Arsenic Hazards in Coal Fly Ash and Its Fate in Indian Scenario

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Resources, Conservation and Recycling 55 (2011) 819–835

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Resources, Conservation and Recycling

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rsenic hazards in coal fly ash and its fate in Indian scenario

imal Chandra Pandeya,∗, Jay Shankar Singha, Rana P. Singha, Nandita Singhb, M. Yunusa

Department of Environmental Science, Babasaheb Bhimrao Ambedkar (Central) University, Raibarelly Road, Lucknow 226025, Uttar Pradesh, IndiaEco-Auditing Group, National Botanical Research Institute, Council of Scientific and Industrial Research, Rana Pratap Marg, Lucknow 226001, Uttar Pradesh, India

r t i c l e i n f o

rticle history:eceived 1 June 2010eceived in revised form 4 April 2011ccepted 8 April 2011

eywords:rsenic hazardsoalnvironmental problemsly ash pollution

a b s t r a c t

Fly ash (FA) generated as a waste produced from thermal power plants globally has started gaining asa potentially significant anthropogenic source of arsenic (As). In India electricity generation is predom-inantly dependent upon coal-based thermal power plants and are being producing huge amount of FA.Coal contains many toxic metals, arsenic is one of those, which is significantly toxic for aquatic and ter-restrial life including human being. Coal used in Indian thermal power plants is mainly bituminous andsub-bituminous and which on combustion generate over 40% of FA. Generated FA is being disposed toopen ash pond in thin slurry form. More than 65,000 acre of land in India is occupied for storage of thismassively generated quantity of FA. Dumping of FA in open ash pond causes serious adverse environ-mental impacts owing to its elevated trace element contents, in particular the As which causes ecologicalproblems. Although, the As problem in our country is not new, in recent years the occurrence of As con-tamination cases of agricultural soil, ground water as well as human health has resulted a great concern
for its mitigation. Very recently India has been charged for being a “dumping hub for As”. Utilization of FAin India is still infancy (more than 38%) as compared to developed countries (more than 70%). In India FAis used particularly in cement production, brick industry, as road base, as amendments in the restorationecology and forestry. This review emphasized on the concentration of As in FA, its fate and behaviour ashazardous element on human health, environment quality and on mitigation strategies to accomplishenvironmental management.
© 2011 Elsevier B.V. All rights reserved.

ontents

. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 820

. Indian coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8202.1. Arsenic forms in coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8212.2. Arsenic existing forms during coal combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 821

. Fly ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8223.1. Arsenic status in fly ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8233.2. Residence mode of arsenic in fly ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8243.3. Fate of arsenic in fly ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8243.4. Arsenic-leaching from fly ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 825

. Arsenic poisoning of biosphere due to fly ash disposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8274.1. Atmosphere and arsenic-poisoning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8274.2. Agricultural soil and arsenic-poisoning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8284.3. Water and arsenic-poisoning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 828

. Analytical points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 829

. History and chemistry of arsenic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 829

. Sources of arsenic contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Arsenic toxicity due to fly ash. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Arsenic in rice agriculture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +91 522 2995605; fax: +91 522 2441888; mobile: +91 94E-mail address: [email protected] (V.C. Pandey).

921-3449/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.resconrec.2011.04.005

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10. Future prospective and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83111. Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 831

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 832. . . . . .

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References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Introduction

India ranks 6th in the world as largest energy consumer,ccounting for 3.4% of the global energy consumption. The govern-ent of India has set an ambitious target to add approximately

8,000 MW installed generation by 2012. The total demand forlectricity in India is expected to cross 950,000 MW by 2030http://www.livemint.com/2009). India has the 4th largest reservesf coal and the world’s 3rd largest coal excavator in India with 292illion tonnes (Mt) in 1998, after United States (951 Mt) and China

1243 Mt) (OECD/IEA, 1999). In India the electricity generation isainly dependent upon the coal-based thermal power plants andill continue in the coming several decades due to its huge reserves.

A generation in India was 112 Mt in 2005–06 and it is expected toike between 150 and 170 Mt per year by the end of 2012 (MoEF,007). In our country, most of the thermal power plants have beenituated in the vicinity of coal reserve area. Major coal fields of Indiare situated in the eastern region (Bihar and West Bengal) whichre densely populated. The three major thermal power plants ofastern India are Kolaghat, Durgapur and Bandel and consume bitu-inous coal (Gondwana coal) which contains low sulfur but high

sh content. The combustion of such coal in these power plantsesults in generation of huge amounts of ash, which is disposed-offn large ash ponds (dykes). The ash, after being dumped in thesepen dykes is left to get dried up. These FA-fine solid particles ofsh, dust, and soot containing arsenic and other toxic elements.t becomes a deadly source of health hazards when carried intohe atmosphere and reached to the domestic area. Therefore, theeavy metal pollution from these FA deposited dykes can affecthe surrounding population, on prolonged exposure. Thus specialttention is to be needed to manage these toxic As emitted from FA.

The Singrauli region in the south-eastern part of Uttar Pradesh,ndia is one of the highly polluted industrial areas of the Asia conti-ent. This region has six thermal power plants (Uttar Pradesh Statelectricity Board (UPSEB), Vindhyachal, Singrauli, Rihand, Anparand Renusagar), situated in the transition zone of Madhya Pradeshnd Uttar Pradesh states of India. Therefore, Singrauli region haseen recognized as “Energy Capital” of our country. This Energyapital generates about 10% of India’s installed generation capac-ty 7500 MW electricity approximately. A huge human populationave been shifted from their native homes from this region duringhe establishment of these thermal power plants. Due to workingf these power plants, some environmental problems such as con-amination of the surface water, groundwater, man-made Rihandater reservoir is rife in this region. Besides these the soil, vege-

ation and air are also contains higher level of toxic heavy metalshan the permissible limits. Consequently, urgent corrective mea-ures are required in this region to save the domestic populationrom arsenic contamination due to FA.

Presently the power plants situated in the Singraulirea accounts 10% of India and 0.3% of the global car-on dioxide emission, a major cause of global warmingwww.pollutedplaces.org/region/south asia/india/singrauli.shtm).hese thermal power plants are responsible for the main source ofollution in this region, emitting 6 Mt of FA per year, making land
nfit for cultivation due to toxic trace elements such as As, Hg, Pb,e, and Cd.
According to the reports in Indian news-papers, FA poses majorealth hazard for people living in Delhi (the capital city of India).

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 832

Driving down the Capital’s Ring Road during summer is a night-mare. Strong westerly winds pick up FA from the Indraprasthapower station, and disperse it over a large area. Residents of thesurrounding areas and commuters are the worst hit. Within fouryears of its commissioning in 1982, the Indraprasatha power plantis today perceived by people living in Delhi as a serious healthhazard (http://sundaytimes.lk/060813/ft/2.1.html).

Recently, the problems of toxicity and occurrence of As havedrawn much attention in Indian scenario. The issue of release ofAs from FA has risen from several thermal power plants and pro-duces air, soil and water pollution. The heavy metal As exists inthe −3, 0, +3 and +5 oxidation states (Smedley et al., 2002). Dueto the existence of different oxidation speciation the arsenic maybe found in many different chemical states and forms. Therefore,the health effects of As may vary widely depending on the chemicalform of the As. The burning of coal and smelting of metals are majorsources of As in the air. The most sensitive ecological indicator ofenvironmental pollution of arsenic, via air, was a mass eradicationof honeybee families, first described as “Tisin’s disease” by Svobodain 1936, which occurred in the vicinity of agglomeration furnacefacilities. Similar situations also have found in the vicinity of differ-ent smelters and power plants that burn coal with a high As content(Bencko, 1987). This may emphasize environmental problem by Asleaching in to soils and groundwater and re-entrance of As in toatmosphere from fly ashes dumped near the plant area. A numberof information has been available on the environmental problemsrelated to FA. However, this review paper will reveal As hazards incoal FA, its fate in India scenario and future strategies to achieveenvironmental concerns and sound management issues.

2. Indian coal

Coal is the principal source of commercial energy in India. Itis combustible sedimentary rock which consists organic and inor-ganic materials and its formation and composition depends on theearth crust composition, climate and flora of its location (Clark andSloss, 1992; Finkelman, 2004; Walker, 1993). Coal quality dependsupon its rank and grade. Ascending order of coal rank on the basisof carbon contents is lignite → sub-bituminous coal → bituminouscoal → anthracite. Indian coal is largely of sub-bituminous rank,followed by bituminous and lignite. Ash content in Indian coalranges from 35% to 50% (Bhattacharjee and Kandpal, 2002;http://www.hvfacprojectindia.com/Summary Report.pdf). Occur-rence of adventitious mineral impurities in these coals has beenfound to be high in comparison to most of the foreign coals. Prop-erties of coal from different regions vary widely. India is the thirdlargest coal producing country in the world after China and USA.Some of the important coal fields in the country from which bulkof the coal production comes are mentioned in Table 1. Indiancoal, albeit has high ash, bears many positive traits in respect ofenvironmental perspectives such as low sulfur content, refractorynature of ash, low chlorine content, low toxic trace elements, highash fusion temperature and low iron content in ash. Arsenic has astrong affinity to concentrate in sulfide minerals and its associationwith pyrite (FeS2) in some coals is well-documented (Yudovich and
Ketris, 2005a,b). Average As content at world level for bituminousand lignite coals are, respectively, 9.0 and 7.4 ppm and maximum Ascontents for the former and later coals are 50 and 49 ppm, respec-tively (Yudovich and Ketris, 2005a,b). In contrary, As content in
http://www.livemint.com/2009

http://www.pollutedplaces.org/region/south_asia/india/singrauli.shtm

http://sundaytimes.lk/060813/ft/2.1.html

http://www.hvfacprojectindia.com/Summary_Report.pdf

V.C. Pandey et al. / Resources, Conservatio

Table 1Coal producing states with major coal fields in India (Banarjee, 1999).

States Percentage share Major coal fields

Madhya Pradesh 31.5 Korba,Mand-Raigarh,Hasdeos-Arrand

Bihar 26 Jharia, East andWest Bokaro, NorthKaranpura

Orissa 9.6 Talcher, Ib valleyAndhra Pradesh 10.2 Godavari ValleyWest Bengal 7.7 RaniganjMaharashtra 8.3 Wardha Valley,

Kamptee

Table 2Arsenic concentration in coal and its derivative substances (Tripodi andCheremissinof, 1980; Baba, 2000).

Element (ppm) Whole Coal ash Bottom Worldwide fly

ItAT

2

Ttfomhiaitts1fraltneCbAo2t1cib

qH2mo

coal (525 ◦C) ash ash average

As 55 320 0.02–168 2.3–1,700

ndian bituminous coal is 22.3–62.5 ppm. For bituminous coal FA,he As concentration ranges from 1 to 1000 mg kg−1 (EPRI, 1987).rsenic element in coal and its derivative substances are given inable 2.

.1. Arsenic forms in coal

There are three dominant forms of As that are found in coal.hese are sulfides, organic and arsenate forms but one of all formshe most common form of occurrence in coal is sulfides. Some otherorms are also possibly occur such as in clay arsenopyrite. The workf Bayet and Slosse (1919) has been considered that pyrite is aain carrier of As, while arsenopyrite is another probable minor

ost. Arsenic enrichment is generally related to sulfide mineral-zation, either syngenetic or epigenetic. The As-bearing sulfidesre epigenetic where as syngenetic pyrites are generally very poorn As (Yudovich and Ketris, 2005). Usually it has been consideredhat As exists in coal with inorganic affinity because As is concen-rated in heavy density fractions enriched in pyrite or clay. Butome researchers (Minchev and Eskenazy, 1972; Kryukova et al.,985; Goodarzi, 1987) proved that As is also present in organicorm in coal. Belkin et al. (1997a,b) reported arsenopyrite and As-ich pyrite in the coals. Later study showed that As is partly presents an arsenate (Ren et al., 1999). A complex procedure (selectiveeaching, SEM, X-ray and microprobe analyses) showed that 85% ofhe gross As is associated with pyrite but some part is in the arse-ate form as a result of an As-bearing pyrite oxidation (Finkelmant al., 1999). From the bright, low-ash and low-sulfur coals of theentral Appalachian (USA) Elkhorn and hazard seams, As is leachedy HCl (30%) and by HNO3 (25%). The second is accounted for ass-bearing pyrite, but the first may be arsenate resulting fromxidation of pyrite containing As, was in the range from 100 to700 ppm (Crowley et al., 1997). The first case of As poisoning inhe Inner Mongolia, Autonomous Region of PR China, occurred in990. Guizhou, China is the only known arsenicosis endemic areaaused by As contaminated coal. In eastern India coal combustions one of the major sources of anthropogenic As emission in theiosphere.

There are many analytical methods that can be employed touantify the various As forms in coals (Abernathy et al., 2001;
uggins et al., 2002; Dai et al., 2004; Goodarzi and Huggins,005; Yudovich and Ketris, 2005a,b; Zielinski et al., 2007). Arsenicay exist in more than 200 species of minerals and its general
ccurrence form is arsenopyrite (Abernathy et al., 2001). The As

n and Recycling 55 (2011) 819–835 821

association with pyrite (FeS2) in few coals has been well demon-strated (Finkelman, 1993; Finkelman et al., 1999; Zielinski et al.,2007). Zhang et al. (2002) studied the occurrence of As in majorminerals that were specially selected and found As high in calciteand pyrite. Belkin et al. (1997a,b) examined wide forms of As in coalsamples. High As coals for their amount, distribution and forms ofAs (Ding et al., 2001; Dai et al., 2005, 2006; Yang et al., 2006) showedthat As was positively correlated with the ash content and indicateshigh sulfide affinity.

2.2. Arsenic existing forms during coal combustion

The information related to As release during coal combustionare very rare. The various known forms of As, released during coalcombustion usually depends on the combustion temperature. Inaddition, upon coal combustion, a large amount of As elementsleave the coal matrix and is distributed between the vapor andparticulate phases in different proportions, depending on factorssuch as the initial concentration in the parent coal, the designand operating conditions of the combustion facility and the par-ticulate control devices. In general, As is more concentrated inparticles as well as also present in volatile form. The concentra-tion of As species in the flue gas depends on a large extent ontemperature and chemical interactions with FA have been shownto be responsible for partial As retention in particulate phases(Wouterlood and Bowling, 1979a,b). The elemental (As) and oxideforms (As2O3) are considered the most probable As species in theoxidizing flue gas environment of a coal combustion process. How-ever As is much more volatile as As2O3 than as the elementalAs and researchers have concluded that As could only be presentin the flue gas in the form of an oxide (Dismukes, 1994; Winteret al., 1994). In coal gasification, the most probable species is As4with traces of arsine (AsH3) (Clark and Sloss, 1992; Helble et al.,1996). On the basis of thermodynamic modeling, As partitioningin combustion wastes at temperature lower than 925 ◦C, up to6.6% As must go into the slag. Arsenic escapes to the gas phasein the form of AsO and As2O3 during low-temperature combus-tion (1000–1200 ◦C) but at high temperature (1200–1600 ◦C), onlythe As2O3 species is released (Shpirt et al., 1990). Arsenic yield ingaseous phase from the high-temperature zone of a furnace is con-trolled by the furnace construction and slag removal coefficient(Ks) where Ks is a ratio: slag yield/gross ash content. Eary et al.(1990) shows that As concentrations in the combustion residuesvary widely depending on the coal quality, pH of ashes and theignition conditions, ranging from 2 to 240 ppm in the FA and from0.02 to 168 ppm in bottom ash. Flue gas temperature at the col-lection point is also a main factor. According to Bool and Helble(1995), equilibrium thermodynamic modeling shows that sulfidicAs should evaporate, presumably as As and in the temperaturerange 1226.85–1726.85 ◦C as As2O3. Condensed As species in FAshould be theoretically controlled by the chemical composition ofthe FA: in acid (high-Si) FA as Fe3 [AsO4]2 and in alkali (high-Ca) FAas Ca3[AsO4]2. However in realty, Fe-arsenate does not occur due tokinetic limitations, because Fe goes into the silicate glass. Arsenicis condensed on the silicate sphere surface as As2O3. Actually, it isonly physical-sorption, as indicated by strong positive linear rela-tion between As concentration in FA and 1/d value (d is mediandiameter of the FA fraction).

Zevenhoven et al. (2007) reported that combustion of fossilfuels gives rise to As emissions into the atmosphere with the lev-els of 0.1–80 mg kg−1 (dry) in coal and biomass fuels. A significantamount of As will end up in fly ashes, but the fate of As depends on
temperature, condensation–vaporization processes, the presenceof calcium, gaseous sulfur and chlorine compounds in flue gases.Fortunately, equipment installed at power plants for the control ofemissions of FA and sulfur oxides (SOx) effectively remove most of


uring

tt2amErtpric1i(cw1itcadrStfadAahAh

flse(baeah

tal problems. Currently from all coal-based thermal power plants,

Fig. 1. A schematic diagram for fly ash formation d

he As from flue gases as well, devices specially designed to con-rol the emissions of trace elements (Zevenhoven and Kilpinen,001). One aspect related to As oxides in flue gases is that “it isproven poison for catalysts” used in catalytic reduction equip-ent for nitrogen oxides (NOx) (Mukherjee, 1994; Kema, 1997).

specially when firing coals with low Ca/As ratio, this may causeapid catalyst deactivation. Zevenhoven et al. (2007) showed that atwo Finnish coal-fired power plants, both feeding around 60 t h−1

ulverized bituminous Polish coal indicated that 94 mass% of theeceived As was captured in electrostatic precipitator (ESP) FA, 6%n the semi-dry flue gas desulfurization (FGD) unit. Arsenic con-entrations in bottom ashes and the flue gases were both below%. With a concentration of around 5 mg kg−1 in the coal, the As

nput to the plant is around 300 g h−1, of which around 1.5 g h−1

i.e. around 0.5%) is emitted into the atmosphere. Arsenic con-entrations in the bottom ashes and ESP ashes and FGD residuesere around 5 and 30 mg kg−1, respectively (Aunela-Tapola et al.,

998). FA analysis from the firing of coal shows that As is enrichedn the finest fractions, which may pass particulate emission con-rol devices (Martinez-Tarazona and Spears, 1996). For example,onsidering 63–462 mg kg−1 in <2.5 mm particulate matter (PM)gainst 45–206 mg kg−1 in >2.5 mm PM (for several US coals), aetailed X-ray absorption fine structure analysis showed that the Asesides in the ashes almost completely as As2O5 (Shoji et al., 2002).terling and Helble (2003) and Seames and Wendt (2000) reportedhat As has a strong affinity for Ca, with the result that As is mainlyound in Ca-rich fly ashes as Ca3(AsO4)2 and Ca2As2O7, presumablyfter being absorbed from the gas phase as As2O3, followed by oxi-ation of As3+ to As5+. During coal combustion, the partitioning ofs between the vapor and solid phases is determined by the inter-ction of As vapors with FA compounds under post-combustion. Itas been suggested that any calcium present in FA can react withs vapor and capture the metal in water-insoluble forms of the lessazardous As (V) oxidation state (Sterling and Helble, 2003).

Several researches on the retention of trace compounds fromue gases produced by coal combustion and gasification using solidorbents has been published (Ghosh-Dastidav et al., 1996; Mahulit al., 1997). These sorbents have been mostly used in two ways:i) by passing the flue gases through a fixed or fluidized bed of sor-ent and (ii) by direct injection of the sorbent as a powder (Gullettnd Ragnunathan, 1994; Ghosh-Dastidav et al., 1996; Mahuli
t al., 1997). Several solid materials, such as clay minerals, flyshes, metal oxide mixtures and specially hydrated lime [Ca(OH)2]ave been tested as sorbents for retaining As and other metal
coal combustion. [Modified from Seames (2003)].

compounds in gases from coal combustion and gasification (Gullettand Ragnunathan, 1994; Ho et al., 1996; Biswas and Wu, 1998). Inmost of these studies, chemical-sorption is considered to be oneof the probable retention mechanisms. Due to the occurrence ofmultiple trace elements in flue gases, recent efforts of the researchcommunity have been directed towards developing a multifunc-tional sorbent capable of reducing the emission of most of thepollutants to acceptable limits. In this direction, Lopez-Anton et al.(2007) reported that activated carbons are able to capture As andother element present in coal combustion and gasification fluegases to a certain extent. Although the ash content of the activatedcarbon may influence the amount of As and other element cap-tured, no correlation between retention and the porous texture ofthe sorbents was found.

The mechanisms of trace element partition between the vaporand particle phases during coal combustion are not well under-stood, but a hypothetical schematic transformation pathway of FAformation during coal combustion is presented in Fig. 1. Partition-ing of trace elements to fine particles is a very crucial phenomenonbecause the concentration of many trace elements is significantlyhigher in fine particles compared to bigger particles (Seames, 2003).So many reports have demonstrated that the distribution of As inthe atmospheric suspended particles (Senior et al., 2000; Seamesand Wendt, 2000), but measurable description about As release islacking during coal combustion. Combustion of two kinds of highAs coal with and without CaO additive was studied by Zhao et al.(2008) in a bench scale drop tube furnace to understand the parti-tion and emission of As in the process. They concluded the calciumbased sorbent is an effective additive to control As emission duringcoal combustion. Further, the behaviour of As during coal burningwill be dominated by its occurrence mode, and most As will bevolatilized in high temperature, being condensed and enriched onthe surface of the particulate matter and finally released into theatmosphere as As oxide vapor or as As particles.

3. Fly ash

The FA is a part of coal combustion residues (CCRs). CCRs includeFA, bottom ash, boiler slag, flue gas desulfurization (FGD) residueand other solid fine particles which possess major environmen-

dry FA has been collected through Electro Static Precipitator (ESP)in dry condition as well as pond ash from ash ponds in semi-wetcondition. Most of the Indian thermal power plants do not have

rvation and Recycling 55 (2011) 819–835 823

tFadmut

spcbghctAKSpcampiorergnfidhbe(Tcc

3

pawameAbc(Uti(Acciobpr

Table 3Attenuation factors of arsenic in leachate of fly ash calculated from laboratory test(Wadge and Hutton, 1987).

Fly ash Arsenic

Concentration of fly ash 148 (�g g−1)(a) Concentration tolerated in leachatea 50 (�g l−1)(b) Concentration in first bed volume 541 (�g l−1)(c) Attenuation required to conform to guidelines (b/a) 11

a Drinking water guidelines from WHO (1984).

Table 4Arsenic speciation data in water-soluble extracts of fly ash samples (Wadge andHutton, 1987).

Arsenic Fly asha (�g g−1 dry weight)

As (III) NDAs (V) 1.7 ± 1.3Total concentration 136 ± 75.7Water-soluble fraction (%) 1.4 ± 0.7

V.C. Pandey et al. / Resources, Conse

he facility for automatic dry ash collection system. Generally bothA and bottom ash together are discharged as slurry form to thesh pond/lagoon. Keeping in view of the alarming future problemsue to this massive quantity of FA to achieve environmental soundanagement, it is very crucial time for confidence building on FA

tilization and increase in suitability of FA based products amonghe end users.

During the burning of coal, minerals undergo thermal decompo-ition, fusion, disintegration and agglomeration. Several elementsresent in a volatile form may be vaporized. A portion of non-ombustible (ash-forming) elements in the boiler enter into slag orottom ash and the rest of the inorganic residue present in the flueas as FA, left after the complete combustion of coal, containingowever also small amounts of carbon and condensed carbona-eous compounds. One of the most prominent features of FA ishe gradation effects of particle size on elemental concentration.number of researchers (Davison et al., 1974a,b; Klein et al., 1975;aakinen et al., 1975) have observed that As, Cd, Cu, Ca, Mo, Pb, S, Sb,e, Ti and Zn tend to increase in concentration with decreasing FAarticle size. The mechanism of such dependence is not known withertainty. During the combustion of pulverized coal, the meltingnd agglomeration of the mineral inclusions results in break up andaking of a number of usually spherical FA particles. A considerable

ortion of inorganic compounds vaporizes in the cooler parts of thenstallation and condenses on FA particles during the combustionf coal particles at high temperature. It has been hypothesized as aesult of volatilization–condensation theory which correlated min-ral concentration with the particle size (Davison et al., 1974a,b),ecognized three groups of elements: Group I elements are cate-orized as ‘litho files’ (Al, Ca, Fe, K, Mg, Na, Ti) showing little oro enrichment in smaller FA particles, group II elements as ‘Chalcoles’ (As, Cd, Mu, Pb, Sb, Se) show increased concentration withecreasing particle size and group III elements (Be, Cu, Ni, V, Co)ave intermediate behaviour in being enriched in smaller particlesut to a lesser extent than those of group II. These elements are pref-rentially enriched in a thin layer (∼1000 A) at the particle surfaceLinton et al., 1975) and are readily extractable (5–40%) in water.he variation in concentration of elements depends on particle size,ombustion conditions and physical and chemical properties of theoal.

.1. Arsenic status in fly ash

FA, a coal combustion by-product of coal based thermal powerlants, contains As at various levels (Kim and Cardone, 1997; Kimnd Kazonich, 2001). The large volume of FA produced around theorld is a potentially significant anthropogenic source of As haz-

rds. Arsenic in FA is present in significant amounts in the silicateatrix (Hulett et al., 1980; Senior et al., 2000). FA minerals in gen-

ral include notably high As are pyrite (FeS2) and inorganic sulfides.rsenic concentrations in FA generally range from 2 to 440 mg kg−1,ut depending on the concentration in the original coals and theombustion methods, however, they can be as high as 1000 mg kg−1

Eary et al., 1990). An investigation of the trace elements from theK revealed that coal FA contained moderately elevated concen-

rations of As (Wadge et al., 1986). The disposal of coal FA to landn the UK results in an annual input of about 700 tonnes of arsenicHutton and Symon, 1986). Estimates of the total quantities of toxics contained in the fly ashes produced at the power plants can bealculated from the total quantity of FA produced and the elementalomposition of the FA. Arsenic 9000 and 3400 kg year−1 producedn the fly ashes of the Mauban and Masinloc coal fired power plants
f the Philippines. These figures are only approximate as they areased upon the composition of only one sample of FA and on ashroduction data given in the Environmental Impact Statement (EIS)eports for these two power plants. These data, however, give an
a Arithmetic means and standard deviations of two sub-samples from 3 powerplants, ND: not detectable.

indication of the large quantities of As element produced in oneof the waste-streams as a result of coal burning at the Maubanand Masinloc plants (Brigden and Santillo, 2002). Fulekar and Dave(1985) studied on coal-fired power plant of Delhi and found thatFA contains a significant amount of As which is potentially toxicif present in a soluble form. The average As content of FA was0.9 g t−1 of ash and a maximum of 18.8% of average metal con-tent were leachable into double distilled water in a period of fivedays.

Arsenic and other trace elements in FA are important concernfor land disposal due to their environmental significance. Elevatedconcentrations of soluble salts and potentially toxic trace elements,including As, are reported to be present in FA, as well (USEPA,1980). The adsorption rate of As increases in the presence of Feand Al oxides (Jacobs et al., 1970). Consequently, arsenate ions areexpected to be strongly adsorbed on these oxide surfaces of FAand/or soil. But the final impact of each trace element will dependupon its state in FA and mobility, toxicity and availability in the bio-sphere. What is essential for As element is a better understandingof the factors which determine the environmental mobility and, inparticular, the potential for ground water contamination and ten-dency for bioavailability. In this view, much emphasis has beenplaced on measuring the chemical species of trace elements in envi-ronmental samples, as these characteristics is considered to play amajor role in determining the behaviour of the element (Florence,1986). Less attention has been paid to the direct measurement ofAs leaching behaviour at landfill disposal FA sites.

A laboratory leaching tests has been used to predict the poten-tial mobility and the attenuation characteristics of As when these flyashes are land filled (Wadge and Hutton, 1987). Due to the absenceof any ground water leachate standards, guidelines for drink-ing water can be used to investigate the dilution of the leachaterequired to produce water of appropriate quality for every traceelement. Table 3 shows attenuation factors of As in leachate of FAwhich was calculated from laboratory test by Wadge and Hutton(1987). Table 4 shows speciation data for As in water-solubleextracts of FA samples. The arsenic concentration in differentwastes is mentioned in Table 5.

Few approaches are used for contents and residence sitesof arsenic in FA. These are thermodynamic modeling (Senioret al., 2000a,b,c,d), lab experiments modeling different combustion
regimes (Huggins et al., 2003) and direct analysis of fly ashes andtheir size fractions (Shpirt et al., 1998; Font et al., 2003). A schemewas devised by Kizilstein (2002) for determining As precipitationon the surface of FA particles. Generally two types of spherical


Table 5Arsenic concentration in different wastes (source: Wright et al., 1998; Killingley et al., 2000; Mitrra et al., 2005).

Element BA SS G

As (mg kg−1) 8.1 98 6.3

FA: fly ash, BA: bed ash, SS: scrubber sludge, G: gypsum, FYM: farm yard manure, PFS: pa

Table 6Detailed arsenic in fly ash in five zones of India during 1997–98 (source of data:Mohan et al., 2001).

Zones Fly ash produced(million tonnes)

Estimated arsenic in flyash (tonnes) [In thisstudy]

Northern 19.080 7.63Southern 18.370 8.08Western 28.547 12.50Eastern 11.943 5.25North-East 00.037 0.016Total 77.980 34.41

Zones: Northern: Jambu and Kashmir, Himachal Pradesh, Haryana, Rajasthan, Delhi,UGN

phdict

cAGo

lb

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ttar Pradesh; Southern: Andhra Pradesh, Karnataka, Tamil Nadu, Kerala; Western:ujarat, Maharashtra, Madhya Pradesh, Goa; Eastern: Bihar, West Bengal, Orissa;orth-East: Meghalaya, Manipur, Sikkim, Mizoram.

articles (average diameter near 0.17 mm) are found in FA such asollow (density about 0.60–0.65 g cm−3) and compact with a higherensity. It was proved by Kizilstein (2002) that As concentration

n hollow spheres must be higher due to their lower density thanompact spheres, though the surface condensation film has equalhickness on each particle (near 2–4 �m).

In this review, based on the other data the mean As in Indianoal can be calculated as 42.4 �g g−1. Whereas the mean values ofs in Indian FA is about 46.85 �g g−1 (Sharma and Tripathi, 2008).enerally As content in FA is variable and depends on the manyther factors.

The relative enrichment (RE) factor in this study may be calcu-ated by the following equation as given by Meij et al. (2002) maye expressed as:

EAs = [AS]FA

[AS]coal× ash content of coal %

100

nd it was found that RE = 0.44 [As in coal = 42.4 �g g−1 (meanalue), As in FA = 46.85 �g g−1 (mean value), ash content ofoal = 40 wt%].

The estimated total amount of As in fly ashes in India is pre-ented in Table 6. Based on the model data calculation indicateshat projected FA generation will be 154 Mt in 2012 in IndiaBhattacharjee and Kandpal, 2002). Whereas, according to our pre-icted estimates As in coal FA in the Indian scenario will be about7.76 tonnes in 2012.

.2. Residence mode of arsenic in fly ash

There are scare of studies on the mode of As-occurrence inA. The form of As residence in FA is influenced by the composi-ion of the parent coal and the conditions during coal combustion,articularly during cooling. Arsenic is initially volatilized at theemperature of coal combustion, but will partition between theapor phase and FA particles in the cooler portions of the flue gastream. Compared to other trace elements in FA, As shows par-icularly strong enrichment (5–10 times) on the finest (<10-�miameter) size fractions of highest surface area (Natusch et al.,974; Smith, 1980).

The concentration of As and other trace elements in FA are
elated to the S content of the coal. Generally, those feed coalsith a high S content contain higher concentrations of these ele-ents. For the same coal-fired power plants, the concentrations
f these elements are also greater for bag-house FA compared to

FYM PFS Acidic FA Alkali FA

5.64 6.85 7.4–25.0 56.0–89.0

per factory sludge.

Electrostatic Precipitator (ESP). The S-content of FA is 0.1% for pul-verized ESP FA and 7% for bag-house FA from the fluidized bed,indicating that most of the S is captured by FA in the fluidized bed.

Arsenic is captured by calcium-bearing minerals and hematite,and forms a stable complex of calcium or a transition metal of ironhydroxy arsenate hydrate [(M2+)2 Fe3(AsO4)3(–OH)4–10H2O] in FA.Maximum elements in FA have enrichment indices of greater than0.7 indicating that they are more enriched in the FA than in the feedcoal, except for Hg in all ESP ashes. The bag-house FA from the flu-idized bed has highest content of other trace elements, indicatingthat CaO, for the most part, captures them (Goodarzi, 2006). Somephases contain structural As (arsenopyrite, crystalline) As(III) oxide(As2O3), calcium orthoarsenate, scorodite (FeAsO4.2H2O), substi-tuted As [arsenian pyrite, arsenian jarosite, arsenian eltrignite], andsorbed As(V) or As(III) ettringite, several ferric oxyhydroxide poly-morphs, amorphous silica, aluminosilicate clays, and manganesehydroxides (Zielinski et al., 2007).

The widespread occurrence of the As forces researchers to findits sources which may negatively impact poison the environment.Coal is one of these sources. Perhaps, the first quantification of Asin coal was made by Daubree (1851). He determined As in twosamples of French bituminous coals (169 and 415 ppm); in three lig-nite samples (37, 793, and 2090 ppm) from the German Saar basincoal (30 ppm); and traces in British Newcastle coal (Daubree, 1851).After this work, analyses of coal and its products for As were pub-lished more frequently. For example, As was found in coke, up to110 ppm (Simmersbach, 1917). Especially significant were the firststudies on cattle diseases in England and Belgium. In these studies,the vegetation in the vicinity of coal-fired plants was As enriched.An important conclusion was made that pyrite was the main carrierof As in coal (Bayet and Slosse, 1919).

3.3. Fate of arsenic in fly ash

There is scarcely report on As hazards in coal FA and its fatein Indian scenario. According to thermodynamic modeling, themain factor of the As yield to gas phase is temperature. At thetime of low-temperature ignition, As forms solid phases such asoxides and elemental As. Maximum arsenic passes into gaseousphase in the forms of As2, As3 and AsS at the temperature abovethan 600 ◦C (Bel’Kova et al., 2000). Arsenic and few of its com-pounds are volatile and therefore, escape in general to the gas andaerosol phase in coal combustion. Leaching studies of 18 elementshave been made from Indian coal FA by Praharaj et al. (2002). Hefound that a few trace elements leach from the ashes in excessof world health organization’s (WHO) limit values (As, Mn andMo) or the USEPA limit values (As, Mn and Fe). Arsenic contam-inated the aquatic and terrestrial environments through leachingand erosion of FA pond. Arsenic amount more than permissible lim-its have been reported in the soil, tube-well and surface water nearthermal power plants. The main sources of As of thermal powerplants in the environments are atmospheric source through fluegas and FA. The ultimate sink for most environmental As is theriver sediment. Because of its reactivity and mobility, As can cycleextensively throughout the abiotic and biotic components of the
aquatic and terrestrial systems and a diagram showing the fate ofFA arsenic cycling in the biosphere has been presented in Fig. 2.It is known that As can undergo a variety of chemical transforma-tions such as oxidation and reduction. In Indian scenario, ongoing

V.C. Pandey et al. / Resources, Conservation and Recycling 55 (2011) 819–835 825

cling i

urtbaptDiLo(PptpiTim

3

uFt(11e2

aeta1WHe2

Fig. 2. A schematic diagram for the fate of arsenic-fly ash cy

se of coal combustion for power production will result in futureeleases of toxic and potentially toxic elements including arsenic tohe environment. Due to the very large quantities of FA producedy coal based power plants, each power plant liberates significantmount of arsenic each year, contaminating the biosphere. Twoower plants at Indraprastha and Rajghat combined are estimatedo contribute annually about 5–6 tonnes of As to the Yamuna, inelhi, from ash leaching alone. The element being non-degradable,

t migrates from a remote corner of the watershed to the discharge.eaching of As in groundwater is also expected in the vicinity areasf landfills containing hazardous waste FA and other waste pilesNriagu and Pacyna, 1988; Tripathi et al., 1997; Pandey et al., 1998;raharaj et al., 2002). India has about 85 coal-based thermal powerlants which jointly liberate many tonnes of As per year. So, atten-ion must be taken for mitigation of such type of anthropogenic Asoisoning, otherwise it will create a big environmental problems

n coming year in compared to other anthropogenic As sources.herefore, further detailed investigation is required to analyse thenfluence on the biosphere and human health due to handling and

anaging of the FA containing As.

.4. Arsenic-leaching from fly ash

Presently the leaching behaviour of As from FA is not wellnderstood. Although much research on laboratory leaching ofA aimed at making a quantitative estimate of the release ofoxic elements have been reported during the last three decadesFulekar and Dave, 1985; Ishiguro et al., 1986; Warren and Dudas,988; Sandhu and Mills, 1991; Querol et al., 1996; Nham et al.,996; Khanra et al., 1998; Wang et al., 1999; Ram et al., 2000; Choit al., 2002; Praharaj et al., 2002; Iyer, 2002; Kim et al., 2003; Kim,006; Soco and Kalembkiewicz, 2007).

From the eco-toxicological point of view, the leached Asnd other heavy metals from FA may become a hazard to thenvironment because of their contribution in the formation ofoxic compounds. This process can lead to health, environmentalnd land-use problems (Davison et al., 1974a,b; Kaakinen et al.,975; Klein et al., 1975; Campbell et al., 1978; Wangen and
illiams, 1978; Gehrs et al., 1979; Hansen and Fisher, 1980;
ulett et al., 1980; Georgakopoulos et al., 1994; Fernandez-Turielt al., 1994; Laumakis et al., 1996; McMurphy et al., 1996; Baba,000; Kamon et al., 2000; Baba and Turkman, 2001; Gulec et al.,

n biosphere generated by coal-based thermal power plants.

2001; Georgakopoulos et al., 2002a; Georgakopoulos et al., 2002b;Mandal and Sengupta, 2002; Baba, 2003; Baba et al., 2003; Baba andKaya, 2004). The extent of the As in FA depends on both the miner-alogy and particle size distribution of the raw material being burntand the combustion temperature. Contact between ash particlesand leachant can lead to partial or complete dissolution of mineralphases that constitute the ash particles, or establishment of partialchemical equilibrium between the ash constituents and leachant.These interactions are well documented by EPRI (1998), Kirby andRimstidt (1994) and Fleming et al. (1996), which are dependent onfactors such as pH, redox state, and leachant ionic strength.

The leachate from unlined FA disposal sites is a potentialanthropogenic source of As to the environment (Burns et al., 2006).So, understanding the leaching process of As in FA during ashdisposal and reuse is important to developing novel strategies forthe control of As in the environment. Many researchers investi-gated the speciation and forms of As in FA and reported that As isassociated with iron and aluminium oxide deposits on the particlessurface of FA (Silberman and Harris, 1984; Xu et al., 2001a). BothAs (III) and As (V) were detected but the latter was present in amuch greater fraction (Silberman and Harris, 1984; Goodarzi andHuggins, 2001; Huggins et al., 2007). To determine the total Asleaching potential from FA, various leachants, including HNO3,H2SO4, sodium citrate, geopolymer, and EDTA have been used ina variety of experimental configurations (Silberman and Harris,1984; Bankowski et al., 2004; USEPA, 2006). Silberman and Harris(1984) found that as much as 78–97% of the total As could beleached from FA with a 0.5 N H2SO4 or a 1 M sodium citrate at pH5. Leaching experiments conducted over a range of very acidic tovery alkaline conditions revealed that total arsenic leaching variedamong different fly ashes, ranging from less than 5% in half of theashes tested, to more than 30% in others (USEPA, 2006). Undertypical environmental conditions, significantly lower As leachingis expected than under extreme acid or alkaline conditions.

Experiments from laboratory leaching test have demonstratedthat As leaching from FA is mainly dependent on the pH of the solu-tion. However, some discrepancies result from differences amongthe fly ashes studied, the released As concentrations are normally
high in the alkaline or acidic pH ranges low in the neutral ranges(Van der Hoek et al., 1994a). Nevertheless, the pH dependencycould not be modeled based on the solubility of As-bearing minerals(Fruchter et al., 1990; Van der Hoek et al., 1994a), even though the

8 rvatio

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F1ddmFppclhfil1ilp(aiotpFraaagtdItunMiepfmelitaiiate

A

26 V.C. Pandey et al. / Resources, Conse

arious studies showed that the leaching of other trace elementsrom FA is generally controlled by mineral solubility (Mattigodt al., 1990). It has been suggested that the sorption to iron oxidesould be the cause of the pH-dependent leaching of As from flyshes (Van der Hoek et al., 1994a). Later, they modeled the Asehaviour based on the sorption to iron oxides. However, undereld conditions, the leaching mechanisms of As are poorly under-tood because most of the previous studies have been performed onasis of lab experiments using fresh ashes. Geochemical behaviourf As in pore-waters of an alkaline FA disposal site has been sug-ested by Kim et al. (2009).

Researchers and planners for a long time are anxious aboutA leachates effects on ground water quality (Milligan and Ruane,980). This problem is more sensitive in India due to large depen-ence on ground water and huge production of FA. Eary et al. (1990)uring his experiment found that As is very leachable because itainly occurs as a surface precipitate in FA. Arsenic leaching from

A was governed by their concentrations, association with the asharticles, leaching time and pH of the leachate (most influencingarameter). The relative concentration of calcite and clays in theoal is one of the most prominent factors on the behaviour of FAeaching. Due to their high pH, fly ashes that contain free lime (CaO)ave leaching properties and potential for contamination very dif-

erent from those ashes where free CaO is non-existent or presentn negligible quantities. The reason for the difference is that theeaching of most trace elements is pH dependent (Goumans et al.,994). Particularly, the very fine particles (<1 �m) in FA play an

mportant role above their weight concentration because of theirarge active surface area. This is significant both for leaching andhysical properties (Nathan et al., 1999). According to Nathan et al.1999), some toxic elements in the leachates have concentrationpproaching relatively high levels, such as As, Cr and Se. However,t is well known that FA is a natural buffer and, even after removalf CaO and its derivatives [Ca (OH)2, CaCO3], its pH is never lowerhan 7.5. So oxyanions of As are not potentially dangerous. The finalH of the leachate is variable depending on the CaO content of theA. So the final pH of the leachate should always be part of theesults; it is necessary data for evaluation of the results. Fulekarnd Dave (1985) measured the leaching of As from FA heaps, thesh in disposal pond and FA suspensions discharged to a river fromcoal-fired thermal power plant in Delhi, India, in order to investi-ate the associated hazard to the environment. About 18.8% of theotal As in the FA was leachable into double distilled water in a 5ay period. The leachability of elements from FA was examined by

shiguro et al. (1986) in an elaborate study using an outdoor long-erm weathering test plant. Both tap water and rain water weresed for leaching. They demonstrated that As, Cu, Pb and Ni wereot detected in the leachate, but a trace amount of Zn was noted.echanisms of mobilization and attenuation of inorganic elements

n coal FA basins were studied by Sandhu and Mills (1991). Wangt al. (1999) investigated the potential of surface and ground waterollution through leaching of elements such as As, Zn, Pb, and Nirom FA at different pH (2.0–6.5). Jonkowski et al. (2006) worked on

obility of As element from selected Australian fly ashes. Arseniclement is mobile under different conditions and over differenteaching times. The concentrations of As element released in leach-ng solution with initial pH values of 4, 7 and 10 were used to assesshe influence of pH conditions on element mobility from the acidicnd alkaline fly ashes. Arsenic concentration increased with timen leachate solutions from acidic and alkaline fly ashes: however,n solutions in contact with alkaline fly ashes the As concentration,fter reaching a maximum, later decreased with time. The pH of
he leaching solution is the key factor affecting the mobility of Aslement in these fly ashes.
Dutta et al. (2009) studied on leaching of ten elements includings from four FA samples which were collected from four different

n and Recycling 55 (2011) 819–835

coal-fired thermal power plants in West Bengal, India. The leachingpattern and its dependence on the pH as well as the solid–liquidratio have been critically analysed. A much higher mobility of Aselements have been expectedly observed at a low pH. Less leach-ing is found at a high pH except for As. The mobilization patternis strongly governed by the well-known phenomenon of dissolu-tion and re-precipitation of iron with co-precipitation of a series ofelements depending upon the pH of the medium. Extraction equi-librium was reached for Ca, Fe, Na and Zn at certain pH values. Amonotonic trend of release for the elements Mn, K, Cu, Pb, Cr andAs persisted over the long-term leaching period of 180 days. Thealkalinity or the calcium content of the FA sample greatly deter-mines the leaching pattern if the solution pH is neutral or mildlyacidic. It appears that the risk pollution of ground water as wellas of surface water may not be avoidable if FA alone is used forabandoned coal mine back-filling in an environment where acidmine drainage is well-known. Nevertheless, blending with lime toenhance the alkalinity appears to offer a practical solution to theproblem.

Arsenic-fly ash interactions are complex and related to surfaceand aqueous chemistry. According to American society for testingand materials, class F type fly ash typically contains more than70% of SiO2, Al2 O3, and Fe2O3 (ASTM, 1992). Class F type FA isgenerated from burning of bituminous coal. Because aluminiumoxide (i.e. activated alumina) and iron hydroxide are common sor-bents for As removal from water (Crittenden et al., 2005), thesetwo oxide minerals in FA are believed to be responsible for Asadsorption. Many factors such as pH, calcium, magnesium, reduc-ing or oxidizing conditions, solid-to-liquid (S/L) ratio, leaching time,temperature and anionic constituents (sulfate and phosphate) havebeen shown to influence the leaching of As from FA (Lecuyer et al.,1996; Qafoku et al., 1999; Xu et al., 2001b; Praharaj et al., 2002).Consequently, several mechanisms have been proposed to inter-pret the As leaching behaviour. Iron oxide was reported as the mainsorbent in acidic ash controlling As leaching (Van der Hoek et al.,1994a,b). A model incorporating the electrostatic effect was usedby Van der Hoek and Comans (1996) to quantify the adsorption ofAs onto FA. Modeling results were strongly dependent on the ini-tial assumptions, namely that amorphous iron oxide was the lonereactive site and that the calculated adsorption constant was pHdependent. Similar models were also used to quantify As adsorp-tion onto other solid media such as soil mineral and metal oxides(Goldberg, 1985, 1986; Goldberg and Glaubig, 1988a,b; Hering andDixit, 2005). Information on how to quantify site density and howto determine the type of reactive surface sites in field samplesis lacking. In addition, little is known regarding applicability ofpreviously developed laboratory-derived adsorption constants tofield samples and settings (Miller, 2001). Improved approaches foranalysing surfaces to rapidly and consistently evaluate site acid-ity constants and density for various media are needed. Since, Asacid is a weak acid, As (V) present in water as different forms are:H3AsO4, H2AsO4

−, HAsO42−, and AsO3

4−. The different As species

are expected to have different adsorption constants with FA sur-face sites. Therefore, a speciation-based approach needs to be usedto properly understand and quantify the interactions between Asand FA surfaces. For this purpose, Wang et al. (2008) used batchmethods to investigate As leaching using a raw ash, and As adsorp-tion using a clean, washed ash. Results indicated that pH had asignificant effect on As leaching or adsorption. It has been foundthat between pH 3 and 7, less As was in the dissolved phase. WhenpH was less than 3 or greater than 7, increasing amounts of Aswere leached or desorbed from FA. The adsorption and leaching
behaviour of As was interpreted with the speciation of FA surfacesites and As. A speciation-based model was developed to quan-tify the As adsorption as a function of pH and FA surface acidityparameters.

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concentrations being higher in those particles less than 10 �m(0.01 mm) in diameter. This may be due to the high surface area of


The storage and disposal of FA can lead to the release of leachedetals into soils, surface and ground waters. The majority of these

oxic elements including As are able to build up in soils and sedi-ent, and many are persistent and high toxic to animals, humans

nd plants through air water and soil uptake. The concentra-ions of these hazardous metals detected in the FA samples areot significantly higher than those typical found in uncontam-

nated soils (Alloway, 1990). However, the leaching of As fromigh Ca-ash wastes (produced by lignites and subbituminous coalsombustion) is far less, than from acid ones. The point is that, atater leaching of high-Ca ash disposals, low-solubility ettringite,a6Al2 (SO4)3(OH)12·26H2O, appeared, although after some timeYudovich and Ketris, 2005a,b). Trace element analysis reveals thats and some toxic elements are sufficiently enriched in FA pond

han their crustal abundances, and preferably in the lighter sizeractions. Chemical analysis of the water samples collected fromube-wells near the FA ponds reveals high concentration of As andome trace elements, whose distribution is mainly controlled byhe ash deposited in the area. Among these toxic elements, As andome other trace elements show higher concentration in the tube-ell waters near the FA pond, implying significant input from the

A pile. The enrichment of As and some elements (Al, Fe and Mn)bove World Health Organization (WHO) guidelines for drinkingater denotes significant contamination of the groundwater from

he toxic elements leached from the FA pile (Mandal and Sengupta,005).

Elements present in FA are not fully bound to the particles, andll the elements quantified are able to leach from FA up to differentevels (Llorens et al., 2001). Reports on the fraction of these ele-

ents that are able to leach from the FA vary between methodsmployed. Studies using sequential extraction methods have beenecognized that a significant fraction of most elements present in FAre able to leach from the ash, including As (57%), Cr (17%), Co (8%),u (7%), Ni (8%), Pb (13%) and Zn (12%) (Querol et al., 1996). Evenhere pure water has been used in FA leaching studies, appreciable

ractions of As and other elements have been shown to leach fromA (Llorens et al., 2001). It has been designed to simulate rainwatereaching of FA (Praharaj et al., 2002), As was detected in the leachatet concentrations up to 260 �g l−1, 26 times the US Environmentalrotection Agency (EPA) limit and the WHO recommended valueor drinking water (USEPA, 2002b). The use of lime flue gas scrub-ing has been demonstrated to reduce the quantity of As present

n FA leachate (Lecuyer et al., 1996).Baba et al. (2008) showed that a decrease in pH of the leachant

avours the extraction of toxic metals from FA. A significant increasen the extraction of As and other metals from the FA is attributedo the instability of the mineral phases. These heavy metals con-entrations increase with respect to increasing acidic conditionsnd temperature. Peak concentrations, in general, were found atround 30 ◦C. Sometimes, the temperature of the FA may increasep to 40 ◦C when the FA is in contact with water. Therefore, the con-entration of some heavy metals such as As may be magnified in theA deposited in the ash disposal site. In other words, the mobilityf trace elements from FA depends not only on the element con-entration, temperature and mode of occurrence, but also on thehemical conditions associated with the leaching process. This inurn depends on the ash chemistry (acid or alkaline ash) as well ashe pH of the leaching solutions used.

However, if precautions will not be taken, contaminants mayell spread into the groundwater, bio-diversity and soil resources

rom FA disposal sites. Therefore, groundwater and soil monitoringeed to be carried out periodically in the FA affected region. Suchonitoring activities should provide better management prac-

ices for the sake of environmental quality. In fact, the leaching
ehaviour of As from the FA needs a separate review for under-tanding the related problems very well.
n and Recycling 55 (2011) 819–835 827

4. Arsenic poisoning of biosphere due to fly ash disposal

Fly ash (FA) is one of the noxious environmental pollutants andsurely with this is associated the major concern in relation to drawthe environmental health strategists. It is a complex mixture witha high degree of leaching potential in aqueous media. It containsmore than a dozen heavy metals but this review emphasized onlyArsenic. In the present time scenario much of As of the atmospherecomes from high-temperature process such as coal combustion,largely from thermal power plants. The As is released into the atmo-sphere primarily as As trioxide where it adheres readily onto theparticle surface. The mode of As occurrence in FA strongly influ-ences the rate of As-release to the environment and As-relatedhealth risks associated with FA disposal. Arsenic is ubiquitous innature and exists in both inorganic and organic compounds. Someof its compounds are highly toxic and because it can be dispersedin the environment (in air, soil and water) as reported by Faustand Aly (1981). It has been reported that the As released fromthermal power plants is of a complex nature. Arsenic is not sub-ject to decomposing process like organic substances (Bencko andSlamova, 2007). Biological oxidation of As contaminated sites hasrecently gained increased importance and application due to theexistence of certain advantages, over the conventional physico-chemical treatment (Zouboulis and Katsoyiannis, 2005). Therefore,its efficient removal by bioremediation process from natural con-taminated sites intended is considered of great importance.

4.1. Atmosphere and arsenic-poisoning

Coal is naturally contaminated with As, and when it is burnedto generate electricity, As is released into the air through thesmokestacks and coal combustion by-products (fly ash). FA con-taining toxic elements as a result of coal burning, some toxicelements are also emitted to the atmosphere in gaseous form. Dur-ing the pulverized coal combustion of thermal power plants, mainpart of initial As evaporates to gaseous phase. Most of the organicAs, pyritic As and some shielded As-bearing micro-mineral phasesescape in a gaseous phase and only a minor part of arsenic clayremains in bottom ash. Maximum escaping arsenic is captured byFA. Because of 97–99% FA is collected by ESP, the atmosphericemission of As (solid-phase and gaseous) is rather minor (exceptsome coals with extremely high As-concentrations). However, Asatmospheric emission data are contradictory: up to 30–40% on theRussian and Bulgarian data and far less on the some USA data.It seems that such difference is caused by the different combus-tion regimes and different ash-hoppers systems (cool-side versushot-side ESP, for example). Besides, the presence of FGD-devices(wet scrubbers, etc.) may greatly effect As emission rate. The frac-tion of certain elements that are emitted in flue gases comparedto the amounts in all waste-streams have been projected; theseinclude Hg (up to 95%), Pb (up to 40%) and As (up to 30%) (Llorenset al., 2001). During the 1990s, As emissions in the European Union(EU-15 states) were estimated to be 575 tonnes, of which 15 wt%(86 tonnes) were contributed by fossil fuel-fired stationary com-bustion plants. Atmospheric emission of As due to fuel combustionis 186 tonnes per year in Europe during 1995 (Pacyna and Pacyna,2001). The use of flue gas scrubbing with lime (calcium oxide) cansignificantly reduce the fraction of gaseous As released to the atmo-sphere through sorption to lime particles (Senior et al., 2000a).

Certain elements are vaporized to varying degrees during coalcombustion and upon cooling of the combustion gases are fullyor partially sorbed onto FA particles. This process results in their

these very fine particles. Elements for which this sorption enrich-ment has been observed include As, Cd, Cr, Co, Pb, Hg and Zn

8 rvatio

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Wadge et al., 1986) The higher concentrations of these elementsn the very fine particles of the FA have implications for the failuref pollution control devices to capture ultrafine particles, and thehreat posed by such particles upon inhalation and thus may havereater impacts on biological systems in the vicinity of coal-firedower plants (Klein et al., 1975). When inhaled, particles <1 �m iniameter may be deposited in the pulmonary tissue of the respi-atory tract and gain entry into the blood stream (Davison et al.,974a,b; Natusch et al., 1974). ESP, bag-house, and other devicesapture more than 99% of most elements emitted during the com-ustion of pulverized coal (Goodarzi, 2006). Due to the considerableuantities of FA produced, however, particulate emissions to thetmosphere can still be considerable. Emitted FA particles can beeposited at distance several to hundreds of kilometers from themission point, depending on factors including chimney height. Theollection efficiencies of pollution control devices are still insignif-cant for particles less than 10 �m in diameter, with the greatestenetration for ultra-fine particles of 0.1–1 �m diameter (Seniort al., 2000b). Particulates in the emission plume can be enrichedith elements including As and Zn by 1.5–3.0 times compared with

tack particles. Estimate on the quantities of toxic elements emittedo atmosphere based on the percentage of FA retained may under-stimate the total quantities due to such enrichment of elementsn the very fine particles (Llorens et al., 2001; Senior et al., 2000b).he other results show that use of one ton of FA in concrete willvoid 2 tonnes of CO2 emitted from cement production and reducesreen-house effect and global warming (Krishnamoorthy, 2000).

.2. Agricultural soil and arsenic-poisoning

Generally low levels of As are naturally present in the soil. Theackground levels are around 5 mg kg−1 worldwide with substan-ial variation depending on the origin of the soil. Arsenic behaviours distinctly different under flooded (anaerobic) and non-floodedaerobic) soil conditions, with flooded conditions being likely the

ost hazardous in terms of uptake by plants and toxicity. Arsenicould be the biggest environmental toxic contaminant in most ofhe agricultural soil where FA is applied. It has been observed thatue to non mobility of arsenic, it is strongly adsorbed on to theoil particles. However, in gypsum and phosphorus amended soilss may be much more mobile. As (III) and As (V) are taken upy different mechanisms from soils by the plants. Arsenic (V) isaken up via the high affinity phosphate uptake system in plantsMeharg, 2004). Phosphate (PO4) applications have therefore beenuggested to reduce uptake because of competition between PO4nd As (V) for uptake. Speciation of inorganic As in the soil is largelyontrolled by reduction and oxidation processes (redox). Undernaerobic (reducing) conditions As (III) predominates, whereas AsV) predominates under aerobic (oxidizing) conditions.

The relationships between soil As and growth of plants dependn the form and availability of the As. The toxicity of As variesith its form and valence, its toxic order being AsH3 > As (III) > As

V) > organic As. In general, As availability to plants is highest inoarse-textured soils having little colloidal material and little ionxchange capacity and lowest in fine-textured soils high in clay,rganic material, iron, calcium and phosphate. The average As con-entration naturally occurring in the soil worldwide is 10 mg kg−1.he food hygiene concentration limit for As is 1.0 mg kg−1 (Das et al.,004). In general, ordinary crops do not accumulate enough As toe toxic to man from nonpolluted agricultural fields. However, ins contaminated soil, the uptake of As by the plant tissue is signifi-
antly elevated, particularly in vegetables and edible crops (Larsent al., 1992). So, there is concern regarding accumulation of As ingricultural crops and vegetables grown in the As affected area dueo FA.
n and Recycling 55 (2011) 819–835

4.3. Water and arsenic-poisoning

Water is the major source of transporting As under natural con-ditions. In oxygenated water As occurs in a pentavalent form, butunder reducing condition the trivalent form predominates. Kimet al. (2009) showed geochemical characteristics of As in pore-waters of an alkaline coal FA disposal area using multilevel samples.When the disposal area was covered with soil, As levels in the pore-waters were very low (average of 10 �g l−1) due to co-precipitationwith siderite. The soil cover has capacity for the creation of anoxicconditions, which raised the Fe concentration by the reductive dis-solution of Fe hydroxides. Due to high alkalinity generated fromthe alkaline coal FA, still a small increase in the Fe concentrationcould cause siderite precipitation (Kim et al., 2009). After removingsoil cover, however, an oxidizing condition was created and trig-gered the precipitation of dissolved Fe as hydroxides. Accordingto result, the dissolution of previously predicated As-rich sideritecaused higher As concentration in the pore-waters (Kim et al.,2009). Therefore, the As concentrations in pore-water of coal FAdisposal area can be regulated and managed by siderite precipi-tation and dissolution as well as could be effectively immobilizedin the presence of soil cover by forming reducing conditions thatwould precipitate dissolved As in cooperation with siderite (Kimet al., 2009). In this view, covering a disposal area with low-permeability soils is a required management idea. Though, theAs that co-precipitates with siderite can also be simply mobilizedagain when the disposal area is disturbed, because carbonate min-erals are very sensitive to changes in the geochemical condition ofwater chemistry. Arsenic immobilization based on siderite may notbe applicable to the disposal area where pore-water pH is acidicbecause carbonate precipitation is mostly suppressed under thiscondition (Kim et al., 2009).

The river, containing waste water from ash deposits of a powerplant, which was burning As-rich coal, reached an As concentrationup to 0.21 mg l−1. Of the micro-organisms occurring in this riveronly diatomaceous survived. Fish can easily survive on this con-centration. In an experiment trout perished at a concentration of20–25 mg As l−1 and carp at a concentration of 25-30 mg As l−1. Par-ticularly sensitive are the pike perch and roach. The eel is supposedto be more resistant (Bencko and Slamova, 2007). The absenceof fish on a long stretch of the polluted river which received theabove-mentioned As-contamination waste water from ash depositwas probably due to the destruction of their natural food chain(Bencko and Slamova, 2007). Ground water As contaminationhas exceeded 2000 �g l−1 in some areas of India and Bangladesh(Hossain, 2006). Table 7 shows that maximum permissible limitof As in drinking water, industrial effluent, leachates and land filldrainage.

The release of FA pond decants into the local water bodiesincreases turbidity, decreases primary productivity, affects fishesand other aquatic biota. Many studies have been exposed impactson amphibians inhabiting locations contaminated with FA. Forexample, both adult southern toads (Bufo terrestris) and freshwatergrassshrimp (Palaemonetes paludosus) have been shown to accu-mulate trace elements from FA polluted areas, including arsenicand cadmium (Hopkins et al., 1999; Rowe, 1998). Larval southerntoads (B. terrestris) and larval bullfrogs (Rana catesbeiana) inhabit-ing similar sites have been shown to suffer elevated incidences ofsurvival threatening physiological impacts (Hopkins et al., 2000).Such effects are believed to result from the complex mixtures ofpollutants in fly ashes, including teratogenic elements such as sele-nium, chromium, cadmium and copper (Hopkins et al., 2000). Since,arsenic is considered as a primary pollutant in drinking water due toits high toxicity (Zouboulis and Katsoyiannis, 2005). So, its efficient
removal from natural contaminated water resources intended fordrinking water is considered of great importance (Tables 7 and 8).

V.C. Pandey et al. / Resources, Conservation and Recycling 55 (2011) 819–835 829

Table 7Maximum permissible limit of arsenic in drinking water and industrial effluent/leachates/land fill drainage (IS, 1983, 1974; Robinson et al., 2003; DWT, 2001; CEC, 1991).

Metal (mg/l) Drinking water (IS = 10,500) Drinking Water (WHO) Industrial effluents (IS = 2,490) Leachates Land fill drainage

Arsenic 0.05 0.01 0.2 0.01 0.2–1.0

All the standards are meant for field conditions.

Table 8Arsenic accumulation in edible crops/food of India.

Edible crops/food State Total As (mg kg−1 dw) References

Rice West Bengal – Roychowdhury et al. (2002)Wheat West Bengal 0.74 Norra et al. (2005)

l

5

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for wood preservation is still common (Nico et al., 2004). Arsenic,potentially toxic heavy metal, is widely distributed in nature and

Wheat flour, turmeric power, beans and green chili West Benga

. Analytical points

Limit of detection (LOD) for As in Atomic Absorption Spec-rophotometer (AAS) is <0.001 mg l−1. Arsenic detection limity atomic emission spectroscopy (Huggins, 2002) has beenenerally satisfactory for determination of the near-Clarke con-entration of As. Though, As can be partly depleted fromoal by high-temperature ashing, especially from high-sulfuroals. In the boiler deposits, boron arsenate has been found,ecause of evaporation of B and As (Swaine and Taylor,970).

Formation of gaseous As compounds can depends on the pres-nce of chlorine in coal. On the basis of experimental data andalculation, arsenic escapes to the gas phase in the form of AsOnd As2O3 during low-temperature combustion (1000–1200 ◦C),ut at high temperature (1200–1600 ◦C), only the As2O3 species

s released (Shpirt et al., 1990). Arsenic yield in gaseous phase fromigh-temperature zone of a furnace is controlled by the furnaceonstruction at slage-removal coefficient (Ks) where Ks is a ratio:lag yield/grass ash content. In contrast, presence of illite orCaCO3upports capturing of As because of the formation some refrac-ory arsenates KAsO4 and Ca3 (AsO4)2 with melting temperaturesf 1310 and 1455 ◦C, respectively. On the basis of assumption, thextent of the As volatility from coal is controlled by mineralogi-al residence of As. Therefore, by the stepped ignition of the coalsSouth Siberia) at temperatures in the range 400–800 ◦C, As wasully evaporated from low-sulfur coals containing the organic formf As in the dominant form. From medium-sulfur coals (whereoth organic form of As and pyritic form of As (arsenopyrite) areresent), 67% As was evaporated, and from coaly argillite only 31%Kryukova et al., 1985). The sorption of As in the FA is controlled byhe combustion rate. If particulate coal is very rapidly fired, kinet-cs limitations on the As sorption exist, and As capture appears ashysical-sorption, with strong relation to particulate size (the finerhe FA particles, the higher the As concentration). If the coal fires

ore slowly, the equilibrium thermodynamics shows a chemical-orption process (and there is no strong relation of As content in FAo the fraction size).

Direct observation of As enrichment on surfaces of FA parti-les is analytically challenging due to the usually low 10–100 ppmoncentration of As and the small particle size. For the sub-ituminous coals, up to 77% of the original As was volatilizeduring semicoking. During ashing, the lower the As and S con-ents, the more As was evaporated. It is important that inoaly argillite with 17 ppm As and more strongly bound: only2.1% As is evaporated at 800 ◦C. These data indicates that anrganic form of As is easily volatilized and probably pyritic
orm of As is more difficult to evaporate (Khankhareev et al.,002).
0.080–0.335 Roychowdhury et al. (2002)

6. History and chemistry of arsenic

Arsenic, first isolated by Albert Magnus in Europe in the year1250, is a metalloid and a member of the nitrogen family. Arsenichas a long history of use in human medicine (Ferguson and Gavis,1972) and over the last 100 years, its use has been documentedin animal feed (to promote growth). Arsenic falls into the peri-odic table of the elements in Group 15 (V) along with nitrogenand phosphorous. It is considered a metalloid and as such has bothmetallic and non-metallic properties. It is a brittle, grey metal inits pure form, but in nature is most often found with other met-als, such as iron, copper, silver and nickel and in combination withoxygen and sulfur. Its atomic number is 33 and its atomic weightis 74.9, placing it as heavier than iron, nickel and manganese butlighter than silver, lead or gold. It has four common redox states,−3, 0, +3 and +5, but the most inorganic forms are the +3 and+5 state when in combination with oxygen as an arsenite or anarsenate. Arsenic is the 20th most common element in the Earth’scrust, 14th in the sea and 12th in the human body. Arsenic isfound naturally in over 150 minerals, one of which is arsenopy-rite, which includes both iron and sulfur; another is realgar(AsS).

7. Sources of arsenic contamination

There are several ways of As contamination in the bio-sphere. Geochemical weathering of rocks, As-bearing mineralsand geothermic sources (e.g. volcanic emissions) and this largelydepends upon microbial induced reduction, oxidation and methy-lation, which contribute to As mobilization in the biosphere (Islamet al., 2004).

In addition anthropogenic activities that result in the local con-tamination of nearby air, soil, sediment and aquatic systems withAs include FA disposal, long-term mining and the associated smelt-ing of the sulfide ores, coal combustion, runoff from mine tailings,pigment production for paints, hide tanning waste, agriculturalpesticides (as As2O3) and herbicides application and poultry feed-lot supplements (Smedley and Kinniburgh, 2002). Moreover, Asis used in the production of semi-conductors, lead-acid batter-ies, the glass industry and the copper refining industry and inthe hardening of metal alloys (Hathaway et al., 1991). While theuse of arsenic-containing products such as pesticides and herbi-cides has decreased significantly in the last few decades, their use

commonly occurs in a variety of natural metal-bearing sulfides(Nriagu, 2002).

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in foliar parts and was not detected in seeds and below groundparts. The order of metal accumulation in selected rice varieties


. Arsenic toxicity due to fly ash

Currently, the toxic effects of As have become a great criticalssue in both developed and developing countries. Arsenic toxicityas become an Indian concern owing to the ever-increasing con-amination of water, soil and crops in many regions of the India. Itas reported in Purulia district of West Bangal, India that a largeumber of people in the area are victims of lung infections andkin disease, caused by FA contamination of air and water. Theffect of the ash was found on local animals and vegetation, too.he cattle feeding on contaminated vegetation are victims of skiniseases and dental disorders. The population of birds and waternimals is also decreasing. Love et al. (2009) showed that higheroliar levels (2–8 fold) of As is present in Cassia occidentalis plantshich was grown on the weathered FA and also reported that ele-

ated levels of DNA damage in leaf tissues of C. occidentalis growingild on FA basin compared to growing to soil. Arsenic is highly

oxic to all forms of its life. Arsenic exists naturally in both organicnd inorganic forms in the environment. The two major forms ofnorganic arsenic are the reduced form, arsenite (As3+) and thexidized form, arsenate (As5+). It occurs predominantly in inor-anic forms. Arsenite (As3+) is known to be more toxic and 25–60imes more mobile than arsenate (As5+) (Dutre and Vanecasteele,995; Pantsar-Kallio and Manninen, 1997). Monomethylarseniccid (MMA) and dimethylarsenic acid (DMA) are the most com-on organic species in the soil, but their natural presence is low

ompared to inorganic As. So, Arsenic speciation in the environ-ent is of significant importance because organic and inorganic

ompounds differ mainly in their toxicity. Arsenic toxicity is alsoelated to the rate that it is metabolized from the body and theegree to which it accumulates in the tissues. Generally As toxic-

ty pattern is AsH3 > As3+ > As5+ > RAs-X. Hence, according to rule,norganic arsenicals are more toxic than organic arsenicals andhe trivalent oxidation state is more toxic than the pentavalentxidation state. Water supplies and soils contaminated with Asre the major sources of drinking water and food-chain con-amination in numerous countries. Especially As2O3 are highlyoxic components of coal, As is both a carcinogen and mutagennd leads to dangerous dermatological, respiratory and diges-ive system diseases. In spite of these, Chronic As poisoning canause serious health effects including cancers, melanosis (hyper-igmentation or dark spots and hypopigmentation or white spots),yperkeratosis (hardened skin), restrictive lung disease, peripheralascular disease (blackfoot disease), gangrene, diabetes melli-us, hypertension, and ischaemic heart disease (Guha-Mazumdert al., 2000; Morales et al., 2000; Rahman, 2002; Srivastava et al.,001).

USPHS (2000) reported that most As compounds can readily dis-olve in water, and so As can enter water bodies such as rivers,akes, pond and by surface runoff. Arsenic is toxic to many plants,nimals and humans, though lethal doses in animals are somewhatigher than the estimated lethal dose in humans (Kaise et al., 1985;SPHS, 2000). Arsenic exposure may not only affect and disablergans of human bodies, especially the skin, but may also interfereith proper functioning of the immune system (Duker et al., 2005).

norganic As has been reported as lethal consequences in mostf the cases. Its long term exposures even in low concentrationsay cause damage to the blood circulatory systems and may cause

njury to the nervous system and other vital organs (USPHS, 2000).ts great risk is concerned with carcinogenicity due to consump-ion for a long duration. The US Department of Health and Humanervices in its 9th Report on Carcinogens lists arsenic compoundss known to be human carcinogens. Skin cancer is the prevalentorm resulting from exposure, though there is also evidence for an
ncreased risk of internal cancers, including liver cancer (USPHS,001; USEPA, 2002b).
n and Recycling 55 (2011) 819–835

9. Arsenic in rice agriculture

About 23% of the country’s total irrigated area being used forrice production. Rice is the major crop in the irrigated areas ofIndia. Nearly 42% of the total food grain production is rice in India.West Bengal is one of the major rice producing states in India andcovers 5,900,000 ha. The Residents of many arsenic-contaminatedrural villages of west Bengal depend mainly on rice for their caloricintake, about 70% of total (Bae et al., 2002; Meharg, 2004). The sub-stantial varietals differences are found with respect to As uptake,accumulation and speciation in rice plants. According to marketsurvey (Williams et al., 2005), Indian basmati rice possessed thelowest mean As level (0.05 �g g−1) in grains, the American long-grain rice had the highest concentration (0.26 �g g−1), whereas theBangladeshi and European cultivars showed intermediate values(0.13 �g g−1 and 0.18 �g g−1, respectively). American long-grainrice contained As mainly in the organic form, whereas inorganicAs formed the higher proportion in all other cultivars. The differ-ences in As accumulation also depended on the growing seasonand agronomic practices (Abedin et al., 2002). Arsenic concentra-tion in that rice which is cultivated in arsenic contaminated soilsunder anaerobic conditions (at which arsenic is highly available forplants uptake) and irrigated with arsenic contaminated water, issupposed to be high compared to other crops and regions (Meharg,2004; Abedin et al., 2002). Due to high As-concentration in paddyrice, it has been considered as an important source of As intake. Atthe time of rice cooking, it can also be increased if As polluted wateris being used. Arsenic amount in cooked rice could be increased bychelation to the rice starch and/or bran (Bae et al., 2002). Abedinet al. (2002) also reported that high As concentration in straw mayhave the potential for adverse health effects on the cattle and anincrease of As exposure in humans via the plant-animal-humanpathway. According to World Health Organization (WHO), the per-missible limit for rice and foodstuff is 1.0 mg kg−1 and 2 mg kg−1,respectively (Rohman et al., 2007). It is reported that 40% of arsenicin human body comes from food chain (BIAM, 2002).

A study was carried out to evaluate the growth performance,elemental composition (Fe, Si, Zn, Mn, Cu, Ni, Cd and As) and yieldof the rice plants (Oryza sativa L. cv. Saryu-52) cultivated underdifferent doses of FA mixed with garden soil (GS) in combinationwith nitrogen fertilizer (NF) and blue green algae biofertilizer (BGA)(Tripathi et al., 2008). Significant enhancement of growth wasobserved in the plants growing on amended soils as compared toGS and best response was obtained in amendment of FA + NF + BGA.Accumulation of Si, Fe, Zn and Mn was higher than Cu, Cd, As and Ni.Arsenic accumulation was detected only in FA and various amend-ments. Inoculation of BGA caused slight reduction in Cd, Ni andAs content of plants as compared to NF amendment. The high lev-els of stress inducible non-protein thiols (NP-SH) and cysteine inFA were decreased by application of NF and BGA indicating stressamelioration. This study demonstrated that the integrated use ofFA, BGA and NF for improved growth, yield and mineral composi-tion of the rice plants besides reducing the high demand of nitrogenfertilizers.

Dwivedi et al. (2007) demonstrated that the three rice culti-vars viz., Saryu-52, Sabha-5204, and Pant-4 cultivated in gardensoil (GS, control) and amendments (10–100%) of FA for a period of90 days. They observed the effect of above treatments on growthand productivity of plants as well as metal accumulation. The tox-icity effect of FA at higher concentration (≥50%) was reflected bythe reduction in various parameters such as protein, growth andphotosynthetic pigments. Arsenic accumulation was found only

was Fe > Si > Mn > Zn > Ni > Cu > Cd > As. Further, they demonstratedthat the rice cultivars Saryu-52 and Sabha-5204 were more tolerant

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nd showed enhanced growth and yield in lower FA applicationoses as compared to Pant-4. Table 8 shows the concentration ofs accumulation in various edible crops/foods of India.

0. Future prospective and recommendations

In Indian scenario, coal is the major natural resources avail-ble in abundance and hence coal based electricity generationbviously will increase in the present and future to meet theemand due to the growing industrialization and urbanization.onsequently, there is a production of enormous quantity of FA fol-

owed by emission of green house gases and intrude significantlyor global warming as well as toxic metal pollution. Bhattacharjeend Kandpal (2002) presented model estimations for projected FAonsumption for 2009–10 mainly in three areas: cement produc-ion, road embankments and brick making. Agricultural utilization

ay offer a partial solution to disposal problems, but the benefitsnd risks associated with using these materials must be assessedPandey et al., 2009a). But in future forestry will attract more FAtilization for growing economically and socio-ecologically impor-ant trees due to the presence of macro- and micro-nutrients inA and having ability to improve physico-chemical and biologicalroperties of the problem soils (Pandey and Singh, 2010). So, usingA for biomass production (Pandey et al., 2009b) will be one of themportant strategies to protect environmental degradation as wells economical importance.

1. Discussion and conclusions

There is not much study about the fate of As hazards in coal FAnd this review is the first attempt on mapping As flows in environ-ent due to FA. In recent years, there is debate about increased As

missions from point sources and diffuse emission of As is alarmingn India. Coal combustion by-products mainly FA is an undesirable

aste whose interaction with air, soil and water has created effectsn human health, agriculture and natural ecosystems. Hence FAafe disposal and utilization is an important concern to safeguardhe cleaner environment. In current years about 38.4% of Indian FAas put to use mainly in civil construction, building materials and

ome in agro-forestry, the presence of toxic metals mostly As in FAas become a subject of much concern (contamination of soil, sur-

ace and ground water bodies), inhibiting more complete utilizationn a sustainable basis (Ram et al., 2007).

Many pilot projects were undertaken in recent years to demon-trate the bulk utilization of FA specifically for Indian perspectives.lso, it has been successfully demonstrated that FA can be utilized

n major construction projects such as dams, ash dyke, landfills,oads and pavements, soil stabilization and for other purposes suchs brick manufacture, cement industry, tiles and paint industry.ealizing huge amount of FA generation and its very low utilization,he Indian Government set up the FA mission under the Depart-

ent of Science and Technology, New Delhi for coordinating alluch efforts. Regulatory law has been enacted in 1999, which spells00% utilization of FA within a stipulated period and making itandatory to use FA for the purpose of road construction, bricks,

tc. within a radius of 50 km from coal-based thermal power plants.here are several reasons for low FA utilization in Indian scenariouch as easy availability of topsoil and which is cheaper than FA foranufacturing bricks, lack of FA promotion, higher costs, dry FA

s not available always, communication gap between FA users andhe management of power plants.

The Indian agencies which have also contributed significantlyowards adoption of new technologies for safe disposal and gain-ul utilization of FA include National Thermal Power Corporation;hmadabad; State electricity Boards (TNEB, PSEB, RSEB, MSEB,

n and Recycling 55 (2011) 819–835 831

APSEB, etc.); Ministry of Environment and Forests; Ministry ofPower; Ministry of Laboratories; IITs and Engineering Institute, etc.Instead of these, Public Work Departments (PWDs), local devel-opment and housing authorities as well as National HighwayAuthority of India is also directed to prescribe use of FA and FAbased products in their respective schedules of specifications andcodes of practice, etc.

India is a developing country and stands 2nd position in theworld’s population and for which it needs more and more energy forindustrialization and urbanization. So, huge FA production in ther-mal power plants is a result of coal combustion for more energyrequirements. It is a challenging issue for Indian people to gen-erate low cost-based energy while promoting the biosphere byminimizing the FA production and also increasing its utilization.Cost-effective technologies should be applied for FA utilization inIndian scenario also.

In India, one of main problems related to FA disposal is prob-ably due to the presence of heavy metals in the residue. Afterthe coal burnings the toxic pollutants mainly As is released intothe atmosphere, which consequently leach out and contaminatesoils, as well as surface water and groundwater (Baba et al., 2010).The As chemistry in the contaminate soils is complex and itstoxicological effects depend on its chemical forms, exposure andbioavailability (Turpeinen et al., 2003). Arsenic can enter the bodythrough the lungs by inhalation, though the skin by direct contact,and through contaminated foods. Ingestion is the most commonnon-occupational exposure. The health effects of As vary usuallydepending on the chemical form of the arsenic, the concentration,the route of exposure, and the duration of exposure. Some As com-pounds present in soil leachate are highly toxic to various organ-isms (Langdon et al., 2001), including humans (Datta and Sarkar,2005). It has been reported that elevated DNA damage are expectedin workers involved in dealing with the FA (Chen et al., 2010).Industrial exposure may appear to be affect the persons workingin industries related to coal consumption are at increased risk ofAs exposure due to FA. Contaminated drinking water resources byAs can increases the risk of various health problems. People livingin the areas with high As content (e.g., emissions from industrialplants) are also at increased risk of As exposure. Therefore, whenthe soil toxicity due to FA is characterised, the remediation of Ascontaminated sites is needed to minimize the As labile pool inexcess, its transfer through the food chain and subsequent risk(s)to plants, animals and humans (Coeurdassier et al., 2010).

The present study shows many potential environmental hazardsrelated to FA generation of coal combustion. Hence, proper mea-sures should be taken to check the release of As and other toxicmetals from the FA pond and subsequent mixing with the ground-water. Using good quality coal and temperature control during coalcombustion is a key factor for controlling arsenic release in theenvironment. Underground lining in the FA ponds to prevent directcontact of the ash pile with the top soil and the local drainage bod-ies is a probable remedy. Green technology should also be adoptedfor minimizing As and other toxic heavy metals pollution due tothe FA disposal around thermal power plants. Re-vegetation of FAdykes, basins, lagoons and landfills serves a variety of function likestabilizing the ash against wind and transforming the area intoaesthetically pleasing landscapes. Use of lime (calcium oxide) asa flue gas scrubbing agent due to its high sorption capacity for Asand other toxic elements vaporized during coal combustion shouldbe applied in coal-based thermal power plants. For the removalof As from flue gases, activated carbon was shown to be effec-tive (trapping As2O3) at temperatures around 200 ◦C (Wouterlood
and Bowling, 1979a,b; Lopez-Anton et al., 2007). Using lime (CaO)for this purpose was found to be most effective at 500–600 ◦C,with little influence of acidic gases such as HCl and SO2 (Jadhavand Fan, 2001). Technology up-gradation is very necessary for

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ecreasing power plants pollution such as clean coal technologynd refurbishment of existing thermal power plants. An inte-rated, i.e. Organic/Biotechnological/Phyto remediative approachor As pollution due to FA need to be applied and application ofhese tools should be made mandatory to conserve or reclaim thenvironment. Further, arsenic risk related to FA can be minimizey creating awareness among the susceptible populations due tohe involvement of governmental and non-governmental organi-ations (NGOs), related industries and others relevant correctiveeasures. In future, this can be avoided through the termination

f coal combustion and the implementation of non-polluting tech-ologies such as solar, wind and atomic power generation.

cknowledgments

Vimal Chandra Pandey is thankful to University Grants Com-ission (UGC), Govt. of India for providing UGC-Dr. D.S. Kothari

ost Doctoral fellowship. Authors are thankful to Vice Chancellor,abasaheb Bhimrao Ambedkar University (Central) for providing

acilities.

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Arsenic Hazards in Coal Fly Ash and Its Fate in Indian Scenario