Open access

Introductory Chapter: Coal Fly Ash and Its Application for Remediation of Acid Mine Drainage

Written By

Mugera Wilson Gitari and Segun Ajayi Akinyemi

Submitted: September 22nd, 2016 Published: January 31st, 2018

DOI: 10.5772/intechopen.70711

Chapter metrics overview

1,282 Chapter Downloads

View Full Metrics

1. Introduction

The coal mining and other metal extraction industry contributes significantly to the country’s economy through export markets of the processed minerals in addition to creation of thousands of jobs in the mining industry. For example, in South Africa, 98% of the country’s electricity supply is from coal combustion. Coal mining and coal combustion for electricity generation leaves in its wake various types of waste streams such as acid leachate generating coal discards, acid generating mine tailings, and acid mine drainage (AMD) in mine voids and huge volumes of coal fly ash (CFA) in power utilities. Environmental regulations require these waste streams to be managed and remediated to reduce any negative impact on the environment. Of most concern and increasing interest is the prevention and control of acid mine drainage (AMD) and its treatment. Acid mine drainage has the potential to impact negatively on aquatic ecosystems and groundwater availability in particular. Acid mine drainage (AMD) is generally regarded as the principal environmental problem caused by the mining of the sulfide ore deposits. In coal combustion in power utilities, huge volumes of coal fly ash (CFA) are produced annually and this is normally stockpiled on land as ash heaps that require management due to possible negative environmental impacts such as generation of leachates with high concentration of toxic metal and non-metal species which could affect the quality of surface and groundwater resources in addition to the generation of particulate pollution.


2. Acid mine drainage generation and negative environmental impacts

Acid mine drainage is generated when sulfide minerals in the mined rock or overburden are exposed to oxygen and water during the mining process. These sulfide minerals include but not limited to pyrite (FeS2), its dimorph marcasite, and pyrrhotite (Fe1-xS). These sulfide minerals undergo bacterially catalyzed oxidation reactions which generated acidity and increased Fe, sulfate, and other toxic metal species concentration in recipient water bodies such as groundwater in mine voids [1] (Eq. (1))


AMD is extremely acidic (as low as pH 2.0) and enriched with iron, manganese, aluminum, sulfate, and metal species such as lead, mercury, cadmium, and zinc depending on the geology of the mined rock [2]. During active mining operations, the groundwater is normally pumped out to maintain water table below mining levels but once mining stops pumping stops and the mine voids fill with acid mine drainage and eventually decant to the surface. This mine water requires management, which involves storage and treatment to the specific country’s disposal guidelines before disposal to surface water bodies.

Influx of acid mine drainage (AMD) into streams can degrade both aquatic habitat and water quality, often producing an environment devoid of most aquatic life. The extent of impact on aquatic ecosystems depends on a variety of factors which includes frequency of influx, volume, and chemistry of the AMD, and the buffering capacity of the receiving aquatic ecosystem [3]. Drainage from underground mines, surface mines, and refuse piles is the oldest and most chronic industrial pollution associated with coal mining. Various impacts of AMD include quality impacts on groundwater, corrosion of water supply infrastructure, and other manmade structures such as pipes, bridges, dams, and pumps. Acid mine drainage can also be toxic to vegetation; however, toxicity depends on discharged volume, pH, total acidity, and concentration of dissolved chemical species. pH is the most critical with respect to aquatic life. Smothering of stream beds from precipitated metal compounds is a common phenomenon in AMD-impacted streams [4, 5]. Acid mine drainage can also cause reduction in diversity and total number of micro-invertebrates and changes in community structure.


3. Coal fly ash generation and its physicochemical properties

Coal fly ash (CFA) is a by-product obtained during the combustion of coal in coal-burning power generation plants. As demand for cheaper electricity increases, huge volumes of coal fly ash will be generated that will require disposal and management. The worldwide annual production of CFA stands at 780 Mt [6] while 415 Mt or 53% is beneficially utilized, but utilization varies across countries. Japan has the highest utilization rate at 96.4%, Europe 90.9%, China 67%, and Middle East and Africa 10.6% [6]. The rest of the CFA is stockpiled on land or slurried to ash dams [7, 8]. South African Bureau of Standards (SABS) [9] defines fly ash as the powdery residue obtained by separation of the solids from the flue gases of furnaces fired with pulverized coal. Coal fly ash consists of many small (0.01–100 µm diameter) glass-like particles of a generally spherical character. The fineness of fly ash particles depends largely on the combustion temperature, the grinding size of introduced coal, and whether the resultant particle is spherical or irregular. The physicochemical and mineralogical properties of CFA depend on the composition of the parent coal, the conditions during coal combustion (temperature, air/fuel ratio, coal pulverization size, and rate of combustion), the efficiency of emission control devices, the storage and handling of the by-products, and the climate [10, 11].

Coal fly ash tends to accumulate toxic chemical species at high temperatures involved during its generation [10, 12] and is considered an environmental hazard in some countries. Anionic species (Cl, SO42), oxy-anionic species (Se, As, Mo, B, and Cr), and cationic species (Al, Fe, Na, K, Ca, Sr, Ba, Zn, Cu, Cd, and Mg) are leached from the ash heaps by the wastewater derived from the ash slurry or by subsequent infiltration by rain upon disposal [8, 10, 13]. This may be of environmental concern due to possible contamination of surface and groundwater in disposal sites and limits beneficial application of CFA. The pH of a fly ash suspension, for example, in water can vary depending on the S content of the parent coal [14]. Fly ashes derived from anthracite coals are generally high in S and produce acidic fly ashes while fly ashes derived from lignite coals are low in S but high in Ca and produce alkaline ashes [15, 16]. South African coal is sub-bituminous and generates fly ash that is characterized by low Fe content. The aqueous extracts of this high Ca coal fly ash are strongly alkaline (pH 12–12.5) due to the free lime content [2, 17]. Mineralogical analysis indicates coal fly ash to be mainly aluminosilicate which forms the basis of its utilization in the synthesis of geopolymers [18], zeolitic adsorbents for water treatment [19]. X-ray diffraction also indicates that CFA has free lime content ranging from 1 to 40% depending on the coal source [6]. This free lime content is the basis of fly ash utilization in acid mine drainage treatment [2] and remediation of acidic mine spoils and control of acid mine drainage generation in sulfidic mine tailings [11].


4. Acid mine drainage treatment using coal fly ash

Acid mine drainage is characterized by high acidity (pH 2–4) and often contains high concentrations of chemical species such as Fe, Mn, and Al and anionic species such as SO4 in addition to elements like Zn, Co, Pb, Cr, and Cu, in trace concentrations which necessitate these waters to be treated before release. Management of mine water pollution demands a range of active and passive remediation engineering technologies to minimize its impact on ground and surface waters which can incur significant expense [20]. Consequently, mining companies are in constant search for innovative and economically viable treatment technologies. Traditionally, the remediation of AMD has been carried out through a range of active and passive technologies. Active treatment technologies involve the use of alkaline reagents such as limestone and lime [21, 22], and magnesite [23]. Some of the limitations of limestone treatment processes include armoring of limestone particles by amorphous ferric hydroxide precipitates which reduces the efficiency of treatment process and attains a maximum pH of 7 which leaves species such as Mg in solution [24]. A limitation of lime is its high cost [24]. Passive treatment systems involve a combination of alkaline reagents and utilization of natural biochemical processes in artificially constructed wetlands, ponds, and alkaline-generating drains [2527]. Innovative acid mine drainage treatment using unconventional alkaline agents such as slag has been evaluated [28, 29]. The steel slags on contact with the AMD increased the pH to circumneutral levels and reduced the levels of most major inorganic contaminants such as Al, Fe, Ti, Ni, Be, and Cu. However, the slag was observed to leach chemical species in the reaction mixture leading to an increased concentration of Ba, V, Mn, Cr, As, Ag, and Se in the leachates. This would be a major drawback of employing the steel slags in AMD treatment due to the secondary contamination of the product water.

Several research studies have reported on the application of coal fly ash for the amendment of acidic coal mine spoils [30] and of acidic soils [31]. These applications were motivated by the alkaline nature of the coal fly ash. Other researchers have reported on the ability of the coal fly ash to remove metal species from aqueous solutions [3234].

Due to the pressure on the mining companies to reduce the cost of acid mine drainage treatment, they are constantly looking for cheaper treatment agents for AMD. On the other hand, most coal combustion power utilities are constantly looking for large volumes of beneficial coal fly ash generated by their coal combustion process. In most instances, the coal mines generating AMD are located close to the power utilities producing coal fly ash. This has motivated researchers to look at the possibility of utilization of coal fly ash as liming agent for AMD. Coal fly ash contains free lime content that can be used in the neutralization of AMD leading to an increase of pH and precipitation of the metal ion contaminants as insoluble hydroxides, oxyhydroxides, or oxyhydroxysulfates [17, 35].

Coal fly ash has also been beneficiated into zeolitic materials that are used as adsorbents for metal species in AMD effluents. These zeolites possess adsorptive properties and provide a combination of ion exchange and molecular sieve properties [36, 37]. Results of the treatment of AMD with CFA zeolitic product indicated an increased pH and a decreased metal concentration. These materials have advantage over traditional liming materials such as lime or limestone since they are cheaply produced and contribute to sustainable management of coal fly ash.

Although coal fly ash will continue to attract researchers working on cheaper options for the treatment of AMD in both active and passive systems, it is important to note some of the shortcomings and strengths of its application for AMD effluent treatment. Several authors [2, 17, 35] observe that coal fly ash can effectively increase the pH of AMD and decrease its metal species concentration leading to much cleaner product water. The process was observed to be effective for the treatment of acidic AMD [2]. However, the application of CFA for AMD treatment leads to the release of Na, Cl, K, Mg, B, Sr, Ba, and Mo leading to an increased salinity of product water. The chemistry of the coal fly ash and AMD being treated will be a significant determinant factor on the success of the treatment process in addition to the contact time employed. The authors further observed that the treatment process will be combination specific, meaning that different CFA:AMD combination ratios will give product water with varying quality.


5. Conclusions

Coal fly ash can be used effectively to treat AMD. However, the process has its strengths and weaknesses. The treatment technology will be dependent on the chemical properties of the coal fly ash and the AMD effluents being treatment. This means that the treatment process has to be optimized for each coal fly ash/AMD combination for effective results. The product water in this treatment process will require secondary treatment such as reverse osmosis to remove the increased salinity of the product water. Most countries have environmental legislation that classify coal fly ash as a hazardous material, hence reducing its development and utilization as a beneficial material. However, there is still a lot to be learned in terms of the application of coal fly ash and its products for remediation of acid mine waters.


  1. 1. Nordstrom SK. The effect of sulfate on aluminum concentrations in natural waters: Some stability relations in the system Al2O3-SO3-H2O at 298°K. Geochimica et Cosmochimica Acta. 1982;46:681-692. DOI:
  2. 2. Gitari MW, Petrik LF, Etchebers O, Key DL, Iwuoha E, Okujeni C. Treatment of acid mine drainage with fly ash: Removal of major contaminants and trace elements. Journal of Environmental Science Health-Part A. 2006;A41:1729-1747. DOI: 10.1080/10934520600754425
  3. 3. Kimmel WG. The impact of acid mine drainage on the stream ecosystem. In Pennsylvania Coal: Resources, Technology Utilization, (S.K. Majumdar and W.W. Miller, eds), The pa. Acad. Sci. Publ. 1983;424-437. ISBN-13: 978-0960667017
  4. 4. Parsons JD. The effects of acid strip-mine effluents on the ecology of a stream. Archiv für Hydrobiologie. 1968;65:25-50
  5. 5. Warren CJ, Dudas MJ. Formation of secondary minerals in artificially weathered fly ash. Journal of Environmental Quality. 1985;14:405-410. DOI: 10.2134/jeq1985.00472425001400030019x
  6. 6. Craig H, Hans-Joachim F, Anne W. Coal combustion products: A global perspective. In: World of Coal Ash (WOCA) Conference; April 22-25, Lexington, KY, USA 2013. Available from: http:/
  7. 7. Abbott DE, Essington ME, Mullen ND, Ammons JT. Fly ash and lime-stabilized biosolid mixtures in mine spoil reclamation: Simulated weathering. Journal of Environmental Quality. 2001;30:608-616. DOI: 200110.2134/jeq2001.302608x
  8. 8. Mattigod SV, Rai D, Eary LE, Ainsworth CC. Geochemical factors controlling the mobilization of inorganic constituents from fossil fuel residues: Review of the major elements. Journal of Environmental Quality. 1990;19:188-201. DOI: 10.2134/jeq1990.00472425001900020004x
  9. 9. South African National Standards (SANS 50413-1/EN 450-1). Fly Ash for Concrete—Part 1: Definition, Specifications and Conformity Criteria. 2015. ISBN 978-0-626-32353-0
  10. 10. Eary LE, Dhanpat R, Mattigod SV, Ainsworth CC. Geochemical factors controlling the mobilization of inorganic constituents from fossil fuel combustion residues: II. Review of the minor elements. Journal of Environmental Quality. 1990;19:202-214. DOI: 10.2134/jeq1990.00472425001900020005x
  11. 11. Xenidis A, Evangelia M, Ioannis P. Potential use of lignite fly ash for the control of acid generation from sulphidic wastes. Waste Management. 2002;22:631-641. DOI:
  12. 12. Spears DA, Lee S. Geochemistry of Leachates from Coal Ash. Geological Society, London Special Publications. 2004;236:619-639. January 1, 2004. DOI:
  13. 13. Plank CO, Martens DC. Boron availability as influenced by application of fly ash to soil. Soil Science Society of America, Proceedings. 1974;38:974-977. DOI: 10.2136/sssaj1974.03615995003800060038x
  14. 14. Barry SR, Daniels WL, Jackson ML. Evaluation of leachate quality from co-disposed coal fly ash and coal refuse. Journal of Environmental Quality. 1997;26:1417-1424. DOI: 10.2134/jeq1997.00472425002600050031x
  15. 15. Furr AK, Parkinson TF, Hinrichs RA, Van Campen DR, Bache CA, Gutenmann WH, St. John RE Jr, Pakkala IS, Lisk DJ. National survey of elements and radioactivity in fly ashes: Absorption of elements by cabbage grown in fly ash soil mixtures. Environmental Science & Technology. 1977;11:1194-1201 ISSN: 0013-936X
  16. 16. Page AL, Elseewi AA, Straughan I. Physical and chemical properties of fly ash from coal-fired power plants with reference to environmental impacts. Residue Reviews. 1979;71:83-120 978-1-4612-6185-8_2
  17. 17. Gitari WM, Petrik LF, Etchebers O, Key DL, Iwuoha E, Okujeni C. Passive neutralisation of acid mine drainage by fly ash and its derivatives: A column leaching study. Fuel. 2008;87:1637-1650. DOI:
  18. 18. Nyale SM, Babajide OO, Birch GD, Böke N, Petrik LF. Synthesis and characterization of coal fly ash-based foamed geopolymer. Procedia Environmental Sciences. 2013;18:722-730. DOI:
  19. 19. Musyoka NM, Petrik LF, Gitari WM, Balfour G, Hums E. Optimization of hydrothermal synthesis of pure phase zeolite Na-P1 from South African coal fly ashes. Journal of Environmental Science and Health, Part A. 2012;47:337-350. DOI: 10.1080/10934529.2012.645779
  20. 20. Younger PL, Banwart SA, Hedin RS. Mine water: Hydrology, pollution, remediation. In: Chapter Two: Mine Water Chemistry. Dordrecht: Kluwer Academic Publishers; 2002. p. 65-126 978-94-010-0610-1
  21. 21. Maree JP, du Plessis P. Neutralisation of acidic effluents with calcium carbonate. Water Science and Technology. 1994;29:285-296. DOI:
  22. 22. Geldenhuys AJ, Maree JP, De Beer M, Hlabela P. An integrated limestone/lime process for partial sulphate removal. South African Institute of Mining and Metallurgy (SAIMM). 2001;103:345-371 ISSN 2411-9717
  23. 23. Masindi V, Gitari WM, Tutu H, De Beer M. Passive remediation of acid mine drainage using cryptocrystalline magnesite: A batch experimental and geochemical modelling approach. Water SA. 2015;41:677-682. DOI:
  24. 24. Maree JP, Van Tonder GJ, Millard P. Underground Neutralization of Mine Water with Limestone. Pretoria, South Africa: Water Research Commission, Report No. 609/1/96; 1996. ISBN: 1-86845-241-7
  25. 25. Cravotta III AC, Trahan MK. Limestone drains to increase pH and remove dissolved metals from acidic mine drainage. Applied Geochemistry. 1999;14:581-606. DOI:
  26. 26. Hedin RS, Watzlaf GR. The effects of anoxic limestone drains on mine water chemistry. In: Proceedings of the International Land Reclamation and Mine Drainage Conference; Pittsburgh. United States of America; 1994. pp. 185-194. DOI: 10.21000/JASMR94010185
  27. 27. Ziemkiewicz PF, Skousen JG, Brant DL, Sterner PL, Lovett RJ. Acid mine drainage treatment with armoured limestone in open limestone channels. Journal of Environmental Quality. 1997;26:1017-1024. DOI: 10.2134/jeq1997.00472425002600040013x
  28. 28. Simmons J, Ziemkiewicz P, Courtney Black D. Use of steel slag leach pads for the treatment of Acid Mine Drainage. Mine Water and the Environment. 2002;21:91-99. DOI:
  29. 29. Zvimba JN, Siyakatshana N, Mathye M. Passive neutralization of acid mine drainage using basic oxygen furnace slag as neutralization material: Experimental and modeling. Water Science Technology. 2017;75:5-6. DOI: 10.2166/wst.2016.579
  30. 30. Bhumbla DK, Keefer RF, Singh RN. Selenium uptake by alfalfa and wheat grown on a mine spoil reclaimed with fly ash. In: Proceedings of Mine Drainage and Surface Mine Reclamation Conference, Washington DC, USA, 1998. pp. 15-21. DOI: 10.21000/JASMR88020015
  31. 31. Phung HT, Lund LJ, Page AL, Bradford GR. Trace elements in fly ash and their release in water and treated soils. Journal of Environmental Quality. 1979;8:171-175. DOI: 10.2134/jeq1979.00472425000800020007x
  32. 32. Erol M, Kucukbayrak S, Ersoy-Mericboyu A, Ulubas T. Removal of Cu2+ and Pb2+ in aqueous solutions by fly ash. Energy Conservation Management. 2005;46:1319-1331. DOI:
  33. 33. Panday KK, Prasad G, Singh VN. Copper (ii) removal from aqueous solutions by fly ash. Water Research. 1985;19:869-873. DOI:
  34. 34. Doye I, Duchesne J. Neutralization of acid mine drainage with alkaline industrial residues: Laboratory investigation using batch-leaching tests. Applied Geochemistry. 2003;18:1197-1213. DOI:
  35. 35. Madzivire G, Gitari WM, Kumar Vadapalli VR, Ojumu TV, Petrik LF. Fate of sulphate removed during the treatment of circumneutral mine water and acid mine drainage with coal fly ash: Modelling and experimental approach. Minerals Engineering. 2011;24:1467-1477. DOI:
  36. 36. Moreno N, Querol X, Ayora C. Utilization of zeolites synthesized from coal fly ash for the purification of acid mine waters. Environmental Science and Technology. 2001;35:3526-3534. DOI: 10.1021/es0002924
  37. 37. Fungaro DA, Izidiro JDC. Remediation of acid mine drainage using zeolites synthesized from coal flyash. Química Nova. 2006;29:735-740. DOI:

Written By

Mugera Wilson Gitari and Segun Ajayi Akinyemi

Submitted: September 22nd, 2016 Published: January 31st, 2018