Aqueous aluminium species in forest soils affected by fluoride emissions from an aluminium smelter in NW Spain

Aluminium speciation was studied in aqueous extracts of forest soils in the vicinity of an aluminium smelter in Galicia, NW Spain. Two soil depths (0-10 and 10-20 cm) were sampled within a 3.5 km radius around the factory. Forest soils samples far away from the factory were used as a control. Aqueous extracts (1:10 soil:water ratio) were analyzed for pH, electrical conductivity (EC), dissolved organic carbon (DOC), sulphate, Ca, Mg, Na, K, free and total fluoride, and aluminium species. The pH of the extracts ranged between 4.50 and 7.85, the EC was always under 0.35 dS/m, DOC ranged between 0.14 and 1.64 mmole C/L, and sulphate between 1.36 and 14.10 mg/L. Most of these values fall within normal ranges for natural Galician soils. Total fluoride concentrations ranged between 0.18 and 1.19 mg/L and tended to be lower in subsurface layers and at greater distances from the factory. Free fluoride and free/total fluoride ratio correlated positively (P < 0.01) with pH. Total aluminium concentrations in the extracts ranged between 0.04 and 3.24 mg/L. Acid-soluble (colloidal, polymeric and/or organically complexed) aluminium often accounted for more than 50% of the total AI. Non-labile (organic monomeric) aluminium constituted 6 to 83% of the total Al and 30 to 96% of the reactive Al. Labile (inorganic monomeric) aluminium was a very small fraction of the total AI in extracts at pH > 5 but prevailed in some more acid samples. Most labile Al in the vicinity of the factory was bound to fluoride and the activity of Al3+ was extremely low. In control samples Al3+ and hydroxylated complexes appeared together with lower concentrations of AI-F complexes.