Comparison of Carbon Capture & Storage (CCS) and an innovating

biomimetic approach to sequester carbon

Caroline Zaoui1, Jean Valayer1 & Gauthier Chapelle1 (all Biomim-Greenloop SA) 1 Biomim-Greenloop, rue d'Alost, 7-11, 1000, Brussels, Belgium.

Abstract: The unprecedented rate of increase of atmospheric carbon dioxide, with its dramatic climate change and ocean life threatening impact, is considered as the most urgent challenge within the sustainability realm. Carbon sequestration is presented as one of the major avenues to mitigate carbon dioxide emissions. The sequestration technology focus for the coming decades is known as Carbon Capture & Storage (CCS). It currently consists in concentrating the CO2 produced by big emitters before storing it at very high pressures in former gas & oil reservoirs. Increasingly significant budgets are devoted to those techniques. Hence, CCS presented a perfect opportunity to test the relevance of a biomimetic approach on this major sustainability issue. Accordingly, as an SME, Biomim-Greenloop has launched a biomimetic CCS project, CO2SolStock, funded by the European Commission to explore the potential of microbiology for carbon sequestration in the form of lime precipitation. Calcium carbonate constitutes of course, the main constituent of shellfish as well as other marine group’s skeleton. Furthermore, the process of lime precipitation can be triggered by the metabolism of bacteria and other micro-organisms. One microbial process is at the source of about 40% of the world chalks (calcite) cliffs, in addition to the remains of calcareous plankton. CO2SolStock is studying a variety of such microbial processes to be found in natural habitats as a potential tool for carbon sequestration. In this paper, we will confront the CCS and biomimetic approaches, both from a theoretical point of view and from the already available data. Amongst the parameters to be compared are the types of carbon source, the energy demand, the stability of the sequestered carbon, the mass balance and flows and the up-scaling challenges.

Conference theme: Keywords: climate change, biomimicry, carbon sequestration, microbial precipitation, calcium carbonate

Introduction

Alteration of local climate by human activities has been long suspected and concerns over a global modification of the Earth's climate by human activities were seriously considered since the 70s (Weart 2008), with the release into the atmosphere of increasing amounts of greenhouse gases (GHG) that are mainly CO2, methane, nitrous oxide and halocarbons. In 2007, scientists participating in the Intergovernmental Panel on Climate Change (IPCC) 4th Assessment report made the statement that global atmospheric concentrations of greenhouse gases (GHG) generated

by human activities had exceeded the pre-industrial values and that the net effect of human activities since 1750 has been one of warming (Pachauri 2007). Since the onset of the industrial era, also referred to as “Anthropocene” by Paul Crutzen, global increases in CO2 concentrations are primarily due to fossil fuel use, as well as land-use change.

Strengthening the IPCC latest conclusions, a 2009 study identified and quantified 10 planetary boundaries that must not be transgressed in order to maintain the Holocene state (Röckstrom 2009, 472-475). The authors demonstrate that 3 boundaries had already been overstepped: climate change, biodiversity loss and the biogeochemical flow boundaries (N & P cycles). However, latest analyses are revealing that the acidification of ocean (one of Earth's major carbon sink) is occurring at an alarming rate, showing that the ocean acidification boundary is already overstepped (Kerr 2010, 1500-1501; Kump et al. 2009, 94-107). As the atmospheric concentrations of GHGs increase when emissions are larger than removal processes (Pachauri 2007), it means that the Earth's buffer systems cannot cope with and offset current atmospheric emission rates of GHGs. Carbon sequestration is one of the key mitigation tools in order to address and offset the impact of climate change on earth. Developing a spectrum of techniques is necessary to first catch the future GHGs emissions, and in a next step, to be able to capture past CO2 emissions, as requested by the IPCC scenarios, so to allow humanity as a whole to become carbon-fixing (van Yppersele, personal communication). If the first batch of carbon sequestration techniques derived mainly from the petro-chemical expertise and experience, new approaches are now emerging, inspired by other disciplines. In particular, biology is slowly entering the field, and the first biomimetic projects are emerging. They are typically the result of the biomimetic approach, in which the desired function – in this case carbon sequestration, with carbonate precipitation in particular – is searched for within the living realm, in order to mimic the biological process, with or without the living organism itself. In this paper, a continuum of carbon sequestration techniques is reviewed, in order to compare the latest biomimetic microbio-assisted approaches to more conventional and advanced physico-chemical methods. When possible, the quantitative aspects have been extracted, focusing e.g. on carbon storage potential, stability, energy needs, operating and investment costs. This review is summarized in table 2. Sources of information, other than classical scientific literature collated during the CO2SolStock project proposal writing (see 3.3), include early scientific publications on demonstration tests and impact, as well as general public releases on current projects and OEM (Original Equipment Manufacturer) brochures.

1 Conventional, non biomimetic end-of-pipe techniques 1.1 CCS leading to supercritical CO2

Carbon Capture and Storage involving intensive capture at the stacks of big emitters such as coal fired power plants involve up to five steps, some of them possibly carried out in conjunction: the actual capture of carbon dioxide at the stack (1), its concentration (2), compression (3) and transportation (4), and finally its injection in deep geological layers (5).

1.1.1 Capture and concentration techniques (steps 1&2)

There are essentially three widely different capture techniques for units functioning mainly for energy production: during the combustion, after or before (Adams & Davison 2007). In addition, industrial processes which involve oxygen and carbon-based fuel as chemical reactant for other purpose than just energy, have tailored approaches, specific to their process, and which are not described here.

1.1.1.1 Oxy-fuel: capture and concentration during the combustion

Instead of using air providing diluted oxygen, pure oxygen is used in the combustion. As a result, the flue gas is already concentrated in CO2 and water. A significant amount of the flue gas is recycled into the combustion to avoid excess temperature. Oxygen is obtained either by membrane separation or by cryogenic distillation, which are both energy intensive. In the case of membrane use, high pressure is needed. In the cryogenic case, cooling energy and a balanced output of the process is needed, for best valorization of the complete distillation.

1.1.1.2 Post-combustion Carbon dioxide is separated from the flue gas by three different approaches, two of them, involving solvent, are similar to oxygen separation: membrane diffusion or cryogenic distillation, with similar problematic as for oxygen separation. However, the most common process uses sorbents: alkanolamines MEA, DEA, MDEA (much used in natural gas “sweetening”). In this process, CO2 is removed in a scrubber; first alkanolamines asborb CO2 and then a regenerator (or stripper) releases the concentrated CO2 and regenerates the alkanolamines which can then be re-circulated into the scrubber (Rubin & Rao 2002). The energy required to reverse the absorption reaction is 0.17 MWh/t CO2 captured and a total of 0.33 MWh/t CO2 captured for the complete ethanolamine capture process (Rao & Rubin 2006, 2 4 2 1- 24 2 9). LCA energy analysis, extra fuel upstream energy cost, ammonia, its derivatives and other products add a further 0.05 to 0.1 MWh for each MWh produced as can be inferred from various authors (Rubin et al. 2005; Energetics 2006; Worrell et al. 2000).

1.1.1.3 Pre-combustion In a first stage a mixture of hydrogen and carbon monoxide (syngas) is produced by reacting the fuel either with steam or oxygen to achieve a partial oxidation which produces a mix of CO and H2 (Metz et al. 2005). A further reaction of this CO with water produces CO2 and H2. Eventually, CO2 gets separated from H2 by one of the techniques used in post combustion capture. Yet, an advantage of this approach is the gasification of the fuel which provides efficient turbine capability.

1.1.2 Compression, transport and injection (steps 3, 4 and 5)

CO2 recovered by one of the above processes, is compressed and liquefied for most effective shipment, most of the time by pipe-line, according to technologies developed to service the enhanced oil recoveries (EOR) (Metz et al. 2005). Storage is achieved already in depleted oil fields through EOR applications, where protocols have been established. Beyond depleted oil fields, saline formations are actively tested (North Sea Sleipner project). Coal seams are also being tested (DOE test finds CO2 storage potential in

coal seams Storage, Nov 05 2010 (Carbon Capture Journal)), where CO2 appears to get at least to some extent, absorbed by coal (Metz et al. 2005). Finally, apart from their technical challenges, steps 1 to 5 are subjected to issues ranging from safety to environmental risks. The table below summarizes the list of identified points of concerns (IEA, 2006).

Table 1. Issues common to CCS techniques leading to storage of supercritical CO2

Storage safety (leak risk) and overall risk assessment

- EOR prior art, limited to naturally gas proof geological structures

- No experience for insurers

EPA framework (Bacanskas, 2009)

- Financial liability

- Geomechanical & geochemistry uncertainties in injection zone and overall confining

- Decision tree for unanticipated leakage (human health, ecosystems, water quality, liability)

Capital intensive - Insufficient funds (diverted from e.g. energy reduction projects)

- ROI uncertainty (dependent on regulations & CO2 value)

Regulatory framework

- International law on marine environment, trans-boundary movements of CO2

- Environmental impact assessment process

- 1/3 party access to storage site, authorization of sites, monitoring, liability, etc.

- Government policies for emission reduction

- Public involvement e.g. independent risk assessments

1.2 CCS leading to mineralization of injected supercritical CO2

Beyond the dissolution and absorption of CO2 in coal seams, conditions where mineralization of CO2 in geostructure occurs after injection are studied among others by a French consortium of major petro-geological institutions and companies. (Ménez et al. 2007). This project addresses the various interactions associated with the thermodynamical disequilibrium caused by the injection of CO2, and the associated potential reactions, including carbonatation (reaction described in paragraph 1.3), dissolutions and reactions. This also includes the role of microbial communities. Furthermore, a complete issue of Chemical Geology addresses the fate and

mineralization possibilities of CO2 in deep layers, which is still an emerging science (Volume 265, Issues 1-2, Pages 1-236 -15 July 2009). At any rate, reaction of CO2 with its surrounding mineral is part of the exploration phase of the considered geological structure (Wilkinson et al. 2009, 486-494).

1.3 Mineralization of CO2 as carbonate at surface This approach can improve the sequestration of CO2 emitted by fossil-fuel-fired thermal systems. Indeed, carbonate minerals constitute a massive CO2 reservoir, thus proving to be a geologically-proven, safe, e.g. environmentally-friendly, long term storage means of CO2. In solution, this approach implies the hydration of CO2 in aqueous solution to form carbonic acid (reaction 1). Carbonic acid can then dissociate as bicarbonate and carbonate as the pH increases (reaction 2). Hence, under sustained alkaline conditions and pH values above 10.2, carbonate ions are formed, and in the presence of cations such as calcium (or magnesium), carbonate ions precipitate to form calcium carbonate (reaction 3) (Bond 2001, 309-316). Reaction 1: Reaction 2: Reaction 3: As precipitation of carbonate depends on the availability of counter-ions such as calcium and magnesium, reactant rocks for atmospheric or captured CO2 are being considered by neutralization with alkaline and alkaline-earth oxides, e.g. serpentine ((Mg,Fe)3Si2O5(OH)4) and olivine ((Mg,Fe)2SiO4). Simulating natural weathering has thus been proposed. For this purpose, olivine or serpentine is grinded, then either reacted with concentrated CO2 close to a power plant (Metz et al. 2005), or spread to react with atmospheric CO2 (Schuilling and Tickell, 2009). Shipments of reactants and reacted products are large, as on-site treatment of CO2 generates large quantities of rock to be transported and disposed, e.g. about 3 tons of rock per ton of carbonate formed. Yet, bulk shipment can be considered as ore. In principle, available serpentine/olivine far exceeds storage needs, however, exploitation of these opportunities are not scoped to the point of sizing. Moreover, a health risk needs to be taken into account: serpentine often contains chrysotile, which is a form of asbestos, a mineral that is known for its carcinogenic properties (Yano et al. 2010, 867-871), and an environmental risk might exist when the large quantities of mined and treated minerals used are returned to quarries. Although such opportunities appear to be attractive in view of the long term stability, they are yet in an early stage of evaluation.

1.4 Storage of CO2 as bicarbonate and/or carbonates in subterranean saline aquifers

Another approach was proposed by Dziedzic et al., calling on the huge volumes of subterranean saline aquifers. Basically, they assessed the feasibility of sequestering CO2 using a strategy based on membranes and gas-to-liquid transfer with the goal of concentrating CO2 in brines. Various CO2 concentrations (5 to 50%) were tested, and capture efficiency of brines was compared to tap

(CO2)g + (H2O)l → ← (H2CO3)aq

HCO3- +

H+ → ← H2CO3 → ← CO3

2- + 2H+

CO32- + Ca2+ → ← CaCO3↓

water, and purified water. The Gas Exchange Module consisted mainly of a hollow-fiber gas exchanger, composed of microporous polypropylene fibers. The simulated brines were made following a formula for “typical” Michigan brines, with a controlled concentration of MgCl2, CaCl2, NaCl & KCl. Capture of the CO2 relied on the same set of chemical reactions already mentioned in paragraph 1.3. CO2 balance was assessed by modeling the energy requirements of pumping the brine from the subterranean reservoirs and to return the liquid to the reservoir. Their experiments show that brines can dissolve CO2 nearly as efficiently as water, even at rather low CO2 concentration. However the key factor, not only to enhance the gas to liquid transfer, but also to mediate carbonate precipitation (as they recommend) is alkalinity. To be fully efficient, this technique thus relies on the availability of either naturally alkaline brine, or of a cheap source of base to be added. Furthermore, Dziedzic and his colleagues assessed the CO2 and energy balance of the whole operation by modeling the energy requirements of pumping the brine from the subterranean reservoirs and then to return the liquid to the reservoir after the CO2 dissolution process (see table 2). As an alternative, they also envisaged an above-ground storage as “mountains”.

Table 2. Comparison of various carbon sequestration techniques. Section number is added to the corresponding column heading.

2 Biomimetic end-of-pipe techniques: biological catalyst-assisted CO2 sequestration 2.1 The early steps

A biomimetic approach has been applied to improve the mineralization of CO2 as carbonate in the end-of-pipe context, according to equations 1, 2 and 3 mentioned in paragraph 1.3. However, in this succession of chemical reactions leading to CO2 fixation as calcium carbonate, CO2 hydration is the rate limiting step (reaction 1). In nature, the catalysts responsible for the reversible hydration of CO2 are carbonic anhydrases (CA) and are known to be among the fastest enzymes (Lindskog et al. 1973, 2505-2508). They are reported to be ubiquitous, as they are present in animals, plants and microorganisms. As an example, the human CA (HCA II) can at least hydrate 1,4x106 molecules of CO2 per second (Dodgson, 1991, 49-70). The use of CA has been proposed and tested by Bond and colleagues from New Mexico Tech in a wet scrubber system to be adapted to fossil-fuel fired thermal systems for the generation of electricity (Bond al. 2001, 309-316). In this system, flue gas is sprayed onto calcium and magnesium containing brine in the presence of the catalyst – bovine carbonic anhydrase – embedded in chitosan-coated alginate beads. By doing so, the enzyme speeds up the processing route of CO2 to carbonate ions formation, which can then precipitate in the presence of calcium and magnesium containing brines. This type of system avoids an energy intensive carbon concentration step. In addition, with this approach, CO2 sequestration takes place in aqueous solution, without necessitating extremes of pH, temperature or pressure. Note that Dziedzic et al have also used with some success carbonic anhydrase to facilitate the gas to liquid transfer in their approach.

The proof of concept of the Bond approach has been demonstrated, taking into account that SOx and NOx contained in the flue gas are first removed in a separate sulfur scrubber (Bond et al. 1999, 603– 619; Bond et al. 2001, 309– 316; Simsek et al. 2001, 74– 90), and the focus of recent research has been to develop an industrial pilot of this CA containing scrubber.

Scale-up challenges are of three sorts: 1) counter ion supply, 2) enzyme costs, 3) how to handle the precipitated carbonate end products. Similarly to the Dziedzic technology, Bond and colleagues proposed to give further value to brines of various origins and preferably brines that are produced in vicinity of the CA scrubber associated power plant. These brines could consist of desalination brines, produced waters from oil & gas industry, as well as brines from soda ash industry (Gao et al. 2007, 1419-1425). It was proposed that the costs linked to the use of purified bovine CA could be lowered by producing human carbonic anhydrase II (HCA II) in a bacterial overexpression system. Finally, in the context of using oil & gas industry produced brines, the formed carbonate precipitates could be stored in former oil & gas reservoirs, together with the used produced waters (Liu et al. 2005, 1615-1625). A bench scale test was conducted based on the salinity of produced waters from two former oil reservoirs, the San Juan and the Permian Basins in New Mexico. It was concluded that precipitation of carbonate is maximized where and when the Ca2+ to Mg2+ ratio of the brines is high. Moreover, it was also pointed out that the resulting carbonate and bicarbonate enriched produced waters could be directly injected in subsurface geological structures without subsequent precipitation with calcium, thus storing CO2 as carbonate and bicarbonates.

2.2 Industrial development More recently, the Québec based CO2 Solution Inc. company has developed a similar biomimetic approach to provide a bio-technological platform for CO2 capture from industrial flue gas by exploiting the enzyme carbonic anhydrase. The platform employs conventional capture solvents

such as amines together with CA to improve solvents performance. Hence, the enzyme improves the dissolution of CO2 from flue gas in the solvents (absorption). Dissolved CO2 can then be processed in two different ways: 1) CA catalyses the transformation of CO2 into bicarbonate ion, for the production of solid carbonate for storage, 2) CA catalyzes the stripping of CO2 from the solvents, to capture and produce pure CO2 for underground storage (desorption). Hence, the bio-inspired technological platform of CO2 Solution allows the capture and purification of CO2 from flue gas, and enables further processing of CO2 and carbonate. While CO2 storage is one major purpose of this technology, it also provides CO2 and solid carbonate aimed for enhanced oil recovery and paper/glass industries, respectively. This technology is described in details by various patents filled by CO2 Solution (Fradette & Ruel 2009; Lalande & Tremblay 2003), and is on its way of being adapted to and exploited by Alcoa, the world’s leading producer of primary aluminum, fabricated aluminum and alumina. This new development was made possible by a $13.5 million fund from the U.S. Department of Energy. It is however to note that patent US6908507 relates to using the CO2Solution technology for the purpose of producing Portland cement, which is an energy intensive process that requires calcination of calcium carbonate. Moreover, data for energy and carbon balance including an exhaustive life cycle assessment of the process, from the equipment construction and production of all reactants to the end product valorization could not be found. Considering the fact that the sequestration impact of CO2 recovered from CaCO3 calcination can only be neutral at best, one could question whether the whole process enables a significant CO2 sequestration.

3 New opportunities for CO2 sequestration: the microbio-assisted biomimetic approach

While some of the C sequestration avenues presented above exploit the fact that calcium carbonate constitutes one of the major carbon sink on Earth, a fundamental question had yet to be addressed: how was this carbon sink formed, and how was biology involved in that process?

By undergoing this biomimetic approach, the following literature assessment was made:

Limestone and other fixed carbonates represent 1,8x1022 g carbon in the Earth lithosphere (Ehrlich 2009);

Mineralization of carbon into carbonates involves bacteria, fungi, algae and some metazoa (Ehrlich 2009; Mann 2001);

40% of the Earth's limestone deposit (CaCO3) that was previously thought to be of abiotic origin are in fact the consequence of heterotrophic bacterial metabolism (Castanier et al. 1999, 9-23; Westbroek 2009);

Civil engineering and building restoration applications make use of limestone producing bacteria (calcifying bacteria) to consolidate structures such as historical buildings, as well as to repair cracks in concrete (Castanier et al. 1995; Le Métayer-Levrela et al. 1999, 25-34; Jimenez-Lopez et al. 2007, 1929-1936; Jonkers 2007, 195-204).

Thus, although the considerable amounts of carbon sequestered as carbonate via bacterial metabolism spanned over 2 billion years, this calcification mechanism could also be used to generate a calcareous biomortar in an efficient and rapid manner. Taken together, these facts all pointed to the conclusion that bacteria could potentially be used to trap carbon as precipitated carbonates, thus providing an exciting new research axis for solutions in CO2 emissions mitigation. As a matter of fact, the latest IPCC reports do no list this sequestration opportunity as

one of the carbon sequestration options to tackle climate change. Therefore, further bibliographical research efforts were undertaken to accumulate evidence and key arguments that would strengthen the attractiveness of exploring a biomimetic, microbio-assisted CO2 sequestration approach. This lead first some of the authors of this paper to the generation of an inventive concept of subterranean storage of CO2 or its derivatives (Chapelle & Valayer 2009, WO/2009/098070) in which the inventors proposed to inject calcifying microorganisms together with nutrients, CO2 and/or carbon and oxygen containing chemical species and cations into subterranean formations so to promote the formation and precipitation of carbonates. That way, calcification would both prevent CO2 leakage by blocking voids and constitute a further stabilization step for the sequestration of CO2. Key aspects and impact estimation identified are described in more detail in the following paragraphs.

3.1 Bacterial metabolism's speed of calcification

Studies on biocementation carried out at the Technological University of Delft showed that the inoculation of 1 m3 of sand by urease producing whole cell bacteria together with an appropriate mix of calcium and nutrients could be solidified as sandstone within 4 to 6 hours. This implied that up to 105 kg/m3 of CaCO3 could be formed under these conditions, thus corresponding to the storage of 40 kg CO2 as precipitated carbonate (Whiffin et al. 2008).

3.2 Microbiogenic precipitation of carbonate in soil Studies on landfill waste disposal sites' leachate aiming at reconstituting the decomposition events of organic matter in natural sediments revealed that carbonate precipitation induced by microorganisms occurred in both subaqueous sediments and in landfill waste disposal sites, as a result of anaerobic microbiological metabolism. The results were therefore pointing to a possible sequestration of carbon as precipitated carbonate due to microbial activity both in soil and in waste disposal sites (Manning 2000, 229-238). Moreover, the same author further demonstrated that oxalate, an organic acid molecule produced by plants, promote the formation and precipitation of carbonates in soil, thus constituting a further stabilization step for CO2 sequestration (Manning 2008, 639-649).

To further that point, studies conducted at the University of Neuchâtel and University of Lausanne revealed the presence of significant amounts of calcium carbonate around and within the roots of Iroko trees (Milicia excelsia) from Ivory Coast and Cameroon, despite the acidic pH of the soils tested (Braissant, 2004; Cailleau, 2004). Evidence showed that the presence of calcium carbonate was the result of intertwined plant, fungal and bacterial metabolisms, hence allowing part of the atmospheric CO2 taken up by plant photosynthesis to end up stored in the soil and plant roots as calcium carbonate. In the context of acidic soil, this biomineralization process enables soil amendment as the presence of calcium carbonate modifies the pH of the soil.

3.3 Further developments in the microbio-assisted approach: the CO2SolStock

project In order to promote the exploration and development of these new biomimetic, microbio-assisted carbon sequestration opportunities, including C sequestration in soils and tailored end-of-pipe industrial applications, CO2SolStock, the first research and development project on CO2 sequestration by microbial carbonatation was launched in April 2009 by some authors of this

paper. They used a typically biomimetic methodology in terms of transdisciplinarity. After the extensive bibliographical review already mentioned, which showed the potential of this new approach for carbon sequestration, they identified the mix of teams necessary to fulfill the 3 main functions of a biomimetic group: biologists specialized on the targeted taxonomic group; engineers able to use the recipe taught by the biologists, or the organisms themselves like in this case; and Biomimicry specialists to feed the dialogue between the other partners. In the first role were the University of Lausanne, which had identified bacteria-fungi stimulated opportunities particularly applicable in forestry, and the University of Granada, with a strong experience in bacterial carbonate precipitation in saline media. On the other hand, TUDelft had excellent expertise both in actual microbiological precipitation of carbonates in civil engineering processes, and in microbiological water treatment. The University of Edinburgh, very close to the CCS activity, had considerable expertise in natural and geological carbonate precipitation. They all agreed to join with Greenloop (as the Biomimicry SME), in applying for an European grant, to identify meaningful avenues. The project is funded for three years by the European Community's Seventh Framework Program (FP7/2007-2013) under grant agreement n°226306. The goal of the CO2SolStock consortium is the exploration and evaluation of various microbial routes and promises for carbonate precipitation, as well as to validate the proof-of-concept of at least two distinct microbial routes for the sequestration of CO2. Furthermore, in agreement between all teams (and in consistence with biomimicry principles), this research program is conducted within the following mandatory requirements: - No use of genetically modified microorganisms will be made; instead research will be focusing on existing microbial guilds; - Any introduction of non native bacterial strains will be considered with extreme care;

- Cheap sources of nutrients and reactants will be favored. In other words, a systematic industrial ecology approach will be carried out to look for the least expensive sources of alkalinity, carbon (either CO2 or organic carbon) and cations.

4 Discussion and conclusion

A comparison table of the techniques listed above was built, based on the published data, such as early scientific publications on demonstration tests and impact, general public releases on current projects and operating equipment manufacturer's brochures as available. As some aspects to be covered had not been quantified yet, the comparison table below aims to provide orders of magnitudes and qualitative comparisons. Our general conclusions put the focus on the benefits and the challenges of each approach, and show the complementarity between them.

From a carbon sequestration impact point of view, it appears that the currently developed end-of-pipe carbon and concentration techniques hold the greatest potential as they have the capacity to store some 2000 Gt of CO2, which represent the estimated CO2 emissions of the next 80 years (IEA, 2006). Considering that 9.28 billion tonnes of CO2 (9.28 Gt CO2) were emitted worldwide in 2009 (Friedlingstein et al. 2010, 811-812), the implementation of such a technology makes sense.

However, the energy and operating costs of the succession of steps required to capture, concentrate, compress and dispose of the CO2 still constitute a major bottleneck for a widespread implementation of such techniques. Indeed:

All steps depend on the use of fossil fuel to operate and to synthesize chemical reactants. It can only sequester future CO2 emissions Still frees 25 % of the emitted CO2 Works only for big CO2 emitters: this means that smaller structures like SMEs, municipal

treatment plants, cities and communities will not have easily the possibility to mitigate their CO2 emissions via these techniques

Huge storage sites are needed to justify the cost of transport Needs a huge infrastructure for transport that is comparable to the one needed for oil and

gas combined Risk of leakage due to storage at high pressure It suffers from poor social acceptability

On the other hand, biomimetic approaches to store CO2 could constitute cheaper alternatives as energy costs can be significantly reduced. Indeed:

These approaches rely – at least partly, if not entirely – on the use of cheap energy and raw material sources, such as solar energy, atmospheric CO2, organic carbon and calcium containing industrial by-products.

No capture needed: this biomimetic approach would fix either atmospheric CO2 or organic carbon, hence saving energy (see box below)

They are in line with some of the most recent IPCC scenarios: it can sequester past emissions, thus opening the avenue for decreasing atmospheric CO2 concentration itself

These biomimetic techniques also rely on the use of biological catalyst (either enzymes or whole cell microorganisms), thereby reducing the energy dependency on fossil fuel-based chemistry, also commonly referred to as the heat, beat and treat chemistry

The limestone produced via CO2SolStock approach can be considered as a biomaterial that could be valorized in the construction and civil engineering sector. For example, large quantities of calcium carbonate slurry could be decanted in polder-like structures, thus providing material for the extension of constructible space in a sea-level rise context. Moreover, as a long term vision, one could imagine combining a CO2SolStock approach with biomimetic techniques that help controlling the shape of biominerals produced to provide a bio-based construction material whose shape and dimensions are biomimetically controlled. However, the development of such methods still requires performing fundamental research on the growth control mechanisms for biomineral formation, as in mollusks shells for example.

Together with other previous works, this review also stresses the key issue of the origin of the counter-ions (usually calcium). Counter-ions are naturally weathered down, and find their way to the ocean. For example, silicate weathering, by its contribution to the production of calcium counter-ions, is associated with 1.5 to 3.3 108 t of natural CO2 sequestration per year (Hilley & Porder 2008, 16855-16859). When an alkaline earth ion, such as calcium reaches the ocean, it supports alkalinity, hence furthering carbonate precipitation somewhere in the ocean. On the other hand, if a process diverts calcium ions from reaching the ocean, it will prevent CO2 sequestration elsewhere in the long run. Similarly, any process relying on calcium extracted from carbonates can theoretically at best be neutral in terms of carbon sequestration. CO2 dissociated in the extraction might be recaptured as carbonate, bicarbonate or dissolved CO2. In the later two cases, this will result in equilibrium changes, which will ultimately dissolve carbonates elsewhere.

Processes which include the global balance of calcium are worthwhile. The CO2SolStock project, mindful of this necessity, develops microbiological carbonate precipitation routes where available sources of calcium are not naturally available for further precipitation down the line.

To conclude, a complete portfolio of complementary techniques, from the most physico-chemical ones like CCS to the biomimetic approaches will be necessary in the rush to first lower CO2 emissions before sucking past emissions from the atmosphere at a global scale. Given the lag in developing the latter, the authors recommend to support the interdisciplinarity studies necessary to fill the gap in terms of R&D between the biomimetic and the CCS techniques. The need to push for the development of such solutions also comes from the better fit offered by biomimetic/microbio-assisted solutions for small-scale emitters. Finally, a special note is given for the Iroko (tree-fungus-bacteria) system, as it is low tech, hence not too difficult to deploy, in line with the IPCC recommendations to build a globally carbon positive economy (unlike conventional CCS) and bringing some motivating extra benefits (soil fertility, biodiversity, climate stability, etc).

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