CHAPTER 1 INTRODUCTION This chapter consists of the overall discussion on the mercury issues in which the research topic is concerned, the aims, motivations, as well as the outline of the research scope. 1.1 Mercury An early document filed about mercury can be related to the death of Emperor Ying Zheng (259-210 BC) that founded the Chinese imperial system and fascinates people with the gigantic Great Wall, the terra-cotta warriors, and horses in China. He was reportedly died of gulping down mercury pills which were believed to lead him an immortal life (Zhao, Zhu, & Sui, 2006). It cannot be denied that the existence of this element has led a long historical lifetime. Mercury with the symbol Hg and atomic number of 80 is also known as quicksilver or hydrargyrum. Mercury has a silvery white colour which freezes into a soft solid at -38.87 °C and boils at 356.9 °C ("Mercury (Hg)," 2011). Being placed in the d- block in periodic table, it is an unusual transition metal which is liquid at room temperature and pressure. It is notable that mercury has high vapor pressure at room temperature (1.9 ×10-3 torr) and the only element that can exist as a monoatomic vapor besides noble gasses (Greenwood & Earnshaw, 1984; Hutchison & Atwood, 2003). 0 Mercury exists in three different forms: elemental- Hg (zero valent), inorganic- II I II Hg2 (Hg ) (monovalent), and Hg (divalent) as well as organic- RHg. It is interesting to II note that the monovalent oxidation state is only found as the diatomic Hg2 , and never straight as HgI (Hutchison & Atwood, 2003). Table 1.1 summarizes the key properties, transport and fate of different forms of mercury in the environment. In aquatic systems, 1 mercury exists in elemental, inorganic, and organic forms. Elemental mercury (Hg0) has high volatility and relatively low water solubility. Aqueous inorganic mercury has two valences, +1 and +2, however mercury with +2 is more widely spread in the environment. Hg(II) consists of both Hg2+ free ions and Hg2+complexes. Chloride, hydroxide, sulfide, dissolved organic matter, and other chemicals are found in Hg2+ complexes. Aqueous organic mercury can be divided in 2 categories: covalently-bonded organomercurials (methylmercury and dimethylmercury) and mercuric complexes with organic matter (humid substances) (Wang, Kim, Dionysiou, Sorial, & Timberlake, 2004).The common mercury transformation is as below (Okoronkwo, Igwe, & Okoronkwo, 2007): Table 1.1: Key properties, transport and fate of different forms of mercury (EPA, 1999). Elemental or Divalent or mercuric Methylmercury Forms 0 2+) + metallic (Hg ) (Hg (CH3Hg ) Bound to airborne particles Lipophylic ion Comprises 5 % of produced by bacteria in 95 % of atmospheric mercury Key the water column or atmospheric Found in soil and water Properties sediment mercury is Hg0 as a number of complex Nearly all mercury in vapor ions fish is methylated. May form inorganic mercuric salts Tends to remain Enters food chain airborne Easily deposited to through aquatic biota Not easily Earth’s surface in dry and uptake into fish deposited form or in precipitation tissue Transport May travel long Once in water, may Bioaccumulates as it and fate distances before volatilize or partition travels up the food conversion to intoparticulates and be chain, reaching highest other forms and transported to concentrations in deposition sediment. organisms at highest trophic level. 2 Mercury also shows a distinct tendency to form strong bonds with sulfur, to the extent that thiol compounds are sometimes known as mercaptans (“mercury capturing”). This can at least partially be explained by the hard soft acid base (HSAB) concept. Sulfur is a quintessential soft base and mercury is one of the best examples of a soft acid (it has even been argued that the methyl mercury ion serves as the “soft” equivalent of the “hard” proton). Therefore, it is expected that mercury and sulfur species would form stable compounds (Hutchison & Atwood, 2003). Where does mercury come from? Mercury can come from two different sources, i.e. natural and anthropogenic sources. Figures 1.1 and 1.2 depict the evaluation of global mercury emissions by natural and anthropogenic sources (Pirrone, et al., 2010). Among the natural sources, it is obvious that ocean is the main contributor for global mercury emission. On average, coastal waters and Mediterranean Sea have the highest evasional flux, 1.83 and 1.96 ng m-2 h-1, respectively (Hedgecock, Pirrone, Trunfio, & Sprovieri, 2006; Pirrone, et al., 2003). Mason reported recent estimates of total mercury evasion from ocean basins and lakes, which account for 2778 Mg yr-1 (37 % GEb) of net gaseous mercury evasion to the atmosphere (Mason, 2009). Numerous studies pinpoint that the evasion of elemental mercury from surface waters is primarily driven by (i) concentration gradient of mercury between the top-water microlayer and air above the surface water, (ii) solar irradiation which is responsible for the photo-reduction of oxidized mercury in the top-water microlayer, and (iii) the temperature of the top-water microlayer and air above the surface water (air-water interface) (Hedgecock, et al., 2006; Pirrone, et al., 2003; Pirrone, Sprovieri, Hedgecock, Trunfio, & Cinnirella, 2005). Mercury emission from biomass burning was reported as the second largest contributor after oceans in year 2008. The most recent estimate suggests that about 675 Mg of mercury is released to the atmosphere from biomass 3 burning every year, (annual average for the period 1997-2006), which accounts for 13 % of the total contribution from natural sources (Friedli, Arellano, Cinnirella, & Pirrone, 2009). On the other hand, the factors which have influenced the mercury emissions from top soil and vegetation are meteorological conditions, historical atmospheric deposition, as well as the type of vegetation and top soil (Pirrone, et al., 2010). According to Rea et al., 2002, mercury emissions from vegetation depend on the mercury uptake from the atmosphere, atmospheric deposition to foliage and mercury uptake from roots (Rea, Lindberg, Scherbatskoy, & Keeler, 2002). Summarizing all the net mercury fluxes from all regions (Forests, Tundra/ Grassland/ Savannah/ Prairie/ Chaparral, Dessert/ Metalliferous/ Non- vegetated Zones and Agricultural areas), the total net global mercury evasion is estimated 1464 Mg yr-1 in 2008. Mercury emissions from natural processes (primary mercury emissions-reemissions), including mercury depletion events, is expected to be 5207 Mg yr-1, which constitutes about 4 % of the total mercury emissions from natural sources in 2008. Meanwhile, the contribution from volcanoes varies over time depending on the degassing or eruption phase (Pirrone, et al., 2010). In general, volcanoes and geothermal activities emit nearly 90 Mg yr-1 of mercury to the atmosphere (Mason, 2009). Mercury emissions from calderas are important natural source of mercury, for instance, the Phlegrean fields (Pozzuoli, Italy) show fluxes of mercury, as Hg-S complexes, in the range of 0.9 to 19 g day-1 (Bagnato, Parello, Valenza, & Caliro, 2009; Ferrara, Mazzolai, Lanzillotta, Nucaro, & Pirrone, 2000). In summary, the global mercury emission by natural sources in 2008 is estimated at 5207 Mg yr-1. On the contrary, mercury is also discharged from man-made sources to the atmosphere which embraces fossil-fuel fired power plants, gold mining production, ferrous and non-ferrous metals manufacturing, caustic soda production, cement production, vinyl 4 chloride monomer (VCM) production, medical and industrial waste. The parameters employed to estimate mercury emission from anthropogenic sources are: the bulk material amount, mercury content of the material and the technology adopted to reduce emissions (Pirrone, et al., 2010). Fossil fuel-fired power plants are the largest mercury contributor to the atmosphere. World coal consumption in 2006 was 6118 Tg, responsible for the primary fuel used in electrical power generation facilities (42 %) and attributed for about 27 % of world’s energy consumption (EIA: International Energy Outlook 2009, 2009). Wood waste is primarily used to produce heat in the industrial sector, while wood is burned in wood stoves in the residential areas with no emission control in most of the developing countries (Mukherjee, Bhattacharya, Sarkar, & Zevenhoven, 2009). Oil burning as part of the fossil fuels category has also contributed to mercury emissions to the atmosphere. The top five countries which are using oil for power generation comprises of USA, Japan, Russia, China, and Germany. Bulky volumes of distillate and residual oils are burned each year to accommodate the worldwide consumption for electric utilities, commercial and industrial boilers, as well as residential boilers (Pirrone, et al., 2010). Fuel oils contain mercury with concentrations that vary with crude oil type (Wilhelm, 2001). The values are in the intervals between 0.007 to 30 g Mg-1, with a typical value of 3.5 g Mg-1 (Mukherjee, et al., 2009; Wilhelm, 2001). It can be envisaged that mercury concentrations in residual oils are higher than those found in distillate oils and the heavier the refinery fractions, the higher the mercury contents (Pirrone, et al., 2010). Natural gas might contain small amount of mercury but it is usually eliminated from the raw gas during the recovery of liquid constituents and hydrogen sulfide removal processes. Hence, it can be assumed that mercury emissions from natural gas combustion are insignificant comparing to other sources (Pirrone, Keeler, & Nriagu, 1996; Pirrone, et al., 2001). The release of mercury 5 from artisanal and gold mining activities is one of the significant environmental issues as most of the activities are carried out in developing countries. The estimation mercury released from this industry is 17 % out of the total of 2320 Mg yr-1. Whilst, smelting processes to produce metals such as copper, zinc, lead, nickel, as well as gold are believed to be one of the largest mercury contributors, especially in developing countries (Telmer & Veiga, 2009; UNEP, 2002). Combustion temperatures in the smelting boilers, furnaces and roasters are key aspects affecting the amount of mercury discharged into the atmosphere.