Zirconium, Hafnium and Rare Earths behaviour during the transport in volcanic fluids. Geochemical effects throughout the sublimation and after interactions with aqueous media

This research indicates the fate of Zr, Hf and Rare Earths during their sublimation from the high-temperature gas phase (100° - 420 °C), in volcanic systems associated with different geodynamic regimes, and processes associated with volcanic sublimation representing the last natural “inorganic” interface where a detailed investigation into the behaviour of Rare Earths had never been conducted. The research was carried out in active volcanic systems at Vulcano (Aeolian Islands, Italy), Santorini (Cyclades Islands, Greece), Phlegrean Fields (Italy), Tenerife (Canary Islands, Spain) and Fogo Island (Cape Verde) where both fumaroles and thermal waters from submarine springs and inland wells were collected and investigated. Both solid newly forming sublimates and the coexisting volcanic gas phase were studied in fumaroles in order to evaluate partitioning of the elements studied during the sublimation. Volatile geochemistry of Zr, Hf and Rare Earths (hereafter reported REE, being the sum of lanthanides and yttrium) data suggested a scenario where REE are transported in the volcanic gas phase as chloride complexes. This indication is apparently also confirmed for Zr and Hf by the Zr partitioning in the volcanic gas phase during sublimation relative to Hf. This evidence is attributed to the formation of [(Zr,Hf)Cl4(H2O)4] and other Cl-complexes that are more stable in the volatile phase when Zr is the coordinating metal relative to Hf. Features of shale-normalised REE patterns show positive Gd anomalies in volcanic gas during the sublimation whose amplitude progressively grows with increasing HCl contents in volcanic gas. These features are consistent with reference values of stability constants for chloride-REE complexes rather than for fluorine- and other REE complexes. This data may explain several instances of medium REE enrichments in natural waters during and immediately after large volcanic eruptions and suggests a particular Gd fate relative to other REE during the emission of volcanic soluble substances. Geochemical evidence coming from hydrothermal waters from Vulcano and Santorini Islands confirm the above-mentioned suggestion about the particular Gd behaviour in volcanic fluids since positive Gd anomalies are recognised in waters from submarine vents. Here Gd/Gd* ii up to 1.6 are associated with clear signs of W-type tetrad effects that allow us to identify these fluids as those leaching the authigenic solids that are immediately formed during the mixing between the hot reducing and acidic hydrothermal fluids with cold, oxidizing and basic seawater close to the vent. This and other geochemical evidence along with model calculations allow us to identify two different water groups in terms of physical-chemical characters, calculated saturation/oversaturation with respect to carbonates (group 1 waters) and Fe-oxyhydroxides (group 2 waters) and reciprocal Zr-Hf behaviour dissolved in natural pools. In both waters, the dissolved speciation of these elements is dominated by [Zr(OH)4]0 e [Hf(OH)5]- complexes. These species being differently charged, they fractionate during interactions with occurring solid surfaces. Our data confirms that Zr and Hf undergo a competition process between dissolved speciation and surface adsorption, but also suggests that the larger Hf surface reactivity, especially onto Fe-oxyhydroxides, can be related to the Hf surface complexation rather than to a simple interaction attributed to electrostatic attractions. Our findings prove that passive volcanic degassing can represent a suitable Gd source in a soluble and bioavailable form. Model calculations based of CO2 fluxes from the studied active volcanic areas indicate that about 1 kg Gd per year is released as a whole into the atmosphere. Comparing this value with reference data, this indication suggests that the volcanic Gd-flux is of the same order of magnitude as the yearly anthropogenic Gd delivered to the hydrosphere from hospital wastewaters in Germany. Therefore, Gd from volcanic source could represent a potential environmental risk under particular conditions

Dissolved major and trace elements in meteoric depositions on the flanks of Mt. Etna (Italy): the impact of volcanic activity on the environment

In the framework of the “Save the Etna World” research project, which investigates the impact of the volcanic activity on the surrounding environment, three bulk collectors were deployed on the flank of the Mt. Etna volcano to collect the meteoric depositions. The sampling sites were at distances between 5.5 and 13 km from the summit vents of the volcano on its eastern flank, that is the most exposed to the volcanic plume due to the high-altitude prevailing winds direction. The sites were selected in order to have a gradient of exposition with respect to the volcanic emissions, the most exposed being CIT, the intermediate ILI and the least NIC. Samples were collected monthly from July 2017 to July 2018 and analysed for major ions and for a large suite of trace elements. The influence of volcanic emissions is evidenced by the low pH of the collected depositions in the most exposed site, showing values mostly below 3.5 and never exceeding 5.72. The lowest values are related to high fluoride, chloride and sulfate concentrations in the collected water, deriving from the acid gases (HF, HCl and SO2) of the volcanic plume. The other two sites show pH values in range from 3.95 to 7.21. While the lowest values indicate a lower but significant volcanic influence, the highest values can be related to the dissolution of geogenic (mainly carbonate) particulate of local or regional (Saharan) origin. The latter process is evidenced by high concentrations of Ca and HCO3 in the samples with the highest pHs. Trace elements show almost all higher concentrations in the most exposed site. Highly volatile elements like Pb, Te and Tl, which are known to have strong enrichment factors in volcanic plumes with respect to the average upper crust composition, are found at CIT at concentrations always at least one order of magnitude higher than at NIC. Also lithophile elements like Si, Al, Ti and Fe are sometimes strongly enriched at CIT deriving from the interaction of the acid gases of the plume with the occasionally emitted volcanic ash. These new results confirm the importance of meteoric deposition as main carrier of volcanogenic elements to earth’s surface. “Etna World” is a fascinating natural laboratory, and the study of atmospheric depositions in this peculiar environment allows to understand better the general processes that influence the cycles of trace metals. Furthermore, the quantitative estimation of both emission and deposition of volcanogenic elements is a key factor for complementary studies on the geochemical mobility of trace elements and their distribution between atmosphere, soils, vegetation, and lastly, animals and humans in active volcanic areas

Weathering of evaporites: natural versus anthropogenic signature on the composition of river waters

Weathering of evaporites strongly influences the chemistry of continental runoff, making surface waters poorly exploitable for civil uses. In south-central Sicily, this phenomenon is worsened by the occurrence of abandoned landfills of old sulphur and salt mines. The industrial evolution of the Bosco-S. Cataldo mining site leaved two landfills from the early exploitation of a sulphur mine followed by that of a kainite deposit. In particular, the weathering of these landfills leads the dissolved salt (TDS) values up to about 200 g l−1 in the Stincone–Salito Stream waters. This process induces the V, Cr and Fe desorption from sediments and particulates in the aqueous phase under reducing conditions. At the same time, the weathering of salt minerals releases Rb and Cs, originally contained in halite. The overall processes lead to the V, Cr, Fe, Rb and Cs enrichment of waters from the Stincone–Salito Stream system accompanied by a sharp growth of As content, up to about 13 µg l−1, caused by As release from Fe-bearing solids due to the high salinity. Therefore, the scenario of the weathering of Bosco-S. Cataldo mine landfills depicts an environment strongly influenced by effects of the growing salinity and euxinic water conditions where the attained TDS, Eh and pH conditions reduce the natural scavenging capability of the interested river system, favouring a growth of residence time of toxic elements in river waters

RARE EARTHS AND TRACE ELEMENTS CONTENTS IN LEAVES: A NEW INDICATOR OF THE COMPOSITION OF ATMOSPHERIC DUST

The relationship between the trace element distribution in atmospheric particles and leaves of some exposed plants in the environment was recently demonstrated. This indication would suggest that the trace element analysis of leaves in these plants could provide information about the composition, nature and origin of the atmospheric dust dispersed in the environment. In order to corroborate this hypothesis, the distribution of trace elements and Rare Earths were studied in leaves of some endemic plants, in the atmospheric fallout and in soils of rural, urban and industrial ecosystems in Sicily. These elements have been chosen to discriminate the source and nature of different source on atmospheric dust and the larger capability of the composition of the latter materials to influence the metal ion distribution in leaves of studied plants rather than the soil composition. These evidences are related to the recognition both of positive La anomaly and trace element enrichments in studied leaves and to their particular V/Th and Co/Ni signature. On the other hand, some particular normalised REE features recognised in leaves suggest that a limited contribution to the REE budget in studied leaves is provided by the REE migration from roots