MODERN BIOGEOCHEMISTRY: SECOND EDITION Phần 6

METALLOGENIC BIOGEOCHEMICAL PROVINCES 217 In some regions this led to natural enrichment of ecosystems by different elements, in others, to natural depletion. It is known that complex biological, geological and chemical influence is jointly determined as the biogeochemical one, and in the be-ginning of the 20th century, studying the behavior of heavy metal in the biosphere led to the new discipline, biogeochemistry. In 1929, Vladimir Vernadsky founded the Biogeochemical laboratory in the USSR Academy of Sciences (at present Insti-tute of Geochemistry and Analytical Chemistry of Russian Academy of Sciences). V. Vernadsky and his colleagues A. Vinogradov and V. Kovalsky have carried out biogeochemical mapping of huge area of northern Eurasia in the former USSR. Bio-geochemicalregionsandprovincesdifferinginheavymetalscontentweredelineated. It was found that in many biogeochemical provinces the enrichment of biogeochem-ical food webs by some heavy metals is accompanied by depletion of other metals, which creates extremely complex biogeochemical structure of terrestrial and fresh water ecosystems in these provinces. Moreover it is shown that the depleted content of many heavy metals is equally dangerous as the excessive contents. Thiscomplexbiogeochemicalstructurewithnon-optimalcontentofheavymetals and some micronutrients induces the development of various endemic diseases of humans and animals (Bashkin, 2002). The biogeochemical structure of the modern biosphere is discussed in more detail in Chapter 2. Metals from the 6th period in Mendeleev’s table are potentially the most toxic (Os, Ir, Pt, Au, Hg, Tl, Pb), however small water solubility of their prevalent salts decreases sharply this toxic influence (Table 1). In the environment, metals are common as a chemical species, and as usual the metal–organic species are more toxic. For example, the inorganic lead and mercury species are less toxic for living organisms than the organic ones (methyl mercury, tetraethyl lead). However inorganic arsenic compounds are more toxic than organic Table 1. Classification of chemical elements according to their water solubility, natural abundance and toxicity. Widely distributed and low toxic metals Toxic but low soluble and rarely distributed metals Very toxic and widely distributed metalsa Na C F Ti K P Li Hf Mg Fe Rb Zr Ca S Sr W H Cl Al Nb O Br Si Ta N Re Ga Be As Au La Se Co Hg Os Te Ni Tl Rh Pd Cu Pb Ir Ag Zn Sb Ru Cd Sn Bi Ba Cr Pt aThe most dangerous species especially upon their accumulation in wastes 218 CHAPTER 11 Table 2. Technophility index of heavy metals. Metal Mn = Fe < Ni < Cr < Zn < Cu = Ag < Hg = Pb < Au < Cd Technophility index 1 1 2 4 10 20 20 30 30 60 140 species, and fishes can accumulate arsenic as arsenolipides that are practically non-toxic. The most organic and inorganic compounds of tin are non-toxic; the known exception is tri-n-butyl species like tri-butyltin that is used as a biocide for preventing the growth of mollusks on the submerged parts of marine ships. 1.2. Sources of Heavy Metals and Their Distribution in the Environment Globaldistributionofheavymetalsinthebiosphereisrelatedtotheirtechnophilitythat is determined as the ratio of global annual exploration to their average concentrations in the Earth’s core (Table 2). The value of technophility indices testifies to a higher actual and potential danger of such metals as Pb, Hg, and Cd in comparison with, let’s say, Mn or Fe. These are also supported by registered changes in the global emissions of heavy metals into the atmosphere and oceans (Table 3). The number of anthropogenic sources includes the followings: r industrial ore treatment; r usage of metals and metal-containing materials; r runoff of heavy metals from wastes; r human and animal excretes. Table 4 shows a typical list of heavy metals and relevant industries. One should note that in some technological processes a wide spectrum of metals is used (for ex-ample,productionofpesticides,electronics,non-ferroussmelting,electrochemistry), Table 3. Global heavy metals emissions into atmosphere and oceans (103 tons per year). Emissions to atmosphere Emissions into oceans Element Cd Pb Cu Zn Natural 0.29 4 19 36 anthropogenic 5.5 400 260 840 F∗ Natural weathering 19 36 100 110 13 250 23 720 Municipal wastes 3 15 42 100 F∗—mobilization factor as a ratio of anthropogenic emission into the atmosphere to the natural one METALLOGENIC BIOGEOCHEMICAL PROVINCES 219 Table 4. Typical use of heavy metals in different technological processes. Technological processes Exploration and treatment of non-ferrous metals Electrochemistry Production of pesticides Electronics Dry cleaning Metal surface treatment Chemical industry Production of explosive substances Rubber and plastics production Batteries and accumulators production Pharmaceutical production Textile production Oil and coal combustion and treatment Pulp and cardboard production Leather processing As Cd Cr Cu Pb Hg Se Zn Ni × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × whereas in others, only one element is used (for example, Cr using in leather-processing or Hg in the pulp industry). Emissions of actually and potentially dangerous toxic elements may influence the human and ecosystem health on local, regional and global scales. Accumulation of toxic metals may be in soils, waters, bottom sediments and biota. For example, the accumulation of heavy metals in the upper layers of bottom sediments and glaciers occurring during the 20th century is shown in many recent studies. The global cycle of lead was anthropogenically changed to the maximal extent owing to the use of TEL as a petrol additive (Table 5). The regional aluminum cycle was changed due to acid depositions (Bashkin and Park, 1998; Bashkin, 2003). Dif-fering from lead and aluminum, chromium influence is local, nearby electrochemical andleather-processingplants;Cr-VIformisthemosttoxicanditisprimaryregulated. As it has been mentioned, formation of organic compounds accelerates the mobility of heavy metals, and accordingly their toxicity is also enhanced. Migration of many heavy metals increases upon soil and water acidification. 220 CHAPTER 11 Table 5. Anthropogenic changes in cycles of heavy metals. Scale of changes Metals Global Regional Local Diagnostic media Mechanism of release Ways to living organisms Pb + + + Al − + − Cr − − + Hg (−) + + Cd (−) + + Glaciers, bottom sediments Soils, waters Soils, waters Fishes, bottom sediments Soils, waters, bottom sediments Volatilization Dissolution Dissolution Alkilation Dissolution, volatilization Air, food Water, food Water Food, air Food One can see that most environmental impacts in global, regional and local scale arerelatedtomercury,lead,andcadmium.Thesemetalsareconsideredinmoredetail further. 2. USAGE OF METALS 2.1. Anthropogenic Mercury Loading Mercury is a relatively rare chemical element. In the lithosphere it occurs mainly as sulfides, HgS. Mercury sulfide comes in two forms: cinnibar, which is black, and vermillion. In some places mercury exists in a small proportion as free chemical species. Mercury refining involves heating the metal sulfide in air in accordance with the following reaction: HgS +O2 → Hg +SO2. Gaseous mercury is condensed in a water-cooled condenser and redistilled for sale. At present industrial mercury uses are connected with electric batteries, electric tungsten bulb, pulp bleaching and agrochemical production. Mercurybatteriesareusedwidelyineverydaylife,inapplicationssuchascameras and hearing aids. About 30% of U.S. production of mercury is used in this way, the reason being the constancy of the voltage in the mercury battery, almost to the point of complete discharge. The electrical uses of mercury include its application as a seal to exclude air when tungsten light bulb filaments are manufactured. Fluorescent light tubes and mercury arclampsthatareusedforstreetlightingandasgermicidallampsalsocontainmercury. METALLOGENIC BIOGEOCHEMICAL PROVINCES 221 Mercury is consumed in the manufacture of organomercurials, which are used in agriculture as fungicides, e.g., for seed dressing. 2.2. Anthropogenic Lead Loading Lead occurs in nature as the sulfide, galena, PbS. Lead is more electropositive than mercury, and roasting the sulfide in air forms lead oxide. 2PbS +3O2 → 2PbO +2SO2. The oxide is then reduced to metal with coke. The impure metal is refined by electrolysis. Major anthropogenic sources of lead include the use of Pb as a petrol additive, Pb miningandsmelting,printing,Pgpaintflakes,sewagesludgeandtheuseofpesticides containing Pb compounds, like lead arsenate. A well-known use of lead is also in the familiar lead–acid storage battery. This device is an example of a storage cell, meaning that the battery can be discharged and recharged over a large number of cycles. The lead–acid battery is familiar as a battery in your car. An important disadvantage of the lead–acid battery is its heavy mass, on account the high Pb density. The second disadvantage is that used car batteries distribute a lot of lead into the environment; despite recycling, they are the major source of lead in municipal waste. Recently, the recycling of lead–acid batteries has created problems in the local environment around recycling plants. Most of these plants are located in developing countries of Asia and Latin America and they process batteries imported from industrialized nations. Levels of Pb as high as 60,000–70,000 ppm have been measured in soils in the vicinity of Pb-battery recycling plants in the Philippines, Thailand and Indonesia. The relevant health effects have been observed. This appears to be one example where trying to conserve resources and minimize pollution has goneseriouslywrong.InCalifornia,soilcontaminating1000ppmofPbisconsidered to be hazardous waste and its disposal is strictly regulated. Humanactivityhaschangedtheintensityofnaturalbiogeochemicalfluxesoflead during industrial development. However, the history of lead use is the longest of any metals. The period of relatively intensive production and application of lead is about 5000 years. Lead has been used as a metal at least since the times of the Egyptians and Babylonians. The Romans employed lead extensively for conveying water, and the elaborate water distribution systems allowed by bending of the soft metal lead. Through the Middle Ages and beyond, the malleability of lead encouraged its use as a roofing material for the most important constructions, like the great cathedrals in Europe. The modern production of lead is n ×106 tons annually (Figure 2). How Risky are the Pb Background Levels? The long-term uses of lead explain why this element should be so widely dispersed in the environment. In this regards one should answer the question as to what is the natural background level of lead. At present this is a question of controversy. Lead ... - tailieumienphi.vn