BiOBr/MoWO4 p-n heterojunction photoanode for photodegradation of norfloxacin in water

Water purification is a constant battle against substances that refuse to break down. While traditional methods like filtering or chemical treatment work well for many impurities, they often fail against stubborn pharmaceutical residues that slip through the cracks. These leftover medicines, such as antibiotics, can linger in rivers and lakes, eventually contributing to the rise of drug-resistant bacteria and posing long-term risks to ecosystems and human health. To solve this, scientists are turning to advanced oxidation processes, which use energy to create highly reactive particles capable of attacking and dismantling these tough pollutants. One promising approach combines light and electricity. In this method, a special material acts as a bridge, absorbing sunlight to create charged particles that are then guided by an electric current. This dual force helps separate the positive and negative charges that are usually generated, preventing them from canceling each other out and allowing them to work efficiently to destroy contaminants.

Researchers at the University of Johannesburg have developed a new material designed to make this process significantly more effective. They created a composite photoanode, a type of electrode that generates electricity when exposed to light, by joining two different semiconductors: bismuth oxybromide and molybdenum tungstate. These two materials were synthesized together using a hydrothermal process, which involves heating water under high pressure to encourage the crystals to grow and bond. The goal was to form a specific type of connection known as a p-n heterojunction. In simple terms, one material naturally carries an excess of positive charges while the other carries an excess of negative charges. When they touch, they create an internal electric field that acts like a one-way street, forcing the charged particles to move in specific directions rather than bouncing around aimlessly. This setup was designed to solve a common problem in photocatalysis where the useful charges recombine and vanish before they can do any work.

The team tested this new material by attempting to break down norfloxacin, a common antibiotic found in wastewater. They placed the photoanode into a solution containing the drug and exposed it to visible light while applying a small electric current. The results showed a clear advantage for the combined material. Under optimized conditions, the bismuth oxybromide-molybdenum tungstate composite removed 85% of the norfloxacin from the water. This performance was notably better than using either of the two original materials on their own, which removed only 71% and 56% respectively. The researchers confirmed that the improvement came from the way the two materials worked together to keep the charges separated, allowing more of them to reach the surface of the electrode and attack the pollutant molecules.

To understand exactly how the destruction happened, the scientists used chemical scavengers, which are substances that neutralize specific types of reactive particles. They found that the most important agents in the breakdown were positively charged holes and superoxide radicals. These particles acted as the primary weapons, stripping electrons from the antibiotic molecules and breaking them apart. The study also measured how much of the drug was completely converted into harmless substances like carbon dioxide and water, a process called mineralization. In a simulated water environment, the material achieved a 41% mineralization rate. However, when tested in real wastewater collected from a treatment plant in South Africa, the efficiency jumped to 70%. This unexpected improvement in real-world conditions suggests that other components naturally present in the wastewater may have assisted the process, perhaps by helping to generate even more of the reactive particles needed for the job.

The durability of the new material was also put to the test. The researchers ran the degradation process five times in a row using the same electrode to see if it would wear out or lose its effectiveness. While the removal rate dipped slightly after repeated use, the material remained structurally sound. Analysis of the crystal structure before and after the experiments showed no significant changes, indicating that the material did not crumble or degrade chemically during the process. Furthermore, the team demonstrated that this electrode was not limited to just one type of drug; it also showed good performance when used to degrade other common antibiotics like tetracycline and ciprofloxacin. By successfully combining two distinct semiconductors into a stable, efficient system, the researchers have provided a viable path toward treating pharmaceutical-contaminated water, offering a sustainable tool that could help protect water supplies from the growing threat of persistent drug residues.