Potential release of hydrogen fluoride from domestic coal in endemic fluorosis area in Guizhou,China

Almost half of the total rural area of Guizhou Province and many regions within the 11 adjacent provinces in southwestern China have a long history (at least 70 years) of endemic fluorosis, including dental fluorosis and osteofluorosis along with its associated deformities and disabilities. Over decades of research, this specific type of endemic fluorosis has been defined as coal-burning fluorosis, which is distinct from drinking-water fluorosis. It is generally acknowledged that indoor burning and combustion of high-fluorine coal leads to food contamination, and fluorine then enters the human body. However, the exact chemical form of fluorine during its release and transfer to the body is still unknown. In the present study, 21 domestic coal samples from outcrop and semi-outcrop coal collected in five villages with fluorosis were analyzed by time-of-flight secondary ion mass spectrometry (TOF-SIMS). The total mass fraction of sulfur in the samples ranged from 0.24%–5.58% and total fluorine content ranged 90.2–149.2 mg/kg. H₃O⁺, H₂SO₄⁺ and HSO₄⁻ were detected in the samples by TOF-SIMS, which indicated that sulfuric acid hydrate (H₂SO₄·H₂O) was present in the samples. F⁻ was detected in all of these, which suggested the samples contain ionic fluorine compounds. Under certain circumstances, such as heating or burning, the prevalence and coexistence of the acid (H₂SO₄·H₂O) and base (F⁻) would lead to a neutralization reaction producing volatile hydrogen fluoride (HF, bp = 19.5°C). This would be the chemical form of fluorine released from the coal. Further studies using HF and SO₂ test tubes on headspace gas over coal samples heated to 200°C in the laboratory and on headspace gas over stoves or chimney tops at rural residences confirmed the release of HF.