Speciation and Mobility of Cr and Ni in Serpentine Rocks and Derived Sediments (Tuscany, Italy)

Large outcrops of ophiolites from exposed land surfaces can potentially impact the geochemistry of much greater areas through transport and weathering. Derived soil and sediments contain significant concentrations of heavy metals, including chromium and nickel. In the context of environmental risk analysis, there is a necessity to obtain more information about the distribution of Cr and Ni in serpentine rocks and their derived associated geological matrices, and about how easily Cr could be released and then oxidized in the environment, causing pollution of groundwater. The aim of this study was to evaluate the distribution of Cr and Ni in the geochemical fractions containing Fe and Mn and the role of Fe and Mn oxides (crystalline and non-crystalline) in redox processes leading to the formation of Cr(VI) during serpentine soil weathering. Through the combination of chemical selective sequential extraction (SSE) and X-ray diffraction, solid samples belonging to ophiolitic rocks and their derived soils and sediments in southern Tuscany were investigated. The applied SSE method followed the established extraction scheme commonly used in sequential selective extraction procedures. The extraction was accomplished in seven successive steps, using appropriate reagents to destroy the binding agents between the target metal and the specific soil fraction to release the heavy metals selectively from their structural context. The results indicated significant differences in the availability and mobility of Cr and Ni in soils, with Cr concentrations ranging from 200 to 950 μg/g and Ni from 274 to 665 μg/g in reactive fractions. Cr is tightly bound to well-crystallized Fe-oxides and primary rock-derived phases, whereas Ni is substantially more mobile, being mainly controlled by Mn-oxides and amorphous Fe-oxides. Weakly acidic solutions or systems with high redox potential increase Cr and Ni mobility in the environment due to Fe/Mn hydroxides produced by the weathering of serpentinites. An ORP higher than 1000 mV leads to the formation of Cr(VI) by oxidation of Cr(III), increasing the mobility of Cr in groundwater and the hazard for human health. The analytical activity carried out in this research can be used to identify the potential risk of Cr(VI) release in groundwater from serpentine and derived geomaterials.