Fluorine in plants
Although fluorine emissions aren’t usually seen as a major environmental problem, they aren’t entirely harmless either. For a start, fluorines can be toxic to humans and animals at high enough concentrations, and such concentrations can build up in plants exposed to fluoride compounds in the atmosphere or soil, providing a route into the food chain.
Normally, plants contain 2–20?g of fluorine per gram of biomass, which is perfectly safe. However, plants next to industrial areas or active volcanoes, which can emit huge amounts of fluorine, can contain fluorine concentrations as high as 4000?g, which is hazardous for any animal feeding on them. What is more, fluorides can react in the atmosphere to form the kind of fluorinated gases known to damage the ozone layer.
The three biggest emitters of fluorine, in the form of various fluoride compounds, are volcanoes, industrial activity, particularly coal burning, and biomass burning. But whereas scientists have reliable estimates for the volume of emissions from the first two sources, no one had tried to work out how much fluoride was produced globally by burning biomass. So a team of chemists and atmospheric scientists in the US, led by Elizabeth Stone at the University of Iowa, decided to do the calculations.
Smoke in the filters
Their first step involved determining the volume of fluorides emitted by a range of different types of biomass when burnt. These included conifers such as pine and spruce, grasses such as African grass and sawgrass, agricultural residues such as wheat straw and sugar cane stalks, and peat. Stone and her colleagues burnt these different types of biomass at the US Department of Agriculture’s Fire Sciences Laboratory in Missoula, Montana. Here, smoke emitted by burning biomass is funnelled into an exhaust stack, where the particles making up the smoke are collected on Teflon and quartz filters.
Stone and her colleagues then extracted the smoke particles from the filters using alcohol and separated the particles into their component cations and anions, including fluoride anions, with ion-exchange chromatography. Finally, they used a conductivity detector to detect the separated anions and cations and determine their concentrations.
They found that the amount of fluorides emitted during burning varied quite bit between the different types of biomass. For a start, whereas 100% of the conifer samples and 94% of the different agricultural residues emitted fluorides on burning, only 36% of the grasses did so, while none of the peat samples emitted fluorides. Even the biomass samples that emitted fluorides tended to emit different amounts of them, with the proportion of fluorides in the smoke particles varying from 0.06% for wheat straw to 0.28% for sugar cane.
Dwarfed by volcanoes
Stone and her colleagues attribute this variation both to compositional differences among the plants and to the effect of different geographical locations and environmental climates. For example, they found that the amount of fluoride emitted by rice straw varied according to where it was grown, with rice straw grown in China emitting the least fluorides and rice straw grown in Malaysia emitting the most.
Saying all that, the average amount of fluorides emitted by the three main biomass groups were remarkably similar: the average proportion of fluoride in smoke emitted by burning conifers and grasses was the same, at 0.10%, while it was 0.15% for agricultural residue. This consistency allowed Stone and her colleagues to estimate the amount of fluorine produced by global burning of biomass each year, which they calculated to be 76,000 tonnes, with a range from 40,000 tonnes to 150,000 tonnes.
This means that burning biomass produces similar amounts of fluorine to industrial processes, which are estimated to produce 50,000–150,000 tonnes a year. Nevertheless, both are still dwarfed by volcanic emissions, which are estimated to produce between 700,000 tonnes and 8.6 million tonnes of fluorine a year, with the upper range corresponding to major volcanic eruptions.
Environmental Science & Technology, 2014, 48, 12636-12644: “Emissions of fine particle fluoride from biomass burning“