TOF-SIMS and FE-SEM/EDS to verify the heavy metal fractionation in serpentinite quarry soils

Abstract

This study aims at extensively defining serpentinite quarry soils and analysing their content and distribution of heavy metals using chemical sequential extraction. The association with the different geochemical phases of the soil was verified using TOF-SIMS and SEM-EDS techniques. Seven soils were chosen in two serpentinite quarries located in Moeche (M) and Silleda (C) (Galicia, northwest of Spain). The selected areas for soil sampling were: dump sites (M1, M2 and C1), rock extraction sites (M3, C2), and quarry boundaries (M4 and C3). The total soil concentration of Co, Cr, Ni, and V varied from 1472 to 7132, 1499 to 4309, 76 to 373, and 21 to 140mgkg−1, respectively. In all cases they exceed the maximum limit permitted in soils. After chemical sequential extraction it was found that the high content of Co, Cr, Ni, and V is associated with the residual fraction of the soils. In addition, Fe and Mn oxides have a high capacity for Co fixation, whereas Cr, Ni, and V are mainly associated with magnesium silicates. The fractions related to organic matter and the soluble or available forms are, respectively low and very low, not exceeding 2.5% in any of the soils. This association of the metals with magnesium silicates and Fe oxides, and the low content of metals bound to organic matter were identified by using the TOF-SIMS and SEM-EDS techniques. Both techniques confirmed and verified the results obtained during the chemical sequential extraction by checking the interaction of heavy metals with the different components of the soil.