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A preliminary study of calcite saturation states with varying pH and salinity

Authors:

Abstract

The present work explores accelerated weathering of limestone (AWL) as an option to combat the climate change problem posed by the still growing carbon footprint. AWL is a promising low-technology solution that sequesters CO2 through the dissolution of calcium carbonate (CaCO3) in water to form calcium bicarbonate. One aspect of the feasibility of its application is whether the newly formed bicarbonate will remain in solution for the long term. This paper examines the carbonate equilibria for various water bodies by combining Bjerrum plots with the calcite saturation value, omega. The hydrochemistry software, aqion, was used to simulate a range of water compositions with different salinities at various conditions. The study compares saturation states of rainwater, limestone quarry water, seawater, and reverse osmosis concentrate, with their respective dissolved inorganic carbon (DIC) concentrations, across the full range of pH values. The areas of stability and instability can be observed from the generated plots in order to be able to design effective AWL processes. A key issue for successful implementation of AWL is operating within the CaCO3 saturation limit and ensuring dissolved material does not subsequently reprecipitate.
1
A preliminary study of calcite saturation states with varying pH and salinity.
Boaz Chung Yi Heng
Department of Chemical & Environmental Engineering
Faculty of Science and Engineering
University of Nottingham Malaysia, Semenyih, Malaysia
email: boazcyh@gmail.com
Jerry Joynson, Steve Willis
Cquestr8 Sdn. Bhd.
D1105, Menara Suezcap 1, KL Gateway
Gerbang Kerinchi Lestari No 2, Jalan Kerinchi
Kuala Lumpur, Malaysia
Ianatul Khoiroh*, Dominic Foo
Department of Chemical & Environmental Engineering
Faculty of Science and Engineering
University of Nottingham Malaysia, Semenyih, Malaysia
email: Ianatul.Khoiroh@nottingham.edu.my
ABSTRACT
The present work explores accelerated weathering of limestone (AWL) as an option to combat
the climate change problem posed by the still growing carbon footprint. AWL is a promising
low-technology solution that sequesters CO2 through the dissolution of calcium carbonate
(CaCO3) in water to form calcium bicarbonate. One aspect of the feasibility of its application
is whether the newly formed bicarbonate will remain in solution for the long term. This paper
examines the carbonate equilibria for various water bodies by combining Bjerrum plots with
the calcite saturation value, omega.
The hydrochemistry software, aqion, was used to simulate a range of water compositions with
different salinities at various conditions. The study compares saturation states of rainwater,
limestone quarry water, seawater, and reverse osmosis concentrate, with their respective
dissolved inorganic carbon (DIC) concentrations, across the full range of pH values. The areas
of stability and instability can be observed from the generated plots in order to be able to design
effective AWL processes. A key issue for successful implementation of AWL is operating
within the CaCO3 saturation limit and ensuring dissolved material does not subsequently
reprecipitate.
KEYWORDS
Accelerated Weathering of Limestone; Carbon Dioxide; pH; Carbonate; Calcium Carbonate
Saturation State; Precipitation.
2
INTRODUCTION
Accelerated weathering of limestone (AWL) is a geochemistry-based sequestration process that
mimics the natural weathering process. It is a potentially highly scalable CO2 sequestration
method that offers a huge potential in reducing the concentration of CO2 in the atmosphere. The
use of AWL for carbon dioxide removal has gained renewed interest from industry because of
the ubiquity of limestone and potential ability to apply the process globally at relatively low
cost. In addition, AWL also helps reduce the impact of ocean acidification by strengthening the
chemical pH buffer (also termed Total Alkalinity) and facilitates the natural cycle of carbon
dioxide removal in the atmosphere by seawater [1].
The oceans of the world are estimated to contain close to 39,000 Gigatonnes (Pg) of dissolved
inorganic carbon, more than 90% of which is HCO3-. By contrast the atmosphere has been
estimated to contain around 800 Gigatonnes of carbon (~ 2,900 Gt CO2) of which around 250
Gt C (912 Gt CO2) [2] has been added since the industrial revolution. Establishing AWL
processes to convert CO2 to calcium bicarbonate at a rate of, say,
0.273 Gt C (1 Gt CO2)/year would be a massive undertaking. And yet at this rate the average
ocean dissolved inorganic carbon (DIC) concentration would increase by only 0.14% 1 after
100 years. Given the approximately 10% variation in DIC concentrations between the Pacific
and Atlantic oceans [3] we may conclude that 0.14% is an insignificant concentration change.
In the AWL reaction, gaseous CO2 is first dissolved in water and is further hydrated to form
carbonic acid in the following reaction:
𝐶𝑂
(
𝑔
)
(
𝑎𝑞
)
(
1
)
(
𝑎𝑞
)
+
𝐻
𝑂
(
𝑙
)
𝐻
(
𝑎𝑞
)
(
2
)
Carbonic acid reacts with solid calcium carbonate (limestone, CaCO3) to form calcium and
bicarbonate ions.
𝐻
𝐶𝑂
(
𝑎𝑞
)
+
𝐶𝑎𝐶𝑂
(
𝑠
)
𝐶𝑎
(
𝑎𝑞
)
+
2
𝐻𝐶𝑂

(
𝑎𝑞
)
(
3
)
The overall reaction can be expressed as:
𝐶𝑂
(
𝑔
)
+
𝐻
𝑂
(
𝑙
)
+
𝐶𝑎𝐶𝑂
(
𝑠
)
𝐶𝑎
(
𝑎𝑞
)
+
2
𝐻𝐶𝑂

(
𝑎𝑞
)
(4)
The tendency for calcium carbonate to form or dissolve is governed by its saturation state,
which often denoted as Ω. Values greater than 1 indicate supersaturation favouring precipitation
while values less than 1 indicate undersaturation favouring dissolution. Ω is defined as:
𝛺
=
[
𝐶
𝑎
]
[
𝐶𝑂

]
𝐾

(5)
where Ω is the product of the concentrations of calcium ([Ca2+]) and carbonate ([CO32−]) ions,
divided by K'sp⁠, the apparent solubility product of CaCO3 [4].
This process is geochemically equivalent to continental and marine carbonate weathering which
naturally consumes anthropogenic CO2. However, the current high rates of anthropogenic
1 This figure also includes the carbon associated with the CaCO3 reacted with the CO2 to form calcium bicarbonate.
3
emissions of CO2 far exceed the rate at which these natural processes consume CO2 which now
would require many millennia to capture all past excess anthropogenic emissions.
By mimicking one of nature’s own CO2 sequestration systems AWL offers a huge potential for
substantially increasing the rate of CO2 sequestration. The process requires corresponding
supplies of the reactants, CO2, limestone and water, and therefore preferred sites for its
implementation will be regions where CO2 is being produced in proximity to limestone close
to coastal regions [4].
Past studies have suggested that AWL may be suited to the capture of coal power plant
emissions. However, with the accelerating closures of coal plants this seems an unlikely source
of CO2 for the future and therefore the process may be better suited to sequestering the hardest
to abate CO2 emissions such as those from cement plants, and eventually CO2 from direct air
capture (DAC) processes, should these become significant.
Cement plants are of particular interest because they are often co-located with limestone
quarries or have an existing system to economically transport limestone to the plant [5]. The
transportation of CO2 over longer distances is associated with significant energy and technology
costs as it involves high compression and often liquefaction. A previous study suggested to
retrofit retired or underutilized vessels with AWL reactors, where the ships would bring both
limestone and the AWL process to coastal CO2 point sources accessible by seagoing vessels
[5].
AWL implementation to sequester CO2 is still mostly in the research phase. Since Rau and
Caldeira first proposed AWL [6], only a limited number of AWL reactors have been constructed
for lab-scale or midscale applications [7]. As of 2020, AWL research has been largely confined
to small-scale experiments, with the largest advancement being a pilot scale application in
Germany. Nonetheless, the feasibility of an upscale application of AWL has been previously
studied by Langer et al. [5].
A lab scale seawater/mineral carbonate gas scrubber was employed to investigate the effect of
concurrent and counter current gas/water flow regimes, flow rate ratio, residence time, as well
as carbonate particle size in AWL reactions [1]. The reactor was found to remove up to 97% of
CO2 in a simulated flue gas stream at ambient temperature and pressure, with a large fraction
of this carbon ultimately converted to dissolved calcium bicarbonate. After full equilibration
with air, up to 85% of the captured carbon was retained in solution, that is, it did not degas or
precipitate. Thus, above-ground CO2 hydration and mineral carbonate scrubbing may provide
a relatively simple point-source CO2 capture and storage scheme at coastal locations. No change
in alkalinity was observed, however, pH value was found to return to the ambient seawater
value due to the presence of ions in seawater that inhibits the precipitation of CaCO3.
Previous studies investigated the feasibility of AWL using different sizes of limestone and rates
of carbonated water using lab scale reactors [8], [9]. They found that better rate of reactions can
be achieved by using smaller grain size and increasing in flow rate of CO2 during AWL process.
A similar study by Haas et al. [10] employed lab-scale apparatus to investigate the effect of
temperature, ion concentration, and pressure of CO2 on AWL. Subsequently, the pilot scale was
developed to verify the repeatability of the lab scale’s tests as well as to investigate the process’
feasibility at the municipal sewage treatment plant in Bad Orb, Germany. The data obtained
from lab-scale study revealed the following relationships: (i) the higher the temperature, the
lower the total alkalinity (TA); (ii) the higher the ion concentration, the shorter the time required
for a specified conversion; (iii) the use of seawater enhances the separation efficiency; and (iv)
4
the higher the partial pressure of CO2, the higher the removal efficiency of CO2. Interestingly,
comparable removal of CO2 achieved at lower quantities of limestone relative to lab-scale has
been achieved in the pilot scale which was attributed to the large contact surface and better
mixing of the suspension. Maximum removal (17.2%) was obtained when synthetic seawater
was used.
The effectiveness of CO2 absorption for one-step and two-step AWL reactor configurations
were evaluated in the previous study by Chou et al. [11]. The impact of AWL effluent solution
on pH and saturation state in receiving seawater was also evaluated by using simulations. They
found that both configurations showed similar changes in DIC values, indicating similar
hydration of CO2. However, the values were found to be much lower than predicted equilibrium
DIC values. Similarly, the two-steps AWL reactor showed a much larger increase in TA,
indicating better CaCO3 dissolution. However, this value was found to be much lesser than the
predicted equilibrium TA. A suggested explanation for the difference between the two reactors
was the captured CO2 in the 1-step reactor remained as molecular CO2, as opposed to forming
carbonic ions. The study also suggested that 10-fold dilution was likely sufficient to maintain
pH within the permissible range of 0.2 which in line with US and Canadian environmental
guidelines.
The largest AWL reactor to date has been demonstrated in Kirchner et al. [7]. The reactor was
capable of processing 200 m3/h of effluent gas stream of the coal-fired power plant. It was used
to examine the limestone weathering efficiency hence long-term storage of the captured carbon
dioxide. The presence of harmful substances in the product waters was also investigated. This
study obtained higher CO2 removal efficiency compared to Haas et al. [10] but lower than that
achieved in their prior work on the lab-scale reactors. Subsequently, higher TA achieved
through circulating product water compared to Chou et al. [11] and Rau [1] but not at
equilibrium due to an unexpected power plant shut down. Unfortunately, precipitation observed
after 24 hours of stirring which contradicted the previous study by Rau [1] to equilibrate CO2
with the atmosphere.
A key issue for successful implementation of AWL is operating within the CaCO3 saturation
limit and ensuring dissolved material does not subsequently reprecipitate. In the present work,
therefore, we aim to evaluate the saturation states of various water bodies, namely, rainwater,
limestone quarry water, seawater, and reverse osmosis concentrate, respectively across the full
range of pH values and various DIC concentrations. This allows us to consider AWL from the
perspectives of CaCO3 solubility, and the effect of its controlling parameters (temperature,
partial pressure of CO2 and system composition).
METHODS
Hydrochemical analysis
The hydrochemical analysis of various water bodies was conducted by employing aqion
software (www.aqion.de) , which uses the open-source PhreeqC database published by United
States Geological Survey [12]. The approach is illustrated as a workflow diagram depicted in
Figure 1.
The inputs to the aqion program comprise the chemical element concentrations, pH,
temperature, and whether the system is open or closed. In this work, either DIC, alkalinity, or
partial pressure of CO2 (pCO2) was specified in aqion. In the closed system mode, where there
is no exchange of matter with the surroundings, cases specifying either DIC or alkalinity were
5
run. In the open system mode, the partial pressure of CO2 was specified equal to the current
atmospheric conditions.
Natural or real aqueous systems have equal sums of positive (cation) and negative (anion)
charges, and so the software calculates the charge balance error (CBE) to allow minor errors to
be identified and adjusted so that the CBE equals zero. The software also recommends which
parameter to adjust to bring the error to zero whilst at the same time reducing the discrepancy
between the measured and calculated total dissolved solids (TDS) or electrical conductivity
(EC).
Upon CBE adjustment, the hydrochemical analysis of the input solution was performed and
two sets of output data were generated by aqion. Output 1 predicts the results based on the
chemical equilibrium of the species in the system assuming there is no precipitation, while
output 2 accounts for both precipitation and any mineral-solution interaction. Therefore, no
differences were observed between the two-output data unless precipitation or dissolution of
minerals was predicted by the software.
Aqion was subsequently used to obtain the data to create Bjerrum plots2 for each water
composition and to calculate the calcium carbonate saturation state values for calcite (Ωcal) vs.
pH. The workflow diagram for the simulations is depicted in Figure 2. The process is essentially
one of repeating the hydrochemical analysis of an input solution at different pH values to obtain
the concentrations of the three DIC species, namely CO2, HCO3-, and CO32-, as well as the
saturation index (SI) values, which are calculated according to the following relationship:
Ω
=
10

(6)
Creation of the Bjerrum plots required the individual carbon species concentrations vs. pH to
be determined, and so the pH was adjusted by the addition of hydrochloric acid to lower the pH
or sodium hydroxide to raise the pH to the desired solution pH.
If the required pH was lower than the pH of the input solution, “Cl” was selected as the adjusted
parameter for the charge balance. Conversely, if the value was higher than the pH of the input
solution, “Na” was selected as the adjusted parameter for the charge balance.
The resulting concentrations of bicarbonate ions and carbonate ions were determined by taking
the sum of the ions and the complexes containing the ions. For example, for bicarbonate, the
concentrations of HCO3-, NaHCO3, CaHCO3+, MgHCO3+, and SrHCO3+ were summed to
obtain the total concentration of HCO3- to be used for the Bjerrum plots.
2 A Bjerrum plot is a graph of the concentrations of the different species of a polyprotic acid in a solution (in this
case for carbonate systems, where the polyprotic acid is carbonic acid), as a function of pH, when the solution is
at equilibrium.
6
RESULTS AND DISCUSSION
Analysis of Different Water Sources
The method to analyse the carbonate chemistry using aqion was used to obtain the properties
of seawater, reverse osmosis concentrate, limestone quarry water, and rainwater. The DIC,
alkalinity and the saturation states are recorded in Table 1.
The carbon species distribution and the calcite saturation state of the water sources were
calculated using the pH, DIC, and concentrations of major ions and trace elements of rainwater
[13], limestone quarry water [14], and average seawater [15]. The concentrations of the ions
and elements in reverse osmosis concentrate (ROC) were calculated from the average seawater
values assuming a maximum water recovery of 53% [16] and a pH of 7.1, which is the average
pH of the ROC from several RO desalination plants in Oman and the UAE [17]. Rainwater has
the lowest pH (5.5) and seawater has the highest pH (8.1).
The saturation states calculated for rainwater, limestone quarry water, reverse osmosis
concentrate, and seawater are 5.52 x 10-6, 1.45, 1.55, and 4.21, respectively.
Figure 1. Schematic diagram of aqion
workflow used in this study. Figure 2. Schematic diagram
to obtain data for
the Bjerrum plots and the saturation state (Ω)
using aqion.
7
Table 1. Analyses of the water sources studied
Variable Units Rainwater Limestone
Quarry Water
Seawater ROC
pH - 5.5 8.09 8.1 7.1
Ionic strength mmol/L 0.62 3.31 674.90 1418.00
TDS mg/L 33.1 177.8 36425.7 77518.9
Electrical
Conductivity
uS/cm 67.5 215.2 52809.2 92773.9
Alkalinity meq/L 0.07 1.97 2.33 5.34
Elements:
Ca mmol/L 0.020 1.023 10.650 22.660
Mg mmol/L 0.049 0.051 54.750 116.490
Na mmol/L 0.409 0.062 486.010 1034.400
K mmol/L 0.000 0.006 10.580 22.511
SO4 mmol/L 0.000 0.103 29.270 62.277
Cl mmol/L 0.480 0.029 565.760 1204.529
Fe mmol/L 0.000 1.43x10
-
0
4
0.000 0.000
Fe(2) mmol/L 0.000 8.49x10
-
06
0.000 0.000
Fe(3) mmol/L 0.000 1.35x10
-
0
4
0.000 0.000
NO3 mmol/L 0.000 0.016 0.000 0.000
Si mmol/L 0.011 0.112 0.000 0.000
B mmol/L 0.000 0.000 0.430 0.915
Br mmol/L 0.000 0.000 0.870 1.851
F mmol/L 0.000 0.000 0.070 0.149
Sr mmol/L 0.000 0.000 0.090 0.191
Carbon Species
CO2 mmol/L 4.92x10
-
1
- 4.27 x10
-
2
1.55 x10
-
2
2.25 x10
-
1
HCO3
-
mmol/L 7.13 x10
-
2
1.93 1.93 4.38
CO3
2
-
mmol/L 1.23 x10
-
6
1.70 x10
-
2
1.68 x10
-
1
5.56 x10
-
2
DIC mmol/L 0.564 1.990 2.110 4.661
Saturation Index
of Calcite
-5.258 0.16 0.624 0.19
Calcite
Saturation
State (Ω)
5.52E-06 1.45 4.21 1.55
Saturation state (Ω) is calculated according to equation (5). It is defined as having a value of 1
at the solubility limit of a mineral (in this case limestone), beyond which precipitation of
calcium carbonate occurs. The concentration at which the limit of solubility is reached varies
with salinity, which is represented by differing values of K'sp. Mook and Rozanski present
values of calcite K'sp for seawater, with an average salinity of 35 mg/g, as 4.27 x 10-7 and 3.31
x 10-9 for fresh water [18]. Other researchers present similar Ksp values for seawater [19].
Mucci summarises measurements of various researchers that show a roughly linear relationship
between the calcite and aragonite Ksp values vs salinity ranging between 5ppt and 45 ppt, and
showing that the aragonite Ksp is roughly 50% higher than the calcite Ksp across the range
[19]. Note that despite the fact that seawater has a much higher salinity than rainwater, Ω is
higher in seawater because rainwater contains almost no calcium ions.
8
AWL, which involves the dissolution of CaCO3, cannot take place when Ω is greater than 1.
We can therefore conclude that for three of the four waters studied, namely limestone quarry
water, seawater, and reverse osmosis concentrate, where the value of Ω is greater than 1, that
dissolution of limestone will not occur at the typical ranges of pH found in these waters. On the
contrary we should expect precipitation of carbon carbonate for these water sources under
normal conditions. Rainwater is the exception, which with a low pH and no calcium present,
has a corresponding value of Ω far below 1.
One concern raised in relation to the application of AWL is the fact that the calculated values
of omega for aragonite (Ωarag) and calcite (Ωcal) in seawater are already typically significantly
above unity in surface waters. Gattuso calculates the preindustrial Ωarag at 446 and the present
value at 387 (values of Ωcal, which are higher, are not given) [20]. The reason why precipitation
is not experienced in surface waters, despite Ω being well above unity, was studied by Rybacki
[21] who showed that magnesium ions (Mg2+), dissolved organic carbon (DOC), and phosphate
ions (PO43-) inhibit calcite precipitation. DOC and magnesium ions act as precipitation
inhibitors by adsorption onto the calcite crystal growth sites. However, the mechanisms and the
roles of these compounds in the inhibition of precipitation are the subjects of continued debate
[22]. Hence, it is difficult to specify the value of the saturation state at which precipitation will
occur while accounting for the effect of inhibition. Given the elevated saturation state of calcite
in average seawater are above values of unity, inhibition must be occurring in seawater.
The kinetics of calcium carbonate dissolution are typically described by the equation:
𝑟 = 𝑘(1 − 𝛺) (7)
where r is the rate of calcium carbonate dissolution, k is the rate constant and n is the reaction
order [23]. This shows that the lower Ω becomes the quicker the dissolution reaction occurs,
and that lowering the pH of the water both accelerates the reaction, and creates a stable solution
containing the newly formed calcium and carbonate ions.
What the Bjerrum plots show is that the concentration of the carbonate ion falls with reducing
pH, and correspondingly the value of Ω also falls by several orders of magnitude, as can be
seen in Figure 3.
The method to obtain Bjerrum plots using aqion was executed to obtain the distribution of the
three carbon species in seawater, reverse osmosis concentrate, limestone quarry water, and
rainwater at different pH as shown in Figure 3. Generally, it is seen that the concentration of
dissolved CO2 (or compound carbonic acid) decreases as pH increases while the concentration
of carbonate ions increases as pH increases. Meanwhile, the concentration of bicarbonate ions
increases as pH increases before decreasing as pH continues to increase.
The interplay between these components is described by the equilibrium relation in equation 8.
𝐶𝑂
(
𝑔
)
+
𝐻
𝑂
𝐻
𝐻
+
𝐻𝐶𝑂

2
𝐻
+
𝐶𝑂

(8)
As the pH increases, the concentration of hydrogen ions decreases, and the equilibrium shifts
towards the right of the equation. Therefore, bicarbonate ions become the dominant ions before
carbonate ions become the dominant carbon species at higher pH values.
9
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0.0
0.2
0.4
0.6
CO
2
HCO
3
-
CO
3
2-
Ω
pH
Carbon Species Concentration (mmol/L)
(a)
0.0
0.5
1.0
Ω
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
CO
2
HCO
3
-
CO
3
2-
Ω
pH
Carbon Species Concentration (mmol/L)
(b)
0
2
4
6
8
10
12
14
Ω
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
CO
2
HCO
3
-
CO
3
2-
Ω
pH
Carbon Species Concentration (mmol/L)
(c)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Ω
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0
1
2
3
4
5
CO
2
HCO
3
-
CO
3
2-
Ω
pH
Carbon Species Concentration (mmol/L)
(d)
-1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Ω
Figure 3. Bjerrum plots and the corresponding saturation states at different pH values for (a)
rainwater, (b) limestone quarry water, (c) seawater, and (d) reverse osmosis concentrate
Previously, it was shown that under normal conditions, AWL reaction cannot take place as Ω
was greater than 1 for limestone quarry water, seawater, and reverse osmosis concentrate. With
the addition of acid, the pH is lowered. Subsequently, due to the shift in equilibrium, the
concentration of CO32- ions decreases, thus lowering the Ω values to below unity. Hence, AWL
reaction can take place at these conditions.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1 .6
1E-7
1E-6
1E-5
1E-4
0.001
0.01
0.1
1
10
Ω
pCO
2
(atm)
(a) (b)
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
4
5
6
7
8
9
pH
pCO
2
(atm)
Seawater
ROC
Limestone
Rainwater
Seawater
ROC
Limestone
Rainwater
Figure 4. Effect of partial pressure of CO2 on (a) saturation state (Ωcal) and (b) pH.
10
Figure 4 describes the effect of the partial pressure of CO2 on pH and Ωcal for various water
bodies. It is seen that both Ωcal and pH decrease rapidly for very small increases in pCO2 above
the background level for all salinities. Precipitation is only of concern in the transition from
acidic AWL reactor conditions to the fully diluted conditions of the open ocean. Dilution is
discussed in more detail below.
Influence of minor ion concentrations on AWL using seawater
The distribution of the carbon species and Ω were determined for seawater in the absence of
several common ions, is shown in Figure 5. When the calcium, magnesium, sulphate,
potassium, and bromide ions were each separately removed from the composition of seawater,
it was found that both the carbon species distribution and Ω were very close to that for average
seawater. This indicates that overall salinity (ionic strength), rather than any one ion species,
influences the shapes and positions of the curves. Magnesium causes the greatest offset as it
has the third highest concentration after sodium and chlorine, which make up the bulk of the
ionic strength in seawater. The only exception is Ω when calcium ions were removed because
Ω is a function of the calcium ion concentration.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
Line Style: Variable
CO
2
HCO
3
-
CO
3
2-
Ω
Line Colour: System
No Calcium Ions
No Magnesium Ions
No Sulfate Ions
No Potassium Ions
No Bromide Ions
Average Seawater
pH
Carbon Species Concentration (mmol/L)
0
2
4
6
8
10
12
14
16
Ω
Figure 5. Bjerrum plots for seawater in the absence of different ions
Bjerrum Plots for an AWL reactor effluent stream
The composition of an AWL reactor effluent can be determined through Equation (4). Three
cases were studied using seawater as the feed solution, with outlet DIC increased by 2, 6 and
10 mmol/L respectively (approximately 2, 4 and 6 times higher than the ocean background).
Ocean DIC varies and so a reference value of 2.11 was arbitrarily selected for this study.
11
5 6 7 8 9 10 11
0
2
4
6
8
10
12 Line Type: Variable
CO
2
HCO
3
-
CO
3
2-
Ω
cal
Line Colour: System
Seawater
DIC=4.11 mmol/L
DIC=8.11 mmol/L
DIC=12.11 mmol/L
pH
Concentration of Carbon Species (mmol/L)
0
2
4
6
8
10
12
14
Ωcal
Figure 6. Bjerrum plots for AWL effluents of increasing DIC concentrations
The results of these three elevated DIC cases are plotted together with seawater in Figure 6,
with DIC on the left axis and Ω on the right axis. The pH of the bicarbonate concentration peaks
and the CO2 / HCO3 and HCO3 / CO3 intersections on the Bjerrum plots show only the slightest
shift to the left with increasing DIC. These represent the values of the equilibrium constants in
Equation (8), which remain relatively unchanged. At higher DIC concentrations, the
concentration of all the carbon species is higher at all pH.
Higher Ω values are also observed at higher DIC concentrations as more Ca2+ and CO32- ions
are present as a result of additional CaCO3 dissolution. This points to the fact that significantly
higher DIC concentrations are unlikely to be stable at background ocean pH of 8.1
Dilution
Comparing the calculated values of pH and Ω of an AWL reactor effluent to that obtained by
experiment, Chou et al. [11] recorded significantly lower pH and Ω at 6.55 and 0.23
respectively, which was likely caused by incomplete CaCO3 dissolution observed during the
experiment, as indicated by the measured total alkalinity, which was much lower than the
predicted value. This may be expected given that acidity is what drives the reaction, and as CO2
is consumed the pH will rise, slowing the rate of reaction, leading to incomplete dissolution.
The immediate assumption is that the excess CO2 be removed from the effluent.
Figure 7 demonstrates a profound difference between degassing before dilution of AWL reactor
effluent vs diluting first. In both cases dilution brings both pH and Ω closer to the background
levels of the dilution water, in this case surface ocean water. However, degassing to reach
equilibrium with the ambient air at pCO2 of 415 μatm before dilution raises the pre-dilution pH
significantly above 8.1 causing Ω to rise far above open-ocean values (dotted lines). In practice,
12
however, these initial very high values of Ω in such a degassed high DIC system would
precipitate much of the initially reacted limestone and the very high values of Ω would not be
experienced. Note that the DIC of average seawater is higher than the equilibrium value with
atmospheric air with pCO2 of 415 μatm [24].
Figure 7. The effect of diluting an AWL reactor effluent with seawater on (a) pH, and (b) Ωcal
without degassing (solid line) and equilibrating with ambient air pCO2 of 415 μatm prior to
dilution (dotted lines)
Dilution with seawater in a closed system (solid lines) holds the pH below 8.1 and at the same
time maintains the lowest values of Ω. Seawater typically has a pH of 8.1 and Ωcal of 4.2 [15]
and values within the AWL effluent come very close to the background ocean values within
dilution ratios of 50 and 100. In the example plotted in Figure 7 it has been assumed that excess
DIC is retained in the water in the form of CO2 which represents incomplete limestone
dissolution, as would be experienced in practice. In each case an assumed nominal excess of
0.5 mmol/l DIC was included to illustrate the point. Values experienced in a reactor will vary
according to residence time, temperature, and other reaction parameters, and so the position of
the curves will vary.
CONCLUSIONS
In the study, the calcium carbonate saturation state, Ω, is a function of the pH and the DIC
concentration, and that the value of Ω changes significantly with small changes in pH, as the
curves of Ω show in Figure 6.
Of the four types of water that were analysed, rainwater, limestone quarry water, seawater and
reverse osmosis concentrate all, apart from the rainwater, have values of Ω which exceed 1,
favouring precipitation. However, within an AWL reactor with a high pCO2 lowering the pH
to drive the reaction with limestone, the calcium carbonate saturation state is depressed far
below 1.
Total salinity has a minor impact on the DIC chemistry, causing the DIC species curves to move
very slightly to the left (lower pH) on a Bjerrum plot as salinity rises. The presence or removal
of any individual ion does not seem to impact the curves beyond their contribution to total
13
salinity. The curves of calcium carbonate saturation state, Ω, are moved to the left as DIC rises,
which reflects the increasing concentration of both calcium and carbonate ions.
This study therefore shows that elevated levels of DIC in an AWL reactor effluent are stable at
reactor conditions of low pH and elevated pCO2. However, allowing the solution to equilibrate
with ambient air (pCO2 = 415 μatm) without dilution increases both pH and Ω to the point of
significant precipitation. Dilution within a closed system allows the product stream to reach
values of Ω that are typical of the surface ocean without passing through higher values of Ω
and hence avoiding reprecipitation of the dissolved limestone. A healthy ocean requires values
of Ωcal that are above 4 for calcifying organisms to thrive. This study shows how that might be
achieved with AWL.
ACKNOWLEDGEMENTS
This work has been made possible by the financial support of Cquestr8 Ltd, a CDR business
backed by Counteract Partners Ltd in the UK.
14
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