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FLY ASH: AN UBIQUITOUS MICRO ENVIRONMENTAL POLLUTANT

The key source of energy in any nation is the coal based thermal power plants. In thermal power plants, the by-product of combustion of coal is predominantly a powdery material called coal fly ash. Coal fly ash is a complex heterogeneous material consisting of both amorphous and crystalline phases. Moreover, the property of ash depends upon the firing condition and the ash content of the coal which depends on the origin of the coal. On the basis of ash content, Indian coal has about 32.16% of ash content. Worldwide about 7575 million tonnes of fly ash was generated in 2020, where 760 million tonnes was added solely by India. Furthermore, the thermal power plant area is the major source of heavy metal pollution as well.

High surface area of fly ash has high concentration of toxic elements, atmospheric mobility and gets deposited over large area around thermal power plant. Atmosphere (air) acts as a transport medium for fly ash containing heavy metals. Fly ash ranges from 10 to 100 micron comes under silt-sized particles and is spherical in shape called spherules. The spherical shape of fly ash reveals that their formation from a viscous melts. These spheres are glassy and mostly transparent, indicating their derivation due to the complete melting of rock- and ore-forming minerals present in coal. The major constituents of fly ash are SiO2, Al2O3, FeOx, TiO2 and CaO. It also contains small quantity of unburnt carbon along with oxides of Mg, Cr, Na, and K. The fine fraction of fly ash makes its tendency to remain in air for longer duration. Moreover, due to their tiny size, it frequently clings to the air for a long time. Fly ash is collected using electrostatic precipitators; however, it can enter into terrestrial and aquatic environment in the form of dry and wet deposition. Fly ash can be categorized into two classes: Class “C” which forms from the burning of lignite and sub-bituminous coal, and contains 20% more CaO. Class “F” produce from burning of anthracite and bituminous coal and contains less amount of CaO (7%).

Fly ash has an adverse effect on the environment when it is either disposed of in large numbers or when small particles are released into the atmosphere and eventually settle in nearby areas. The latter quantities are typically difficult to estimate due to the obvious challenges in measuring them. Several researchers have highlighted the effects of tiny fly ash particulates on human health in their research. Fly ash contains a number of heavy metals, including Pb, Cd, Sb, Se, Ni, V, Zn, Co, Br, Mn, and SO2, which are small and potentially hazardous particles that deposits in the lungs. Nearly all particles with dimensions ≤ 1 micron are deposited almost exclusively in the lung. The main sources of emission of these particles are high temperature combustion sources.

Several research has been done on the effects of introducing fly ash into the soil which depict that by raising the quantities of soluble major and minor inorganic elements, fly ash enhance the electrical conductivity of the soil mixture by a significant amount. The quick release of Ca, Na, Al, and OH- ions from fly ash accounts for the immediate rise in soil pH following the addition of alkaline fly ash. Applying small amounts of fly ash can increase soil hydraulic conductivity, but when fly ash intake surpasses 20% (v/v) in calcareous soils and 10% in acidic soils, the soil’s hydraulic conductivity quickly degrades. Fly ash is an inorganic substrate that is high in electrolytes but low in organic matter, hence adding it to soil causes an increase in hydraulic activity.

Fly-ash dust adheres to the leaves or fruits of numerous crops, including green beans, turnips, cabbage, and tomatoes, and enhance chemical as well as physical injuries. Small necrotic dark brown spots also occur on the leaves of many plants. Fly ash particles build up on the guard cell surface at lower fly ash deposition rates, where they activate the mechanism of controlling stomata opening and closure and prevent them from closing, and hence preventing higher rates of transpiration. Fly ash accumulations that are too thick blocks the light needed for photosynthesis, which lowers the rate of photosynthesis. It also coats the leaves to absorb heat more efficiently, which raises the leaf temperature and raises transpiration rates.

Although, fly ash can be used for waste reclamation, cement, brick etc., however, still large amount of fly ash is present and unused in the environment. Therefore, the identification of different pollutants from coal combustion is needed. In some barren or sterile areas, fly ash could be used as a potential soil amendment for cultivated plants, as previously mentioned. In the small-scale cultivation of plants with potential for ornamental, medicinal, agricultural, or forestry use, fly ash can be directly utilized as a cost-effective fertilizer and soil amendment. However, in order to establish its quality and safety, its widespread application to field crops, vegetables, and other edible plants must wait for more extensive field experiments.

With regard to fly-ash disposal, the landfills should be vegetated or covered by green plants to control erosion, water pollution and dust pollution. In this regard, it is interesting to note that press mud, when used as a bio-fertiliser, farm yard manure, paper mill wastes and several other organic amendments have been used as alternative amendment materials in lieu of standard fertilisers to aid re-vegetation. Several studies have shown the potential benefits of using sewage sludge compost as a fly-ash ameliorant.