Heavy Metal Contamination Of Soil And Associated Agricultural And Environmental Problems

Table 16.1 shows the yearly figures of production of heavy metals. Although the figures are of much environmental concern, these are of little importance as far as contamination of soil is concerned. This is because the use of heavy metals as industrial produce by mankind remains confined to cities and suburban areas, which may constitute less than 10 to 15% of the total inhabitable land mass. More importantly, the heavy metals used as industrial produce mostly find their way into aquatic environments through the drainage system and run-off water during the rainy season; from there, their return to the atmosphere and landmass through biogeochemical cycling is very slow [4].

Furthermore, it may also be noted that the use of heavy metals like Hg and As as components of pesticides in agriculture has been nearly discontinued, and the contamination of the land mass by these through agricultural practices is now only history. Also, although the use of fertilizers may result in contamination of the environment by various heavy metals present in them [26,27], this is unlikely to be of much significance because these are continuously removed from the soil along with each harvest.

Considering the network of the roads connecting one city to another, a significant source of heavy metals in the terrestrial environment, particularly of Pb, could be automobile exhaust [28-30]. Chamberlain [31] estimated that, since 1946, automobile-generated aerosol lead added about 3 |g g-1 lead to the topsoil in rural areas and <10 |g g-1 in urban areas in the U.K. However, this is said to be small compared to natural levels and the lead added from industrial discharges [4]. Furthermore, with increasing use of unleaded gasoline, the threat of contamination of the land mass may not remain of much importance. In fact, the threat of heavy metal pollution of the land mass, which may require some remedial measures such as phytoremediation, mainly results from two sources: (1) mining activities in the region for ore rich in one or more heavy metals; and (2) atmospheric emission by industries.

Mining of the earth for ore is the first step towards increasing contamination of the landmass by various heavy metals depending upon the type of the ore. The mining operation lets the ore particles loose; otherwise, they are bound tightly among each other, remaining virtually immobile. They become prone to be blown away by wind, thus contaminating a vast area around the mine, particularly in the windward direction. Additionally, the mining operation leaves stretches of mined lands devoid of vegetation because of their high metal contents. This problem of contamination of uncontaminated agricultural lands will further increase with increase in the area of mining and the mining operation. It is generally in practice to use only ores rich in metal for cost-effective extraction, but when the currently available stock of the metal-rich ores ends, the ores less rich in metal content may eventually be processed, leading to spatial increase in heavy metal-contaminated/polluted agricultural and other lands.

Processing of the ores for the extraction of metals is the second major step during which metals find their way into land mass; the metals escaping out of the chimneys of smelters are ultimately deposited in agricultural fields or other land, which may be far from the smelting unit. Atmospheric metal enrichment, leading subsequently to pollution of soil, is also associated with other higher temperature anthropogenic activities, like burning of fossil fuels, production of cements, etc. For illustration, the emission of a few heavy metals due to burning of coal is given in Table 16.2. Despite modern technological advances, smelting operations and fossil fuel burning in industries continue to be important sources of heavy metals in the terrestrial environment.

The environmental problem associated with Al needs special mention, although it is not a heavy metal when its specific gravity is taken into consideration. The two sources of metals to the terrestrial environment described earlier hold true for this metal also. In addition, Al as such occurs in high levels in soil, which may be appreciated from its high crustal abundance [1]. Wherever the soil pH is acidic, this causes serious agricultural losses. It has been estimated that approximately 40% of the world's cultivated lands and up to 70% of potentially arable lands are acidic [34], which speaks of the gravity of environmental problems and economical losses associated with Al contamination of soil. Al is most often found as oxide or silicate precipitates that are not toxic to plants. However, in acidic soil (pH < 5.0), Al speciates to soluble octahedral hexahydrate form, Al(H2O)63+ (commonly called Al3+ [35]) — the phytotoxic species responsible for agricultural losses.

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