Sugarcane bagasse biochar functionalized with ZnO via zinc hydroxychloride thermal conversion for enhanced photocatalytic degradation of paracetamol

Water treatment plants are designed to clean our rivers and lakes, but they often struggle with a specific type of invisible pollution: the tiny traces of medicines we flush away. When people take painkillers like paracetamol, their bodies do not absorb every molecule. The rest passes through the sewage system and ends up in the environment, where it can harm fish and other wildlife. Because these chemicals are so persistent and exist in such small amounts, standard cleaning methods cannot remove them effectively. Scientists have turned to a powerful solution called photocatalysis, a process that uses light to trigger a chemical reaction that breaks down pollutants. One common material used for this is zinc oxide, a white powder that acts like a sponge for light energy. However, this material has a flaw: when it absorbs light, the energy it captures often cancels itself out before it can do any cleaning work, and the tiny particles are difficult to gather up after the job is done.

To solve these problems, researchers at the Federal University of Itajubá in Brazil developed a new way to combine zinc oxide with a material made from agricultural waste. They took sugarcane bagasse, the fibrous residue left over after crushing sugarcane to make juice, and turned it into a porous, charcoal-like substance known as biochar. Instead of mixing the materials together in a liquid, they grew the zinc oxide directly onto the surface of this biochar using a heat treatment process. The result was a composite material where the light-absorbing zinc oxide particles were anchored firmly to the carbon-rich biochar. This setup allowed the biochar to act as a support structure that kept the zinc oxide particles separate and stable, while also helping to manage the flow of energy so that the cleaning reaction could happen more efficiently.

The team tested this new material by trying to remove paracetamol from water under a light source. They found that the biochar-supported material was far superior to using plain zinc oxide powder. In their experiments, the new composite removed more than 78 percent of the paracetamol from the water in just 90 minutes, whereas the standard zinc oxide powder managed to remove less than half that amount in the same timeframe. The researchers observed that the porous nature of the biochar helped trap the medicine molecules close to the active zinc oxide sites, giving the reaction a head start. Furthermore, the material showed a much faster speed of degradation, breaking down the contaminant at a rate more than double that of the unmodified powder.

To understand exactly how this cleaning happened, the scientists used special chemicals to block different parts of the reaction process. They discovered that the breakdown of the medicine was driven by a team of reactive agents. The most important helpers were hydroxyl radicals and superoxide radicals, which are highly energetic particles that attack and dismantle the drug molecules. The biochar played a crucial role in generating these radicals by helping to separate the electrical charges created by the light, preventing them from wasting their energy. While the material showed some decline in performance after repeated uses, likely because the pores became clogged with leftover debris, it remained stable enough to be considered a promising candidate for future water treatment systems. This work suggests that turning agricultural waste into a high-tech cleaning tool could offer a low-cost, effective way to protect our water supplies from the growing threat of pharmaceutical pollution.