Mercury in global environment

The rapid growth of industry since the beginning of modernization era is followed by the increasing of problem awareness caused by it. One of them is the heavy metal pollution problem, especially the mercury compound. Mercury is known as a heavy metal which used for industrial purposes. However, behind its usefulness, just like other heavy metals, mercury is very dangerous, moreover if it enters the human and other organism’s body. Mercury is released to the environment from natural sources and processes and also as a result of human activities. Mercury can go through a cycles between air, land, and water once it has entered into the natural environment, and it will be removed through burial in deep ocean or lake sediments or through entrapment in stable mineral compounds. However, after it is buried in sediment, actually mercury can be re-suspended by physical and chemical process, such as currents or wave and bacteria activities. Especially the methylmercury (MeHg), the most toxic and bioaccumulative form of mercury, which presents the greatest health risk to humans and wildlife is mainly formed in aquatic environments through natural microbial processes.

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2.1.1 Source of mercury emission to air and water

The sources of mercury emission can be divided into three sources, first is natural sources, second is anthropogenic sources, and last is the re-emission/re-mobilization of it. Basically, mercury can be found throughout the world as a natural element. It is contained in some minerals, such as cinnabar, an ore mined to produce mercury. Then, the anthropogenic is a human activity such as mining and burning of coal which also has increased the mobility of mercury in the environment. In addition the mercury itself will go through the re-emission and re-mobilization (UNEP, 2013).

Natural process of mercury emission, namely such as the mercury-containing rocks which experiences weathering continuously will allows mercury to escape to air and to be washed into lakes and rivers. Furthermore, eruption of volcano can emit and release mercury and geothermal activity can take mercury from underground and emit it to the air and release it to the deep ocean. This natural emission of mercury contributes around 10% of the estimated mercury in the atmosphere from all sources.

Mercury emission by human activity has already begun since 1800 when the revolution of industry occurred. At that time, coal burning, mining, cement production, and oil refining have increased the mercury emission to atmosphere and release to water environment. Moreover, the emission can be caused by wastes from consumer products such as batteries, paints, switches, electronic devices, thermometers, blood-pressure gauges, fluorescent lamps, pesticides, fungicides, medicines, and cosmetics. In addition, mercury is also used in a number of industrial processes. A major industrial use is in the chlor-alkali industry where mercury-cell technology may be used in the production of chlorine and caustic soda. Mercury is also used as a catalyst in the production of vinyl chloride monomer (VCM) from acetylene, mainly in China. This anthropogenic sources of mercury contributes for about 30% of the total amount of mercury entering the atmosphere each year and 50% of the atmosphere emission is from Asia region, namely East and Southeast Asia (UNEP, 2013).

The last source of mercury emission is the re-emission and re-mobilization processes of mercury. It contributes around 60% of mercury emission to the air. Re-emission is a

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result of natural processes that convert inorganic and organic forms of mercury to elemental mercury, which is volatile and therefore readily returns to the air. Mercury deposited to plant surfaces can be re-emitted during forest fires or biomass burning.

Mercury may be deposited and re-emitted many times as it cycles through the environment. In the aquatic environment, re-mobilization of mercury occurs when mercury deposited on and accumulated in soils or sediments is re-mobilized by, for example, rain or floods that cause the mercury to enter or re-enter the aquatic system.

Resuspension of aquatic sediments due to wave action or storm events is an additional way for mercury to re-enter the aquatic ecosystems.

2.1.2 Global mercury budget

The presence of mercury in the land, air, and ocean is in unsteady state and it can change over between those 3 reservoirs. The model of mercury budget in the air, land and atmosphere has been presented by Selin et al. (2008) and modified in Selin (2009).

Figure 2.1 is the result of global 3D land-ocean-atmosphere model for mercury (Selin, 2009). The black line is indicated natural fluxes of mercury involves atmospheric transport, deposition to land and ocean, and re-volatilization. The red line is the anthropogenic sources, and the red-black dotted line is the combination of both natural and anthropogenic sources, all number unit are in Megagram (Mg)/year or ton/year.

In preindustrial time, the anthropogenic and biomass emission are not included yet, the budgets are written by black color (Figure 2.1). At the present day, the anthropogenic and biomass burning emission are included due to the industrial activities which use mercury. Human contribution of mercury emission to the air and ocean gives significant number of the mercury amount in the atmosphere and ocean. The most dominant source is the anthropogenic sources that has made the atmosphere budget increases as well as the deposition to land and ocean, and made mercury budget in surface ocean also increases. The ultimate sink of mercury is burial to the deep-ocean sediments, which occurs very slowly. The overall (e-folding) lifetime of mercury in the combined

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atmosphere-ocean-terrestrial system against transfer to the sediments is 3,000 years (Selin, 2009).

Figure 2.1. Global Mercury Budget. [Unit: Mg/year]

Reprinted from “Global Biogeochemical Cycling of Mercury : A Review,” by Selin, N.E, 2009, Annu.

Rev. Environ. Resour, 34, p. 43-63. Copyright 2009 by Annual Reviews

The geogenic, antrophogenic, biomass burning, soil and vegetation contribute for mercury emission to the atmosphere resulting around 5,600 Mg/year mercury stays in the atmosphere. In soil, the mercury amount is around 1.15x10⁶ Mg/year, also there is small fraction that released from land to ocean through river and ground water. Rivers are estimated to carry more than 2,800 tons of mercury each year, but only about 380 tons of this is transported offshore. The rest is trapped by particles in estuaries.

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Groundwater and re-mobilization from sediments provide 100-800 tons of mercury to the oceans each year (UNEP, 2013). From atmosphere, the elemental mercury Hg(0) and particulate mercury Hg(II)P will be deposited again to the land and ocean, where the ocean receives more than land. In ocean the mercury budget is divided in 3 area, first is the surface which directly interact with atmosphere deposits around 7,000 Mg/year mercury. Then it will through exchange with deep ocean and it deposited around 350,000 Mg/year. In the end it will be buried in very slow process to the deep-ocean sediment which deposits 3x10¹¹ Mg/year mercury (Selin et al., 2008; Selin 2009).

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