Heavy metals polluting the soil. Heavy metals in soils. Danger of heavy metal contamination

Heavy metals (HMs) are already ranked second in terms of danger, behind pesticides and well ahead of such well-known pollutants as carbon dioxide and sulfur. In the future, they may become more dangerous than nuclear power plant waste and solid waste. HM contamination is associated with their widespread use in industrial production. Due to imperfect purification systems, HMs enter the environment, including the soil, polluting and poisoning it. HMs are special pollutants, the monitoring of which is obligatory in all environments.

Soil is the main medium into which HMs enter, including from the atmosphere and the aquatic environment. It also serves as a source of secondary pollution of surface air and waters that enter the World Ocean from it.

HMs are absorbed from the soil by plants, which then get into food.

The term "heavy metals", which characterizes a wide group of pollutants, has recently become widely used. In various scientific and applied works, the authors interpret the meaning of this concept in different ways. In this regard, the number of elements assigned to the group of heavy metals varies over a wide range. Numerous characteristics are used as membership criteria: atomic mass, density, toxicity, prevalence in the natural environment, the degree of involvement in natural and technogenic cycles.

In works devoted to the problems of environmental pollution and environmental monitoring, today more than 40 elements of D.I. Mendeleev with an atomic mass of more than 40 atomic units: V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Cd, Sn, Hg, Pb, Bi, etc. According to the classification of N. Reimers, heavy metals should be considered with with a density of more than 8 g/cm3. At the same time, the following conditions play an important role in the categorization of heavy metals: their high toxicity to living organisms in relatively low concentrations, as well as their ability to bioaccumulate and biomagnify. Almost all metals that fall under this definition (with the exception of lead, mercury, cadmium and bismuth, the biological role of which is not clear at the moment), are actively involved in biological processes, are part of many enzymes.

The most powerful suppliers of waste enriched in metals are non-ferrous metal smelting enterprises (aluminum, alumina, copper-zinc, lead-smelting, nickel, titanium-magnesium, mercury, etc.), as well as non-ferrous metal processing (radio engineering, electrical engineering, instrument-making, galvanic, etc.).

In the dust of metallurgical industries, ore processing plants, the concentration of Pb, Zn, Bi, Sn can be increased compared to the lithosphere by several orders of magnitude (up to 10-12), the concentration of Cd, V, Sb - tens of thousands of times, Cd, Mo, Pb, Sn, Zn, Bi, Ag - hundreds of times. Wastes from non-ferrous metallurgy enterprises, paint and varnish factories and reinforced concrete structures are enriched with mercury. The concentrations of W, Cd, and Pb are increased in dust from machine-building plants (Table 1).

Table 1. Main technogenic sources of heavy metals

Under the influence of metal-enriched emissions, areas of landscape pollution are formed mainly at the regional and local levels. The influence of energy enterprises on environmental pollution is not due to the concentration of metals in waste, but to their huge amount. The mass of waste, for example, in industrial centers, exceeds their total amount coming from all other sources of pollution. A significant amount of Pb is released into the environment with car exhaust gases, which exceeds its intake with waste from metallurgical enterprises.

Arable soils are polluted with elements such as Hg, As, Pb, Cu, Sn, Bi, which enter the soil as part of pesticides, biocides, plant growth stimulants, structure formers. Non-traditional fertilizers made from various waste products often contain a wide range of contaminants at high concentrations. Of the traditional mineral fertilizers, phosphate fertilizers contain impurities of Mn, Zn, Ni, Cr, Pb, Cu, Cd.

The distribution in the landscape of metals released into the atmosphere from technogenic sources is determined by the distance from the pollution source, climatic conditions (strength and direction of winds), terrain, and technological factors (the state of waste, the method of waste entering the environment, the height of pipes of enterprises).

HM dissipation depends on the height of the source of emissions into the atmosphere. According to M.E. Berland, with high chimneys, a significant concentration of emissions is created in the surface layer of the atmosphere at a distance of 10-40 chimney heights. Six zones are distinguished around such pollution sources (Table 2). The area of influence of individual industrial enterprises on the adjacent territory can reach 1000 km2.

Table 2. Zones of soil contamination around point sources of pollution

	Distance from pollution source in km	Excess of HM content in relation to the background
Security zone of the enterprise

Soil pollution zones and their size are closely related to the vectors of the prevailing winds. The relief, vegetation, urban buildings can change the direction and speed of movement of the surface layer of air. Similarly to the zones of soil pollution, zones of vegetation cover pollution can also be distinguished.

Soil is the surface of the earth, which has properties that characterize both living and inanimate nature.

The soil is an indicator of the total. Pollution enters the soil with atmospheric precipitation, surface waste. They are also introduced into the soil layer by soil rocks and groundwater.

The group of heavy metals includes all with a density exceeding the density of iron. The paradox of these elements is that they are necessary in certain quantities to ensure the normal functioning of plants and organisms.

But their excess can lead to serious illness and even death. The food cycle causes harmful compounds to enter the human body and often cause great harm to health.

Sources of heavy metal pollution are. There is a method by which the allowable metal content is calculated. This takes into account the total value of several metals Zc.

admissible;
moderately dangerous;
high-dangerous;
extremely dangerous.

Soil protection is very important. Constant control and monitoring does not allow growing agricultural products and grazing livestock on contaminated lands.

Heavy metals polluting the soil

There are three hazard classes of heavy metals. The World Health Organization considers lead, mercury and cadmium to be the most dangerous. But no less harmful is the high concentration of other elements.

Mercury

Pollution of the soil with mercury occurs with the ingress of pesticides, various household wastes, such as fluorescent lamps, and elements of damaged measuring instruments into it.

According to official data, the annual release of mercury is more than five thousand tons. Mercury can enter the human body from contaminated soil.

If this happens regularly, severe disorders of the work of many organs can occur, including the nervous system.

With improper treatment, a fatal outcome is possible.

Lead

Lead is very dangerous for humans and all living organisms.

It is extremely toxic. When one ton of lead is mined, twenty-five kilograms are released into the environment. A large amount of lead enters the soil with the release of exhaust gases.

The soil pollution zone along the routes is over two hundred meters around. Once in the soil, lead is absorbed by plants that are eaten by humans and animals, including livestock, whose meat is also on our menu. Excess lead affects the central nervous system, brain, liver and kidneys. It is dangerous for its carcinogenic and mutagenic effects.

Cadmium

Soil contamination with cadmium is a huge danger to the human body. When ingested, it causes skeletal deformities, stunted growth in children, and severe back pain.

Copper and zinc

A high concentration of these elements in the soil causes growth to slow down and the fruiting of plants to deteriorate, which ultimately leads to a sharp decrease in yield. In humans, changes occur in the brain, liver and pancreas.

Molybdenum

Excess molybdenum causes gout and damage to the nervous system.

The danger of heavy metals lies in the fact that they are poorly excreted from the body, accumulate in it. They can form very toxic compounds, easily pass from one environment to another, do not decompose. At the same time, they cause severe diseases, often leading to irreversible consequences.

Antimony

Present in some ores.

It is part of the alloys used in various industrial fields.

Its excess causes severe eating disorders.

Arsenic

The main source of soil contamination with arsenic are substances used to control pests of agricultural plants, such as herbicides, insecticides. Arsenic is a cumulative poison that causes chronic. Its compounds provoke diseases of the nervous system, brain, and skin.

Manganese

In the soil and plants, a high content of this element is observed.

If an additional amount of manganese enters the soil, a dangerous excess of it is quickly created. This affects the human body in the form of destruction of the nervous system.

An excess of other heavy elements is no less dangerous.

From the foregoing, we can conclude that the accumulation of heavy metals in the soil entails severe consequences for human health and the environment as a whole.

The main methods of combating soil pollution with heavy metals

Methods for dealing with soil contamination with heavy metals can be physical, chemical and biological. Among them are the following methods:

An increase in soil acidity increases the possibility. Therefore, the introduction of organic matter and clay, liming help to some extent in the fight against pollution.
Sowing, mowing and removing some plants, such as clover, from the soil surface significantly reduces the concentration of heavy metals in the soil. In addition, this method is completely environmentally friendly.
Underground water detoxification, its pumping and cleaning.
Prediction and elimination of migration of soluble form of heavy metals.
In some particularly severe cases, complete removal of the soil layer and its replacement with a new one is required.

Heavy metals are biochemically active elements that enter the cycle of organic substances and affect mainly living organisms. Heavy metals include elements such as lead, copper, zinc, cadmium, nickel, cobalt and a number of others.

The migration of heavy metals in soils depends, first of all, on alkaline-acid and redox conditions, which determine the diversity of soil-geochemical conditions. An important role in the migration of heavy metals in the soil profile is played by geochemical barriers, which in some cases enhance, in others weaken (due to the ability to conserve) the resistance of soils to pollution by heavy metals. At each of the geochemical barriers, a certain group of chemical elements with similar geochemical properties lingers.

The specifics of the main soil-forming processes and the type of water regime determine the nature of the distribution of heavy metals in soils: accumulation, conservation, or removal. Groups of soils with the accumulation of heavy metals in different parts of the soil profile were identified: on the surface, in the upper, in the middle, with two maxima. In addition, soils in the zone were identified, which are characterized by the concentration of heavy metals due to intra-profile cryogenic conservation. A special group is formed by soils where, under the conditions of leaching and periodically leaching regimes, heavy metals are removed from the profile. The intra-profile distribution of heavy metals is of great importance for assessing soil pollution and predicting the intensity of accumulation of pollutants in them. The characteristic of the intra-profile distribution of heavy metals is supplemented by the grouping of soils according to the intensity of their involvement in the biological cycle. In total, three gradations are distinguished: high, moderate and weak.

The geochemical environment of the migration of heavy metals in the soils of river floodplains is peculiar, where, with increased watering, the mobility of chemical elements and compounds increases significantly. The specificity of geochemical processes here is due, first of all, to the pronounced seasonality of the change in redox conditions. This is due to the peculiarities of the hydrological regime of rivers: the duration of spring floods, the presence or absence of autumn floods, and the nature of the low-water period. The duration of flood water flooding of floodplain terraces determines the predominance of either oxidative (short-term floodplain flooding) or redox (long-term flooding) conditions.

Arable soils are subjected to the greatest technogenic impacts of an areal nature. The main source of pollution, with which up to 50% of the total amount of heavy metals enters arable soils, is phosphate fertilizers. To determine the degree of potential contamination of arable soils, a coupled analysis of soil properties and pollutant properties was carried out: the content, composition of humus and particle size distribution of soils, as well as alkaline-acid conditions were taken into account. Data on the concentration of heavy metals in phosphorites of deposits of different genesis made it possible to calculate their average content, taking into account the approximate doses of fertilizers applied to arable soils in different regions. The assessment of soil properties is correlated with the values of agrogenic load. The cumulative integral assessment formed the basis for identifying the degree of potential soil contamination with heavy metals.

The most dangerous in terms of the degree of contamination with heavy metals are multi-humus, clay-loam soils with an alkaline reaction of the environment: dark gray forest, and dark chestnut - soils with a high accumulative capacity. The Moscow and Bryansk regions are also characterized by an increased risk of soil pollution with heavy metals. The situation with soddy-podzolic soils does not contribute to the accumulation of heavy metals here, but in these areas the technogenic load is high and the soils do not have time to "self-purify".

Ecological and toxicological assessment of soils for the content of heavy metals showed that 1.7% of agricultural land is contaminated with substances of hazard class I (highly hazardous) and 3.8% - hazard class II (moderately hazardous). Soil contamination with heavy metals and arsenic content above the established norms was detected in the Republic of Buryatia, the Republic of Dagestan, the Republic of Mordovia, the Republic of Tyva, in the Krasnoyarsk and Primorsky Territories, in Ivanovo, Irkutsk, Kemerovo, Kostroma, Murmansk, Novgorod, Orenburg, Sakhalin, Chita regions.

Local contamination of soils with heavy metals is associated primarily with large cities and. The assessment of the risk of soil contamination by heavy metal complexes was carried out according to the total indicator Zc.

FEDERAL AGENCY OF MARINE AND RIVER TRANSPORT
FEDERAL BUDGET EDUCATIONAL INSTITUTION
HIGHER PROFESSIONAL EDUCATION
MARITIME STATE UNIVERSITY
named after Admiral G.I. Nevelskoy

Department of Environmental Protection

ABSTRACT
in the discipline "Physical and chemical processes"

Consequences of soil pollution with heavy metals and radionuclides.

Checked by teacher:
Firsova L.Yu.
Performed by student gr. ___
Khodanova S.V.

Vladivostok 2012
CONTENT

Introduction
1 Heavy metals in soils

2 Radionuclides in soils. Nuclear pollution
Conclusion
List of sources used

INTRODUCTION

Soil is not just an inert environment on the surface of which human activity is carried out, but a dynamic, developing system that includes many organic and inorganic components, which have a network of cavities and pores, and they, in turn, contain gases and liquids. The spatial distribution of these components determines the main types of soils on the globe.
In addition, soils contain a huge number of living organisms, they are called biota: from bacteria and fungi to worms and rodents. Soil is formed on rock parent rocks under the combined influence of climate, vegetation, soil organisms, and time. Therefore, a change in any of these factors can lead to changes in soils. Soil formation is a long process: it takes 1,000 to 10,000 years for a 30 cm layer of soil to form. Consequently, soil formation rates are so low that soil can be considered a non-renewable resource.
The soil cover of the Earth is the most important component of the Earth's biosphere. It is the soil shell that determines many processes occurring in the biosphere. The most important importance of soils is the accumulation of organic matter, various chemical elements, as well as energy. The soil cover functions as a biological absorber, destroyer and neutralizer of various contaminants. If this link of the biosphere is destroyed, then the existing functioning of the biosphere will be irreversibly disrupted. That is why it is extremely important to study the global biochemical significance of the soil cover, its current state and changes under the influence of anthropogenic activity.

1 Heavy metals in soils

Heavy metals (HM) include more than 40 chemical elements of D.I. Mendeleev, the mass of atoms of which is more than 50 atomic mass units (a.m.u.). These are Pb, Zn, Cd, Hg, Cu, Mo, Mn, Ni, Sn, Co, etc. The current concept of "heavy metals" is not strict, because TMs often include non-metal elements, such as As, Se, and sometimes even F, Be, and other elements whose atomic mass is less than 50 a.m.u.
There are many trace elements among HMs that are biologically important for living organisms. They are essential and irreplaceable components of biocatalysts and bioregulators of the most important physiological processes. However, the excessive content of HMs in various objects of the biosphere has a depressing and even toxic effect on living organisms.
The sources of HM entry into the soil are divided into natural (weathering of rocks and minerals, erosion processes, volcanic activity) and technogenic (extraction and processing of minerals, fuel combustion, the impact of vehicles, agriculture, etc.) Agricultural lands, in addition to pollution through the atmosphere, HMs are also polluted specifically, when using pesticides, mineral and organic fertilizers, liming, and using wastewater. Recently, scientists have paid special attention to urban soils. The latter experience a significant technogenic process, an integral part of which is HM contamination.
HMs reach the soil surface in various forms. These are oxides and various metal salts, both soluble and practically insoluble in water (sulfides, sulfates, arsenites, etc.). In the composition of emissions from ore processing enterprises and non-ferrous metallurgy enterprises - the main source of HM environmental pollution - the bulk of metals (70-90%) is in the form of oxides.
Getting on the soil surface, HMs can either accumulate or dissipate, depending on the nature of the geochemical barriers inherent in the given territory.
Most of the HMs that have entered the soil surface are fixed in the upper humus horizons. HMs are sorbed on the surface of soil particles, bind to soil organic matter, in particular in the form of elemental organic compounds, accumulate in iron hydroxides, are part of the crystal lattices of clay minerals, give their own minerals as a result of isomorphic substitution, and are in a soluble state in soil moisture. and gaseous state in the soil air, are an integral part of the soil biota.
The degree of HM mobility depends on the geochemical environment and the level of technogenic impact. The heavy particle size distribution and high content of organic matter lead to the binding of HMs by the soil. An increase in pH values enhances the sorption of cation-forming metals (copper, zinc, nickel, mercury, lead, etc.) and increases the mobility of anion-forming metals (molybdenum, chromium, vanadium, etc.). Strengthening oxidizing conditions increases the migration ability of metals. As a result, according to the ability to bind most HMs, the soils form the following series: gray soil > chernozem > soddy-podzolic soil.

Soil pollution with HMs has two negative sides at once. First, HMs enter the food chains from the soil to plants, and from there to the organisms of animals and humans, causing serious diseases in them. An increase in the incidence of the population and a reduction in life expectancy, as well as a decrease in the quantity and quality of crops of agricultural plants and livestock products.
Second, by accumulating in the soil in large quantities, HMs can change many of its properties. First of all, the changes affect the biological properties of the soil: the total number of microorganisms decreases, their species composition (diversity) narrows, the structure of microbial communities changes, the intensity of the main microbiological processes and the activity of soil enzymes decrease, etc. Heavy contamination with HMs also leads to changes in more conservative features of the soil, such as the humus state, structure, pH of the medium, etc. This results in a partial, and in some cases, complete loss of soil fertility.

In nature, there are territories with insufficient or excessive content of HMs in soils. The anomalous content of HMs in soils is due to two groups of reasons: the biogeochemical features of ecosystems and the influence of technogenic fluxes of matter. In the first case, areas where the concentration of chemical elements is above or below the optimal level for living organisms are called natural geochemical anomalies or biogeochemical provinces. Here, the anomalous content of elements is due to natural causes - features of soil-forming rocks, the soil-forming process, and the presence of ore anomalies. In the second case, the territories are called technogenic geochemical anomalies. Depending on the scale, they are divided into global, regional and local.
The soil, unlike other components of the natural environment, not only geochemically accumulates pollution components, but also acts as a natural buffer that controls the transfer of chemical elements and compounds into the atmosphere, hydrosphere and living matter.
Various plants, animals and humans require a certain composition of soil and water for life. In places of geochemical anomalies, the transmission of deviations from the norm of the mineral composition occurs, aggravated, throughout the food chain. As a result of violations of mineral nutrition, changes in the species composition of phyto-, zoo- and microbial communities, disease of wild-growing forms of plants, a decrease in the quantity and quality of crops of agricultural plants and livestock products, an increase in the incidence of the population and a decrease in life expectancy are observed.
The toxic effect of HMs on biological systems is primarily due to the fact that they easily bind to sulfhydryl groups of proteins (including enzymes), inhibiting their synthesis and, thereby, disrupting the metabolism in the body.
Living organisms have developed various mechanisms of resistance to HM: from the reduction of HM ions into less toxic compounds to the activation of ion transport systems that efficiently and specifically remove toxic ions from the cell to the external environment.
The most significant consequence of HM impact on living organisms, which manifests itself at the biogeocenotic and biospheric levels of the organization of living matter, is to block the processes of organic matter oxidation. This leads to a decrease in the rate of its mineralization and accumulation in ecosystems. At the same time, an increase in the concentration of organic matter causes the binding of HMs, which temporarily removes the load from the ecosystem. A decrease in the rate of decomposition of organic matter due to a decrease in the number of organisms, their biomass and intensity of vital activity is considered a passive reaction of ecosystems to HM pollution. Active opposition of organisms to anthropogenic loads is manifested only during lifetime accumulation of metals in bodies and skeletons. The most resistant species are responsible for this process.
The resistance of living organisms, primarily plants, to elevated concentrations of HMs and their ability to accumulate high concentrations of metals can pose a great danger to human health, since they allow the penetration of pollutants into food chains.

The issue of rationing the content of HMs in soil is very complicated. The basis of his decision should be the recognition of the multifunctionality of the soil. In the process of regulation, the soil can be considered from various positions: as a natural body, as a habitat and substrate for plants, animals and microorganisms, as an object and means of agricultural and industrial production, as a natural reservoir containing pathogenic microorganisms. Rationing of the content of HMs in the soil should be carried out on the basis of soil-ecological principles, which deny the possibility of finding uniform values for all soils.
There are two main approaches to the issue of sanitation of soils contaminated with HMs. The first one is aimed at cleansing the soil from HMs. Purification can be carried out by washing, by extracting HM from the soil with the help of plants, by removing the top contaminated soil layer, etc. The second approach is based on the fixation of HMs in the soil, their conversion into forms insoluble in water and inaccessible to living organisms. For this, it is proposed to introduce organic matter, phosphorus mineral fertilizers, ion-exchange resins, natural zeolites, brown coal into the soil, liming the soil, etc. However, any method of fixing HMs in the soil has its own period of validity. Sooner or later, part of the HM will again begin to enter the soil solution, and from there into living organisms.

Soils contain almost all chemical elements known in nature, including radionuclides.
Radionuclides are chemical elements capable of spontaneous decay with the formation of new elements, as well as the formed isotopes of any chemical elements. The consequence of nuclear decay is ionizing radiation in the form of a stream of alpha particles (a stream of helium nuclei, protons) and beta particles (a stream of electrons), neutrons, gamma radiation and X-rays. This phenomenon is called radioactivity. Chemical elements capable of spontaneous decay are called radioactive. The most commonly used synonym for ionizing radiation is radioactive radiation.
Ionizing radiation is a stream of charged or neutral particles and electromagnetic quanta, the interaction of which with the medium leads to ionization and excitation of its atoms and molecules. Ionizing radiations have electromagnetic (gamma and X-ray radiation) and corpuscular (alpha radiation, beta radiation, neutron radiation) nature.
Gamma radiation is electromagnetic radiation caused by gamma rays (discrete beams or quanta, called photons), if after alpha or beta decay the nucleus remains in an excited state. Gamma rays in air can travel considerable distances. A photon of high-energy gamma rays can pass through the human body. Intense gamma radiation can damage not only the skin, but also internal organs. Protect from this radiation dense and heavy materials, iron, lead. Gamma radiation can be created artificially in contaminated particle accelerators (microtron), for example, bremsstrahlung of fast accelerator electrons when they hit a target.
X-rays are similar to gamma rays. Cosmic X-rays are absorbed by the atmosphere. X-rays are obtained artificially, they fall on the lower part of the energy spectrum of electromagnetic radiation.
Radioactive radiation is a natural factor in the biosphere for all living organisms, and living organisms themselves have a certain radioactivity. Soils have the highest natural degree of radioactivity among biospheric objects. Under these conditions, nature prospered for many millions of years, except in exceptional cases with geochemical anomalies associated with a deposit of radioactive rocks, for example, uranium ores.
However, in the 20th century, humanity was confronted with radioactivity beyond the limits of natural, and therefore biologically abnormal. The first victims of excessive doses of radiation were the great scientists who discovered radioactive elements (radium, polonium) spouses Maria Sklodowska-Curie and Pierre Curie. And then: Hiroshima and Nagasaki, testing of atomic and nuclear weapons, many disasters, including Chernobyl, etc.
The most significant objects of the biosphere, which determine the biological functions of all living things, are soils.
The radioactivity of soils is due to the content of radionuclides in them. There are natural and artificial radioactivity.
The natural radioactivity of soils is caused by natural radioactive isotopes, which are always present in varying amounts in soils and soil-forming rocks. Natural radionuclides are divided into 3 groups.
The first group includes radioactive elements - elements, all isotopes of which are radioactive: uranium (238
etc.................

S. Donahue - Soil pollution with heavy metalsSoils are one of the most important components of the agricultural and urban environment, and in both cases, sound management is the key to soil quality. This series of technical notes looks at human activities that cause soil degradation, as well as management practices that protect urban soils. This technical note focuses on soil contamination with heavy metals

Metals in the soil

The extraction, production and use of synthetic substances (eg pesticides, paints, industrial wastes, domestic and industrial waters) can result in heavy metal contamination of urban and agricultural land. Heavy metals also occur naturally, but rarely in toxic amounts. Potential soil contamination can occur in old landfills (especially those used for industrial waste), in old orchards that have used pesticides containing arsenic as the active ingredient, in fields that have been used for sewage or municipal sludge in the past, in or around dumps and tailings, industrial areas where chemicals may have been dumped on the ground in areas downwind of industrial facilities.

Excess accumulation of heavy metals in soils is toxic to humans and animals. The accumulation of heavy metals is usually chronic (exposure over a long period of time), along with food. Acute (immediate) heavy metal poisoning occurs by ingestion or skin contact. Chronic problems associated with long-term exposure to heavy metals include:

Lead - mental disorders.
Cadmium - affects the kidneys, liver and gastrointestinal tract.
Arsenic - skin diseases, affects the kidneys and central nervous system.

The most common cationic elements are mercury, cadmium, lead, nickel, copper, zinc, chromium and manganese. The most common anionic elements are arsenic, molybdenum, selenium, and boron.

Traditional methods of remediation of contaminated soils

Soil and crop remediation practices can help prevent pollutants from entering plants by leaving them in the soil. These remediation methods will not result in the removal of heavy metal contaminants, but will help to immobilize them in the soil and reduce the likelihood of negative impacts from metals. Please note that the type of metal (cation or anion) must be considered:

Increasing soil pH to 6.5 or higher. Cationic metals are more soluble at lower pH levels, so raising the pH makes them less available to plants and therefore less likely to be incorporated into plant tissues and ingested by humans. Raising the pH has the opposite effect on anionic elements.
Drainage in wet soils. Drainage improves soil aeration and will allow metals to oxidize, making them less soluble and available. The opposite will be observed for chromium, which is more readily available in its oxidized form. The activity of the organic matter is effective in reducing the availability of chromium.
. The use of phosphates. Phosphate applications can reduce the availability of cationic metals but have the opposite effect on anionic compounds such as arsenic. Phosphate must be applied wisely as high levels of phosphorus in the soil can lead to water pollution.
Careful selection of plants for use in metal-contaminated soils Plants move more metals in their leaves than their fruits or seeds. The greatest risk of food contamination in the chain is leafy vegetables (lettuce or spinach). Another danger is the eating of these plants by livestock.

Environmental treatment plants

Studies have shown that plants are effective in cleaning up contaminated soil (Wentzel et al., 1999). Phytoremediation is a general term for the use of plants to remove heavy metals or to keep the soil clean, free of contaminants such as heavy metals, pesticides, solvents, crude oil, polycyclic aromatic hydrocarbons. For example, steppe grass can stimulate the breakdown of petroleum products. Wildflowers have recently been used to degrade hydrocarbons from the Kuwait oil spill. Hybrid poplar species can remove chemicals such as TNT as well as high levels of nitrates and pesticides (Brady and Weil, 1999).

Plants for processing metal-contaminated soils

Plants have been used to stabilize and remove metals from soil and water. Three mechanisms are used: phytoextraction, rhizofiltration and phytostabilization.

This article talks about rhizofiltration and phytostabilization, but the main focus will be on phytoextraction.

Rhizofiltration is the adsorption on plant roots or absorption by plant roots of contaminants that are in the solutions surrounding the root zone (rhizosphere).

Rhizofiltration is used to disinfect groundwater. Plants grown in greenhouses. Polluted water is used to acclimatize plants in the environment. Then, these plants are planted in place of polluted groundwater, where the roots filter the water and pollutants. Once the roots are saturated with pollutants, the plants are harvested. At Chernobyl, sunflowers were used in this way to remove radioactive substances in groundwater (EPA, 1998)

Phytostabilization is the use of perennial plants to stabilize or immobilize harmful substances in soil and groundwater. Metals are absorbed and accumulated in the roots, adsorbed on the roots, or deposited in the rhizosphere. Also, these plants can be used for revegetation where natural vegetation is lacking, thereby reducing the risk of water and wind erosion and leaching. Phytostabilization reduces the mobility of pollutants and prevents further movement of pollutants into groundwater or air, and reduces their entry into the food chain.

Phytoextraction

Phytoextraction is the process of growing plants in metal-contaminated soil. The roots transport the metals to the aboveground parts of the plants, after which these plants are harvested and burned or composted to recycle the metals. Several cycles of crop growth may be necessary to reduce pollution levels within acceptable limits. If the plants are burned, the ashes must be disposed of in landfills.

Plants grown for phytoextraction are called hyperaccumulators. They absorb an unusually large amount of metal compared to other plants. Hyperaccumulators can contain about 1,000 milligrams per kilogram of cobalt, copper, chromium, lead, nickel, and even 10,000 milligrams per kilogram (1%) of manganese and zinc in dry matter (Baker and Brooks, 1989).

Phytoextraction is easier for metals such as nickel, zinc, copper, because these metals are preferred by most of the 400 hyperaccumulator plants. Some plants from the genus Thlaspi (pennycress) are known to contain about 3% zinc in tissues. These plants can be used as ore due to the high concentration of the metal (Brady and Weil, 1999).

Of all metals, lead is the most common soil contaminant (EPA, 1993). Unfortunately, plants do not accumulate lead in natural conditions. Chelators such as EDTA (ethylenediaminetetraacetic acid) should be added to the soil. EDTA allows plants to extract lead. The most common plant used for lead extraction is Indian mustard (Brassisa juncea). Phytotech (a private research company) reported that they had cleared plantations in New Jersey, under industry standards 1 to 2, with Indian mustard (Wantanabe, 1997).

Plants can remove zinc, cadmium, lead, selenium and nickel from soil in projects that are medium to long term.

Traditional site cleanup can cost between $10.00 and $100.00 per cubic meter (m3), while removal of contaminated materials can cost $30.00 to $300/m3. In comparison, phytoextraction can cost $0.05/m3 (Watanabe, 1997).

Future prospects

Phytoremediation has been studied in the process of researching small and large scale applications. Phytoremediation may move into the realm of commercialization (Watanabe, 1997). The phytoremediation market is projected to reach $214 to $370 million by 2005 (Environmental Science & Technology, 1998). Given the current efficiency of phytoremediation, it is best suited for cleaning larger areas where contaminants are present in low to moderate concentrations. Before full commercialization of phytoremediation, further research is needed to ensure that plant tissues used for phytoremediation have no adverse effects on the environment, wildlife, or humans (EPA, 1998). Research is also needed to find more efficient bioaccumulators that produce more biomass. There is a need for the commercial recovery of metals from plant biomass so they can be recycled. Phytoremediation is slower than traditional methods of removing heavy metals from soil, but much less expensive. Prevention of soil pollution is much cheaper than remediation of catastrophic consequences.

List of used literature

1 Baker, A.J.M., and R.R. Brooks. 1989. Terrestrial plants which hyperaccumulate metallic elements - a review of their distribution, ecology, and phytochemistry. Biorecovery 1:81:126.
2. Brady, N.C., and R.R. Weil. 1999. The nature and properties of soils. 12th ed. Prentice Hall. Upper Saddle River, NJ.
3. Environmental Science & Technology. 1998 Phytoremediation; forecasting. Environmental Science & Technology. Vol. 32, issue 17, p.399A.
4. McGrath, S.P. 1998. Phytoextraction for soil remediation. p. 261-287. In R. Brooks (ed.) Plants that hyperaccumulate heavy metals their role in phytoremediation, microbiology, archeology, mineral exploration and phytomining. CAB International, New York, NY.
5. Phytotech. 2000. Phytoremediation technology.

Heavy metals polluting the soil. Heavy metals in soils. Danger of heavy metal contamination