Esperanza Alvarez Rodríguez

Comparison of methods for fluoride extraction from forest and cropped soils in vicinity of an aluminum smelter in Galicia (NW Spain)

Four extracting solutions (1 M NH4Cl, 0.02 M NH4Cl, 0.01 M CaCl2, and water) have been tested to determine mobile fluoride in forest and cropped soils having high fluoride contents resulting from emissions by an aluminum smelter in Galicia, NW Spain. Seventy-two acid forest soil samples and 47 cropped soil samples were analyzed. Linear regression analysis was performed by SPSS 9.0 statistical package. Significant correlations (P < 0.01) were found between fluoride concentrations in different extracts provided that soils had similar pH and exchange aluminum (Al) and calcium (Ca) values. The 1 M NH4Cl, 0.02 M NH4Cl, and 0.01 M CaCl2 extractants were more efficient than water in extracting fluoride from acid forest soils, due to displacement of exchange aluminum and, therefore, Al-bound fluoride. Highest extractions were achieved by 1 M NH4Cl, due to its higher ionic strength. Water extracted more fluoride than salt solutions from cropped soils, usually fertilized and limed, and consequently having high (>10 cmolec kg−1) exchange Ca values. When extracting cropped soils by NH4Cl and CaCl2, the CaF2 solubility product is presumably reached due to displacement of exchangeable calcium, and fluoride is removed from solution. Water extractions seem adequate to estimate mobile F from soils high in exchange Ca (>10 cmolec kg−1), while salt solutions seem suitable for soils high in exchange Al (>1 cmolec kg−1).