Remediating the sky: the role of geoenvironmental engineers and applied geoscientists in geochemical carbon dioxide removal

Societies have grappled with environmental impacts resulting from their own advancements and interactions with the natural world, and the evolution of environmental engineering has been intertwined with the challenge of harmonizing human progress with ecological equilibrium. As societies progressed, so did the environmental challenges they faced. The industrial revolution of the eighteenth and nineteenth centuries brought about unprecedented environmental degradation as a consequence of rapid industrialization and urbanization. Growing concerns over pollution and public health sparked the need for innovative solutions and paved the way for the emergence of environmental geosciences and geoenvironmental and environmental engineering as specialized fields.

Since the industrial revolution our civilization has released over 2400 billion tonnes (Bt) of carbon dioxide (CO₂) into the atmosphere, increasing concentrations from 280 to more than 410 ppm (by volume) (IPCC 2023), constituting the largest, by mass, of chronic pollution in human history. The 2015 Paris Agreement, under the United Nations Framework Convention on Climate Change (UNFCCC 2015), was negotiated between 196 national or regional governments with the intention of limiting the impacts of dangerous climate change. Parties to the agreement account for nearly 99% of greenhouse gas emissions. The first aim of the 2015 Paris Agreement is to maintain global average temperatures to well below 2°C and to pursue efforts to limit temperature increases to 1.5°C, over pre-industrial averages (UNFCCC 2015), such ‘climate action’ is one of 17 sustainable development goals promoted by the United Nations (UN 2015).

These climate targets require a dramatic reduction in net greenhouse gas emissions, equivalent to an average 3–4% per year reduction between now and 2050. For comparison, CO₂ emissions have grown by 1.3% annually over the last 10 years (IPCC 2023). Such emissions reduction will be the primary focus of efforts over the coming decades. However, the climate targets expressed in the Paris Agreement can only be achieved if the greenhouse gas concentrations in the atmosphere are stabilized and that will require the annual removal of c. 5–15 Bt of CO₂ from the atmosphere, as suggested by the United Nations Intergovernmental Panel on Climate Change (IPCC 2022) (termed carbon dioxide removal or ‘CDR’). This briefing introduces CDR and new geochemical-based methods (gCDR) before exploring how geoscientist and geoenvironmental engineers may contribute to this new field.

CDR encompasses a broad range of approaches that aim to actively remove CO₂ from the atmosphere (Fig. 1): for instance, through land-management practices (Smith et al. 2019), ocean-based approaches (National Academies of Sciences, Engineering, and Medicine 2022) or terrestrial engineered approaches (National Academies of Sciences, Engineering, and Medicine 2019). These approaches face numerous challenges for successful implementation (see Renforth and Wilcox 2019).

Some CDR technologies are potentially expensive, making large-scale implementation financially challenging without appropriate incentivization. For example, direct air capture (DAC) technologies remove and concentrate CO₂ from the atmosphere using machines that require substantial energy inputs, which can be expensive (potentially $100–600 tCO₂⁻¹ with technology improvements: Young et al. 2023) and may contribute to the CO₂ emissions if derived from fossil fuels (Deutz and Bardow 2021). While these are currently being developed on a small scale through nascent voluntary markets (Michaelowa et al. 2023), the 2022 Inflation Reduction Act in the US provides tax credits for CO₂ removal, and recent developments in UK, EU and US policy suggest scalable incentivization may be imminent (Smith et al. 2023), and thus the potential rapid growth of CDR investment and deployment.

To effectively address climate change, CDR needs to operate at a scale such that it can remove large quantities of CO₂. This involves scaling up CDR technologies to operate globally, and for novel approaches a factor of 10³–10⁴ scale-up is required (Smith et al. 2023). Achieving the scale and speed necessitates investment (which may equate to up to 1% of GDP by 2050: Bednar et al. 2021), coordinated international efforts and policy innovation supportive of CDR deployment (National Academies of Sciences, Engineering, and Medicine 2019).

Implementing large-scale CDR strategies requires a comprehensive understanding of potential environmental impacts. Some CDR methods may have unintended consequences, or secondary benefits, for ecosystems and biodiversity (Smith et al. 2019). In addition, long-term storage of removed carbon is required. Robust risk assessments, regulatory frameworks and monitoring systems are essential to mitigate these risks and ensure the sustainability of CDR strategies. Overcoming these technical, environmental, social and economic hurdles, and making CDR technologies economically viable, is crucial for their widespread adoption.

Geochemical CO₂ removal (gCDR) methods leverage chemical reactions within the Earth's natural carbon cycle. Unlike other forms of CDR, which rely exclusively on biological or chemo-mechanical processes, gCDR focuses on the interaction between CO₂ and minerals or alkaline substances (Campbell et al. 2022). There are a range of archetypal gCDR approaches, including adding crushed minerals to the land surface (‘enhanced weathering’: Hartmann et al. 2013; Beerling et al. 2020), injecting CO₂ into reactive alkaline rock formations (Kelemen et al. 2020), and, in some approaches, CO₂ is transformed and stored as bicarbonate in the ocean (Renforth and Henderson 2017).

Some gCDR approaches propose to react atmospheric CO₂ with reactive alkaline materials (e.g. lime, slag, cement waste or mine waste) in distributed heaps or layers (Power et al. 2014; Renforth 2019; Kelemen et al. 2020; McQueen et al. 2020) or through integration with wastewater management (Cai and Jiao 2022). Accidental or unintended atmospheric CO₂ uptake in these materials is well known in legacy deposits (Renforth et al. 2009; Wilson et al. 2009; Washbourne et al. 2015) but only a small fraction of the CO₂ capture potential may currently be realized in some of these (Pullin et al. 2019). These materials are by-products and wastes of carbon-intensive industries, and as such their reaction with CO₂ only partially offsets the emissions during production. However, these industries are under substantial pressure to decarbonize, and in so doing alkaline products and wastes may be able to offset 3–8 Bt of residual CO₂ emissions (Renforth 2019). Some have already commercially exploited the reaction of wastes with CO₂ to produce construction materials (Gunning et al. 2011).

Alkaline materials may be produced specifically for reaction with atmospheric CO₂ (Dubey et al. 2002; McQueen et al. 2020; Samari et al. 2020). In these processes, oxide or hydroxide minerals (e.g. Ca(OH)₂ or Mg(OH)₂) are manufactured from carbonate rock, the process emissions are captured and stored, and the resulting ‘zero-carbon lime’ is distributed in heaps or layers, or within engineered structures (Abanades et al. 2020) to facilitate its reaction with atmospheric CO₂. The reaction may be integrated into wastewater management (Masindi et al. 2023). The few studies that have explored CO₂ uptake in distributed lime suggest that tens of tonnes of CO₂ up to more than 1000 tCO₂ per hectare per year may be possible (Erans et al. 2020), which is considerably, but necessarily, larger than CO₂ uptake during CDR land management (e.g. tree planting and blue carbon: Bernal et al. 2018). McQueen et al. (2020) suggest that such approaches may cost of the order of $50–150 tCO₂⁻¹ removed, which is competitive with other CDR technologies.

A range of early state commercial operators, non-profit organizations and research activities have been created over the last 5 years exploring a wide range of gCDR approaches, and the field is rapidly expanding (Campbell et al. 2022; Maesano et al. 2022) (Fig. 2).

Geoenvironmental engineers and applied geoscientists have developed solutions to managing and remediating humanity's waste efficiently and economically, often leading to the creation of engineered systems (e.g. landfills, tailings ponds, filtration beds, cascades and settling tanks) that are orders of magnitude more voluminous than what might be found in chemical reactor engineering. The combination of environmental engineering and geosciences has been essential in the management of pollution from legacy industries: for instance, the identification and remediation of acid mine water (e.g. Younger 1993; Whitehead et al. 2005), the removal of phosphate from wastewater (e.g. Zheng et al. 2023), and contaminated land remediation (e.g. Cundy et al. 2008). Given the ability to design and operate at scale and to integrate across disciplines, environmental engineers and geoscientists have a critical role to play in gCDR (as they do in delivering wider sustainable development: Lagesse et al. 2022). However, few geoenvironmental engineers have worked in this field. It is time that these skills were brought to bear on the removal of CO₂ from the atmosphere. Table 1, while not exhaustive, provides a range of examples that map some key gCDR activities to disciplines within the geosciences. Broadly these activities include:

• Research and development – the emerging field of gCDR can harness geoenvironmental engineering and geosciences to develop fundamental data on reaction rates and their controlling mechanisms, and to prototype new technologies. Geochemical models are a core component for gCDR engineering process models, which feed directly into technology assessment (and thus are able to estimate the cost of a particular approach). For instance, Renforth et al. (2022) embedded a geochemical model coded in PHREEQC into a bespoke techno-economic model to size reactors, and thus determine the capital costs of a gCDR approach. Lagesse et al. (2022) suggest that improved understanding in hydromechanical–chemical coupling is required for ‘conventional’ carbon capture and storage (i.e. the injection of CO₂ into sedimentary formations), which is also true for in situ mineralization in basic rock formations.

• Technology design and operation – professionals with experience in the implementation of environmental solutions may use their skills in the development of ground models that can predict the behaviour of granular reactive materials, site selection for deployment, detailed design of the reaction systems, specification for raw material processing and protocols for waste recycling or disposal.

• Environmental impact and life-cycle assessment – in addition to specifying the function of gCDR, environmental and geoprofessionals will assess and monitor the environmental impact of the deployed technology. This might include environmental monitoring and assessment at deployment locations, managing application for permits and assessing compliance. Professionals will also be responsible for assessing and improving the environmental performance through life-cycle assessment.

• Monitoring, reporting and verification – critical to the success of any CDR approach is the ability to monitor and verify carbon removal. Environmental technology monitoring, and reporting the outcomes, is a routine operation for applied geoscientists; although widely-adapted or formalized best practice has yet to be developed within gCDR.

The emergence of a gCDR industry within the coming decades presents challenges and opportunities for applied geoscientists. The opportunity to apply existing skills and work in a new and growing market, while helping to prevent climate change, is appealing (Maesano et al. 2022). However, gCDR requires a subtle conceptual shift in how geoscientists apply these skills. For instance, geochemists would benefit from additional skills in chemical engineering to understand how geochemical models may integrate with engineering process models. Exploration geologists would need to develop new tools and models for assessing the potential of basic or ultrabasic rock formations for CO₂ injection (Kelemen et al. 2020). Environmental geoscientists would need to measure inorganic carbon dynamics and accumulation at gCDR sites to monitor CO₂ removal. Currently, gCDR is missing from most university geoscience programmes, and there is minimal awareness of it in professional practice. Yet, demand for skills with this space is already growing, and the gCDR industry's ability to grow may be curtailed if the geosciences community does not rise to the challenge. As Maesano et al. (2022) suggest, a range of training opportunities (e.g. university modules) and awareness-raising activities (e.g. facilitated by learned societies) would be an important first step.

There is growing interest in geochemical carbon dioxide removal (gCDR) that involves the reaction of CO₂ with rocks, minerals and alkaline materials, which has the potential to operate at global scales at competitive costs. There is already an ecosystem of emerging commercial operators developing first projects in this space. Environmental engineers and applied geoscientists will have an important role to play in the research and development of these technologies, their design and implementation, monitoring their environmental impact, and assessing and verifying CO₂ removal for long-term storage. Learned societies and university academics have an important role in helping to build awareness and skills within gCDR. Pioneering professionals have an opportunity to become early practitioners within this emerging industry by joining early-stage commercial activities or beginning their own. It is also vital that geoscience organizations reflect on the skills of their workforce and understand what additional training might be required if they are to be competitive in the field of gCDR.

The artistic rendering of afforestation was created using Midjourney through the command prompt ‘a reforestation programme photorealistic --ar 3:2’.

PR: conceptualization (lead), funding acquisition (lead), writing – original draft (lead), writing – review & editing (lead).

This work was funded by the Engineering and Physical Sciences Research Council (EPSRC) (grant No. EP/V027050/1) through the UK's Industrial Decarbonization Research and Innovation Centre.

The author declares that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data sharing is not applicable to this article as no datasets were generated or analysed during the current study.