Mercury is a rare element. Its average content in the earth’s crust and main types of rocks is estimated at 0.03-0.09 mg / kg, ie, 1 kg of rock contains 0.03-0.09 mg of mercury, or 0.000003-0, 000009% of the total mass (by comparison, one mercury lamp, depending on the design, may contain from 20 to 560 mg of mercury, or from 0.01 to 0.50% by weight). The mass of mercury, concentrated in the surface layer of the earth’s crust with a capacity of 1 km, is 100,000,000,000 tons (one hundred billion tons), of which only 0,02% are in its own deposits. The remaining part of mercury exists in a state of extreme dispersion, mainly in rocks (41.1 million tons of mercury is dispersed in the waters of the World Ocean, which determines a low average concentration of mercury in its waters – 0.03 μg / l). It is this scattered mercury that creates a natural geochemical background, on which mercury contamination is imposed due to human activities and leading to the formation of technogenic pollution zones in the environment.
More than 100 mercury and mercury-containing minerals are known. The main mineral, which determines the industrial significance of mercury deposits, is cinnabar. Native mercury, metacinnabarite, living-stonite and mercury-containing faded ores have a sharply subordinate value and are extracted along with cinnabar.
In total, about 5000 mercury deposits, ore sites and ore occurrences, which have been given an independent name, have been discovered in the world; Of these, about 500 have been developed at different times. But in the history of the mercury industry, the vast majority of mercury (more than 80%) was obtained from 8 deposits: Almaden (Spain), Idrija (Slovenia), Monte Amiata (Italy), Huancavelica (Peru), New Almaden and New Idrija (USA), Nikitovka (Ukraine), Khaidarkan (Kyrgyzstan).
In the industry, two versions of the technology for its extraction from ores are used to produce metallic mercury: oxidation-distillation firing with the release of mercury from the gas phase, and a combined process involving preliminary enrichment and subsequent pyrometallurgical processing of the concentrate. According to experts, about 700,000 tons of mercury were produced by man, a significant part of which is scattered on the earth’s surface. The amount of mercury that has entered the habitat during other types of human activity (when mining various minerals, smelting metals, producing cement, burning fossil fuels, etc.) is also great.
Mercury concentrates not only in mercury minerals, ores and their enclosing rocks. According to the Clark-Vernadsky law on the universal dispersion of chemical elements, in some quantities, mercury is found in all objects and components of the environment, including meteorites and lunar soil samples. In high concentrations, mercury is found in the ores of many other minerals (polymetallic, copper, iron, etc.). The accumulation of mercury in bauxites, some clays, combustible shales, limestones and dolomites, in coals, natural gas, oil.
Modern data indicate a high content of mercury in the mantle (the second from the surface, after the earth’s crust, the Earth’s shell), as a result of its degassing, as well as the natural process of evaporation of mercury from the earth’s crust (rocks, soils, waters), there is a phenomenon called “Mercury respiration of the Earth.” These processes go on constantly, but are activated by volcanic eruptions, earthquakes, geothermal phenomena, etc. The supply of mercury to the environment as a result of the mercury respiration of the Earth (natural emission) is about 3000 tons per year. The supply of mercury to the atmosphere caused by human industrial activity (man-made emissions) is estimated at 3600-4500 tons per year.
In natural conditions, mercury usually migrates in the three most common states – Hg0 (elemental mercury), Hg2 + (divalent mercury ion), CH3Hg + (methylmercury ion), and also in the form of the less abundant Hg22 + ion. Hg (ll) , Rather than Hg (l). In the waters between Hg0, Hg22 + and Hg2 +, an equilibrium is established, which is determined by the oxidation-reduction potential of the solution and the concentration of various substances forming complexes with Hg2 +. Hg (II) ions form stable complexes with biologically important molecules. It is the high chemical affinity of mercury (II) and its methylated compounds to biomolecules that determines to a significant extent the toxicological hazard of mercury under environmental conditions.
Distribution and migration of mercury in the environment are carried out in the form of a cycle of two types. First, a global cycle involving the circulation of mercury vapor in the atmosphere (from land-based sources to the World Ocean and vice versa). Secondly, the local circulation, based on the processes of methylation of inorganic mercury, coming mainly from technogenic sources. Many stages of the local circulation are not yet sufficiently clear, but it is believed that it involves circulation in the habitat of dimethylmercury. It is with the second type of circulation that formation of dangerous situations from environmental positions is most often connected.
Entering the environment from natural and man-made sources, mercury and its compounds undergo various transformations in it. Inorganic forms of mercury (elemental mercury Hg0 and inorganic ion Hg2 +) undergo transformation as a result of oxidation-reduction processes. The mercury vapor is oxidized in water in the presence of oxygen by inorganic divalent mercury (Hg2 +), which is significantly facilitated by the organic substances present in the aqueous medium, which are especially abundant in the contamination zones. In turn, ionic mercury, entering or forming in water, is capable of forming complex compounds with organic matter. Along with the oxidation of mercury vapors, the formation of Hg2 + can occur when mercury-organic compounds are destroyed.
Inorganic mercury Hg2 + undergoes two important types of transformations in the environment. The first is a recovery with the formation of mercury vapor. This process, which is key to the global mercury cycle, has been poorly studied. It is known that some bacteria are able to perform this transformation. The second important reaction to which Hg2 + is exposed in nature is its conversion into methyl- and dimethyl derivatives and their subsequent interconversion into each other. This reaction plays a key role in the local circulation of mercury. Importantly, methylation of mercury occurs in a variety of conditions: in the presence and absence of oxygen, different bacteria, in various water bodies, in soils and even in atmospheric air. Especially intensively, the methylation processes take place in the upper layer of the sediment rich in organic matter in water bodies, in suspended matter in water, as well as in mucus covering the fish. Methylation leads to the formation of monomethyl- and dimethyl mercury compounds. Monomethylmercury (CH3-Hg +), usually spoken and written just “methylmercury”), having, as already said, a high affinity for biological molecules, is extremely actively accumulated by living organisms. The factors of bioconcentration, i.e. the ratio of methylmercury content in fish tissues to its concentration in water, can reach 10,000-100,000. Dimethylmercury (CH3) 2Hg, differing in its high solubility and volatility, volatilizes out of the water into the atmosphere, where it can be converted into monomethyl mercury, removed from rainfall and returned to water and soil, thus completing the local circulation of mercury.
Typical natural (background) concentrations of mercury vapors in the surface layer in the ambient air are usually 10-15 ng / m3 with fluctuations from 0.5-1 to 20-25 ng / m3. Apparently, such content is practically safe for living organisms. In pollution zones, concentrations increase tens and hundreds of times, and in industrial or mercury-contaminated rooms they can reach extremely high values (up to 1-5 mg / m3). The main form of mercury in the atmosphere is the metal vapor (Hg0), the ionic form, organic and inorganic (chloride, iodide) compounds are of less importance. It also binds to aerosols. In zones of pollution, mercury concentrations in rainwater reach 0.3-0.5 μg / l and even more (with a background usually not more than 0.1 μg / l). In cities, there is an increase in the amount of mercury carried with aerosols and atmospheric dust.
The background levels of mercury in natural soils depend on their type, but in most cases they are in the range 0.01-0.1 mg / kg. The lower limits are characteristic of sandy soils, the upper ones for soils rich in organic matter. The contents exceeding these values are related to the influence of pollution. In contaminated zones, mercury levels, especially in the upper horizons of soils, increase tens of times, sometimes even thousands of times. In soils, mercury is actively accumulated by humus, clay particles, it can migrate down the soil profile and enter groundwater, be absorbed by vegetation, including agricultural, and also be released as fumes and dust into the atmosphere. If the soils are heavily soiled, mercury concentrations in the air can reach dangerous levels for humans.
In surface waters, mercury migrates in two main phase states – in water solution (dissolved forms) and in suspension (suspended forms). In turn, in a solution of water it can be in the form of a divalent ion, mercury hydroxide, complex compounds (with chlorine, organic matter, etc.). Among the Hg (II) compounds, we already know about this, a special role belongs to the organochlorine compounds in terms of their ecological and toxicological significance. The most important accumulators of mercury, especially in conditions of pollution, are suspended matter and bottom sediments of water objects. The highest concentrations of mercury are characterized by man-made muds actively accumulating in rivers and reservoirs, where industrial wastewater enters. Mercury levels in them reach 100-300 mg / kg and more (against a background of up to 0.1 mg / kg). There are cases when the amount of mercury, supplied with sewage and accumulated in such mud, was tens and hundreds of tons. The normal functioning of such rivers and reservoirs, their practical use is possible only with the removal of contaminated sediments. The use of mercury-contaminated water for irrigation of agricultural land resulted in accumulation in agricultural products to levels exceeding the MPC.
Typical background levels of total mercury (dissolved forms) in natural fresh waters are 0.03-0.07 μg / l; In bottom sediments of rivers and freshwater lakes – 0,05-0,1 mg / kg; In freshwater plants -0.04-0.06 mg / kg dry weight. Usually, where there is no indication of mercury contamination, its levels in drinking water rarely exceed 0.1 μg / l. Mercury, primarily methylmercury, refers to substances that accumulate in the food chain, a simple example of which can be, for example, the following series: larva – minnow – perch – cat – cat. This means that in each subsequent organism the content of methylmercury is usually many times higher than in the previous one. Food products grown and obtained under the necessary conditions are usually characterized by the permissible content of mercury.