Control of nickel availability by nickel bearing minerals in natural and anthropogenic soils

The aim of this paper was i) to determine the Ni-bearing minerals and localize Ni in natural and contaminated Ni-rich soils, ii) to characterize Ni availability with isotopic exchange kinetics (IEK) and iii) to study its interactions with soil mineralogy and characteristics along a gradient of weathering intensity. We sampled 16 soils varying from a recently exposed surface serpentinite in cold regions, to Ferralsols (laterites) from a humid tropical climate including two highly contaminated soils (Ni industry). The minerals identified ranged from primary minerals to secondary phyllosilicates and lastly to Mn/Fe oxides, according to weathering intensity. Primary minerals inherited from the parent materials and secondary phyllosilicates formed in low leaching conditions had concentrations of Ni similar to the rock (0.2–0.3%). When compared to other secondary minerals, Fe oxides displayed slight Ni enrichment in moderate leaching conditions (0.4–0.8%) up to 10-fold enrichment in highly weathered Ferralsols (4–6%). Full characterization of the three factors of Ni availability in soils: the intensity (CNi), the quantity (Et) and the capacity (CF) factors was achieved with IEK. For most of the soils, CNi and Et varied conjointly: elevated values of these two parameters were found in soils dominated by both phyllosilicates and amorphous Fe oxides (high exchange capacity); low values were found in soils with significant amounts of well-crystallized Fe oxides (high retention capacity). In the case of anthropogenic origin, control of soil Ni availability also depends on the type of Ni-bearing minerals.