Soil fluoride enrichment process and the possible adaptation prevention principle in coal-burning fluorosis area in Southwest China

Coal-burning fluorosis prevails in southwest China and other provinces. Although clay used as binder of briquettes was proven to cause coal-burning fluorosis, its enrichment processes remain unknown. The soils and rocks on typical geological units were sampled and simulation experiments were performed to detect the forming process of high-fluoride clay. The surface and mineral soils, farmland soils and rocks have fluoride levels of 157.9-1076.76, 334.58-1419.28, 227.52-1303.11 and 46.05-964.11 mg/kg respectively. Fluoride levels of surface soils, mineral horizon soils and farmland soils are significantly positively correlated, while those between soils and rocks are not significantly correlated. The soils overlying carbonates have substantially higher fluoride levels than those overlying non-carbonates although the carbonates have extremely lower fluoride levels. The fluoride levels in acid insoluble substances are significantly positively correlated with soil fluoride levels. The acid insoluble substances in carbonates have obviously higher fluoride levels than those in non-carbonates. High Ca(Mg) levels in carbonates restrict fluorine leaching into the water and facilitate fluorine deposition in soils. Fluoride enriches in soils with numerous Ca(Mg)CO₃ leaching during carbonate weathering, which is a new insight into the cause of high-fluoride clay. An exposure pathway of fluoride is forwarded. The best prevention principle and policy are proposed.