Methylene blue is a particularly troublesome target. The dye is a workhorse of the textile world, coloring cotton, calico, tannin, and leather, and it also serves as a sensitive redox indicator in chemistry. Yet once it enters waterways, its persistence becomes a liability. It is non-biodegradable, resists breakdown by natural processes, and conventional treatment methods often fail to remove it completely. Because its molecules contain multiple aromatic rings, traditional techniques struggle to cleave its structure, and its low biological degradability means it can linger in the environment for long periods, threatening marine life and potentially accumulating in organisms. Trace metals such as chromium, copper, and lead, which can accompany dyes and finishing agents in textile effluent, add to the toxic burden.

Existing treatment technologies, from ultrafiltration and advanced oxidation processes to solvent extraction, chemical precipitation, ion exchange, reverse osmosis, and electrochemical and bio-electrochemical methods, share a common drawback: they tend to demand high capital investment and heavy energy consumption, placing them beyond the reach of small dyeing operations. Photocatalysis offers an appealing alternative. In a photocatalytic process, light excites electrons in a semiconductor from the filled valence band to the empty conduction band, generating electron–hole pairs on the catalyst surface that drive redox reactions capable of mineralizing organic pollutants into harmless carbon dioxide and water. The catalyst itself is not consumed, and water or atmospheric oxygen supply the oxidants and reductants, eliminating the need for additional chemical inputs. The approach can even support a closed-loop system in which treated effluent is reused within the dyeing process.

The challenge lies in designing the elusive ideal photocatalyst: highly active, selective, and durable. A key enemy of efficiency is the recombination of photo-induced electron–hole pairs, which occurs far faster than the charge carriers can reach the surface to do useful work. The new study tackles this problem with a carefully chosen trio of materials. Praseodymium, the third element of the lanthanide series, forms the mixed oxide Pr₆O₁₁, formally a combination of 4PrO₂·Pr₂O₃, whose oxygen vacancies grant it high hole and oxygen ion conductivity. Neodymium oxide introduces surface defects that act as mediators, impeding electron–hole recombination and prolonging the lifetime of charge carriers so they can participate in more redox reactions. Graphene oxide, with its two-dimensional sheets decorated with carboxyl, hydroxyl, and epoxy groups, serves as an electron acceptor and transporter, whisking photo-generated electrons away from the oxides and suppressing recombination.

The synthesis began with graphene oxide produced by a modified Hummer’s method, in which expandable graphite powder was stirred in 98 percent sulfuric acid, oxidized with potassium permanganate kept below 20 degrees Celsius, and then diluted and quenched with hydrogen peroxide before repeated centrifugation and filtration. The composites were assembled in stoichiometric ratios: single oxides, binary pairs of Pr₆O₁₁ with GO, Nd₂O₃ with GO, and Pr₆O₁₁ with Nd₂O₃, and the ternary Pr₆O₁₁/Nd₂O₃/GO containing one gram of each oxide and 0.05 grams of GO. Each mixture was dispersed in deionized water by ultrasonication for 15 minutes and collected by centrifugation at 6000 rpm.

Characterization confirmed the purity and structure of the materials. X-ray diffraction revealed the characteristic reflections of Pr₆O₁₁ and Nd₂O₃, while the intense GO peak at 2θ = 10.54° disappeared in the composites, indicating successful integration of the oxide particles with the carbon sheets. Crystallite sizes calculated by the Scherrer equation ranged from 29 to 43 nanometers across the four composites. Fourier-transform infrared spectroscopy identified the Pr–O and Nd–O vibrational bands along with hydroxyl features, and energy-dispersive X-ray analysis of the ternary composite showed carbon at 28.71 percent, oxygen at 41.54 percent, praseodymium at 28.35 percent, and neodymium at 1.4 percent. X-ray photoelectron spectroscopy verified the coexistence of Pr, Nd, O, and C on the surface, with the Nd 3d₅/₂ peak near 971 electron-volts confirming Nd³⁺ states and Pr 3d envelopes at roughly 933 and 953 electron-volts.

Microscopy showed that both oxides adopt a rod-like shape, with Nd₂O₃ rods averaging 313.6 nanometers in length and 61.7 nanometers in width, distributed homogeneously across the GO sheets. This morphology matters: rod-shaped nanocomposites have been shown to outperform spherical or cubic counterparts because their larger surface area exposes more active sites and enhances light absorption and scattering. Brunauer–Emmett–Teller analysis classified the ternary composite as macroporous, with a type III isotherm, a pore volume of 1.99 × 10⁻² cubic centimeters per gram, and a mean pore diameter of 136.59 nanometers. Optical measurements revealed a deep ultraviolet fundamental band gap of about 5.6 electron-volts for the ternary composite, but critically, the lanthanide ions introduce localized 4f states at 1.4–1.8, 2.0–2.6, 3.6–3.8, 4.2, and 5.0 electron-volts, allowing the material to absorb lower-energy photons through stepwise excitation. Photoluminescence emission at 2.64 electron-volts indicated that radiative recombination occurs through mid-gap defect states and oxygen vacancies rather than direct band-to-band transitions.

Thermal analysis demonstrated that the composites remain stable up to 600 degrees Celsius, with the ternary composition losing only about 5 percent of its mass at that temperature, compared with roughly 8 percent for the binary GO-containing composites. The GO flakes appear to restrict the volatilization of degraded fragments, boosting thermal resilience. Then came the decisive test: dye degradation under a 500-watt visible light source. The individual oxides managed removal efficiencies of only 46.2 percent for Pr₆O₁₁ and 48.1 percent for Nd₂O₃ after 150 minutes. Adding GO lifted the binary composites to 73.1 percent for Pr₆O₁₁/GO and 71.2 percent for Nd₂O₃/GO, while the oxide pair reached 76.9 percent. The ternary Pr₆O₁₁/Nd₂O₃/GO composite outperformed them all, achieving 92.3 percent removal.

The proposed mechanism explains the synergy. Under illumination, electrons are excited from the valence band into the localized sub-bandgap defect states and then promoted into the conduction band. Interfacial band alignment drives a thermodynamically favorable downhill transfer of these photo-excited electrons into the conductive two-dimensional graphene oxide matrix, which acts as an electron sink. Electrons trapped on the GO surface reduce dissolved oxygen into superoxide radicals, while holes in the deep defect states oxidize adsorbed water into hydroxyl radicals. Together, these reactive oxygen species progressively mineralize the methylene blue molecules. The composite also proved reusable: after five recycling cycles, the ternary material retained 79.5 percent activity, a decline of only about 12.8 percent, compared with steeper drops for the single oxides, which fell to 35.2 and 38.6 percent.

The findings point toward eco-friendly water treatment technologies that harness light, reusable catalysts, and minimal chemical inputs to tackle dye pollution. By combining the oxygen-vacancy chemistry of praseodymium oxide, the defect-mediated charge separation of neodymium oxide, and the electron-transporting power of graphene oxide, the study offers a blueprint for designing next-generation photocatalysts. The author notes that further research and optimization are warranted to explore broader applications in wastewater treatment, but the message is clear: rare-earth oxide–graphene hybrids could play a significant role in the pursuit of clean and safe water resources, turning sunlight into a tool for dismantling some of the textile industry’s most persistent pollutants.

Subject of Research: Development of a Pr₆O₁₁/Nd₂O₃/graphene oxide photocatalytic nanocomposite for methylene blue dye degradation in textile wastewater

Article Title: Physicochemical investigation and development of photocatalytic composite containing Praseodymium (III, IV) oxide (Pr₆O₁₁), Neodymium (III) oxide (Nd₂O₃), and graphene oxide (GO) for methylene blue degradation

Article References: Domyati, D. (2026). Physicochemical investigation and development of photocatalytic composite containing Praseodymium (III, IV) oxide (Pr₆O₁₁), Neodymium (III) oxide (Nd₂O₃), and graphene oxide (GO) for methylene blue degradation. Journal of Saudi Chemical Society, 30(3), Article 29. https://doi.org/10.1007/s44442-026-00078-6

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00078-6

Keywords: photocatalysis, methylene blue, graphene oxide, praseodymium oxide, neodymium oxide, rare-earth oxides, textile wastewater, dye degradation, nanocomposite, water treatment, reactive oxygen species, band gap