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Aluminium smelters are major sources of F emission to the environment. We studied, in laboratory experiments, the sorption and desorption of fluoride on organic and mineral horizons of soils located within 2 km from one of these factories, situated in the northern coast of Galicia (NW Spain). The soils, developed from granite, are acid (pH HO 3.9-5.5), rich in organic matter (4-16 % C in the A horizon) and most A horizons have high Al saturation in the exchange complex. All samples showed a notable F sorption, between 1,066 and 1,589 mg kg, after adding 200 mg F L, which accounts for 53-80 % of F added. The sorption was slightly higher in the A horizons than in the respective organic horizons (differences of up to 194 mg kg). The fluoride sorption upon addition of 200 mg F L correlated significantly ( p < 0.05) with soil pH in water ( r = −0.77), iron extracted by acid ammonium oxalate ( r = 0.68), aluminium plus iron extracted by acid ammonium oxalate ( r = 0.63), exchange aluminium ( r = 0.52) and clay percentage in soil ( r = 0.76). The F sorption fitted to both Langmuir and Freundlich models. Desorbed F accounted for only 12-22 % of sorbed fluoride and correlated ( p < 0.05) negatively with non-crystalline (extracted by acid ammonium oxalate) Fe ( r = −0.51) and clay content ( r = −0.74) and positively with organic matter ( r = 0.69) and with the effective cation exchange capacity of the soil ( r = 0.50). [ABSTRACT FROM AUTHOR]

Abstract: Fluorine is a phytotoxic element that can reach the soil from various industrial activities. Fluorine sorption by soil is crucial to protect water and food chain from fluorine pollution. In Galicia (NW Spain), various activities emit fluorine into the atmosphere, mainly ceramic industries and an aluminium smelter. This study, aiming to investigate fluorine sorption by Galician soils, was conducted on natural soil horizons representative of the area. Most soils were acid and rich in organic matter and showed strong fluorine sorption. The lowest sorptions were exhibited by a near-neutral serpentinite-derived soil (1001mgkg−1) and the B horizons of soils developed from quartz schist (989mgkg−1), and the highest by the A horizons of amphibolite-derived soils (1783mgkg−1). In soils developed from quartz schist, biotitic schist and amphibolite, A horizons sorbed more fluoride than the corresponding B horizons (average 1621 and 1324mgkg−1, respectively), while the opposite is true in granite-derived soils (average 1644 and 1324mgkg−1, respectively). In the A horizons, the F sorption significantly correlated to soil pH (r =−0.79), pH in NaF (r =0.83) and oxalate Al (r =0.81). In the B horizons, sorption correlated to soil pH (r =−0.78), oxalate Fe (r =0.71) and organic C (r =0.66). F sorption can be described by both the Langmuir and Freundlich equations. The concentration of free fluoride in the equilibrium solutions increased above pH 6. [Copyright &y& Elsevier]