Nickel, chromium, and cobalt in soils developed on nickel laterites near an abandoned mining area in southern Czech Republic

Abstract

Geochemical anomalies are gaining importance due to the Europe's renewed prospecting activities for technologically critical metals, such as nickel (Ni) and cobalt (Co). In this context, soils developed on Ni laterites near old open pit mining and exploration works near Křemže in southern Czech Republic were investigated using multi-method approach to assess the distribution, solid speciation, and (bio)availability of the trace metals, with a particular focus on Ni, chromium (Cr), and Co. The total concentrations of metals in the studied soils (Ni: 170–4950 mg/kg, Cr: 56–1190 mg/kg, Co: 14–424 mg/kg) exceeded the median world regulatory guideline values (Ni: 112 mg/kg; Cr: 250 mg/kg; Co: 50 mg/kg) as well as the Czech concentration limits for agricultural soils for most of the samples. The concentration of metals in the soil profiles generally increased as a function of depth with surface horizons in agricultural plots homogenized by ploughing. The effect of a former open pit mine (already closed for ca 80 years) on the vertical distribution of metallic elements has not been demonstrated. The extractable metals were relatively low (extraction efficiency order: water < DTPA < EDTA). The mean EDTA-extractable values corresponded to 4.2 % Ni_tot, 8.6 % Co_tot, and only 0.14 % Cr_tot. This contrasting metal availability is strictly related to the metal speciation in the solid phase. The primary minerals (olivine, clinopyroxene) are highly weathered to secondary phyllosilicates (lizardite, talc) and Fe oxyhydroxides and Mn oxides, all representing important carriers for Ni. Cobalt was exclusively hosted by Mn oxides, and the less mobile Cr was mainly bound in insoluble phases, likely spinel-family oxides. Despite the relatively low metal availability, elevated concentrations of Ni in the crop biomass (86 mg/kg) collected in the agricultural area suggest a metal uptake from the soil, which should be further investigated in detail.