There is no universal best material for ocean alkalinity enhancement (OAE). In a 31-day coastal experiment published by Li et al. in Marine Pollution Bulletin (2026), fly ash released alkalinity faster than the tested olivine and basalt, while steel slag produced a sharp initial pH increase followed by alkalinity consumption from carbonate precipitation. Olivine released alkalinity slowly, and lime follows a separate, processed route that can deliver alkalinity faster but adds calcination energy and emissions. The useful comparison is therefore not a simple ranking: it is a site-specific assessment of dissolution, net lifecycle emissions, contaminants, secondary reactions, ecology, scale and monitoring.
What ocean alkalinity enhancement is designed to do
OAE adds alkaline material to seawater or coastal water to increase total alkalinity. Higher alkalinity shifts seawater carbonate chemistry so it can absorb more atmospheric carbon dioxide (CO2). The absorbed carbon is converted mainly into dissolved bicarbonate and carbonate rather than remaining as gaseous CO2.
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That mechanism creates two different accounting questions. Gross alkalinity delivery asks how much usable alkalinity a material adds to seawater. Verified net carbon removal subtracts emissions from mining or sourcing, grinding, calcination, transport, spreading, monitoring and any other operation. A material can dissolve quickly yet deliver little net removal if its preparation is energy-intensive or if the added alkalinity is later consumed by secondary reactions.
OAE is also not the same as dumping an alkaline solid and assuming permanent storage. Dissolution rate, carbon speciation, gas exchange, carbonate precipitation, sediment chemistry and ecosystem exposure determine what actually happens at a deployment site.
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How the materials compare in the 2026 coastal experiment
The following figures come from Li et al.’s 31-day coastal comparison. The CO2 values are hypothetical deployment projections for coastal China, not field removal records. They should not be treated as commercial rates or as global potential.
| Material | Observed result in the study | Study projection or process detail | Main uncertainty |
|---|---|---|---|
| Olivine | Alkalinity enhancement of 12 μmol/kg/day | 0.57 ± 0.06 Tg CO2 per month in a hypothetical coastal-China deployment | Slow, condition-dependent dissolution; nickel, chromium and other metal release; limited coastal field evidence |
| Basalt | Negligible alkalinity release during the tested period and conditions | No comparable removal projection reported | Short test duration and site conditions may not represent longer-term weathering |
| Fly ash | Alkalinity release of 30 μmol/kg/day | 1.24 ± 0.05 Tg CO2 per month in a hypothetical coastal-China deployment | Source-specific chromium and phosphorus release, eventual surface passivation and lifecycle-allocation assumptions |
| Steel slag | Early pH increase of about 1.5 ± 0.3 pH units, declining toward a stable increase around 0.7 ± 0.4 | Secondary carbonate precipitation consumed alkalinity | A transient pH rise is not proof of durable atmospheric CO2 removal |
| Lime-derived alkalinity | Not tested alongside those four materials in the 2026 comparison | Limestone is calcined above 800 °C to CaO, then hydrated to Ca(OH)2 | Calcination energy, process emissions, transport and any carbon-capture or low-carbon-heat strategy |
The table shows why a single “winner” is misleading. Fly ash had the fastest measured alkalinity release among the tested materials, but the result depends on that feedstock, the experimental water and the study’s system boundaries. Lime may be engineered for rapid delivery, yet its upstream process burden is fundamentally different from an unprocessed mineral or an industrial residue.
Olivine: natural mineral with slow, variable dissolution
What makes olivine attractive
Olivine is a magnesium-iron silicate proposed for marine enhanced rock weathering. Its chemical composition gives it theoretical alkalinity and CO2-uptake potential. It is a naturally occurring mineral, so proponents often focus on the possibility of using abundant rock rather than manufacturing a reagent.
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Why theoretical capacity is not delivered capacity
Olivine must dissolve before its alkalinity affects seawater. Dissolution depends on grain size, mineral surface area, water chemistry, temperature, flow, oxygen conditions and the receiving sediment. Fine grinding can increase reaction rates but also increases energy use, dust-control requirements and transport mass. A 2025 review by Geerts et al. concluded that field-scale coastal evidence remains limited and identified sediment type as a critical control on dissolution.
In Li et al.’s 31-day experiment, olivine produced only 12 μmol/kg/day of alkalinity enhancement. The authors projected 0.57 ± 0.06 Tg of CO2 sequestration per month for a hypothetical coastal deployment in China. That number is a model output from the study, not a measured deployment result. It cannot be generalized to every olivine source, grain size or coastline.
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Metal exposure requires source-specific testing
Olivine contains iron and can contain potentially toxic metals, notably nickel and chromium. Zhuang et al. (2025) discussed possible metal migration and interactions with manganese oxides in sediments and called for environmental assessment and mitigation before large-scale use. Concentrations vary among deposits, so an assay of the actual feedstock is essential. A generic statement that “olivine is safe” or “olivine is contaminated” is technically unjustified without source and site data.
Basalt: a natural comparator, not a fast-delivery material
Basalt is another silicate considered for enhanced weathering. In the 31-day coastal comparison, it produced negligible alkalinity release under the tested conditions. That finding does not prove basalt never weathers in seawater; it shows that the tested sample and period did not produce a measurable release comparable to fly ash or olivine.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsBasalt decisions therefore require longer time-series measurements and site-specific hydrodynamic and sediment analysis. Grinding, transport and repeated application may be needed to obtain a useful rate, and those operations must be included in the lifecycle calculation. Basalt’s low result in this experiment is a warning against using a mineral’s theoretical weathering capacity as a short-term deployment estimate.
Fly ash: fast alkalinity release with residue-specific liabilities
What the experiment found
Fly ash released alkalinity at 30 μmol/kg/day in Li et al.’s experiment, more than twice the measured olivine rate. The study projected 1.24 ± 0.05 Tg of CO2 uptake per month for a hypothetical coastal-China deployment. The projection reflects the paper’s assumptions and should not be presented as an observed removal rate.
Why byproduct status can change the carbon balance
The study attributed part of fly ash’s lifecycle advantage to its byproduct status and to avoided upstream production emissions. That benefit depends on the chosen system boundary and allocation method. If a residue already exists and would otherwise require disposal, assigning no new upstream production burden can improve its calculated balance. Different accounting choices, additional processing, testing, storage or transport distances can change the result. “Byproduct” is therefore not synonymous with low-carbon.
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Passivation and leaching concerns
Fly ash surfaces can eventually become passivated, slowing further dissolution. The study also flagged possible chromium and phosphorus release. A deployment would need feedstock assays, leaching tests, controlled dosing and monitoring of water and sediment concentrations. The low aggregate heavy-metal pollution index reported in the comparison is bounded by the tested samples and conditions; it does not establish universal ecosystem safety for all fly ash.
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Steel slag: a strong initial pH response that can reverse chemically
Steel slag caused the largest early pH change in the reported experiments: approximately 1.5 ± 0.3 pH units. The increase later declined toward a stable rise around 0.7 ± 0.4. The study linked this pattern to secondary carbonate precipitation that consumed alkalinity.
This sequence matters for carbon accounting. A rapid pH increase can look like successful alkalinity delivery, but precipitation may remove dissolved alkalinity from the water before it drives additional atmospheric CO2 uptake. The resulting solid carbonate may represent a different carbon pathway from persistent dissolved alkalinity, and its formation, stability and carbon source must be measured rather than assumed.
Steel slag chemistry varies with furnace operation, cooling and processing. Before use, operators would need composition testing, leaching characterization, particle-size control, dose limits and a plan for observing pH, total alkalinity, dissolved inorganic carbon and precipitated solids over time.
Lime and ocean liming: a processed route to rapid alkalinity
How lime is made
Ocean liming generally starts with limestone. Heating limestone above 800 °C produces calcium oxide (CaO), or quicklime. Adding water then produces calcium hydroxide, Ca(OH)2, commonly called hydrated lime. This processing addresses the slow dissolution limitation of raw limestone and creates a material that can deliver alkalinity more readily.
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The process emissions that must be counted
Calcination requires substantial heat and also releases CO2 from limestone decomposition. Grinding, hydration, transport and marine distribution add further energy demand. A lime project could reduce its net burden with low-carbon heat, carbon capture or carefully chosen transport, but those measures must be demonstrated and included in the lifecycle boundary.
The cited evidence does not provide a directly comparable lime trial alongside the 2026 olivine, basalt, fly ash and steel-slag experiment. It is therefore not valid to rank lime numerically against those materials using Li et al.’s figures. Lime’s relevant comparison is a full lifecycle calculation that reports feedstock source, kiln energy, emissions treatment, transport distance, application method and measured carbon uptake.
Seven tests that should determine a material choice
- Measure alkalinity release under seawater conditions. Use the intended grain size, temperature, salinity, flow and contact time. Report total alkalinity and dissolved inorganic carbon, not pH alone.
- Characterize the actual feedstock. Test major elements, trace metals, mineral phases, particle-size distribution and moisture. Results from one mine, ash pond or slag batch cannot automatically represent another.
- Run leaching and exposure tests. Examine water and sediment concentrations for chromium, nickel, phosphorus and other elements of concern. Compare results with applicable local marine-water and sediment standards.
- Check for passivation and precipitation. Continue measurements long enough to detect surface coatings, carbonate formation or declining release rates. A short initial spike can overstate durable delivery.
- Complete a transparent lifecycle assessment. Include mining or collection, grinding, calcination, hydration, storage, transport, spreading, monitoring and end-of-life handling. State how byproduct burdens and avoided disposal are allocated.
- Assess the receiving ecosystem. Establish baseline pH, alkalinity, dissolved metals, plankton, benthic organisms, habitats and sediment permeability. The same dose can have different consequences in open water, an estuary or a confined sediment system.
- Design measurement, reporting and verification before deployment. Define sampling locations, frequency, analytical methods, uncertainty ranges, mass balance, stop criteria and public reporting. Do not scale from a laboratory result without field validation.
Monitoring requirements and failure modes
When pH rises but removal does not follow
Measure total alkalinity and dissolved inorganic carbon alongside pH. If carbonate precipitation is consuming alkalinity, the pH signal may fade while solids accumulate. Record the solids and determine whether the carbon is atmospheric or simply rearranged from dissolved carbonate already present.
When dissolution is slower than expected
Check grain size, mineralogy, sediment burial, flow and water chemistry before increasing the dose. More material may raise exposure without solving a passivation or transport limitation. A slower-release design may require a different site or material rather than simply more feedstock.
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When contaminants appear in water or sediment
Pause additions, compare concentrations with baseline and permit limits, trace the result to a feedstock batch and expand sampling down-current and in sediments. Remediation and reversal options should be defined before any field release.
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How to compare a proposed project responsibly
Ask for a side-by-side evidence package rather than a headline removal number. It should show measured alkalinity-release curves; total alkalinity, dissolved inorganic carbon and pH; feedstock composition and leaching results; secondary-mineral observations; lifecycle assumptions; transport and energy data; ecological baselines; and uncertainty bounds.
Then separate three claims that are often conflated: what the material did in a controlled experiment, what a model projects for a specified site and what has been verified in an operating deployment. Li et al.’s 2026 results support the first category and, for two materials, the second. They do not establish large-scale field removal.
Local permitting is part of the technical decision. Requirements can cover waste classification, marine dumping, water quality, chemical handling, worker exposure, habitat protection and monitoring. A low pollution index in one experiment cannot substitute for the approvals and site-specific impact assessment required for a real coastline.
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- Need a rapid alkalinity response: Investigate processed lime or a tested industrial residue, but quantify calcination or residue-specific emissions and monitor pH excursions.
- Prefer an unprocessed natural mineral: Olivine or basalt may fit that objective, provided dissolution tests, sediment studies and metal assessments support the chosen source and site.
- Consider an existing industrial residue: Fly ash or steel slag may avoid some upstream production burdens, but only after batch testing, leaching controls and a defensible allocation method.
- Need durable, auditable removal: Choose the option with the strongest time-series evidence for alkalinity persistence, carbon speciation, lifecycle accounting and measurement, not the largest initial pH change.
The practical conclusion is conditional: fly ash was the fastest alkalinity releaser in one 31-day coastal study, olivine was slower but potentially useful under suitable conditions, basalt was negligible in that test, steel slag showed a chemically complicated pH response, and lime offers faster processed delivery at a calcination cost. Material names alone cannot establish net removal or environmental safety.
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