Recently, the issue of environmental problems has become the most acute, and one of them is heavy metals.
Heavy metals are elements of a periodic system with a relative molecular mass greater than 40. Not an exception, Group II of the periodic table, in particular mercury, zinc, cadmium.
Thus, more than 40 chemical elements with a relative density of more than 6 belong to heavy metals. The number of dangerous pollutants, if one considers the toxicity, persistence and ability to accumulate in the external environment, and also the scale of the distribution of these metals, is much less.
First of all, those metals that are most widely and in considerable quantities used in production activity are of primary interest and, as a result of accumulation in the external environment, represent a serious danger from the point of view of their biological activity and toxic properties. These include lead, mercury, cadmium, zinc, bismuth, cobalt, nickel, copper, tin, antimony, vanadium, manganese, chromium, molybdenum and arsenic.
Forms of being in the environment. In the air, heavy metals are present in the form of organic and inorganic compounds in the form of dust and aerosols, as well as in gaseous elemental form (mercury). In this case, the aerosols of lead, cadmium, copper and zinc consist mainly of their submicron particles with a diameter of 0.5-1 μm, and the aerosols of nickel and cobalt – from coarse particles (more than 1 μm), which are formed mainly by burning diesel fuel.
In aqueous media, metals are present in three forms: suspended particles, colloidal particles and dissolved compounds. The latter are represented by free ions and soluble complexes with organic (humic and fulvic acids) and inorganic (halides, sulfates, phosphates, carbonates) ligands. A great influence on the content of these elements in the water has hydrolysis, which largely determines the form of the element in aquatic environments. A significant part of heavy metals is transferred by surface waters in a suspended state.
Sorption of heavy metals by bottom sediments depends on the features of the composition of the latter and the content of organic substances. Ultimately, heavy metals in aquatic ecosystems are concentrated in bottom sediments and biota.
In soils, heavy metals are contained in water-soluble, ion-exchange and weakly adsorbed forms. Water-soluble forms, as a rule, are represented by chlorides, nitrates, sulfates and organic complex compounds. In addition, heavy metal ions can be bound to minerals as part of the crystal lattice.
Extraction and processing are not the most powerful source of environmental pollution by metals. Gross emissions from these enterprises are much less than emissions from thermal power plants. Not metallurgical production, namely coal combustion is the main source of many metals entering the biosphere. In coal and oil, all metals are present. Much more than in the soil, toxic chemical elements, including heavy metals, in the ashes of power plants, industrial and household furnaces. Emissions to the atmosphere during fuel combustion are of particular importance. For example, the amount of mercury, cadmium, cobalt, arsenic in them is 3-8 times higher than the amount of metals produced. It is known that only one boiler of a modern coal-fired power plant emits an average of 1-1.5 tons of mercury vapor per year. Heavy metals are contained in mineral fertilizers.
Along with burning of mineral fuel, the most important way of technogenous dispersion of metals is their release into the atmosphere during high-temperature technological processes (metallurgy, burning of cement raw materials, etc.), as well as transportation, dressing and grading of ore.
The technogenic supply of heavy metals to the environment occurs in the form of gases and aerosols (sublimation of metals and dust particles) and in the composition of sewage.
Metals are relatively quickly accumulated in the soil and are very slowly evolved from it: the period of zinc removal is up to 500 years, cadmium – up to 1100 years, copper – up to 1500 years, lead – up to several thousand years.
A significant source of soil contamination with metals is the use of fertilizers from slurries derived from industrial and sewage treatment plants.
In emissions of metallurgical industries, heavy metals are mainly in an insoluble form. As the distance from the source of pollution, the largest particles settle, the proportion of soluble metal compounds increases, and the relationships between soluble and insoluble forms are established. Aerosol contaminants entering the atmosphere are removed from it through natural self-cleaning processes. An important role is played by atmospheric precipitation. As a result, industrial emissions into the atmosphere, wastewater discharges create the prerequisites for heavy metals to enter the soil, groundwater and open water, plants, sediments and animals.
The range of distribution and levels of air pollution depend on the source capacity, emission conditions and meteorological conditions. However, in conditions of industrial-urban agglomerations and urban development, the parameters of the distribution of metals in the air are still poorly predicted. With the removal of pollution sources, the decrease in the concentration of metal aerosols in atmospheric air often occurs exponentially, so that the zone of their intensive exposure, in which there is an excess of MPC, is relatively small.
In urbanized areas, the total effect of the detected air pollution is the resultant addition of many scattered fields and is due to the removal from the emission sources, the urban planning structure and the presence of the necessary sanitary protection zones around the enterprises. The natural (background) content of heavy metals in an uncontaminated atmosphere is a thousandth and ten thousandths of a microgram per cubic meter and lower. Such levels in modern conditions on any inhabited territories are practically not observed. The background content of lead is assumed to be 0.006 μg / m3, mercury – 0.001-0.8 μg / m3 (in cities – several orders of magnitude higher). The main industries with which environmental pollution is associated with mercury include mining, metallurgical, chemical, instrument-making, electrovacuum and pharmaceutical. The most intensive sources of cadmium environmental pollution are metallurgy and electroplating, as well as the burning of solid and liquid fuels. In the uncontaminated air above the ocean, the average concentration of cadmium is 0.005 μg / m3, in rural areas – up to 0.05 μg / m3, and in the areas where the enterprises that contain it are emitted (non-ferrous metallurgy, thermal power plants operating on coal and oil, Plastics, etc.), and industrial cities – up to 0,3-0,6 mkg / m3.
The atmospheric path of the entry of chemical elements into the environment of cities is the leading one. However, even at a small distance, particularly in suburban agricultural areas, the relative role of sources of environmental pollution with heavy metals can change and the greatest danger will be represented by sewage and waste accumulated in landfills and used as fertilizers.
The maximum ability to concentrate heavy metals is suspended matter and bottom sediments, then plankton, benthos and fish.
Precipitation. The zone of maximum concentrations of metals in the air extends up to 2 km from the source. In it, the content of metals in the surface layer of the atmosphere is 100-1000 times higher than the local geochemical background, and in snow – by 500-1000 times. At a distance of 2-4 km is the second zone, where the metal content in the air is approximately 10 times lower than in the first. A third zone with a length of 4-10 km is planned, where only individual samples show an increased content of metals. As far as the distance from the source, the ratios of the different forms of the scattered metals change. In the first zone, water-soluble compounds make up only 5-10%, and the main mass of deposition is formed by small dust-like particles of sulfides and oxides. The relative content of water-soluble compounds increases with distance.