ERE1.6

The production of iron and steel has generated substantial volumes of slag as waste, with estimates of up to 384 million tonnes (Mt) of iron slag and 280 Mt of steel slag produced globally, to date. Whilst the majority of these by-products (approximately 70 %) have seen bulk re-use in a number of applications (e.g. as ballast in construction applications), a significant volume of slag has been disposed of in environmental settings, where the release of metal-rich alkaline leachates can cause enduring pollution. However, the mineralogical and physical properties of slags also offer opportunity for environmental benefits, namely; through sequestration of atmospheric carbon dioxide, by acting as stockpiles of critical raw materials, and in certain situations performing as incidental coastal defences.

Findings are presented from national-scale field investigations, laboratory experiments, and spatial data analyses, which aim to explore the resource potential of iron and steel slags in the United Kingdom (UK). Within the 236 Mt of slags disposed of in the UK environment, projected carbonation rates revealed a potential for uptake of 138 Mt CO₂ under enhanced weathering conditions. Notable masses of technologically-critical elements were also estimated within UK slags, with reserves in the region of 1.55 Mt of V₂O₅, 1.58 Mt of TiO₂, and 1.26 Mt of Cr₂O₃ estimated. Further to acting as a resource, in areas of coastal deposition, slag banks were observed to offer tidal protection. At a number of sites this allowed the development of nationally-significant ecological communities, whilst laboratory leaching experiments reveal a very low risk of chemical leaching in saline conditions. The integration of spatial analyses with chemical composition and leaching data can help to inform decisions on maximising resource potential whilst minimising the potential environmental risks associated with slag reworking.

How to cite: Riley, A., Onnis, P., Gandy, C., MacDonald, J., Burke, I., Renforth, P., Jarvis, A., Hudson-Edwards, K., and Mayes, W.: Iron and steel-making slags as an environmentally-beneficial resource; a UK perspective., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4137, https://doi.org/10.5194/egusphere-egu22-4137, 2022.

Mineral sequestration using solid alkaline by-products, such as steel slag, is a feasible technology to capture carbon dioxide. This silicate weathering reaction forms solid carbonates, mineralizing the atmospheric CO₂ into calcite, which can occur passively under ambient environmental conditions over monthly to decadal timescales. The passive mineralization of carbon dioxide in steel slag is a not well-known reaction, particularly when climate factors influence the mineral carbonation. Non-destructive quantification of CO₂ mineralization is necessary to set underpinning knowledge on capturing rates.

The utilisation of X-ray micro-Computed Tomography (XCT) allows the 3D spatial visualisation and quantification of carbon dioxide precipitated as calcite in steel slag pores. We used XCT to analyse samples of legacy steel slag collected in Mexico and Scotland, to determine the effects of environmental factors on mineral carbonation. The XCT data were analysed with image processing to classify the slag volume into three phases (slag, pores, and calcite). The classification of the data into different phases allows the determination of the volume of each phase in the sample as well as its 3D spatial extent, thus enabling the quantification of mineralized CO₂ characterized as the calcite phase.

We will present a comparison between the volumes of carbon dioxide passively mineralized in the samples from the Mexican and Scottish collection sites, in the context of contrasting environmental factors. Preliminary results from one of the Scottish samples shows that calcite (mineralized CO₂) accounts for  ~ 5 vol. % of the sample, and it is localised across the whole sample.  A comparison of these results between the Mexican and Scottish samples will provide a better understanding of how climatic factors influence the volumes of atmospheric CO₂ mineralized by the samples. 

How to cite: Del Angel Lozano, J. P., Macente, A., and MacDonald, J.: The utilisation of X-ray Micro-Computed Tomography (XCT) for the quantification of carbon dioxide in passively carbonated steel slag., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4645, https://doi.org/10.5194/egusphere-egu22-4645, 2022.

Legacy deposits of by-product slag from iron and steel making create significant volumes of artificial ground around the world. Composed mainly of calcium-silicate mineral phases, experimental studies have shown the potential of slag for capturing atmospheric CO₂ by mineralisation (e.g. Huijgen et al. 2005). Renforth (2019) calculated that steel slag could capture ~370-400 kg CO₂ per tonne of slag, depending on the type of slag. ~0.5 Gt of steelmaking slag is produced every year (USGS 2018) and this could potentially reach ~2 Gt yr^-1 by the end of the century (Renforth 2019). In addition to new slag, there is an estimated 160 million m³ of legacy slag in the UK alone (Riley et al. 2020), stockpiled or dumped from historical steelmaking.

Artificial ground poses challenges around ground stability but slag-dominated artificial ground also offers opportunities for atmospheric CO₂ drawdown. In this contribution, we document the lithification of legacy slag deposits – conversion of loose gravelly slag material into a rock-like mass through cementation of calcite via drawdown of atmospheric CO₂.

Parts of slag heaps at our case study sites (Glengarnock and Warton, UK) have a lithified nature: gravel-to-cobble sized lumps of slag are visible but have been cemented together with a mineral cement, with an appearance not unlike a natural breccia rock. We present field, X-Ray Diffraction and δ¹³C data from these case study sites to document the lithification of slag-dominated artificial ground through mineralisation of atmospheric CO₂ as a cementing phase; we present scanning electron microscope data to show the microstructural evolution of this lithification. This understanding has implications for artificial ground stabilisation and how atmospheric CO₂ drawdown can be harnessed in this process.

References

Huijgen et al., 2005, ES&T, 39, 9676-9682

Renforth, 2019, Nat. Comms., 10, 1401

Riley et al., 2020, J. Geochem. Exp., 219, 106630

USGS, 2018, USGS Minerals Yearbook

How to cite: MacDonald, J. and Brolly, C.: Lithification of slag-dominated artificial ground through atmospheric CO2 drawdown, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-790, https://doi.org/10.5194/egusphere-egu22-790, 2022.

CO₂ mineralization is a natural process that occurs during weathering of silicate materials that are calcium/magnesium-rich and aluminum-poor (Kelemen et al., 2020). During this process, silicates convert to carbonates, making silicate-rich materials such as ultramafic rocks and alkaline wastes suitable for CO₂ removal from air.  Using slag to sequester CO₂ is particularly attractive as it is a by-product of a key industry, and it can utilize CO₂ from the emission source, therefore reducing the need for CO₂ and slag transportation, or draw down of CO₂ already in the atmosphere. It is estimated that steel slag has the potential to capture ~150-250 Mt CO₂ yr^-1 now, and ~320-870 Mt CO₂ yr^-1 by 2100 (Renforth, 2019).

Although the chemical composition of alkaline wastes shows that CO₂ capture can significantly offset emissions from corresponding industries, recent observations reveal that the CO₂ uptake in alkaline wastes in underutilized (Pullin et al., 2019). Here, we use image-based analysis to understand the microstructures of CO₂ mineralization in slag. We use X-ray Computed Tomography (XCT) to visualize slag internal structures and to calculate reactive surface area and pore connectivity. We then use scanning electron microscopy (SEM), coupled with energy dispersive spectroscopy (EDS) to study the distribution of elements within the studied sample.

In our study, we use a slag sample collected from the former Ravenscraig Steelworks in Lanarkshire, Scotland, where steelmaking took place from 1950s until 1992 (Stewart, 2008), leaving behind a slag heap that has been weathering since then. Our analysis demonstrates that calcium carbonate precipitates as pore-lining. Surface passivation and low surface-connected porosity were identified as processes that can cause reduction in CO₂ uptake.

References

Kelemen, P.B., McQueen, N., Wilcox, J., Renforth, P., Dipple, G., Vankeuren, A.P., 2020. Engineered carbon mineralization in ultramafic rocks for CO2 removal from air: Review and new insights. Chem. Geol. 550, 119628. https://doi.org/10.1016/j.chemgeo.2020.119628

Pullin, H., Bray, A.W., Burke, I.T., Muir, D.D., Sapsford, D.J., Mayes, W.M., Renforth, P., 2019. Atmospheric Carbon Capture Performance of Legacy Iron and Steel Waste. Environ. Sci. Technol. 53, 9502–9511. https://doi.org/10.1021/acs.est.9b01265

Renforth, P., 2019. The negative emission potential of alkaline materials. Nat. Commun. 10, 1401. https://doi.org/10.1038/s41467-019-09475-5

Stewart, D., 2008. Fighting for Survival: The 1980s Campaign to Save Ravenscraig Steelworks. J. Scottish Hist. Stud. 25, 40–57. https://doi.org/10.3366/JSHS.2005.25.1.40

How to cite: Khudhur, F. W. K., Macente, A., MacDonald, J., and Daly, L.: Passive CO2 mineralisation in slag: evidence from a slag heap in Lanarkshire, Scotland., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6525, https://doi.org/10.5194/egusphere-egu22-6525, 2022.

Soils have high pCO₂ because of the decay of organic matter and plant root respiration. Some of this CO₂ dissolves to form carbonic acid in soil waters. Natural weathering partially neutralizes this carbonic acid, but in recent years there has been much interest in using soil mineral and rock amendments (e.g. added olivine and basalt) to accelerate weathering-driven atmospheric CO₂ drawdown to counter rising atmospheric CO₂.  To be effective, accelerated weathering should result in increased dissolved bicarbonate (the main dissolved inorganic carbon carrier at circum-neutral pH) in drainage waters, as well as enhanced land-to-ocean fluxes of divalent metal cations such as Ca and Mg, ultimately to lock up the soil-derived inorganic carbon in marine limestones. Here we present new results for field experiments that investigate a novel soil amendment to sequester CO₂ from soil-gas via weathering; crushed returned concrete (CRC). Unlike previously investigated mafic and ultramafic materials for accelerated weathering approaches that generally require energy- and carbon-intensive mining, grinding and long-distance transport operations, CRC is a waste product that requires minimal crushing after post-return solidification at concrete plants to achieve high measured specific surface areas (c. 10 m²/g).  CRC is widely available globally because a few % of the >10Gt/year of concrete produced is typically returned unused to concrete ready-mix plants.  Engineering codes preclude the reincorporation of this waste as aggregate in new concrete in many jurisdictions. This results in the widescale availability of this highly weatherable alkaline waste product that is often landfilled or sold as a low-value construction fill.  Local availability of the material facilitates short haulage distances and relatively small energy and carbon footprints to transport the material to nearby field sites.  In this pilot study, CRC was added to the upper 15 cms of a one-hectare trial tillage field in SE Ireland at a rate of 10 tonnes/hectare. Soil-water solutions were extracted for analysis using suction-cup lysimeters at monthly intervals from the amended and adjacent non-amended control sites to determine the geochemical impact of CRC on soil waters and to calculate weathering and therefore CO₂ uptake rates via carbonic acid neutralisation. Relative to adjacent control sites, concrete-amended sites exhibited significant increases in soil-water pH (by 0.2 to 0.5 pH units), a two- to three-fold increase in electrical conductivity (total ion load) and similar increases in soil-water Ca²⁺, reflecting the weathering of portlandite and calcium silicates in the concrete. Field experiments are ongoing to assess the long-term effects of the concrete amendment on soil-water chemistry, soil pH and nutrient status. No increases in detrimental heavy metals (e.g. Ni, Cr) often associated with the use of mafic and ultramafic materials as soil amendments have been detected. Weathering is attributable entirely to carbonic acid neutralization, with no evidence for weathering by strong acids. Methods for the calculation of likely rates of CO₂ capture (tonnes CO₂ per tonne of amendment) and their associated uncertainties based on alkalinity increases, divalent metal exports and enhanced soil-leachable Ca will be discussed. 

How to cite: McDermott, F., Bryson, M., and van Acken, D.: An investigation of crushed returned concrete (CRC) as a soil amendment for atmospheric CO2 removal, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4824, https://doi.org/10.5194/egusphere-egu22-4824, 2022.

Clinker substitutes are frequently used in the cement and concrete industries to reduce CO₂ emissions associated with production, improve physico-chemical properties and performance, and reduce costs. Pulversized Fly Ash (PFA), a fine waste residue produced in coal-fired power stations, is the commonly used partial clinker substitute in Ordinary Portland cement (OPC) for cements for the immobilisation of low-level nuclear waste (LLW). Because of the global trend to shut-down coal-fired power stations, the production of PFA is decreasing and will eventually cease. Alternative sustainable clinker substitutes can be used must meet strict performance standards for the safe enclosure of LLW for the final disposal. These include physical, chemical, and mechanical properties; performance and suitability for use.

This study investigates the suitability of different materials (natural and anthropologic) as a substitute of PFA in OPC in LLW immobilisation, and compares the behaviour of these substituted cements to those of the current standard. The focus of the study is on the cementing and physico-chemical properties of the cement, and the interaction between groundwater, the cement, and the stored waste.

Here we present the characterisation the standard PFA+OPC (samples provided by Low-Level Waste Repository Ltd.) using X-ray computed tomography (XCT), and the latest data from the ongoing analysis elemental composition of the alternative materials and the leaching tests. Over the leaching period the samples undergo repeated XCT analysis to link structural changes to the chemical evolution. Future work will include studying the long-term leaching effects and the interaction of the LLW (usage mock waste formulation) with concrete.

These studies will allow us to identify changes to the cement microstructure and physico-chemical properties arising from the PFA substitutes, and the chemical and physical interaction of the cements, especially with groundwater Such understanding is critical for the adoption of clinker alternatives in LLW encapsulation.

How to cite: Kozlowski, A., Renshaw, J., and Dobson, K.: Replacing Pulversized Fly Ash in cement with natural and anthropogenic geomaterials identifying the corresponding physico-chemical properties used for the encapsulation of Low-Level Waste, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11448, https://doi.org/10.5194/egusphere-egu22-11448, 2022.

Ultramafic (Mg-rich) mine wastes are produced in vast quantities, are of no economic value, and their storage impoundments can be susceptible to catastrophic failure.¹ Carbon dioxide (CO₂) mineralization of these wastes to form carbonate cement can reduce greenhouse gas emissions² and assist in de-risking storage impoundments through physical stabilization.³ CO₂ mineralization and cementation have been documented at asbestos,⁴ nickel,² and diamond mines, occurring unintentionally over long periods (e.g., decades).^2,4 This research aimed to 1) better understand carbonation and cementation processes by examining historic kimberlite mine wastes from diamond mines and 2) accelerate these processes in experiments using brucite-bearing serpentinite mine wastes. 

We collected physical, mineralogical, and geochemical data of cemented historic kimberlite wastes 70 to >110 years old. Analysis of cemented fine- and coarse-grained residues from the Voorspoed and Cullinan diamond mines (South Africa) revealed the presence of a fine-grained (<63 µm) cement matrix with greater total inorganic carbon (TIC; +0.08–0.34% relative to clasts), secondary clays (e.g., Mg-Al silicates), and some minor carbonates (e.g., calcite). Unconfined compressive strength varied considerably between fine- and coarse-grained wastes (UCS; 0.13–4.45 MPa). Furthermore, kimberlite clasts and cements were isotopically distinct, suggesting that mineral weathering by meteoric water drove cementation over decades after the deposition of these wastes. 

In experiments, coupling organic and inorganic carbon cycling accelerated carbonation of synthetic tailings that contained brucite [Mg(OH)₂], a minor yet reactive mineral. In cylindrical test experiments (2.5 × 5 cm; 40 weeks), waste organics were either mixed (0–10 wt.%) or kept separate from brucite-bearing serpentinite mine wastes to provide an additional source of CO₂. In the mixed cylinders, brucite consumption ranged from 3–30% and was limited by CO₂ generation, as evidenced by minor increases in TIC (+0.02–0.22%). Compressive strengths amongst the cylinders reached 0.51 MPa with few cylinders becoming sufficiently stabilized; however, in experiments that exposed cylinders to CO₂ generated from organics separate from cylinders, brucite carbonation (64–84% consumption) and compressive strengths were substantial (0.4–6.9 MPa).³ Our research demonstrates the role of long-term weathering for sequestering CO₂ within ultramafic mine wastes, and how coupling organic and inorganic carbon cycling can accelerate CO₂ sequestration and physically stabilize these wastes. 

References: 

1. Rourke and Luppnow (2015), Tailings Mine Waste Manag., 225–230. 
2. Wilson et al. (2014), Int. J. Greenh. Gas Control 25, 121–140. 
3. Power et al. (2021), Environ. Sci. Technol. 55, 10056–10066. 
4. Wilson et al. (2009), Econ. Geol. 104, 95–112.

How to cite: Lockhart, J., Power, I., Paulo, C., Stubbs, A., and McDonald, D.: Carbonation and cementation of ultramafic mine wastes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6308, https://doi.org/10.5194/egusphere-egu22-6308, 2022.

Bauxite residue, the by-product produced in the alumina industry, is generated at an estimated global rate of approximately 150 million tonnes per annum. Currently, the reuse of bauxite residue is low (∼2%), and consequently the bulk is stored in land-based impoundments.

If not adequately managed, exposed residue may be prone to dusting, wind and water erosion and can contaminate surrounding areas. Establishing vegetation covers (rehabilitation) is viewed as an effective strategy for mitigating against pollution risk and approaches used can be broadly divided into capping with inert soil material or establishing vegetation in the tailings surface (revegetation). Revegetation provides a strategy where topsoil material is scarce and avoids the sourcing of large volumes from other sites.

While bauxite residue is typically alkaline (pH 10-12), saline and lacking in nutrients the implementation of effective rehabilitation strategies can promote favourable soil conditions for plant growth. Results also show establishment of soil microbial communities and soil faunal activity. These positive rehabilitation effects are maintained for several years and demonstrate that residue can be transformed to a soil-like medium capable of supporting ecosystem function.

How to cite: Courtney, R.: Technosols derived from bauxite residue tailings for effective revegetation and rehabilitation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2318, https://doi.org/10.5194/egusphere-egu22-2318, 2022.

Anthropogenic substrates are produced as waste materials and/or by-products of various industries. Such substrates include: blast furnace/steel slag; colliery spoil; oil shale spoil; and paper mill sludge (Ash et al. 1994; Riley et al. 2020). Historically, in the UK, these substrates were dumped in or close to the sites where they were being produced (Riley et al. 2020). Many examples of anthropogenic substrate sites still exist in the UK, despite the fact that there has been much cultural motivation to restore these sites (Ash et al. 1994; Bradshaw, 1995; Riley et al. 2020). This often results in either the total removal of anthropogenic substrate, or the covering of anthropogenic substrate with a clay cap/similar natural substrate. However, if left undisturbed, such sites could potentially provide undisturbed spaces for wildlife.

Various studies have been carried out which demonstrate that wildlife, including unusual and/or important species communities, can colonise and live on anthropogenic substrate sites (Ash et al. 1994; Riley et al. 2020). It is important to note that because anthropogenic substrate often differs greatly from the natural substrate in the surrounding area, such sites can support species and communities which might not otherwise survive in that area. For example, plants that rely on calcareous substrates might settle on slag sites or on Solvay process waste sites, but might not otherwise settle in the area if natural calcareous substrate is absent (Ash et al. 1994). Anthropogenic substrate sites can, therefore, act as refugia for many species and communities.

This study investigates three important aspects of anthropogenic substrate sites: substrate chemistry and mineralogy; plant species and communities; and certain invertebrate species. The investigation of these aspects allows for a detailed study of both anthropogenic geodiversity and biodiversity. For the substrates, various analyses will be carried out to determine the minerals, elements and pH levels present, including X-ray Diffraction, ICP and pH analysis for the six study sites. Different plant communities, as well as the species within them, were recorded in 2021 using quadrats in the six study sites. Different invertebrate species were recorded in 2021, throughout three of the study sites. Due to the current biodiversity crisis, it is more important than ever before to record and assess the biodiversity of places, especially if such places are often overlooked in terms of biodiversity potential. Additionally, very few studies have investigated the relationships between plant species and the mineralogical and elemental composition of the substrates on which they are growing – this work helps us to investigate plant establishment, survival and growth on anthropogenic substrates in a novel manner.

Ash et al., 1994, J. Appl. Ecology, 71, 74-78

Riley et al., 2020, J. Geochem. Explor., 219, 106630

Bradshaw, 1995, Can. J. Fish. Aquat. Sci. 53, 3-9

How to cite: van Mesdag, S., MacDonald, J., Neill, I., and Jump, A.: Anthropogenic biodiversity and geodiversity – can legacy industrial waste help offset falling global biodiversity?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1352, https://doi.org/10.5194/egusphere-egu22-1352, 2022.

Uncontrolled leaching from legacy industrial waste may release toxic elements, which poses long-term risks of water and soil contamination. In some situations, secondary mineralisation from the leachates may occur downstream from waste sites, thus potentially limiting contaminant migration. An example is tufa, a surface freshwater CaCO₃ (calcite) deposit which forms as a result of atmospheric CO₂ absorption into Ca-rich hyperalkaline leachates. The tufa develops a range of morphologies and varies in hardness across the deposit. Moreover, it may also incorporate other elements into its mineral structure during precipitation. Understanding the processes of secondary mineralisation which are able to capture toxic metals would provide beneficial insights into controlling hazardous leaching.

This work characterises tufa occurring within anthropogenic contexts. Several tufas were found forming on or adjacent to anthropogenic sites (colliery spoil and steel slag heaps) in central Scotland, UK and studied for their geochemistry. A combination of direct field measurements of water physico-chemistry is complemented by alkalinity and elemental analyses of leachate source, water and tufa by ion chromatography (IC) and ICP-OES. The results from these analyses will help understand the processes involved in tufa formation and can be applied to the re-creation of tufa with the purpose of metal capture under controlled laboratory conditions. Early experiments have focused on CaCO₃ precipitation onto different media by bubbling CO₂ into CaCl₂ solutions. The aim of these experiments is to create an engineered metal-capturing tufa system which can be applied across different post-industrial settings as a low-cost technique which beneficially captures CO₂.

How to cite: Kalabová, M., Cumberland, S., Renshaw, J., and MacDonald, J.: Anthropogenic tufa at legacy industrial sites: Potential for metal capture, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8116, https://doi.org/10.5194/egusphere-egu22-8116, 2022.

The leaching of heavy metals from post-industrial slag and other anthropogenic waste sites is detrimental for human health and the wider environment. Remediation of these sites can be costly and sustainable low carbon solutions are preferably sought. Examining natural analogues which stabilize metals could provide valuable insights into low-cost solutions to the legacy problems of aquatic environments that are impacted by leaching. Calcareous tufa, sometimes known as travertine limestone, forms naturally when calcium-rich groundwater is exchanged with atmospheric CO₂ at mid to hyperalkaline pH resulting in a calcite (CaCO₃) precipitation. Tufa has also been observed to form at a small number of old industrial sites (e.g. mining, steel works, paper mills) across northern England and Scotland. One site of interest is at Consett, N.E England, UK. Here tufa precipitates in the Howden Burn stream, a tributary of the River Derwent, as it emerges from the slag heaps from old steel work’s. Bulk analysis shows lead, arsenic, vanadium and zinc are present in the Howden Burn up to several 100 ppm.  Analysis of the water downstream of the tufa shows metal concentrations are considerably reduced compared to concentrations upstream. High spatial resolution LA-ICP-MS analysis of the solid tufa sampled reveal metals present within the tufa structure. This leads to the hypothesis that the metals are precipitated together with the tufa during its formation. However, little is known about metal capture processes during tufa formation and the form that these metals are in.  Here we present synchrotron micro X-ray fluorescence (μ-XRF) element maps of the tufa in cross-section that show the distributions of the metal within the laminations of the tufa structure. Understanding and exploitation of artificial tufa for metal capture could have potential as a CO₂ positive solution for sustainable in-stream remediation. 

How to cite: Cumberland, S., Tierney, K., Renshaw, J., Geraki, K., and MacDonald, J.: Analysis of metal entrapment within anthropogenic tufa using synchrotron micro-XRF, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4044, https://doi.org/10.5194/egusphere-egu22-4044, 2022.