New insights on the chemical speciation of nickel and copper in a naturally metal-rich soil from the Thetford Mines area, Quebec

The objective of this research was to investigate trace metal speciation in a naturally metal-rich soil from the Thetford Mines area (Quebec, Canada) by (1) identifying the principal mineral host(s) of Ni, and (2) characterizing relative vertical changes in the kinetic reactivity of Ni and Cu in the soil profile. SEM-EDS was used to characterize the mineralogy of individual silt- and clay-sized (< 0.063 mm) soil particles. ToF-SIMS analysis of these grains found several Mg-bearing phyllosilicates (i.e. biotite and chlorite) to contain Ni. Kinetic fractionation studies were used to characterize the change in the kinetics of Ni and Cu extraction from the soil as a function of depth. The results indicated that Ni became less labile towards the surface, while Cu displayed the opposite trend. These trends likely reflect the differential effects of soil weathering processes with depth, which act to varying extents on the two elements because of differences in their chemical forms. This research has important implications for interpretation of natural background levels of trace metals in soils and the associated environmental risk.