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lable at ScienceDirect
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
All rights reserved.
mailto:[email protected]/science/journal/03601285http://www.elsevier.com/locate/pecs
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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
-
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(2010) 327363330
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
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M. Ahmaruzzaman / Progress in Energy and Combustion Science 36
(2010) 327363 331
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
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M. Ahmaruzzaman / Progress in Energy and Combustion Science 36
(2010) 327363332
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]
M. Ahmaruzzaman / Progress in Energy and Combustion Science 36
(2010) 327363 333
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]
M. Ahmaruzzaman / Progress in Energy and Combustion Science 36
(2010) 327363334
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.
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M. Ahmaruzzaman / Progress in Energy and Combustion Science 36
(2010) 327363 335
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
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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)
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M. Ahmaruzzaman / Progress in Energy and Combustion Science 36
(2010) 327363 337
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
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M. Ahmaruzzaman / Progress in Energy and Combustion Science 36
(2010) 327363338
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
M. Ahmaruzzaman / Progress in Energy and Combustion Science 36
(2010) 327363 339
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