Almost all of the CO 2 that has ever leaked out of the planet has been removed from the atmosphere and the ocean, and sustainably stored in rocks, mainly by weathering, and also in the later part of the Earth’s history by storage as organic carbon. During weathering, which is the reaction of rocks with CO 2 and water, CO 2 is first converted to bicarbonate solutions. In the ocean corals, shellfish, and plankton convert them to carbonate sediments, which form the ultimate sustainable storage of CO 2 (Figure 1).

Figure 1 A Karst landscape in China is one of the many stores for CO 2 of the world. Full size image

Six solutions

Six low-cost or financially self-supporting ways are described in which we can store large volumes of CO 2 and/or significantly diminish CO 2 emissions:

1. A switch from nickel mining, ore dressing, and nickel recovery to nickel farming by the use of nickel hyperaccumulator plants. This switch will cut down CO 2 emissions because it avoids the energy-intensive steps in the nickel cycle and enhances the weathering of olivine or serpentine which captures additional CO 2 . 2. Diatoms contain approximately 50% of lipids, which makes them an ideal starting material for the production of biodiesel. They grow fast, provided they have a source of silica. They do not suffer from drawbacks of land-grown biofuel crops. They do not occupy vast tracts of land that are urgently needed for food production, and they do not require vast amounts of scarce irrigation water and fertilizer. The use of biodiesel from diatoms will reduce the CO 2 emissions from fossil fuels. 3. Quenching of forest fires. Forest fires are the second largest emitter of CO 2 , after fossil fuels. It was demonstrated that quenching such fires with a slurry of serpentine powder is considerably more effective than quenching with water. This reduces the emissions of CO 2 by the fires and the associated financial losses. The serpentine that was calcined by the fire reacts very fast with CO 2 and water afterwards, thereby compensating part of the emitted CO 2 during the fire. A better and quicker mastery over forest fires may also help to save lives. 4. If the heat that is released by the weathering of olivine is trapped, this would represent a huge alternative source of energy that additionally captures large volumes of CO 2 , hence the name supergreen energy. A basic scenario is described how this could be achieved. 5. When olivine is used for coastal protection (breakwaters, artificial reefs, sand replenishment on beaches) this has a direct effect against ocean acidification. CO 2 is absorbed as bicarbonate, and the pH of the surrounding waters rises. 6. Olivine in high-energy marine environments. Large areas of shallow seas are subjected to strong currents that can transport gravel. When olivine grit is spread on the sea floor, the grains are kept in motion and bump and rub against each other. This destroys reaction-limiting silica coatings on the grain surfaces and releases micron-sized slivers that rapidly react with sea water. It is the most direct way to counter ocean acidification.

Nickel farming

All mining operations have an impact on the environment. This also holds for nickel, independent of the type of ore, whether nickel laterite or nickel sulfide. Nickel laterites must be leached and nickel sulfides must be roasted and dissolved. These steps are energy-intensive and polluting. These disadvantages can be reduced if part of the world nickel production is gradually replaced by a switch to nickel farming. A fairly large number of plant species from different families are known to exhibit the remarkable property that they very effectively extract nickel from nickel-rich soils and store it in their tissues (Figure 2). Soils on serpentinized peridotites often contain no more than 0.2% of nickel, but the ash of these plants may contain 10% or more of nickel, much richer than the richest nickel ores. If some NPK fertilizer is spread over nickel-rich soils and such plants are sown, these nickel hyperaccumulator plants can be harvested at the end of the growing season, and their nickel content can be recovered after ashing. Several of these plants are perennial, so they do not need to be sown every year.

Figure 2 Alyssum corsicum , a nickel hyperaccumulator. Full size image

A first estimate[1] shows that the recovery of nickel by using these plants will cost no more than the current way of nickel production. This means that all the savings on CO 2 expenditure and CO 2 storage are essentially cost-free. In instances where an appropriate value may be associated with the CO 2 savings compared to conventional nickel production, nickel farming may economically outcompete incumbent nickel production processes.

Once such nickel hyperaccumulation systems will have been fully developed and become deployable, it is hoped that governments adopt incentive structures that oblige mining companies with nickel mining assets to conduct at least part of their businesses with these methods and that associated CO 2 savings and removals are quantified and verified. In addition, because the nickel obtained from phytomining is not extracted from nickel ores but from common peridotite rocks, nickel farming will extend the lifetime of nickel ore deposits.

Biodiesel from diatoms

Diatoms (siliceous algae) make up a large part of the biomass in the oceans. They consist for about 50% of lipids, which makes them an ideal raw material for the production of biodiesel. The process to make biodiesel from algae is already known. They grow fast and can outcompete their competitors in the algal world in the fight for food, provided that there is sufficient silica available in their environment. Diatoms need silica for the construction of their silica skeleton (Figure 3). An extensive discussion of the role of dissolved silica in promoting the growth of diatoms at the cost of other plankton like dinoflagellates can be found in[2].

Figure 3 Diatoms have delicate silica skeletons. Full size image

Land-grown biofuels (among others oil palm, sugarcane, sweet sorghum, soybean, maize) occupy vast tracts of land that would normally be used for food production, or land that used to be the territory of threatened species like the orangutan. They also need vast volumes of scarce irrigation water and fertilizer. This results in higher prices for these fertilizers, which will push up their price as well as the price of food.

Diatoms do not have these drawbacks, but before they can be used as an alternative source of biofuel, the problems of mass culturing and harvesting them must be solved. A large-scale mariculture of diatoms might take the following shape. An artificial lagoon can be sectioned off by a dike around a sector of shallow seawater in front of a beach section.

The beach between the low-tide mark and the high-tide mark must be covered by a layer of olivine sand with a thickness of about 0.5 m. One or more U-shaped tubes are left in the dike that connects the lagoon with the open sea, permitting the tide to reach the lagoon and to alternatively wet and drain the olivine beach. These tubes should be closed by a perforated metal plate covered with a plankton net. This would permit the exchange of water, but prevent the diatoms to be carried out of the lagoon by the ebb. The olivine will weather, and the weathering solution, including the silica that is set free during the olivine reaction, will be distributed in the lagoon. In addition, the bicarbonate that is captured during olivine weathering will be used by the diatoms for photosynthesis.

When the silica limitation is removed, diatoms will form a quasi-monoculture in the lagoon. Nutrients should, of course, be added, mainly for their ammonia and phosphate requirements. A cheap way to do this would be by the use of struvite, an ammonium-magnesium phosphate that is produced by a simple and robust technology in the treatment of organic wastes, including manure, urban waste, and urine[3]. Struvite is a slow-release fertilizer that will steadily add ammonium and phosphate to the lagoon. The addition of nutrients should be limited, however, because diatoms react to a slight starvation by raising their lipid content, which increases their value for biodiesel production. The diatom production can be increased by underwater lighting at night.

Harvesting the diatoms efficiently is a major problem. The following possibility may provide a solution. Dig a hole inside the lagoon. Dead diatoms will collect in this pit, also thanks to the fact that they are relatively heavy due to their silica skeletons. From time to time, this mass of dead diatoms can be sucked up, drained and transported to the biofuel plant.

When the culture and harvesting of vast volumes of diatoms can be successfully accomplished, this application will become financially self-supporting and will reduce CO 2 emissions from the burning of fossil fuels. It can be set up in any country with marine coastlines, preferentially in dry climate zones with abundant sunshine.

Fire fighting

Forest fires (Figure 4) are the largest CO 2 emitters after the burning of fossil fuels. Forest fires and, to a lesser extent, other forest losses account annually for about 6 Gt of extra CO 2 emissions on a total of somewhat more than 30 Gt of human CO 2 emission[4]. They cause every year not only huge financial losses but also the deplorable loss of human lives. Experimental fires at the test site of Brandbeveiliging BV (Fire Protection) in the Netherlands were considerably faster and completely extinguished by spraying with a suspension of serpentine powder than with plain water. Serpentinite powder from the PASEK mine in North-West Spain and from the Isomag Mine in Austria was used with equally positive results.

Figure 4 Forest fires are the second largest emitter of CO 2 in the world. Full size image

Serpentine can be considered the hydrated equivalent of olivine. Huge massifs of serpentinite are formed by the interaction of olivine with hydrothermal waters and also on the ocean floor along mid-ocean ridges. Serpentinite is a soft rock and serpentine is similar to a clay mineral. Like any other clay, it can be baked into a hard, brick-like substance. When this calcined serpentine is pulverized, it turns out that the powder reacts fast with CO 2 and water, considerably faster even than olivine. It would be an excellent material to rapidly remove CO 2 from the atmosphere, but baking it costs a lot of energy and associated CO 2 emission. So, it is a pity, but using calcined serpentine against climate change is out…, except in cases where one wants to quickly remove as much heat as possible, like in extinguishing forest fires. When serpentine slurries were tested in test fires, they not only removed a considerable amount of heat from the fire, but they displayed another property which is probably more decisive. The serpentine that was sprayed over the fire turned into a thin baked impermeable skin that prevented the access of oxygen to the burning material, and also prevented the emission of the inflammable gases from the burning wood.

So, when forest fires are raging, the spraying of serpentine slurries (almost as simple as spraying water, because a 40% serpentine slurry is still very fluid) can reduce the extent and severity of such fires. When a reduction of 10% in forest losses could be achieved worldwide, this would already be a major breakthrough, since this represents a reduction of 0.6 Gt of CO 2 emissions each year.

Moreover, after extinction of the fire, the calcined serpentine will quickly react with CO 2 and the first rainwater, thereby compensating part of the CO 2 that was emitted by the fire. It is clear that the spraying of serpentine (serpentine powder is a cheap and ubiquitous material) is a very cost-effective way of reducing the huge financial losses from forest fires, and it holds the promise of reducing losses of life as well. It pays amply for the reduction in CO 2 emission by limiting the areal extent of burnt forest and by the capture of CO 2 by the reaction of the calcined serpentine afterwards. It also limits the required volumes of water considerably, which is important in hot dry summers in countries that are most vulnerable for forest fires and have only limited fresh water resources.

It should be considered whether the spraying of serpentine slurries can also be used in the containment of tunnel fires.

Supergreen energy

A property of olivine weathering that is commonly overlooked is its energy production. When olivine is weathering under conditions of limited water flow, it weathers according to

Mg 2 SiO 4 + CO 2 + 2 H 2 O → Mg 3 Si 2 O 5 OH 4 + MgCO 3 Olivine Serpentine Magnesite

Serpentine is like a clay mineral, and magnesite is similar to limestone. It is well known that baking clays to make bricks costs a lot of energy and the same holds for burning lime to make quicklime. If we follow the reverse route and make clays and carbonates, such energy is set free. Unfortunately, weathering reactions are notoriously slow, so there are no technological applications for this energy yet, because under normal conditions this heat will be radiated or conducted away. That is a pity, because the energy that is produced by the weathering of olivine is considerable. The heat flow anomalies along the mid-ocean ridges may be due, for a large part, to the widespread serpentinization of mantle rocks when they react with infiltrating sea water[5].

In a system that is very well isolated and has a large volume-to-surface ratio, it might be possible to recover most of that energy. Rocks are excellent thermal isolators, as shown by caves. If one visits a cave in summer, it feels nice and cool, and in winter it feels pleasantly warm. This is because the surrounding rocks provide a good thermal isolation and keep the cave at a fairly constant temperature throughout the year. The larger the volume of olivine sand under good isolating conditions, the better it will be able to develop and keep a high temperature. One might say, volume stands for heat production and surface area stands for heat loss; thus, the larger the volume (and the thicker the isolation), the lesser the heat loss.

A scenario that provides these conditions could be the following. An existing 550-m deep lignite mine in Germany (Figure 5) will be taken as an example; but in fact, any deep open pit mine could serve, whether in operation or left as a scar in the landscape after closure.

Figure 5 A lignite mine in Germany. Full size image

The lignite mining goes on at the front end of the mine, while the mined-out rear part is filled with the overburden that was first removed to reach the lignite seams. This way the mine moves slowly through the landscape. Villages are torn down in front of the mine and rebuilt at the backside. Instead of refilling the whole mine with the overburden, the lower 250 m may be filled with olivine sand and then topped off with the remainder of the overburden. This setup provides thermal isolation and also sufficient counter-pressure to maintain the pore waters in a liquid state. Before doing this, a network of perforated pipes and heat exchangers should be installed in the olivine sand, through which water (or steam) and CO 2 can be injected. A set of thermistors inside the olivine mass will make it possible to follow its thermal evolution.

As long as the temperature is low, the reaction will be slow. In order to kickstart the process, it is advisable to first inject steam to heat the inside of the mass. This will increase the reaction rate, and as the reaction takes off, the temperature will rise further and the reaction accelerates.

When the system has reached a sufficiently high temperature to be of interest for power production, water is passed through the heat exchangers and converted to high-pressure steam.

It should be evident that such a system will require a lot of additional and rather unusual engineering before it can be operational. On the other hand, the potential reward is huge because it represents an almost unlimited amount of energy. This energy is called supergreen energy because it does not produce CO 2 , but, on the contrary, it traps it in a safe and solid form. The question asked by the author in[6] is relevant 'So what would we prefer, a CCS infrastructure that uses a quarter of a power station’s electricity to sequester its CO 2 emissions under the North Sea or one that generates additional electricity and useful materials products?’.

A major technical problem may arise if silica that is released during the olivine reaction would form a layer on the olivine grains, preventing the reaction to proceed. A possible way out is to mix the olivine sand with some minute quartz grains. Quartz has a much lower solubility than amorphous silica, so the dissolved silica that is released in solution will tend to diffuse to the quartz grains and precipitate as an overgrowth on quartz surfaces instead of on the olivine grains, leaving the olivine surfaces free for continued reaction.

Coastal protection

Olivine can be used in several ways to protect coastlines against erosion. Olivine is considerably heavier than normal quartz sand (specific masses of 3,400 versus 2,650 kg/m3), which makes it more resistant to physical erosion. Olivine blocks can be used in the construction of permeable breakwaters. In a permeable triangular breakwater, pointing into the sea, the force of the longshore (flood and ebb) currents is weakened because part of the water passes through the breakwater and loses momentum in doing so, while another part is deviated from the coast. Both effects reduce coastal erosion. If the sections at either side of the breakwater are covered with olivine sand, it will resist erosion even better. Another way of using olivine for coastal protection is the construction of olivine reefs at strategic points to keep waves and currents away from the coast. If the seawater that is enclosed in the reefs is only slowly refreshed, its pH will rise as a consequence of the olivine reaction. This may lead to the precipitation of calcite, so that these reefs are self-cementing. They will become hatching and hiding places for fish and a place for mussels and oysters to settle (Figure 6).

Figure 6 The sea as a threat: the Hondsbossche Zeewering along the Dutch Coast. Full size image

Stretches of beach that lose sand can be restored by spreading olivine sand on the beaches. Olivine sand on beaches feels well, and children love to build their sand castles with it and make sand sculptures of dolphins and seals (Figure 7).

Figure 7 The sea as an ally. Children making sand sculptures of olivine sand that will merge with the sea at high tide and help in counteracting ocean acidification. Full size image

Very rough coastal stretches can be covered with olivine grit, preferably of various sizes. In imitated surf experiments, we have shown that mixtures of different grain sizes become rounded and are abraded faster than single grain sizes by the multiple grain-to-grain collisions[7]. During this polishing in the surf, small micron-sized slivers of olivine are knocked off (see also Section 'Olivine in high-energy marine environments’). These slivers react very rapidly with sea water and add alkalinity to counteract ocean acidification. It was even found that brucite (Mg(OH) 2 ) formed already after a few days in experiments with olivine and seawater. From the observations on white smokers[8], it is known that brucite is rapidly transformed into aragonite (Figure 8).

Figure 8 Sixty-meter tall aragonite (replacing brucite) chimneys on Lost City Seamount. Full size image

Coastal protection with olivine, instead of with the usual basalt blocks, will add alkalinity to the ocean and also provide places of interest to tourists. This makes this combined function of CO 2 capture and alkalinity provider also financially attractive. Rough stretches of beach covered with olivine grit can serve as natural tumbling devices, where nicely rounded green grit can be produced by the surf. These may serve for applications as diverse as chicken grit and covering material for driveways. Tourists may also find these polished marbles attractive collector’s items. Using the surf which is free of charge, instead of mechanical crushing and tumbling devices, is an additional modest saving. Another financial advantage is that olivine cargo ships can unload their olivine directly in front of the coast, thus avoiding harbor dues.

Olivine in high-energy marine environments

It is a paradigm that weathering on land, and under marine conditions, always would be a slow process. When olivine grains, preferably of different sizes, are free to be kept in motion by currents, their weathering is a fast process. The grains are quickly rounded and abraded by mutual collisions (Figure 9), producing myriads of micron-sized slivers (see picture in Additional file1; see also[9]).

Figure 9 Angular olivine grains are quickly rounded and abraded by mutual collisions when kept in motion. Full size image

In experiments where modest current action was imitated by letting olivine grains rotate slowly along the bottom of an Erlenmeyer, the water had become opaque white after a few days of rotation, the pH of the solution had gone up, and neoformed grains of brucite, a mineral known to transform into carbonate fast, had evolved.

Many shallow sea floors are covered with gravel. When 700,000 km2 of such sea bottoms are covered each year with a 1-cm thick layer of olivine grit, this would compensate the entire anthropogenic CO 2 emissions, and raise the pH of the oceans. To make it more concrete, the following example may serve. Part of the continental shelf between the Shetland Isles and France, i.e. the Southern Bight of the North Sea, the English Channel and the Irish Sea, is covered with sand waves, and in and around the Channel, an area of well over 100,000 km2 experiences bed stresses capable of transporting gravel[10, 11]. If a volume of 0.35 km3 coarse olivine is spread over 35,000 km2 of this area, this would compensate 5% of the world’s CO 2 emissions, that is more than the combined emissions of the adjoining countries, the UK, France, Ireland, Belgium and the Netherlands together[9].

Another site where the spreading of coarse olivine grit may work out well is the Maelstrom, with very strong tidal current in the Lofoten Islands, Norway, and there are many more suitable areas in shallow shelf seas.