Mercury (Hg), the most fascinating, rare and potent neurotoxic heavy metal along with distinctive, singular properties like a liquid at room temperature distinguishes it from other elements, belongs to d-group elements. It is non-essential, highly toxic and persistent pollutant that is globally distributed due to its strong persistence and bioaccumulative nature in the environment. Mercury has a high vapor pressure and low solubility, results in freely departure into the atmosphere. Also, Hg is a non-biodegradable element and hence remains persistent in the atmosphere for several years. Now these days the climax of mercury is ascending due to post-industrial activities like combustion of coal, fossil fuel and petroleum, the operation of mercurial fungicides in farming, paper manufacturing industry, mercury catalysts used in industries, Chlor-alkali plants , gold mining, manufacturing of non- ferrous metals, remission from previously deposited mercury on various surface environments like terrestrial region, and cement production leads to a significant increase in global mercury pollution. While other major factors of mercury pollution are natural sources like hydrological cycle, soil erosion, geothermal activities, and wildfires. Notably, mercury released by volcanic sources and burning of coal is an estimated global total of 60,000 kg and 3,000 tons of mercury per year and that is same as the amount released by all the industrial activities. Different forms of mercury are toxic at different levels, among all organic mercury (methyl mercury) is highly toxic while other naturally occurring compounds like mercury sulfide also known as cinnabar is non-toxic. Furthermore, methylmercury was responsible behind the Minamata disease in Japan (1952). During the Minamata disaster, industrial wastewater contaminated with methyl mercury was continuously discharged into the bay, which affected the aquatic life followed by human life and another case was held in Iraq (1971) where organo mercury fungicides were used in grains treatment which further consumed by humans, huge population was disturbed by these accidents . Anthropogenic activities, fossil fuel combustion, and atmospheric circulation have enhanced 3 to 10 times Hg in soil and sediments. In 2005, the global Hg emission was reported to be 3000 tons. Moreover, 800 tons Hg per year alone released by the burning of fossil fuel and become the superior anthropogenic source of Hg in the atmosphere. Being recalcitrant in nature, removal of Hg is quite difficult and through bioaccumulation, it transfers to the food chain and causes a threat to the human being. As mercury is easily absorbed by alimentary tracts, it penetrates into the placenta, with the passage of the bloodbrain barrier; it disrupts the function of the membrane, protein compounds, nucleic acids, and other enzymes. Heavy metal does not degrade easily like organic pollutants. Because of having versatile nature and the best conductor of electricity, it is also used in various applications like a thermometer, thermostats, catalysts, electrode materials, electrical switches, reflective liquid in liquid mirror telescope, medicine (dental amalgam), fluorescent light bulbs and ballast for submarines. Immense increase in the level of mercury in the terrestrial as well as aquatic ecosystem decreases plant yield and also disturbs the stability of the food web. There are 3 different categories of heavy metals to which we should show concern about, primarily the heavy metals like Hg, Cd, As, Sn, Pb, Co, Cu, Ni, Mn, Fe, etc., radionuclides like Ra, U, Th, Am etc., and the adored metals like Ag, Ru, Pt, Au, Pd etc. Among all mercury, cadmium, and lead is regarded as ‘toxic trio’ and have no biological importance, also considered as highly toxic and threatening. Distribution of mercury in the environment Natural occurrence and chemistry of mercury: Mercury is commonly known as Quicksilver and was formerly named as hydrargyrum. It is silver-white liquid metal belongs to the D group element with extraordinary properties like a liquid at room temperature other than bromine. It exists in elemental, organic and inorganic form, in which organic mercury is highly toxic (methylmercury). It is used in various scientific research applications and in amalgam for dental restoration in some locales. The Mercury cycle: During biogeochemical cycle of mercury, it endures many physical and chemical transformations and circulated into the atmosphere. There are two cycles is involved in the distribution and transportation of mercury cycle, i.e., global and local. These cycles involve in the atmospheric circulation of elemental mercury and methylation of inorganic mercury from contaminated environment. The mercury cycle in the environment continues until it gets deposited into reservoirs like deep-ocean sediments and this cycle in the environment is circulated by biological and geological processes. The concentration of mercury in the surface of the earth’s crust varies and found 21 ppb in the lower crust to 56 ppb upper crust in different organic, inorganic and elemental form. Generally, in nature, mercury occurs in 3 valence states, i.e., Hg0 (zero oxidation state, metallic mercury), Hg2+ (inorganic mercury, mercuric mercury), and Hg22+ (mercury mercury) and these states of mercury balance stability among themselves by the processes of chemical dismutation: 〖Hg〗^(2+)→〖4Hg〗^0+〖Hg〗^(2+) Besides elemental form (Hg0), the inorganic and organic form (monomethyl mercury) of mercury is found in soil, water, sediments, and biota but dimethyl mercury is only organic form of mercury found in very less concentration in marine ecosystem. Mercury toxicity in an aquatic ecosystem is affected by its salinity, dissolved oxygen, temperature, and water hardness. Elemental mercury is the predominant species, with a residence time of 0.5 to 2 years in atmosphere due to high solubility and chemical inertness in water while mercuric mercury is reactive gaseous mercury, highly water-soluble, less volatile than elemental mercury with a residence time of days in the atmosphere . The natural and anthropogenic source of global mercury discharge into the atmosphere is mercury vapor (Hg0) and this mercury vapor as a result of its interaction with ozone in presence of water further photo-oxidized to ionic mercury. The gaseous elemental form of mercury is easily dispersed into the atmosphere. However, inorganic mercury reaches to the surface of earth through rain precipitation where micro-organism present in aquatic as well as soil ecosystem converts it in different oxidation form of mercury. The elemental mercury has ability to vaporize easily into the air, so new mercury starts once elemental mercury reaches into the air. Hg2+ plays a fundamental role in the biogeochemical cycle of mercury and toxicology of living things as it is a resultant (product) of the metabolism of vapor Hg and other organic compounds of Hg. Methylmercury is highly toxic among all compounds of mercury, if methylmercury is formed, it gets bio accumulated and translocated in the food chain. As a result, the bio magnification of the organic form of methylmercury causes the predator to have a higher mercury concentration . By methylation from inorganic to organic form, several bacteria or fungi transforms available mercury in the environment Hg2+ and form a more potent toxic compound than its precursor, mainly sulfur reducing bacteria, iron reducing and methanogenic bacteria are capable of Hg methylation in comparison to other microorganisms. For transformation of Hg species, sulfur cycle plays an important role as mercury has a high affinity towards sulfur-containing compounds and proteins such as glutathione and metallothionein.
