Environmental impact of magmatic fluorine emission in the Mt. Etna area

Abstract

Fluorine is the most reactive and the most electronegative of all elements, meaning that it has a powerful attraction for electrons and that it is able to attack all other elements, with the exception of oxygen and nitrogen, so it is not found in the free elemental state in nature. Fluorine is widely distributed throughout the earth’s crust as the fluoride ion. Fluorine is reported to be the 13th most abundant element in the earth’s crust (Smith and Hodge, 1979), with an average concentration of 0.032% by weight. Fluorides are released into the environment naturally through the weathering and dissolution of minerals, the emissions from volcanoes and from marine aerosols (WHO, 2002). Fluorides are also released into the environment via coal combustion and process waters and waste from various industrial processes, including steel manufacture, primary aluminium, copper and nickel production, phosphate ore processing, phosphate fertilizer production and use, petroleum refining, glass, brick and ceramic manufacturing, and glue and adhesive production (WHO, 2002). Based on available data, phosphate ore production and use as well as aluminium manufacture are the major industrial sources of fluoride release into the environment. According to Wellburn (1997), fluorine (in the form of HF) occupies - after O3, SO2 and nitrogencontaining air pollutants - the fourth place with regard to its detrimental effects on harvest, at least in the US. Relative to its weight fluorine even has the highest level of phytotoxicity of all air pollutants. Wellburn (1997) reports that F-related damages at sensitive plants can develop at concentration levels 10 to 10.000 times lower than other pollutants. There is no doubt that inorganic fluoride was one of the major air pollutants of the 20th century damaging crops, forests and natural vegetation, and causing fluorosis in factory workers, livestock and wild mammals. However there have been enormous improvements during the last 40 years in the containment and scrubbing of emissions, so that modern fluoride emitting industries generally have little or no environmental impact outside the factory fence at the present time (Weinstein and Davison, 2003). On the other hand, fluoride emissions from volcanoes and the natural occurrence of excessive amounts of fluoride in drinking water have affected the health of humans and livestock for centuries, if not millennia. For example some historical report tells that Pliny the Elder was dispatched by fluoride-containing fumes from a Vesuvian eruption, although other state that the cause of its death had actually no relation to volcanic activity. Whether the story is true or not,fluoride was certainly the agent responsible for the death of sheep after the volcanic eruption described in the Icelandic sagas, and fluoride emissions from volcanoes continue to affect the health of humans and livestock today (Georgsson and Petursson, 1972; Fridriksson, 1983; Araya et al.,1990; Cronin et al., 2002). Fluorine is emitted by volcanoes mostly as HF, but emissions from Vesuvius and Vulcano in Italy have been shown to contain also NH4F, SiF4, (NH4)2SiF6, NaSiF6, K2SiF6 and KBF4 (Weinstein and Davison, 2003). Volcanoes are also an important source of organo-fluorides, including some CFCs (Schwandner et al., 2004). Estimations of the global release of fluorine to the atmosphere by volcanic activity ranges from 50 to 8600 Gg/a (Cadle, 1980; Symonds et al., 1988; Halmer et al., 2002) with the lowest figures being probably an underestimate. Average HF emission rates from Mt. Etna can be estimated to about 75 Gg/a (Aiuppa et al., 2004a). This makes Mt. Etna the largest known point atmospheric source of fluorine (Francis et al., 1998), even stronger than todays estimated anthropogenic release over whole Europe (Preunkert et al., 2001). The environmental impact of anthropogenic fluorine emissions have long been studied considering all main type of activity, for example coal burning (Ando et al., 2001), aluminium smelters (Egli et al., 2004) or phosphate fertiliser production (Klumpp et al, 1996) and all types of receptors (air – Liu, 1995; glaciers - Preunkert et al., 2001; surface waters – Skjelkvale, 1994; vegetation – Weinstein, 1977; Weinstein and Davison, 2003; soils – Polomski et al., 1982; wildlife – Kierdorf and Kierdorf, 2000; etc.). Considerably fewer studies have been devoted to the consequences of volcanic fluorine emissions and most of them were focussed on the impact of fluorine released through explosive volcanic eruptions (Georgsson and Petursson, 1972; Oskarsson, 1980; Thorarinsson, 1979; Cronin et al., 2002). Recent researches have highlighted that passive degassing – quietly but persistently releasing volcanogenic pollutants - may also have profound impact on the ecosystems downwind, sometimes disrupting the social and economic activities of populations (Delmelle et. al., 2002; Delmelle, 2003). In this context, the impact of volcanogenic fluorine has been assessed on vegetation growing along the flanks of volcanoes (Guadeloupe – Garrec et al., 1977; Masaya – Garrec et al., 1984; Etna – Garrec et al., 1984; Notcutt and Davies, 1989; La Palma – Davies and Nottcut, 1989; Hawaii - Notcutt and Davies, 1993; Furnas - Notcutt and Davies, 1999) on rainwater chemistry (Hawaii - Harding & Miller, 1982; Vulcano Island – Capasso et al., 1993; Etna – Aiuppa et al., 2001; Stromboli Island – Bellomo et al., 2003) and on soils (Delmelle et al.,2003). The aim of the present PhD thesis is to provide original data on the geochemical cycling of fluorine of an active volcanic system like Mt. Etna. An assessment of the impact of volcanic fluorine on the local environment is also attempted by analysing different media (atmospheric air, rainwater, volcanic ashes, vegetation and soil).