Differential influence of solution pH on the adsorption of fluoride in Icelandic volcanic soils varying in weathering degree and organic matter content

Fluorine, in the form of fluoride, is a minor but ubiquitous element found in magmas. It is released into the environment during volcanic degassing and exists as various salts on the surfaces of ash particles. Fluoride is also liberated during the weathering of magmatic rocks. The presence of fluoride in surface/ground waters and soils can pose a health hazard to humans and livestock. Additionally, fluoride enhances the dissolution of aluminosilicates during water-rock interaction. However, fluoride interacts strongly with iron (oxy)hydroxides (notably, ferrihydrite) and allophanes, secondary poorly crystallised minerals commonly found in volcanic soils. In volcanic regions, these reactions limit the mobility of fluoride significantly while causing its accumulation in the soil. We lack a quantitative description of the factors that control fluoride adsorption in volcanic soils, which hinders a comprehensive assessment of the geochemical behaviour of fluoride in these environments. Here we measured the fluoride adsorption envelopes (pH 2.8–7) of Icelandic volcanic soils with different weathering degrees and organic matter contents. The experiments were performed with a 1.3 mmol l^-1 NaF solution and using pH-stat titration. The minimum and maximum fluoride adsorption typically occurs at pH of 2.8 and ≥6, respectively, reflecting the combined influence of pH and soil anion exchange capacity (AEC). At pH <6, fluoride forms positively charged alumino-fluoride complexes (AlF_x^(3-x)). Since the AEC of allophanes (point of zero charge, PZC = 6) and ferrihydrite (PZC = 6.5) increases with solution acidity, adsorption of AlF_x^(3-x) becomes restricted at lower pH values. At higher pH, the fluoride ion (F^-) predominates in solution, but its adsorption is limited as the AEC decreases. More weathered soils, characterised by higher allophanes+ferrihydrite content, exhibit a greater capacity to adsorb fluoride. However, we also emphasise the role of organo-aluminium/iron complexes in reacting with fluoride. Based on the results of our adsorption/desorption experiments, we have developed a constant capacitance model to predict fluoride adsorption in volcanic soils.