­Soil pollutants-sources-urban and industrial-Heavy metal-pesticides-PAHs and PCB’s-E-waste

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­Soil pollutants-sources-urban and industrial-Heavy metal-pesticides-PAHs and PCB’s-E-waste

Types of pollutants

Soil pollutants can be included into two main groups, the organic pollutants (OPs) and the inorganic pollutants (IPs). One of the best examples of the latter are the potentially toxic elements (PTEs). Both PTEs and OPs can have natural or anthropogenic origins and the majority of polluted soils in the world contain complex mixtures of each or from both. Although these two types of pollutants highly differ in their behavior in soils. This are mainly related to the nondegradability of PTEs in relation to OPs, which can be decomposed by living organisms. Thus, PTEs may experience increased or decreased availability over time, depending on their form when deposited in soil, and changes in physicochemical conditions during their accumulation in soil. On the other hand, the availability of OPs tends to decrease over time and is closely related to their degradation susceptibility, usually resulting in decomposition into simple units or functional groups of lowest toxicity, but not necessarily.

Potentially toxic elements (PTEs)

PTEs is a general collective term applied to the group of metals and metalloids with an atomic density greater than 4 g cm⁻³, which toxicity for living organisms are well reported. This group of elements is originally derived from natural sources through the weathering of rocks and minerals, or other geological events (e.g., volcanic emissions). However, anthropogenic activities are gaining special relevance as the main sources of PTEs pollution in the soil, since the rates of PTEs generation via manmade cycles are more rapid relative to natural ones.

PTEs are considered priority pollutants, since their nondegradability and persistence in environment for long periods allows their transfer from the contamination sources (e.g., mining or industrial areas) to other locations where direct exposure of living organisms may be more favored. The new conditions of the receiving system may induce changes in chemical form (species) and render it more (bio)available.

In soil system, PTEs can occur in the soil solution under ionic, molecular, chelated, and colloidal forms, conferring them high mobility, or associated with the solid fractions. Thus, from the ready to less available PTEs forms are (1) exchangeable ions in mineral or organic particles; (2) complexed or chelated to organic colloids; (3) sorbed to inorganic constituents; (4) incorporated in supergenic phases as (oxy)hydroxides, clay minerals, or insoluble salts; and fixed in crystal lattice of the minerals. Even when PTEs are not dissolved, their mobility in soil is always dependent of an aqueous phase, as well as the intrinsic characteristics of each element. Both will determine the ease of release (dissolution), or sorption onto surfaces of soil components, according to the physicochemical conditions of the medium, being the pH and ion interactions determining factors. In addition, the mobility of PTEs are highly dependent on the source, and usually PTEs from anthropogenic inputs are more mobile than from pedogenic or geogenic ones.

Therefore, the knowledge of PTEs support phases are a crucial step to assess their migration and availability to other environmental compartments, highlighting the bioavailability for plants, animals, and humans, since concentrations in these bioavailable fractions are not necessarily proportional to the total concentration of PTEs. In general, the free ionic species are the most bioavailable forms of PTEs and good indicators of toxicity. However, this is not a rule since there are cases, such as the organic forms of Hg (methylmercury), that are more toxic than the free ionic species. In addition, the toxicity of some PTEs change according to their oxidation states. A good example is the Cr(VI), which is toxic and carcinogen for living organisms and also very mobile in soils, whereas Cr(III) is not toxic to plants and is necessary in animal nutrition. Thus, reductants materials in soils, as the case of organic matter, are very important in the reduction of Cr(VI) to more stable Cr(III) complexes.

Organic pollutants (OPs)

Although OPs can occur naturally, associated to some volcanic emission, forest fires, or related to fossil fuels, in fact, the greatest harmful impacts are associated with the manmade production of a huge number of chemical compounds. For many of these chemicals, the environmental and health impacts are not even known, while for others the risks are well known and result in prohibition of their use as the typical example of DDT.

The most important factors regarding OPs toxicity are the persistence, the solubility (in water or organic solvents), the volatilization and the by-products resulting from their (bio)degradation.

The OPs that are being deserve more attention in last decades are the persistent organic pollutants (POPs), which, as the name indicates, have long half-lives in environment due to their resistance to biological, chemical, and photolytic degradation. The hydrophobic that they present causing their retention in the particle soil fraction, particularly in organic matter due to being lipophilic compounds. In addition, the great affinity for lipids allows the accumulation in food chains by storage in fatty tissues of organisms. The semivolatile character of these pollutants also enable them to move long distances in the atmosphere, which cause a high spatial distribution and away from the source.

Considering the high diversity of POPs, they can be grouped into two main categories considering the production purpose or the result (by-product) of industrial activity. As main chemical synthesis products are that for industrial uses (e.g., PCBs) and that ones for agrochemicals (pesticides) uses (e.g., DDT and its metabolites, toxaphene, chlordane). As examples of combustion or industrial by-products of chemical synthesis are dioxins/furans (PCDDs/Fs) and polycyclic aromatic hydrocarbons (PAHs), which can be direct contaminants as they do not have a specific application purpose. PAHs are considered priority pollutants by US EPA and EU and their distribution in the environment, as well as the potential risks to human health (some of them are probable human carcinogens) have been the focus of much attention. These compounds, which inputs can be natural or anthropogenic, have soil as the most important environmental compartment sink. This may result from their hydrophobicity, which make them readily adsorbable by soil particles, particularly SOM, and difficult of being degraded. On the other hand, more polar POPs are phenolscompounds, making easier their transport in soils, since they can exist as dissolved in soil solution, sorbed to soil particles, or polymerized in humic compounds. For this reason, they are easily degraded under aerobic conditions 

Most Important Sources of Soil Pollution

Some of the most important sources of land or soil pollution are: 1. Domestic and Municipal Wastes 2. Industrial and Mining Wastes 3. Agricultural Wastes 4. Radioactive Materials and Biological Agents.

1. Domestic and Municipal Wastes:

One of the main causes of land and soil pollution is the growing quantity of domestic and municipal wastes. Household garbage includes food scraps, old newspapers, and a variety of plastic items, bottles, discarded papers, wood, lawn trimmings, glass, canes, old appliances, tyres, worn-out furniture, broken toys and a host of other items.

The total quantity of solid wastes is large and increasing. In the United States, municipal solid wastes averaged 1.2 kg per person per day in 1920. The quantity rose to 2.3 kg in 1970 and 3.6 kg in 1980 and now it is more than 4.5 kg. Urban wastes comprise both commercial and domestic wastes including dried sludge of sewage. In general, all the urban solid wastes are referred to as ‘refuse’.

The amount of solid wastes generated is directly related with prosperity. In contrast to the 3.6 kg per person per day in the United States, residents of Australia produce only 0.8 kg per person per day. The average person in India produces only about 0.2 kg per day.

It has been estimated that in 45 major cities of India, the quantity of average per day municipal wastes is about 50,000 tonnes.

In our cities 90 per cent wastes include ash, dust, mixed material and carbon, while in developed countries paper, plastic, glass, metal, etc., dominates.

The dumping of domestic and municipal wastes is a serious problem in cities because of its impact on environment and public health. Solid wastes may or may not cause diseases in man but are hazardous to health. Diseases such as dysentery, diarrhoea, plague, malaria and numerous others are the result of the indiscriminate dumping of wastes.

Industrial and Mining Wastes:

The disposal of industrial solid wastes is the major source of soil pollution by toxic chemicals.

The industrial wastes are mainly discharged from coal and mineral mining industries, metal processing industries and engineering industries. They contain toxic metals such as lead, copper and chemicals having acids and are responsible for soil pollution.

It has been reported that about 50 per cent of raw materials ultimately become waste products in industry and about 15 per cent of it are toxic. The chemicals discharged from the industries often enter the surface or groundwater or poison the soil or crops. The production of consumer goods also involved environmental problems, including land and soil pollution.

Effects of the production of consumer goods

The expansion of mining activities in many countries has now become a major cause of land pollution due to loss of soil and destruction of land. Mining has become a threat to the environment because it leads to the huge quantity of waste generally not useful to man and its reprocessing is not economical. Mining operations produce about 1.35 billion tonnes of debris each year.

Agricultural Wastes:

Agricultural practices also pollute the soil. According to an estimate, agricultural activities produce more than 1.8 billion tonnes of waste, each year.

About three-quarters of this is manure. Much of this manure is piled in dumps where it pollutes streams and waterways. Yet, at the same time, farmers across the continent are suffering from worn-out and depleted soils. Other agricultural wastes include branches and slash left over from logging apart from animal wastes.

In addition to fertilisers and pesticides, soil condi­tioners and fumigants are used in agriculture. Organic compounds containing lead, mercury and arsenic, when applied to a land, accumulate on the soil permanently and introduce these toxic metals into plant products.

Radioactive Materials:

The radioactive wastes produced by nuclear testing laboratories and industries reach the soil and accumulate there. Wastes from nuclear reactors contain ruthenium-106 and rhodium-106, iodine-131, barium-140, lanthanum-140, cerium-144, etc. All the radionuclides deposited on the soil emit gamma radiations, and are harmful to soil as well as for plant growth.

Biological Agents:

The excreta of humans, animals and birds are also a source of soil pollution by biological agents. Digested sewage sludge, which is used as manure, also causes soil pollution. In the developing countries, intestinal parasites constitute the most serious soil pollution problems. Faulty sanitation, waste water and wrong methods of agriculture also induce soil pollution.

There are three groups of pathogenic organisms that pollute the soil, viz.:

(i) Pathogenic organisms excreted by man, (ii) pathogenic organism excreted by animals, and (iii) pathogenic organisms found in contaminated soil.