A Copper–Iron Hybrid Turns Air’s Oxygen Into a Water-Purifying Weapon

A new catalyst that converts ordinary molecular oxygen from air into highly reactive forms of oxygen could offer a cleaner route for removing antibiotics, dyes and hormone-disrupting chemicals from polluted water. The material, made by anchoring zero-valent copper onto amorphous iron oxyhydroxide, uses a carefully engineered interface to generate singlet oxygen and hydroxyl radicals without relying on added peroxide, persulfate or other chemical oxidants. In experiments, the hybrid catalyst substantially outperformed bulk copper, producing 2.91 times more singlet oxygen and 1.44 times more hydroxyl radicals. The researchers report that the system rapidly degraded several representative organic contaminants and remained stable, reusable and resistant to interference from the complex chemical mixtures found in real water.

The study, published in Engineering, addresses a persistent problem in advanced water treatment: molecular oxygen is abundant, inexpensive and environmentally benign, but it is surprisingly difficult to activate. Oxygen molecules in their ground state have a triplet electronic configuration, which makes them relatively unreactive toward many organic pollutants. Catalysts can transfer electrons to oxygen or otherwise weaken its O–O bond, creating reactive oxygen species, or ROS. These include superoxide radicals, hydroxyl radicals and singlet oxygen, an electronically excited form of oxygen. ROS can attack the chemical bonds of pharmaceuticals and industrial compounds, breaking large molecules into smaller products and, ideally, ultimately converting them into carbon dioxide, water and inorganic ions.

The team, led by researchers at the University of Jinan in China with collaboration from Tongji University, designed a crystalline–amorphous hybrid called a-FeOOH-Cu⁰. FeOOH is an iron oxyhydroxide, a class of minerals and nanostructured materials known for their redox activity and ability to interact with oxygen-containing molecules. The “amorphous” portion lacks the long-range atomic order found in a conventional crystal, leaving a disordered surface rich in structurally diverse sites. Onto this material, the researchers anchored zero-valent copper, written as Cu⁰, in which copper atoms are in their elemental oxidation state. The resulting interface combines the chemical flexibility of disordered FeOOH with the electron-rich properties of metallic copper.

That interface appears to be the central feature of the catalyst. According to the researchers’ characterization results, the hybrid contains abundant low-valent copper species and exhibits electronic redistribution between copper and the iron oxyhydroxide support. In practical terms, electrons are not confined to one component. Instead, the contact between the two phases changes the local electronic environment around the copper and iron atoms, creating sites that can interact more effectively with dissolved oxygen. The amorphous FeOOH also supplies a high density of irregular coordination environments, while the anchored copper provides active centers for electron transfer. Together, these effects help the catalyst bypass a commonly proposed reaction route involving surface-bound hydroperoxyl intermediates, written as *OOH.

The researchers propose that the modified copper sites can directly cleave the O–O bond in activated oxygen species rather than proceeding primarily through the formation of *OOH. This distinction matters because the identity and lifetime of the intermediate determine which ROS are produced. A catalyst that directs oxygen toward singlet oxygen can favor selective, nonradical oxidation, while one that promotes hydroxyl radicals can generate extremely aggressive but less selective reactions. The a-FeOOH-Cu⁰ material generated both types of oxidant, along with superoxide, but singlet oxygen emerged as the most important species for degrading oxytetracycline, a widely used antibiotic and a frequent concern in aquatic pollution. Low-valent copper sites were identified as crucial contributors to this pathway.

Singlet oxygen, denoted as ¹O₂, is not simply “more oxygen.” It is a higher-energy electronic state of the oxygen molecule in which the arrangement of electrons changes its chemical behavior. Unlike hydroxyl radicals, which react rapidly and broadly with nearby molecules, singlet oxygen can show greater selectivity toward electron-rich functional groups, including certain sulfur-, nitrogen- and carbon-based structures in pharmaceuticals and dyes. That selectivity may reduce wasted oxidant and limit some unwanted side reactions. At the same time, hydroxyl radicals, represented as •OH, remain powerful oxidation agents capable of attacking a wide range of organic bonds. By generating both ¹O₂ and •OH, the hybrid offers complementary chemical routes for pollutant breakdown.

To test the material’s performance, the researchers exposed contaminated water to the catalyst and molecular oxygen supplied through air. The experiments included oxytetracycline, rhodamine B and bisphenol A, representing three chemically distinct classes of pollutants: an antibiotic, a synthetic dye and a phenolic endocrine-disrupting compound. The abstract reports significantly enhanced removal of all three contaminants compared with bulk Cu⁰. For oxytetracycline specifically, mechanistic investigations indicated that singlet oxygen was the dominant ROS responsible for degradation. The study therefore moves beyond simply measuring disappearance of a pollutant, linking the treatment effect to a proposed reaction mechanism and to the copper sites that make oxygen activation possible.

The potential environmental advantage is that the catalyst uses oxygen already present in air rather than requiring continuous dosing with sacrificial oxidants. Conventional advanced oxidation processes can be highly effective, but they may involve energy-intensive ozone generation or the storage and transport of peroxides and persulfates. Those reagents can add cost, create residual chemicals or produce secondary transformation products if the process is poorly controlled. A system based on air and a reusable solid catalyst could simplify operation, particularly in decentralized treatment settings. However, the researchers’ results come from laboratory experiments, and performance at large scale will depend on factors such as oxygen-transfer rates, catalyst recovery, hydraulic flow, pollutant concentration and the composition of natural waters.

The material also showed properties that could help with practical deployment. The researchers report high stability, good reusability and strong resistance to matrix interference. In water-treatment chemistry, matrix interference occurs when dissolved organic matter, salts, bicarbonate, chloride or other background constituents compete with target pollutants for reactive species or deactivate catalyst sites. A catalyst that performs only in purified laboratory water may lose much of its effectiveness in wastewater or surface water. Resistance to these effects is therefore important, although the abstract does not provide the detailed operating conditions, pollutant concentrations or number of reuse cycles needed to evaluate industrial readiness. Further work will also need to identify transformation products and assess their toxicity rather than relying solely on the disappearance of the original compounds.

The researchers describe their work as evidence that crystalline–amorphous hybrid materials can be used to tune oxygen activation and steer ROS production. The concept could extend beyond this particular copper–iron combination: interfaces between ordered and disordered phases are increasingly being explored because they can redistribute charge, expose unusual atomic configurations and create multiple reaction environments in one material. The study’s most striking result is not that copper can participate in pollutant oxidation, but that its chemical behavior changes when it is anchored to amorphous FeOOH. By engineering the boundary between the two phases, the team turned oxygen from a relatively inert background molecule into a source of reactive chemistry capable of attacking persistent contaminants. The approach is promising, but its ultimate value will be determined by durability, safety, cost and performance in complex, continuously operating water-treatment systems.