Geoscientist Online

The soil of brownfield sites can be engineered so as to help mitigate climate change. David Manning, Elisa Lopez-Capel and Phil Renforth* explain the passive sequestration of atmospheric CO₂

Geoscientist 19.1 January 2009

“…this Cinderella of the geological world suddenly dons glass slippers…”

It isn’t every day that a Northumberland quarry gets into the international press, but the French newspaper Le Monde of 27 May this year carried a special report from Tarmac’s Barrasford Quarry. The item described experiments begun by Mineral Solutions Ltd in 2002 (though for a purpose quite different from that being reported).

A site unprepossessing

Duly warned what to expect, Le Monde’s correspondent Hervé Kempf travelled all the way from Paris and braved the cold and wet to see what he described (loosely translated) as “three mounds of soil, covered with weeds, in an abandoned corner”. It was a long way to travel for such an unprepossessing sight. But these mounds of soil (above) had done a very unexpected thing.

The original experiment (using funding from the Minerals Industry Sustainable Technology scheme administered by MIRO) was to blend of quarry fines with compost to produce soils for land restoration. But once that project had ended, the soils stayed put - and five years later Phil Renforth’s final year undergraduate project (MEng Civil Engineering) took us back to Barrasford, to see how they had changed.

To our great surprise, we discovered that calcite precipitation had occurred, and as a result these humble plots opened the door to a new process for the passive capture of CO₂ from the atmosphere, in which (once set up) plants, and the microbes that infest their roots ,do all the work with minimal human input.

It has long been known that plants pump carbon through their roots in substantial quantities, as organic acids such as citric acid¹. Although difficult to quantify, the amount of carbon introduced to soils in this way is equivalent to the ‘body mass’ produced above-ground by the plant each year². These organic acids play a vital role in the plant’s acquisition of nutrients, and sequester potentially harmful elements such as aluminium. They make short work of minerals such as feldspars, which corrode much more rapidly in their presence than they would in water alone. But despite their effectiveness, organic acids are short-lived in soils, and rapidly decay - ultimately to CO₂ and water, giving heightened levels of CO₂ in soil gases, a proportion of which is partitioned into the soil solution where it occurs as bicarbonate (the dominant anion in the majority of shallow terrestrial groundwaters). As long as the concentration of calcium is high enough, calcium carbonate precipitates as calcite.

Pedogenic carbonates are well known from many soils, occurring naturally in areas with low rainfall (Figure 2). It is generally considered that they do not occur in UK soils - the assumption being that our climate is too humid. But it is possible that this view arises as an artefact of our procedures for soil mapping and description, which historically has focused on assessing soils’ suitability for agriculture.

Generally, soil mapping extends down to a depth of 1m; there has been little need to go deeper, which in any case costs more. But work in the US has shown that as rainfall increases pedogenic carbonates form at greater depths³. So in the UK pedogenic carbonates might be expected to occur at less than 1m depth beneath any land capable of growing wheat (whose distribution roughly matches areas with a mean annual rainfall below 750mm). Additionally, soils’ calcium carbonate content is not routinely reported in descriptions from England and Wales. North of the border however, when the same soil series are mapped, calcium carbonate contents are reported. This introduces a second reporting-based artefact into our understanding of the occurrence of carbonates in British soils.

Chalk talk

There are many possible origins for calcium carbonate in soils, the major source being limestone or chalk. Fortunately, these geological sources are readily distinguished from pedogenic carbonates by their carbon isotope signatures. Depending on their photosynthetic pathway, plant tissue is dominated by carbon that is isotopically light (δ¹³C is -12 to -13‰ PDB for C4 plants, such as grasses, and -27‰ PDB for C3 plants, such as trees). Plant root exudates have very similar negative ¹³C values, and (although becoming more positive due to fractionation during the soil precipitation process) the δ ¹³C values for pedogenic carbonates are distinctly negative (typically -3‰ to -7‰ PDB5). Geological carbonate sedimentary rocks typically have δ ¹³C values between -2 and +2‰, and are readily distinguished.

Our work at Barrasford Quarry has shown that pedogenic carbonates can form in a neglected artificial soil within five years. Carbon and oxygen isotope analysis has shown that the carbon in the carbonates is isotopically light compared with what is expected for limestones (which are quarried at the same location), and so demonstrate a pedogenic origin for at least a proportion of the carbonate in these plots.

We have also sampled a brownfield site at Byker in Newcastle where a modern concrete office complex once stood (Figure 3). Trial pits were dug down to 3m, through different types of made ground. The first trial pit was into coarse rubble, a clast-supported ‘conglomerate’, and yielded bricks on whose surface small concretions of calcite were clearly visible (Figure 4).

We were delighted and thought - how lucky was that? How often in any survey do you find what you are looking for within 30 minutes of going on site? Needless to say, the following four trial pits yielded nothing as visually exciting, and we are now performing the lab analysis necessary to find out the story that these samples have to tell.

Calcite to the rescue

How can we exploit and promote the development of pedogenic calcite? The experience at Byker has shown that made ground clearly has a carbon capture function, and we need to capitalise on this. Whenever demolition takes place, recovery of secondary aggregates by crushing, on or off site (Figure 5), generates a significant volume of fines (perhaps 30% of the material being crushed). Much of this remains on site - contributing to the subsoil beneath landscaping. The fines are dominated by highly reactive sources of calcium: portlandite (Ca(OH)₂) within mortars, calcium silicates from cement, and gypsum (CaSO₄.2H₂O) from plaster.

Add carbonate derived (ultimately from plants growing above this material or from associated microbial activity within the soil) and calcite precipitates. To estimate the ultimate potential for carbon capture by these materials requires knowledge of the amount of reactive calcium that is present, and currently there are no standard tests for doing this. The most optimistic estimate would be based on X-ray fluorescence (XRF) analysis of a sample; typically, fines from the demolition of a concrete building can contain up to 20% CaO₂. Thus every tonne of fines from a concrete crusher has the potential to sequester 180kg of CO2 as a consequence of the carbonate-forming reaction, in which 1 mole (56g) of CaO reacts with 1 mole (44g) CO₂ to give 1 mole (100g) CaCO₃, according to the simple equation CaO + CO₂ = CaCO₃.

So, what difference will this approach make? When all is said and done, the use of fines in artificially prepared soils provides no more than a way of compensating for the CO₂ released into the atmosphere by the manufacture of cement in the first place by the decarbonation of limestone. But, for a full Life Cycle Analysis, the burial of fines from concrete crushing helps close the loop (ashes to ashes, dust to dust and all that), meaning that cement, once made, is either reused as a component of secondary aggregates, or goes back into a soil where it recarbonates.

In the UK, cement manufacturing contributes 6 MT/year of CO₂ to the atmosphere. Compensating for this would require 40 million tonnes of demolition waste running at a ‘grade’ of 20% CaO. Coincidentally, the UK also generates 40 million tonnes of demolition waste each year - though only a proportion of this (less than 30%) will end up in soils, giving a CO₂ sequestration potential of about 2MT CO₂ per year.

So - fines from crushed concrete have a role to play in reducing atmospheric CO₂, and as these often ‘end up’ in made ground, this Cinderella of the geological world suddenly appears to don glass slippers. Does made ground have an inherent carbon-capture value? If so, how can we assess and exploit that value?

Obviously, the design of major redevelopment sites could include an assessment of the carbon capture value of retaining on site the crusher fines from aggregate recycling. If we suppose that a hectare of made ground on a site previously occupied by concrete buildings has a one-metre layer of soil dominated by crusher fines, covered by top soil, we are looking at of the order of 20,000 tonnes of demolition ‘waste’ that has the capacity to absorb a total of 3600 tonnes of CO₂ - on top of carbon sequestered as plant tissue. On this basis, a 2.5km² site (such as the London Olympics, for example) could absorb almost a million tonnes (0.9 MT) of CO₂ as calcium carbonate. This surely adds to the value of such a development.

Beyond limestone

We also need to look at the scope for designing soils based on blends of quarry fines (especially basalt and dolerite) with composts, and to evaluate the potential of blast furnace slags within soils as carbon capture mechanisms. Although less CaO-rich than cement, basaltic rocks typically contain 9% CaO, and an estimated nine million tonnes per year are produced as crusher fines – with the potential to capture about half a million tonnes per year of CO₂.

About 4MT per year of basic slags are produced in the UK, containing over 45% CaO. If all of this material were available for carbonation, it could sequester 1.5 MT CO₂ every year. More realistically, fines from crushing slag will constitute no more than a third of crusher feed; and so if limited to these materials, the CO₂ sequestration potential is reduced to no more than 0.5 MT CO₂ per year. The total CO₂ sequestration potential in the UK for soils developed from fines arising from demolition, silicate rock quarrying and steel manufacture, comes out at about 3 MT CO₂ per year.

The artificial use of soils that have the capacity to remove atmospheric CO₂ will not solve the world’s greenhouse emission problems at a stroke. But every little helps. The UK’s Renewables Obligation target, which increases annually, is currently 16 MT CO₂ per year, to be met by finding alternatives to fossil fuels. If that is worth chasing, then it is certainly worth maximising the use of soils as a comparatively inexpensive way of sequestering CO₂.

Acknowledgements

This work is funded by the Engineering and Physical Sciences Research Council, award reference EP/F02777X/1.

References

1. Ryan, P R, Delhaize, E and Jones, D L (2001) Function and mechanism of organic anion exudation from plant roots. Annual Reviews in Plant Physiology and Plant Molecular Biology, 52, 527-560.
2. Manning, DA C (2008) Biological enhancement of soil carbonate precipitation: passive removal of atmospheric CO2, Mineralogical Magazine, in press
3. Jenny, H (1980) The Soil Resource. Springer Verlag, New York, 377pp.
4. Cerling, T E (1984) The stable isotopic composition of modern soil carbonate and its relationship to climate. Earth and Planetary Science Letters, 71, 229-240.

* School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, NE1 7RU