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Water from precipitation runoff or groundwater finally transports the dissolved calcium and bicarbonate ions into rivers and the ocean. Due to the higher pH and lower CO2 content in seawater, the bicarbonate ion is permanently locked up in solid calcium carbonate (CaCO3), which is formed by the interaction of the bicarbonate ion with calcium ions already present in seawater. On the ocean floor, these freshly created calcium carbonate minerals precipitate and settle as sediments.

Sedimentary rocks, such as chalk, limestone, and dolomite, can be created when these sediments are compressed and cemented over time (Lewis et al., 2021).

The MRV process flowchart, which encompasses all phases from planning to acknowledgment, was developed based on previous articles to provide a thorough framework for assessing EW as a CDR technique.

Sites are chosen for EW application based on a comprehensive study of geological, climatic and soil conditions combined with a baseline evaluation of carbon stocks and soil characteristics in the initial “Pre-implementation” phase.

The "Implementation" phase comprises applying EW materials precisely to specific places and carefully documenting the application rates, procedures, and timing.

The “Monitoring” phase is crucial, involving analysis of soil and water for changes in alkalinity, electrical conductivity, and other indicators of carbon sequestration. It also includes measuring greenhouse gas flux and utilizing remote sensing to assess broader impacts. Innovative and cost-effective monitoring proxies, such as electrical conductivity and volumetric water content, are utilized as well.

After gathering data, the "Reporting" stage compiles information into thorough reports that analyse environmental effects and sequestration rates. These reports are subsequently distributed to stakeholders. To ensure environmental integrity, verification is needed for confirmation of the reported data against that of established MRV protocols and validation of carbon sequestration claims.

To continue to improve EW practices, the MRV process concludes with “Feedback and Adjustment”. In “Certification and Recognition”, a successful CDR is recognised by issuing carbon credits or certifications that justify the contribution made to mitigate climate change.

This systematic approach to MRV emphasises the precision and detail required in the documentation and verification of effectiveness of EW as a CDR technique. The creation of effective MRV frameworks for EW require the integration of policy, scientific methodologies and innovative measurement techniques. Significant challenges include the need for scalable and fair verification procedures, methodological uncertainties, and variability in carbon sequestration efficacy. On-going research and cutting-edge methodologies aim to address these challenges which demonstrate MRV's potential to accurately assess EW project outcomes and guarantee transparency, comparability, and credibility in CDR claims associated with EW endeavours (Abdalqadir et al., 2024).

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Silicate rocks contain phosphorous (P), magnesium (Mg), potassium (K) and calcium (Ca) which are limiting nutrients for plant growth, thus their release due to the dissolution of silicate rocks via EW can lead to the fertilisation of crops. Regions with heavily weathered tropical soils such as Brazil or Africa have a history of using ground silicate rocks to enhance soil quality and boost crop productivity. For example, the addition of basalt to cocoa plants has led to an increased level of nutrients such as K, Mg, Ca compared to untreated plants. This application of basalt often lead to the treated plants being 50 percent taller and 60 percent thicker-stemmed than untreated plants after 2 years. In some regions such as Mauritius, the application of basalt was often used in combination with standard nitrogen (N), P, K fertiliser treatments which results in a yield increase of 29 percent over five successive crops and only 17 percent over three successive crops in separate replicated trials compared to plots receiving only basalt, indicating a positive correlation between basalt and fertiliser.

EW allows for the absorption of silica in the form of silicic acid for major tropical crops such as rice, oil palm, sugar cane, maize and sorghum. Through the mechanical strengthening of cell walls (via silicon deposition in tissues) and the priming of defence systems, silica absorption in these crops boosts resistance to important pests and diseases. By lowering leaf transpiration rates, silicon can increase water-use efficiency and possibly increase crop resistance to drought. Consequently, employing silicate rocks for EW might lower the need for pesticides and their related expenses while also enhancing food security in areas vulnerable to drought.

The application of crushed basalt can raise pH levels in highly weathered tropical soils, aiding in mitigating soil acidification in agricultural areas more broadly and addressing production challenges in crops grown on acidic soils (e.g., heavy metal toxicity in plants, as seen in oil palm cultivation on drained peatlands in Southeast Asia). The impact of EW on soil pH aligns with that of liming agricultural soils to counter acidification. Substituting silicate EW for lime application helps reduce CO2 emissions associated with lime's reaction with soil water and its production.

Land sparing

Deforestation is causing tropical agricultural areas to continue expanding rapidly. Silicate rock application, however, may lessen the need for new agricultural land if crop yields rise. This might lead to reduced deforestation and/or greater natural forest recovery on abandoned marginal farms. Land and greenhouse gas emissions were reduced during the Green Revolution in Asia and Latin America, indicating that higher yields brought about by EW may allow for even more land savings. Absence of effective market regulation and land planning, however, may cause perverse outcomes of higher-yielding, cheap tropical crops, including further deforestation.

Reduced risk of phytoplankton blooms in rivers and reefs

The ratios of silicon (Si) to nitrogen (N) and silicon (Si) to phosphorus (P) in runoff water determine the threat of eutrophication. When algal blooms are caused by high N and P but low Si, cultural eutrophication takes place. High Si: P and Si: N ratios in runoff from the erosion of silicate rocks are expected to result from their EW. This will raise the amount of diatoms in the water, limiting cultural eutrophication and promoting a variety of productive food webs. Along with the potential advantage of increased diatom production contributing to an increase in CO2 drawdown in the seas, this might also have a positive impact on heavily polluted riverine, reef, and marine ecosystems downstream of large tropical agricultural producing regions.

(Edwards et al., 2017)

Mafic igneous rocks such as basalt are composed of abundant calcium and magnesium-rich silicate minerals widely proposed to be suitable for scalable carbon dioxide removal (CDR) by EW (Lewis et al., 2021).

Basalt is viewed as an alternative to dunite since it contains fewer harmful elements and may even function as fertilizer by supplying nutrients (such as calcium and magnesium) which are lacking in many tropical regions. Although it has a poorer weathering efficiency, the increase in plant biomass carbon caused by nutrient release can offset this (Strefler et al., 2018).

Depending on grain size, global analyses show that the energy expenditures (i.e., CO2 emissions) related to extracting, processing, and dispersing rock dust may reduce the effectiveness of EW's CO2 sequestration by 10 percent to 25 percent. However, as the globe moves toward more decarbonized energy sources, this cost should go down. NOx emissions would rise with more crushed rock being transported. This might cause ground-level ozone (O3) to rise to dangerous levels for plant and human health in 16 Mha of oil palm plantations, which are known to be major emitters of isoprene.

Yield quality

EW suggests the possibility that harmful substances found in certain silicate minerals might become accessible and reduce yields, or they could build up in the food chain and cause problems for human health. As was previously indicated, the breakdown of olivine may release nickel (Ni) and chromium (Cr), which might be troublesome in significant mines and in agriculture when these elements are present in minerals associated to asbestos. To prevent these unintentional negative effects, EW with basalt is favored instead; nevertheless, basalt is less efficient than olivine in capturing CO2 (around 0.3 tCO2 t−1 vs 0.8 tCO2 t−1 olivine). The lack of heavy metal toxicity and ancillary advantages of basalt for crop productivity, soil enhancement, and greenhouse gas emission reduction that are less likely to result from olivine would reduce the practical hurdles to adoption by farmers in tropical agroecosystems.

Biodiversity impacts within plantations and adjacent forest

Wildlife in tropical farmland provide essential ecosystem services such as pollination and pest control. The effect of the application of silicate is still unknown. In particular, increasing pH could negatively impact species adapted to low pH soils which are widespread, especially peatland. Forest edges are first affected by environmental changes (e.g. increased wind, higher nutrient loads). The effect of crushed silicates on forest from farmland is still uncertain. If the consequences are negative, this would be a pressing issue given that 25% of the Amazon and Congo and 91% Brazilian Atlantic forest is within 1 km of farmland edge.

Reduced water quality in rivers and reefs

Reduced reproduction and recruitment in river fish populations may occur if unweathered silicates are carried into rivers, possibly during strong tropical rainstorms. This might lead to increased inorganic turbidity and sedimentation. Reef diversity and depth limit are reduced, coral death is caused by increased sediment loads, and inorganic turbidity. Therefore, there might be very detrimental effects on nearby fisheries and conservation efforts; however, these losses would have to be balanced against any advantages from less organic turbidity (i.e., lower eutrophication). Riverine plants and animals may also suffer from elevated pH levels in the water, particularly in naturally acidic drainage systems like peatlands.

Mining and infrastructural expansion

Although silicates are considered a waste product from mining and steel and iron production, the increase in demand of silicates for EW may require larger mines. For example, rock application to 670 Mha of tropical cropland at 10 t ha−1 yr−1 would require 6.7 Pg of rock per year, and at 50 t ha−1 yr−1 would need 33.5 Pg annually [5]. By comparison, global silicate production is 7-17 Pg [46] and global aggregate production is 40 Pg [47,48]. Mine creation is environmentally harmful, driving deforestation across the tropics and often occurring within or near to areas of high biodiversity value. Development and expansion of road and rail infrastructure for mining can increase access to biodiverse and remote ecosystems, which combined with employment opportunities, encourage population immigration, land clearing for agriculture and hunting.

(Edwards et al., 2017)

After taking into consideration the hydrological changes across the country and scaling up the CDR potential of basalt over UK agriculture, it is estimated that applying 50 t ha−1 of basalt for ten years may consume 1.8 ± 0.9 MtCO2 yr−1.

Buckingham, F.L. and Henderson, G.M. (2024)

The maximum CDR rates predicted by the model were primarily influenced by the geographic scope of ERW application, expanding with increased resource availability (Fig. 1a–c). Additionally, ongoing effects from previous years also played a significant role. CDR rates per unit area showed a continual increase over time due to successive annual applications of rock dust, even if the deployment land area remained constant.

(Kantzas et al., 2022)

One significant factor contributing to the higher CDR potential in Kantzas et al. (2022) is likely the use of elevated water fluxes in their UK model (461–849 mm yr−1), which far exceed actual net water fluxes observed from river flow in agricultural regions.

Assuming a linear relationship between water flux and CDR potential, and increasing the flux above for all arable land accordingly, suggests 1.3 ± 0.1 MtCO2 yr−1 as a best estimate for the drawdown potential of EW in the UK when accounting for variation in hydrology.

(Buckingham et al., 2022)

• EW materials

• particle size

• application rate

• soil properties

• land management

• weather and climate.

EW materials

EW materials, derived from natural silicate formations and industrial byproducts, are crucial for enhanced weathering (EW) strategies aimed at CO2 sequestration. These materials, ranging from acidic to basic compositions, are chosen based on cation richness, mineralogy, dissolution rates, trace elements, life cycle assessments, availability, and cost-effectiveness. Basalt, dunite, olivine, and wollastonite are notable EW materials due to their chemical compositions and mineralogical characteristics, influencing their effectiveness in capturing CO2.

Particle size

Smaller particles have larger surface areas, which accelerate weathering and CO₂ absorption rates. Research suggests that particles measuring between 10 to 20 µm in diameter work best for efficient weathering, with smaller particles demonstrating notably improved CO2 absorption rates. However, reducing particle sizes through comminution processes requires substantial energy.

Application rate

This rate varies based on the EW material used, CO2 removal goals, and the land area. Although the optimal application rates are still debated, higher rates generally lead to more significant CO2 sequestration per hectare due to increased reactive surface areas. However, this relationship is complex, as seen in studies where factors like mineral composition and particle size also influence sequestration rates. For example, different application rates of olivine, wollastonite, and basalt in various experiments demonstrate varying CO2 sequestration efficiencies, highlighting the need for nuanced considerations beyond just the application rate.

Soil properties

Factors like pH, texture, organic matter, microbial activity, CEC, porosity, temperature, and moisture content all influence the effectiveness of EW processes. For instance, soil pH affects mineral solubility and CO₂ availability, while organic matter and microbial activity can accelerate weathering. Soils with higher CEC and porosity tend to facilitate more efficient weathering reactions, leading to increased CO₂ removal. Understanding these soil properties helps optimize EW strategies for effective CO₂ sequestration, although maintaining appropriate moisture levels and considering other soil characteristics are vital for long-term success.

Weather and climate

Warmer temperatures generally accelerate mineral dissolution, potentially enhancing CO2 sequestration rates, whereas colder climates can impede this process. Regional variations in humidity, precipitation, and seasonal changes further modulate weathering rates; high rainfall and wetter soils typically promote faster weathering, while dry climates slow down the process. Extreme weather events and microclimate variations from agricultural activities also play roles in altering local weathering environments

Land management

Incorporating EW materials can alter soil pH and nutrient availability, impacting weathering rates significantly. Practices that enhance soil organic matter and moisture retention, coupled with erosion control and proper irrigation management, contribute to optimal conditions for efficient weathering reactions. Additionally, reforestation and afforestation efforts indirectly support EW by improving soil health and biological processes, showcasing the interconnectedness of land management and carbon sequestration through EW.

(Abdalqadir et al., 2024)

Silicate weathering rates depend on:

• temperature

• run-off

• rate of physical erosion

Though silicate rock weathering should theoretically be accelerated by warm, humid tropical circumstances, lowland tropical habitats are typically characterized by thick, mature soils that experience minimal physical disturbance, meaning that natural rates are frequently quite low.

As a result, in contrast to other temperate zones where kinetics largely determine the speed of silicate weathering, the availability of new mineral surfaces in the tropics limits the rate of natural rock weathering.

(Edwards et al., 2017)

The monetary value of EW application may be an insignificant incentive but an additional financial return for the work carried out.

Buckingham, F.L. and Henderson, G.M. (2024)

This is clearly a greater carbon sink potential than that observed in the studies for the UK.

Rock Type

• Basalt can be classified as a mafic rock depending on its mineral content.

• Weathering of mafic rocks produces concentrated waters with higher Ca2+/Na2+ and Mg2+/Ca2+ ratios than felsic rocks such as granite, rhyolite

• weathering rates are significantly higher than at lower pH values

Climate

• Temperature

  • Chemical reactions proceed more rapidly

    at higher temperatures

• Precipitation

  • More water allows

    more chemical reactions and therefore more

    weathering.

  • Chemical weathering is moisture-limited in

    arid and semi-arid regions.

    
    

Relief

• Weathering limited regime

  • Weathering rate limited by chemical

    processes where erosion is active.

• Transport limited regime

  • Weathering products accumulate where

    erosion is minimal.

• In high-relief and mountainous areas,

  landsliding and other erosion processes

  remove weathering products.

• In low-relief areas, these weathering

  products build up and inhibit further

  weathering

Vegetation

• Vegetation also stabilises the soil which

  • prevents physical erosion → decreases weathering

  • increases contact time of initial weathering products and incoming precipitation → increases weathering

• Deforestation → high TDS in runoff waters → colonisation of plants of a previous un-vegetated area

Time

• Contact time between rock and water is an important

  variable that control water chemistry.

• High rainfall usually results in shorter contact times but

  depends on how rainfall is distributed in time.

• In arid areas soil/weathering zone may dry out for part of year – salts precipitate in soils.

• Silicate rocks (like basalt) are rich in essential nutrients and micronutrients.

• When these rocks undergo weathering, they release minerals such as calcium, magnesium, potassium, phosphorus, sulfur, and micronutrients like copper, iron, manganese, molybdenum, and zinc.

• These nutrients are crucial for plant growth and soil health.

• This may reduce the reliance of N-rich fertilisers.

Addressing Soil Deficiencies

• Crushed silicate rocks can help address deficiencies in potassium and micronutrients in agricultural soils.

• Depletion of these nutrients is often due to continuous farming practices and soil nutrient stripping.

Improved Crop Yields

• EW has been linked to increased crop yields, especially in areas with highly weathered, acidic soils.

• Faster weathering minerals applied in temperate climates have also shown positive results, leading to improvements in yields for crops like maize and potatoes.

Controlled Nutrient Release

• Silicate rock amendments release nutrients slowly, reducing the risk of nutrient leaching and surface run-off.

• This controlled release is beneficial for sustained plant nutrition without harming water sources.

pH Neutralization

• Silicate rocks can neutralize acidic soils, raising pH levels to more optimal ranges for plant growth.

• Improved pH levels also enhance nutrient availability in the soil, further contributing to increased crop yields.

It is estimated it takes up to 180 kWh t−1 to produce 10 μm particles. (Renforth, 2012)

Furthermore, it was found that the Cr(VI) form poses a health risk to humans, especially through acute and long-term inhalation exposures that exacerbate respiratory tract issues, the major target organ for toxicity.

(Prasad et al., 2021)

(Genchi et al., 2020)

This mitigation measure while scalable, should be used in combination with a number of other measures such as reforestation, rewetting peatlands, direct air capture with carbon storage (DACCS), etc.