Potentially toxic elements in ultramafic rocks and soils: A case study from the Voltri Massif (NW Italy)

Description

High concentrations of potentially toxic elements (PTEs) in surface and near-surface environment may be attributed to both anthropogenic sources, including industrial and agricultural activity, and geogenic source, mainly due to natural weathering of rocks. Considering the geogenic sources, ultramafic rocks (e.g., dunite, peridotite, pyroxenite, and serpentinites) are among the most critical from the environmental point of view; in fact, they are characterized by high contents of Cr, Ni, Co, and other PTEs which can have potential harmful impact on ecosystems and human health if released into soils and waters during weathering and pedogenic processes. As a matter of the fact, weathering of ultramafic rock produces ultramafic soil containing high concentration of PTEs, including Cr, Ni, and Co compared to soils derived from non-ultramafic bedrock. Although ultramafic rocks and relative soils cover approximately 1% of the Earth's surface, they are distributed worldwide and are commonly associated with ophiolite complexes; for this reason, they play an important role in environmental management. The main objective of this PhD thesis was to determinate the mineralogy and the chemistry of PTEs of ultramafic soil profiles from the Voltri Massif and to evaluate how lithological, textural, and structural properties of the ultramafic bedrock with various degree of serpentinization and deformation may affect the PTEs re-distribution and the fate in the soils during pedogenesis, as well as to assess their environmental implications in the ecosystem. The multidisciplinary and multiscale approach, used in this PhD thesis, has allowed to highlight a series of considerations that led to the following conclusions: • In the studied rocks, Cr, Ni, and, subordinately, Co are invariably the PTEs with the highest concentrations; in addition, V, Cu, and Zn are generally found in high concentrations. The main factors controlling the PTEs distribution within the studied ultramafic rocks appeared to be the serpentinization degree and the deformation style and intensity. The main source of the PTEs are spinel-group minerals. Moreover, PTEs-bearing phases are also represented by the other rock-forming minerals (such as serpentines, olivines, pyroxenes, and chlorites) and some accessory phases (e.g., ilmenite and other oxides, sulfides). • The studied ultramafic soil profiles vary in thickness from 35 to 80 cm and are characterized by weakly developed A and C horizons and a very thin O horizon (up to 5-10 cm). In general, in the soils the mineralogy of the skeleton (sandy and silty fractions) is closely related to bedrock mineralogy as expected for primitive A-C soils (in order of abundance: antigorite, chlorite, spinel-group minerals, pyroxenes, authigenic phases, quartz, olivine, and tremolite). The clay fraction is mainly composed by amorphous or low-crystalline Fe-oxyhydroxides, serpentines, smectite and illite/smectite mixed-layer clay, and clinochlore. Cr and Ni values have very variable concentrations over a wide range. Cr and Ni decrease according to serpentinization degree of bedrock. Co, Zn, and Cu do not show a clear correlation with bedrock serpentinization. V increase with the serpentinization of the bedrock. The PTEs concentrations in the studied ultramafic soil profiles are linked both to the primary minerals, inherited by bedrocks (e.g., serpentines, spinel-group minerals, pyroxenes, chlorites), and to their stable authigenic products (Fe-oxyhydroxides and clay minerals). The PTEs leached from the primary minerals is mainly scavenge by goethite, and subordinate by clay. • The results evidence that Cr, Ni, and, Co systematically exceed (up to one order of magnitude) the residential and industrial threshold values (CSC) according to Italian law (D.M. 471/1999; D.Lgs 152/2006) both in rocks and soils. However, combining all my results, I have demonstrated that the critical PTEs concentrations in the studied profiles have a geogenic origin and are linked both to the primary minerals, inherited by bedrocks, and to their stable authigenic products. Considering the high stability of authigenic products in supergenic environment, it is evident that this mineral species are effective and often permanent traps for the most important PTEs of ultramafic soils and bedrocks, thus reducing its bioavailability. A study of this kind that combine the bulk chemistry of outcropping rocks and the relative soil profiles with their geological, structural, mineralogical, and crystallochemical data, can be also a useful tool in environmental concerns to determine the PTEs distribution, to evaluate their potential mobility and bioavailability as well as to discriminate the natural geochemical background from possible source of contamination.

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