Bauer1981 Componentes Carbonatados en Ceniza Volante

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Identification and Quantitation of Carbonate Compounds in Coal

Fly Ash

Christopher F. Bauert and David F

S.

Natusch'

Department of Chemistry, Colorado State Un iversity, Fort Collins, Colorado 80523

rn

Alkali, alkaline earth, an d ferrous carbonates have been

identified a nd q uantitatively d etermined in several coal fly

ashes at total carbonate concentrations of 0.058-0.86%.

Th erm al evolved gas analysis for Con, a selective acid-leaching

proced ure for carbonates, and meta l analyses of th e leachates

provide da ta for a mass balance between metals and carbon-

ate. I n some cases the leachable metal comes predominantly

from a ca rbonate species. High relative hu midity promoted

COz absorp tion by fly ash regardless of th e COz partial pre s-

sure. Although this study used ash removed by electrostatic

precipitators, flue-gas characteristics ar e likely t o promo te

carbonate formation on th e ash in the plume.

The projected continued use of coal for power generation

reaffirms the need to stud y particulate emissions from coal

combustion, t o assess and, if possible, to m inimize adverse

environm ental imp acts. Most stud ies ,have focused on th e

behavior of potentially toxic metals

1-41,

organic compounds

5 , 6 ) ,

nd sulfur gases (7) with little information being gen-

erated on the inorganic compounds present. Techniques

giving compound-specific information have been ham pered

by inade quate detection limits. Only major species have been

unequivocally identified, primarily by X-r ay powder dif-

fraction

(8-10):

quartz, mullite (3A1~03.2Si02),hematite,

magnetite , h e , nd anhydrite (CaS0.d. X-ray photoelectron

spectrometry (X PS ) has shown tha t binding energies for el-

eme nts in fly ash are consistent with oxides for Al, Si, and Fe

and with Cas 04 for Ca and S,but oth er compounds may have

th e same binding energies 11,12).Evidence for sulfates of

Al, Ca, and Fe has been provided by Fourier transform in-

frared spectroscopy 13 ) . Differential thermal analysis has

been ap plied widely t he m inerals in coal 14 )and to fly ash

9),

bu t t o our knowledge in only one instance were mixed

carbonates of Ca, Mg, and F e found (15).

Knowledge of the chemical forms of metals in fly ash is

highly desirable in order t predict t he m obility of the metal.

Mobility has implications for inhala tion toxicology and fly-ash

disposal because both require information on solubility. In

addition , utilization of ash

as

a raw material may benefit from

more com plete information on ash chemistry.

Th e present stu dy demonstrates that , in fact , carbonate

compounds occur in fly ashes, th at th e concentrations of in-

dividual comp ounds are trace level, and that the m etals bound

as carbon ates can be a significant fraction of the totalmobile

metal.

t

Present address: Water Chemistry Laboratory, University

of

Wisconsin, Madison, WI 53706.

Experimental Section

Thermal Evolved as Analysis. Because carbonate

minerals decompose with t he release of C02 a t characteristic

temperatures

16),

volved gas analysis (EG A) with carbon-

specific detection was applied. The instrumen t is illustrated

in Figure 1.Th e furnace (L eco induction furnace, Model 521)

was temperature-programmed from room temp erature t 1200

OC a t -190 OC/min by means of a moto r-driven variable

transformer (I7). Calibration of the tem perature scale was

perform ed by using th e melting points of several pure solids

a t th e stat ed programming rate. Ca. 100 mg of fly ash was

spread in a th in layer (


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T M O l O

C A V I T Y

IH

Flgure 1.Thermal evolved gas analysis instrument.

DROPPING FUNNEL

h

C A V l T Y

9

H

U

Figure

2.

Acid evolved gas analysis system.

Leaching Procedure.

By controlling th e carbonate equi-

librium w ith the am ount of acid added, it is possible

to

dissolve

selectively the carbonate compounds a s a group. For a given

am oun t of fly ash, we needed to determine the stoichiometric

number

of

moles of acid to add and the duration of mixing

required to assure complete dissolution. This was done easily

with the acid EGA apparatus (Figure

2).

For 100 mg of fly ash (C orrette) the to tal amo unt of C02

released by

25

mL of acid was determined for several con-

centrations of acid a t a constan t mixing time of 10 min. If th e

acid streng th is sufficient, the carbonate equilibrium will shift

completely to gaseous COz. Th e CO z-release peak sha pe will

reflect only mass-transfer effects and will be independen t of

acid concentration. If,however, acid strength is too small, only

part of the total carbonate will be present as

COz,

the rest

existing as bicarbon ate or carbon ate ions. Eventually , as COz

is purged, th e solution equilibrium will shift, releasing more

COz, unt il equ ilibrium with t he COz level in helium is reached.

T h e net effect is peak broadening and delay. One molar acid

was chosen to assu re complete dissolution.

Duration of mixing was addressed by mixing 25

mL

f

1

M

HC1 with

100

mg of fly ash (C orre tte) for variable lengths of

time before sweeping th e C0 2 out of the reaction cell. If dis-

solution is complete, the peak width a nd th e time at peak

maximum will be independent of mixing duration; for in-

complete dissolutiofi the time a t peak maximum and peak

width will increase with shorter mixing times because the COB

generation

is

incomplete. Kinetic effects were absent after 10

min of mixing.

Metal Determinations.Iron was determined by atomic

absorption spectrometry with background correction (Varian,

Model 1250),

Li

and R b were determined by flame emission

(Varian, Typ e AA-5), and t he rest of the metals were deter-

mined by argon dc plasma emission using the Spectraspan I11

Echelle spectrom eter (Sp ectraMetrics, Inc.). Th e first three

elements were assayed by the met hod of stan dard addition s.

Since the S pectraspan gives direct concentration readout, the

results from i t were verified by stan dard a ddition s. All analyte

solutions contained 1000 ppm Cs as an ionization suppres-

sant.

All thermal EGAs, acid EGAs, and leaching procedures

were performed on duplicate samples of four

fly

ashes:

Crawford and IGS from Illinois, Niagara from New York, and

Corrette from Montana. In addition, IGS was separated into

“magnetic” and “no nmagn etic” fractions by using a magnet.

Th e ashes were obtained from hoppers beneath th e electro-

static p recipitators and were sieved manually (Buckbee-Mears

Co. sieves) to ob tain the

2

--

C ORR

E

T T

E

CRAWFORD

25

600

1200 25 600 1200

Figure

3.

Thermal evolved gas analyses

for

carbon

in

fly

ashes.

TEMPERATURE ( ’ C )

784

Environmental Science

&

Technology


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Table 1 Comparison of Total Carbonate by Thermal

and Acid Evolved Gas Analysis

c%*-,'

ppm

dgnlflcant

f ly

a&

thermal acld difference?

Crawford

230 0 (26) 4100 (49) no

Niagara 1700 (32) 580 (39) no

Corrette

290 0 (22) 2400 (40) no

IGS 5200 (27)

8600 (52) no

Percent relative average deviations

are

given

in

parentheses.

calibration of the sensitivity of response has been minimal,

th at only duplicate samp les of each ash were analyzed, and

th at there may be considerable variability between the as h

aliquots. For these reasons, the numerical results must be

considered only semiquantitative. This interpretation is re-

flected by the large relative deviations associated with the

measured values. Consequ ently, th e differences between th e

two EGA methods are no t significant. This indicates th at bo th

yield reasonable estim ates of carbon ate content .

Identification of the compounds contributing

to

each of the

peaks in Figure

3

was obtained by running therm al EGA s of

the carbonates of Fe, Li, Na, K , Rb , Mg, Ca,

Sr,

and Ba, some

of which are show n in Figure

4.

Th e decomposition tem pera-

tures

of

th e pure compounds were consistent with literature

values allowing for use of a faster hea ting rat e 16 ,24 )herein,

which shifts obseqved peak tem peratu res upward . All of the

major and minor elements present in fly ash tha t form stable

carbonates are included in this se t of standards. T he identi-

fications are in Tabl e

11.

Bicarbona tes of Na an d K were also

considered; however, since these compounds were found to

release COz by decomposition to th e carbonates below

-350

C, their contribution to the fly-ash EGA profiles was con-

sidered negligible, except perha ps for Crawford. Th is possi-

25

100 400

8 0 0

1200

Figure 4.

Thermal evolved gas ana lyses of alkaline earth carbon-

ates.

T E M P E R A T U R E r c )

bility is acknowledged in Tab le I1 bu t is not included in the

mass balance.

For m ost bases several compounds may be contributo rs to

each peak in t he EGA profile of th e fly ashes. Also listed ar e

the amounts of carbonate represented by each peak. Some

carbonates may, in fact, exist as mixed-metal mineral entities

or alkaline ear th hydroxy carbonates. For most of these min-

erals, th e

COS

elease temperatures are the sam e as those for

the single-metal species within the t emp erature resolution of

this instrum ent (see dolomite in Figure

4).

As a result, al-

though specific mineral forms cannot be identified from t he

COz evolution profiles, a given m etal may be associated w ith

a specific COz peak, allowing a mass balance t o be perform ed.

For example, a COZ

peak

around 8 OC could indicate calcite,

dolomite, or ankerite, but in any case it does indicate an as-

sociation between Ca a nd CO32-. H enceforth, reference to a

particular metal carbonate refers to an association an d no t

necessarily to a particular crystalline form. The only major

mineral for which this app roach may fail is anke rite because

its Fe com ponent decomposes

-150

OC higher than in pure

FeC03.

Th e leachate analyses a re presented in T able 111. For each

element, the stoichiometric amo unt of carbonate th at

it

could

bind was calculated and compared with th e C0 32- present

(Table

11)

in the appro priate EG A peak. If leaching removed

carbonates only, the sum of the contributions to each peak

calculated from the metal determination s would equal th e

carbonate determined by EGA in t ha t peak. In some cases,

however, species in addi tion to carbon ates a re dissolved. Al-

though t he m ass balance is imperfect, limits can be placed on

Ta b 6 I I. Concentrat ions

of

Carbonate on

Fly

Ashes

and Possible Counterions

fly ash

Crawford

Niagara

Cbrrette

IGS

IGS,

Mag

IGS, Nonmag

reglon

total

1

2

3

4

total

1

2

total

1

2

3

4

total

1

2

4

total

1

2

3

4

total

1

2

3

4

cq2- a ppm

2300 (26)

18 (26)

1400 (26)

620 (26)

250 (25)

1700 (32)

420 (26)

290 (35)

1000 (34)

2900 (22)

240 (23)

1300 (22)

540 (23)

810 (24)

5200 (27)

230 (25)

2300 (26)

720 (29)

1900 (27)

5300 (22)

290 (22)

2600 (22)

860 (22)

1500 (24)

3600 (22)

210 (23)

550 (23)

890 (24)

1900 (22)

p o d b l e

cou l

e on8

NaH,

KH

Mg,

Ca, Fe

Li

Na,

Rb,

Sr

K, Ba

Fe, Mg

Ca

Li

Na, K, Rb, Sr, Ba

Fe

Mg, Ca, Li

Na, K ,

Rb,

Sr

Ba

Fe

Mg,

Ca,

Sr,

Li

Na,

K, Rb,

Ba

7

Fe

Mg, Ca, Sr, Li

Na, K.

Rb,

Ba

7

Fe

Mg, Ca,

Li

Sr,

Ba, Na, K ,

R b

7

Percent relative average

deviations

are

given

in

parentheses.

Volume

15,

Number

7,

July

1981

785


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the possible contributio ns from each metal (Tab le IV). An

exam ple of th e calculation is instructive.

Consider peak 3 of Corret te fly ash (Tab le 11), which con-

tains contributions from Na, K, Rb, and Sr.Calculated from

th e leachable metals in Ta ble 111, th e stoichiometric amoun ts

of carbon ate tha t could be bound are 73,140,


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a t less than 1 .n mo st cases, th e carbonate is a significant

fraction of the total leachable metal and for Na, K , and Ba

may even rep rese nt all of th e leachable metal.

To determ ine the conditions under which fly ash would

ads orb COz, we decomposed all of the carb onates in a batc h

of IGS ash by heating. Th e ash, which had p artly fused, was

ground to a fine powder in a boron carbide mortar and pestle,

and portions were stored under various conditions. Afterward,

thermal EGAs were run to determine how much C02 had been

readsorbed. Tab le VI lists the exposure conditions and car-

bona te found. Th e capped bottle, representing dry exposure

conditions, showed relatively little C02 uptake, even over a

long period of time. Humid air, regardless of C 0 2 content,

prom oted significant absorption of C02 after jus t

1

week.

Apparently, humid atmospheres promote C02 adsorption.

A further experiment supports this observation. IGS fly ash

was leached with

1M

HCl, which has been shown to remove

all carbonates, in air and inside a glovebox containing argon.

A titrimetric method involving aqueous Ba(0 H) z absorption

(27)

demonstrated that the COz concentration was -0.5-1

pp m on a volume basis. Air contains -350 ppm . Th e ash was

recovered by filtration and dried, and a thermal EGA was run.

Carrying out the leaching procedure in air yielded 0.73%

Cos2-; performed under low-CO2 conditions,

0.5

C032 - was

found.

It is

apparent tha t moisture facilitates the absorption of C02

by fly ash despite low parti al pressures of COB.This obser-

vation is consistent with studies of CaO and MgO, which in-

dicate that limited chemisorption of COa occurs under dry

conditi ons below 400OC 28,29). oisture, however, catalyzes

the absorption even a t relative humidities as low as

15

(28).

D i s c u s s i o n

Th e occurrence of carbonates in fly ash is not surprising;

however, it is importan t t o consider the significance of this

observation. Since the ashes studied were obtained from

electrostatic precipitator hopp ers, two questions arise:

(1)

Are

the carbonates formed in the flue gas or in the hopper?

(2)

Can

the analytical results for hopper ash be extrapolated to th e ash

which reaches the atmospheric environment as pa rt of the

plume? To answer these, the conditions within the power

pla nt need to be considered.

If

one assumes the boiler to be fired with -20 excess air

30),

he volume percen t of C02 in th e flue gas will be 16% ,and

th at of water vapor 7-10% (fo r air of 0-90 relative humidity).

Ratios of

H

to C in coal were used to calculate th e lower value

31).Th e fly ash leaving the boiler will be devoid of carbon ates

since the tem perature and th e particle size opmineral car-

bonates in coal allow very rapid decomposition

10,32).

We

will use CaO

as

a model compound. Above -400 “C and below

the decomposition temperature of CaC03, COz can be ad-

sorbed rapidly a nd extensively by CaO

(28)

even though th e

relative humidity is much less than

1 .

Much of the fly ash

is removed in th e electrostatic precipitator where tempera -

tures are -200 OC 33)and the relative humidity is


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(20) Beenakker, C. I. M. Spectrochirn. Acta, Part B , 1977, 32,

173-87.

(21) I. G. Farbenind. A&. Belgian Paten t 450 648, Jun e 1943; Chern.

Abstr. 1947,41, P7063d.

(22) Fisher, G. L.; Prentice, B. A.; Silberman, D.; Ondov, J. M.;

Bierm ann, A. H.; Ragaini, R. C.; McFarland, A. R. Enuiron. Sci.

Technol. 1978,12,447-51.

(23) McCrone, Walter C.; Delly, John G. “T he Particle Atlas”, 2nd

ed.; Ann Arbor Science Publishers: Ann A rbor, MI, 1973; Vol. 2, pp

(24) Duval. C. “Inoreanic Thermoeravimetric Analvsis”: Elsevier:

546-9.

-

“ ,

New York, 1963.

(25) Habashi. F . “Pr in ci de s of Extractive Metallurev”: Gordon

”

Breach: New York, 1964; p 95.

(26) Gibson, Everett K., Jr.; Johnson, Suzanne M. Thermochirn. Acta

(27) Snell, F. D., Ettre, L. S., Eds. “Encyclopedia of Industrial

Chem ical Analysis”; Interscience: New York, 1969 ; Vol. 8, p 261.

1972,4,49-56.

(28) Boynton, Robert S. “Chemistry an d Technology of Lime and

Lim estone”; Interscience: New York, 1966; p 191.

(29) Gregg, S. J.; Ramsay, J. D. J . Chern. SO C. 19 70 ,Il , 2784-7.

(30) Sherm an, R. A,; Landry, B. A. In “Chemistry

of

Coal Utilization.

Supp lem ent”; Lowry, H. H., E d.; Wiley: New Y ork, 1963; p 802.

(31) Gluskoter, H. J.;Ruch, R . R.; M iller, W. G.; Cahill, R. A.; Dreh er,

G. B.; Kuhn, J. K. Ill . St at e Geol. Suru. Circ. 1977,

No.

499.

(32) Gallagher, P.

K.;

Johnson , D. W., Jr. Therrnochirn. Acta 1973,

6 ,67 .

(33) Ray,

S. S.;

Parker, F. G. “Characterization of Ash from Coal-

Fired Power Plants”: U.S. Environmental Protection Amncv.

_ .

EPA-600/7-77-010,1977; p 87.

College Park , M D, 1976.

(34) Small, John A. Ph.D. Dissertation, University of Maryland,

(35) Gillott, J. E. J . Appl. Chern. 1967,17,185-9.

Received for review April 3,1979. Accepted M arch 2,19 81,

Determination

of

the Biodegradability

of

Organic Compounds

Dickson Liu,* Wil l iam M J. Strachan, Karen Thomson, and Kazim iera Kwasniews ka

Environmental Contaminants Division, N ational Water Res earch Institute, Canada Centre for Inland Waters, Burlington, Ontario,

Canada L7R

4A6

w

A standard procedure and a new app aratus for determining

the relative biodegradability of both water-soluble and -in-

soluble organic compoun ds under various laboratory envi-

ronmental conditions has been developed. The degradation

test system is based on the me asurement of the primary bio-

degradation rates of such substances in cyclone fermentors

under aerobic and anaerobic conditions, with and without

cometabolites. Th e degradation is accomplished with a mix-

tur e of microorganisms from activ ated sludge, soil, an d sedi-

ments, and abiotic processes are corrected for by the use of

controls. Fenitrothion

O,O-dimethyl-O- 3-methyl-4-

nitrophenyl) phosphorothioate) and 2,4-D ((2,4-dichloro-

phenoxy)acetic acid) were tested in the system. The former

was found to be more readily degraded under anaerobic

cometabolism co nditions

Tl/2

= 1.0 day) than when the sam e

system was aerobic T1/2= 5.5 day s); when fenitro thion was

th e sole carbon source (me tabolism ), its stability was greater

still, especially under aerobic conditions. The herbicide 2,4-D

was easily degraded und er aerobic conditions T1/2=

1.8

and

3.1 days for cometabolism and metabolism, respectively);

however, under anaerobic conditions, its degradation rate was

greatly decreased

Tl/z

=

69

and 135 days, respectively) to th e

point where the rates were comparable to those of abiotic

processes. Th e precision of the t est system w as checked with

aniline and found to be f3 2 % elative standard deviation on

T1/2= 0.21 day for aerobic cometabolism.

Introduction

Th e fate of synthe tic organic compounds in the environ-

men t is an area of interna tional concern since the contami-

nation of natural waters with these man-m ade substances has

constituted a major impact on aquatic ecosystems and hence

represents a serious problem to the management of water

quality. Therefore, it is app ropriate to examine for biodeg-

radability of organic compound s, many of which may be ex-

pected to enter the aquatic environments in the course of their

use or ultim ate disposal. Such an examination is required as

an integ ral part of the hazard ev aluation of new chemicals and

in one form or another is proposed in the toxic substances

legislation testing schem es

of

many of th e world’s industrial

nations.

Because of th e many variables involved, it is impractical to

study the persistence of such compounds in all natural envi-

ronments, and consequently several standard laboratory

procedures have been developed for the assessment of a

compound’s persistence 1-4). Th e shaker flask system of the

OECD te st used a chemically defined mineral mediu m with

the t est substance as th e carbon and energy source

for

the

microorganisms

I

. In a different system, Gledhill employed

a continuous-flow activated sludge system representative of

a treatmen t plant o peration for the evaluation of a substance’s

persistence (2).Stu rm developed a screening test for the as-

sessment of th e biodegradability of nonionic sur fact ant s by

following the C 0 2 evolution 3) .The model ecosystem of

Metcalf et al. was basically an aquarium with terrestrial-

aquatic interface and a seven-element food chain

4 ) .

Unfor-

tunate ly, examination of all of these tests reveals tha t there

are basic differences in th e degradation environments und er

which the t est substances are idvestigated. As a result, com-

parison between different studies in the literature is extremely

difficult, and m uch of the dat a have only limited value with

regard to assessing the relative stability of chemicals. For

example, the p ersistence of 2,4-D in soils has been reported

to be between 4 weeks and 3 yr 5 ) , eflecting the impact of

anoxic conditions on th e chemical’s persistence. T his p artic-

ular problem is further compounded by th e fact that certain

compound s, such as DD T, do not provide a suitable sole source

of carbon and energy to su ppo rt microbial growth (metabo-

lism) and consequently other organic nutrients are needed to

support the biodegradation microorganisms (cometabolism),

a fact which is frequently ignored in test systems. Another

common drawback in interpreting most persistence studies

788 Environmental S cience

&

Technology

0013-936X/81/0915-0788 01.25/0 @

1981 American Chemical Society

Documents

Bauer1981 Componentes Carbonatados en Ceniza Volante