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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 )
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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