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Progress in Energy and Combustion Science 36 (2010) 327363

Contents lists avai

Progress in Energy and Combustion Science

journal homepage: www.elsevier .com/locate/pecs

A review on the utilization of fly ash

M. Ahmaruzzaman*

Department of Chemistry, National Institute of Technology Silchar, Silchar-788010, Assam, India

a r t i c l e i n f o

Article history:Received 8 August 2009Accepted 10 November 2009Available online 28 December 2009

Keywords:Fly ashAdsorptionWastewaterHeavy metalsDyeOrganicsZeoliteConstruction

* Tel.: 91 3842 233 797.E-mail address: [email protected]

0360-1285/$ see front matter 2009 Elsevier Ltd.doi:10.1016/j.pecs.2009.11.003

a b s t r a c t

Fly ash, generated during the combustion of coal for energy production, is an industrial by-product whichis recognized as an environmental pollutant. Because of the environmental problems presented by the flyash, considerable research has been undertaken on the subject worldwide. In this paper, the utilization offly ash in construction, as a low-cost adsorbent for the removal of organic compounds, flue gas andmetals, light weight aggregate, mine back fill, road sub-base, and zeolite synthesis is discussed.A considerable amount of research has been conducted using fly ash for adsorption of NOx, SOx, organiccompounds, and mercury in air, dyes and other organic compounds in waters. It is found that fly ash isa promising adsorbent for the removal of various pollutants. The adsorption capacity of fly ash may beincreased after chemical and physical activation. It was also found that fly ash has good potential for usein the construction industry. The conversion of fly ash into zeolites has many applications such as ionexchange, molecular sieves, and adsorbents. Converting fly ash into zeolites not only alleviates thedisposal problem but also converts a waste material into a marketable commodity. Investigations alsorevealed that the unburned carbon component in fly ash plays an important role in its adsorptioncapacity. Future research in these areas is also discussed.

2009 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3282. Properties of coal fly ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329

2.1. Physical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3292.2. Chemical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330

3. Properties of biomass ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3304. Fly ash utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3315. Adsorbents for cleaning of flue gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .331

5.1. Sulphur compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3315.2. Adsorption of NOx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3325.3. Removal of mercury . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3325.4. Adsorption of gaseous organics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333

6. Removal of toxic metals from wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3336.1. Adsorption of various types of heavy metals on fly ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3346.2. Adsorption mechanism and kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3356.3. Adsorption isotherms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3366.4. Factors affecting adsorption of metal on fly ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337

7. Removal of other inorganic components from wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3377.1. Removal of phosphate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3377.2. Removal of fluoride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3387.3. Removal of boron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338

8. Removal of organic compounds from wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3388.1. Removal of phenolic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338

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M. Ahmaruzzaman / Progress in Energy and Combustion Science 36 (2010) 327363328

8.2. Removal of pesticides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3398.3. Removal of other organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340

9. Removal of dyes from wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3409.1. Azo dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3409.2. Thiazine dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3409.3. Xanthene dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3429.4. Arylmethane dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3429.5. Other dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342

10. Leaching of fly ash in water system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34311. Synthesis of zeolite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344

11.1. Application of zeolite synthesised from fly ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34612. Construction work/industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34813. Lightweight aggregate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35314. Road sub-base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35315. Mine backfill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35416. Cost estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35417. Barriers to utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35518. Future research and prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35519. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356

Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356

1. Introduction

Since wide scale coal firing for power generation began in the1920s, many millions of tons of ash and related by-products havebeen generated. The current annual production of coal ash world-wide is estimated around 600 million tones, with fly ash consti-tuting about 500 million tones at 7580% of the total ash produced[1]. Thus, the amount of coal waste (fly ash), released by factoriesand thermal power plants has been increasing throughout theworld, and the disposal of the large amount of fly ash has becomea serious environmental problem. The present day utilization of ashon worldwide basis varied widely from a minimum of 3% toa maximum of 57%, yet the world average only amounts to 16% ofthe total ash [1]. A substantial amount of ash is still disposed of inlandfills and/or lagoons at a significant cost to the utilizingcompanies and thus to the consumers.

Coal is a dominant commercial fuel in India, where 565 minesare operated by Coal India and other subsidiaries. In 2003,production of hard coal was 358.4 Mt.; while utilization was 407.33Mt. India is the sixth largest electricity generating and consumingcountry in the world. Fly ash can be considered as the worlds fifthlargest raw material resource [2]. An estimated 25% of fly ash inIndia is used for cement production, construction of roads and brickmanufacture [3]. The fly ash utilization for these purposes isexpected to increase to nearly 32 Mt by 20092010. Currently, theenergy sector in India generates over 130 Mt of FA annually [4] andthis amount will increase as annual coal consumption increases by2.2%. The large-scale storage of wet fly ash in ponds takes up muchvaluable agricultural land approximately (113 million m2), and mayresult in severe environmental degradation in the near future,which would be disastrous for India.

Fly ash particles are considered to be highly contaminating, dueto their enrichment in potentially toxic trace elements whichcondense from the flue gas. Research on the potential applicationsof these wastes has environmental relevance, in addition toindustrial interest. Most of the fly ash which is produced is disposedof as landfill, a practice which is under examination for environ-mental concerns. Disposal of fly ash will soon be too costly if notforbidden. Considerable research is being conducted worldwide onthe use of waste materials in order to avert an increasing toxicthreat to the environment, or to streamline present waste disposal

techniques by making them more affordable. It follows that aneconomically viable solution to this problem should include utili-zation of waste materials for new products rather than landdisposal.

Fly ash is generally grey in color, abrasive, mostly alkaline, andrefractory in nature. Pozzolans, which are siliceous or siliceous andaluminous materials that together with water and calciumhydroxide form cementitious products at ambient temperatures,are also admixtures. Fly ash from pulverized coal combustion iscategorized as such a pozzolan. Fly ash also contains differentessential elements, including both macronutrients P, K, Ca, Mg andmicronutrients Zn, Fe, Cu, Mn, B, and Mo for plant growth. The geo-technical properties of fly ash (e.g., specific gravity, permeability,internal angular friction, and consolidation characteristics) make itsuitable for use in construction of roads and embankments, struc-tural fill etc. The pozzolanic properties of the ash, including its limebinding capacity makes it useful for the manufacture of cement,building materials concrete and concrete-admixed products. Thechemical composition of fly ash like high percentage of silica (6065%), alumina (2530%), magnetite, Fe2O3 (615%) enables its usefor the synthesis of zeolite, alum, and precipitated silica. The otherimportant physicochemical characteristics of fly ash, such as bulkdensity, particle size, porosity, water holding capacity, and surfacearea makes it suitable for use as an adsorbent.

From the perspective of power generation, fly ash is a wastematerial, while from a coal utilization perspective, fly ash isa resource yet to be fully utilized; producers of thermal electricityare thus looking for ways to exploit fly ash. The cement industrymight use it as a raw material for the production of concrete. Coalfly ash discharged from power plants can also be utilized as a by-product, and its use in recycling materials for agriculture andengineering is also being studied [5,6]. The conversion of fly ashinto zeolite has also been widely examined [7].

Another interesting possibility might be use as a low-costadsorbent for gas and water treatment. Several investigations arereported in the literature on the utilization of fly ash for adsorptionof individual pollutants in an aqueous solution or flue gas. Theresults are encouraging for the removal of heavy metals andorganics from industrial wastewater. This paper will review thevarious applications of fly ash, including low-cost adsorbents forflue gas cleaning, wastewater treatment for removal of toxic ions

Nomenclature

A RedlichPeterson constantB RedlichPeterson constantCe equilibrium concentration of the solutionCs surface concentrationCt solution concentrationk1 pseudo-first order rate constantk2 pseudo-second order rate constantks mass transfer co-efficientki rate parameter of intraparticle diffusion control stageqe amount of adsorbate adsorbed at equilibrium (mg/g)qt amount of adsorbate adsorbed at any time t (mg/g)DG0 standard Gibbs free energy of adsorption (kJ/mol)DH0 standard enthalpy change of adsorption (kJ/mol)K Langmuir equilibrium constantKd distribution co-efficientKf Freundlich constantm mass of the adsorbent1/n adsorption intensityq heat of adsorptionR universal gas constantS specific surface areaDS0 standard entropy change of adsorption (JK1 mol1)t1/2 half-life periodV eluted volume (ml)Vb volume of effluent at break point (ml)Vm Langmuir monolayer adsorption capacityx/m amount adsorbed per unit of the adsorbentb heterogeneity factor

AbbreviationsAASHTO American association of state highway and transport

officialsABS acrylonitrile butadiene styreneACC autoclaved cellular concreteACCG activated carbon-commercial gradeACLG activated carbon laboratory gradeAcid Orange 7 p-(2-hydroxy-1 naphthylazo)benzene sulfonic

acidAEA air entraining admixtureAMD acid mine drainageASR alkalisilica reactionASTM American society for testing of materialsBDTDA benzyldimethyl tetradecylammoniumBFA bagasse fly ashBG Brilliant greenCANMET Canada centre for mineral and energy technologyCC char-carbonCCP coal combustion productsCCB coal combustion by-productsCEC cation exchange capacityCFA coal fly ashCFS chemical fixation and solidification

CPC cityl pyridinium chlorideCR Congo redDDD 2,2-bis (4chloro-phenyl)-1,1,-dichloro ethaneDDE 2,2-bis (4chloro-phenyl)-1,1,-dichloro ethaneDEF delayed ettringite formationDNP di-ntrophenolDTA differential thermal analysisEDTA ethylene diamine tetraacetic acidEPA Environmental protection agencyFA fly ashFAZ-Y fly ash based zeoliteFGD flue gas desulphurizationFTIR Fourier Transform infrared spectroscopyGGBFS ground granulated blast furnace slagHDTMA hexadecyl tetramethyl ammoniumHeCB 2,21,3,31,4,5,6-heptachlorobiphenylHVFA high-volume fly ashHSFA high-sulphate fly ashIFA impregnated fly ashLCA life cycle assessmentsLOI loss on ignitionMB methylene blueMSWI Municipal solid waste incinerator bottom ashMV methyl violetNMR nuclear magnetic resonanceNPC normal Portland cementOG Orange-GOPC ordinary Portland cementPPC Pozzolana Portland cementRB rhodamine BRBB Remazol brillant blueRCC reinforced concrete constructionRHFA rice husk fly ashRPC reactive powder concreteRY rifacion yellow HEDSDS sodium dodecyl sulphateSSA sewage sludge ashSEM scanning electron microscopeSMZ-Y surface modified fly ash based zeoliteTCB 2,3,4-trichloro biphenylTCLP Toxicity Characteristic Leaching ProcedureTEA tetramethyl ammoniumTEM transmission electron microscopeTOC total organic carbonTPABr tetraporpyl ammonium bromideUSEPA United States environmental protection agencyUHPC ultra high-performance concreteTNT tri-nitro tolueneUHPC ultra high-performance concreteWC wood charcoalXRD X-ray diffractionXRF X-ray fluorescenceZFA zeloite fly ash

M. Ahmaruzzaman / Progress in Energy and Combustion Science 36 (2010) 327363 329

and organic matters, synthesis of zeolite, mine backfill, light weightaggregate, road sub-base and construction/cement applications.

2. Properties of coal fly ash

Characterisation of fly ash in terms of composition, mineralogy,surface chemistry and reactivity is of fundamental importance inthe development of various applications of fly ash.

2.1. Physical properties

Fly ash consists of fine, powdery particles predominantlyspherical in shape, either solid or hollow, and mostly glassy(amorphous) in nature. The carbonaceous material in the fly ash iscomposed of angular particles. The particle size distribution of mostbituminous coal fly ash is generally similar to that of silt (less thana 0.075 mm or No. 200 sieve). Although sub-bituminous coal fly ash


is also silt-sized, it is generally slightly coarser than bituminous coalfly ash. The specific gravity of fly ash usually ranges from 2.1 to 3.0,while its specific surface area may vary from 170 to 1000 m2/kg [811]. The colour of fly ash can vary from tan to gray to black,depending on the amount of unburned carbon in the ash.

2.2. Chemical properties

The chemical properties of fly ash are influenced to a great extentby the properties of the coal being burned and the techniques used forhandling and storage. There are basically four types, or ranks, of coal,each vary in heating value, chemical composition, ash content, andgeological origin. The four types (ranks) of coal are anthracite, bitu-minous, sub-bituminous, and lignite. In addition to being handled ina dry, conditioned, or wet form, fly ash is also sometimes classifiedaccording to the type of coal from which the ash was derived.

The principal components of bituminous coal fly ash are silica,alumina, iron oxide, and calcium, with varying amounts of carbon,as measured by the loss on ignition (LOI). Lignite and sub-bitumi-nous coal fly ash is characterized by higher concentrations ofcalcium and magnesium oxide and reduced percentages of silicaand iron oxide, as well as lower carbon content, compared withbituminous coal fly ash. Very little anthracite coal is burned inutility boilers, so there are only small amounts of anthracite coal flyash. Table 1 compares the normal range of the chemical constitu-ents of bituminous coal fly ash with those of lignite coal fly ash andsub-bituminous coal fly ash. From the table, it is evident that ligniteand sub-bituminous coal fly ash has a higher calcium oxide contentand lower loss of ignition than fly ash from bituminous coals.Lignite and sub-bituminous coal fly ash may have a higherconcentration of sulphate compounds than bituminous coal fly ash.According to the American Society for Testing Materials (ASTMC618) [12], the ash containing more than 70 wt% SiO2 -Al2O3 Fe2O3 and being low in lime are defined as class F, whilethose with a SiO2Al2O3 Fe2O3 content between 50 and 70 wt%and high in lime are defined as class C. Briefly, the high-calciumClass C fly ash is normally produced from the burning of low-rankcoals (lignites or sub-bituminous coals) and have cementitiousproperties (self-hardening when reacted with water). On the otherhand, the low-calcium Class F fly ash is commonly produced fromthe burning of higher-rank coals (bituminous coals or anthracites)that are pozzolanic in nature (hardening when reacted withCa(OH)2 and water). The chief difference between Class F and ClassC fly ash is in the amount of calcium and the silica, alumina, andiron content in the ash. In Class F fly ash, total calcium typicallyranges from 1 to 12%, mostly in the form of calcium hydroxide,calcium sulphate, and glassy components, in combination withsilica and alumina. In contrast, Class C fly ash may have reportedcalcium oxide contents as high as 3040%. Another differencebetween Class F and Class C is that the amount of alkalis (combinedsodium and potassium), and sulphates (SO4), are generally higherin the Class C fly ash than in the Class F fly ash.

Table 1Normal range of chemical composition for fly ash produced from different coaltypes.

Component (wt.%) Bituminous Sub-bituminous Lignite

SiO2 2060 4060 1545Al2O3 535 2030 1025Fe2O3 1040 410 415CaO 112 530 1540MgO 05 16 310SO3 04 02 010Na2O 04 02 06K2O 03 04 04LOI 015 03 05

The mineralogical composition of fly ash, which depends on thegeological factors related to the formation and deposition of coal,its combustion conditions, can be established by X-ray diffraction(XRD) analysis. The dominant mineral phases are quartz, kaolinite,ilite, and sideraete. The less predominant minerals in the unreactedcoals include calcite, pyrite and hematite. Quartz and mullite arethe major crystalline constituents of low-calcium ash, whereashigh-calcium fly ash consists of quartz, C3A, CS and C4AS.

The several distinct end uses of fly ash differ considerablyamong themselves in the stringency of the properties required inthe fly ash for its successful utilization. The success of fly ash instructural fill applications rests primarily on the ability of thematerial to be compacted to a reasonably strong layer of low unitweight. This is primarily a function of particle size distribution, andto some extent of the content of spherical particles. The chemicalcharacteristics of fly ash are secondary, although the postcompaction cementation provided by some high-calcium fly ash islikely to prove beneficial.

With highway bases chemical considerations come into play,although not in an important way. Stabilization of some basecourses (and stabilized sub grades) may rest on lime fly ashchemical reactions, i.e. the classical pozzolanic reaction, withlime. Low-calcium fly ash may be entirely satisfactory or evenpreferred, especially where sufficient time is available for theseslow reactions to take place. The only real chemical requirement isthat fly ash has a sufficient content of glass that eventually willreact with added lime. Some road base applications of fly ashdepend on the physical effects of fly ash incorporation rather thanits reaction with lime.

The cement and concrete end-use areas are by far the mostdemanding of the fly ash in terms of adherence to strict criteria andrequirements. However, the requirements differ considerablydepending on the specific end use involved.

Fly ash for use as a raw material in cement manufacture is soldand used primarily on the basis of its chemical composition, asexpressed in the usual oxide convention. Such factors as glasscontent, the type of crystalline matter present, size distribution, etc.,are relatively immaterial. Even high carbon content, which may belimiting in most other end uses, may actually be beneficial in cementraw material use, since it provides a definite (although modest)proportion of the fuel needed. Uniformity and chemical consistencyfrom day to day and week to week is the prime necessity.

Fly ash, as a blended cement component shares some of therequirements for both raw material and direct concrete admixtureuse. Since such fly ash eventually is incorporated in concrete, itschemical and physical characteristics must be suitable for thatpurpose. However, since little or no adjustment can be provided atthe concrete mixing stage, fly ash for use in blended cements mustbe of consistent and uniform chemical and physical characteristics,the consistency and predictability being as important as thenumerical values of the various parameters involved. Since theblended cement manufacturer has little control over the concurrentuse of chemical admixtures or of mixing and curing conditions, thefly ash used should be relatively insensitive to such variations.Especially to be considered here are rheological effects, strengthdevelopment characteristics, and possibilities for developingefflorescence. The color of the ash and its effect on the color of thefinal concrete to be produced by the blended cement may also be ofimportance.

3. Properties of biomass ash

The use of biomass as fuel generates large amount of residualash which causes serious environmental problems. Biomass ashdoes not contain toxic metals like in the case of coal ash. The ash


forming constituents in biomass fuels are quite diverse dependingon the type of biomass, type of soil and harvesting [13]. In general,the major ash forming inorganic elements in biomass fuels are Ca,K, Na, Si and P and some of these act as important nutrients for thebiomass [14]. However, some biomass fuels have high siliconcontent (e.g. rice husk) while some have high alkali metal content(wood). While the elemental composition of the ash is deter-mined by the inorganic constituents in the parent biomass, thecrystallinity and mineralogy depends on the combustion tech-nique used.

Typically, fly ash from neat biomass combustion has more alkali(Na and K) and less alumina (Al2O3) than coal fly ash [13,15]. Asa class, biomass fuels exhibit more variation in both compositionand amount of inorganic material than is typical of coal. Therefore,biomass fly ash varies more than coal fly ash, which depends on thevarieties of origin from woody to herbaceous and other resources[14,16]; furthermore, even for the same type of biomass, theproperties of its fly ash depends also on some growth andproduction factors including weather, season, storage andgeographic origins [16,17].

Compared to coal fly ash where significant research has alreadytaken place and high utilisation figures are already reported inseveral countries [16,18], commercial utilisation of biomass ash isnot widely reported. However, several research efforts areunderway for applications such as adsorbent, raw material forceramics, cement and concrete additive, material recovery, etc.based on its characteristics. The composition, surface area, andpresence of unburnt material play an important role in determiningthe application.

Many kinds of biomass fly ash have similar pozzolanic proper-ties as coal fly ash, such as those from rice husk, wood, wheat strawand sugar cane straw [1921] among which have been added inconcrete as mineral admixtures, improving the performance ofconcrete.

Bagasse fly ash has been examined as an adsorbent as well as anadditive in cement and concrete [1921]. However, its high carboncontent can cause a hindrance in its application for concrete. Ricehusk with its high silica content has been used as an insulator,adsorbent, cement and concrete additive and as a substitute forsilica [22]. Studies on ash from arecanut shell, cashewnut shell andgroundnut shell ash are limited [21].

Fig. 1. Schematic plant view of flue gas desulfurization using coal ash [23].

4. Fly ash utilization

There are many reasons to increase the amount of fly ash beingre-utilized. A few of these reasons are given below.

Firstly, disposal costs are minimized; secondly, less area isreserved for disposal, thus enabling other uses of the land anddecreasing disposal permitting requirements; thirdly, there may befinancial returns from the sale of the by-product or at least an offsetof the processing and disposal costs; and fourthly, the by-productscan replace some scarce or expensive natural resources.

Utilization of coal combustion by-products, namely fly ash, canbe in the form of an alternative to another industrial resource,process, or application. These processes and applications include,but are not limited to, addition to cement and concrete products,structural fill and cover material, roadway and pavement utiliza-tion, addition to construction materials as a light weight aggregate,infiltration barrier and underground void filling, and soil, water andenvironmental improvement. The following is a brief description ofeach of the previously mentioned alternative uses of fly ash andassociated research that has been conducted and how it relates toeach alternative use. In this section, the application of fly ash hasbeen discussed.

5. Adsorbents for cleaning of flue gas

5.1. Sulphur compounds

Effort has been made to reduce SOx emissions by installingequipment for flue gas desulphurization (FGD). The wet-typelimestone scrubbing processes is widely used because of its highDeSOx efficiency and easy operation. However, these processeshave drawbacks, such as high water consumption and the need forwastewater treatment [6]. Dry-type FGD does not require waste-water treatment; however, it requires a large amount of absorbentcompared to wet-type FGD. This may be due to the fact thata higher molar ratio of calcium to-sulphur is required to obtaina high DeSOx efficiency. The reactions are represented below.

NO D 1=2 O2 / NO2 (1)

SO2 D NO2 / SO3 D NO (2)

CaOH2 D SO3 / CaSO4 D H2O (3)

As shown in the above chemical formulas, the sulfur dioxides inthe flue gas are fixed as gypsum. On the other hand, they are fixedas sulfite in other conventional dry processes such as limestoneinjection and active manganese. Some of the spent absorbent dis-charged from the desulfurization process can be used as the rawmaterial for the absorbent pellets. In addition, this spent absorbentis reused as a solidification agent for sludge and as a deodorant forrefrigerators, pet litter and so on [23]. The process flow is explainedas follows: the system is composed of an absorber body, anabsorbent feeder and a drawout facility, and an absorbentmanufacturing facility. The absorbents in a fixed process are fedinto an absorber and drawn out of its lower part. Both absorptionand removal in sulfur dioxide are conducted during the time whenthe absorbents move down from the upper part to the lower part ofthe absorber. Flue gas containing sulfur dioxide is introduced to theabsorber to make contact with the absorbents, and then the treatedgas is discharged from a stack to the atmosphere [23]. A simplifiedplan view is shown in Fig. 1.

Activated carbon was used to oxidize reduced sulphurcompounds; however, it is too costly for large-scale environmentalremediation applications. Coal fly ash is a cheap absorbent fordry-type FGD. Fly ash recycling in the flue gas desulphurizationprocess has shown promising results. Fly ash treated with calciumhydroxide has been tested as a reactive adsorbent for SO2 removal[24]. A mixture of fly ash and calcium hydroxide for


desulphurization was also studied by Davini [25,26]. It was foundthat Ca(OH)2-fly ash mixtures were a low-cost SO2 control option.Davini [27] also tested a process using activated carbon derivedfrom fly ash for SO2 and NOx adsorption from industrial flue gas;this mixture exhibited similar characteristics to typical activatedcarbon for flue gases.

The FGD process using coal ash has been commercialized, andsome industrial plants have achieved DeSOx efficiencies of over90%, such as the Ebetsu power station (50,000 Nm3/h) and theTomtoh Atsuma power station (644,000 Nm3/h) under a high molarratio of calcium to sulfur (1.01.2) [23]. During operation, there isno need for wastewater treatment or gas reheating, and so thisprocess is considered to be an ideal choice for controlling theemission of sulfur dioxide and an environmentally friendly methodfor reuse of coal ash. Since the introduction of FGD in the late 1960s,global market demand for FGD has been steady at between 5000and 10,000 MW per year, and mainly wet-type limestone FGD unitshave been installed [28]. As described in this part, wet limestoneFGD requires a wastewater treatment facility. Furthermore, it emitscarbon dioxide (greenhouse gas) into the atmosphere as follows.

CaCO3slurryDSO2 D 1=2 O2 / CaSO4slurryD CO2 (4)

Dry-type FGD using fly ash is one of the processes that providea solution to the above-mentioned problems, but this FGD has notyet spread worldwide.

5.2. Adsorption of NOx

Fly ash has also been proposed as adsorbents for NOx removalfrom flue gases [29]. The properties of fly ash particularly withrespect to NOx adsorption were closely examined for carboncontent and specific surface area. It was found that unburnedcarbon remaining in the fly ash particles contribute the mainsurface area to fly ash, and the carbon can be activated to furtherimprove the adsorption performance of the fly ash. Experimentalresults on activating coarser fly ash particles showed that adsorp-tion capacity can be increased through controlled gasification of theunburned carbon. So, this carbon present in fly ash can bea precursor of activated carbons since it has gone through devola-tilization during combustion in the power station furnace, and it,requires only a process of activation [30]. The adsorption of NOxusing activated chars recovered from fly ash was reported [31].Carbon-rich fractions from a gasifier were adsorbed one-third ofthe NOx compared with a commercial carbon. Recently, activatedcarbon from unburned carbon in coal fly ash has also been used forremoval of NO [32]. It was found that mineral matter must beremoved efficiently from unburnt carbon of fly ash before activa-tion, to obtain a more suitable activated carbon for environmentalapplications in the gas phase.

5.3. Removal of mercury

Mercury has long been known as a potential hazard to-healthand environmental hazard; it is identified as one of the 189 toxic airpollutants by the Clean Air Act Amendments of 1990. Becausemercury accumulates in the biosystem it is of particular concern; itis very difficult to monitor and capture, and is high in the publicawareness.

To cope with the mercury emission problem, efforts have beenmade to remove various types of mercury from the flue gas ofutility boilers. However, due to technical and economic limitations,no process has been commercially utilized beyond pilot scale tests.Among the current technologies being evaluated, activated carboninjection is the process most promising for removing mercury from

flue gas, due to its high removal efficiency. In this process, activatedcarbon powder is injected into the flue gas stream and collected,after adsorption, with a particulate matter control device. However,the high cost of activated carbon hinders large-scale applications inutility boilers [33]; therefore, it is desirable to find an alternativecarbon.

Usually the unburned carbon content in fly ash is in the range of212%. However, with the introduction of the 1990 Clean Air ActAmendments, caps have been established on the emission ofnitrogen oxides (NOx). Many coal-fired utilities have begun toretrofit with low NOx burners to meet the emission requirements.As a result of such transition, the carbon content of fly ash increasessignificantly, up to 20% in some cases, due to the low oxygen and/orlow temperature combustion conditions required by those low NOxcombustion units. Since the unburned carbon separated from flyash is a by-product, any practical application of such materialwould be economically and environmentally advantageous to theoverall fly ash beneficiation process. Researchers at The Pennsyl-vania State University have developed a method to economicallyseparate unburned coal from fly ash [34]. Preliminary study showsthat some unburned carbon from fly ash has certain capabilities foradsorbing elemental mercury. Such findings triggered the idea ofusing fly ash carbon as a low-cost adsorbent in removing elementalmercury from gas phases, such as utility flue gas, to replace costlyactivated carbons.

The retention of hazardous elements by fly ash produced incombustion plants has been extensively studied in recent years. Inthe case of mercury it has been observed that some fly ash maycapture this element which would otherwise be emitted into theatmosphere. Although the role of inorganic components of fly ashin this capture is still unclear, considerable attention has been paidto the capture of mercury by unburned fly ash carbons [3542].A relationship has been reported between Hg content and thepercentage of carbon in fly ash derived from the combustion ofbituminous coals [37] and coal blends containing anthracites[42,43]. The role that the different types of unburned carbons playin mercury capture in fly ash has also been a matter of interest forsome studies associating types of particles with the amount of Hg,captured [35,42,43]. The concentration of unburned carbons andtheir respective ability to capture Hg have also been related to theirtextural properties [37,4345], given that the BET surface areasuccessively increased from inertinite, isotropic coke (isotropic flyash carbons) to anisotropic coke (anisotropic fly ash carbons) [37].

The exact nature of Hgfly ash interactions is still unknown andthe variables affecting the mercury adsorption need to be identi-fied. In view of the significant variations in the properties of fly ashobtained from different coals [43,4648], and to better understandthe properties of the materials influencing the capture of Hg, Lopez-Anton et al. [49] have tried to establish a relationship between Hg0

and HgCl2 retention and the characteristics of fly ash samples takenfrom the combustion of feed coal blends of different characteristics.The relationship between the types of particles, the BET (BrunerEmmett Taylor) surface area and the quantities of mercury retainedwas studied. It can be seen that the fly ash exhibit differentretention capacities depending on the species in gas phase (Hg0 orHgCl2). A comparison of the results obtained demonstrates that Hg

0

is retained in fly ash in a greater proportion than HgCl2. When theraw fly ash samples are compared with the fractions enriched inunburned carbons it can be observed that retention capacityincreases slightly as the unburned carbon content (LOI) increased.The mercury values recorded were compared to the content of eachtype of organic component and total inorganic matter present inthe fly ash. Because mercury retention depends on the mode ofoccurrence of this element in gas phase the evaluation was basedon each individual mercury species. When the retention of Hg0 was

Table 2Summary of adsorption of metals on fly ash.

Metals Adsorbent Adsorptioncapacity (mg/g)

Temperature (C) References

Zn2 Coal fly ash 6.513.3 3060 [81]Fe impregnated fly ash 7.515.5 3060Al impregnated fly ash 7.015.4 3060Coal fly ash 0.252.8 20 [83]Coal fly ash(I) 0.251.19 20 [84]Coal fly ash(II) 0.071.30 20Bagasse fly ash 2.342.54 3050 [93]Bagasse fly ash 13.21 30 [94]Fly ash 4.64 23 [104]Fly ash 0.27 25 [105]Fly ash 0.0680.75 055 [106]Fly ash 3.4 [87]Rice husk ash 5.88 [86]Bagasse fly ash 7.03 [85]Fly ash 11.11 [71]Rice husk ash 14.30 [63]Fly ash 7.84 [71]

Cd2 Fly ash 198.2 25 [79]Fly ash-washed 195.2 25Fly ash-acid 180.4 25Fly ash 1.68.0 [80]Fly ash zeolite 95.6 20Fly ash 0.670.83 20 [83]Fly ash (I) 0.080.29 20 [85]Fly ash (II) 0.00770.22 20Bagasse fly ash 1.242.0 3050 [95]Fly ash 0.05 25 [105]Coal fly ash 18.98 25 [70]Rice husk ash 3.04 [86]


compared to the amount of each type of unburned carbons in thefly ash, no correlations were found. However, a general tendencycould be observed with the anisotropic, fused and porous struc-tures (which are mainly network structures in all cases). The fly ashsamples that have a greater surface area retain a higher quantity ofHgCl2, but this tendency shows several exceptions in the case of Hg.

The adsorption of mercury on carbon can be explained by thephysical and chemical interactions which occur between thecarbon surface and mercury. According to the theory proposed byDubini [50] the carbon surface contains some adsorption centers,called primary sites. When a molecule of the adsorbate adsorbs ona primary site, the adsorbed molecule can then act as a secondarycenter for the adsorption of more molecules. The enhancement ofmercury adsorption after oxidizing unburned carbon at 400 C inair shows that oxygen-containing functional groups may have animportant role, which is also suggested by Hall et al. [51]. Masakiet al. [52] utilized synthetic fly ash, consisting of calcium chloridewith 5% activated carbon, which showed very high efficiency ofover 99% for mercury removal at 120 C. When the calcium chloridecontent was more than 0.5% in the synthetic fly ash with 5% acti-vated carbon, mercury vapor was completely removed. However,the most efficient removal was obtained when the activated carboncontent ranged from 5 to 7% in synthetic fly ash with 1% calciumchloride. The removal of mercury was affected by temperature, ifthe activated carbon content was very small. It was assumed thatthe complex chemical action with activated carbon and calciumchloride was most significant for metallic mercury removal byactual fly ash.

Afsin-Elbistan fly ash 0.29 [64]Seyitomer fly ash 0.21Bagasse fly ash 6.19 [85]Fly ash 207.3 [79]Fly ash 1.38 [68]

Pb2 Fly ash 444.7 25 [79]Fly ash-washed 483.4 25Fly ash-acid 437.0 25Fly ash 753 32Bagasse fly ash 285566 3050 [92]Fly ash 18.8 [87]Fly ash 18.0 [75]Treated rice husk ash 12.61 30 [73]

Cu2 Fly ash 1.39 30 [68]Fly ashwollastonite 1.18 30Fly ash 1.78.1 [80]Fly ash (I) 0.341.35 20 [84]Fly ash (II) 0.091.25 20Fly ash 207.3 25 [79]Fly ash-washed 205.8 25

5.4. Adsorption of gaseous organics

Apart from the adsorption of NOx, SOx and mercury in flue gas,fly ash has also been used for adsorption of organic gas. Theadsorption of toluene vapours on fly ash was investigated by Pelosoet al. [53]. It was found that fly ash product obtained after particleaggregation and thermal activation showed satisfactory adsorptionperformance for toluene vapours [54]. The adsorption kinetics ofrepresentative aromatic hydrocarbon and m-xylene, on fly ash hasalso been studied [55]. The results indicated that the kinetics of m-xylene adsorption by fly ash resembled kinetics reported forpenetration of absorbates into porous adsorbents. No increase inadsorption rates was observed with increased temperature, andrate constants decreased with increased vapour pressure. Thissuggested that adsorption was diffusion-controlled.

Fly ash-acid 198.5 25Fly ash 0.630.81 25 [69]Bagasse fly ash 2.262.36 3050 [93]Fly ash 0.76 32 [100]Fly ash 7.5 [87]Coal fly ash 20.92 25 [70]Fly ash 7.0 [75]CFA 178.5249.1 3060 [74]CFA-600 126.4214.1 3060CFANAOH 76.7137.1 3060

Ni2 Fly ash 9.014.0 3060 [81]Fe impregnated fly ash 9.814.93 3060Al impregnated fly ash 1015.75 3060Fly ash(I) 0.400.98 20 [84]Fly ash(II) 0.061.16 20Bagasse fly ash 1.121.70 3050 [95]Fly ash 3.9 [87]Bagasse fly ash 6.48 [85]Fly ash 0.03 [67]

Cr3 Fly ash 52.6106.4 2040 [65]

(continued on next page)

6. Removal of toxic metals from wastewater

Fly ash has potential application in wastewater treatmentbecause of its major chemical components, which are alumina,silica, ferric oxide, calcium oxide, magnesium oxide and carbon,and its physical properties such as porosity, particle size distribu-tion and surface area. Moreover, the alkaline nature of fly ash makesit a good neutralising agent. Generally, in order to maximise metaladsorption by hydrous oxides, it is necessary to adjust the pH ofwastewater using lime and sodium hydroxide [56,57].

Today, heavy metals are most serious pollutants, becominga severe public health problem. Heavy metal and metalloid removalfrom aqueous solutions is commonly carried out by severalprocesses such as, chemical precipitation, solvent extraction, ionexchange, reverse osmosis or adsorption etc. Among theseprocesses, the adsorption process may be a simple and effectivetechnique for the removal of heavy metals from wastewater.

Table 2 (continued )

Metals Adsorbent Adsorptioncapacity (mg/g)

Temperature (C) References

Cr6 Fly ash wollastonite 2.92 [61]Fly ash China clay 0.31 Fly ash 23.86 [62]Rice husk Ash 25.64 Fly ash 1.38 3060 [82]Fe impregnated fly ash 1.82 3060Al impregnated fly ash 1.67 3060Fly ash(I) 0.55 20 [85]Fly ash(II) 0.82 20 [85]Bagasse fly ash 4.254.35 3050 [97]

Hg2 Fly ash 2.82 30 [76]Fly ash 11.0 3060 [82]Fe impregnated fly ash 12.5 3060Al impregnated fly ash 13.4 3060Sulfo-calcic 5.0 30 [87]Silico-aluminous ashes 3.2 30 [87]Fly ash-C 0.630.73 521 [77]Treated rice husk ash 6.72 30 [73]

As3 Fly ash coal-char 3.789.2 25 [109]As5 Fly ash 7.727.8 20 [107]

Fly ash coal-char 0.0234.5 25 [109]


6.1. Adsorption of various types of heavy metals on fly ash

Fly ash has been widely used as a low-cost adsorbent for theremoval of heavy metal. Table 2 summarizes the results of theimportant metals investigated using fly ash. Among these metalions, Ni, Cr, Pb, As, Cu, Cd and Hg are the most often investigated.The use of fly ash for removal of heavy metals was reported as earlyas 1975. Gangoli et al. [58] reported the utilization of fly ash for theremoval heavy metals from industrial wastewaters.

Removal of chromium ions, including Cr(VI) and Cr(III) using flyash has been investigated by several researchers [59,60]. The effectsof chromium concentrations, fly ash dosage, contact time, and pHon the removal of chromium were reported. A homogeneousmixture of fly ash and wollastonite (1:1) was also reported toremove Cr(VI) from aqueous solutions by adsorption [61]. Bhatta-charya et al. [62,63] studied the removal of Cr(VI) and Zn (II) fromaqueous solution using fly ash. Turkish fly ash was also used for theremoval of Cr(VI) and Cd(II) from an aqueous solution on [64]. Flyash was found to have a higher adsorption capacity for Cd(II), ascompared to Cr(VI). The lime (crystalline CaO) content in the fly ashseemed to be a significant factor influencing the adsorption ofCr(VI) and Cd(II). Fly ash obtained from the combustion of poultrylitter was also utilized as an adsorbent for the removal of Cr(III)from aqueous solution [65]. Yadava et al. [66] investigated theremoval of cadmium by fly ash by varying contact time, tempera-ture and pH. The removal of Cd(II) has been found to be contacttime, concentration, temperature and pH dependent. The processof removal follows first order adsorption kinetics and the ratecontrolling step is intraparticle transport into the pores of fly ashparticles. The temperature dependence of Cd(II) adsorption on flyash indicates the exothermic nature of adsorption. Alkalineaqueous medium favors the removal of Cd(II) by fly ash. Theincrease in adsorption of Cd(II) with pH has been explained on thebasis of surface complex formation approach. Raw bagasse and coalfly ash have also been used as low-cost adsorbents for the removalof chromium and nickel from aqueous solutions [67]. The extent ofadsorption at equilibrium was found to be dependent on thephysical and chemical characteristics of the adsorbent, adsorbateand experimental system.

Fly ash was also utilized for the removal of copper from aqueoussolution. Removal efficiency was found to be dependent onconcentration, pH and temperature [68]. The kinetics of adsorption

indicated the process to be diffusion controlled. Fly ash withdifferent quantities of carbon and minerals was also used forremoval of Cu(II) from an aqueous solution [69]. The carbon fractionin fly ash was important in the removal of Cu(II). The specificadsorption capacities of carbon ranged from 2.2 to 2.8 mg Cu/gcarbon, while the capacities for mineral were only about 0.630.81 mg Cu/g mineral. Fly ash can also be shaped into pellets andused for the removal of copper and cadmium ions from aqueoussolutions [70]. The calculated adsorption capacities for copper andcadmiumwere found to be 20.92 and 18.98 mg/g, respectively. It wasfound that fly ash shaped into pellets could be considered asa potential adsorbent for the removal of copper and cadmium fromwastewaters. Equilibrium studies for the adsorption of zinc andcopper from aqueous solutions were carried out using sugar beetpulp and fly ash [71]. The removal characteristics of Pb(II) and Cu(II)from aqueous solution by fly ash were investigated by Alinnor [72].The utilization of rice husk ash was investigated for the adsorption ofPb(II) and Hg(II) from aqueous water [73]. The Bangham equationcan be used to express the mechanism for adsorption of Pb(II) andHg(II), by rice husk ash. Its adsorption capability and adsorption rateare considerably higher and faster for Pb(II) than for Hg(II). The finerthe rice husk ash particles used, the higher the pH of the solution andthe lower the concentration of the supporting electrolyte, potassiumnitrate solution, the more Pb(II) and Hg(II) absorbed on rice husk ash.

Raw and modified coal fly ash effectively adsorbs Cu(II) fromwastewater [74]. These adsorptions were endothermic in nature;the values of activation energy (between 1.3 and 9.6 kJ mol1) wereconsistent with an ion exchange adsorption mechanism. Theadsorptions of Cu(II) onto coal fly ash (CFA), CFA-600, and CFANaOH followed pseudo-second order kinetics. Changing the natureof CFA did not improve its ability to adsorb Cu(II).

The presence of organic pollutants significantly affected theremoval of heavy metals from wastewater. Wang et al. [75] inves-tigated the competitive adsorption of heavy metals and humic acidusing fly ash as adsorbent. It is found that, for a single pollutantsystem, fly ash can achieve adsorption of lead ion at 18 mg/g,copper ion at 7 mg/g and humic acid at 36 mg/g, respectively. Forco-adsorption, complexation of heavy metals and humic acid playsan important role. The presence of humic acid in water will provideadditional binding sites for heavy metals, thus promoting metaladsorption on fly ash. For PbHA and CuHA systems, Pb(II) andCu(II) adsorption can increase to 37 and 28 mg/g, respectively. Theheavy metal ions present in the system will compete with theadsorption of humic acid on fly ash, thus resulting in a decrease inhumic acid adsorption.

Fly ash was also found to be effective for the removal of mercury.The adsorption capacity of coal fly ash for mercury was comparableto that of activated powdered charcoal [76]. The effectiveness of flyash in adsorbing mercury from wastewater has been studied [77].Selective adsorption of various metal ions (Na, K, Mg, Ca, Cu, Cd,Mn, Hg, Cr, Pb, and Fe) by fly ash was also reported [78]. Lead ionswere found to be selectively adsorbed at a mean value of 19 meq ofPb(II) per 100 g of fly ash. This selective adsorption could be due tothe formation of crystalline ettringite mineral after the hydration ofthe fly ash. Coal fly ash has also been used for the removal of toxicheavy metals, i.e. Cu(II), Pb(II) and Cd(II) from water [79]. Thebreakthrough volumes of the heavy metal solutions have beenmeasured by dynamic column experiments in order to determinethe saturation capacities of the adsorbents. The adsorptionsequence is Cu> Pb> Cd in accordance with the order of insolu-bility of the corresponding metal hydroxides. Similar results on theadsorption of Cd and Cu by fly ash were also reported [80]. Thepresence of high ionic strength or appreciable quantities of calciumand chloride ions does not have a significant effect on theadsorption of these metals by fly ash.


Banerjee et al. [60,61], studied the adsorption of various toxicmetal ions, Ni(II) and Zn(II), Cr(II) and Hg(II)], on fly ash and Al and Feimpregnated fly ash. The impregnated fly ash showed much higheradsorption capacity for all the ions, as compared to that of untreatedfly ash. The adsorption capacity of FA, AlFA, and FeFA for Cr(VI)was found to be 1.379, 1.820, and 1.667 mg/g and that of Hg(II) was11.00,12.50, and 13.40 mg/g. Bayat investigated the removal of Zn(II)and Cd(II) [83], Ni(II) and Cu(II) [84], using lignite-based fly ash andactivated carbon, and found that fly ash was effective as activatedcarbon. The percent adsorption of Zn(II) and Cd(II) increased with anincrease in concentration of Zn(II) and Cd(II), dosage of fly ash andtemperature; maximum adsorption occurred in the pH range of 7.07.5. The effectiveness of fly ash as an adsorbent improved withincreased calcium content (CaO). Fly ash was found to have a higheradsorption capacity for Cd(II), compared to Cr(VI). Bagasse fly ashand rice husk ash were also utilized for the removal of Ni(II),Cd(II)and Zn(II) from an aqueous solution [85,86].

Fly ash and fly ash/lime mixture were investigated for theremoval of Cu, Ni, Zn, Cd and Pb [87,88]. The extent of removal wasachieved in the order of Pb(II)> Cu(II)>Ni(II)> Zn(II)> Cd(II).Formation of calcium silicate hydrates (CSH) was assumed to beresponsible for increasing removal, and for decreasing desorption.Two fluidized-bed-sourced fly ashes with different chemicalcompositions, silico-aluminous fly ash and sulfo-calcic fly ash, weretested to remove Pb(II), Cu(II), Cr(III), Ni(II), Zn(II), Cr(VI) [89] andHg(II) [90] from aqueous solutions. The percentage of adsorbed ionswas greater when they were in contact with silico-aluminous flyash than sulfo-calcic fly ash, except in the case of the ion Ni(II).Mercury is bound to the ash surface due to several chemical reac-tions between mercury and various oxides (silicon, aluminium andcalcium silicate), on the surface of the ash.

Gupta and Terres [91] measured the changes in toxicity andheavy metals in a municipal wastewater treatment plant effluentby treatment with fly ash. After the treatment with fly ash, theeffluent showed a significant reduction in toxicity, Cu, Pb andPO43 and NO3

contents. Fly ash removed Cu and Pb from theeffluent; the removal of these toxic heavy metals resulted ina reduction of toxicity. The Gupta research group conducted a seriesof investigations on the adsorption of heavy metals, using bagassefly ash as adsorbents. They used bagasse fly ash from sugarindustries for the removal of lead [92], copper and zinc [93,94],cadmium, nickel [95] and chromium [96,97] from aqueous solu-tions. Copper and zinc are adsorbed by the developed adsorbent upto 9095% in batch and column experiments. The batch test showed90% removal for Cd and Ni, in about 60 and 80 min, respectively.The removal of Zn is 100% at low concentrations, whereas removalis 6065% at higher concentrations. The uptake decreases withincreased temperature, indicating that the process is exothermic innature. Lead and chromium are also adsorbed by the developedadsorbent up to 9698%. The removal of these two metal ions (up to9596%), was achieved by column experiments at a flow rate of0.5 mL/min. The adsorption capacities of sewage sludge ash (SSA),with fly ash for copper ions were compared [98]. The estimatedmaximum capacity of copper adsorbed by SSA was 3.24.1 mg/gclose to that of fly ash. The adsorption isotherm of SSA for copperions generally followed the Langmuir model and depends onparticle size, loading, pH etc. The primary mechanisms of copperremoval by SSA included electrostatic attraction, surface complexformation, and cation exchange. The precipitation of copperhydroxide occurred only when the dosage of SSA and the equilib-rium pH of wastewater were at a high level (30/40 g/l and greaterthan 6.2, respectively).The feasibility of using fly ash for theremoval of Cu(II) and Pb(II) from wastewater was investigated[99,100]. The cation exchange capacity and specific surface area offly ash increased with increased carbon content. The adsorption of

metal ions onto the surface of fly ash was found to be proportionalto the carbon contents. This is because the amounts of adsorptionor ion exchange sites on carbon soot are higher than on mineralsurface. This is consistent with cation exchange capacity andspecific surface area. Consequently, carbon residual in the fly ashplay a more important role than mineral matter in the removal ofmetals by the fly ash.

Fly ash was found to be good adsorbent for removal of zinc fromaqueous solutions [101]. Gashi et al. [102] reported that fly ashshowed good adsorptive properties for removal of lead, zinc,cadmium and copper from effluents in the battery and fertilizerindustries. Removal efficiencies were greater than 70%. Adsorptionstudies carried out to estimate heavy metal removal, using fly ashon wastewater at Varnasi, India, showed that removal was in thefollowing order: Pb> Zn> Cu> Cr> Cd> Co>Ni>Mn [103].Adsorption of Cd (I), Ni(II), Cd(II), Pb(II), Zn(II) and Ag(I) on fly ashwas investigated and found that the process was spontaneous andendothermic [104]. A process for the treatment of industrialwastewater containing heavy metals, using fly ash adsorption andcement fixation of the metal-laden adsorbent, was investigated byHuang research group [104106]. Results showed that fly ash couldbe an effective metal adsorbent, at least for Zn(II) and Cd(II) indilute industrial wastewaters. A 10% metal-laden fly ash was testedfor leaching and it exhibited metal concentrations lower than thedrinking water standards.

Fly ash was also effective for the removal of arsenic fromaqueous solution. Fly ash, obtained from coal power stations, wasexamined for removal of As (V) from water [107]. Kinetic andequilibrium experiments were performed to evaluate As(V)removal efficiency by lignite-based fly ash. Maple wood ashwithout any chemical treatment was utilized to remediate As(III)and As(V) from contaminated aqueous streams in low concentra-tions [108]. Static tests removed 80% arsenic, while the arsenicconcentration was reduced from 500 to


be controlled by the slowest step that would be either film diffusionor pore diffusion controlled.

Various kinetic models have been suggested for adsorption,including the Lagergren pseudo-first order kinetics, the pseudo-second order kinetics, external diffusion model, and intraparticlediffusion model, which are expressed in Eqs. (5)(8) as listedbelow:

logqe qt logqe k1

2:303t (5)

tqt 1

k2q2e 1

qet (6)

dCtdt ksSCt Cs (7)

qt ki

t1=2

(8)

where k1, k2, ks and ki are the pseudo-first order, pseudo-secondorder rate constant, mass transfer coefficient, and rate parameter ofthe intraparticle diffusion control stage, respectively, qe the amountof solute adsorbed (mg/g) at equilibrium and qt the amount ofsolute on the surface of the adsorbent (mg/g) at any time t, Cs and Ctare surface and solution concentration, and S is the specific surfacearea. Adsorption kinetics of heavy metals on fly ash was investi-gated by several researchers. Most investigations reported thatadsorption of metal usually follows the first order kinetics[68,81,82], and that adsorption is pore diffusion controlled[68,81,82,95,97]. Kelleher et al. [65] investigated adsorption ofCr(III) on fly ash, and kinetic studies suggested that overall rate ofadsorption was pseudo-second order.

6.3. Adsorption isotherms

The Langmuir, Freundlich, RedlichPeterson, DubininKaganerRadushkevich (DKR), Tempkin, and Sips isotherms were generallyused to describe observed adsorption phenomena of various metalions on fly ash. The Langmuir isotherm applies to adsorption oncompletely homogenous surfaces with negligible interactionbetween adsorbed molecules. For a single solute, it is given by

xm VmKCe

1 KCe(9)

However, the linear form of the equation can be written as

Cex=m

1KVm

CeVm

(10)

Where Ce is the equilibrium concentration of the solution, x/m isthe amount adsorbed per unit mass of adsorbent, m is the mass ofthe adsorbent, Vm is the monolayer capacity, and K is an equilibriumconstant that is related to the heat of adsorption by equation:

K Koexpq

RT(11)

where, q is the heat of adsorption. Langmuir model can describemost adsorption phenomena of heavy metals on fly ash[76,77,83,84,91]. In most cases, Vm and K increase with tempera-ture, suggesting that adsorption capacity and intensity of adsorp-tion are enhanced a higher temperature. A linear plot from Eq. (10)can be drawn for a particular metal adsorption, and the values, Vmand K for isotherms of the metal under study can be obtained, byusing least squares method. The Freundlich model, which is an

empirical model used to describe adsorption in aqueous systems,was also used to explain the observed phenomena of adsorption ofmetal on fly ash materials. The Freundlich isotherm is shown in thefollowing equation.

xm Kf C

1=ne (12)

The linear form of the equation can be written as:

logxm logKf logC

1=ne (13)

where, Kf is the measure of sorption capacity, 1/n is sorptionintensity, and other parameters have been defined as in Eq. (13).

The RedlichPeterson model was also used to describe theadsorption phenomenon. The RedlichPeterson equation has threeparameters, A, B and b. Parameter b ranges between 0 and 1.Theequation is represented below:

Cex=m

BA 1

ACbe (14)

This isotherm describes adsorption on heterogeneous surfaces,as it contains the heterogeneity factor b. It can reduce to Langmuirequation as b approaches one. Using Eq. (14), the parameters A, B,and b were determined by curve fitting.

The DKR equation can be represented as

ln Qe ln Qm b32 (15)

where, Qe is the amount adsorbed (mol/gm), Qm (mol/gm) is theDKR monolayer capacity, b (mol2/J2) is a constant related to theadsorption energy, and e is the Polanyi potential, which is related tothe equilibrium concentration through the expression:

3 RTln1=C (16)

where T is the temperature and C is the equilibrium concentrationof the adsobate in solution. When lnQe was plotted against e

2,a straight line will be obtained. The value of b is related to theadsorption energy, E, through the following relationship:

E 1=2b1=2 (17)

Tempkin and Pyzhev considered the effects of some indirectadsorbate/adsorbate interactions on adsorption isotherms andsuggested that because of these interactions the heat of adsorptionof all the molecules in the layer would decrease linearly withcoverage. The Tempkin isotherm has been used in the followingform:

qe RT=blnACe (18)

Eq. (18) can be expressed in its linear form as:

qe RT=blnA RT=blnCe (19)

B RT=b (20)

A plot of qe versus ln Ce enables the determination of theconstants A and B. The constant B is related to the heat of adsorp-tion [111].

Sips model suggests that the equilibrium data follow Freundlichcurve at lower solute concentration and follows Langmuir patternat higher solute concentration. The equation can be represented asfollows [112]:

Q KsCeb=1 asCeb (21)


where Ks (L/g) and as (L/mg) are Sips isotherm constants and b isthe exponent which lies between 1 and 0.

6.4. Factors affecting adsorption of metal on fly ash

The adsorption of heavy metals on fly ash is dependent on boththe initial concentration of heavy metals and contact time. It wasreported that the initial concentration of heavy metal has a strongeffect on the adsorption capacity of the fly ash. The adsorptioncapacity of fly ash depends on the surface activities, such as, specificsurface area available for solute surface interaction, which isaccessible to the solute. The adsorption affinity of fly ash for heavymetal depends on the equilibrium between competitive adsorptionfrom all the cations, ionic size, stability of bonds between heavymetals and fly ash. The other important factor for the adsorption ofheavy metal on fly ash is pH. In a certain pH range, most metaladsorption increased with increased pH up to a certain value, andthen decreases with further increase in pH.

It is apparent that by increasing the adsorbent dose theadsorption efficiency increases, but adsorption density, the amountadsorbed per unit mass, decreases. It is readily understood that thenumber of available adsorption sites increased with increasedadsorption dose and resulted in increased removal efficiency. Thedecrease in adsorption density with increased adsorbent dose ismainly due to unsaturation of adsorption sites through adsorptionreaction. Another reason may be because of the particle interaction,such as aggregation, resulted from high adsorbent concentration.Such aggregation would lead to decrease in total surface area ofadsorbent and an increase in diffusional path length [78]. Particleinteraction may also desorb some of adsorbate that is only looselyand reversibly bound to carbon surface.

Thermodynamic parameters such as standard free energychange (DG0), standard enthalpy change (DH0) and standardentropy change (DS0), are calculated using the following equation.

ln Kc DG0

RT DS

0

R DH

0

RT(22)

where, Kc is equilibrium constant that is resulted from the ratio ofequilibrium concentrations of metal ion on adsorbent and in thesolution, respectively. Linear property of ln Kc against 1/T wasproved in a number of studies on adsorption of heavy metal by flyash materials [79,98]. DG0 or DS0 and DH0 are calculated from a plotof ln Kc versus 1/T. A negative value of DG

0 indicates the process tobe feasible and spontaneous nature of adsorption. A positive DH0

suggests the endothermic nature of adsorption, and DS0 is used todescribe randomness at the solidsolution interface duringadsorption.

Fly ash can be regenerated after the adsorption, using suitablereagents. Batabyal et al. [113] regenerated the used saturated fly ashwith 2% aqueous H2O2 solution. The regenerated fly ash was dried,cooled and used for further adsorption. The adsorption rate andequilibrium time were same as the fresh fly ash particles.

7. Removal of other inorganic components from wastewater

Apart from heavy metals in wastewater, some other inorganiccontaminants, such as phosphorous, fluoride, and boron also existsin waters and dangerous for human health. Phosphorous loading tosurface and groundwater from concentrated agricultural activities,including soil fertilization, feed lots, diaries, and pig and poultryfarms is causing water quality problems in rivers, and lakes.Because fly ash is enriched with oxides of aluminum, iron, calcium,and silica, fly ash emerges as a potential candidate to treat phos-phate-laden effluents since aluminum, iron and calcium are

strongly adsorb or precipitate phosphates in many agricultural,industrial and environmental applications.

7.1. Removal of phosphate

Kuziemska first reported an investigation using water extract ofbrown coal fly ash as coagulant for precipitation of phosphate in1980. It was found that phosphate precipitation occurs immedi-ately after introduction of coagulant, and after a short and intensivemixing because of very high total alkalinity of extract [114].

Coal fly ash was paid great attention as a potential material forremoval of phosphate, since it is easily available and cost effective[115120]. Ugurlu and Salman [116] found that a Turkish fly ash isan efficient adsorbent for removal of phosphate due to highconcentration of calcite (33.83%). The influence of temperature,phosphate concentration, and fly ash dosage on phosphate removalwas investigated. Tsitouridou and Georgiou [120] compared threefly ash with different calcium contents, and indicated that phos-phate removal involved an adsorption and/or precipitation process.Vordonis et al. [121] determined that uptake of orthophosphate byfour calcium-rich (1032%) Greek fly ash exceeded the amountpredicted by monolayer coverage, suggesting either multilayeradsorption or precipitation. Interaction of inorganic orthophos-phate at water/solid interface was investigated.

Cheung and Venkitachalam [122] investigated the removal ofphosphate by fly ash with high- and low-calcium contents andconcluded that phosphate removal was primarily due to theprecipitation of phosphate with Ca2 ions in solution. The removalof phosphate by a medium calcium fly ash (with CaO content of11.57%) predominantly took place by precipitation mechanism, ionexchange and weak physical interactions between the surface ofadsorbent and the metallic salts of phosphate [123]. Grubb et al.[124] carried out batch equilibration experiments using lowcalcium, acidic fly ash for phosphate immobilization on the order of10075% for 50 and 100 mg P/L solutions, respectively. For theamorphous and crystalline phases studied, the immobilization ofphosphate in the fly ash is attributed to the formation of insolublealuminum and iron phosphates at low to medium values of pH. Theremoval of phosphate ion from aqueous solution was comparedwith fly ash, slag and ordinary Portland cement (OPC) and relatedcement blends [125]. The rate and efficiency of PO4

3 removal werefound to increase in the order: fly ash, slag, OPC, apparentlymimicking the order of increasing percent CaO in the adsorbents.Blending OPC with fly ash or slag evidently resulted in diminishedPO4

3 removal efficiency. Recently, Chen et al. [126] investigated theremoval of phosphate on different fly ash. The sorption maxima ofphosphate (Qm) ranged from 5.51 to 42.55 mg/g. The Qm valueshowed a significantly positive correlation with total Ca content(r 0.9836) and total Fe content (r 0.8049), but negative corre-lation with total Si and total Al content. Fractionation of Phosphorusadsorbed by fly ash revealed that loosely bound Phosphorus frac-tion and/or CaMg-P fraction were the dominant form of immo-bilized phosphate. Higher removal of phosphate occurred atalkaline conditions for high-calcium fly ash, at neutral pH levels formedium calcium fly ash, while low-calcium fly ash immobilizedlittle phosphate at all pH values. This behavior was explained by thereaction of phosphate with Ca and Fe related components. It wasconcluded that P immobilization by fly ash was governed by Caingredient (especially CaO and CaSO4) and Fe ingredient (especiallyFe2O3d). The selection of a fly ash with a high phosphate sorptioncapacity is of utmost importance to obtain a sustained phosphateremoval in the long term in practice.

Acid modified fly ash was effective in the removal of phosphatefrom contaminated antibiotic wastewater. Adsorption, chemicalprecipitation, and increase of BET were main mechanisms of


removal of phosphate with modified fly ash. The addition of fly ashto water produces insoluble or low solubility salt when combinedwith phosphate. Solid phase phosphate compounds are separatedfrom water by sedimentation or classical filtration. But some fly ashparticles may remain in the water and cause turbidity. Membraneprocess is being used for various water and wastewater treatmentapplications. Crossflow microfiltration was effective in a number ofprocesses, including removal of colloidal organic and inorganicsolids, and various anions, and cations from aqueous streams withthe aid of surfactants, and macromolecules [127,128]. Removal ofphosphate ions from water using fly ash in a crossflow micro-filtration membrane unit was examined [129].

7.2. Removal of fluoride

Fluoride in water is essential for protection against dental caries,and weakening of the bones, but higher levels can have an adverseeffect on health. The presence of excessive fluorides in drinkingwater is a matter of serious concern. The fluoride toxicity indicatesthat it cause weight loss, dental skeletal changes, indicators ofcarcinogenesis, hypocalcemia (low blood calcium), hyperkalemia(excess blood potassium) which will affect spine, cerebral impair-ment, and damage of soft tissues. Excess fluoride consumption alsoleads to cancer, osteoporosis, neurological, cerbrovascular effects,and other physical ailments. Besides natural geological enrichmentof fluoride in ground waters, there can also be formidable contri-butions from industries. High fluoride containing wastewater isgenerated by coal power plants, semiconductor manufacturing,glass and ceramic production, electroplating, rubber and fertilizermanufacturing. Fluoride concentration in industrial effluent isgenerally higher than in natural waters, ranging from tens tothousands of mg/L.

Wastewaters from phosphate fertilizer plants may contain up to2% of fluoride. Increased levels of fluoride can also be present ineffluents from fluorine industry, glass etching and in ground wateraround aluminum smelters. The problem of high fluoride concen-tration in ground water resource was an important health-relatedgeo-environmental issue. Examples include the state of Rajasthan,India, where nearly 3 million people are reported to consumeexcess fluoride containing water, and upper regions of Ghana,where 23% of wells have fluoride concentrations above WHO rec-ommended maximum guideline limit of 1.5 mg/L. In the Gdanskregion, high fluoride levels (1.903.00 mg/L) were detected inMalbork drinking water. There is a need for defluoridation ofindustrial wastewaters, because of excessive amounts of fluoridemay cause adverse health effects to humans and animals. Variousmethods have been used to remove fluoride from wastewaters.These methods were divided into two groups: (a) precipitationmethods based on addition of chemicals to water and (b) adsorp-tion methods in which fluoride is removed by adsorption or ionexchange reactions on some suitable substrate, capable of regen-eration and reuse [130]. Several investigations were reported forthe removal of fluoride from waters by using fly ash. Chaturvediet al. [131] examined fly ash for removal of fluoride from water andwastewaters at different concentrations, times, temperatures andpH of the solution. Removal of fluoride is favourable at lowconcentration, high temperature and acidic pH. Nemade et al. [132]carried out batch adsorption studies to determine removal effi-ciency of fluoride by fly ash. Retention of fluoride ion in dynamicexperiments on columns packed with fly ash was studied inaqueous solutions [133]. At lowest F concentration, F level in theeffluent initially increased and then gradually decreased down to0 mg/L after 120 h. With higher F concentrations in the feedsolutions, F concentration in the effluent steadily decreased

reaching 0 mg/L after 120168 h. Coal fly ash is an effectiveadsorbent for F ions, especially at high concentrations in water.

7.3. Removal of boron

Boron occurs naturally in environment, and it is commonlyfound in oceans. It is present as boric acid and borate ions inaqueous solution. Boric acid and boron salts have extensiveindustrial use in the manufacture of glass and porcelain; in wiredrawing; production of leather, carpets, cosmetics and photo-graphic chemicals; for fireproofing fabrics; and weatherproofingwood. Very few investigations were reported on boron adsorptionusing fly ash. Hollis et al. [134] examined the effect of ash particlesize, pH, and Ca(OH)2 on dissolution and adsorption of boron by flyash in aqueous media. A small amount of born was adsorbed by flyash at pH 7. This was attributed to a ligand exchange mechanism.Adsorption of boron increased with increased pH, up to 12, whichcould not be explained by co-precipitation with CaCO3. Adsorptionof boron from aqueous solution using fly ash was investigated inbatch and column reactors [135]. The Thomas and YoonNelsonmodels were applied to experimental data to predict breakthroughcurves, and to determine characteristics parameters of the columnuseful for process design.

8. Removal of organic compounds from wastewater

8.1. Removal of phenolic compounds

Phenols are important organic pollutants discharged into envi-ronment causing unpleasant taste, and odour of drinking water.Major sources of phenol pollution in aquatic environment arewastewaters from paint, pesticide, coal conversion, polymeric resin,petroleum, and petrochemicals industries. The chlorination ofnatural waters for disinfection produces chlorinated phenols. Thereare several methods reported for the removal of pollutants fromeffluents. Fly ash has a good adsorption potential for phenoliccompounds. Table 3 presents a summary of adsorption capacity ofvarious organic compounds on fly ash.

Fly ash has good adsorption potential for phenolic compounds.Khanna and Malhotra [136] first examined the potential of fly ashfor the removal of phenol. They reported kinetics and mechanismof phenol removal on fly ash and provided useful data in the designof phenolfly ash adsorption systems. Adsorption of phenol, andcresol, and their mixtures from aqueous solutions on activatedcarbon and fly ash were compared [137]. The effects of contact timeand initial solute concentration have been studied and isothermparameters were evaluated. The Freundlich isotherm was moresuitable for all the systems investigated. Removal of phenoldepends markedly on temperature and pH value of treatmentsolution [138]. Adsorption isotherms for phenol, 3-chlorophenol,and 2,4-dichlorophenol from water onto Texas Municipal PowerAgency (TMPA) fly ash were determined [139]. The fly ash adsorbed67, 20, and 22 mg/g for phenol, chlorophenol, and 2,4-dichlor-ophenol, respectively, for the highest water phase concentrations.The affinity of phenolic compounds for fly ash is above the expectedamount corresponding to a monolayer coverage considering thatthe surface area of fly ash is only 1.87 m2/g. However, moleculeswith strong functional groups align themselves vertically on thesurface; moreover, these adsorbed molecules can interact withother molecules, making the next adsorption layer energeticallyand statistically more favorable. They explained that the threephenolic compounds, having a very strong functional group as wellas strong molecular interaction, display this type of behavior. Theisotherms examined were unfavourable (BET Type III) or coopera-tive (Curve S), indicating that adsorption becomes progressively

Table 3Comparison of organic pollutant adsorption on fly ash.

Organic compounds Adsorbent Capacity (mg/g) References

Phenol FA 67 [139]Sugar fly ash 0.470.66 [143]FA-C 0.26 [148]Wood FA 5.4 [142]

Ortho-chloro phenol Coal-FA 0.81.0 [147]FA-C 98.7 [141]Fly ash 98.7

2,4-Dichloro phenol FA 22 [147]Coal FA 1.51.7 [142]

3-Chloro phenol FA 20 [139]Para-chloro phenol FA-C 118.6 [141]

Fly ash 118.62-Nitro phenol Wood FA 143.8 [151]

FA 5.806.44 [143]3-Nitro phenol FA 6.528.06 [142]4-Nitro phenol Sugar fly ash 0.761.15 [151]

Wood FA 134.9 [143]FA 7.809.68 [142]

Para-nitro phenol Bagasse fly ash 8.3 [143]Cresol Coal FA 85.496.4 [146]m-Cresol Wood FA 34.5 [134]p-Cresol Wood FA 52.52,4 Dimethyl phenol Fly ash 1.39 [113]DDD Sugar FA (7.57.7) 103 [160]DDE Sugar FA (6.56.7) 103Lindane Bagasse FA (2.42.5) 103 [161]Malathion Bagasse FA (2.02.1) 103Carbofuran FA 1.541.65 [162]TCB FA 0.35 [164]HeCB FA 0.15


easier as more solutes are taken up. Phenols have a strong hydroxylfunctional group which interacts with the adsorbent surfaces,resulting in vertical alignment of the molecule on the surface.Moreover, additional adsorption is motivated and consequentlystrengthened by the interaction between the adsorbed molecules.This phenomenon is known to contribute significantly to thecooperative nature of adsorption and hence an S type curve.

The potential of fly ash as a substitute for activated carbon forthe removal of phenolic compounds from wastewater was exam-ined [140,141]. The maximum phenol loading capacity of eachadsorbent was 27.9 mg/g for fly ash and 108.0 mg/g for granularactivated carbon. Adsorption of phenolic compounds on a mixtureof bottom and fly ash was reported [142]. The effect of molecularweight and molecular configuration on adsorption of phenol (Ph),m-cresol (m-Cr), p-cresol (p-Cr), 2-nitrophenol (2-NP) and4-nitrophenol (4-NP) from aqueous solution was investigated. Theultimate capacity of the adsorbent is considerably less than thatpredicted from summing the single-component data; this wasattributed to increased competition for adsorption sites. Bagasse flyash was converted into a low-cost adsorbent and used for removalof phenolic compounds [143145]. The uptake increases whenlarger quantities of adsorbent are used. The presence of an anionicdetergent Manoxol-IB reduces uptake of phenol and p-nitrophenol.Adsorbent prepared from fly ash was successfully used to removecresol from an aqueous solution in a batch reactor [146].

Kao et al. [147] utilized fly ash for removal of 2-chlorophenol (2-CP) and 2,4-dichlorophenol (2,4-DCP). More adsorption takes placewith fly ash of higher carbon content and larger specific surface area.Adsorption of chlorophenol is not influenced by matrix in waste-water. Chlorophenols in wastewater were also removed efficientlythrough a fly ash column. The breakthrough time was inverselyproportional to flow rates. The effectiveness of less expensiveadsorbents such as peat, fly ash and bentonite in removing phenolfromwastewater was also examined [148]. Peat, flyash and bentonitewere found to adsorb 46.1%, 41.6%, and 42.5% phenol, respectively.

Sarkar et al. [149] investigated the adsorption of some priorityorganic pollutants, viz., phenol (hydroxybenzene), o-hydroxyphenol(1,2-dihydroxybenzene), m-hydroxyphenol (1,3-dihydrox-ybenzene), and 4-nitrophenol (1-hydroxy-4-nitrobenzene), on flyash. The process was complex consisting of both surface adsorptionand pore diffusion. Activation parameter data for ultimate adsorptionand pore diffusion are also evaluated. The data indicate that externaltransport mainly governs rate-limiting process.

Batch adsorption experiments were conducted to estimate thepotential of fly ash (FA) for removal of phenols from aqueoussolution [150,151]. Polar substituted phenol, having less sterichindrance is better adsorbed than others. Substituted phenol withhindered group is less adsorbed than phenol (m-nitrophenol>o-nitrophenol> phenol>m-cresol> o-cresol). This order is relatedto electron-withdrawing properties of substituents of phenoliccompound. Therefore, electron withdrawal or deactivation ofbenzene ring favors formation of electron-donoracceptorcomplexes between these rings and basic groups on the surface offly ash. The removal mechanism of phenol is explained due tochemical coagulation with metallic oxides. Bagasse fly ash (BFA),rice husk fly ash (RHFA) and activated carbon (AC) were alsoinvestigated for adsorption of 2,4-dichlorophenol and tetra-chlorocatechol [152,153].

The potential of rice husk and rice husk ash for phenoladsorption from aqueous solution was examined [154]. Rice huskash is very effective than rice husk for phenol removal. Rice huskash (RHA) obtained from a rice mill in Kenya was used for removalof some phenolic compounds in water [155]. Adsorption capacitiesof 1.53104, 8.07105, and 1.63106 mol/g were determinedfor phenol, resorcinol and 2-chlorophenol, respectively. Coal fly ashwas used successfully to remove 2,4-dimethyl phenol by adsorptionfrom aqueous solutions [113]. Both diffusional and kinetic resis-tances affect the rate of adsorption and their relative effects varywith operating temperatures. The rate of adsorption is controlledby both diffusional and kinetic resistances at higher temperature,whereas at low temperature, rate of adsorption is dominated bydiffusion effect. Regeneration of used fly ash with H2O2 indicatesthat fly ash may be a useful cheap industrial adsorbent for waste-water treatment. Srivastava et al. [156] studied the adsorption ofphenol on carbon rich bagasse fly ash (BFA) and activated carbon-commercial grade (ACC) and laboratory grade (ACL). The highnegative value of change in Gibbs free energy (DG0) indicatesfeasible and spontaneous adsorption of phenol on BFA. The overalladsorption process is controlled by intraparticle diffusion ofphenol. Activated carbon (AC), bagasse ash (BA) and wood charcoal(WC) were also used as adsorbents for removal of p

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