Fluoride sorption and desorption on soils located in the surroundings of an aluminium smelter in Galicia (NW Spain)

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 H2O 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−1, after adding 200 mg F L−1, 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−1). The fluoride sorption upon addition of 200 mg F L−1 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).