Mercury In Fluorescent Bulbs Has A Unique Isotope Fingerprint

To reduce electricity bills, consumers have started switching from incandescent to compact fluorescent light bulbs. As more people make the swap, increasing numbers of fluorescent bulbs end up in landfills, where the mercury contained in the bulbs could leach into groundwater. A new study suggests that researchers could track the mercury from fluorescent bulbs by looking for its unique isotopic signature (Environ. Sci. Technol., DOI: 10.1021/es303940p).

Human generated sources of mercury, including coal-fired power plants, pump 2,000 tons of the element into the atmosphere each year. Mercury can damage the human central nervous system; in particular, it disrupts brain development in fetuses and young children.

Scientists analyze the ratios of mercury isotopes to identify the source of the metal found in lakes, oceans, and the atmosphere. Although researchers know the unique ratios found in sources such as coal, they currently don’t have an isotope fingerprint for mercury found in compact fluorescent light bulbs (CFLs).

Chris Mead of Arizona State University and his colleagues thought that chemical processes inside CFLs might produce a unique isotopic composition for mercury lodged in the bulbs’ glass. Previous studies have shown that when mercury vapor within a fluorescent bulb gets excited it not only generates light but also causes a small fraction of the mercury to become trapped in the bulb’s glass. This process might affect some mercury isotopes more than others, leading to a characteristic ratio of isotopes in the glass.

With this in mind, the researchers turned on 14-watt consumer bulbs and kept them burning continuously for 1,700, 3,600, 10,000 or 16,000 hours. Afterwards, they broke each bulb and separated the mercury trapped in the glass from the remainder of the mercury. Using a multicollector inductively coupled plasma mass spectrometer, the team measured the isotopic composition of this trapped mercury. Then they compared the mercury isotope ratios in their samples to those from a known mercury standard. The difference between these sets of ratios represents a unique isotopic signal for the CFL mercury. Mead was surprised by the findings: The isotope ratios found in the CFL mercury were much larger than those found in other sources.

Joel D. Blum, of the University of Michigan, Ann Arbor, who helped to establish the method for using isotope ratios to track mercury emitted by coal burning, calls determining an isotope fingerprint for CFL mercury “a good idea.” He says the study presents a unique signal that researchers could use when tracking mercury in the environment.