Carbon Sequestration to Stabilise Legacy Chemical Waste and Capture Local CO2 Emissions

Shortlisted for Brownfield Awards Category 2: Best Scientific/Technical/Digital Advance or Innovation

Carbon Sequestration to Stabilise Legacy Chemical Waste and Capture Local CO2 Emissions

Alkaline waste materials can pose significant health, environmental and geotechnical constraints for site redevelopment; however, they can also offer opportunities. This submission presents the development and pilot trial application of innovative in-situ carbon sequestration to provide long-term physical and chemical stabilisation of legacy alkaline chemical waste.  The integration of carbon sequestration within the remedial strategy provides a robust, sustainable and defensible solution with real environmental, economic and social benefits, most notably the opportunity to capture and use the CO2 produced by local industrial facilities, thereby significantly reducing the CO2 emissions of those facilities.


Since 2016, AECOM has undertaken site characterization and risk-management works at this legacy area of chemical process waste. Slaked lime (calcium hydroxide) forms the dominant component of the waste and has a very high alkalinity. The remediation options appraisal for the site introduced the concept of incorporating in-situ carbon sequestration to: treat and reduce the high pH source term by converting the slaked lime to calcite; promote long term chemical and physical stabilisation of the site and hence reduce long term maintenance requirements; and provide a “carbon sink” to capture carbon dioxide (CO2) that would otherwise be released to the atmosphere as a greenhouse gas.  In 2020/21 AECOM undertook a pilot trial of in-situ carbon sequestration coupled with further site characterization to inform the sequestration potential of the waste. The pilot trial successfully demonstrated carbon sequestration indicating that up to 85,000 tonnes of CO2 could be sequestered within the waste.


The 8-hectare site is located adjacent to a former chemical processing facility in Northern Ireland. In the 1960s, the facility deposited chemical process waste on top of low-lying mudflats. The alkaline chemical waste extends to depths of 6m, with an estimated volume of approximately 265,000 m3 (Image 1).


Image 1: Legacy alkaline chemical waste.

Although the site is a brownfield site, it has re-vegetated and is bounded by ecologically sensitive rivers and coastal habitats. The scenic area is used for walking, birdwatching and fishing. 

Highly alkaline (pH 12-13) surface soils of the site pose a potentially acute risk to human health and highly alkaline groundwater discharges to the river foreshore.

Site characterization data provided the first indication of the potential for sequestration: CO2 was not detected during soil gas monitoring despite the presence of methane; and X-ray diffraction (XRD) samples of waste left exposed to air showed increasing proportions of carbonated products. 

XRD analysis of the waste identified three minerals with the potential to react and sequester CO2 into carbonated products:

  • Portlandite (Ca(OH)2 + CO2 → CaCO3 + H2O)

  • Ettringite (Ca6Al2(SO4)3(OH)12∙26H2O + 3CO2 → 3CaCO3 + 3[CaSO4∙2H2O] + Al2O3∙xH2O + (26-x)H2O)

  • Hydrocalumite (2[Ca2Al(OH)6Cl∙2H2O] + 3CO2 → 3CaCO3 + Al2O3∙xH2O + CaCl2 + (10-x)H2O)

The products of these reactions are calcite (CaCO3) and water, with ettringite also producing gypsum (CaSO4∙2H2O), hydrocalumite producing calcium chloride (CaCl2) and both ettringite and hydrocalumite producing amorphous aluminium (gel) material (Al2O3∙xH2O).

CO2 sequestration capacity decreases in the order portlandite > hydrocalumite > ettringite.

AECOM completed a remediation options appraisal (ROA) that identified pathway interception solutions (capping and cut-off wall) as the preferred solution and introduced the concept of pairing these engineering solutions with innovative in-situ carbon sequestration. The ROA concluded that carbon sequestration could be potentially beneficial due to overall sustainability / natural capital gains, but also as a means to reduce the high pH source and potentially reducing the long-term reliance on the engineered capping solution.  Further work was recommended to assess the feasibility, cost and potential for carbon sequestration. 

Pilot Trial

A pilot trial was carried out to assess the potential for carbonation through surface delivery of CO2 to the waste.  The trial was designed to mimic low pressure injection of CO2 within a gas distribution layer incorporated within the proposed cap (Image 2).


Image 2: Conceptual design for incorporation of CO2 distribution within the cap.

The scope of works comprised:

  • Exposing the surface of the waste to CO2 at two (active) locations

  • Comparison of active and control locations for changes in mineralogy

  • Further sampling of the waste to inform the carbon sequestration potential

  • Re-evaluation of the cost-benefit of incorporating carbon sequestration into the remediation solution

Works were completed between December 2019 and December 2020 and comprised:

  • Installation of four casings (two active; and two control): to isolate columns of waste within the casing

  • Installation and operation of a CO2 injection system within the two active casings by Cornelson Ltd. 

  • Removal of borehole headspace casings and drilling of cores at each location

  • Drilling of two additional boreholes within the waste for XRD analysis

The CO2 injection system operated for a total of 108.5 days.  The system was solar powered and included telemetry to allow continuous monitoring of the rate of CO2 injection to the headspace of the active casing and the corresponding CO2 concentration within the headspace (Image 3).  To mimic diffusion and density-driven advection conditions as far as practical, CO2 was injected at very low pressure (5-15 millibar above ambient) into the headspace to top-up the headspace as CO2 migrated into and reacted with the waste.

Image 3: Pilot trial implementation.


A total of 1,137 litres of CO2 were injected into the waste at the active locations (Figure 4).  The average rate of injection over the duration of the trial (normalized to unit surface area of the waste) ranged between 0.21-1.28 kg/ with the slower rate (BH_CO2_01A) attributed to the presence of cohesive made ground at surface.


Image 4: Cumulative CO2 injection into waste at active locations.

On completion of the trial, the four casings isolating the active and control locations were removed and cores recovered from each location to a depth of 4 metres using a Dando Terrier 200 percussive drilling rig.  Each core was photographed and sub-sampled for XRD analysis and a range of other geotechnical and environmental tests. 

Significant variability was noted in minerals with the potential for carbon sequestration and calcite contents between locations and within locations.  This variability was inferred to result from two mechanisms: (1) natural atmospheric carbon sequestration at the surface of the waste; and (2) carbon sequestration resulting from mineralization of tars co-disposed with the alkaline waste (Figure 5a).  Whilst a comparison of calcite content and total PAH concentrations (as a marker of tar impact) within the waste did not indicate a clear correlation,  the highest calcite contents (>40%) were detected in samples with total PAH concentrations greater than 15 mg/kg (Figure 5b) providing support for the hypothesis of carbon sequestration resulting from tar mineralization. 


Image 5: (a) Mineralogy of control location 01; (b) Comparison of total PAH concentrations to calcite composition 

At the active location with the lower rate of CO2 injection (BH_CO2_01A), the mass of carbon sequestered during the trial was masked by the high variability in carbon sequestration minerals and calcite at this location.

Where higher CO2 injection rates were achieved (BH_CO2_02A), XRD analysis demonstrated that the pilot trial had completely carbonated minerals within the waste in the upper 0.1 m and had increased the proportion of carbonated minerals in the upper 0.4 m (Figure 6).  A total of 0.53 kg of CO2 was calculated to have been sequestered from the increase in calcite content.


Image 6: Mineralogy of waste at active and control location 02 following pilot trial.

The carbon sequestration capacity of the waste was calculated from the waste volume, bulk density and the average concentration of the three carbon sequestration minerals from 49 XRD analyses across the waste body.  The volume of alkaline waste was estimated using a three-dimensional geological model (Figure 1) collating information from trial pits, boreholes and cone penetration tests.  Accounting for the potential contribution to carbonation from CO2 produced by residual organic contaminant mineralization (5,000 tonnes), the remaining carbon sequestration capacity of the waste was estimated to be 85,000 tonnes.

From the range of observed rates of CO2 injection/carbonation during the pilot trial, complete carbonation of the waste was estimated to take between four and 20 years.

A review of local CO2 emitters was undertaken that identified several industries in close proximity to the site where carbon capture and storage (CCS) could be implemented.  Cost-benefit analysis (CBA) was undertaken using the Treasury Green Book toolkit including capital expenditure (capex) and operation and maintenance costs of CCS.  The CBA assumed carbon sequestration would commence in 2025 and considered the current price of CO2 allowances (European Union Emission Trading Scheme) and sensitivity to future carbon price.  

A net benefit was identified for a number of cost-carbon price combinations with further work planned to address uncertainties, including: longer-term rate of carbon sequestration; cost for small-scale CCS; CO2 price evolution/development; and funding models.


Notable innovation / exemplary best practice

Through observations during site characterization AECOM recognized the opportunity that the alkaline chemical waste offers for carbon sequestration and have successfully demonstrated both the feasibility and the capacity for in-situ sequestration.  Rather than sole reliance on engineered entombment solutions this project couples engineered pathway interception solutions (cap/cut off wall) with innovative in-situ carbon sequestration to provide long-term physical and chemical stabilisation of the legacy waste.  AECOM is currently finalizing the detailed design for the pathway interception remediation solutions at the site.  The cap design has been modified so that the aggregate layer supporting the geosynthetic clay liner can be employed as a CO2 gas distribution layer.

Real environmental/economic/social benefit

The incorporation of in-situ carbon sequestration within the remediation solution for the site offers real environmental/economic and social benefits.  In addition to the natural capital benefit offered by the potential sequestration of 85,000 tonnes of CO2, cost-benefit analysis indicates that the cost of implementing CCS may be outweighed by the carbon price avoided, indicating both an economic benefit and a social benefit from CCS implementation.

Cost effectiveness

The pilot trial was run entirely using solar power to reduce power costs.  Careful design of the control system and telemetry allowed the trial to be monitored and troubleshooting carried out remotely - with the associated cost savings, sustainability and safety benefits of reduced travel. 

Compliance with legislation, codes and guidance

All works have been undertaken in full compliance with legislation and appropriate guidance.

Throughout all the works on-site AECOM has ensured safety as the highest priority, an approach actively supported by our client.  Pre-works planning calls were held with the client, drilling contractor and Cornelsen to discuss each step of the works and consider potential risks and mitigation measures.  Key considerations included acute risks posed by the alkaline waste and working with compressed gas.  Covid-19 travel restrictions were adhered to and working practices were reviewed and revised to implement additional control measures.    

Effective public/stakeholder engagement

The integration of carbon sequestration within the remedial strategy was achieved through AECOM’s advocacy, supported by a robust conceptual site model, an engaged, forward-thinking client and effective stakeholder engagement.

Key stakeholders included the client team, the Northern Ireland Environment Agency and the Local Authority.  A close, collaborative working relationship ensued from the onset of the project.  All stakeholders have been involved at relevant stages of site characterisation and the development of the remedial strategy, with regular updates, workshops and clear communication and collaboration in developing and executing the remedial strategy.  

A robust, sustainable and defensible solution

Through the combination of in-situ carbon sequestration with engineered pathway interception solutions, AECOM has developed a robust, sustainable and defensible solution for the site.  The engineered pathway interception solutions address immediate risks, whilst carbon sequestration adds resilience through reducing reliance on the long-term performance of the engineered solutions (through neutralization of the high alkalinity waste).

Further works are underway to take forward this innovative opportunity to couple brownfield remediation with the sequestration of 85,000 tonnes of CO2.  This marrying of contaminant risk mitigation and wider climate change impact by capturing and sequestering locally produced CO2 is a breakthrough in remediation design that has broad potential in brownfield remediation strategies in the future.