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Enhanced rock weathering and Mineral Carbonation: How significant it can be in battling climate change

Sapneswar Sahoo, PhD

IIT(ISM) Dhanbad

Earth's geological past provides insight into how the planet has historically controlled global warming through natural geological processes. Nonetheless, in contrast to the current anthropogenic global warming that has caused climate change, geological global heating and cooling is a longer-lasting process. The long-term shift in temperature and variations in local, regional, and global weather patterns (such as precipitation, wind patterns, heat waves, and sea currents) are referred to as climate change. The average global temperature has increased by almost 1 °C over the previous century, according to recent studies. The main factor influencing Earth's weather patterns is an increase in the planet's average global temperature. Global net anthropogenic GHG emissions are expected to reach ~59.6 Gt CO2 in 2019, up 12% (or 6.5 Gt CO2) from 2010 and 54% (or 21 Gt CO2) from 1990. Every year, the ocean and terrestrial biosphere absorb almost half of these emissions from the atmosphere. The residual emissions, on the other hand, accumulate in the atmosphere and fuel global warming. Considering that the Earth's climate system is being forced by global warming to experience more frequent and intense weather, environmental degradation, sea level rise, etc.

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Figure 1. Rise in the concentration of three major greenhouse gases (GHG) in the atmosphere since 1850. Source: Figure 2.1 IPCC Report 2023

There are a number of ways to lower the amount of carbon dioxide released into the atmosphere, including capturing and permanently storing greenhouse gasses as well as reducing greenhouse gas emissions at their source. Negative emissions technology (NET) refers to the science and process utilized to capture and store greenhouse gasses. In order to combat global warming and meet "net zero" commitments, negative emissions technologies, often referred to as carbon dioxide removal (CDR) or greenhouse gas removal technologies (GGRTs), are expected to be essential. A prospective carbon dioxide removal (CDR) method called enhanced rock weathering (ERW) (Beerling et al., 2020) speeds up natural weathering in order to boost CO2's capacity for reactivity with the target minerals and produce bicarbonate. Similar to ERW, mineral carbonation (MC) (Snæbjörnsdóttir  et al., 2020) produces carbonates through the reaction of trapped CO2 with minerals that contain metal oxides. MC uses more energy during the process but sequesters CO2 more quickly.

 

Mechanism of Mineral Carbonation (MW) and Enhanced Rock Weathering (ERW)

The increased pace of weathering and chemical alteration (mineral reaction) that occurs naturally in geology is replicated by the ERW method. Rainwater becomes slightly acidic (carbonic acid, pH 5–5.5) when carbon dioxide dissolves in water droplets. This solution can react with various minerals at the surface depending on their chemical stability. ERW is a chemical process that involves dispersing finely ground or powdered silicate rocks (rich in calcium and magnesium) with a highly reactive surface to accelerate the chemical interaction between the rocks, water, and CO2 in the air (Beerling et al., 2020). The bicarbonate that is formed stores the CO2. Eventually, this bicarbonate washes into the oceans, where the carbon is either trapped for hundreds of thousands of years on the sea floor or stored in soluble form. Alkaline minerals that take part in the extended residual waste (ERW) reaction are found in igneous rocks like basalt and dunite as well as in the byproducts of several industrial processes such the manufacturing of cement, lime, steel, and iron. In order for atmospheric CO2 to be transformed into dissolved inorganic carbon (hydrogen carbonate ions, or HCO3) that can be eliminated by soil drainage fluids, the ERW process releases base cations that provide alkalinity. In contrast to ERW, mineral carbonation forms insoluble carbonates by reacting CO2 with minerals such as calcium, magnesium, and iron.

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Figure 2. The schematic diagram shows the uptake of CO2 by weathering and its transportation to the ocean to get deposited as carbonate. This similar principle is applied in the ERW methodology where the process is accelerated by pulverizing the basalt (or Mg, Ca-rich rocks). Source: Wikimedia, https://en.wikipedia.org/wiki/Carbonate%E2%80%93silicate_cycle#

These elements are found in high concentrations in the silicate rocks dunite, serpentinite, and wollastonite. For rapid mineralisation into calcite (CaCO3), dolomite (CaMg(CO3)2), or magnesite (MgCO3), the carbon in MC is stored by adding it to reactive rocks such as mafic or ultramafic lithologies that contain high quantities of divalent cations including Ca2+, Mg2+, and Fe2+. These components are also present in fly ash, steel slag, and cement kiln dust, among other industrial wastes. Mineral carbonation reactions can occur either in situ, or below earth, or ex-situ, or above ground. Because of its composition, basalt is very common on Earth and has great potential for use in MC and ERW carbon sequestration technologies. Basalt is the best rock for ERW because it weathers quickly and has the necessary mineralogy that is changeable chemically. Since natural rock weathering now absorbs roughly 0.3% of global fossil fuel emissions, increasing weathering can help remove even more CO2 from the atmosphere. Studies have indicated that although though ERW is a relatively new technique for sequestering carbon, it has the ability to combat climate change by sequestering CO2. A study's models indicate that, over a 75-year period, spanning all croplands worldwide, ERW can retain more than 200 gigatons of CO2 at a fixed rate of 10 tons of basalt dust per hectare (Baek et al., 2023). Much research is still needed in the relatively young fields of improved rock weathering and mineral carbonation for carbon sequestration.

 

In addition to its capacity to store carbon, ERW offers significant co-benefits and environmental benefits in the fight against climate change. Modern farming practices have the potential to worsen the condition of the soil, which will ultimately lower the soil's ability to store carbon and release CO2 back into the sky. When applied to agricultural land, crushed basalt releases essential nutrients like potassium, calcium, and magnesium into the soil, which can enhance crop productivity in addition to aiding in the sequestration of CO2. Increased weathering raises the alkalinity of water, which aids in the ocean's acidification process as well. The phrase "ocean acidification" refers to a long-term drop in ocean pH that is mostly caused by carbon dioxide (CO2) absorption from the atmosphere. 


Scaling ERW and MC are scientific endeavours that require attention. Businesses that address climate change, such as UNDO in the UK and Carbfix in Iceland, are utilising these technologies to practically store carbon dioxide on a commercial scale. However, additional participation from other businesses and the government is needed. A number of mitigation techniques have been proposed by the Intergovernmental Panel on Climate Change (IPCC) to keep global warming below 1.5–2°C of preindustrial levels by the year 2100 (IPCC, 2018, 2023). Crucially, in order to keep global warming below 1.5°C, all mitigation plans require the active removal of atmospheric carbon dioxide, or roughly 100–1,000 gigatons (billion tons), within the next century (IPCC, 2018, 2023).

References:

Beerling, D. J., Leake, J. R., Long, S. P., Scholes, J. D., Ton, J., Nelson, P. N., et al. (2018). Farming with crops and rocks to address global climate, food and soil security. Nat. Plants 4, 138–147.

 

Baek, S. H., Kanzaki, Y., Lora, J. M., Planavsky, N., Reinhard, C. T., & Zhang, S. (2023). Impact of climate on the global capacity for enhanced rock weathering on croplands. Earth's

Future, 11, e2023EF003698.

 

Snæbjörnsdóttir, S.Ó., Sigfússon, B., Marieni, C. et al. Carbon dioxide storage through mineral carbonation. Nat Rev Earth Environ 1, 90–102 (2020).

 

Campbell JS, Foteinis S, Furey V, Hawrot O, Pike D, Aeschlimann S, Maesano CN, Reginato PL, Goodwin DR, Looger LL, Boyden ES and Renforth P (2022) Geochemical Negative Emissions Technologies: Part I. Review. Front. Clim. 4:879133.

 

IPCC Report 2023: doi: https://www.ipcc.ch/report/ar6/syr/

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