetracycline in their system. Four culprits emerged: sulfate radicals (SO₄•⁻), hydroxyl radicals (•OH), superoxide radicals (•O₂⁻), and the photogenerated holes themselves. Interestingly, their relative contributions were not what a casual reading of persulfate chemistry might suggest. The ordering, from least to most important, was sulfate radical, then hydroxyl radical, then superoxide radical, with photogenerated holes contributing the most. That result underscores a key insight of the study: in a well-designed photocatalytic persulfate system, the direct oxidative power of the holes left behind in the catalyst’s valence band can rival or exceed the contribution of the radicals generated indirectly. It also suggests that strategies maximizing hole availability, such as the charge-separation engineered into the homojunction, pay double dividends.

Durability and real-world robustness are where many promising photocatalysts stumble, and the researchers addressed both concerns directly. Over five consecutive cycling tests, the CN/BCN system retained a tetracycline removal efficiency above 90 percent, indicating that the catalyst does not rapidly deactivate, foul, or lose its boron dopant under working conditions. Equally important, the team examined how the system performs in the presence of common coexisting anions, the dissolved salts such as chloride, nitrate, and carbonate species that are ubiquitous in natural waters and wastewater. These anions exerted only minor effects on photocatalytic performance, a significant finding because many advanced oxidation processes are notoriously sensitive to water chemistry, with scavenging by background ions sharply curtailing pollutant destruction in anything other than ultrapure laboratory water.

The broader significance of the work lies in its elegance and economy. Both the urea-derived and melamine-derived carbon nitrides are made from inexpensive commodity chemicals, and the two-step calcination strategy is a straightforward thermal process rather than an exotic synthesis requiring rare metals or complicated precursors. The entire catalytic system is metal-free, which avoids the leaching of toxic metal ions that complicates many other advanced oxidation catalysts. Because the homojunction is built from two members of the same material family, the approach is also conceptually transferable: the same logic of pairing a doped and an undoped carbon nitride, or two differently doped variants, could be applied to degrade other persistent organic pollutants, from dyes to pesticides to pharmaceutical compounds beyond tetracycline.

Challenges remain before such systems treat actual wastewater at scale, including reactor engineering, catalyst recovery, and performance under variable sunlight. But the study offers a clear demonstration that careful electronic-structure engineering, boron doping to widen light harvesting and a homojunction to keep charges separated, can transform a famously limited photocatalyst into a high-performance pollutant destroyer. As antibiotic resistance accelerates into a global health emergency, technologies that can inexpensively and reliably eliminate antibiotic residues from water are urgently needed. This boron-doped carbon nitride homojunction, coupled with persulfate and plain visible light, is a compelling step in that direction.

Subject of Research: Boron-doped graphitic carbon nitride homojunction photocatalysis for persulfate-activated tetracycline degradation

Article Title: Promoting tetracycline degradation in persulfate-based photocatalytic advanced oxidation processes by construction of B-doped g-C₃N₄/g-C₃N₄ homojunction

Article References: Zhu, Z., Zheng, G., Zhou, Y., Chen, L., Wu, Q., & Yuan, T. (2026). Promoting tetracycline degradation in persulfate-based photocatalytic advanced oxidation processes by construction of B-doped g-C₃N₄/g-C₃N₄ homojunction. Journal of Nanoparticle Research, 28(10), Article 261. https://doi.org/10.1007/s11051-026-06782-z

Image Credits: AI Generated

DOI: 10.1007/s11051-026-06782-z

Keywords: graphitic carbon nitride, photocatalysis, boron doping, homojunction, persulfate activation, sulfate radicals, tetracycline, antibiotic pollution, water treatment, advanced oxidation processes, charge carrier separation, visible light

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