Pr6O11/ZrO2 Photocatalytic Coating Removes 85.97% of Oxytetracycline
Oxytetracycline is one of the most heavily used antibiotics in livestock farming and human medicine, and it is also one of the most stubborn pollutants to get out of water. Most of the dose never gets absorbed. It passes straight through the body and lands in wastewater. And the treatment plants we have today were never built to catch it. This article walks through a practical fix: a Pr6O11/ZrO2 photocatalytic coating that breaks oxytetracycline down using nothing more than visible light.
Why oxytetracycline is hard to remove from wastewater
Only a small share of an antibiotic dose is actually used by the body. The rest is excreted, mostly unchanged, and ends up in rivers, lakes, and groundwater. That alone would be bad enough. Worse, antibiotic residues lingering in the environment push bacteria to develop resistance genes. The genes spread through ecosystems and climb the food chain, until human infections become harder to treat. So removing antibiotics from wastewater is not a niche concern. It is one of the few direct ways to slow down antimicrobial resistance.
Conventional treatment handles organic matter, nitrogen, and phosphorus quite well. Antibiotics are a different story. Their molecules are stable enough to survive most standard treatment steps, which is why removal in ordinary plants tends to be poor.
Photocatalysis takes another route. Light generates reactive species that tear pollutant molecules apart. It is efficient, inexpensive, and leaves nothing behind to dispose of. For those reasons it has become one of the most studied approaches to eliminating antibiotics from water.
Why zirconia needs modification before it can work
Zirconium dioxide, usually just called zirconia (ZrO2), looks like a natural photocatalyst. It is an n-type semiconductor, chemically inert, non-toxic, cheap, and reusable. Industry already uses it as a catalyst, a dielectric, a chemical sensor, and a photocatalyst support.
But there is a catch, and it is a big one. The bandgap is too wide. Pure ZrO2 sits at around 4.05 eV, so it can only absorb light below roughly 317 nm, which is the UV range. Sunlight reaching the ground contains only about 4 percent UV. That means unmodified zirconia wastes nearly the entire solar spectrum, and no one can afford that in real water treatment.
People have tried noble metal doping, up-conversion modification, and composite photocatalysts. The last two keep proving themselves in practice.
What up-conversion materials actually do
Up-conversion materials take in low-energy photons, visible or near-infrared light, and re-emit higher-energy photons. In effect, they push light energy up the spectrum. Pair one with a wide-bandgap semiconductor like ZrO2, and the semiconductor suddenly gets to use light it could never touch on its own.
Earlier work pointed the way. One group doped ZrO2 with cerium and erbium using a hydrothermal route and got a composite with a lower bandgap, stronger visible-light absorption, and better separation of photo-generated electrons and holes. Under visible LED light it performed impressively on methylene blue. Another group built a lanthanum-modified TiO2-ZrO2 composite that wiped out 10 mg/L of tetracycline in 120 minutes, because the lanthanum changed the surface chemistry and boosted both adsorption and photocatalytic activity.
What the Pr6O11/ZrO2 coating is and how it is made
The material covered here pairs zirconia with praseodymium oxide (Pr6O11), a rare-earth up-conversion material. The powder comes from an ultrasound-assisted co-precipitation route. Zr(NO3)4·5H2O and the surfactant CTAB dissolve in water, praseodymium nitrate goes in at different Pr3+:Zr4+ molar ratios, and sodium hydroxide solution is dripped in under stirring plus ultrasound until precipitation finishes. The solids are washed, dried, and calcined. That yields samples with Pr6O11 doping ratios from 0.15 to 0.75.
The powder then becomes a coating you can actually use. It is blended with water-based aliphatic polyurethane resin, a silane coupling agent, defoamer, thickener, film-forming aid, light calcium carbonate, and talc. The slurry spins at 150 r/min for an hour, gets filtered, and is brushed onto concrete. Three coats keep the layer even, and the total thickness stays below 1 mm. End result: a photocatalytic surface on ordinary building material, ready to sit in a concrete tank or channel.
What the characterization showed
XRD, HR-TEM, SEM, EDS, BET, and UV-Vis DRS were all brought in to check what the composite actually is and how it behaves optically.
XRD found the expected tetragonal ZrO2 peaks and separate Pr6O11 peaks, with the zirconia structure basically untouched by the modification. Crystallite sizes measured 28.52 nm for ZrO2 and 38.66 nm for Pr6O11. Here is a detail worth pausing on: the composite’s measured particle size, 58.68 nm, comes out smaller than the sum of its two parts. That is a classic sign that ZrO2 and Pr6O11 form a heterojunction. HR-TEM confirmed it directly, with lattice fringes from the (101) plane of ZrO2 and the (113) plane of Pr6O11 overlapping in the same image.
SEM shows ZrO2 as regular spheres while the composite turns blocky, since the praseodymium reshapes the microstructure. EDS detects only Zr, Pr, and O, in an atomic ratio matching a doping ratio around 0.45. The material is clean.
Surface area is where photocatalysis lives or dies. BET gave the composite 88.12 m2/g, up from 50.73 m2/g for pure ZrO2. Average pore size falls from 7.12 nm to 3.16 nm, and both materials count as mesoporous. More surface means more active sites exposed, and more active sites means faster oxytetracycline breakdown.
The optical results are where the up-conversion pays for itself. Pure ZrO2 absorbs nothing past 317 nm. The composite shows clear absorption across the visible region, with a strong red shift. Bandgap drops from 4.05 eV to 2.19 eV, and the usable spectral range extends to 625 nm. Translation: this coating runs on sunlight or an ordinary lamp. No UV rig required.
How well it degrades oxytetracycline
Every degradation test used visible light (xenon lamp, wavelength above 420 nm), with the coated concrete sitting in OTC solution. Before the lamp came on, the system stirred in the dark for 30 minutes to reach adsorption-desorption equilibrium. Plain adsorption accounted for about 5.35 percent removal.
With unmodified ZrO2 in the coating, removal was dismal, 11.36 percent, because the coating barely uses visible light. Add Pr6O11 and the picture flips. Screening runs went from 11.36 to 55.43 percent after modification, and at the best doping ratio, 0.45, removal hit 76.47 percent with a rate constant of 1.002 min-1.
Why 0.45 and not higher? Below it, there is too little Pr6O11 to soak up enough light. Above it, the extra praseodymium speeds up recombination of electron-hole pairs, which makes things worse. Doped photocatalysts almost always have this sweet spot.
Two more single-factor trends showed up. Raising the dosage from 0.4 to 1.4 g/L steadily improved removal and rate constant, because more coating absorbs more light, produces more photogenerated electrons, and generates more radicals. OTC concentration matters too: performance climbed as the initial level rose from 10 to 30 mg/L, then fell. At low concentrations the radicals rarely meet the pollutant. At very high concentrations, excess OTC stuck on the catalyst surface blocks the light.
The optimal conditions, found with response surface methodology
The single-factor results fed into a three-factor, three-level Box-Behnken design. The variables were Pr6O11 doping ratio, catalyst dosage, and initial OTC concentration; the response was removal percentage. A quadratic model fit the 17 runs cleanly, with a highly significant F value (168.87, p < 0.0001) and a non-significant lack of fit. The model can be trusted.
It predicted the best recipe as doping ratio 0.45, dosage 1.4 g/L, and OTC at 30.45 mg/L, for a theoretical maximum of 87.12 percent. Validation runs under those conditions delivered 85.97 percent. Prediction and experiment agree.
The response surfaces also say which lever matters most. The doping ratio has the steepest slope by far. It influences removal more than dosage or concentration, and that makes physical sense. The amount of Pr6O11 sets the bandgap, controls electron-hole separation, and therefore decides how many hydroxyl radicals (·OH) and superoxide radicals (O2·-) get produced.
Reusability and long-term durability
A photocatalyst earns its place in water treatment only if it keeps performing. The coated concrete came out of the reaction, got rinsed with methanol and ultrapure water, dried with nitrogen, and went back in. After six cycles, removal was still above 80 percent.
Durability got its own test. Coated samples were prepared and measured after 1, 10, and 20 days. At day 20, removal stayed above 80 percent. The credit goes to the composite’s stable crystal structure, which keeps the catalyst active through prolonged use. Easy recovery plus steady output is exactly what field work demands.
How the degradation actually happens
Radical trapping experiments named the active species. Isopropanol (IPA) scavenges hydroxyl radicals, 1,4-benzoquinone (1,4-BQ) scavenges superoxide radicals, and ammonium oxalate scavenges holes. Under optimized conditions, removal ran at 83.25 percent with no scavenger present, rate constant 1.278 min-1. Adding IPA or 1,4-BQ made removal fall sharply. Adding ammonium oxalate barely moved it (80.66 percent). Holes are bit players here. The workhorses are hydroxyl radicals and superoxide radicals.
LC-MS/MS on the reaction mixture sketched a plausible degradation path. Radical attack cleaves the C=C double bond in the oxytetracycline molecule first, forming an intermediate. The hydroxyl group oxidizes, the amino group’s carbon chain snaps off, the six-membered carbon ring opens, and further oxidation finally reduces the fragments to water and carbon dioxide. So the drug is not just adsorbed or stashed away. It gets dismantled, piece by piece, into harmless end products.
How it stacks up against other photocatalysts
Set against published oxytetracycline photocatalysts, this coating lands in an interesting spot. ZnO (74.45 percent), N-ZnO/p-BN (92.20 percent), TiO2 (95.02 percent), and BiYO3 (100 percent) all post higher peak removals. All of them need UV light, and all of them lose steam over 5-6 reuse cycles. The visible-light TiO2/CuFeO2 composite manages only 72.45 percent.
Pr6O11/ZrO2 delivers 85.97 percent under visible light, and its performance does not drop across six cycles. None of the UV-driven materials can claim that. There is a real trade-off, though: visible light carries less energy than UV, which caps the ceiling. The work itself flags an obvious next step, combining the coating with stronger UV irradiation where that is available.
Frequently asked questions
What is a Pr6O11/ZrO2 photocatalytic coating? A paint-like layer made by mixing a praseodymium oxide (Pr6O11) modified zirconia photocatalyst with polyurethane resin and additives, applied to concrete. Exposed to light, it produces reactive radicals that break down pollutants such as oxytetracycline in water.
Why is oxytetracycline pollution a problem? Most of the oxytetracycline given to humans or animals is excreted unmetabolized into wastewater. Antibiotic residues in the environment encourage resistance genes in bacteria, which spread through ecosystems and eventually make human infections harder to treat.
How does Pr6O11/ZrO2 work under visible light? Pr6O11 is an up-conversion material. It converts low-energy visible light into higher-energy photons zirconia can actually use. The modification narrows the bandgap from 4.05 eV to 2.19 eV and extends absorption to 625 nm, so ordinary visible light drives the reaction.
What removal rate does it achieve? Under optimized conditions (doping ratio 0.45, dosage 1.4 g/L, initial OTC 30.45 mg/L), the coating removes 85.97 percent of oxytetracycline, matching the model-predicted 87.12 percent.
Can the coating be reused? Yes. After six consecutive degradation cycles it still removes over 80 percent of oxytetracycline, and it stays above 80 percent even after 20 days of storage.
Which reactive species degrade oxytetracycline? Hydroxyl radicals (·OH) and superoxide radicals (O2·-). Trapping experiments show that holes (h+) contribute very little.
The bottom line
Oxytetracycline slips through conventional treatment, and plain zirconia cannot take it on because it only responds to UV. Combining ZrO2 with the up-conversion material Pr6O11 fixes both problems at once. The composite coating absorbs visible light up to 625 nm, removes up to 85.97 percent of oxytetracycline under optimized conditions, survives six reuse cycles without losing performance, and brushes onto ordinary concrete. It is not the highest-removal photocatalyst on paper. But it is one of the most practical: simple to make, sun-driven, easy to recover, stable over time. For a facility dealing with antibiotic-laden wastewater, that combination beats a headline removal number every time.
