Fluoride pollution can take many forms. Thus, the fluorinated compound best known as an atmospheric pollutant is hydrofluoric acid (HF), but less often we think of alkali, alkaline earth or metal fluorides, and single or double fluorides. It should be highlighted that apart from the gaseous effluents (SiF4, H2SiF6 ,…), there is a particulate fluorinated pollution where compound size is extremely variable.
Origin of pollution
Fluorinated products present in the atmosphere can have a natural origin, such as during volcanic eruptions, but most often they are found in the immediate vicinity of certain industries:
- The aluminium industry. This industry requires the use of a flux, cryolite (AlF33 - 3 NaF), which produces fluorinated emissions.
- Industries that fire clay (cement factories, tile factories, brickyards, factories of ceramics and refractory products). Clays generally contain 0.03 to 0.09% fluorine, which can be released in part during the firing of the clay.
- The phosphoric acid and phosphate industry, which processes natural phosphates, which may contain up to 5% fluorine.
- The steel industry, glassworks, and certain metal refining plants. These industries are sources of fluorinated pollution, but often in negligible quantities, either in absolute terms or in relation to another pollutant emitted simultaneously, such as sulphur dioxide.
- The combustion of coal and certain fuels that are two combustibles containing fluorine. Given the large quantities of fuels sometimes burned in an installation, this source of emissions is far from negligible.
While useful for life in low doses, fluorides are toxic at higher concentrations and can cause a condition known as fluorosis, which affects teeth and bones. This toxicity comes from the ability of fluorine ions to form complexes with calcium and magnesium, thus disrupting various metabolic balances. Fluoride can also have effects on the kidneys, nerves and muscles.
The toxicity of fluorine also impacts plants, which can suffer damage (necrosis of leaves, flowers, etc.) and see their growth impaired. Certain plants, such as vineyards, for example, are particularly sensitive and can suffer damage at concentrations much lower than those having an impact on human health.
Animals can also be affected by fluoride pollution. Herbivores, by consuming contaminated plants, can accumulate fluoride and suffer from cavities, degradation of bones, claws, hairs or hooves. In the case of cattle or sheep, a decrease in milk production has been reported.
Finally, inorganic objects are not spared: corrosion of metals, damaged windows, etc.
The situation in Wallonia
Air pollution by fluorinated compounds is pollution of a local nature, in the immediate vicinity of specific sources. In Wallonia, two sites are constantly monitored: Battice (glass fibres) and Engis (manufacture of phosphates).
In Battice, fluoride concentrations are low and have been for years. In Engis, no improvement has been observed and the World Health Organization's guide value is still being exceeded.
The fluorides are sampled on filters (capturing the solid part), and filled with sodium formate (capturing the gaseous part). Back in the laboratory, the fluorides are eluted with a solution of known concentration of fluorides and potassium citrate, then analysed potentiometrically using a specific electrode.