After reading this article you will learn about the properties and effects of coal fly ash on soil properties.
Properties of Coal Fly Ash:
The physical and chemical properties of coal fly ashes depend on the geological origin of coal, combustion conditions, efficiency of particulate removal and degree of weathering before final disposal. Comparatively, fly ash contain relatively large amount of Si, Al, Fe and Ca, intermediate amounts of Mg, K, Na and Ti and lesser amount of B, Zn, Mn, Cu and numerous other elements.
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These elements occur in coal fly ash as silicates, oxides, sulphates and carbonates. During combustion, a large portion of these elements in coal fly ash is fused into glassy spheres having ‘cenospheres’ (hollow spherical particles) and ‘plerospheres’ (large spheres containing small spheres).
Aitken (1984) tested thirteen coal fly ashes from Australian power stations and reported that the ashes consisted of non-coherent particles predominantly in the silt plus fine sand fraction size (67-98 per cent) having both amorphous and crystalline aluminosilicates and quartz, minor amounts of Fe oxides and lesser amounts of Ca, Mg, Na, K, Ti and P oxides and variable levels of incompletely combusted carbon.
In general, these coal fly ashes were alkaline (pH 8.0-12.8), saline (0.63-7.0 mS. Cm-1 in saturation extracts) and contained adequate levels of sulphate-S and DTPA extractable Cu, Zn, Fe and Mn. Cation sorption and exchange and sorption of anions except for monovalent ones is also exhibited by fly ash material.
In India, coal fly ash samples originating from different hoppers of Koradi Thermal Power Station were characterized their chemical characteristics, mineralogical phases and petrologic properties. These workers reported that in fly ash samples the major matrix elements like Fe, Al and Si showed little variation while trace elements showed high variability in their concentration; their concentration increased with the decrease in particle size (Table 1). Altered mineral phases like a-quartz, mullite and vitrified silica (glass) were also identified in fly ash samples by X-ray diffraction and petro logical examinations.
Laboratory characterization of some Indian lignitic and non-lignitic coal fly ashes; both weathered and un-weathered; revealed that these fly ashes comprise of mainly silt size particles with a particle density ranging from 1.83 to 2.27 Mg.m-3 (Table 2). The water holding capacity of these fly ash samples is high and varies from 29.6 to 42.4%.
Some chemical properties of Indian coal fly ashes derived from lignitic or non-lignitic coal; both un-weathered and weathered samples, which are important from plant growth point of view, are indicated in table 3. The pH of un-weathered fly ash samples derived from non-lignitic coal is reported to be acidic in reaction while that of weathered samples is generally neutral to alkaline.
On the other hand, the pH of un-weathered lignite coal fly ash is alkaline, but the pH of the weathered sample is acidic. The pH of Indian coal fly ash samples can be better related to the ratio of exchangeable Ca to exchangeable Al rather than to the C: S ratio in them as reported by Ainsworth and Rai (1987). A ratio of exch. Ca/exch. Al below and above 5.0 yields acidic and alkaline pH in Indian coal fly ashes.
The acid neutralization capacity of Indian coal fly ashes is low and ranges from 14 to 36 mM H–1.kg–1 for weathered non-lignitic and un-weathered lignitic coal fly ashes. The acid neutralization value is more closely related to the sum of exchangeable Ca and Mg in these ashes. The base neutralization value varies from 22.5 to 66.0 mM OH-1 kg-1 for un-weathered non-lignitic and weathered lignitic coal fly ashes and is related to the sum of total Fe and Mn in them.
The electrical conductivity (E C.) of Indian coal fly ashes ranges from 2.8 to 9.4 dS.m-1 for un-weathered samples and from 2.2 to 3.4 dS.m–1 for weathered samples. All Indian coal fly ashes are reported to have higher content soluble salts than normal soil indicating that re-vegetation of coal fly ash dumps with salt sensitive plants could be difficult in the beginning. Cation exchange capacity of Indian coal fly ashes is invariably low in the range of 1.5 to 4.2 cmol (p+) kg-1 and their mixing in cultivated soil may not increase the cation exchange capacity of soils.
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As most of C and N present in coal are likely to be lost during combustion, these elements are likely to be present in small concentration in fly ash. Most of the carbon in un-weathered fly ash sample could be expected to be present in the form of charcoal-like substances with fewer characteristics resembling to that of soil organic matter.
The weathering of fly ash has been reported to increase the organic C content of fly ash material possibly by secondary enrichment. Indian coal fly ashes are deficient in N, medium to high in P, high in K, Ca, Mg and S from plant availability point of view. A decrease in the content of extractable S in weathered samples of fly ashes is likely due to leaching losses of SO4—S from them during the course of weathering.
Comparing the contents of DTPA extractable micronutrient cations in fly ashes with the suggested critical limits in soils, viz. 4.5 mg, Fe, 1.0 mg Mn, 0.8 mg Zn and 0.2 mg Cu.kg-1 soil, Indian coal fly ashes are also rich in micronutrient cations like Fe, Cu, Mn, and Zn from plant growth point of view.
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Considering 0.5 mg Bkg-1 soil as the critical level for B efficiency in soil, all Indian coal fly ashes are also rich in B. Keeping the toxic limit of B (4-10 mg B. kg-1 fly ash) as suggested by Townsend and Hodgson (1973) in view un-weathered sample of non-lignitic coal fly ash from Kanpur and weathered lignitic coal fly ash sample from Neyveli have toxic B concentration which may limit their agronomic use in B sufficient soils.
The content of exchangeable Al in Indian coal fly ashes is reported to vary widely from 138 to 340 mg.kg-1 fly ash. Considering 252 mg exchangeable Al. Kg– soil as the critical level for Al toxicity for crops, excess levels of Al are in un-weathered fly ashes from Aligarh, Kanpur, and weathered fly ash from Neyveli, which also have a pH of 5.0 or below.
The exchangeable Na content in Indian coal fly ash fly ash ranges from 74 to 251 mg kg-1. The calculated exchangeable Na percentage (ESP) varies from 9.1 to 24.6%. As ESP values above 9.0 are critical to most crops, a problem of Na toxicity can be expected on fly ash deposits. Weathering decreases the ESP value of fly ash. The content of total and DTPA extractable Cd in samples of Indian coal fly ashes ranges from 1.59 to 2.57 mg and from 0.2 to 1.1 mg kg-1 fly ash, respectively. Comparing the content of Cd with the suggested critical limit for Cd toxicity (3 mg.kg-1), all fly ash samples are low in Cd.
Effect of Fly Ash on Soil Properties:
1. Effect of fly ash on soil physical properties:
Being fine in size, fly ash application improves soil texture of coarse textured soils and also increases soil porosity and water holding capacity, available water capacity, water infiltration rate and overall drainage.
The application of fly ash decreases bulk density, permeability, hydraulic conductivity and modulus of rupture but did not harmfully affect the soil aggregation. Roberts (1966) also observed that incorporation of 2.5 per cent of fly ash by weight in sandy soil greatly improved the emergence and establishment of subterranean clover (Trifolium subterraneum L.) by improving soil water conditions.
2. Effect of fly ash on soil chemical properties:
The changes in soil chemical properties brought about by the application of coal fly ash depend upon the chemical composition of fly ash being used for the purpose. Normally, the addition of fly ash to the soil of poor buffering capacity increases soil pH due to the presence of basic metal oxides and alters the availability of some nutrients.
With the addition of fly ash the increase in soil pH is more in acidic soils than alkaline soils because CO2 evolved soon reacted with CaO to produce CaCO3 shifting the pH towards neutrality (Hofmann and Wolf, 1958). This property of fly ash has been used in the reclamation of acidic soils and coal mine spoils.
Though many workers have proposed use of fly ash as potential liming agent showed highly variable buffering capacity of fly ash samples and stressed the need for assessment of individual samples.
In case of calcareous soil, addition of un-weathered fly of acidic reaction could decrease the soil pH to effect higher availability of micronutrient cations. Several workers have reported increase in available P, K, Ca, Mg, B and base saturation and cation exchange capacity, organic carbon content of soil treated with fly ash.
The benefits of small application of fly ash to correct S, B and Mo deficiency in soils have been demonstrated. Elseewi and Page (1984) also demonstrated a decrease in Fe, Mn, Ni, Co and Pb in acid soil with application of fly ash. However, application of fly ash samples rich in As (0.58%) could be hazardous due to fear of topsoil contamination.
3. Effect of fly ash on microbiological and biochemical properties of soils:
As regards the effect of coal fly ash on soil microbiological and biochemical properties of soil, very little work has been done. Application of fly ash at higher rates (400 to 700 t/ha) inhibits the activity of heterotrophic microorganisms. However, even with heavy application of fly ash, the development of mycorrhiza on seedlings of broad leaved plants has found to be normal.
The application of fly ash @ 30000 lbs/acre on highly leached coarse sand has been reported to increase the survival Rhizobium trifolii and nodulation by 45 per cent in subterranean clover (Trifolium subterraneum L.).
Eijsaker (1983) studied the effect of fly ash addition on soil fauna and reported higher number of Acrina on young ash but poor development of earthworm population for several years. It was also noted by these workers that Allolobophora. calinosa was more sensitive than Lumbricus rubellus for survival on young ash.
4. Effect of fly ash application on nutrient content of plants:
The application of fly ash to soil affects both macro- and micro- nutrients content of growing plants depending upon application rate, stage of weathering, composition of fly ash and the nature of crop as well. The presence of Al in fly ash material induces P deficiency in plants.
Hill and Lamp (1980) reported that the treatment with pulverized fuel ash to soil at rate equivalent to 20, 60 and 150 kg Mg/ha increased Mg ( > 0.4%) and Na ( > 1.0%) in the rye grass but decreased Ca content from 0.9 to 0.6%, However, Malanchuk (1980) reported that fly ash application did not result high concentration of Ca, Mg and Na in Bird foot trefoil (Lolium corniculatus) in any harvest but increased K content with increasing rate of fly ash application in the range of 0 to 672 t/ha.
The contents of K, B and Zn were low in the first harvest but increased in the second harvest, Mn content however, decreased in the second harvest with increasing rate of fly ash application. In general, the availability of B and S from fly ash material is easier. Furr (1979) reported that fly ash application increased the absorption of B, Cu, Co, Fe, Mg, Mn, Mo, Se and Zn in apple, millet and vegetable crops.
Gutenmann (1979) noted that in five successive cuttings of lucerne, bird foot trefoil, brome grass, cocksfoot and timothy grown on fly ash amended soil (112.5t fly ash/ha), B content increased mainly in legumes while Se increased mainly in grasses and Mo showed consistent increase in all cuts of all crops.
Arsenic content increased mostly in the first cut of crops grown but crop yields were not adversely affected by fly ash application. The major proportion (90%) of absorbed As in plants grown on fly ash amended soil stays in roots.
5. Effect of fly ash application on crop yields:
Depending upon the composition, properties and rate of application, the response of fly ash application on crop yields varies. The application of fly ash in acid soils has been reported to increase the yield of Lucerne, sugar beet, barley and wheat.
The yield increasing effect of fly ash application has been also reported for several crops in neutral sandy loam soil, calcareous soil, brown clay soil and heavy clay soil. Application of un-weathered fly ash is reported to have favorable effect on yields of rice, soybean and black gram grown on calcareous soil.
A combined application of 120 t fly ash + 9 t FYM ha-1 could increase the grain/seed yield of rice and black gram grown in a calcareous soil by 56 and 45% over control, respectively. Application of weathered fly ash to an acid soil has been reported to increase the yields of rice, soybean and black gram crops with no adverse effect on yields even up to 20 per cent application rate; on weight basis.
The reduction in crop yield due to fly ash application, particularly at high rates of application of fly ash in toxic elements, may occur. Lobl (1971) reported that fly ash application even up to 33.3 per cent by weight increased the yields of mustard, oat and sunflower but application of fly ash at 66.6 per cent by weight reduced the yields of buckwheat and maize. Salter and Williams (1967) noted a reduction of 9 per cent in yield of red beet with application of 200 t fly ash/acre due to B toxicity.
Mulford and Martens (1971) also reported reduction in yield of Lucerne at heavy rate of application of B rich fly ash due to Zn deficiency. Khandkar (1996) attributed possible Na toxicity to the observe reduction in yield of soybean crop at higher doses of fly ash (20% by soil weight). Such yield depressing effects of heavy application of fly ash are not permanent but gradually diminish with time.
Thus, coal fly ash is an industrial waste that has potential to serve as a substitute source of micronutrient fertilizers and amendments for soils. As fly ash as such is poor in certain key nutrients such as N and P, its utilization in conjunction with organic manure can fit very well in the philosophy of integrated nutrient supply management system. However, fly ash materials rich in toxic heavy metals should not be used for agricultural purposes as a safeguard against contamination of soil and water resources.