Newcastle University

The UK has a target of reducing CO₂ emissions by 80% by 2050 (www.decc.gov.uk, 2012), compared with a 1990 baseline. This is an enormous task that will require contributions from a wide range of societal sectors. Although the ‘heavy lifting’ is likely to be done through improved energy efficiency, low carbon energy and fuels, land use change will play a significant role.

One aspect of land use that so far has been omitted from calculation of the carbon budget is urban land, which represents 7% or 1.7 million of the UK’s 25m ha area (UK National Ecosystem Assessment). Although at any one time much of this is occupied by buildings and hard infrastructure, the next 40 years will see demolition and construction activity that involves increasing attention to the impact of redevelopment on the national carbon budget.

The scale of the problem of compensating for artificial emissions of CO₂ is clear: by 2050, emissions will exceed 2000 levels by 7 GT C. There is no single process or activity that could compensate for this, and it has been suggested that 7 different approaches, each accounting for 1 GT C (approx. 3.7 GT CO₂), might be a reasonable target. Current UK emissions are 456 MT CO₂ (590 MT CO₂ in 1990; www.decc.gov.uk), and so the UK’s target for CO₂ emissions reduction, whilst ambitious, is small compared with potential global targets.‌

Recently we have investigated the role of urban soils as a sink for atmospheric CO₂. We have shown that pedogenic carbonate minerals, normally regarded as a phenomenon of natural arid soils, form in artificial urban soils in the UK and North America. For example, our work at the 10 ha Newcastle Science Central site (photograph shows Carbon Capture Gardens; St James Park in background), formerly occupied by the Newcastle Brewery, has shown that the soils onsite remove over 80 T CO₂ per hectare monthly. Assuming that the UK has 1.7 million ha of urban land, only 12500 ha of land (or 5 Olympic parks) in a similar state to that of Science Central will remove 1M T CO₂ annually.

Pedogenic carbonates are predominantly composed of the mineral calcite (CaCO₃). Their formation depends on the availability of calcium and carbonate in solution. In most natural occurrences, calcium is derived from the weathering of silicate minerals (plagioclase feldspars, pyroxenes etc) that commonly occur in basic igneous rocks (e.g. basalts and dolerites). Stable isotope studies have demonstrated that carbonate C is derived from photosynthesis.

The model for formation of pedogenic carbonates involves plant roots exuding organic acid anions which combine with other biogenic carbon inputs in soils and decompose to give CO₂ as the end product of aerobic decomposition; a proportion of this partitions into the soil solution as bicarbonate or carbonate (depending on pH). Dissolved bicarbonate combines with Ca derived from silicate mineral weathering to precipitate CaCO₃ as a pedogenic carbonate. Once formed, this is a stable sink for CO₂ in the sense that naturally it will only be removed by dissolution, after which C normally stays in solution, entering surface and groundwater systems.

Our work at Science Central4 has shown that the origin of pedogenic carbonates in urban soils is not necessarily as simple as explained above. Very few plants are colonizing the ground at this site, and so the photosynthesis inputs are lacking for the period in which we have observed the increase in soil carbonate. Instead, the high pH of the soil (up to pH 10) suggests that non-biological processes of carbonation are taking place, in which CO₂ dissolved in rain water enters the soil, speciates as carbonate, and precipitates by combining with Ca derived, in this case, from the cement mineral portlandite (Ca(OH)₂) as well as weathered cement-derived calcium silicates in materials generated by the demolition process.

These observations lead to critical research questions that are addressed by SUCCESS, defined as specific work packages following consultation with the stakeholder community:

1) What controls the maximum amount of carbon that can be captured in a soil?  Is it porosity, or mineralogy, or something else?

2) What is the rate of the process of carbonate-carbon capture for different settings and sites?

3) How can you demonstrate that carbon has accumulated as predicted, and so validate carbon capture claims?

4) Is it possible to specify vegetation that maximizes the accumulation of carbon in the underlying soils, and how does the balance between organic and inorganic C change with time?

5) What are the effects of soil composition (e.g. pH, possible contaminants) on the ability of the soil to support desired planting schemes?

6) What are the effects of CaCO₃ precipitation on geotechnical properties of the soil, such as strength?

7) Is there a positive or negative effect on the ability of urban soils to absorb rainwater, thus affecting flood resilience?

8) What is the feasibility of the process, given the availability of materials and the overall carbon costs associated with their sourcing, transport and use in construction, and existing regulation?

To illustrate the potential of the concept, an area of about 29000 ha of vegetated land is associated with the UK highway network. If strategically managed over the next 40 years to optimize carbon capture through carbonate precipitation alone, hypothetically this area could remove 25M tonnes CO₂ per year from the atmosphere, equivalent to 5% 2011 total UK emissions or 20% of current transport emissions.

However, carbonation of material derived from Portland cement in demolition products does not give net removal of atmospheric CO₂, as it simply replaces CO₂ driven off by calcining during manufacture. The challenge addressed in SUCCESS is to identify non-calcined, calcium silicate rocks, that weather sufficiently rapidly to provide a net sink for CO₂, taking into account all emissions during production. Carbon capture through carbonation will add to other CO₂ removal processes associated with plant growth, biomass production and incorporation of organic C into soils.

The overarching research idea in SUCCESS is this: is it possible to design a carbon capture function into soils that permits the phenomenon of pedogenic carbonate formation to be exploited to enable urban and restored soils to remove significant amounts of CO₂ from the atmosphere?

The urgent need to compensate for emissions derived from fossil fuel combustion means that this work is timely, and drivers affecting the behavior of developers require research inputs through partnerships with universities.

The aims of the project are to specify a practical design for soils used in engineering, restoration and construction works that maximizes sequestration of atmospheric CO₂ through natural soil processes, including the use of ‘Carbon Capture Gardens’.

The objectives are:
1) to build research/demonstration plots at suitable locations, using (a) artificial materials (slag, cement-based) and (b) quarried natural rocks (basalt, dolerite)

2) to identify up to 6 existing brownfield sites suitable for investigation of the process

3) to monitor carbon capture at the research/demonstration sites over a period of 2 years, and at the brownfield sites for up to 2 years

4) to measure the effect of carbonate precipitation on physical, chemical and geotechnical properties of the soils at the sites

5) to review current and planned regulatory controls on soils in the context of redevelopment and construction, identifying key constraints

6) to review market constraints, including availability and geographical distribution of materials

7) to engage with a wide range of stakeholders, recording and developing progress in building public acceptance of the concept

The research resolves into the following work packages:

Work package 1: Field and laboratory investigation of the carbonation process

1.1) Identification and construction of short and long-term field sites suitable for investigation of carbonation. Candidates include existing sites with known history of fill, and new sites built with specific known fill (reclaimed concrete aggregate or natural dolerite/basalt aggregate) or other materials, including quarry wastes and steel slag. In addition to quarry restoration.   It is intended that the site will be suitable for long-term monitoring, beyond the end of the project.

1.2) Measurement (in some cases to 3m depth) of pedogenic (i.e. formed within soil) calcium carbonate (calcimeter) and total organic carbon (LECO) in soils at sites identified in collaboration with partners, recording vegetation type and integrating with existing site investigation reports. Thermal analysis (Netzsch TG-QMS) will be used for a subset of samples to confirm carbon hosts, a combination of petrography, scanning electron microscopy and X-ray diffraction used to determine soil mineralogy, and X-ray fluorescence to determine material compositions.

1.3) Production of an inventory of existing carbon capture within selected sites

1.4) Estimation of rates of carbon capture on the basis of site history and repeated sampling where possible

1.5) Laboratory experiments to determine rate of calcium carbonate precipitation. Column and batch experiments will be used, with solutions designed to simulate soil pore waters and solids selected from natural (dolerite, basalt) and artificial materials (concrete, slag). Materials will be characterized using techniques in 1.2, and solutions analysed using ICP-AES, ion chromatography and aqueous TOC/TC determination.

1.6) Output WP1: Integration of investigative work to produce a predictive model of carbon capture through pedogenic carbonate precipitation.

Work package 2: Validation of the process

2.1) Stable isotope (C, H, O) analysis of a subset of samples from WP1.1, WP1.2 and WP1.5 to determine relative importance of C sources (atmosphere, biogenic, geological; organic matter such as compost). This will use accredited commercial laboratories with standard procedures for carbonate C and O isotope determination.

2.2) Investigate the feasibility of using laboratory-scale tomographic methods on products from WP1 (e.g. X-ray computerized tomography, XRCT) to monitor accumulation of carbonate precipitates. We will use the EPSRC state of the art XRCT system (Durham University).

2.3) Investigate potential of field sensors to monitor carbon uptake: chemical sensors, field-scale tomography. We are aware of a number of techniques that might enable in-situ monitoring of key parameters, and will investigate the feasibility of using these to monitor soil carbon accumulation.

2.4) Output WP2: Determination of a validation process that enables the success of the carbonation process to be measured.

Work package 3: Optimisation of planting schemes

3.1) Survey vegetation present at newly-created, restored and natural sites.

3.2) Measure pH and other key variables for soils (using standard soil analytical protocols) that determine plant performance, and relate this to species abundance and community structure.

3.3) Assess amounts of carbon accumulated through other processes (such as soil organic matter, plant biomass volumes), using GIS/remote sensing/3d geometrical modelling to assess areas (especially for linear assets) and t integrate spatial variability in soil composition.

3.4) Review management of existing planting schemes at selected sites.

3.5) Growth trials with specific commercial seed mixes in soils of known specification, at sites identified and/or created in WP1.1.

3.6) Observation of performance of long-lived plants, especially trees, in long-standing sites.

3.7) Output WP3: Recommendations for selection and management of planting and vegetation to optimize carbon capture.

Work package 4: Impact on the receiving environment
4.1) Evaluate human and environmental risks associated with the process in the context of potential pre-existing ground contamination and other site-specific factors, reviewing and using existing risk assessment protocols.

4.2) Determine impact on specific geotechnical properties relating to strength (CPT, shear box, CBR, shear vane), permeability and other key parameters (including laboratory scale carbonation experiments), using standard test procedures.

4.3) Assessment of impact of soil carbonation on urban flood risk. For selected case studies in Newcastle (which has a history of urban flooding) and elsewhere, we will integrate outputs of WP4.2 and WP3.3 for specific areas prone to flooding, modeling the impact of soil carbonate formation on through drainage.

4.4) Identify monitoring protocols for field scale application (e.g. resistivity, seismic and other geophysical procedures; soil solution and soil gas compositional monitoring). Following review and discussion with experts, specific tests will be carried out using rented field equipment ourselves, or using service consultancy companies.

4.5) Trial of monitoring protocols and soil strength measurements at sites where carbonate precipitation has taken place (from WP1). Experience gained in WP4 will be applied to selected sites.

4.6) Output WP4: Report on geotechnical properties of specific sites where carbonate precipitation has taken place, and technical aspects of site creation and management.

Work package 5: Feasibility assessment (a) technology sustainability assessment

5.1) Economic assessment, including cost analysis, material flow analysis, assessment of market for this technology, drivers and barriers to investment; regulatory status. Drawing on a range of previous experience, we will collate information concerning material flows, transport and material costs, and market conditions nationally, including international material flows. We will review current regulation (cross referring to WP4.1), and synthesize the impact of these on investors and commercial decision makers.

5.2) Environmental assessment, including life cycle analysis, risk analysis of use of materials classified as wastes; evaluation of carbon costs and benefits, to ensure that process design is optimized from the perspective of sustainability.

5.3) Societal acceptability, in context of other carbon management activities. We will work closely with local community groups (catalyzed by Newcastle Science City and other stakeholders) to
communicate the project and gauge reactions, assessing public perception, synergies and conflicts with other urban land uses and services.

5.4) Output WP5: Business-focused evaluation of the technology, its viability and its acceptability.

Work package 6: Feasibility assessment (b) implementation

6.1) A SWOT analysis will be carried out, with stakeholders and other interested parties, to assess the feasibility of the approach from a wide range of perspectives.

6.2) Scale of application: availability and geographical distribution of suitable materials, and potential for national/international adoption; identification of minimum cost-effective scale for implementation. This will consider the distribution and magnitude of demolition and quarry in activities, and the proximity of construction, in the British Isles and appropriate export markets (e.g. those importing quarried aggregates from the UK).

6.3) Comparison with other C mitigation methods/CCS, to identify possible synergies or conflicts, comparing industrial processes for mineral carbonation with passive soil-based processes.

6.4) Identification of more effective alternatives, building on the experience gained from carbon capture gardens. We will carry out an options analysis, enabling a decision process to compare different options for specific cases.

6.5) Output WP6: Policy-focused evaluation of the technology and its implementation.

Work package 7: Dissemination of findings and implementation plan

7.1) Creation of project website/blog, providing dialogue with interested parties and providing an on-line carbon capture prediction tool so that interested parties can assess amount of C stored per square meter (or hectare) as a consequence of their activities.

7.2) Preparation and distribution of materials for dissemination (a) to stakeholders and (b) to non-experts.

7.3) Regular stakeholder meetings.

7.4) Attendance at and contribution to key stakeholder-focused conferences and events, such as contaminated land fora and professional body regional group meetings.

7.5) Attendance at and contribution to academic research-focused conferences and events, in the UK and internationally

7.6) Integration of stakeholder dialogue with research outputs to produce a practical implementation plan in 2 stages: stage 1 a pilot, reviewed in stage 2 to generate a final product.

7.7) Output WP7: Reporting; Implementation plan, 2 stages, at month 21 and end of project.