Project Summary

United States

Environmental Protection

Agency

Calcium Carbonate Dissolution

Rate in Limestone Contactors

Raymond D. Letterman

Risk Reduction

Engineering Laboratory

Cincinnati, OH 45268

Research and Development

The project summarized here investigated some of the parameters and

relationships used to predict the performance of limestone contactors. The

purpose of the project was to study the

effect of limestone composition, especially the dolomite [CaMg(CO3

)2] and

impurity content of the stone, on the

kinetics of carbonate mineral dissolution and to determine the effect of temperature on the rate of dissolution. The

rate of dissolution was determined by

using a rotating disk apparatus and

samples of limestone of varied composition.

The limestone samples included a

white marble and a selection of sedimentary stones. The white marble contained a significant amount of silica

(approximately 35%). The major mineral constituents of the sedimentary

limestones ranged from approximately

100% calcite (CaCO3

) to essentially pure

dolomite. The approximate iron (Fe)

content of the stones ranged from 15

to 377 mg Fe/100g and the approximate aluminum content (Al) from 1 to

134 mg Al/100g.

A heterogeneous reaction model for

mineral dissolution effectively explained

the results of the rotating disk experiments for all samples except the two

with the highest dolomite content. The

magnitude of the dissolution rate constant for fresh stone decreased by approximately 60% as the calcite content

of the stone decreased from 0.92 to

0.09 g CaCO3

/g stone. The rate of dissolution of stones with a high dolomite

content may have been enhanced by

the presence of small amounts of calcite. The rate of solubilization of magnesium (Mg) was negligible in all

samples except the two with the highest 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 calcium dissolved per unit area of stone

surface, the magnitude of the percentage decrease in the dissolution rate

constant increased as the iron and aluminum 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 aluminum 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 (calcite) surface area of the stone in proportion to the mass of silica in the

sample, but it did not appear to affect

the dissolution rate of the calcite surface.

The dissolution rate constant for 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 was 101±8 kJ/mol for

the surface reaction rate constant and

EPA/600/SR-95/068 May 1995
2

17±0.3 kJ/mol for the mass transfer rate

constant in the heterogeneous reaction 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 calcium carbonate) from a packed bed of

crushed limestone. Dissolution of calcium

carbonate (under a closed-to-atmosphericcarbon dioxide condition) increases the

pH, alkalinity, and dissolved inorganic carbon concentration of the water and depletes 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 supplies.

In an earlier study (Letterman et al.,

1987), limestone contactors effectively reduced the dissolution of corrosion

byproducts, such as lead, copper, and

zinc, from piping system surfaces. A mathematical model related the depth of limestone needed to reach the desired effluent

water chemistry to the influent water chemistry, the limestone particle size and shape,

the limestone bed porosity, and the temperature 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 containing somewhat impure, high-calcium

limestone. Here, the author concluded that

as the calcium carbonate dissolved, the

rate of dissolution decreased because relatively insoluble impurities formed a residue 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 temperature of the water flowing through a limestone 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 relationship.

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 dolomite quarry near York, PA.

A sample of each stone was powdered

and dissolved in concentrated hydrochloric acid. Dilutions of this solution were

used to determine the calcium, magnesium, iron, and aluminum content of the

stone with a direct current plasma spectrometer and an atomic absorption spectrophotometer. For a number of the

samples, some translucent material, probably quartz, remained after 2 days of dissolution. 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 content was set equal to zero.

Samples WM, SL and BR as well as a

number of the samples from the York dolomite 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 insoluble residue content (36 g/100g) but

relatively low amounts of iron and aluminum (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 quarries. 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 abrasive. The back face and edge of the disks

were coated with plastic so that only the

polished face was available for dissolution. Each disk was mounted in a Tefloncoated1

 brass holder (Figure 1). Between

dissolution rate experiments, each stone

sample was “aged” by controlled dissolution in dilute acid solution. The cumulative

amount of calcium and magnesium dissolved during aging was determined by

measuring their concentrations in the dilute acid solution.

Rotating Disk Apparatus

The reactor used in the rotating disk

apparatus (Figure 1) was 14 cm in diameter 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°

Table 1. Estimated Major and Minor Constituents of the Stone Samples (g/100g)

Stone ID Calcite Dolomite Insoluble Fe Al

WM 64 1 35 0.071 0.034

SL 92 4 4 0.101 0.114

BR 99 2 0 0.019 0.044

A 89 16 0 0.024 0.012

B 92 4 4 0.029 0.001

C 17 68 15 0.189 0.093

D 79 18 3 0.040 0.037

E 71 29 0 0.041 0.025

F 89 9 2 0.015 0.005

G 23 59 18 0.294 0.129

H 38 53 9 0.154 0.134

I 9 93 0 0.377 0.032

J 0 100 0 0.189 0.010

1 Mention of trade names or commercial products does

not constitute endorsement or recommendation for

use.
3

grade (ACS Certified) chemicals were used

(KCl, N/10 HCl). The background electrolyte was 0.079 M KCl.

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 solutions (600 mL) were prepared as needed

by adding potassium chloride and the required 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 vessel 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 concentration 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,

ko, was determined for each experimental

run by using the measured calcium concentrations and, in some cases, the measured magnesium concentrations. For the

stones that released negligible amounts

of magnesium, the calcium concentrations

(Ct

) were substituted in the relationship,

M = ln {(Ceq - Ct

)/(Ceq - Co)}(Vt/A) (1)

where Ceq and Co

 are the equilibrium

and initial calcium concentrations, respectively, and A is the surface area of the

stone sample disk exposed to the solution. Co

 was zero in all experiments. Vt 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.

Limestone disk

Teflon-coated

disk holder

Water jacketed reactor

Drive shaft

Reactor support

Plexiglass

cover

Drive shaft support

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 measurement. 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

Figure 1. Schematic diagram of the rotating disk apparatus.
4

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

5.0

6.0

7.0

8.0

9.0

10.0

Time (hours)

pH

The magnitude of Ceq was determined

for each experimental run by using a

chemical equilibrium model and effective solubility products determined for a

number of the stone samples (Table 2).

For the stone samples that were not included in the solubility product experiments, i.e., samples B, D, E, G, H and J,

the average value of the effective solubility products (pKsp = 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.

Table 2. Effective Solubility Products for Calcium

Carbonate and Calcium-Magnesium Carbonate

in Selected Limestone Samples.*

Negative log

of the effective

Stone sample ID solubility product

WM 8.20 ± 0.07

SL 8.35 ± 0.06

A 8.76 ± 0.09

C 8.72 ± 0.07

F 8.88 ± 0.05

I 8.89 ± 0.04

*

Values are for 25°C and infinite dilution.

As samples were withdrawn during an

experiment, the magnitude of Vt

 decreased. A value of Vt

 was calculated for

each value of Ct

 using the relationship,

Vt

 = Vo - nv (2)

where Vo

 is the volume of the solution

in the reactor at the start of the experiment, v is the volume of each sample

withdrawn for the calcium and magnesium measurements, and n is the total

number of samples withdrawn from the

reactor up to that sample. In the dissolution rate experiments, Vo

 was 600 mL and

v was either 2 or 5 mL.

A straight line was fitted to the M versus 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 experiment, 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 3.3x10-3 cm/s.

Effect of Insoluble Residue on

the Dissolution Rate

The overall dissolution rate constant for

fresh calcitic stones (stones with low dolomite content) tended to decrease as the

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.

Figure 3. pH versus time for the rotating disk experiment of Figure 2. pH

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0.0

0.5

1.0

1.5

2.0

2.5

Time (hours)

Calcium Concentration (mg/L)
5

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-12

-10

-8

-6

-4

-2

0

Time (hours)

m M (c

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

Table 3. Comparison of Experimental and Corrected Overall Dissolution Rate Constants (ko)for

Essentially Fresh Limestone Disks.*

Stone Mass % Experimental kox103 Corrected kox103

Calcite (cm/s) (cm/s)

Calcium dissolved in “aging” = 0.2 mg Ca/cm2

 WM 64 1.99 3.12

 SL 92 3.26 3.51

 BR 100 3.75 3.75

Calcium dissolved in “aging” ≈ 0 mg Ca/cm2

 B 92 4.39 4.77

 F 89 3.46 3.89

*

Small amounts of calcium had been dissolved from samples WM, SL, and BR before the first rate

constants were determined.

than the values of 3.51x10-3 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 measured value divided by the initial, fresh

stone, value, ko

/koi) plotted versus the

amount of calcium dissolved from the surface of the stone, Cad

, for stones A through

H. (The results for stones I and J were

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 ko

/koi were interpolated from

Figure 5 at Cad

 = 2 mg Ca/cm2 and then

listed in Table 4 in rank order, from the

highest (ko

/koi = 0.90 for stone F) to the

lowest (ko

/koi = 0.23 for stone G). The

stones with the highest aluminum content

(> 25 mg Al/100g of stone) had the greatest decrease in the overall dissolution rate

constant for this amount of calcium dissolved. For several stones, especially

stone I with ko

/koi = 0.36, the iron content

seemed to be an additional factor.

Since both the iron and aluminum content of the stone seem to determine how

sample aging affects the overall dissolution rate constant, a composite parameter

that includes a weighted combination of

the iron and aluminum concentrations (aCAl

+ bCFe) was derived, where CAl is the

aluminum concentration in mg Al/100 g

and CFe is the iron concentration in mg

Fe/100 g. The highest linear correlation

between ko

/koi and the Fe+Al parameter

(r2

 = 0.92) was obtained with weighting

factors a=1 and b=0.3, i.e., (CAl + 0.30

CFe). The quantity ko

/koi and corresponding values of (CAl + 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 CAl + 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 analyzed for total soluble aluminum. The

soluble aluminum expressed as the

amount per area of disk was 0.97 μmoles/

cm2

 (26 μg/cm2). The scraped residue did

not dissolve completely in acid which suggested the presence of alumino-silicates.

The overall dissolution rate constant for

the SL stone increased to 90% of its original value when the residue layer was

scraped from the disk surface.

Conclusions and

Recommendations

A heterogeneous reaction model for mineral dissolution, in which the rate of dissolution is controlled by a surface reaction

and a cation mass transfer resistance acting in series, effectively explained the results of the rotating disk experiments for

all samples except the two with the highM (cm)
6

Table 4. Effect of the Iron and Aluminum Content on the Fractional Decrease in the Overall Dissolution

Rate Constant (ko) at Cad = 2 mg Calcium Dissolved/sq cm of Limestone Surface.

Stone % ko x 103 * Fe Al

ID Calcite (cm/s) ko/koi

+ (mgFe/100g) (mgAl/100g)

F 88.9 3.5 0.90 15 5

A 88.9 4.7 0.74 24 12

B 92.1 4.4 0.73 29 1

J 0 0.9 0.70 189 10

E 70.7 3.3 0.65 41 25

D 78.9 4.2 0.61 40 37

C 16.5 2.7 0.43 189 93

I 8.8 2.8 0.36 377 32

H 38.3 3.3 0.35 154 134

G 22.9 3.2 0.23 294 129

*

 Interpolated from Figure 5 at Cad = 2 mg Ca/cm2 .

+ koi = 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 CAl + 0.30 CFe ID ko/koi

*

 (mg/100 g)

F 0.90 10

A 0.74 19

B 0.73 10

J 0.70 67

E 0.65 37

D 0.61 49

C 0.43 149

I 0.36 145

H 0.35 180

J 0.23 217

*

koi = value when negligible Ca dissolved from the stone.

Figure 5. Effect of amount of calcium dissolved from disk surface on the fractional decrease in the

overall dissolution rate constant.

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

cm2/s (at 25°C, can be used in predicting

the mass transfer resistance.

The stone samples with the highest calcite 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 content of the stone decreased from 0.92 to

0.09 g CaCO3

/g stone. The rate of dissolution 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 dolomitic stone with approximately 9% calcite.

When the high dolomite content samples

were fresh, it appeared that the calcium

carbonate component of the dolomite dissolved faster than the magnesium carbonate 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 dissolved from the surface of the stone increased. 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 increased 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 aluminum content of the stone increased. The

results suggest that the effect of sample

aging on the rate of dissolution is a minimum 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

ko/ko (initial)

0 2 4 6 8 10

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Ca dissolved per unit area of disk (mg/sq cm)

sample A sample B sample C sample D

sample E sample F sample G sample H

1.2

1.0

0.8

0.6

0.4

0.2

0.0

0 2 4 6 8 10

Ca dissolved per unit area of disk (mg/sq cm)

sample A sample B sample C sample D

sample E sample F sample G sample H
7

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 determined for the surface reaction rate constant in the heterogeneous reaction model

was 101±8 kJ/mol, a value that is significantly larger than literature values (46 to

63 kJ/mol). The apparent activation energy for the mass transfer rate constant

was 17±0.3 kJ/mol, which is consistent

with values in the literature for mass transfer controlled kinetics.

References

Haddad, M., 1986. Modeling of Limestone 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, Cincinnati, OH (EPA/600/S2-86/099).

Letterman, R D., M. Haddad and C. T.

Driscoll, 1991. Limestone Contactors:

Steady-State Design Relationships, Journal of Environ. Eng., Am. Soc. of Civil

Engineers, 117:339-358.

The full report was submitted in fulfillment of CR 814926 by Syracuse University under the sponsorship of the U.S.

Environmental Protection Agency.
8

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 Limestone Contactors," (Order No. PB95-222733; 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

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