Sorption and desorption of fluoride in soil polluted from the aluminium smelter at Ardal in Western Norway

Fluoride (F) accumulation and transport in soil columns near the aluminium smelter at Ardal in Western Norway are studied together with fluoride sorption capacity, and the effect of pH and ionic strength on F solubility in soil. Unpolluted soil columns of 50 cm height placed at different distances from the smelter(1-0 km) accumulated from 0.27 g F m(-2) to 1.5 g F m(-2) during a 5 months period. Fluoride accumulation was high in the upper 0-10 cm of the soil columns where 50-90% of the accumulated F was found. Laboratory sorption experiments showed that the sorption of added F in the Ah-horizons increased with increased distance from the smelter at Ardal, in accordance with decreasing previous pollution from the smelter. The B-horizons sorbed considerably more F than the Ah-horizons, due to higher content of Al-oxides/hydroxides. Maximum sorption of F occurred at pH 4.8-5.5. Fluoride solubility increased with increased ionic strength. Continued deposition of F may increase the availability to plants and soil organisms. Leaching of F from soil to groundwater or surface water will be strongly dependent on the presence and thickness of a B-horizon, and probably also on underlying horizons.