Leather Waste Turned Into a Sunlight-Driven Catalyst That Breaks Down Drug Pollutants

Researchers have transformed vegetable-tanned leather scraps from a tannery into a porous carbon support for a hybrid photocatalyst that degrades two common pharmaceutical pollutants under natural sunlight. The material, a zinc oxide-doped titanium dioxide/carbon nanocomposite, removed 97 percent of sulfasalazine and 96 percent of losartan potassium from 10 ppm solutions within 120 minutes of sunlight exposure, according to a study published in the journal Collagen and Leather. The work addresses two problems at once: the substantial solid waste generated by leather processing, and the growing presence of biologically active pharmaceutical residues in freshwater systems around the world.

The leather industry produces large volumes of trimmings, shavings and buffing dust whose disposal poses an environmental challenge because of their complex composition and sheer quantity. Existing management strategies, including chromium recovery from tanned residues, do not fully exploit the available biomass. The research team, led by Ravichandran Sushmitha and Palanisamy Thanikaivelan, instead used a thermochemical route: vegetable-tanned leather shredding was first heated to 600 degrees Celsius under nitrogen for four hours to form a pre-carbon, then chemically activated with potassium hydroxide and heated again at 800 degrees Celsius for two hours. The resulting treated carbon had a high BET surface area of 444.46 square metres per gram and retained nitrogen from the collagen-rich protein structure of the leather, yielding intrinsically nitrogen-doped carbon with 67.8 percent carbon content by elemental analysis.

The photocatalysts were synthesized through a sol-gel method in which controlled amounts of zinc oxide sol were incorporated into a titanium dioxide/carbon sol matrix. The researchers systematically varied the TiO2:ZnO mass ratio from 1:0.015 to 1:0.075, deliberately keeping zinc oxide loading low to maintain titanium dioxide as the dominant photoactive phase while enabling heterojunction formation. Higher zinc oxide contents were avoided because of their reported tendency to induce particle agglomeration and charge-carrier recombination. The final products, labelled CTZ1 through CTZ5 with increasing zinc oxide content, were dried at 100 degrees Celsius and calcined at 500 degrees Celsius for two hours under nitrogen to induce crystallinity while preserving the carbon framework.

Characterization revealed how zinc oxide incorporation modified the material’s electronic and structural properties. X-ray diffraction confirmed the anatase phase of titanium dioxide and, in the best-performing sample, the wurtzite phase of zinc oxide. Band-gap energy decreased gradually from 3.31 to 3.23 electronvolts as zinc oxide content rose, compared with 3.34 electronvolts for pristine titanium dioxide, indicating that even small zinc oxide fractions effectively modulate the electronic structure. The absorption edge shifted from 325 nanometres for the carbon-titanium composite to 357 nanometres for CTZ5, extending light absorption toward the visible region. High-resolution transmission electron microscopy showed crystalline lattice fringes with interplanar spacings of about 0.352 and 0.289 nanometres, corresponding to the anatase (101) plane and the zinc oxide (002) plane, with an average particle size of 8.79 nanometres.

Photocatalytic performance was tested on both analytical-grade compounds and commercial tablet formulations, including Saaz-DS sulfasalazine tablets and Losawyn losartan potassium tablets, to better simulate practical contamination scenarios. Degradation experiments ran for 120 minutes under either a 200-watt lamp filtered to remove ultraviolet radiation or natural sunlight, at native solution pH values between 6.9 and 7.9, with all experiments performed in triplicate. The degradation followed pseudo-first-order kinetics, and the highest rate constant of 0.0396 per minute was achieved for pure sulfasalazine under sunlight. Across all conditions, the CTZ5 nanocomposite consistently outperformed treated carbon alone. In a comparison against a zinc oxide-titanium dioxide composite made without carbon, the carbon-free material managed only 66.6 percent degradation of pure sulfasalazine and just 37.8 percent of pure losartan potassium, underscoring the carbon matrix’s role in enhancing charge separation, light harvesting and adsorption of pharmaceutical molecules.

To identify which reactive species drove the degradation, the team ran radical scavenger experiments and electron paramagnetic resonance spin-trapping studies. Adding benzoquinone, a superoxide scavenger, caused the most dramatic suppression: at 270 micromolar, sulfasalazine removal dropped to 18.9 percent. Isopropanol, which scavenges bulk hydroxyl radicals, had only a marginal effect, reducing removal to about 84.6 percent. Potassium iodide and sodium azide produced intermediate suppression, implicating valence-band holes and surface-adsorbed hydroxyl radicals as secondary contributors. The EPR measurements, taken without any pharmaceutical present, detected the characteristic four-line signal of the DMPO-superoxide spin adduct in methanol dispersions after ten minutes of sunlight exposure, while no distinct hydroxyl radical signatures appeared in aqueous dispersions. Together, these results establish superoxide radicals as the principal reactive oxygen species in the system.

Electrospray ionization mass spectrometry of reaction aliquots revealed the degradation pathway. The initial step involves superoxide-assisted reductive cleavage of the azo bond in sulfasalazine, yielding two key intermediates: 4-nitro-N-(2-pyridinyl)benzenesulfonamide and sulfapyridine. The sulfonamide intermediate then undergoes denitration and desulfonation to form p-nitrobenzenesulfonic acid and benzenesulfonic acid, followed by progressive oxidation and ring-opening toward small oxygenated products such as dihydroxyacetone. Notably, the superoxide-dominated pathway differs from hydroxyl-radical-driven mechanisms reported in earlier studies of sulfasalazine degradation. Total organic carbon analysis showed partial rather than complete mineralization: the initial TOC value of 4.46 milligrams per litre fell to 2.78 milligrams per litre after 120 minutes, corresponding to roughly 36 percent mineralization, consistent with the persistence of resistant aromatic and sulfonamide-containing intermediates.

The catalyst’s practical performance was further probed in a synthetic wastewater matrix containing representative inorganic ions and natural organic matter, including chloride, bicarbonate, sulphate, calcium, magnesium and humic acid. Degradation efficiencies dropped from 97 to 82 percent for sulfasalazine and from 96 to 56 percent for losartan potassium under sunlight. The decline was attributed primarily to scavenging of reactive oxygen species by dissolved ions, competitive adsorption on catalyst active sites, and light attenuation and surface fouling by humic acid. Losartan potassium proved more susceptible to this interference than sulfasalazine, with its rate constant falling from 0.0296 to 0.0075 per minute. The authors note that evaluating photocatalysts under such realistic conditions is essential for assessing practical applicability, and that the catalyst retains appreciable degradation capability that could be optimized further for real wastewater treatment.

Reusability testing revealed both strengths and limitations. Over three consecutive cycles treating fresh 10 ppm sulfasalazine solutions, removal declined from approximately 92 percent in the first cycle to about 61 percent by the third, while the required degradation time lengthened from roughly 80 to 140 minutes. X-ray diffraction of the recovered catalyst showed no significant alteration in crystalline phase composition, and electron microscopy confirmed that the overall morphology remained largely unchanged. However, surface elemental analysis revealed a drastic drop in surface carbon content from 44.95 to 11.05 percent after three cycles, accompanied by increases in titanium and zinc signals, indicating surface reconstruction and partial masking of the carbonaceous phase. The authors attribute the performance decline to surface deactivation, including active-site blocking by adsorbed intermediates and disruption of interfacial interactions, rather than to crystalline phase degradation or catalyst dissolution.

Benchmarked against previously reported systems, the composite performs competitively: magnetized molecularly imprinted titanium dioxide achieved 92 percent sulfasalazine degradation under ultraviolet light, while copper oxide-cadmium sulfide and zinc oxide-nickel oxide composites reached 82 and 87 percent under ultraviolet or visible light, and complete losartan removal was reported using nitrogen-doped porous carbon with peroxymonosulfate activation, an approach requiring external oxidants. The new nanocomposite achieves its high efficiencies under natural sunlight without added oxidants, though the authors caution that direct quantitative comparisons are constrained by differences in experimental conditions. By converting leather waste into a functional, sunlight-active photocatalytic material, the study offers a route that couples industrial by-product valorization with the treatment of pharmaceutical-contaminated water, while the observed limitations in complex water matrices and catalyst reuse mark the remaining steps toward real-world deployment.

Subject of Research: Sunlight-driven photocatalytic degradation of pharmaceutical pollutants using a ZnO-doped TiO2/carbon nanocomposite derived from leather waste

Article Title: Insight into the sunlight-mediated photodegradation of active pharmaceuticals using ZnO-doped TiO2/Carbon nanocomposite derived from leather waste

Article References: Sushmitha, R., & Thanikaivelan, P. (2026). Insight into the sunlight-mediated photodegradation of active pharmaceuticals using ZnO-doped TiO2/Carbon nanocomposite derived from leather waste. Collagen and Leather, 8(1), Article 40. https://doi.org/10.1186/s42825-026-00270-9

Image Credits: AI Generated

DOI: 10.1186/s42825-026-00270-9

Keywords: photocatalysis, leather waste, titanium dioxide, zinc oxide, porous carbon, sulfasalazine, losartan potassium, superoxide radicals, pharmaceutical wastewater, band gap engineering, wastewater treatment, circular economy