The geochemical cycle of Tellurium in volcanic environments

This research is focused on the geochemistry of Tellurium (Te) in active volcanic environments. To this end concentrations of Te have been measured on different matrices (plume, ashes, soils, atmospheric depositions and plants). Samples were collected from different volcanoes around the world: volcanic aerosols from Etna and Vulcano (Italy), Turrialba (Costa Rica), Myakejima, Asama and Aso (Japan), Mutnovsky and Gorely (Kamchatka, Russia), Copahue (Argentina), Nyamuragira and Nyiragongo (D.R. Congo); atmospheric depositions from Etna and Vulcano (Italy), Nyiragongo (D.R. Congo); ash from Etna (Italy), Copahue (Argentina), San Miguel (El Salvador). Among with plume and rain samples, several leaves of plants from volcanic areas were collected for biomonitoring investigations: Etna, Vulcano and Stromboli (Italy), Nyiragongo (D.R. Congo), Nisyros (Greece), Gorely (Kamchatka), Turrialba (Costarica), Masaya (Nicaragua), Soufriere (Guadalupe); for comparison plant samples were collected also from an industrial area (Augusta, Italy) and a rural forest (Ficuzza, Italy). Samples of soils were also collected from Nisyros (Greece) to better understand the soil-plant distribution. From a literature review, there is a scarcity of data available about Tellurium abundances in the environment, expecialy for volcanic areas. This fact can be ascribed to the scarce use of this element in the past. During the last decades, the use of Te increased considerably due to its importance for electronics and solar photovoltaic manufacturing. Moreover, the recent natural disaster of Fukushima in March 2011 drew attention to the environmental impact of isotope Tellurium-132, produced after the nuclear power plant failure. As consequence, a lot of new studies are focused on Te behaviour in the environment. Regarding Te plume concentrations and fluxes from active volcanoes, only few estimations were reported in previous studies. Sixty-five plume samples were collected and analysed thus significantly increasing the existing worldwide dataset. The concentrations of Te detected range from <0.003 to 0.15 µg/m3, significantly higher than the background level. The atmospheric depositions are the most important way to transfer the elements from the atmosphere to the ground and consequently to all the terrestrial environments (soils, groundwater, plants, animals and humans). Nowadays very few data are available about Te abundances in atmospheric deposition and no data in volcanic environments. The data here reported are the first results of the concentrations of Te in rainwater collected close to active volcanoes. The samples of rainwater samples (bulk deposition) representative of the bulk deposition, were collected from three different volcanoes (Etna, Vulcano and Nyiragongo). A total of 98 rainwater samples were analysed for the major and minor constituents including Te, so creating a complete and unique dataset. The results show clearly higher Te contents close to the crater with values up to 3.20 µg/l with respect to far away background values below 0.04 µg/l, suggesting a volcanogenic origin. In particular, the concentrations of Te, among with other volatile metals e.g. As, Bi, Cs, Cu, Cd, Tl, Pb, are inversely correlated with pH values. This trend confirms that the volcanic emissions can significantly impact the chemical composition of atmospheric deposition. In the same way it influences the geochemical cycle of Te, causing a major accumulation into the soil-plants system. Interesting results were obtained by chemical analysis of Te contents in ash leachates. As well as for rainwaters, no data of volcanic ashes leachates are reported in the literature. For this study we analysed the fresh ashes ejected from: Copahue (Argentina) emitted in December 2012, after 12 years of inactivity; Etna (Italy), collected during 20 paroxysmal events occurred in 2011 and 2012; San Miguel (El Salvador) erupted in December 2013. The samples, when possible, were divided in the different grain size, otherwise the bulk grain size was studied. The results highlight the rapid dissolution of Te from the ash surface indicating high mobility of this element among with other known mobile metals. Te is probably carried out during the volcanic explosion forming hydro-soluble salts with the main acid volatile species (SO2, HCl, HF), Te(SO4)2, TeCl4 and TeF4 but also in elemental form. As expected, the experiments showed that the Te concentrations increase with decreasing grain size, due to the increase of specific surface of the ash. This process, including Te dissolution, have important implications on atmospheric deposition chemistry: when the smaller particles are injected into the atmosphere they can be transported for long distance from the source-point, participating in the cloud processing and affecting sequentially the chemistry of the rain, plants and soils in the surrounding of active volcanoes. The plants commonly contain low contents of Te with variable range. The highest concentration of Te is reported in onion and garlic (up to 300 µg/g) and the characteristic garlic odour is caused by vapours of dimethyl telluride. In the last frame of this study, leaves of 11 species were collected and analysed from 9 different volcanic areas; for comparison samples from an industrial area (Augusta) and a rural area (Ficuzza) were also investigated. The same species but in different volcanoes are featuring the same range, while plants that bioaccumulate more Te are Senecio sp., mosses and lichens with values up to 0.575 µg/g. At Nisyros island (Greece), concentrations of Te in two indigenous plants (Cistus sp. and Erica sp.) were compared with those in the soils. For this study, the soil samples were taken in the Lakki Plain caldera inside and outside the main fumarolic areas. Also in this case, the data show the influence of volcanic emissions on Te concentrations, recording the highest values within the active fumarolic areas. Anyway, in general the concentrations inside the caldera (0.056 – 0.746 µg/g) are higher than the local background (0.004 µg/g). Comparing the soil with the plants, the values of Te in plants tend to grow with the increase of this element in soils. The plants, particularly Cistus, tend to take up more Te near the active fumarolic areas. The results showed very high enrichments of Te in volcanic emissions comparable with other volatile elements like selenium, arsenic, thallium and bismuth. Concluding, the results of this study increase our knowledge about abundances and distribution of Te in volcanic areas, providing a great amount of unpublished data regarding Te, among with many others major and trace elements, in different matrices. We highlighted the prevailing volcanogenic origin of Te and the potential impact of this toxic metalloid on the environment. The results also suggest a primary transport of Te in the volatile phase, probably in gaseous form (as also suggested by recent studies) and/or as soluble salts (halides and/or sulfates) adsorbed on the surface of volcanic particles and ashes. Comparison between different volcanic systems, Etna and Nyiragongo with persistent passive degassing from one side and Nisyros without a volcanic plume, suggest a bioaccumulation of Te in vegetation both from soils and directly from the atmosphere, confirming the high geochemical mobility of this element in volcanic emissions. New flux estimations confirm the relevance of this element in the total volatile output from active volcanoes. The first estimates of volcanic flux of tellurium from Etna range from 0.26 to 12.3 tons per year, confirming that this volcano is one of the biggest point sources to the atmosphere also for this element. The new dataset, among with previous data, allowed to extrapolate a Global Volcanogenic Flux of Te (GVF-Te) through the Te/SO2 ratios and the global budget of SO2 from active volcanoes taken from literature. The total output estimated range from 15.3 to 316 ton/a with a median value of 98, coherently with the estimation obtained from previous authors. Moreover, this value is significantly high if compared with the global production of Te (about 500 ton/a in 2010) by human industrial activities.