In water treatment, most contaminants follow familiar rules. Metals can be precipitated. Bacteria can be disinfected. Organic pollutants can often be broken down or filtered out. PFAS do not behave like most contaminants. Their unusual chemistry makes them both useful in consumer products and stubbornly difficult to remove from water. Over the last decade, the spread of PFAS contamination in groundwater and drinking water has forced utilities and regulators to confront a problem that is as much about physics and chemistry as it is about public health. Engineers now know how to capture many PFAS compounds. What they still struggle with is doing it efficiently, consistently, and affordably. At the heart of the challenge is a molecular structure that resists the very processes water treatment systems normally rely on. The bond that refuses to break PFAS owe their persistence to a specific chemical feature: the carbon–fluorine bond. In PFAS molecules, carbon atoms are surrounded by fluorine atoms, creating one of the strongest bonds in organic chemistry. Fluorine is extremely electronegative, meaning it pulls electrons tightly …