enhanced weathering

Enhanced weathering has been proposed for both terrestrial and ocean-based carbon sequestration. Ocean methods are being tested by the non-profit organization Project Vesta to see if they are environmentally and economically viable.{{Cite web|last=Peters|first=Adele|date=2020-05-29|title=Ever been to a green sand beach? The newest geohack to fight climate change|url=https://www.fastcompany.com/90510254/ever-been-to-a-green-sand-beach-the-newest-geohack-to-fight-climate-change|access-date=2020-11-06|website=Fast Company|language=en-US|archive-date=2020-10-29|archive-url=https://web.archive.org/web/20201029201256/https://www.fastcompany.com/90510254/ever-been-to-a-green-sand-beach-the-newest-geohack-to-fight-climate-change|url-status=live}}{{Cite web|last=Delbert|first=Caroline|date=2020-06-11|title=How This Strange Green Sand Could Reverse Climate Change|url=https://www.popularmechanics.com/science/environment/a32799266/green-sand-carbon-dioxide-climate-change/|access-date=2020-11-06|website=Popular Mechanics|language=en-US|archive-date=2020-10-12|archive-url=https://web.archive.org/web/20201012122839/https://www.popularmechanics.com/science/environment/a32799266/green-sand-carbon-dioxide-climate-change/|url-status=live}}

In July 2020, a group of scientists assessed that the geo-engineering technique of enhanced rock weathering, i.e., spreading finely crushed basalt on fields – has potential use for carbon dioxide removal by nations, identifying costs, opportunities, and engineering challenges.{{cite news |title=Applying rock dust to croplands could absorb up to 2 billion tonnes of CO₂ from the atmosphere |url=https://phys.org/news/2020-07-croplands-absorb-billion-tonnes-co2.html |work=phys.org |language=en |access-date=28 August 2020 |archive-date=12 September 2020 |archive-url=https://web.archive.org/web/20200912060345/https://phys.org/news/2020-07-croplands-absorb-billion-tonnes-co2.html |url-status=live }}{{cite journal|last1=Beerling|first1=David J.|last2=Kantzas|first2=Euripides P.|last3=Lomas|first3=Mark R.|last4=Wade|first4=Peter|last5=Eufrasio|first5=Rafael M.|last6=Renforth|first6=Phil|last7=Sarkar|first7=Binoy|last8=Andrews|first8=M. Grace|last9=James|first9=Rachael H.|last10=Pearce|first10=Christopher R.|last11=Mercure|first11=Jean-Francois|date=July 2020|title=Potential for large-scale CO₂ removal via enhanced rock weathering with croplands|url=https://www.nature.com/articles/s41586-020-2448-9|journal=Nature|language=en|volume=583|issue=7815|pages=242–248|doi=10.1038/s41586-020-2448-9|issn=1476-4687|archive-url=https://web.archive.org/web/20200716021952/https://www.nature.com/articles/s41586-020-2448-9|archive-date=16 July 2020|access-date=16 August 2020|via=|last18=Pidgeon|first15=Madhu|last12=Pollitt|first12=Hector|last13=Holden|first13=Philip B.|last14=Edwards|first14=Neil R.|last16=Koh|first18=Nick F.|first16=Lenny|last17=Quegan|first17=Shaun|first21=Steven A.|last21=Banwart|first20=James|last20=Hansen|first19=Ivan A.|last19=Janssens|last15=Khanna|pmid=32641817|bibcode=2020Natur.583..242B|s2cid=220417075|url-status=live|hdl=10871/122894|hdl-access=free}}

{{See also|Weathering|Ocean acidification}}

File:Gletsjerpad naar de Morteratschgletsjer 12-09-2019. (d.j.b) 28.jpg on the mountain path to the tongue of the Morteratsch glacier.]]

File:CO2 pump hg.svg

Weathering is the natural process of rocks and minerals dissolving to the action of water, ice, acids, salts, plants, animals, and temperature changes.{{Cite web |url=https://www.nationalgeographic.org/encyclopedia/weathering/ |title=National Geographic - Weathering |access-date=2020-11-30 |archive-date=2017-01-25 |archive-url=https://web.archive.org/web/20170125215232/http://www.nationalgeographic.org/encyclopedia/weathering/ |url-status=live }} It is mechanical (breaking up rock—also called physical weathering or disaggregation) and chemical (changing the chemical compounds in the rocks). Biological weathering is a form of weathering (mechanical or chemical) by plants, fungi, or other living organisms.

Chemical weathering can happen by different mechanisms, depending mainly on the nature of the minerals involved. This includes solution, hydration, hydrolysis, and oxidation weathering.{{Cite web |url=https://redrockcanyonopenspace.org/education/geology/rock-weathering/ |title=Brandon Vogt, "Rock Weathering" |date=17 October 2012 |access-date=2020-11-30 |archive-date=2020-10-27 |archive-url=https://web.archive.org/web/20201027184413/https://redrockcanyonopenspace.org/education/geology/rock-weathering/ |url-status=live }} Carbonation weathering is a particular type of solution weathering.

Carbonate and silicate minerals are examples of minerals affected by carbonation weathering. When silicate or carbonate minerals are exposed to rainwater or groundwater, they slowly dissolve due to carbonation weathering: that is the water (H₂O) and carbon dioxide ({{CO2}}) present in the atmosphere form carbonic acid (H₂CO₃) by the reaction:{{Cite web |url=https://www.britannica.com/topic/evolution-of-the-atmosphere-1703862/Biological-carbon-cycle#ref390494 |title=Encyclopædia Britannica - Biological carbon cycle |access-date=2020-11-30 |archive-date=2020-12-12 |archive-url=https://web.archive.org/web/20201212015052/https://www.britannica.com/topic/evolution-of-the-atmosphere-1703862/Biological-carbon-cycle#ref390494 |url-status=live }}

:H₂O + {{CO2}} → H₂CO₃

This carbonic acid then attacks the mineral to form carbonate ions in solution with the unreacted water. As a result of these two chemical reactions (carbonation and dissolution), minerals, water, and carbon dioxide combine, which alters the chemical composition of minerals and removes {{CO2}} from the atmosphere. Of course, these are reversible reactions, so if the carbonate encounters H ions from acids, such as in soils, they will react to form water and release {{CO2}} back to the atmosphere. Applying limestone (a calcium carbonate) to acid soils neutralizes the H ions but releases {{CO2}} from the limestone{{clarify|date=December 2023}}.

In particular, forsterite (a silicate mineral) is dissolved through the reaction:

:Mg₂SiO₄(s) + 4H₂CO₃(aq) → 2Mg²⁺(aq) + 4HCO₃⁻(aq) + H₄SiO₄(aq)

where "(s)" indicates a substance in a solid state and "(aq)" indicates a substance in an aqueous solution.

Calcite (a carbonate mineral) is instead dissolved through the reaction:

:CaCO₃(s) + H₂CO₃(aq) → Ca²⁺(aq) + 2HCO₃⁻(aq)

Although some of the dissolved bicarbonate may react with soil acids during the passage through the soil profile to groundwater, water with dissolved bicarbonate ions (HCO₃⁻) eventually ends up in the ocean, where the bicarbonate ions are biomineralized to carbonate minerals for shells and skeletons through the reaction:

:Ca²⁺ + 2HCO₃⁻ → CaCO₃ + {{CO2}} + H₂O

The carbonate minerals then eventually sink from the ocean surface to the ocean floor. Most of the carbonate is redissolved in the deep ocean as it sinks.

Over geological time periods these processes are thought to stabilize the Earth's climate.{{cite journal |doi=10.2475/ajs.301.2.182 |title=GEOCARB III: A revised model of atmospheric CO₂ over Phanerozoic time |year=2001 |last1=Berner |first1=Robert A. Berner |first2=Zavareth |last2=Kothavala |journal=American Journal of Science |volume=301 |issue=2 |pages=182–204|bibcode=2001AmJS..301..182B |citeseerx=10.1.1.393.582 }} The ratio of carbon dioxide in the atmosphere as a gas (CO₂) to the quantity of carbon dioxide converted into carbonate is regulated by a chemical equilibrium: in case of a change of this equilibrium state, it takes theoretically (if no other alteration is happening during this time) thousands of years to establish a new equilibrium state.

For silicate weathering, the theoretical net effect of dissolution and precipitation is 1 mol of {{CO2}} sequestered for every mol of Ca²⁺ or Mg²⁺ weathered out of the mineral. Given that some of the dissolved cations react with existing alkalinity in the solution to form CO₃²⁻ ions, the ratio is not exactly 1:1 in natural systems but is a function of temperature and {{CO2}} partial pressure. The net {{CO2}} sequestration of carbonate weathering reaction and carbonate precipitation reaction is zero.{{clarify|date=November 2020}}

File:Carbon-Slicate Cycle Feedbacks.jpg

Weathering and biological carbonate precipitation are thought to be only loosely coupled on short time periods (<1000 years). Therefore, an increase in both carbonate and silicate weathering with respect to carbonate precipitation will result in a buildup of alkalinity in the ocean.{{clarify|date=November 2020}}

Enhanced weathering was initially used to refer specifically to the spreading of crushed silicate minerals on the land surface.{{cite journal |doi=10.1007/s10584-005-3485-y |title=Enhanced Weathering: An Effective and Cheap Tool to Sequester CO₂ |year=2006 |last1=Schuiling |first1=R. D. |last2=Krijgsman |first2=P. |journal=Climatic Change |volume=74 |issue=1–3 |pages=349–54|bibcode=2006ClCh...74..349S |s2cid=131280491 }}{{cite journal |doi=10.1180/minmag.2008.072.2.639 |title=Biological enhancement of soil carbonate precipitation: Passive removal of atmospheric CO₂ |year=2008 |last1=Manning |first1=D. A. C. |journal=Mineralogical Magazine |volume=72 |issue=2 |pages=639–49|bibcode=2008MinM...72..639M |s2cid=94528533 }} Next to geochemical constraints, I.e., reactive surface area of the rock feedstock, and solution pH and pH buffer capacity of the receiving soil,{{cite journal |last1=Van Der Bauwhede |first1=Robrecht |last2=Muys |first2= Bart |last3=Vancampenhout |first3=Karen |last4=Smolders |first4=Erik |title=Accelerated weathering of silicate rock dusts predicts the slow-release liming in soils depending on rock mineralogy, soil acidity, and test methodology |journal=Geoderma |date=2024 |volume=441 |issue=116734 |doi=10.1016/j.geoderma.2023.116734|doi-access=free }} the biological activity in soils has been shown to promote the dissolution of silicate minerals,{{cite journal |doi=10.1021/es301250j |title=Passive Sequestration of Atmospheric CO₂ through Coupled Plant-Mineral Reactions in Urban soils |year=2013 |last1=Manning |first1=David A. C. |last2=Renforth |first2=Phil |journal=Environmental Science & Technology |volume=47 |pages=135–41 |pmid=22616942 |issue=1|bibcode = 2013EnST...47..135M }} but there is still uncertainty surrounding how quickly this may happen. Because weathering rate is a function of saturation of the dissolving mineral in solution (decreasing to zero in fully saturated solutions), some have suggested that lack of rainfall may limit terrestrial enhanced weathering,{{cite journal |doi=10.1073/pnas.1000545107 |title=Geoengineering potential of artificially enhanced silicate weathering of olivine |year=2010 |last1=Köhler |first1=Peter |last2=Hartmann |first2=Jens |last3=Wolf-Gladrow |first3=Dieter A. |last4=Schellnhuber |first4=Hans-Joachim |journal=Proceedings of the National Academy of Sciences |volume=107 |issue=47 |pages=20228–33 |bibcode=2010EGUGA..12.6986K |jstor=25756680 |pmid=21059941 |pmc=2996662|doi-access=free }} although others{{cite journal |doi=10.1073/pnas.1019024108 |title=Enhanced silicate weathering is not limited by silicic acid saturation |year=2011 |last1=Schuiling |first1=Roelof D. |last2=Wilson |first2=Siobhan A. |last3=Power |first3=lan M. |journal=Proceedings of the National Academy of Sciences |volume=108 |issue=12 |pages=E41 |bibcode=2011PNAS..108E..41S |pmid=21368192 |pmc=3064366|doi-access=free }} suggest that secondary mineral formation or biological uptake may suppress saturation and promote weathering.

The amount of energy that is required for comminution depends on the rate at which the minerals dissolve (less comminution is required for rapid mineral dissolution). A 2012 study suggested a large range in potential cost of enhanced weathering largely due to the uncertainty surrounding mineral dissolution rates.{{cite journal |doi=10.1016/j.ijggc.2012.06.011 |title=The potential of enhanced weathering in the UK |year=2012 |last1=Renforth |first1=P. |journal=International Journal of Greenhouse Gas Control |volume=10 |pages=229–43 |bibcode=2012IJGGC..10..229R |s2cid=96612612 |url=http://orca.cf.ac.uk/60892/1/Renforth%25202012%2520-%2520IJGGC.pdf |access-date=2019-12-10 |archive-date=2020-12-05 |archive-url=https://web.archive.org/web/20201205103205/http://orca.cf.ac.uk/60892/1/Renforth%25202012%2520-%2520IJGGC.pdf |url-status=live }}

To overcome the limitations of solution saturation and to use natural comminution of sand particles from wave energy, silicate minerals may be applied to coastal environments,{{cite journal |doi=10.1016/j.ijggc.2010.04.012 |title=Coastal spreading of olivine to control atmospheric CO₂ concentrations: A critical analysis of viability. Comment: Nature and laboratory models are different |year=2010 |last1=Schuiling |first1=R.D. |last2=de Boer |first2=P.L. |journal=International Journal of Greenhouse Gas Control |volume=4 |issue=5 |pages=855–6|bibcode=2010IJGGC...4..855S }} although the higher pH of seawater may substantially decrease the rate of dissolution,{{cite journal |doi=10.1016/j.ijggc.2009.07.001 |title=Coastal spreading of olivine to control atmospheric CO₂ concentrations: A critical analysis of viability |year=2009 |last1=Hangx |first1=Suzanne J.T. |last2=Spiers |first2=Christopher J. |journal=International Journal of Greenhouse Gas Control |volume=3 |issue=6 |pages=757–67|bibcode=2009IJGGC...3..757H }} and it is unclear how much comminution is possible from wave action.

Alternatively, the direct application of carbonate minerals to the upwelling regions of the ocean has been investigated.{{cite journal |doi=10.1029/2007JC004373 |title=Mitigating the atmospheric CO₂ increase and ocean acidification by adding limestone powder to upwelling regions |year=2008 |last1=Harvey |first1=L. D. D. |journal=Journal of Geophysical Research |volume=113 |issue=C4 |pages=C04028 |bibcode=2008JGRC..113.4028H|doi-access=free }} Carbonate minerals are supersaturated in the surface ocean but are undersaturated in the deep ocean. In areas of upwelling, this undersaturated water is brought to the surface. While this technology will likely be cheap, the maximum annual CO₂ sequestration potential is limited.

Transforming the carbonate minerals into oxides and spreading this material in the open ocean ('Ocean Liming') has been proposed as an alternative technology.{{cite journal |doi=10.1016/0360-5442(95)00035-F |title=Sequestering atmospheric carbon dioxide by increasing ocean alkalinity |year=1995 |last1=Kheshgi |first1=Haroon S. |journal=Energy |volume=20 |issue=9 |pages=915–22|bibcode=1995Ene....20..915K }} Here the carbonate mineral (CaCO₃) is transformed into lime (CaO) through calcination. The energy requirements for this technology are substantial.

The enhanced dissolution and carbonation of silicates ('mineral carbonation') was first proposed by Seifritz in 1990,{{cite journal |bibcode=1990Natur.345..486S |title=CO₂ disposal by means of silicates |last1=Seifritz |first1=W. |volume=345 |year=1990 |pages=486 |journal=Nature |doi=10.1038/345486b0 |issue=6275|s2cid=38210921 |doi-access=free }} and developed initially by Lackner et al.{{cite journal |doi=10.1016/0360-5442(95)00071-N |title=Carbon dioxide disposal in carbonate minerals |year=1995 |last1=Lackner |first1=Klaus S. |last2=Wendt |first2=Christopher H. |last3=Butt |first3=Darryl P. |last4=Joyce |first4=Edward L. |last5=Sharp |first5=David H. |journal=Energy |volume=20 |issue=11 |pages=1153|bibcode=1995Ene....20.1153L }} and further by the Albany Research Center.{{cite book |last1=O'Connor |first1=W. K. |last2=Dahlin |first2=D. C. |last3=Rush |first3=G. E. |last4=Gedermann |first4=S. J. |last5=Penner |first5=L. R. |last6=Nilsen |first6=D. N. |date=March 15, 2005 |title=Aqueous mineral carbonation, Final Report |url=https://www.netl.doe.gov/File%20Library/Research/Coal/NETLAlbanyAqueousMineralCarbonation.pdf |publisher=National Energy Technology Laboratory |access-date=December 29, 2015 |archive-date=March 4, 2016 |archive-url=https://web.archive.org/web/20160304114358/https://www.netl.doe.gov/File%20Library/Research/Coal/NETLAlbanyAqueousMineralCarbonation.pdf |url-status=live }}{{Page needed|date=March 2013}} This early research investigated the carbonation of extracted and crushed silicates at elevated temperatures (~180 °C) and partial pressures of CO₂ (~15 MPa) inside controlled reactors ("ex-situ mineral carbonation"). Some research explores the potential of "in-situ mineral carbonation" in which the CO₂ is injected into silicate rock formations to promote carbonate formation underground (see: CarbFix).

Mineral carbonation research has largely focused on the sequestration of {{CO2}} from flue gas. It could be used for geoengineering if the source of {{CO2}} was derived from the atmosphere, e.g. through direct air capture or biomass-CCS.

Soil Remineralization contributes to the enhanced weathering process. Mixing the soil with crushed rock such as silicate benefits not only plants' health, but also carbon sequestration when calcium or magnesium are present.{{Cite journal|last1=Lefebvre|first1=David|last2=Goglio|first2=Pietro|last3=Williams|first3=Adrian|last4=Manning|first4=David|last5=Azevedo|first5=Antonio|last6=Bergmann|first6=Magda|last7=Meersmans|first7=Jeroen|last8=Smith|first8=Pete|date=2019-10-01|title=Assessing the potential of soil carbonation and enhanced weathering through Life Cycle Assessment: A case study for Sao Paulo State, Brazil|url=https://research.wur.nl/en/publications/assessing-the-potential-of-soil-carbonation-and-enhanced-weatheri|journal=Journal of Cleaner Production|language=en|volume=233|pages=468–481|doi=10.1016/j.jclepro.2019.06.099 |s2cid=197776092 |doi-access=free|bibcode=2019JCPro.233..468L |hdl=2164/12752|hdl-access=free}} Remineralize The Earth is a non-profit organization that promotes rock dust applications as natural fertilizers in agriculture fields to restore soils with minerals, improve the quality of vegetation and increase carbon sequestration.

Where abundant electric surplus electricity is available, the electrolytic dissolution of silicate minerals has been proposed{{Cite journal|last1=Scott|first1=Allan|last2=Oze|first2=Christopher|last3=Shah|first3=Vineet|last4=Yang|first4=Nan|last5=Shanks|first5=Barney|last6=Cheeseman|first6=Chris|last7=Marshall|first7=Aaron|last8=Watson|first8=Matthew|date=2021-02-04|title=Transformation of abundant magnesium silicate minerals for enhanced {{CO2}} sequestration|journal=Communications Earth & Environment|language=en|volume=2|issue=1|page=25|doi=10.1038/s43247-021-00099-6|bibcode=2021ComEE...2...25S|s2cid=231793974|issn=2662-4435|doi-access=free}} and experimentally shown. The process resembles the weathering of some minerals. In addition, hydrogen produced would be a carbon-negative.{{Cite journal|last1=Rau|first1=Greg H.|last2=Carroll|first2=Susan A.|last3=Bourcier|first3=William L.|last4=Singleton|first4=Michael J.|last5=Smith|first5=Megan M.|last6=Aines|first6=Roger D.|date=2013-06-18|title=Direct electrolytic dissolution of silicate minerals for air {{CO2}} mitigation and carbon-negative {{chem|H|2}} production|journal=Proceedings of the National Academy of Sciences|volume=110|issue=25|pages=10095–10100|doi=10.1073/pnas.1222358110|pmid=23729814|pmc=3690887|bibcode=2013PNAS..11010095R|doi-access=free}}

In a 2020 techno-economical analysis, the cost of utilizing this method on cropland was estimated at US$80–180 per tonne of CO₂. This is comparable with other methods of removing carbon dioxide from the atmosphere currently available (BECCS (US$100–200 per tonne of CO₂)- Bio-Energy with Carbon Capture and Storage) and direct air capture and storage at large scale deployment and low-cost energy inputs (US$100–300 per tonne of CO₂). In contrast, the cost of reforestation was estimated lower than US$100 per tonne of CO₂.{{Cite journal|last=Beerling|first=David|date=2020-07-08|title=Potential for large-scale CO₂ removal via enhanced rock weathering with croplands|url=https://www.nature.com/articles/s41586-020-2448-9|journal=Nature|language=en|volume=583|issue=7815|pages=242–248|bibcode=2020Natur.583..242B|doi=10.1038/s41586-020-2448-9|pmid=32641817|access-date=2021-02-09|s2cid=220417075|archive-date=2020-07-16|archive-url=https://web.archive.org/web/20200716021952/https://www.nature.com/articles/s41586-020-2448-9|url-status=live|hdl=10871/122894|hdl-access=free}}

[https://un-do.com/ UNDO], a UK-based Enhanced Weathering company, spreads crushed silicate rock, such as basalt and wollastonite, on agricultural land in the United Kingdom, Canada and Australia. They claim to have spread more than 200,000 tonnes of crushed rock to date, which will capture over 40,000 tonnes of CO2 as their rock weathers. In March of 2024, they published a peer-reviewed paper{{Cite journal |last1=Skov |first1=Kirstine |last2=Wardman |first2=Jez |last3=Healey |first3=Matthew |last4=McBride |first4=Amy |last5=Bierowiec |first5=Tzara |last6=Cooper |first6=Julia |last7=Edeh |first7=Ifeoma |last8=George |first8=Dave |last9=Kelland |first9=Mike E. |last10=Mann |first10=Jim |last11=Manning |first11=David |last12=Murphy |first12=Melissa J. |last13=Pape |first13=Ryan |last14=Teh |first14=Yit A. |last15=Turner |first15=Will |date=2024-03-27 |title=Initial agronomic benefits of enhanced weathering using basalt: A study of spring oat in a temperate climate |journal=PLOS ONE |language=en |volume=19 |issue=3 |pages=e0295031 |doi=10.1371/journal.pone.0295031 |doi-access=free |issn=1932-6203 |pmc=10971544 |pmid=38536835|bibcode=2024PLoSO..1995031S }} in partnership with Newcastle University in PLOS ONE journal concerning the agronomic co-benefits of crushed basalt in a temperate climate. They are 1 of 20 XPRIZE Carbon Removal{{Cite web |title=20 Teams Bring Cutting-Edge Solutions to XPRIZE Carbon Removal Final |url=https://www.xprize.org/prizes/carbonremoval/articles/20-teams-bring-cutting-edge-solutions-to-xprize-carbon-removal-finals |access-date=2024-06-11 |website=XPRIZE |language=en}} finalists, a $100 million competition hosted by the Musk Foundation.

An Irish company named Silicate has run trials in Ireland and in 2023 is running trials in the USA near Chicago. Using concrete crushed down to dust it is scattered on farmland on the ratio 500 tonnes to 50 hectares, aiming to capture 100 tonnes of CO2 per annum from that area. Claiming it improves soil quality and crop productivity, the company sells carbon removal credits to fund the costs. The initial pilot funding comes from prize money awarded to the startup by the THRIVE/Shell Climate-Smart Agriculture Challenge.{{cite web |title=Can concrete dust help to fight climate change? This Irish startup is trying it out on US farmland |url=https://www.euronews.com/green/2023/10/27/can-concrete-dust-help-to-fight-climate-change-irish-startup-is-trying-it-out-in-us |date=27 October 2023}}{{cite web |title=CONGRATULATIONS TO OUR THRIVE SHELL CLIMATE-SMART AGRICULTURE CHALLENGE WINNERS |url=https://thriveagrifood.com/shell-challenge/ |access-date=3 November 2023}}

• {{Section link|Olivine#Experimental_uses}}

{{reflist}}

• [https://climatecleanup.org/ew-conference-2022/ Enhanced Weathering Conference 2022]

Category:Carbon dioxide removal

Category:Geoengineering

Category:Weathering

Category:Enhanced weathering