Serpentinite Carbonation
for CO2 Sequestration
Ian M. Power1, Siobhan A. Wilson2, and Gregory M. Dipple1
1811-5209/13/0009-115$2.50
DOI: 10.2113/gselements.9.2.115
S
erpentinites offer a highly reactive feedstock for carbonation reactions
and the capacity to sequester carbon dioxide (CO2 ) on a global scale.
CO2 can be sequestered in mined serpentinite using high-temperature
carbonation reactors, by carbonating alkaline mine wastes, or by subsurface
reaction through CO2 injection into serpentinite-hosted aquifers and serpentinized peridotites. Natural analogues to serpentinite carbonation, such as
exhumed hydrothermal systems, alkaline travertines, and hydromagnesite–
magnesite playas, provide insights into geochemical controls on carbonation
rates that can guide industrial CO2 sequestration. The upscaling of existing
technologies that accelerate serpentinite carbonation may prove sufficient
for offsetting local industrial emissions, but global-scale implementation
will require considerable incentives and further research and development.
carbonation of alkaline industrial
waste, such as serpentinite mine
tailings, and (2) in situ mineral
carbonation processes initiated
by the injection of CO 2 into
serpentinite-hosted aquifers or
the injection of CO2 and seawater
into peridotite and serpentinite
at higher temperatures (>150 ºC)
(FIG. 1). The challenge is to develop
carbonation technologies that
operate at a scale and rate that are
relevant to anthropogenic GHG
emissions locally and globally.
Serpentinite carbonation occurs
naturally under geochemical
conditions that are similar to those
currently being investigated for
the development of industrial and
geoengineering strategies for CO2
sequestration. Carbonate minerals are produced during
serpentinization (Evans et al. 2013 this issue) and during
hydrothermal alteration and weathering of serpentinite.
This propensity to form carbonate minerals in ultramafic
terranes reflects the thermodynamic stability of these
minerals in the presence of CO2 . In natural systems, the
extent of carbonation may be limited by the availability
of CO2 or by sluggish reaction kinetics (e.g. Zhang et al.
2000; Power et al. 2009; Beinlich et al. 2012). Natural
analogues provide valuable insight into the mechanisms
of carbonation and the geochemical controls on the scale
and rate of carbonation reactions. Thus, understanding
the processes and limitations of mineral carbonation in
natural systems is a useful guide for the development of
serpentinite carbonation for CO2 sequestration.
KEYWORDS : carbon sequestration, climate change, mineral carbonation,
geoengineering, serpentinite
INTRODUCTION
Carbon sequestration research and technology is motivated
by concerns that increasing atmospheric carbon dioxide
(CO2) concentrations will drive long-term change to Earth’s
climate (IPCC 2005). The concentration of atmospheric
CO2 is expected to continue to rise due to the availability
of inexpensive fossil fuels and increasing energy demands
driven by population growth. Mineral carbonation is an
approach for sequestering CO2 that utilizes the reaction
of alkaline earth silicate and hydroxide minerals with
CO2 to form carbonate minerals. It has been an active
area of greenhouse gas (GHG) mitigation research since
the mid-1990s. This research seeks to capitalize on the
large global reserves and wide distribution of ultramafic
rocks, such as serpentinite, and the long-term stability of
carbonate minerals (Lackner et al. 1995).
Global estimates of accessible serpentinized peridotite
reserves are in the hundreds of thousands of gigatonnes
(Lackner 2002); carbonation of this resource could
mineralize the CO2 produced from the combustion of all
known coal reserves (10,000 Gt C; Lackner et al. 1995).
GHG mitigation strategies that employ serpentinite
include (1) ex situ mineral carbonation reactions in
high-temperature industrial reactors and low-temperature
1 Mineral Deposit Research Unit
Department of Earth, Ocean and Atmospheric Sciences
The University of British Columbia
Vancouver, British Columbia V6T 1Z4, Canada
E-mail of corresponding author: [email protected]
2 School of Geosciences, Monash University
Clayton, VIC 3800, Australia
E LEMENTS , V OL . 9,
PP.
115–121
CARBONATION REACTIONS
In nature, carbonate minerals can form during
serpentinization or during hydrothermal carbonation and
weathering of serpentinites (FIG. 2A). Industrial mineralcarbonation processes are commonly represented by
the reaction of olivine or serpentine with CO2 to form
magnesite + quartz ± H 2O. Similar reaction pathways are
preserved in serpentinized peridotites and in carbonate-altered serpentinites. Serpentinization reactions may
proceed by the hydration of olivine to produce serpentine +
brucite or the hydration–carbonation of olivine to produce
serpentine + magnesite (FIG. 2 A). Subsequent carbonate
alteration of serpentinite can produce talc + magnesite
and magnesite + quartz (listwanite) assemblages (Hansen
et al. 2005) (FIG. 2A). These reactions are relevant to carbon
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commonly controlled, at least in part, by microbial activity.
Microorganisms are ubiquitous in the environment and
have a variety of metabolisms that may enhance dissolution
of the mineral constituents of serpentinite. For instance,
biological effects (e.g. microbial acid production) enhance
weathering on Earth by at least 100 to possibly >1000
times (Schwartzman and Volk 1989). Microorganisms may
also contribute to carbonate precipitation while playing
an important role in the global carbon budget (Ferris et
al. 1994). Microbial processes may alter water chemistry
by affecting factors such as pH and the concentrations
of cations and dissolved inorganic carbon, which can
induce carbonate precipitation. Such processes include
photosynthesis, ammonification, denitrification, sulfate
reduction, and manganese and iron oxide reduction (Ferris
et al. 1994; Dupraz et al. 2009). Some of these processes
are now known to play an active role in the formation
of landscape features, such as hydromagnesite–magnesite
playas, that result from serpentinite weathering.
Mg2Si2O6
0.0
5.0
Reactants to products
(A) Pertinent reaction pathways at high and low
temperatures that lead to carbonation of peridotite
and serpentinite.
FIGURE 2
E LEMENTS
1.0
4.0
3.0
MgCO3
Mineral carbonation:
2.0
magnesite
magnesite + quartz
MgCO3 + SiO2
CaMgSi2O6
MgCO33·3H22O
hydromagnesite + amor. silica
Mg5(CO3)4(OH)2·4H2O + SiO2
diopside
Mg(OH)2
nesquehonite
g
& rin
tic
e
o
i
ab ath
e
W
Mg2SiO4
brucite
lansfordite
talc + magnesite
Mg3Si4O10(OH)2 + MgCO3
ic
ot
bi
forsterite
MgCO3·5H2O
CARBONATION
Carbonate alteration
3.0
Mg33Si22O55(OH)44
artinite
serpentine + magnesite
MgCO3
Mg3Si2O5(OH)4 + M
4.0
serpentine
magnesite + CaCO3
enstatite
serpentine + brucite
Mg3Si2O5(OH)4 + Mg(OH)2
Mg2CO3(OH)2·3H2O
olivine
Mg2SiO4
B
HYDRATION
Tonnes reactant mineral/tonne CO2
Se
rpe
nti
ni z
ati
on
A
The mineralogy of both feedstock and carbonate precipitate
affects the footprint of an industrial carbon-sequestration
operation. The mass ratio of CO2 sequestered to reactant
mineral depends on the amount of Mg present in the
dypingite + CaCO3
Serpentinite carbonation also occurs as a consequence of
weathering at the lower temperatures that dominate at
Earth’s surface (FIG. 2A). Globally, continental weathering
sequesters an estimated 300 million tonnes of CO2
each year (Seifritz 1990). While serpentinite represents
only a small proportion of crustal rocks, it contributes
a disproportionately high share of weathering products
because of its reactivity. The weathering of ultramafic rocks is
hydromagnesite + CaCO3
sequestration technologies that use high-temperature
carbonation reactors and the geothermal carbonation of
serpentinite (FIG. 1).
hydromagnesite
Conceptual diagram depicting geoengineered systems
designed for serpentinite carbonation. Graphical
elements are courtesy of the Integration and Application Network,
University of Maryland Center for Environmental Science (http://ian.
umces.edu/symbols).
FIGURE 1
Mg5(CO3)4(OH)2·4H2O
Cumulates
Magnesite precipitation, although thermodynamically
stable, is inhibited in near-surface environments by the
strong hydration shells surrounding aqueous Mg2+ ions.
Consequently, hydrated magnesium carbonate minerals,
such as hydromagnesite, dypingite and nesquehonite
(FIG. 2), form instead (e.g. Königsberger et al. 1999). The
variability in the hydration states (i.e. the H 2O content)
of Mg carbonate minerals reflects the efficiency with
which natural geochemical systems are able to break the
hydration shells surrounding Mg2+ ions. Natural systems
that produce hydrated Mg carbonate minerals may therefore provide valuable insight into reaction pathways that
could be used to improve the rate and efficiency of mineral
carbonation. Hydrated Mg carbonate minerals form most
commonly in hydromagnesite–magnesite playas and
alkaline spring systems, which represent natural analogues
to low-temperature mineral carbonation in alkaline wastes,
such as serpentinite mine tailings (FIG. 1).
artinite + CaCO3
nesquehonite + CaCO3
Gabbros
Serpentinized harzburgite
+ dunite
Mélange
dypingite
Geothermal carbonation
of serpentinized peridotite
Sheared
serpentinite
Mg5(CO3)4(OH)2·5H2O
Electrical
generation
and CO2 capture
CO2 injection into
serpentinite-hosted
aquifer
lansfordite + CaCO3
High-T carbonation
reactors
Carbonation of
serpentinite tailings
2.0
Tonnes product mineral/tonne CO2
(B) Efficiency of carbonating the major mineral components
of serpentinite and peridotite to form Mg carbonate minerals
(see text)
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starting mineral. The mass ratio of CO2 sequestered to
product mineral depends on (1) the ratio of CO2 to MgO
in the mineral precipitate and (2) the amount of H2O in
the carbonate-mineral precipitate (FIG. 2B). For instance,
less reactant mineral is required to sequester 1 tonne of
CO2 in minerals with a 1:1 CO2 :MgO ratio (e.g. magnesite,
lansfordite, nesquehonite) than in minerals with lower
CO2 :MgO ratios (e.g. hydromagnesite, dypingite, artinite)
(FIG. 2B). Importantly, the mass ratio also depends strongly
upon the hydration state of the carbonate mineral. The
hydration state of the carbonate mineral impacts the water
budget for mineral carbonation as well as the stability of CO2
fi xation, with less hydrated minerals in general being more
stable. Thus, reactant mineral feedstocks and carbonation
products need to be carefully selected and controlled in
order to manage the stability of carbonate minerals, water
resources, and the cost of transportation and processing
associated with serpentinite carbonation. The rate of CO2
fi xation is related to dissolution kinetics, which favor the
reaction of brucite or olivine over serpentine. However,
many factors contribute to dissolution rates, including the
extent of silica polymerization in alkaline earth silicates,
metal cation size, and the availability of reactive surface
area (Brantley 2003). The inherently high reactive surface
areas of serpentine-group minerals, often enhanced by
comminution during mineral processing, contribute to a
relatively high dissolution rate for serpentine in industrial
wastes. Carbonation rates are dictated by the rates of several
processes, including the initial dissolution of silicate and
hydroxide minerals, the passivation of mineral surfaces by
secondary minerals, the availability of CO2 in solution, and
the dehydration of Mg2+ ions.
NATURAL ANALOGUES
Diverse reaction pathways and limits on reaction rates are
evident in natural examples of serpentinite carbonation,
the study of which provides a framework for developing
geochemical and biogeochemical strategies for enhancing
carbonation rates in geoengineered systems (FIG. 3).
A
Listwanite
B
Talc–Magnesite and Listwanite Alteration
Talc–magnesite and magnesite–quartz (listwanite) rocks,
essentially fossil analogues of mineral-carbonation systems,
are common in ultramafic terranes on Earth. Their abundance
suggests that conditions favorable for mineral carbonation
exist at shallow levels of the crust. While modest carbonation may accompany serpentinization, extensive carbonate
alteration typically represents a separate hydrothermal event
(Naldrett 1966), producing talc–magnesite and magnesite–
quartz assemblages (FIG. 3A). Carbonate alteration occurs
at temperatures (200–300 °C) and pressures (equivalent to
3–5 km depth) that may be within reach of deep, industrial
CO2 -injection systems, which currently extend to 2.5 km.
In ultramafic terranes, the carbonation reaction is driven by
CO2 infiltration (Naldrett 1966), and the reaction sequences
correlate with changes in the nature of permeability (Hansen
et al. 2005). Phase-equilibrium calculations indicate that
reaction sequences preserved in carbonate-altered serpentinite can be attributed to changes in the CO2 content of
the fluid phase, which could be controlled in a CO2 injection scenario.
Talc–magnesite alteration of serpentinite is nearly isovolumetric, because the volume gain of carbonation is largely
balanced by the volume loss associated with dehydration
(Naldrett 1966). Thus porosity and permeability may be
maintained during reaction. In partially serpentinized rocks,
talc–magnesite alteration of olivine produces rock volume
increases that can be preserved as extension veins (Hansen
et al. 2005). Sharp reaction fronts in exhumed systems
indicate that reaction rate and progress are governed by
the rate of CO2 supply to the reaction site (Beinlich et al.
2012). Listwanite formation is generally associated with a
large increase in rock volume and is typically associated
with extension vein systems attributed to reaction-induced
cracking. This self-cracking mechanism may be desirable for
injection of CO2 into serpentinites, as it would perpetuate
mineral carbonation reactions by generating permeability
and reactive surfaces.
Springs & travertines
C
Hydromagnesitemagnesite playas
Processes:
High-T reactions
Self-cracking
Subsurface dissolution
Surface precipitation
Weathering (abiotic & biotic)
Limitations:
Reactive surface area
CO2 supply
Carbonate precipitation
Water and CO2 supply
High-T &
geothermal
carbonation
Acid generation (silicate dissolution)
Alkaline pH (carbonate precipitation)
Magnesite precipitation
CO2 supply
CO2 injection into
serpentinite-hosted aquifer
Carbonation of
serpentinite mine tailings
Geoengineered
systems:
Natural analogues for serpentinite carbonation.
(A) Listwanite formed under high-temperature and
high-pressure conditions showing a self-cracking mechanism
FIGURE 3
E LEMENTS
(British Columbia, Canada). (B) Alkaline spring in travertine in
Oman. (C) Hydromagnesite–magnesite playas formed from
weathering of serpentinite in British Columbia, Canada.
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Springs and Travertines
Springs and travertines are associated with serpentinites
and variably hydrated peridotites. Near-surface carbonation
of serpentinites and less hydrated peridotites is known to
occur in low-temperature spring systems in Italy, California,
and Oman (O’Neil and Barnes 1971; Kelemen and Matter
2008). Natural carbonation of these Mg-rich rocks produces
alkaline springs that form large travertine deposits (FIG. 3B),
carbonate fracture and vein fillings, and carbonate chimneys
at submarine hydrothermal vents. Weathering of ultramafic
rocks produces two types of waters: (1) shallow, circumneutral Mg–HCO3 groundwater, which develops from the
interaction of meteoric water with serpentinite and can
produce hydromagnesite–magnesite playas; and (2) deep,
high-pH (≥11) Ca–OH groundwater, which develops from
Mg–HCO3 water when isolated from the atmosphere during
carbonate alteration of serpentinite at depth (Barnes and
O’Neil 1969). Although both Mg2+ and Ca2+ are released
from primary forsterite and pyroxenes in peridotite, Mg2+
is preferentially incorporated into reaction products such as
serpentine, brucite, and magnesite, thereby concentrating
Ca2+ in solution. The resultant Ca–OH groundwaters are
extremely depleted in aqueous CO2. Thus, upon discharging
at the Earth’s surface as springs, they react rapidly with
the atmosphere by absorbing CO2 and precipitating Ca
carbonate minerals. As an example, in Oman ~104 tonnes
CO2 per year are consumed by peridotite carbonation
(Kelemen and Matter 2008). The long-term precipitation
of calcite from emergent Ca–OH spring water can produce
extensive carbonate travertine deposits and highlights
the capacity for alkaline solutions to absorb CO2 directly
from the atmosphere. The rate of carbon fi xation is likely
limited by the flow rate of the springs. Spring and travertine
systems associated with serpentinites and variably hydrated
peridotites represent natural analogues for carbonation in
low-temperature, serpentinite-hosted aquifers (FIG. 1).
Hydromagnesite–Magnesite Playas
At near-surface conditions, serpentinite carbonation can
proceed in two spatially separated stages: (1) weathering
of serpentinite that produces Mg–HCO3 groundwater and
(2) Mg carbonate precipitation that typically occurs in
closed basins where groundwater discharges. This process is
exemplified by the formation of hydromagnesite–magnesite
playas (FIG. 3C) (Power et al. 2009). Mg carbonate precipitation is mediated by microorganisms and abiotic kinetic
processes that include CO2 degassing and evaporation of
groundwaters discharged at the surface. For instance, cyanobacteria are able to induce the precipitation of dypingite
through the alkalinization of their microenvironment in
wetlands (Power et al. 2009). The playa deposits produced
by these wetlands are dominated by hydrated Mg carbonate
minerals such as hydromagnesite, which is a common
dehydration product of dypingite. In this near-surface
environment, serpentinite carbonation is limited by the
slow kinetics of silicate-mineral dissolution and carbonate
precipitation. In terms of stability, the transformation of
hydromagnesite to magnesite, the most stable of the Mg
carbonate minerals, occurs either by dissolution–precipitation or by dehydration of hydromagnesite. It is estimated
that this process requires on the order of tens to hundreds
of years (Zhang et al. 2000). Early Holocene glaciolacustrine sediments underlie hydromagnesite–magnesite playas
in northern British Columbia, which suggests that these
natural deposits are stable on the timescale of millennia
and that artificial carbon sinks designed to mimic these Mg
carbonate deposits should be stable for thousands of years,
a period long enough to produce a greenhouse gas benefit.
Hydromagnesite–magnesite playas highlight the important
E LEMENTS
role that biogeochemical reactions might play in carbon
sequestration in alkaline wastes such as serpentinite mine
tailings (Power et al. 2009).
EX SITU INDUSTRIAL CARBONATION
High-Temperature Carbonation Reactors
Carbonation in high-temperature chemical reactors (FIG. 1)
has received by far the most research activity (Huijgen and
Comans 2005; Zevenhoven et al. 2011) and also bears the
least resemblance to natural systems. High-temperature
mineral carbonation targets the rapid conversion of silicate
minerals, including serpentine and olivine, to magnesite
and quartz at rates sufficient to neutralize the CO2 emissions
of a coal-burning electricity-generation facility (~100–1000
tonnes CO2 per hour). A mineral-carbonation plant would
operate using mineral feedstock delivered from a mine to
a chemical reactor that is supplied with high-purity CO2
captured from power plant emissions. The accommodation
of power plant CO2 fluxes requires rapid reactions and short
residence times for mineral feedstock (i.e. on the timescale
of hours).
Most high-temperature carbonation strategies employ either
gas–mineral or gas–water–mineral reactions. Both dry and
wet reaction systems have merit. Direct gas reaction offers
simple process design and greater potential for recovering
heat but suffers from slow reaction rates. The addition of
water greatly enhances reaction rates but requires significant energy inputs and the conscription of water resources.
Direct aqueous carbonation involves (1) dissolution of CO2
gas into an aqueous solution, (2) dissolution of a Mg silicate
mineral, and (3) precipitation of a carbonate mineral. The
release of Mg from silicate minerals is likely a rate-limiting
step in mineral carbonation. Serpentine carbonation is
slow relative to olivine carbonation but can be enhanced
(Balucan and Dlugogorski 2013). However, serpentinite
continues to be a focus for mineral carbonation because
of the accessibility of large deposits to serve as feedstock.
Thermal pretreatment to dehydrate serpentine enhances
reactivity substantially. Serpentine dissolution rates are also
accelerated with chemical additives, including salts, acids,
and complexing agents (Carey et al. 2004). In some hightemperature systems, serpentine carbonation is limited by
the rate of CO2 dissolution into the aqueous solution, as
it is during the weathering of mine wastes and in other
natural low-temperature environments. Other controls on
rate are mineral-surface passivation and kinetic limitations
to carbonate precipitation, which may also have relevance
to accelerating low-temperature carbonation processes.
Hybrid wet–dry reaction systems include indirect carbonation, which converts serpentine to magnesium hydroxide
(brucite) in an aqueous reactor and then dry-carbonates
brucite to magnesite. Dry-carbonation of brucite occurs
rapidly, yet is limited by the rate of production of brucite
from serpentine or olivine. Similarly, pulverized brucite,
such as that found in mine wastes, carbonates rapidly at low
temperatures when exposed to fluids with elevated concentrations of dissolved CO2 (Harrison et al. 2013).
Mineral carbonation using high-temperature reactors is
technically and energetically feasible; however, the cost
of existing technology (~$100 per tonne CO2 ) exceeds
current carbon prices (currently about $5/tonne through the
European Union Emissions Trading System). The expense
reflects the costs of mining, energy requirements, chemical
inputs, and pretreatments to enhance mineral reactivity.
These penalties could be reduced if the exothermic nature of
mineral carbonation reactions were harnessed. Alternatively,
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carbonation reactors could possibly be used effectively in
industries that produce alkaline wastes, which would eliminate the cost of material extraction.
Carbonation of Serpentinite Mine Tailings
Industrial wastes such as serpentinite mine tailings are rich
in the alkaline earth metals necessary for the production
of stable carbonate minerals (Renforth et al. 2011; Bobicki
et al. 2012). Consequently, these materials are of considerable interest as feedstocks for ex situ mineral carbonation.
Serpentinite mine wastes are typically characterized by
large reactive surface areas and, until recently, have been
stockpiled in significant quantities without consideration
for future use (Wilson et al. 2009; Bobicki et al. 2012). The
annual production of ultramafic tailings from a large serpentinite-hosted ore deposit can be greater than 10 Mt, and
correspondingly large amounts of mine waste accumulate
over the life of a mine. For instance, the serpentinite-hosted
chrysotile mines of Québec, Canada, produced approximately 2 Gt of chrysotile- and brucite-rich mine wastes,
which could be conscripted for CO2 sequestration (Larachi
et al. 2010).
Hydrated Mg carbonate minerals have long been recognized as low-temperature weathering products of serpentine
minerals and brucite (e.g. O’Neil and Barnes 1971). These
Mg carbonate minerals commonly form in two distinct
environments within mine tailings storage facilities (FIG.
4A): (1) in surficial environments as efflorescent crusts and
hardpans and (2) at depth within tailings as a cement that
forms between mineral grains (Wilson et al. 2009). The
surface expression of mineral carbonation is likely related to
uptake of atmospheric CO2 into near-surface tailings waters
coupled with evapoconcentration of these Mg-rich brines.
Significant abundances of hydrated Mg carbonate minerals
may develop at depth as carbonated surfaces become buried
or as a consequence of ongoing reaction after burial (e.g.
Pronost et al. 2012).
The scale and rate of passive carbonation of serpentinerich mine tailings can be significant: on the order of 0.1
to 10 kg of CO2 per tonne of tailings per year, which can
represent a substantial, but incomplete, offsetting of a mine’s
greenhouse gas emissions (Harrison et al. 2013). A major
limitation on passive carbonation in mine tailings is the
sluggish rate of CO2 uptake into tailings waters. Harrison
et al. (2013) have demonstrated that a dramatic acceleration over passive rates of carbonation may be achieved by
delivering CO2 -rich gas to tailings. Thus, storage facilities
for serpentinite mine tailings could be redesigned to take
advantage of this by injecting CO2 from onsite energy
generation into the tailings (FIG. 4B). A tenfold increase in
passive carbonation rates would offset total greenhouse gas
emissions for a mine. A hundredfold acceleration would
sequester carbon at a rate commensurate with the largest
carbon capture and storage demonstration projects currently
operating (~1 million tonnes of CO2 per year). CO2 injection
into mine tailings could offer valuable insights by acting as
a testing facility for injection into geological formations,
such as serpentinite-hosted aquifers.
Enhanced carbonation of mine tailings could also
be achieved using microbial processes that facilitate
serpentinite dissolution and Mg carbonate precipitation
by creating optimal conditions for growth of selected
microbes. The layering of waste sulfides and elemental sulfur
colonized by Acidithiobacillus spp. onto serpentinite tailings
would increase the rates of silicate mineral dissolution
by at least one order of magnitude (Power et al. 2010).
Cyanobacteria are able to induce carbonate precipitation,
typically by the alkalinization of their microenvironment
(Ferris et al. 1994; Power et al. 2011). The utilization of
photoautotrophic microbes, such as algae and cyanobacteria,
for mineral carbonation is particularly attractive because
they use sunlight as an energy source and CO2 as a carbon
source. Mine sites hosting serpentinite tailings could be
geoengineered to take advantage of bioleaching to produce
Mg-rich waters that may be directed into specially designed
carbonate precipitation ponds (Power et al. 2011) (FIG.
4C). Photoautotrophs would not only produce carbonate
minerals in these ponds; their biomass might also be
harvested for biofuel or valuable by-products. Bioleaching
of serpentinite tailings and subsequent microbial carbonate
precipitation have a geologically stable analogue in natural
hydromagnesite–magnesite playas and offer a low-energy,
low-temperature strategy for sequestering atmospheric CO2.
Providing industries that produce alkaline wastes (e.g.
serpentinite mine tailings) with incentives to manage their
carbon footprint will drive innovation and technology
development, creating new approaches beyond those
described here.
IN SITU INDUSTRIAL CARBONATION
Serpentinite-Hosted Aquifers
Mineral carbonation occurs naturally in the subsurface as a
result of fluid–rock interactions within serpentinite, which
occur during serpentinization and carbonate alteration
(e.g. listwanite). In situ carbonation aims to promote these
Serpentinite
tailings
A
A) Passive carbon sequestration at mine sites
Tailings
deposition
Water reclamation pond
Water
infiltration
Carbonate precipitation
Impermeable
barrier
B
B) CO2 injection into serpentinite tailings
Evapoconcentration
and gas exchange
CO2 injection
C
C) Microbially accelerated carbonation
Bioleaching
Acid-generating
substance
Carbonate precipitation pond
Microbial
mats
A schematic of (A) passive mineral carbonation
in serpentinite mine tailings, (B) an abiotic
geoengineering strategy employing CO2 injection,
FIGURE 4
E LEMENTS
and (C) a geoengineered tailings facility using bioleaching and
microbial carbonate precipitation.
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reactions by injecting CO2 into porous, subsurface geological
formations, such as serpentinite-hosted aquifers (FIG. 1). The
rate of CO2 sequestration in this geological environment is
controlled primarily by reactive surface area, temperature,
pH, and the partial pressure of CO2. Geothermal heating
accelerates serpentine dissolution and may allow for the
precipitation of magnesite. Cipolli et al. (2004) modelled
CO2 sequestration in a serpentinite aquifer at 60 ºC and
250 bars pCO2 ; they estimated a CO2 sequestration rate of
33 g of CO2 per kilogram of H2O per year.
The advantage of in situ carbonation is that there is no
need to mine, grind, and activate serpentinite, nor transport
reactants or products, because CO2 is injected on site. The
major obstacles for implementation are the generation of
reactive surface area, loss of porosity, and passivation of
reactive surfaces. By analogy with the volume changes
recorded in exhumed carbonated serpentinite, porosity
losses will be minimized if injection is carried out in
fully serpentinized rocks and uses the CO2 content of
injected fluid to promote formation of talc + magnesite
rather than magnesite + quartz assemblages (Hansen et al.
2005). Alternatively, serpentine dissolution and magnesite
precipitation may occur in separate zones, with hydraulic
fracturing limiting reactive surface passivation (Boschi et
al. 2009). Technologies that have been employed for CO2
injection into deep saline reservoirs (e.g. Sleipner, Norway)
could potentially be adapted for serpentinite-hosted aquifers.
One possible means to further accelerate carbonation rates
is to promote the growth of biomineralizing biofilms in
the subsurface. The microorganisms in these biofilms could
enhance carbonate precipitation and solubility trapping,
while providing a physical barrier to minimize CO2 leakage
(Mitchell et al. 2010).
Geothermal Mineral Carbonation
Kelemen and Matter (2008) envision more vigorous in
situ mineral carbonation processes in which the heat
production and solid volume gain of carbonation reactions
are exploited to accelerate uptake of CO2 in peridotite and
serpentinite (FIG. 1). In one method, a rock volume at depth
is hydraulically fractured and heated to optimal reaction
temperature. Injection of CO2 -rich fluid drives carbonation
at a rate of up to 1 tonne of CO2 per m3 per year. The fluid
injection rate employed in this scenario must be controlled
to balance exothermic heat generation. This is required in
order to maintain the rock mass near optimal-reactionrate conditions (185 ºC, pCO2 > 70 bar, pH ~8 for olivine;
Kelemen et al. 2011). Permeability and reactive mineral
surfaces would be maintained through reaction-induced
cracking, induced by volume increases commensurate with
carbonation reactions, such as those observed in listwanite.
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We acknowledge funding by the Carbon Management
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