Chapter 16

Developments in Clay Science, Vol. 3. DOI: 10.1016/B978-0-444-53607-5.00016-5# 2011 Elsevier B.V. All rights reserved.

Sepiolite and Palygorskite asSealing Materials for theGeological Storage ofCarbon Dioxide

Emilio Galan, Patricia Aparicio and Adolfo MirasDepartamento Cristalografıa, Mineralogıa y Quımica Agrıcola, Facultad de Quımica, University

of Sevilla, Professor Garcıa Gonzalez 1. 41012 Seville, Spain

1. INTRODUCTION

1.1. The Geological Storage of Carbon Dioxide

Atmospheric concentrations of greenhouse gases (e.g. carbon dioxide, meth-

ane and nitrous oxides) have increased significantly as a result of human

activity since the pre-industrial era (AD 1000–1750). Fundamentally, carbon

dioxide (CO2) has increased from a pre-industrial level of 275–285 ppm to

379 ppm in 2005 (Solomon et al., 2007). This increase has been caused

mainly by fossil fuel consumption and, to a lesser extent, concrete production

and changes in land use. The increase in average global temperatures since the

mid-twentieth century may be ascribed to increased emission of anthropo-

genic greenhouse gases (Metz et al., 2005). International concern about cli-

mate change led to the establishment in 1992 of the United Nations

Framework Convention on Climate Change (UNFCCC). The ultimate objec-

tive of UNFCCC is the ‘stabilization of greenhouse gas concentrations in

the atmosphere at a level that prevents dangerous anthropogenic interference

with the climate system’. One mean for reducing net greenhouse gas emis-

sions is ‘Capture and storage of CO2’.

Carbon dioxide capture and storage (CCS) include technologies to capture,

transport and store CO2. The storage of CO2 may be effected through a num-

ber of mechanisms, including ex situ mineral carbonation, oceanic storage,

underground injection for enhanced fossil fuel recovery and injection into

saline aquifers or other geological reservoirs, an approach known as in situmineral carbonation (Giammar et al., 2005; Metz et al., 2005; Xu et al., 2005).

375

Developments in Palygorskite-Sepiolite Research376

The subsurface constitutes the largest reservoir of carbon on Earth. The

vast majority of the world’s subsurface carbon is held in the form of coal,

oil, gas, organic-rich shales and carbonate rocks. Geological CO2 storage

has occurred naturally in the Earth’s upper crust for hundreds of millions of

years. Carbon dioxide derived from biological activity, igneous activity and

chemical reactions between rocks and fluids accumulates in the natural sub-

surface environment as carbonate minerals, in solution, or in a gaseous or

supercritical state, either as a gas mixture or as pure CO2.

Geological storage of anthropogenic CO2, as a greenhouse gas mitigation

strategy, was first proposed in the 1970s. No significant research, however,

was done until the early 1990s, when the idea gained credibility through the

work of individuals and research groups (Bachu et al., 1994; Baes et al., 1980;

Gunter et al., 1993; Holloway and Savage, 1993; Kaarstad, 1992; Koide et al.,

1992; Korbol and Kaddour, 1994; Marchetti, 1977; van der Meer, 1992).

Geological CO2 storage in sedimentary basins may be achieved within a

variety of geological settings, the most suitable formations being oil fields,

depleted gas fields, deep coal seams and saline formations. To this end,

CO2 gas must first be compressed to a dense fluid state known as ‘supercriti-

cal’. Depending on pressure and temperature increases with depth, the density

of CO2 increases to a depth of 800 m or more. At this point, the injected CO2

will be in a dense supercritical state (Figure 1).

According to Hitchen (1996), the geological storage of CO2 through injection

into deep reservoirs involves three different processes: (i) hydrodynamic trapping

as a gas or supercritical fluid below a cap rock of low permeability; (ii) solubility

trapping, through dissolution of CO2 in aqueous solutions; (iii) mineral trapping,

through the precipitation of secondary carbonates formed by dissolution of pri-

mary silicates and Al silicates upon injection of CO2 into aquifers.

Other authors (e.g. Metz et al., 2005) have differentiated between physical

and geochemical trapping:

(i) physical trapping, comprising both the stratigraphic and the residual

trapping. The former occurs below low-permeability seals or cap rocks,

whereas the second takes place in saline formations, where fluids migrate

very slowly over long distances even in the absence of closed traps;

(ii) geochemical trapping, encompassing solubility trapping and mineral

trapping.

Mineral trapping is especially attractive because CO2 is permanently

‘fixed’ (as stable carbonate minerals) in relatively deep geological formations,

preventing its return to the atmosphere.

1.2. CO2 Reactivity and Integrity of the Cap Rock

When CO2 is injected in a sedimentary basin, it has a strong tendency to react

with rocks.

100

20

11

3.8

3.2

2.8

2.7

2.7

800600400

Density of CO2 (kg/m3)

200

Assuming a geothermal gradientof 25 �C/km from 15 �C at the

surface, and hydrostatic pressure

0

0

1000

0.5

1

1.5Dep

th (

km)

2

2.5

FIGURE 1 Variation of CO2 density with depth, assuming hydrostatic pressure and a geother-

mal gradient of 25 �C km–1 from 15 �C at the surface (based on data of Angus et al., 1973). Car-

bon dioxide density increases rapidly to a depth of �800 m when CO2 reaches a supercritical

state. The cubes represent the relative volume occupied by CO2. To a depth of 800 m, this volume

dramatically decreases with depth. At depths below 1.5 km, the density and volume become

nearly constant (Metz et al., 2005).

Chapter 16 Sepiolite and Palygorskite as Sealing Materials for CCS 377

Much research effort has been focused on the mineral trapping of CO2,

through carbonate precipitation. A prerequisite for carbonate precipitation is

the availability of aqueous metal cations, derived from non-carbonate miner-

als, and their ability to combine with dissolved CO2. The dissolution of metal-

bearing silicate minerals is a very important potential source of these cations.

The dissolution rate of such minerals is mainly controlled by the pH and tem-

perature of the medium in contact with the mineral surfaces, whereas the

influence of hydrodynamic conditions is nil, at least for surface-controlled

processes.

Kaszuba et al. (2005) attempted to replicate the active processes in a typi-

cal CO2 sequestration site by reacting a mixture of quartz, feldspar, biotite and

shale with a CO2-rich NaCl brine at 200 �C. Magnesite and siderite (FeCO3)

were precipitated validating their potential for mineral trapping. They

observed that shale actively participates in coupled dissolution/precipitation

processes, indicating the potential of CO2-rich fluids for decreasing rock

integrity. Substantial amounts of aqueous Si were released into solution,

which could serve as a source for cement in sandstones or Si mineralization

Developments in Palygorskite-Sepiolite Research378

in veins. More recently, Kohler et al. (2009) have investigated the mineralog-

ical and chemical changes in clays (as possible clay-rich sealing cap rock)

after exposure to partial CO2 pressure. The principal alterations were illitiza-

tion of clay minerals combined with formation of anhydrite, dolomitization

and an increase in dissolved CO2 in the porous media.

Xu et al. (2005) have examined the effects of injecting carbon dioxide into

a common sedimentary basin sequence, a shale-bounded sandstone. The total

amount of CO2 trapped in carbonate minerals depends mainly on the compo-

sition of the rock. For a representative rock composition, 90 kg/m3 of CO2 can

be trapped during 100,000 years, mainly in sandstone. A great deal of CO2

trapping is a consequence of the presence of an adjacent shale unit, providing

many of the cations that form trapped secondary carbonates. The interaction

of acidic CO2-rich fluids with shale, however, tends to be a two-edged sword.

Although it provides the essential metals for trapping CO2 in carbonate miner-

als, the leaching of these metals may increase shale permeability, favouring

the release of CO2 into the atmosphere as Moore et al. (2005) have suggested.

In developing reactive transport models for the interaction between CO2-

rich solutions and cap rock of the Sleipner sequestration site, Gaus et al.

(2005) came to a similar conclusion. They found that the porosity and perme-

ability of the cap rock can be either increased or decreased depending on the

exact composition of the rock.

Marini and Accornero (2009) have identified several drawbacks in the geo-

chemical modelling of reactions occurring during the geological storage of

CO2. The main target of the geological storage of CO2 is represented by sedi-

mentary basins where brines are commonly present. Thus, it is necessary to

describe specific interactions among solute species at the pertinent salinities,

compute correct activity coefficients and extrapolate these interaction para-

meters to the temperature and pressure conditions of the aquifer of interest.

Unfortunately, the computer-stored thermodynamic and kinetic databases,

required for describing chemical reactions of CO2 with the aqueous solution

and aquifer solid phases, are incomplete (Marini and Accornero, 2009). Con-

sequently, the geochemical evolution of the system, following injection of

pressurised CO2, cannot be precisely described using computer-based models.

Nevertheless, geochemical modelling is the only available tool to evaluate the

long-term reactive effects of geological CO2 storage. This is because the dura-

tion of laboratory and field experiments and their duration cannot be extended

up to multiples of the life time of human beings. In contrast, computer experi-

ments have no time limitation and are open to future improvements.

The sealing rock must be capable of preventing the escape of CO2 that

forms when low-permeability rocks are dissolved by local reaction with acidic

CO2-rich fluids.

The aim of this contribution is to evaluate the potential of sepiolite- and

palygorskite-rich rocks to serve as sealing materials in the cap rock for the

geological storage of CO2. Because of their structural characteristics, such

Chapter 16 Sepiolite and Palygorskite as Sealing Materials for CCS 379

rocks would be capable of trapping CO2 physically. However, the Mg ions

present in their structures, and the exchangeable cations, could react with

CO2 to form carbonates.

1.3. Characteristics of Palygorskite and Sepiolite

Palygorskite and sepiolite contain ribbons with a 2:1 type layer structure, each

ribbon being linked to the next inverted SiO4 tetrahedral sheet along a set of

Si—O bonds. Thus, tetrahedral apices point in opposite directions in adjacent

ribbons. The ribbons are aligned parallel to the X-axis and have an average

width along the Y-axis of three linked pyroxene-like single chains in sepiolite

and two linked chains in palygorskite. In this framework, rectangular channels

run parallel to the X-axis between opposing 2:1 ribbons.

As the octahedral sheet is discontinuous at each tetrahedral inversion, oxygen

atoms in the octahedra at the edge of the ribbons are coordinated to cations on the

ribbon side only, while coordination and charge balance are completed along the

channels by protons, coordinated water and a small number of exchangeable

cations (Brigatti et al., 2006). For details on the structure, see Chapter 1.

Sepiolite and palygorskite can take up extraneous liquids, gases and vapours

into their microporous channels. These minerals also contain macropores when

individual (unit) particles combine to form aggregates (Galan, 1996)

Because of their extensive surface area and porosity, these minerals have

found useful applications as absorbents of gaseous toxic compounds, deco-

lourizing agents of oils and solid supports of enzymes and anaerobic bacteria

(Galan, 1996).

2. INTERACTION OF SEPIOLITE AND PALYGORSKITE WITHSUPERCRITICAL CO2

2.1. Material Characterization and Methodology

Sepiolite (SEP) from Vicalvaro, Madrid basin (Spain) and palygorskite (PAL)

from Theis (Senegal), both commercialized by TOLSA, S.A., were selected

for this study.

The mineralogical composition of the raw materials was determined by

X-ray diffraction (D8 Advance model, Bruker) and the chemical composition

performed by X-ray fluorescence (Axios model, Panalytical). The elemental

carbon content was measured using an elemental analyser (LECO CHNS 932).

The sepiolite sample is composed of sepiolite (75%) together with quartz

and calcite. The palygorskite sample contains about 50% palygorskite,

together with carbonates (calcite and dolomite), minor amounts of quartz

and traces of sepiolite and smectite. The chemical composition of the samples

is consistent with their mineralogy (Tables 1 and 2). Elemental carbon con-

tents are 0.45 wt% for sepiolite and 1.91 wt% for palygorskite.

TABLE 1 Mineralogical Composition (wt%).

Sample Quartz Calcite Dolomite Sepiolite Palygorskite Smectite

SEP 16 5 Traces 75 – –

PAL 6 34 10 <5 47 Traces

Developments in Palygorskite-Sepiolite Research380

To evaluate the reactions of sepiolite and palygorskite with CO2, several

experiments were made in a high-pressure and high-temperature Paar reactor.

Depending on the water content of the material, temperatures between 50 and

150 �C were used to give pressures commensurate with supercritical condi-

tions (Figure 2, Table 3).

We also determined the specific surface area of the samples (before and

after reaction with CO2) by adsorption of N2 and applying the BET equation

(Gemini 2360 equipment). In addition, soluble ions were measured by ICP-

OES (Horiba Jobin Yvon, mod ultima2). The mineralogical composition of

reacted materials was also investigated by XRD, and their content of total ele-

mental carbon was evaluated by the elemental analyzer above mentioned.

A profile-fitting peak decomposition program, part of MacDiff 4.1.2 by

Petschick (2004), was used to assess changes in the main representative peaks

in the XRD patterns. A Pearson VII function was used to obtain the following

parameters: peak position, height above the baseline, full width at half height

and the mixing parameter for the function. The initial fit results were iterated

until the difference between the experimental and decomposed patterns

was <5%.

2.2. Potential of Sepiolite and Palygorskite for the Physical andGeochemical Trapping of CO2

The results of experiments, carried out for a maximum of 5 days reaction

time, indicated that physical CO2 trapping occurred on the surface of sepiolite

and palygorskite, causing the BET–N2 surface area decreased by about 50%

for both minerals. To evaluate the influence of reaction time on CO2 trapping,

we carried out a series of experiments over a period of �43 days. The results

indicate differences between sepiolite and palygorskite in their behaviour

towards CO2 (Table 4).

The BET–N2 area of sepiolite increased (the increase for SEPSCH45d is

not as much as for SEPSC46d), suggesting mineral decomposition by car-

bonic acid. The increase in porosity after prolonged reaction in the presence

of water (Figure 3A) lends further support to this suggestion. However, paly-

gorskite can apparently trap CO2 physically, as the BET–N2 area decreases by

TABLE 2 Chemical Composition (wt%).

SiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O TiO2 P2O5 SO3 LOI Total

SEP 54.99 1.94 0.46 0.01 22.54 0.14 0.15 0.63 0.10 0.03 0.05 19.60 100.62

PAL 52.19 3.47 1.75 0.03 7.39 12.06 0.05 0.12 0.21 0.36 0.05 20.85 98.52

Chapter

16

Sepiolite

andPalygo

rskiteas

SealingMaterials

forCCS

381

1000

100

Critical point(31.1 �C, 7.38 MPa)

CO2 solid10

1 Triple point(−56.6 �C, 0.51 MPa)

0.1 CO2 gas

CO2 liquid

Sup

ercr

itica

lC

O2

0.01

Pre

ssur

e (M

Pa)

0.001

0.0001−140 −100 −60 −20 20

Temperature (�C)

60 100

Solid and liquid

Liquid and gasSo

lid a

nd g

as

FIGURE 2 Phase diagram of CO2 (Schutt et al., 2005).

TABLE 3 Pressure, Temperature, Humidity and Reaction Time for the

Experiments.

Sample Label Pressure

(MPa)

Temperature

(�C)Relative

Humidity

(%)

Reaction

Time (h)

Sepiolite SEPSC46d 4 60 <1 1104

SEPSCH45d 6 50 20 1080

Palygorskite PALSC43d 3.5 150 <1 1032

PALSCH43d 8 80 20 1032

Developments in Palygorskite-Sepiolite Research382

more than 70% and the porosity changes significantly after prolonged contact

with CO2 with a reduction of pore volume (Figure 3B).

There was a marked increase in the concentration of soluble ions after pro-

longed exposure of sepiolite to CO2, particularly in the presence of water. The

content of soluble Mg and Si was almost twice that of the original sample

(Table 5). The content of soluble Ca, however, decreases significantly (from

1370 to 377 mequiv./100 g). These observations may be ascribed to chemical

degradation of sepiolite by carbonic acid. The increase in the width-at-half-

height (H) of the XRD reflections of sepiolite (Figure 4) provides further

A

0.00

0.25

0.50

0.75

1.00

0.0 0.15 0.30 0.45 0.65 0.80 0.95

0.0 0.15 0.30 0.45 0.65 0.80 0.95

P/P0

V/V

0

SEP SEPSCH45d SEPSC46d

B

0.00

0.25

0.50

0.75

1.00PAL PALSCH43d PALSC43d

P/P0

V/V

0

FIGURE 3 N2 adsorption isotherms for sepiolite (A) and palygorskite (B) before and after CO2

treatment (P/P0: Relative Pressure; V/V0: Relative Volume).

TABLE 4 Changes in BET–N2 Surface Area of Sepiolite and Palygorskite

Following Long-term (>43 days) Reaction with CO2.

Sample Label Conditions BET (m2/g)

Sepiolite Original sample Unpressured 108

SEPSC46d 4 MPa, 60 �C, 1104 h 281

SEPSCH45d 6 MPa, 50 �C, 20% RH, 1080 h 263

Palygorskite Original sample Unpressured 331

PALSC43d 3.5 MPa, 150 �C, 1032 h 77

PALSCH43d 8 MPa, 20 �C, 20% RH, 1032 h 98

Chapter 16 Sepiolite and Palygorskite as Sealing Materials for CCS 383

TABLE 5 Soluble Ions (mequiv./100 g) for the Samples Before and After CO2 Treatment.

Samples Ca

(mequiv./

100 g)

K

(mequiv./

100 g)

Mg

(mequiv./

100 g)

Na

(mequiv./

100 g)

S

(mequiv./

100 g)

Si (mequiv./

100 g)

Sr (mequiv./

100 g)

Al (mequiv./

100 g)

Startingsepiolite

1370 19 741 206 678 2629 2 9

SEPSC46d 609 22 774 135 69 120 2 0

SEPSCH45d 377 40 1288 290 120 4126 1 0

Startingpalygorskite

722 58 250 318 151 3979 2 0

PALSC43d 1023 33 426 280 349 1038 2 0

PALSCH43d 968 18 683 228 401 1846 1 0

Note: In bold are the results with show significant changes.

Develo

pmen

tsin

Palygo

rskite-Sepiolite

Research

384

Chapter 16 Sepiolite and Palygorskite as Sealing Materials for CCS 385

evidence for this process. The sepiolite degradation is confirmed by the XRD

fitting procedure that indicated the presence of new reflections, especially in

the presence of water during the reaction (Table 6, Figure 5).

Figure 4 also shows that the intensity of the peak due to calcite increases

after CO2 treatment in the presence of water. This observation is consistent

with the increase in elemental carbon content from 0.451 wt% for the original

sample to 0.569 wt% for the material after treatment with CO2 in the presence

of water. In the absence of water, the elemental carbon content of the treated

sample is practically identical with that of the original sample.

These results would indicate that physical trapping of CO2 does not occur

with sepiolite because this mineral decomposes in an acid medium, releasing

Ca ions to form calcite (mineral trapping). But released Mg did not enter a

carbonate lattice, evidently because formation of calcite is favoured over pre-

cipitation of magnesite and dolomite. Hence, no magnesite or dolomite was

precipitated.

Concerning palygorskite, we can say that a significant increment of solu-

ble cations (Ca, Mg) was detected after carrying out several experiments in

the same conditions adopted for sepiolite Table 5. These values were asso-

ciated with a decrease on soluble K, Na and Si. We can interpret these chemi-

cal variations as due to the effect of carbonic acid, which destroys

palygorskite. The XRD fitting procedure confirmed the destruction of paly-

gorskite, because the intensity of the reflections decreased significantly and

new reflections appeared in the presence of water (Table 7, Figure 6).

Sep

After 46d CO2 reactionCa

Q

Sep

Sep Sep

Sep

Sep

Sep

H = 0.488

H = 0.531

H = 0.466

H = 0.830

H = 0.771

H = 0.576

1200

1300

1100

1000

900

800

700

600

Lin

(cou

nts)

500

400

300

200

100

04 10 20 30 40

2-Theta - scale50 60

After 42d CO2+ H2O

Natural

FIGURE 4 XRD patterns of sepiolite before and after reaction with CO2. Sep¼sepiolite;

Q¼quartz; Ca¼calcite (H: width-at-half-height).

TABLE 6 Summary of XRD Peak Fitting Results for Sepiolite.

dhkl I H

Original sepiolite (Figure 5A)

12.728 67 0.611

11.946 671 0,536

After CO2 reaction without water (Figure 5B)

12.722 56 0.581

11.991 700 0.528

After CO2 reaction with water (Figure 5C)

12.335 193 0.487

11.862 540 0.384

11.461 122 0.425

I, intensity; H, width-at-half-height.

6

1 1

1

A B C

7 8 9 6 7 8 9 6 7 8 9

FIGURE 5 XRD patterns and peak fitting results for sepiolite: (A) original sample, (B) after

CO2 reaction without water, (C) after CO2 reaction with water.

Developments in Palygorskite-Sepiolite Research386

However, the elemental carbon content increases from 2 to 4 wt% after

prolonged exposure to CO2. The XRD patterns indicate a parallel increment

in carbonates (calcite and dolomite; Figure 7, Table 8).

These results indicate that palygorskite is capable of trapping CO2 both

physically and geochemically. In fact, secondary carbonates formed immedi-

ately with partial destruction of the mineral structure through attack by

carbonic acid.

TABLE 7 Summary of XRD Peak Fitting Results for Palygorskite.

dhkl I H

Original palygorskite sample (Figure 6A)

10.855 30 0.619

10.419 1893 0.461

9.944 613 1.547

After CO2 reaction without water (Figure 6B)

10.978 25 0.519

10.472 214 0.42

10.109 48 0.42

After CO2 reaction with water (Figure 6C)

10.686 68 0.628

10.491 132 0.17

10.303 58 0.071

10.286 103 0.48

I, intensity; H, width at half height.

8

2A

B

C

2

3

3

1 4

2

1

3

1

10 8 10 7 8 9 10

FIGURE 6 XRD patterns and peak fitting results for palygorskite: (A) original sample, (B) after

CO2 reaction without water, (C) after CO2 reaction with water.

Chapter 16 Sepiolite and Palygorskite as Sealing Materials for CCS 387

Pa

800

700

600

500

400

300

200

100

03 10 20 30 40 50 60

H = 0.488

H = 0.368Lin

(cou

nts)

H = 0.359

Pa

Do

PAL-SCH-43D

DoDo

Natural

2-Theta-scale

Pa

CaCa

Ca

CaCa Ca

After 46d CO2 reaction

After 42d CO2+ H2O

Q

FIGURE 7 XRD patterns of palygorskite before and after CO2 reaction. Pa¼palygorskite;

Q¼Quartz; Ca¼calcite; Do¼dolomite.

TABLE 8 Variation of Carbonates Content on Palygorskite Samples Before

and After CO2 Reaction in Relation with Quartz.

Calcite Area/

Quartz Area

Dolomite Area/

Quartz Area

Natural 4.18 3.13

After CO2 reaction without water 4.71 3.36

After CO2 reaction with water 10.54 5.64

Developments in Palygorskite-Sepiolite Research388

To determine if mineral carbonation also occurs at room temperature and

humidity, several experiments were carried out in situ using a D8 Advance

diffractometer equipped with an Anton Par Reaction Camera. The experi-

ments were run for a week in the presence of water. No changes in the

XRD profile were detected for either sepiolite or palygorskite after treatment

with CO2, indicating that high pressures and temperatures are required to pro-

mote chemical reactions with CO2.

Chapter 16 Sepiolite and Palygorskite as Sealing Materials for CCS 389

3. MODELLING THE ROLE OF SEPIOLITE ANDPALYGORSKITE IN THE GEOLOGICAL STORAGE OF CO2:A TASK FOR THE FUTURE

The sepiolite material used here has a BET–N2 surface area of 108 m2/g

(Table 4). This value can increase up to twofold after treatment with CO2 pos-

sibly due to the formation of amorphous phases through the following

reaction:

Sepiolite þ H2CO3 ! sepioliteþ }amorphous sepiolite}þ amorphous silicaþ CaCO3

Prolonged attack by carbonic acid can lead to: (a) a reduction in the volume of

structural micropores (radius <1.5 nm), that are those contributing to generate a

higher specific surface; (b) an increase in the concentration of surface silanol

(SiOH) groups through rupture of SiOSi bonds on the sepiolite surface, which to

compensate their residual charge accept either a proton or a hydroxyl ion (proton-

ation and hydroxylation) and (c) a progressive destruction of the sepiolite structure.

Thus, sepiolite can become amorphous when it reacts with supercritical CO2

in the presence of water, including surface-adsorbed water and zeolitic water

within the channels of the sepiolite structure, while Mg carbonate precipitation

is inhibited probably by kinetic reasons (Saldi et al., 2009). Although amor-

phous sepiolite and amorphous silica might be expected to hold CO2 physically

because of their extensive surface area, this was not borne out by experiment.

It can therefore be predicted that when sepiolite and sepiolite-rich rocks

are used as sealing materials, these minerals would decompose within a short

time, at least in the very first few metres of the cap rock overlaying the geo-

logical reservoir where CO2 is stored in supercritical conditions. The resultant

variations in volume and decreased mechanical resistance of the cap rock

would cause operational problems.

In the case of palygorskite, the BET–N2 surface area decreased after treat-

ment with CO2. This might be because the carbonic acid produced led to the

formation of amorphous phases, partial destruction of the structure and reduc-

tion in micropore volume. The exchangeable cations and the Mg ions released

into solution induce precipitation of dolomite and calcite. The overall process

may be visualized by the following reaction:

Palygorskiteþ H2CO3 ! palygorskiteþ }amorphous palygorskite}

þ amorphous s�ilicaþ dolomiteþ calcite

Thus, palygorskite behaves similarly to sepiolite, but the presence of calcite in

the original material acts as a pH buffer. Supposedly, if palygorskite is des-

tructed, calcite is destructed as well and the Ca released together with the

Mg that comes from the destroyed palygorskite, are both used to form

Developments in Palygorskite-Sepiolite Research390

dolomite. Hence, there is a CO2 physical adsorption that occurs together with

the precipitation of dolomite, which is relatively more resistant to an acid

ambient.

The formation and relative stability of dolomite will offset problems

related to volumetric variations or mechanical resistance of the cap rock over-

laying the geological reservoir of CO2. Thus, palygorskite and palygorskite-

rich rocks could be used as sealing materials for the storage of CO2. Further

research and data testing, however, are required for the development of math-

ematical models (both reaction path and reactive transport models), based on

the experimental evidence and the conceptual model presented here, describ-

ing the reactions that may occur over a specified period of time.

4. CONCLUDING REMARKS

Our research suggests that sepiolite and palygorskite are capable of sequestering

CO2 through both a physical and a mineral mechanism. At the same time, these

minerals are subject to attack by CO2 in either a dry or a wet environment, lead-

ing to partial decomposition of their structure together with a reduction in vol-

ume and mechanical resistance. This attack may lead to the partial destruction

of the cap rock and possible loss of CO2. In the case of palygorskite, however,

the carbonates formed during reaction with CO2 can precipitate. Thus, a portion

of the CO2 is chemically bound (trapped). More importantly, the structural

integrity of palygorskite is largely preserved, and CO2 release is inhibited.

A mathematical model needs to be developed to describe the sequence of

reactions between CO2 and sepiolite- and palygorskite-rich rocks (reaction

path and reactive transport) and then validated by experimental measurements

for long-term reactive effects.

ACKNOWLEDGEMENT

This researchwas granted by INSTALACIONES INABENSASAunder the Proj-

ect ‘Estudio y Modelado de la Interaccion entre el CO2 supercrıtico y las arcillas

del subsuelo’, with the collaboration of Ings. Enrique Moreno and Marıa Perez.

Diffraction patterns and chemical analysis were performed using the facilities

of the General Research Centre at the University of Seville (CITIUS). Authors

want to thank Profs. Theng and Marini, their comments which improved the

manuscript.

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