Fluorine immission to acid soil in the vicinity of an aluminium smelter in Galicia (NW Spain) and its influence on aluminium dynamics

The aluminium smelter located in the northern coast of Galicia (NW Spain) is a source of fluoride pollution. Previous papers Gago et al. (Commun Soil Sci Plant Anal 32:2503-2517, 2001, Fluoride 35:110-121, 2002) have identified an area of 3-km radius around the factory being affected by fluoride emissions. This study aims to investigate the influence of fluoride emissions on fluorine concentration and speciation as well as on aluminium speciation in soils in the vicinity of the smelter. PVC cylinders containing soil material from the A horizon of an unpolluted forest soil, similar to that in the vicinity of the factory, were placed within the A horizon of soils surrounding the smelter. The cylinders were placed at various distances (up to 3 km) from the factory in the direction of predominant winds. The PVC columns were removed after 6 months, and the soil material inside was divided into two slices (0-10 and 10-20 cm) and analysed for pH, total and available fluorine, exchangeable cations, effective cation exchange capacity (CEC) and oxalate-extracted aluminium. The soil solution was mimicked by aqueous extracts and analysed for pH, electrical conductivity, Ca, Mg, Na, K, total and free fluorine, total Al, acid-soluble Al, non-labile Al, labile Al and labile Al species. In the area closest to the factory (0.5 km), total and available fluorine in the upmost 10 cm soil increased by twofold and eightfold, respectively, while soluble fluoride increased by fourfold. Increases were also observed in exchangeable Ca, Mg, Al and CEC. Noncrystalline Al increased significantly at any distance from the smelter. Fluorine, pH, sulphate, aluminium and organic matter increased in the soil solution of surface (< 10 cm) samples closest to the factory. Monomeric Al-organic matter complexes prevailed in the liquid phase at 0.5-1 km from the factory whereas inorganic Al complexes did at distances over 1 km. Among the inorganic Al complexes, fluoride complexes prevailed at distances less than 1 km and hydroxylated complexes at distances greater than 1 km. Al3+ concentration was always very low. Emissions from an aluminium smelter led to increased concentrations of fluorine and various forms of aluminium in the nearby soil. These emissions mobilised aluminium, organic matter and probably sulphate from the soil. In the closest neighbourhood of the smelter, it was found that most soluble aluminium was bound to organic matter, hence having low toxicity, while at distances farther than 1 km Al-F and Al-OH complexes prevailed.