United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-95/068 May 1995
&EFA Project Summary
Calcium Carbonate Dissolution
Rate in Limestone Contactors
Raymond D. Letterman
The project summarized here inves-
tigated some of the parameters and
relationships used to predict the per-
formance of limestone contactors. The
purpose of the project was to study the
effect of limestone composition, espe-
cially the dolomite [CaMg(CO,)2] and
impurity content of the stone, on the
kinetics of carbonate mineral dissolu-
tion and to determine the effect of tem-
perature on the rate of dissolution. The
rate of dissolution was determined by
using a rotating disk apparatus and
samples of limestone of varied compo-
sition.
The limestone samples included a
white marble and a selection of sedi-
mentary stones. The white marble con-
tained a significant amount of silica
(approximately 35%). The major min-
eral constituents of the sedimentary
limestones ranged from approximately
100% calcite (CaCO,) to essentially pure
dolomite. The approximate iron (Fe)
content of the stones ranged from 15
to 377 mg Fe/IOOg and the approxi-
mate aluminum content (Al) from 1 to
134mg Al/100g.
A heterogeneous reaction model for
mineral dissolution effectively explained
the results of the rotating disk experi-
ments for all samples except the two
with the highest dolomite content. The
magnitude of the dissolution rate con-
stant for fresh stone decreased by ap-
proximately 60% as the calcite content
of the stone decreased from 0.92 to
0.09 g CaCO3/g stone. The rate of dis-
solution of stones with a high dolomite
content may have been enhanced by
the presence of small amounts of cal-
cite. The rate of solubilization of mag-
nesium (Mg) was negligible in all
samples except the two with the high-
est dolomite content (93 and 100 mass
percent dolomite).
The overall dissolution rate constant
decreased as the amount of calcium
dissolved from the surface of the stone
increased. For a given amount of cal-
cium dissolved per unit area of stone
surface, the magnitude of the percent-
age decrease in the dissolution rate
constant increased as the iron and alu-
minum content of the stone increased.
The effect of sample aging on the rate
of dissolution was lowest when the
weighted sum of the iron and alumi-
num content of the stone was less than
about 10 mg/g. The weighted sum is
equal to the aluminum content in mg
Al/g plus 0.30 times the iron content in
mg Fe/g. The presence of silica as the
principal impurity in the white marble
seemed to reduce the effective (cal-
cite) surface area of the stone in pro-
portion to the mass of silica in the
sample, but it did not appear to affect
the dissolution rate of the calcite sur-
face.
The dissolution rate constant for cal-
cite increased with increasing tempera-
ture, from 0.38x10-9 cm/s at 5°C to
2.80x10-3 cm/s at 25°C. The apparent
activation energy was 101 ±8 kJ/mol for
the surface reaction rate constant and
Printed on Recycled Paper
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17±0.3 kJ/mol for the mass transfer rate
constant in the heterogeneous reac-
tion model.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at the
back).
Background
A limestone contactor is a treatment
device in which water flows through and
dissolves carbonate minerals (typically cal-
cium carbonate) from a packed bed of
crushed limestone. Dissolution of calcium
carbonate (under a closed-to-atmospheric-
carbon dioxide condition) increases the
pH, alkalinity, and dissolved inorganic car-
bon concentration of the water and de-
pletes the amount of calcium carbonate in
the bed. Limestone contactors are simple,
low-cost devices, which usually require
minimal maintenance and are, therefore,
especially suitable for small water sup-
plies.
In an earlier study (Letterman et al.,
1987), limestone contactors effectively re-
duced the dissolution of corrosion
byproducts, such as lead, copper, and
zinc, from piping system surfaces. A math-
ematical model related the depth of lime-
stone needed to reach the desired effluent
water chemistry to the influent water chem-
istry, the limestone particle size and shape,
the limestone bed porosity, and the tem-
perature and superficial velocity of the
water. Limited field experiments showed
that contactor performance decreases as
the water temperature decreases.
Another study (Haddad, 1986) monitored
the long-term operation of a contactor con-
taining somewhat impure, high-calcium
limestone. Here, the author concluded that
as the calcium carbonate dissolved, the
rate of dissolution decreased because rela-
tively insoluble impurities formed a resi-
due layer. As the thickness of this layer
increased, the rate of transport of calcium
ion from the calcium carbonate surface to
the bulk solution decreased, and, thus,
contactor performance decreased with
time.
Field experiments have shown
(Letterman et al., 1987) that the tempera-
ture of the water flowing through a lime-
stone contactor can affect its performance.
For a given set of design and operating
conditions, contactor performance tends
to decrease with decreasing temperature.
One of the objectives of this study was to
obtain a better understanding of this rela-
tionship.
Experimental Materials,
Apparatus, and Methods
Limestones
The study was conducted using 13
samples of limestone including a white
marble (sample WM) from a quarry in
Proctor, VT, a sedimentary limestone
(sample SL) from a quarry near Boonville,
NY, Black River limestone (sample BR)
from a quarry near Watertown, NY, and
10 samples (samples A-J) from a dolo-
mite quarry near York, PA.
A sample of each stone was powdered
and dissolved in concentrated hydrochlo-
ric acid. Dilutions of this solution were
used to determine the calcium, magne-
sium, iron, and aluminum content of the
stone with a direct current plasma spec-
trometer and an atomic absorption spec-
trophotometer. For a number of the
samples, some translucent material, prob-
ably quartz, remained after 2 days of dis-
solution. The measured calcium and
magnesium content of the samples was
used to estimate the calcite, dolomite, and
insoluble residue content of the samples.
In these calculations the magnesium was
assumed, based on x-ray diffraction and
thin-section photomicrography results, to
be associated only with dolomite.
The results of these calculations and
the measured iron and aluminum values
are listed in Table 1. In several cases,
where the sum of the calcite and dolomite
fractions was slightly greater than 100g/
100g of stone, the insoluble residue con-
tent was set equal to zero.
Samples WM, SL and BR as well as a
number of the samples from the York do-
lomite quarry (samples A, B, D, E and F)
are high calcium content limestones. Other
York samples (samples C, G, H, and I)
are predominately dolomite, and sample J
is essentially pure dolomite.
The WM sample had the highest in-
soluble residue content (36 g/100g) but
relatively low amounts of iron and alumi-
num (34 mg Al/100g and 71 mg Fe/100g
of stone). It is very likely that the insoluble
residue in this sample is quartz. Sample I,
from York, had the highest amount of iron
(377 mg Fe/100g) and sample H had the
highest amount of aluminum (134 mg Al/
100g).
The stone disks used in the rotating
disk apparatus were prepared by cutting
3.10- or 2.45-cm-diameter, cylindrical cores
from pieces of rock collected at the quar-
ries. Each core was cut into a number of
3-mm-thick disks using a rock saw. The
disk faces were smoothed and polished
on a lapwheel with a silicon carbide abra-
sive. The back face and edge of the disks
were coated with plastic so that only the
polished face was available for dissolu-
tion. Each disk was mounted in a Teflon-
coated1 brass holder (Figure 1). Between
dissolution rate experiments, each stone
sample was "aged" by controlled dissolu-
tion in dilute acid solution. The cumulative
amount of calcium and magnesium dis-
solved during aging was determined by
measuring their concentrations in the di-
lute acid solution.
Rotating Disk Apparatus
The reactor used in the rotating disk
apparatus (Figure 1) was 14 cm in diam-
eter and the clearance between the disk
and the walls of the vessel was greater
than 4 cm. The disk was centered about 3
cm above the bottom of the vessel, and
its rotational speed was varied over the
range 200 to 1200 rpm.
The reactor was constructed with
double-glass walls. A water bath was used
to circulate water between the walls to
maintain the reactor contents at
preselected temperatures in the range 4°
Mention of trade names or commercial products does
not constitute endorsement or recommendation for
use.
Table 1. Estimated Major and Minor Constituents of the Stone Samples (g/100g)
Stone ID Calcite Dolomite Insoluble Fe
Al
WM
SL
BR
A
B
C
D
E
F
G
H
1
J
64
92
99
89
92
17
79
71
89
23
38
9
0
1
4
2
16
4
68
18
29
9
59
53
93
100
35
4
0
0
4
15
3
0
2
18
9
0
0
0.071
0.101
0.019
0.024
0.029
0.189
0.040
0.041
0.015
0.294
0.154
0.377
0.189
0.034
0.114
0.044
0.012
0.001
0.093
0.037
0.025
0.005
0.129
0.134
0.032
0.010
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Plexiglass
cover
Drive shaft
Drive shaft support
L Li,
Limestone disk
Water
L
Reactor support
iackete<
reactor
Figure 1. Schematic diagram of the rotating disk apparatus.
to 25±0.2°C. The plexi-glass cover on the
reactor had holes for inserting the rotating
shaft, pH electrode, and wetted nitrogen
inlet tube. Additional holes were provided
for measuring the temperature and
pipetting samples for the calcium mea-
surement. The bulb of the pH electrode
was located 1.5 cm from the rotating disk
and 3 cm above the bottom of the vessel.
All solutions used in the rotating disk
experiments were made with distilled and
deionized water that had been boiled for a
few minutes, several hours before use, to
remove carbon dioxide. Fisher analytical
grade (ACS Certified) chemicals were used
(KCI, N/10 HCI). The background electro-
lyte was 0.079 M KCI.
Methods
A free-drift method, in which the pH
was allowed to increase as the carbonate
minerals dissolved from the stone, was
used in all experiments. Experimental so-
lutions (600 ml) were prepared as needed
by adding potassium chloride and the re-
quired volume of acid to boiled water and
then transferring this to the reactor.
Each experiment began by raising the
vessel and solution into place beneath the
rotating disk and against the plexi-glass
cover. Samples of solution (either 2- or 5-
ml volume) were withdrawn from the ves-
sel at 6 or 9 min intervals for a period of
1.5 hr with the use of an automatic pipette
(1 to 5 ml). The samples were stored in
polyurethane disposable test tubes at 4°C
for no longer than 2 days before the ion
concentrations were measured by atomic
absorption spectrophotometry.
Experimental Results
The calcium concentration and pH
change with time in a typical rotating disk
experimental run are shown in Figures 2
and 3. In this example, the WM stone
sample was used, the rotational speed
was 600 rpm, and the initial acidity was
0.01 meq/L. At the end of the experiment
the pH was 9.04 and the calcium concen-
tration was about 2 mg/L. For an initial
acidity of 0.01 meq/L and with no calcium
in the solution at t = 0, the calculated
equilibrium calcium concentration is 11.6
mg/L and the calculated equilibrium pH is
10.02.
The overall dissolution rate constant,
k0, was determined for each experimental
run by using the measured calcium con-
centrations and, in some cases, the mea-
sured magnesium concentrations. For the
stones that released negligible amounts
of magnesium, the calcium concentrations
(Ct) were substituted in the relationship,
M = In {(C., - C,)/^ - CJKV/A) (1)
where C^ and Co are the equilibrium
and initial calcium concentrations, respec-
tively, and A is the surface area of the
stone sample disk exposed to the solu-
tion. Co was zero in all experiments. V, is
the volume of the solution in the rotating
disk apparatus. For sample WM and
samples A-J, the limestone disk was 3.6
cm in diameter and, therefore, A was 10.17
cm2. For the 3.1-cm diameter SL sample
and the 2.5-cm-diameter BR sample, A
was 7.91 cm2 and 4.71 cm2, respectively.
-------
0.0
Time (hours)
Figure 2. Calcium concentration in the rotating disk apparatus as a function of time: WM sample;
w = 600 rpm and initial acidity of 0.01 meq/L.
10.0
9.0 -
60 -
5.0
0.0 02
0.4
0.6 08 1.0
Time (hours)
1.2 1.4
Figure 3. pH versus time for the rotating disk experiment of Figure 2.
The magnitude of C was determined
for each experimental run by using a
chemical equilibrium model and effec-
tive solubility products determined for a
number of the stone samples (Table 2).
For the stone samples that were not in-
cluded in the solubility product experi-
ments, i.e., samples B, D, E, G, H and J,
the average value of the effective solubil-
ity products (pK^ = 8.81) for the samples
from the same quarry was used. pKsp =
8.35 was used for sample BR because of
its similarity to sample SL.
Tables. Effective Solubility Products forCalcium
Carbonate and Calcium-Magnesium Carbonate
in Selected Limestone Samples.'
Stone sample ID
WM
SL
A
C
F
1
Negative log
of the effective
solubility product
8.20 + 0.07
8.35 ±0.06
8. 76 ±0.09
8.72 ±0.07
8.88 ±0.05
8.89 ±0.04
'Values are for 25°C and infinite dilution.
As samples were withdrawn during an
experiment, the magnitude of V, de-
creased. A value of V, was calculated for
each value of C, using the relationship,
V = V - nv
(2)
where Vo is the volume of the solution
in the reactor at the start of the experi-
ment, v is the volume of each sample
withdrawn for the calcium and magne-
sium measurements, and n is the total
number of samples withdrawn from the
reactor up to that sample. In the dissolu-
tion rate experiments, Vo was 600 ml_ and
v was either 2 or 5 ml_.
A straight line was fitted to the M ver-
sus time points using the method of least
squares (M is given by Equation 1). The
slope of this line is equal to the overall
dissolution rate constant.
Figure 4 is an M versus time plot for a
fresh sample of WM stone. In this experi-
ment, the disk rotational speed was 600
rpm, the water temperature was 25°C,
and the initial acidity was 0.01 meq/L. The
slope of the least squares line in Figure 4
yields an overall dissolution rate constant
of 3.3x1 O^cm/s.
Effect of Insoluble Residue on
the Dissolution Rate
The overall dissolution rate constant for
fresh calcitic stones (stones with low dolo-
mite content) tended to decrease as the
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-2 -
-4 -
-8 -
-10 -
-12
0.0 0.2 0.4 0.6 0.8 1.0
Time (hours)
1.2
1.4
Figure 4. Determination of the overall dissolution rate constant (ko) for the experiments of
Figures 2 and 3 (ko = o.oo33 cm/s).
estimated amount of insoluble residue in
the stone increased. We concluded that
the insoluble impurities reduced the area
of calcite exposed to the solution. To test
this hypothesis, we assumed that the area
of exposed calcite is proportional to the
mass percent of calcite in the stone. The
rate constants were "corrected" for the
residue content by dividing them by the
mass percent of calcite in the stone. The
results of this calculation, listed in the
right-hand column of Table 3, show that
this correction reduces the effect of the
residue content on the overall dissolution
rate constant.
The corrected overall dissolution rate
constant for the coarse-grained WM
sample (3.12x10~3 cm/s) is somewhat less
than the values of 3.51x10-° and 3.75x10"3
cm/s for the fine-grained SL and BR
samples.
Effect of Aluminum and Iron
Content on the Dissolution Rate
We observed that the extent to which
the dissolution of calcium from the stone
surface reduced the overall dissolution rate
constant depended on the aluminum and
iron content of the stone.
Figure 5 shows the normalized overall
dissolution rate constant (i.e., the mea-
sured value divided by the initial, fresh
stone, value, ko/kj plotted versus the
amount of calcium dissolved from the sur-
face of the stone, Cad, for stones A through
H. (The results for stones I and J were
Table 3. Comparison of Experimental and Corrected Overall Dissolution Rate Constants (kjfor
Essentially Fresh Limestone Disks.'
Stone
Mass % Experimental kjdO3 Corrected k^xtO3
Calcite (cm/s) (cm/s)
Calcium dissolved in "aging" = 0.2 mg Ca/crrf
WM 64
SL 92
BR 100
Calcium dissolved in "aging" = 0 mg Ca/crrf
B 92-
F 89
1.99
3.26
3.75
4.39
3.46
3.12
3.51
3.75
4.77
3.89
'Small amounts of calcium had been dissolved from samples WM, SL, and BR before the first rate
constants were determined.
also plotted but are not shown in Figure
5.) For stones C, I, G, and H, the overall
dissolution rate constant decreased by
more than 60% as the amount of calcium
dissolved increased from 0 to 4 mg Ca/
cm2. For stones B, F, and J, the decrease
was less than 30%.
Values of k/k^ were interpolated from
Figure 5 at Cad = 2 mg Ca/cm2 and then
listed in Table 4 in rank order, from the
highest (kyk^ = 0.90 for stone F) to the
lowest (k^ = 0.23 for stone G). The
stones with the highest aluminum content
(> 25 mg Al/100g of stone) had the great-
est decrease in the overall dissolution rate
constant for this amount of calcium dis-
solved. For several stones, especially
stone I with kyk^ = 0.36, the iron content
seemed to be an additional factor.
Since both the iron and aluminum con-
tent of the stone seem to determine how
sample aging affects the overall dissolu-
tion rate constant, a composite parameter
that includes a weighted combination of
the iron and aluminum concentrations (aC^
+ bCFe) was derived, where C^ is the
aluminum concentration in mg AI/100 g
and CFe is the iron concentration in mg
Fe/100 g. The highest linear correlation
between k^ and the Fe+AI parameter
(r2 = 0.92) was obtained with weighting
factors a=1 and b=0.3, i.e., (CAI + 0.30
CFe). The quantity k/k^ and correspond-
ing values of (CA1 + 0.30 Cfe) are listed in
Table 5.
According to the results in Table 5, the
effect of iron and aluminum on the overall
dissolution rate constant will be minimized
if the quantity CAI + 0.30 CFe for the stone
is less than about 10 mg/100g.
In a special experiment, the brownish
residue layer that formed on the SL disk
was scraped into concentrated nitric acid,
ultrasonicated and the solution was ana-
lyzed for total soluble aluminum. The
soluble aluminum expressed as the
amount per area of disk was 0.97 nmoles/
cm2 (26 ng/cm2). The scraped residue did
not dissolve completely in acid which sug-
gested the presence of alumino-silicates.
The overall dissolution rate constant for
the SL stone increased to 90% of its origi-
nal value when the residue layer was
scraped from the disk surface.
Conclusions and
Recommendations
A heterogeneous reaction model for min-
eral dissolution, in which the rate of disso-
lution is controlled by a surface reaction
and a cation mass transfer resistance act-
ing in series, effectively explained the re-
sults of the rotating disk experiments for
all samples except the two with the high-
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1.2
sample A samples sample C sample D
—B— -•&•- —O-- —X--
sample E sample F sample G sample H
468
Ca dissolved per unit area of disk (mg/sq cm)
10
Figure 5. Effect of amount of calcium dissolved from disk surface on the fractional decrease in the
overall dissolution rate constant.
Table 4. Effect of the Iron and Aluminum Content on the Fractional Decrease in the Overall Dissolution
Rate Constant (kj at Cad = 2mg Calcium Dissolved/sq cm of Limestone Surface.
Stone
ID
F
A
B
J
E
D
C
1
H
G
%
Calcite
88.9
88.9
92.1
0
70.7
78.9
16.5
8.8
38.3
22.9
kox10>'
(cm/s)
3.5
4.7
4.4
0.9
3.3
4.2
2.7
2.8
3.3
3.2
w
0.90
0.74
0.73
0.70
0.65
0.61
0.43
0.36
0.35
0.23
Fe
(mgFe/100g)
15
24
29
189
41
40
189
377
154
294
Al
(mgAI/100g)
5
12
1
10
25
37
93
32
134
129
' Interpolated from Figure 5 at Cad = 2mg Ca/crrf* kol = value when negligible Ca dissolved from the
stone.
Table 5. Effect of the Weighted Sum of Iron and Aluminum in the Limestone on the Fractional Decrease
in the Dissolution Rate Constant at 2 mg Calcium/sq cm of Limestone Surface.
Stone
+ 0.30C
ID
(mg/100g)
F
A
B
J
E
D
C
1
H
J
0.90
0.74
0.73
0.70
0.65
0.6J
0.43
0.36
0.35
0.23
10
19
10
67
37
49
149
145
180
217
'k = value when negligible Ca dissolved from the stone.
est dolomite content. For calcite and the
experimental conditions of this study, the
surface reaction rate was relatively large
and the rate of dissolution was essentially
mass transfer controlled. The results show
that a calcium ion diffusivity of 0.8 x 10~5
cnrWs (at 25°C, can be used in predicting
the mass transfer resistance.
The stone samples with the highest cal-
cite content and lowest dolomite content
had the highest initial rates of dissolution.
The magnitude of the overall dissolution
rate constant for fresh stone decreased
by approximately 60% as the calcite con-
tent of the stone decreased from 0.92 to
0.09 g CaCO/g stone. The rate of disso-
lution of stones with high dolomite content
may be enhanced by the presence of small
amounts of calcite. For example, the stone
that was essentially pure dolomite had a
dissolution rate constant that was 66%
less than the constant for another dolo-
mitic stone with approximately 9% calcite.
When the high dolomite content samples
were fresh, it appeared that the calcium
carbonate component of the dolomite dis-
solved faster than the magnesium car-
bonate component. The rate of dissolution
of magnesium was negligible in all samples
except the high dolomite content samples
(93 and 100 mass percent dolomite).
The overall dissolution rate constant
decreased as the amount of calcium dis-
solved from the surface of the stone in-
creased. Analysis of several stone
surfaces, by scanning electron microscopy
and x-ray energy spectroscopy, indicated
that the density of calcium atoms on the
surface of the stone decreased and the
density of aluminum, silicon, and iron in-
creased as calcium dissolved. For a given
amount of calcium dissolved per unit area
of stone surface, the magnitude of the
decrease in the overall dissolution rate
constant increased as the iron and alumi-
num content of the stone increased. The
results suggest that the effect of sample
aging on the rate of dissolution is a mini-
mum if the weighted sum of the iron and
aluminum content of the stone is less than
about 10 mg/g. The weighted sum is equal
to the aluminum content in mg Al/g plus
0.30 times the iron content in mg Fe/g. To
minimize the negative effect of mineral
dissolution and residue-layer buildup on
the performance of a limestone contactor
during long-term operation, the iron and
aluminum content should be less than this
weighted sum.
The presence of silica as the principal
impurity in the white marble reduced the
effective surface area of the calcite in
proportion to the mass of silica in the
sample but did not appear to cause a
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reduction in the dissolution rate of the
calcite surface.
The dissolution rate of calcite increased
with increasing temperature, from
0.38x10"3 cm/s at 5°C to 2.80x10'3 cm/s at
25°C. The apparent activation energy de-
termined for the surface reaction rate con-
stant in the heterogeneous reaction model
was 101 ±8 kJ/mol, a value that is signifi-
cantly larger than literature values (46 to
63 kJ/mol). The apparent activation en-
ergy for the mass transfer rate constant
was 17±0.3 kJ/mol, which is consistent
with values in the literature for mass trans-
fer controlled kinetics.
References
Haddad, M., 1986. Modeling of Lime-
stone Dissolution in Packed Bed
Contactors Treating Dilute Acidic Water.
Ph.D. Dissertation, Department of Civil
Engineering, Syracuse University.
Letterman, R. D., C. T. Driscoll, Jr., M.
Haddad and H. A. Hsu, 1987. Limestone
Bed Contactors for Control of Corrosion
at Small Water Utilities. A Report for the
Water Engineering Research Laboratory,
Office of Research and Development, U.S.
Environmental Protection Agency, Cincin-
nati, OH (EPA/600/S2-86/099).
Letterman, R D., M. Haddad and C. T.
Driscoll, 1991. Limestone Contactors:
Steady-State Design Relationships, Jour-
nal of Environ. Eng.. Am. Soc. of Civil
Engineers, 117:339-358.
The full report was submitted in fulfill-
ment of CR 814926 by Syracuse Univer-
sity under the sponsorship of the U.S.
Environmental Protection Agency.
-------
Raymond D. Letterman is with the Department of Civil and Environmental
Engineering, Syracuse University, Syracuse, NY 13244-1190.
Jeffrey Q. Adams is the EPA Project Officer (see below).
The complete report, entitled "Calcium Carbonate Dissolution Rate in Lime-
stone Contactors," (Order No. PB9S-222733; Cost: $27.00, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
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