Mobility of Heavy Metals in Tropical Land

Heavy metals exist in two forms in nature. As stated above, microbes can convert contaminants to less harmful products; however, they can also immobilize contaminants (National Research Council 2003). Metal immobility is primarily achieved through reactions that cause the metal to precipitate or that keep the metal in a solid phase (Evanko Cynthia and Dzombak 1997). Chemical and physical properties affect the mobility of metals in soils and groundwater. Under acidic conditions (pH between 4.0 and 8.5) metal cations are mobile while anions tend to bind into oxide minerals. At high pH values, cations are adsorbed onto mineral surfaces and metal anions are mobilized. Hydrous metal oxides of iron, aluminum, and manganese can affect metal concentrations because these minerals can remove cations and anions.

A "biocurtain" is a term used to describe a process in which large amounts of biomass stop or slow contaminant movement. The biomass can absorb hydrophobic organic molecules (National Research Council 2003). A large biomass also can hinder the migration of a contaminant. When a microorganism oxidizes or reduces a species, this reaction causes metals to precipitate (National Research Council 2003). Mercury is an example of a metal that can be precipitated. This process begins when mercury (Hg2+) is reduced to mercuric sulfide, causing mercury to transform into a precipitated form. Chromium is another metal that can be converted into a precipitated form through the use of microorganisms. This process involves the reduction of hexavalent chromium (Cr6+) to trivalent chromium (Cr3+), which can then be precipitated as chromium oxides, sulfides, or phosphates

(National Research Council 2003). Current research is focusing on other metal and radioactive contaminants that can undergo precipitation processes.

Soil acidity is a major problem to agriculture in the tropics. Estimates of the world's potentially arable land resources indicate that only 10.6% of the total land area of the world is cultivated while about 24.2% is considered cultivable (US President's Advisory Committee Report 1967; FAO 1991; Buringh et al. 1975). Of these 2.5 billion hectares of potentially cultivable land, 68% is located in the humid tropics (Von Uexkull and Mutert 1995). The acid soils of the tropics, especially those in the savannas, have the greatest potential for future agricultural development (Dunal 1988). On a global scale, there are two main geographical belts of acid soils: the humid northern temperate zone that is covered by coniferous forest, and the humid tropics, which are (or in some cases were) covered mainly by savanna and tropical rainforest. Soil acidification can develop naturally in humid climates when basic cations are leached from soils, but can also be considerably accelerated by certain farming practices and by acid rain (Kennedy 1986). Approximately 43% of the world's tropical land area is classified as acidic, comprising about 68% of tropical America, 38% of tropical Asia, and 27% of tropical Africa (Pandey et al. 1994; Von Uexkull and Mutert 1995). Tropical forests are invaluable with regard to their role in local, regional, and global ecosystems and to the biodiversity found within them (over 90% of plant and animal species live in forest ecosystems). Indiscriminate conversion of tropical forest into agricultural land will have far-reaching ecological consequences; in spite of these consequences, 11 million or so hectares of forest are cleared each year, of which only a small fraction is converted into productive agricultural land, and most of which becomes unproductive grassland (Von Uexkull and Mutert 1995). Policies to use acid soils for agriculture should be directed at the acid savannas of the world such as the Cerrado in Brazil, Los Llanos of Venezuela and Colombia, the savannas in Africa, and the largely anthropic savannas of tropical Asia. These acid savannas cover an area of over 700 million hectares (which is approximately 50% of the global area that is currently under cultivation), and their potential for human and animal food production could account for a large portion of that required to satisfy the needs of the growing population in the next millennium. There are good examples in Brazil and Asia of the successful development of acid savanna into productive land for the cultivation of sugarcane and soybean (Von Uexkull and Mutert 1995). The use of biotechnology could hugely facilitate the conversion of low-productivity acid savannas into productive croplands.

Aluminum toxicity, poor crop productivity, and soil fertility in acid soils are mainly caused by a combination of aluminum and manganese toxicity and nutrient deficiencies (mainly deficiencies in P, Ca, Mg, and K). Among these problems, aluminum toxicity has been identified as the most important constraint on crop production in acid soils. Aluminum toxicity problems are of enormous importance for the production of maize, sorghum, and rice in developing countries located in tropical areas of Asia, Africa, and Latin America. Most of the maize, sorghum, and rice cul-tivars currently in use are susceptible to toxic aluminum in the soil, and decreases in yield of up to 80% resulting from aluminum toxicity have been extensively reported in the literature (Brenes and Pearson 1973; Lopes and Cox 1977). In tropical South America, aluminum toxicity is a problem shared by several countries, where about 850 million hectares, or 66% of the region, has acid soils. In Brazil alone, acid savannas with low cation exchange capacity and high toxic aluminum saturation cover 205 million hectares, of which 112 million are suitable for maize and sorghum production (Pandey et al. 1994). Aluminum has a clear toxic effect on roots, disturbing plant metabolism by decreasing mineral nutrition and water absorption. Therefore, crop production in acid soils is, to a great extent, limited by nutrient uptake deficiency caused by the inhibition of root growth and function that results from the toxic effects of Al (Kochian 1995). Moreover, in some acid soils, plant growth is affected not only by aluminum toxicity but also by the low availability of some essential elements, such as P, Ca, Mg, and Fe, some of which form complexes with Al and thus are not readily available for root uptake (Haug 1984). It is well documented that many plant species exhibit significant genetic variability in their ability to tolerate Al. Although it is clear that certain plant genotypes have evolved mechanisms that confer Al resistance, the cellular and molecular basis for Al resistance is still poorly understood (Kochian 1995). Two basic strategies by which plants can tolerate Al have been proposed: (1) the ability to exclude Al entry into the root apex and root hairs, and (2) the development of mechanisms that allow the plant to tolerate toxic concentrations of Al within the cell.

A major environmental concern due to the dispersal of industrial, urban and periurban wastes generated by anthropogenic activities is the contamination of agricultural land in tropical regions. Controlled and uncontrolled methods of disposing of waste, accidental and process spillage, mining and smelting of metalliferous ores, and the application of sewage sludge to agricultural soils are responsible for the migration of contaminants into uncontaminated sites as dust or leachate, thus contributing towards the overall contamination of the ecosystem. The tropical regions of the world have the greatest area of agricultural land. However, the population density in such regions is also very high. Hence, remediation of contaminated land is very important in this region in order to maximize the acceptable agricultural land. In the light of these severe problems, this chapter proposes several methods of remediating tropical land.

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