**2. Physicochemical properties of mercury**

various chemical forms can differ in bioavailability, transport, persistence, and toxicity. Still, every mercury species is toxic with methyl mercury being the most toxic species. The World Health Organization (WHO) recommends a maximum methyl mercury intake of 1.6 µg Kg-1 per week, while the Environmental Protection Agency (EPA) lists a maximum recommended intake of 0.1 µg Kg-1 of body weight per day for adults. Due to high bioaccumulation, mercury is found on many levels of the food chain (Hinton and Veiga, 2001; Bengtsson, 2008). Any form of mercury in the environment may evolve into a more toxic species (methyl mercury) under biogeochemical transformation processes (Figure 1). Due to these processes and the high mobility of mercury species, a good understanding of how mercury species transform and accurate monitoring are essential for assessing the risk of mercury in the environment.

828 Environmental Risk Assessment of Soil Contamination

The impact of mercury depends strongly on its chemical species; understanding mercury transformations and the impact of its various chemical forms are vital to preventing harmful effects on humans and the environment. Nevertheless, the physicochemical characteristics of mercury are either useful or necessary for many industrial and agricultural applications, and mercury may be scattered over large area, depending on the source (Leopold et al, 2010).

**Figure 1.** Biogeochemical transformation processes of mercury (Leopold et al, 2010).

Mercury concentrations in ground water indicate that the highest concentration of mercury in groundwater comes from the soil and from aquifers. While simulating mercury predictions can often be difficult, mercury can be estimated in experiments conducted in batch mode or in columns. The percentage of Hg that can potentially leach from the soil was previously estimated in batch experiments. Distribution analyses of species in leachate confirmed the

Mercury (Hg) is a chemical element with an atomic number of 80. Mercury is a silver plated heavy metal, liquid and odorless at normal conditions. It easily alloys with many other metals like gold or silver producing amalgams, is insoluble in water and soluble in nitric acid. The main source of Hg is cinnabar or mercury sulfide (HgS), a stable compound and insoluble usually recovered as a byproduct of ore processing. Mercury in this form is found in the earth's crust average concentrations of 0.5 ppm (Hinton and Veiga, 2001).

Mercury is one of the most toxic elements to human health and ecosystem. At temperatures above 40 °C mercury produces toxic and corrosive fumes. It is harmful by inhalation, ingestion and contact, is a very irritating to skin, eyes and respiratory tract, even to nervous system, its gaseous form is absorbed by lung tissues (Hinton and Veiga, 2001, Bengtsson, 2008).

A wide variety of mercury species exist in the environment and its various chemical forms can differ in bioavailability, transport, persistence, and toxicity. Still, every mercury species is toxic with methyl mercury being the most toxic species. This element can exist in the environment as elemental (Hg0 ), oxidized inorganic (Hg2+ -mercuric, Hg2 2+-mercurous) or oxidized organic (methyl/ethyl mercury) forms. Mercuric and mercurous forms are more stable under oxidizing conditions. In moderately reducing conditions, the organic or inorganic mercury can be reduced to its elemental form and be converted to forms leased by biotic or abiotic processes: these are the most toxic forms of mercury, as well as being soluble and volatile. Hg (II) forms strong soluble complexes with a variety of organic and inorganic ligands oxidized in aqueous systems. Hg sorption in soil, sediment and humic materials is an important mechanism for the removal of mercury from solutions, another mechanism, a high pH is their co-precipitation sulfide (HgS) (Leopold et al, 2010).

Any form of mercury in the environment may evolve into a more toxic species (methyl mercury) under biogeochemical transformation processes. Due to these processes and the high mobility of mercury species, a good understanding of how mercury species transform and accurate monitoring are essential for assessing the risk of mercury in the environment. The impact of mercury depends strongly on its chemical species; understanding mercury trans‐ formations and the impact of its various chemical forms are vital to preventing harmful effects on humans and the environment. Nevertheless, physicochemical characteristics of mercury are either useful or necessary for many industrial and agricultural applications, and mercury may be scattered over large area, depending on the source (Leopold et al, 2010; Nick, 2012).

The metal mercury (Hg0 ) is mainly used to produce chlorine gas and caustic soda, and is part of some types of alkaline batteries, fluorescent lamps, electrical contacts, and instruments such as pressure gauges and thermometers, among others. Hg salts are used in antiseptic ointments and creams and skin lightening. Among the activities that generate the most pollution by Hg, is the burning of coal and chlor-alkali plants: other important sources are mining and metal‐ lurgy and the burning of municipal solid waste, which may contain instruments such as pressure gauges, thermometers, alkaline batteries and fluorescent lamps. The mercury released into the air tends to settle and adhere to soil organic matter (Hinton and Veiga, 2001; Nick, 2012).

Natural and anthropogenic mercury emissions are mainly in the form of elemental mercury (Hg0 ), which makes up about 99 % of total atmospheric mercury. However, biogeochemical transformations can oxidize it, forming Hg+ and Hg2+. Most inorganic Hgcompounds are water soluble in small doses, and can be found in soil and sediments. In contrast, the presence of inorganic forms of Hg2+ bonded to organic and/or inorganic species ([HgClx] 2-x; [HgII-DOC]; [HgS]) depends on the local chemical environment. The life time of these compounds in air is very short (on the scale of minutes) and they are rapidly removed by deposition processes because of high water solubility and surface activity. Figure 2 shows the main mercury species in the atmosphere, hydrosphere and sediment (Leopold et al, 2010; Nik, 2012; Slowey et al, 2005; Wartel et al, 1999; Shi et al, 2005).

Over 90 % of surface water mercury is from atmospheric deposition. Hg2+ usually undergoes a biomethylation process that forms methylmercury (MeHg, CH3Hg+ ) and dimethyl mercury (DMeHg, (CH3)2Hg), though these reactions can be reversed using microorganisms and/or photolytic decomposition. All these species are highly mobile.

Three main forms of mercury are found in natural waters: elemental mercury (Hg0 ), inorganic Hg2+ (Hg2+and its complexes) and organic mercury (MeHg, MeHg complexes and DMeHg). With solubility (at 25°C) of 0.08 mg L-1, Hg0 can be found at all depths. Inorganic mercury (Hg2+) and MeHg forms complexes with other dissolved compounds in fresh water, but for the most part, only forms complexes with chlorine in sea water. DMeHg is found in the deep sea. (Wartel et al, 1999, Shi et al, 2005, Slowey et al, 2005).
