Sequential extraction of heavy metals in soils from a copper mine: Distribution in geochemical fractions

• The total Cu contents in minesoils exceed the intervention limits.
• Most of the heavy metal content is associated with the residual fraction.
• The Fe-oxides play an important role in the fixation of the studied metals.
• Soil acidity and low organic matter contents increase the mobility of heavy metals.
• TOF-SIMS and SEM-EDS techniques confirm the sequential chemical extraction results.

We evaluated heavy metal contamination (Cr, Cu, Mn, Ni, Pb, and Zn) in soils from the copper mine in Touro, NW Spain. Their total content and geochemical phase distribution were determined using sequential chemical extraction. Representative soils from the area were selected and the objectives were to analyse the total content of heavy metals and their distribution in the different geochemical soil phases, and to determine their association with the soil components using XRD, FE-SEM/EDS and TOF-SIMS. Results show that the Cu total contents (104–2924 mg kg− 1) exceed the intervention limits. Most of the heavy metal content is in the residual fraction, Cr (82–95%), Cu (30–70%), Mn (60–96%), Ni (77–95%), Pb (47–68%) and Zn (85–97%). Fe-oxides play an important role in the fixation of metals, especially Cu. The amounts associated with soil organic matter and in the exchangeable form are very low (> 10%). TOF-SIMS and FE-SEM/EDS techniques confirm the fractionation results, showing that only a very small part of the studied metals are sorbed, mainly in the iron oxides and interacting with several soil mineral components. FE-SEM/EDS combined with TOF-SIMS, sequential extraction assays and statistical analyses are an effective tool to check the affinity of the soil components for heavy metal cations.