Predicting and modelling availability of fluoride in soil from sorption properties

Usage of alkaline and saline groundwater with elevated concentrations of fluoride (F-) for irrigation of pastures requires an assessment of the mobility of F- within the plant-water-soil continuum. Factors influencing F- sorption and desorption in 95 Australian soils were examined. Pronounced differences in F- sorption were observed across the soils, but these differences could not be explained by differences in soil pH, electrical conductivity or organic carbon. Rather, sorption was correlated with the content of Fe/Al hydrous oxides and kaolinite in soil. Sorption of F- onto soils increased the solution pH and the ratio of (F- adsorbed) to (OH- desorbed) was consistently below 1, thereby indicating that adsorption of F- results in the release of water and hydroxyl groups from, or co-adsorption of protons to, the sorbent surface. Maximum sorption occurred at pH values of approximately 5-6, whilst sulphate slightly increased (<5%) F- sorption. Desorption was slightly decreased (similar to 3%-7%) in presence of sulphate anions. Hence, it is unlikely that irrigation of soil with alkaline and saline groundwater, in combination with soil applications of gypsum and sulphur to limit pH fluctuations, would increase mobility of F- in soil. Finally, the irrigation of soil columns with 1200-1500 mm of alkaline and saline groundwater containing 0.18 mM F-, as would commonly occur in irrigation systems using coal seam gas associated water in Australia, resulted in a F- concentration of 0.05 mM at 10 cm depth and the measured values were in excellent agreement with modelled F- movement based on sorption parameters. Thus, sorption parameters can be used to identify soils which minimize movement of F- because of their strong F- sorption. Based on these results, safe limits for irrigation of soil can be established which avoid F- toxicity risks to plants, animals and contamination of water resources.