Anthropogenic Sources Of Heavy Metals

Heavy metals are released into the environment by many human activities. They are also used in a large variety of industrial products, which in the long term have to be deposited as waste. Heavy metal release into the environment occurs at the beginning of the production chain, whenever ores are mined, during the use of products containing them, and also at the end of the production chain. Table 6 gives an overview on the multiple uses and products, which contain heavy metals. The natural sources are dominated by parent rocks and metallic minerals, while the main anthropogenic sources are agricultural activities, where fertilizers, animal manures, and pesticides containing heavy metals are widely used, metallurgical activities, which include mining, smelting, metal finishing, and others, energy production and transportation, microelectronic products, and finally waste disposal. Heavy metals can be released into the environment in gaseous, particulate, aqueous, or solid form and emanate from both diffuse or point sources.

5.1. Agricultural Activities

The ever growing world population requires intensive land use for the production of food, which includes repeated and heavy input of fertilizers, pesticides, and soil amendments. Fertilizers are added to the soil in order to provide additional nutrients to crops or by changing soil conditions such as pH to make nutrients more bioavailable. Pesticides are used to protect crops. Soil amendments are often derived from sewage sludge, animal manure, and dredged sediments from harbours and rivers. Heavy metals from these sediments can be mobilized during dredging [34] as the reducing environment changes to oxidizing condition, thus remobilizing heavy metals. As soil, surface, and groundwaters are closely interconnected systems, metals introduced into soils can also affect aquifers or surface waters by infiltration. Also irrigation may trigger release of heavy metals.

An example of heavy metal release by the use of aquifer water for irrigation occurred in West Bengal, India and East Bangladesh. The local water level had been lowered due to increased water use by wells and strata containing pyrite (FeS2) were exposed to oxidation. In pyrite, As substitutes in part for Fe, and this metal has been released into the aquifer by oxidation and decomposition of this mineral. In the aquifer, the toxic ar-senite complex was formed, which heavily affects the local population [3539].

5.1.1. Phosphatic Fertilizers

Phosphatic fertilizers contain various amounts of Zn, Cd, and other heavy metals depending from which parent rock the fertilizer has been produced. Those made from sedimentary rocks tend to have high levels of Cd, while those made from magmatic rocks have only small Cd concentrations [2], The differences in heavy metal content are caused by impurities copre-cipitated with the phosphates. Therefore, Cd input into agricultural soils varies considerably according to the Cd concentration of the fertilizer used [40,41],

5.1.2. Pesticides

Pesticides are used for insect and disease control in high-production agriculture and can be applied as seed treatment, by spraying, dusting, or by soil application. Although metal-based pesticide are no longer in use, their former application lead to increased accumulation of heavy metals, especially of Hg from methyl mercurials, of As, and of Pb from lead arsenate into soils and groundwater. Table 7 lists some pesticide containing metals recommended in Ontario, Canada, from period 1892-1975 [42].

5.1.3. Sewage Effluents

Land application of waste water is widely used in industrialized countries for the last 50 to 100 years [43].

Table 6

Anthropogenic sources and uses of heavy metals, through which they can be introduced into the environment_

As: Additive to animal feed, wood preservative (copper chrome arsenate), special glasses, ceramics, pesticides, insecticides, herbicides, fungicides, rodenticides, algicides, sheep dip, electronic components (gaalium arsenate semiconductors, integrated circuits, diodes, infra-red detectors, laser technology), non-ferrous smelters, metallurgy, coal-fired and geothermal electrical generation, texile and tanning, pigments and anti-fouling paints, light filters, fireworks, veterinary medicine.

Be: Alloy (with Cu), electrical insulators in power transistors, moderator or neutron deflectors in nuclear reactors Cd: Ni/Cd batteries, pigments, anti-corrosive metal coatings, plastic stabilizers, alloys, coal combustion, neutron absorbers in nuclear reactors Co: Metallurgy (in superalloys), ceramics, glasses, paints Cr: Manufacturing of ferro-alloys (special steels), plating, pigments, textiles and leather tanning, passivation of corrosion of cooling circuits, wood treatment, audio, video, and data storage Cu: good conductor of heat and electricity, water pipes, roofing, kitchenware, chemicals and pharmaceutical equipment, pigments, alloys Fe: Cast iron, wrought iron, steel, alloys, constmction, transportation, machine-manufacturing

Hg: Extracting of metals by amalgamation, mobile cathode in the chloralkali cell for the production of NaCl and Cl2 from brine, electrical and measuring apparatus, fungicides, catalysts, pharmaceuticals, dental fillings, scientific instruments, rectifiers, oscillators, electrodes, mercury vapour lamps, X-Ray tubes, solders

Mn: Production of ferromanganese steels, electrolytic manganese dioxide for Use in batteries, alloys, catalysts, fungicides, antiknock agents, pigments, Dryers, wood preservatives, coating welding rods

Table 6 (continued)

Anthropogenic sources and uses of heavy metals, through which they can be introduced into the environment_

Mo: Alloying element in steel, cast irons, non-ferrous metals, catalysts, dyes, lubricants, corrosion inhibitors, flame retardants, smoke repressants, electroplating

Ni: As an alloy in the stell industry, electroplating, Ni/Cd batteries, arc-welding, rods, pigments for paints and ceramics, surgical and dental protheses, molds for ceramic and glass containers, computer components, catalysts Pb: Antiknock agents, tetramethyllead, lead-acid batteries, pigments, glassware, ceramics, plastic, in alloys, sheets, cable sheathings, solder, ordinance, pipes or tubing

Sb: Type-metal alloy (with lead to prevent corrosion), in electrical applications, Britannia metal, pewter, Queen's metal, Sterline, in primers and tracer cells in munition manufacture, semiconductors, flameproof pigments and glass, medicines for parasitic diseases, as a nauseant, as an expectorant, combustion of fossil fuels

Se: In the glass industry, semiconductors, thermoelements, photoelectric and photo cells, and xerographic materials, inorganic pigments, rubber production, stainless steel, lubricants, dandruff treatment Sn: Tin-plated steel, brasses, bronzes, pewter, dental amalgam, stabilizers, catalysts, pesticides

Ti: For white pigments (TiCb), as a UV-filtering agents (suncream), nucleation

Agent for glass ceramics, as Ti alloy in aeronautics Tl: Used for alloys (with Pb, Ag, or Au) with special properties, in the electronics industry, for infrared optical systems, as a catalyst, deep temperature thermometers, low melting glasses, semiconductors, supraconductors V: Steel production, in alloys, catalyst

Zn: Zinc alloys (bronze, brass), anti-corrosion coating, batteries, cans, PVC Stabilizers, precipitating Au from cyanide solution, in medicines and chemicals, rubber industry, paints, soldering and welding fluxes Modified after Ref. 1.

There are some benefits of using the water and nutrients in sewage effluents such as recycling of nutrients, restoring of groundwater, preventing stream pollution, and cut down of commercial fertilizers. Nevertheless, several factors such as the transportation costs, the land use, the soil type, etc. have to be taken into account. Several reviews of hazards from heavy metal concentration in waste water have been conducted and phytotoxic symptoms when using waste water containing Cd, Zn, Cu, Ni, Pb, and especially B have been observed [44, 45]. Nevertheless, long-term observations in Germany indicate that even after use of sewage effluents since 1895, heavy metal concentrations in soil is still within tolerable limits [46].

Table 7

Pesticide-containing metals recommended in Ontario, Canada, from period 1892-1975

Table 7

Pesticide-containing metals recommended in Ontario, Canada, from period 1892-1975

Chemical

Metal composition

Period of

Crops

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  • cinzia
    What are the anthropogenic sources of heavy metals?
    2 years ago
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    How to use anthropogenic activities to reduce heavy metals?
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