Electrolytic Seawater Mineralization

This Protocol provides the requirements and procedures for the calculation of net carbon dioxide equivalent (CO₂e) removal from the atmosphere via Electrolytic Seawater Mineralization. Electrolytic Seawater Mineralization is a marine Carbon Dioxide Removal (mCDR) technique that uses electrolysis of seawater, i.e. the process of using electricity to split water into hydrogen and oxygen gas, to facilitate the Removal and Storage of carbon as solid minerals and dissolved bicarbonates and carbonate ions^1,2. This CDR process occurs within a closed system and has a co-benefit of on-site hydrogen production (Figure 1).

Figure 1 Example process diagram for an Electrolytic Seawater Mineralization facility.

Electrolysis of seawater starts with preprocessing seawater, then passing it through an electrolyzer. The electrolyzer consists of an electrochemical cell, where the negative electrode is called the cathode and the positive electrode is called the anode. When a voltage is applied, the seawater is split to generate aqueous acids and bases, as well as hydrogen, oxygen, and chlorine gas. The fluid surrounding the anode (anolyte) becomes more acidic with pH < 2, while the fluid surrounding the cathode (catholyte) becomes more alkaline with pH > 10. Typically, there will be a semi-permeable barrier such as an ion-selective membrane between the anode and cathode, so that the acidic and alkaline streams remain separated. The catholyte and anolyte are each separately processed, before being recombined and discharged into the ocean.

In the catholyte, pH and saturation indices for calcium carbonate and brucite are significantly increased above typical seawater levels. As a result, calcium ions and magnesium ions are precipitated as calcium carbonate and magnesium hydroxide, leading to a decrease in dissolved inorganic carbon (DIC) in the seawater. As the catholyte is carbonated with atmospheric CO₂, calcium carbonate is further precipitated, while magnesium hydroxide is dissolved back into solution, enabling additional uptake of DIC. Catholyte processing restores and increases DIC above influent levels due to the increased capacity for DIC from increased alkalinity and DIC lost from mineral precipitation. The pH of the catholyte decreases after catholyte processing but remains above influent pH levels. In some cases, solid carbonates may be separated out and stored on land. In the acidic anolyte, alkaline feedstocks are added to neutralize the acidity and restore the alkalinity that was removed in the catholyte. Heavy metals which may be released during anolyte processing must be remediated to environmentally safe levels. The anolyte and catholyte are recombined so that near natural seawater pH (e.g., around pH. 8.1 ³) is achieved prior to discharge into the ocean.

The atmospheric CO₂ removal resulting from this process can be quantified by the amount of CO₂ stored in solid carbonate minerals plus the change in DIC between the effluent and influent, minus losses of CO₂ back to the atmosphere. Approximately 90% of the CO₂ removal is removed as DIC and 10% is removed as carbonate minerals. The CO₂ removal occurs fully within the facility, and the effluent will have elevated alkalinity, DIC, altered concentrations of Ca and Mg, potentially elevated total suspended solids (TSS), and similar pH compared to the ambient seawater. The removed atmospheric CO₂ can be stored as DIC in the ocean, solid carbonate minerals in the ocean, and solid carbonate minerals on land.

While Electrolytic Seawater Mineralization has its similarities to other mCDR approaches such as Ocean Alkalinity Enhancement and Direct Ocean Removal, a key difference is the equilibration of CO₂ with seawater, which occurs within the closed system as opposed to the open ocean. This offers two unique advantages, by enabling direct measurement of CO₂ uptake and ensuring the transient period of large perturbations to the carbonate system occur within a controlled facility rather than the open ocean.

Note that throughout this Protocol, use of the word "must" indicates a requirement, whereas "should" indicates a recommendation.

Specific standards and protocols which are utilized as the foundation of this Protocol and for which this Protocol is intended to be fully compliant with are the following:

• Isometric Standard
• ISO (International Organization for Standardization) 14064-2: 2019 – Greenhouse Gases – Part 2: Specification with guidance at the project level for quantification, monitoring and reporting of greenhouse gas emission reductions or removal enhancements

Additional reference standards that inform the requirements and overall practices incorporated in this Protocol include:

• ISO 14064-3: 2019 - Greenhouse Gases - Part 3: Specification with Guidance for the verification and validation of greenhouse gas statements
• ISO 14040: 2006 - Environmental Management - Lifecycle Assessment - Principles & Framework
• ISO 14044: 2006 - Environmental Management - Lifecycle Assessment - Requirements & Guidelines
• A Code of Conduct for Marine Carbon Dioxide Removal Research, Aspen Institute, 2021
• Methodology for Measurement, Reporting, and Verification of Electrolytic Oceanic Carbon Dioxide Removal, EcoEngineers, 2024
• BS EN 15978:2011 Sustainability of construction works - Assessment of environmental performance of buildings - Calculation method

This Protocol was developed based on the current state of the art and publicly available science regarding Electrolytic Seawater Mineralization, and mCDR generally. As Electrolytic Seawater Mineralization is a novel CDR approach, with limited published literature, the Protocol incorporates requirements that may be highly stringent to minimize risk and for the purposes of environmental and social safeguarding.

This Protocol will be altered in future versions as the science underlying Electrolytic Seawater Mineralization evolves, and the overall body of knowledge and data across all processes is increased, for example regarding feedstock supply, conversion, ecosystem impacts and durable storage.

This Protocol will be reviewed when there is an update to scientific published literature which would affect net CO₂e removal quantification or the monitoring and modeling guidelines outlined in this Protocol.

This Protocol applies to Projects and associated operations that meet all of the following project conditions:

• utilizes electrolysis to split seawater into an alkaline catholyte and acidic anolyte, while co-producing hydrogen.
• precipitates carbonate minerals at the cathode and equilibrates catholyte with air to remove atmospheric CO₂, producing dissolved inorganic carbon and carbonate minerals.
• neutralizes the anolyte acidity with alkaline feedstock, recombines anolyte and catholyte streams, and restores seawater pH greater or equal to ambient seawater pH prior to discharge.
• alkaline feedstock is appropriately characterized, per the requirements in the Rock and Mineral Feedstock Characterization Module v1.2.
• provides a net-negative CO₂e impact (net CO₂e removal) as calculated in compliance with Section 8, on a cradle to grave Greenhouse Gas (GHG) assessment basis.
• is operating within all relevant and applicable regulations and permitted operation levels.
• is considered additional, in accordance with the requirements of Section 5.4.
• provides long duration storage (1,000 yr) of CO₂ in seawater and/or carbonate minerals.

This Protocol only quantifies CDR within the project facility (i.e., within a closed system). Additional uptake of CO₂ that occurs in the open ocean after discharge is not eligible for crediting under this Protocol.

The following topics are covered briefly in this Protocol due to their inclusion in the Isometric Standard, which governs all Isometric Protocols. See in-text references to the Isometric Standard for further guidance.

For each specific Project to be evaluated under this Protocol, Project Proponents must document project characteristics in a Project Design Document (PDD) as outlined in Documentation Section of the Isometric Standard. The PDD will form the basis for project verification and evaluation in accordance with this Protocol, and must include consideration of processes unique to Electrolytic Seawater Mineralization Projects, such as:

• detailed feedstock characterization, if applicable, see Rock and Mineral Feedstock Characterization Module v1.2,
• description of the mitigation plan according to the environmental and social risk assessment in adherence with Section 11.3.5, including an accompanying robust monitoring plan to ensure efficacy,
• description of the quantification strategy for gross CO₂e removal following Section 8,
• description of all measurements and methods used to quantify processes relevant to the calculation of net CO₂e removal, cross-referenced with relevant standards where applicable,
• documentation of official permitting.

Projects must be validated and net CO₂e removals verified by an independent third party, consistent with the requirements described in this Protocol as well as in Validation and Verification Requirements Section of the Isometric Standard.

The Validation and Verification Body (VVB) must consider the following requisite components:

1. Validate that the feedstock adheres to the requirements listed in the Rock and Mineral Feedstock Characterization Module v1.2.
2. Verify that the quantification approach adheres to requirements of Section 8, including demonstration of required records.
3. Verify that the Environmental & Social Safeguards outlined in Section 6 are met.
4. Verify that the Project is compliant with requirements outlined in the Isometric Standard.

The threshold for Materiality, considering the totality of all omissions, errors and mis-statements, is 5%, in accordance with Materiality Threshold of the Isometric Standard.

Verifiers should also verify the documentation of Uncertainty of the GHG Statement as required by the Isometric Standard. Qualitative Materiality issues may also be identified and documented, such as:

• control issues that erode the verifier’s confidence in the reported data;
• poorly managed documented information;
• difficulty in locating requested information;
• noncompliance with regulations indirectly related to GHG emissions, removals or storage.

Project Validation and Verification must incorporate site visits to project facilities in accordance with the requirements of ISO 14064-3, 6.1.4.2, including, at a minimum, site visits during validation and initial verification. Validators should, whenever possible, observe project operations to ensure full documentation of process inputs and outputs through visual observation (see the Isometric Standard).

A site visit must occur at least once during each project validation. Additional site visits may be required if there are substantial changes to operations over the course of a Project's validation period, or if deemed necessary by Isometric or the VVB.

Verifiers and validators must comply with the requirements defined in the Isometric Standard. In addition, VVB teams shall maintain and demonstrate expertise associated with the specific technologies of interest.

All VVBs are approved by Isometric independently and impartially based on alignment with Conflict of Interest policies, rotation of VVB policies, oversight on quality and the following requirements:

• VVBs must be able to demonstrate accreditation from:
  • an International Accreditation Forum member against ISO 14065: 2020 or other relevant ISO standard, including but not limited to ISO 14034:2016, ISO/IEC 17020:2012, ISO/IEC 17029:2019; or
  • a relevant governmental or intergovernmental regulatory body.
• Alternatively, on a case-by-case basis, if VVBs are able to demonstrate to Isometric that they satisfy all required Verification needs and competencies required for the relevant Protocol and follow the guidelines of ISO 19011:2018 or other relevant standards, they may be approved.

Refer to the Validation and Verification Requirements Section of the Isometric Standard for more information on verifier qualifications and requirements.

CDR via Electrolytic Seawater Mineralization can often be a result of a multi-step process (such as quarrying, alkaline feedstock processing, transporting and on-site operations, hydrogen production), with activities in each step managed and operated by a different operator, company or owner. When there are multiple parties involved in the process, and to avoid double counting of net CO₂e removal, a single Project Proponent must be specified contractually as the sole owner of Credits. Contracts must comply with all requirements defined in Ownership Section the Isometric Standard.

The Project Proponent must be able to demonstrate additionality through compliance with the Isometric Standard. The counterfactual scenarios and baselines utilized to assess additionality must be Project-specific, and are described in Section 8 of this Protocol.

Additionality determinations must be reviewed whenever project operating conditions change significantly, such as the following:

• regulatory requirements or other legal obligations for project implementation change or new requirements are implemented
• project financials indicate Carbon Finance is no longer required, potentially due to, for example:
  • increased tipping fees for waste feedstocks
  • sale of co-products (such green hydrogen) that make the business viable without Carbon Finance
  • reduced rates for capital access

Any review and change in the determination of additionality shall not affect the availability of Carbon Finance and Credits for the current or past Crediting Periods, but, if the review indicates the Project has become non-additional, the Project will be ineligible for future Credits.

The uncertainty in the overall estimate of net CO₂e removal as a result of the Project must be accounted for. The total net CO₂e removal for a specific Reporting Period must be conservatively determined, and Projects must conduct an uncertainty analysis for the net CO₂e removal calculation in compliance with requirements outlined in Uncertainty in Removals and Reductions Section of the Isometric Standard.

Projects must report a list of all key variables used in the net CO₂e removal calculation and their uncertainties, as well as a description of the uncertainty analysis approach, including:

• required measurements used for net CO₂e removal calculation
• emission factors utilized, as published in public and other databases used
• values of measured parameters from process instrumentation, such as truck or pallet weights from weigh scales, electricity usage from utility power meters and other similar equipment
• laboratory analyses, including analysis of seawater chemistry and alkaline feedstocks

The uncertainty information should at least include the minimum and maximum values of each variable that goes into the net CO₂e removal calculation (see Section 8 for more details). More detailed uncertainty information should be provided if available, as outlined in the Isometric Standard.

In addition, a sensitivity analysis that demonstrates the impact of each input parameter’s uncertainty on the final CO₂e removal uncertainty must be provided. Details of the sensitivity analysis method must be provided such that a third party can reproduce the results. Input variables may be omitted from an uncertainty analysis if they contribute to a < 1% change in the net CO₂e removal. For all other parameters, information about uncertainty must be specified.

In accordance with the Isometric Standard, all evidence and data related to the underlying quantification of net CO₂e removal and environmental monitoring will be available to the public through Isometric’s Science Platform. That includes:

• Project Design Document
• GHG Statement
• Measurements taken
• Model specifications and output
• Emission factors used
• Scientific literature used

The Project Proponent can request certain information to be restricted (only available to authorized Buyers, the Registry and VVB) where it is subject to confidentiality. However, that does not apply to any numerical data produced or used as part of the quantification of net CO₂e removal.

Following the Isometric Standard, Credits issued under this Protocol are contingent on the implementation, transparent reporting and independent verification of comprehensive safeguards. These safeguards encompass a wide range of considerations, including environmental protection, social equity, community engagement and respect for cultural values. The process mandates that safeguard plans be incorporated into all major Project phases, with detailed reports made accessible to stakeholders. Adherence to and verification of environmental and social safeguards is a condition for all Crediting Projects.

Due to the novelty of marine CDR Projects, in many cases, the international, regional and local legal frameworks have yet to catch up with this new industry⁴, although the U.S. Environmental Protection Agency (EPA) has released guidance and frameworks on mCDR permitting structures in the US. ⁵ There are also existing regulations on ocean discharges at the international, national, regional and local level that may apply for Electrolytic Seawater Mineralization activities. Additionally, specific permits may be required for the installation of an ocean intake, outfall or effluent pipe.

An environmental and social risk assessment in adherence with Environmental and Social Safeguards Section of the Isometric Standard must be completed to identify potential risks, followed by the development of tailored mitigation plans. These plans must encompass specific actions to avoid, minimize or rectify identified impacts. Effective implementation of these measures must also be accompanied by a robust monitoring plan to detect negative impacts and stop Projects when necessary (see Section 11).

For additional guidance and resources on identifying and assessing risk of CDR Projects in coastal and marine contexts, we refer Project Proponents to:

• Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration
  • Chapter 2.1 Legal and Regulatory Landscape⁶
  • Chapter 2.2 Social Dimensions and Justice Considerations⁶
• Guide to Best Practices in Ocean Alkalinity Enhancement Research
  • Chapter 10 (Legal considerations relevant to research on Ocean Alkalinity Enhancement)⁴
  • Chapter 11 (Social considerations and best practices to apply to engaging publics on ocean alkalinity enhancement)⁷

The Project Proponent must conduct an environmental risk assessment which adheres to the Isometric Standard. Specific risks to be considered for Electrolytic Seawater Mineralization are listed below. The list will be updated in future iterations of the Protocol as new research emerges.

Potential additional environmental risks associated with Electrolytic Seawater Mineralization are listed below. The severity of these risks vary based on site specificities, and the intensity and duration of marine CDR activities. Environmental and social risk identification, assessment, avoidance, and mitigation planning will be unique to each Project’s technological, environmental, and social contexts. This list is a minimum to which Isometric and the supplier can add risks on a case by case basis, which would be included in the PDD:

Feedstock sourcing:

• Potential risks associated with feedstocks include land use impacts from sourcing, production, preparation, storage, and distribution, such as land degradation, land occupation, dust pollution, deforestation and localized watershed contamination.

Co-products and waste:

• Generation of co-products and waste must be accompanied by a plan which ensures safe handling, containment and disposal.

Marine pollution prevention:

• Metal contamination (as nutrients or toxic elements) from dissolution of feedstock(s) and potential for resulting bioaccumulation of metals in the food chain. ^8,9
• If silica is a product of feedstock dissolution, changes in silicate concentrations could alter diatom growth rates and shift diatom community composition.¹⁰
• Increased particulates and suspended solids loading due to the mCDR activity could have unknown consequences on ambient light, consumption by filter feeders and flocculation. For example, increased turbidity can generate turbid plumes, which may affect water quality while suspended and affect benthic habitats upon settling.
• Removal of inorganic nutrients due to secondary precipitation could drive nutrient limitation, resulting in a decrease in biological productivity or shift in ecosystem composition.¹⁰

Marine biodiversity:

• Changes in carbonate chemistry, such as pH, could directly help or harm aquatic life depending on the magnitude and direction of the pH shift, and can have a disproportionate impact on calcification, which could lead to positive or negative ecological regime shifts.^11,10,12,13
• Cascading impacts of altered carbonate chemistry, nutrient fields, or particle deposition on biodiversity, organismal and microbial assemblages, and ecosystem functions (including non-native species, pathogens, migration routes and patterns, food webs, productivity) in upper-ocean, mid-ocean, and benthic regions both at project sites and in their vicinity.
• For Projects with seawater intake pipes, marine biota may be adversely impacted by impingement, entrainment and entrapment from pumping, pre-treatment and the CDR process.^14,15
• Shock to the ecosystem due to rapid or sudden changes in carbonate system parameters.

When assessing marine environmental risks, it is important to holistically consider the context, for instance keeping in mind the project impacts in relation to the risk of climate change. Projects are not expected to demonstrate zero changes to the ocean ecosystem due to:

• Difficulty in defining the appropriate baseline. The current measured “baseline” in the ocean is rapidly evolving due to climate change. For instance, changes to the ocean state due to climate change increasingly threaten aquatic life through warming temperatures, acidification, increased prevalence of marine disease, mass mortalities and ecological regime shifts.¹⁶
• Challenges in attributing causation. Observed changes to the marine environment may arise as a result of a CDR Project, or from another reason such as a marine heat wave, pollution from a nearby source, etc.
• Risk-benefit analysis of scaling CDR. Scaling CDR technologies, including ocean-based approaches, is critical to meet Paris Agreement targets. Marine CDR technologies are designed to alter the ocean state, so some changes should be expected, particularly at scale.

However, it is important to try to minimize as much as possible any large and/ adverse impacts.

The Project Proponent must conduct a social risk assessment which adheres to the Isometric Standard on Social Impacts. Marine CDR Projects must conduct an environmental justice review, which considers the historical inequities in the distribution of coastal amenities and disamenities prior to site selection.

Per Stakeholder Input Process Section of the Isometric Standard, Project Proponents must demonstrate active stakeholder engagement throughout Project planning and operation, ensuring that all risk mitigation strategies contribute to sustainable project outcomes. Local stakeholders situated in the vicinity of the project site may contribute an in-depth understanding of the local system and provide invaluable insights and recommendations on the potential risks, necessary safeguards and specific monitoring needs. Relevant local stakeholders may include local members of academia, Indigenous groups, environmental groups, citizen associations and other users of the marine space, such as commercial and recreational fishermen, shellfish farmers, boaters and recreational users. The Stakeholder Input Process must adhere to requirements outlined in the Isometric Standard, and evidence of these meetings must be submitted in the PDD.

• instrument malfunctions lead to data-gaps in required monitoring
• effluent exceeds thresholds outlined in the PDD
• regulatory non-compliance, e.g. danger to ecosystem health detected (such as by the local community or government agency)
• compromised health and/or safety of workers and/or local stakeholders

The scope of this Protocol includes GHG sources, sinks and reservoirs (SSRs) associated with an Electrolytic Seawater Mineralization Project which meet the applicability criteria described in Section 4.

A cradle-to-grave GHG Statement must be prepared encompassing the GHG emissions relating to the activities outlined within the system boundary.

GHG emissions and removals associated with The Project may be direct emissions from a process, or indirect emissions from combustion of fuels, electricity generation, or other sources. Emissions must include all GHG SSRs within the system boundary, from the construction or manufacturing of each physical site and associated equipment, closure and disposal of each site and associated equipment, and operation of each process, including embodied emissions of equipment and consumables used in the project. The Project Proponent is responsible for identifying all sources of emissions directly or indirectly related to project activities.

Any emissions from sub-processes or process changes that would not have taken place without the CDR Project must be fully considered in the system boundary. Any activity that ultimately leads to the issuance of Credits should be included in the system boundary.

The system boundary must include all relevant GHG SSRs controlled and related to The Project, including but not limited to the SSRs set out in Figure 2 and Table 1. If any GHG SSRs within Table 1 are deemed not appropriate to include in the system boundary, they may be excluded provided that robust justification and appropriate evidence is provided in the PDD.

Figure 2 System boundaries for an Electrolytic Seawater Mineralization Project which meets Applicability criteria described in Section 4.

Table 1. Scope of activities and GHG SSRs to be included by the removal Project

Miscellaneous GHG emissions are those that cannot be categorized by the GHG SSR categories provided in Table 1. The Project Proponent is responsible for identifying all sources of emissions directly or indirectly related to project activities and must report any outside of the SSR categories identified as miscellaneous emissions.

Emissions associated with The Project's impact on activities that fall outside of the system boundary of The Project must also be considered. This is covered under Leakage in Section 8.5.4.

In line with the GHG Accounting Module v1.1, the Project must:

• Consider all GHGs associated with SSRs, in alignment with the United States Environmental Protection Agency’s definition of GHGs which includes: carbon dioxide (CO₂), methane (CH4), nitrous oxide (N20) and fluorinated gasses such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6) and nitrogen trifluoride (NF3). For CO2 stored, only CO2 shall be included as part of the quantification. For all other activities all GHGs must be considered. For example, the release of CO2, CH4, and N2O is expected during diesel consumption;
• Quantify emissions in tonnes CO₂ equivalent (t CO₂e) using the 100-year Global Warming Potential (GWP) for the GHG of interest, based on the most recent volume of the IPCC Assessment Report (currently the Sixth Assessment Report); and
• Consider materiality of SSRs in line with Isometric requirements.

In some instances, ESM project activities may be integrated into existing activities. Activities or portions of activities that were already occurring in the baseline and would have continued to occur without the ESM project may be omitted from the system boundary, subject to the conditions set below.

For the purpose of this provision, an "activity" may refer to an operational sub-unit, such as an individual trip leg, a single pass of field equipment, or a discrete processing step, where such sub-units can be cleanly delineated by equipment, timing, and physical scope. Where a pre-existing activity is partially modified by the project (for example, a transport trip whose route is extended to include project-specific stops, or a field operation whose pass is lengthened to include project inputs), the activity must be partitioned into:

• portions that are materially unchanged by the project, which may be excluded from the system boundary under this provision; and
• portions that are new, extended, or altered as a result of the project, which must remain within the system boundary and be quantified in the relevant sub-section.

An activity, or portion of an activity, may only be excluded where the Project Proponent can demonstrate all of the following:

• it was occurring as part of routine operations prior to project activities;
• it would have continued to occur in the absence of the ESM project; and
• its scope, frequency, equipment, route, payload, and intensity are not materially altered as a result of project activities.

Evidence supporting these conditions must be provided in the PDD. This must include either:

• Historic records documenting the activity prior to project start. Acceptable records include management logs, operational records, applicator invoices, or equivalent documentation; or
• A signed affidavit from the relevant operator (e.g., farmer, land manager, applicator, or equivalent party) confirming the activity was part of routine operations prior to the project.

And the following:

• A signed affidavit from the relevant operator (e.g., farmer, land manager, applicator, or equivalent party) confirming:
  • the equipment, route, timing, cadence, and intensity of the activity are not materially changed as a result of the ESM project; and
  • the activity would have continued at a comparable level absent the project.

Where these conditions are met, only the emissions associated with the activity as it would have occurred in the baseline may be excluded. Any incremental emissions attributable to the project must remain within the system boundary and be accounted for in the relevant emissions sub-section.

The baseline scenario for an Electrolytic Seawater Mineralization Project assumes the activities associated with the Project do not take place and any associated infrastructure is not built. Qualifying Projects under this Protocol are not expected to significantly alter the natural ocean uptake of CO₂, and Projects are required to assess potential ocean losses of CO₂ in the removal quantification (see Section 8.3.3).

For most Projects meeting applicability conditions in Section 4, the counterfactual is expected to be 0 (See Section 8.4). However, there may be instances in which the counterfactual weathering of waste alkaline feedstocks should be considered.

The Reporting Period represents an interval of time over which removals are calculated and reported for verification. The total net CO₂e removal is calculated using a series of measurements for a specified Reporting Period, and is written hereafter as $CO_2e_{Removal,\ RP}$. Only CO₂ is included for the quantification of gross removal, while all GHG emissions are included when calculating the net removal (see Section 7.1 and Table 1).

GHG emission calculations must include all emissions related to the project activities that occur within the Reporting Period. This includes:

• any emissions associated with project establishment allocated to the Reporting Period (See Section 8.5.1);
• any emissions that occur within the Reporting Period (See Section 8.5.2);
• any anticipated emissions that would occur after the Reporting Period that have been allocated to the Reporting Period (See Section 8.5.3); and
• leakage emissions that occur outside of the system boundary that are associated with the Reporting Period (See Section 8.5.4).

Total net $CO_2e_{Removal}$ is calculated for each Reporting Period, and is written hereafter as $CO_2e_{Removal,\ RP}$. The final net $CO_2e_{Removal}$ quantification must be conservatively determined, giving high confidence that at a minimum, the estimated amount of CO₂e was removed.

In line with the Isometric Standard, this Protocol requires that Removal Credits are issued ex-post. Credits may be issued once CO₂ has been durably stored in the identified storage reservoir.

Net $CO_2e_{Removal}$ from Electrolytic Seawater Mineralization for each Reporting Period, $RP$, must be calculated conservatively so as to give high confidence that, at minimum, the estimated net $CO_2e$ was removed.

The net $CO_2e$ removal equation is:

$$CO_2e_{Removal,\ RP} = CO_2e_{Stored,\ RP} - CO_2e_{Counterfactual,\ RP} - CO_2e_{Emissions,\ RP}$$

Equation 1

Where

• $CO_2e_{Removal,\ RP}$ represents the total net $CO_2e$ removal for Reporting Period, $RP$, in tonnes of $CO_2e$.
• $CO_2e_{Stored,\ RP}$ represents the total $CO_2$ removed as solid carbonates or Dissolved Inorganic Carbon (DIC) in seawater due to the project activity for Reporting Period, $RP$, in tonnes $CO_2e$.
• $CO_2e_{Counterfactual, RP}$ represents the total counterfactual $CO_2$ removed from the atmosphere for the Reporting Period, $RP$, in tonnes of $CO_2e$. This term is expected to be 0 in most cases (see Section 8.4 for more information).
• $CO_2e_{Emissions, RP}$ represents the total GHG emissions associated with the Project including leakage, over a Reporting Period, $RP$, in tonnes of $CO_2e$.

The net CO₂e that is removed and durably stored is determined using direct measurements at multiple locations throughout the Electrolytic Seawater Mineralization process (Figure 3) and subtracting any potential losses upon discharge into the ocean. Electrolysis splits seawater into a catholyte and anolyte stream which are alkaline and acidic respectively. The catholyte and anolyte streams are processed independently before being recombined and discharged to the ocean.

Note that reversals of CO₂ stored in carbonate minerals may occur if the carbonates are exposed to acidic or high-temperature conditions. Requirements for open- and closed-system storage of carbonates produced through Electrolytic Seawater Mineralization, including reversal risk monitoring, are detailed in the Carbonated Materials Storage and Monitoring Module v1.1; requirements for storage of carbonates in the built environment are given in the CO₂ Storage via Carbonation in the Built Environment Module v1.1.

Catholyte

The catholyte stream has elevated total alkalinity (TA) and pH, resulting in the precipitation of calcium carbonate (CaCO₃) and brucite (Mg(OH)₂). Carbon removal occurs during catholyte processing where the catholyte is equilibrated with ambient air. The catholyte processing results in brucite dissolution, calcium carbonate precipitation, increased DIC, and restoration of pH levels similar to or above the seawater intake. Carbonate minerals which are precipitated out of solution can be separated from the catholyte stream. Measurements of the system seawater intake (A) and outflow (E), as well as the precipitated separated solids, are used to quantify carbon removal (Figure 3). Additional measurements throughout the catholyte stream before and after catholyte processing are used for independent validation and reconciliation checks on the carbon removal calculation (see Section 8.3.5.2.3).

Anolyte

The anolyte stream is acidic, and is neutralized to pH levels similar to or above the seawater intake via the addition and dissolution of alkaline feedstock in the anolyte processing chamber. The decreased pH of the anolyte stream may lead to outgassing of CO₂ in the anolyte chamber, which is a loss of CO₂ removal. Ingassing may also occur during anolyte processing, with the restoration of pH which reduces pCO₂. The extent of ingassing will depend on the duration and extent of air-liquid interaction. While it is recommended that these potential CO₂ fluxes be measured and recorded as a sense check on the carbon mass flow, the carbon accounting in this Protocol relies on direct measurements of DIC after recombination (E, Figure 3), which will include any outgassing and ingassing fluxes that occur during anolyte processing. The alkaline feedstock is required to be characterized prior to being used as an input (see Rock and Mineral Feedstock Characterization Module v1.2), and additional measurements of the anolyte stream before and after anolyte processing are used to confirm acid neutralization (see Section 11.1).

Ocean discharge

The system effluent, comprising the recombined catholyte and anolyte streams, has a similar pH to the intake stream and elevated levels of total alkalinity and DIC, altered concentrations of Ca²⁺ and Mg²⁺ ions, and potentially elevated levels of TSS. Upon discharge into the ocean, there may be additional losses of CO₂ that must be considered and subtracted from the removal quantification if determined to be relevant. These losses are described in Section 8.3.3.

Carbonate minerals

Calcium carbonate that is formed during catholyte processing is a stable sink for CO₂ on millenial timescales under appropriate storage conditions. Reversals may occur if the carbonates come into contact with acidic fluids or high temperatures. Guidance on carbonate mineral storage in closed- and open-system settings, including storage location selection and reversal risk montioring, is given in the Carbonated Materials Storage and Monitoring Module v1.1. Carbonate minerals may also be stored as additives to concrete; requirements and reversal risk assessment for this storage pathway are given in the CO₂ Storage via Carbonation in the Built Environment Module v1.1.

Figure 3 Process diagram for Electrolytic Seawater Mineralization Removal facility with measurement locations shown in green.

The total amount of $CO_2e_{Stored}$ can be determined as follows:

$$CO_2e_{Stored,\ RP} = \Delta CO_2e_{DIC,\ RP} + CO_2e_{CarbonateMinerals,\ RP} - CO_2e_{OceanLosses,\ RP}$$

Equation 2

Where

• $\Delta CO_2e_{DIC,\ RP}$ represents the change in $CO_2e$ stored in marine dissolved inorganic carbon between the seawater inflow and the processed water discharge.
• $CO_2e_{CarbonateMinerals,\ RP}$ represents the total $CO_2e$ stored in carbonate minerals over Reporting Period $RP$.
• $CO_2e_{OceanLosses,\ RP}$ represents the sum of losses which occur upon effluent discharge to the ocean and reduces the total $CO_2e$ stored by the Project activity for Reporting Period, $RP$, in tonnes $CO_2e$.

$\Delta CO_2e_{DIC,\ RP}$ represents the cumulative change in $CO_2e_{Stored}$ in marine dissolved inorganic carbon over a Reporting Reriod, $RP$. This term can be calculated as the change in $DIC$ between the seawater inflow and the processed water discharge, summed over the Reporting Reriod. This term can be calculated as follows:

$$\Delta CO_2e_{DIC,\ RP} = \Delta t \sum_{i=1}^{T} \Big[\big(DIC_{E,i}\times Flowrate_{E,i} \times \rho_{sw-E,i}) - (DIC_{A,i}\times Flowrate_{A,i} \times \rho_{sw-A,i}\big) \Big] \times 1mol CO_2/1mol DIC \times MW_{CO_2}$$

Equation 3

Where

• $\Delta CO_2e_{DIC,\ RP}$ represents the change in marine $DIC$ between the seawater inflow and the processed water discharge, in tonnes of $CO_2e$.

• $\Delta t$ is the time period over which measurements are averaged, in minutes.

• $T = RP/\Delta t$ is the total number of time intervals in the Reporting Period $RP$ to be summed over.

• $DIC_{X,i}$ represents the average concentration of $DIC$ at measurement point $X$ over time interval $i$, in moles of $C$ per kilogram of seawater

• $Flowrate_{X,i}$ represents the average flow rate of water at measurement point $X$ over time interval $i$, in liters per minute.

• $\rho_{sw-X,i}$ represents the average density of seawater at measurement point $X$ over time interval $i$, in kilograms per liter (may be derived from observed temperature and salinity).

• $1mol\ CO_2/1mol\ DIC$ represent the ratio of $CO_2e$ removed from the atmosphere per unit of $DIC$ added.

• $MW_{CO_2}$ represents the molecular weight of $CO_2$, $4.401 \times 10^{-5}$ tonnes per mole.

:::

$CO_2e_{CarbonateMinerals,\ RP}$ represents the total amount of new $CO_2e$ stored in carbonate minerals as a result of Electrolytic Seawater Mineralization removal. These carbonate minerals are precipitated in the cathode chamber and during catholyte processing.

• Option 1: direct weighing of mineral precipitate combined with carbonate content and phase distribution analysis, or
• Option 2: measurement of the change in primary carbonate-forming cations before and after catholyte processing.

Project Proponents are recommended to use both approaches. Project Proponents must designate one of the above options as the primary approach and use another approach for reconciliation (See Section 8.3.5.1). The reconciliation calculation can be the other option listed, or Project Proponents can propose an alternate approach which must be agreed upon in consultation with Isometric. The primary and reconciliatory approaches must be in agreement within the bounds of analytical precision (1 Standard Deviation). See Section 8.5.2 for more information on reconciliation calculations.

$$CO_2e_{CarbonateMinerals,\ RP} = Mass_{WetSolids} \times \frac{Mass_{CO_2}}{Mass_{WetSolids}} + 1/1000\times \Delta t\sum_{i=1}^{T} \Big[TSS_{E,i} \times Flowrate_{E,i} \times \Big(\frac{Mass_{CO_2}}{Mass_{TSS}}\Big)_{E,i} -  TSS_{A,i} \times Flowrate_{A,i} \times \Big(\frac{Mass_{CO_2}}{Mass_{TSS}}\Big)_{A,i}\Big]$$

Equation 4

Where

• $CO_2e_{CarbonateMinerals, RP}$ represents the total $CO_2e$ stored in carbonate minerals over Reporting Period $RP$, in tonnes of $CO_2e$.
• $Mass_{WetSolids}$ represents the total mass of separated material removed from the cathode processing stream, in tonnes.
• $\frac{Mass_{CO_2}}{Mass_{WetSolids}}$ represents the amount of $CO_2e$ stored per mass of separated carbonate minerals, in % wt. Acceptable methods for measuring this term include calcimetry or thermogravimetric analysis (TGA) of representative samples.
• $1,000$ represents the conversion factor between tonnes to kilograms.
• $\Delta t$ is the time period over which measurements are averaged, in minutes.
• $T = RP/\Delta t$ is the total number of time intervals in the Reporting Period $RP$ to be summed over.
• $TSS_{X,i}$ represents the concentration of total suspended solids ($TSS$) at measurement point $X$ over time interval $i$, in kilograms per liter.
• $Flowrate_{X,i}$ represents the average flow rate of water at measurement point $X$ over time interval $i$, in liters per minute.
• $\Big(\frac{Mass_{CO_2}}{Mass_{TSS}}\Big)_{X,i}$ represents the amount of $CO_2e$ stored per mass of suspended solids, in % wt, at measurement point $X$ over time interval $i$. Acceptable methods for measuring this term include calcimetry or thermogravimetric analysis of representative grab samples.

$$CO_2e_{CarbonateMinerals,\ RP} = \Delta t\sum_{i=1}^{T}\Big[([Ca]_{A,i} \times \rho_{sw-B1,i} \times Flowrate_{B1,i}) -([Ca]_{D1,i} \times \rho_{sw-D,i} \times Flowrate_{D1,i})\Big]\times mole CO_2/moleCa\times MW_{CO_2}  + \Big[([Mg]_{A,i} \times \rho_{sw-B1,i} \times Flowrate_{B1,i}) -([Mg]_{D1,i} \times \rho_{sw-D1,i} \times Flowrate_{D1,i})\Big]\times mole CO_2/moleMg\times MW_{CO_2}$$

Equation 5

Where

• $CO_2e_{CarbonateMinerals,\ RP}$ represents the total $CO_2e$ stored in carbonate minerals over Reporting Period $RP$, in tonnes of $CO_2e$.

• $\Delta t$ is the time period over which measurements are averaged, in minutes.

• $T = RP/\Delta t$ is the total number of time intervals in the Reporting Period $RP$ to be summed over.

• $[Ca]_{X,i}$ represents the average $Ca$ concentration at measurement point $X$ over time interval $i$, in moles $C$ per kilogram of seawater.

• $\rho_{sw-X,i}$ represents the average density of processed seawater at measurement point $X$ over time interval $i$, in kilogram per liter.

• $Flowrate_{X,i}$ represents the average flow rate of water at measurement point $X$ over time interval $i$, in liters per minute.

• $mole CO_2/moleCa$ represents the molar ratio of $CO_2$ removed for every $Ca^{2+}$ cation removed from seawater, determined from grab sample after catholye processing (ie. at D1 or from separated solids).

• $MW_{CO_2}$ represents the molecular weight of $CO_2$, $4.401 \times 10^{-5}$ tonnes per mole.

• $[Mg]_{X,i}$ represents the average $Mg$ concentration at measurement point $X$ over time interval $i$, in moles $C$ per kilogram of seawater.

• $mole CO_2/moleMg$ represents the molar ratio of $CO_2$ removed for every $Mg^{2+}$ cation removed from seawater, determined from grab sample after catholye processing (ie. at D1 or from separated solids).

Note that the above equation assumes an identical frequency of sampling for $[Ca]$ and $[Mg]$. An alternative formulation of the equation may be necessary if sampling frequencies are not the same.

Upon discharge of pre-equilibrated, TA- and DIC-enriched effluent in the ocean, the following processes can cause losses that may reduce the efficiency of Electrolytic Seawater Mineralization:

• CO₂ outgassing upon initial mixing with ocean seawater
• CO₂ outgassing from secondary precipitation
• CO₂ outgassing from biotic calcification
• Changes in natural DIC and TA fluxes or buffering of natural TA release from interactions of carbonate minerals and TA-enriched, DIC-enriched effluent with sediments

• avoiding the likelihood of these losses by identifying avoidance strategies around conditions which lead to non-negligible loss terms, with corresponding monitoring to demonstrate adherence to those guardrails
• estimating a conservative upper limit of alkalinity loss based on scientific literature, first principles calculations, and/or experimentation
• process-based modeling studies
• direct measurements
• alternative approaches that are sufficiently justified

Initial Mixing

Nonlinear mixing of the Electrolytic Seawater Mineralization effluent and receiving ocean waters may result in ingassing or outgassing depending on the pCO₂ of the resultant mixture relative to ambient seawater pCO₂.

Net ingassing relative to the ocean baseline is more likely than outgassing. This is because if the effluent stream is at or below atmospheric pCO₂, nonlinear mixing of the effluent and ambient seawater results in a pCO₂ deficit with respect to ambient seawater. This may result in a net additional uptake or reduced natural outgassing in the ocean through air-sea gas equilibration. CO₂ uptake in the open ocean is not considered for crediting under this Protocol, however this effect acts as a buffer against initial mixing losses.

The recommended quantification approach for outgassing upon initial mixing is to use the two endmember mixing model to estimate the DIC of the effluent and ambient water mixture. The difference between the measured DIC of the effluent (ie. point E) and DIC of the mixture is the outgassing loss.

Secondary Precipitation

Secondary precipitation of calcium carbonate in seawater could cause CO₂ outgassing. In the open ocean, abiotic calcium carbonate precipitation is rare because spontaneous nucleation is strongly inhibited in seawater^18,19,20 and most carbonate production is thought to be biologically mediated²¹. There are very few areas of the ocean where spontaneous carbonate precipitation is observed (e.g. the Great Bahama Bank and the Persian Gulf²¹). Such locations typically have exceptionally high saturation states (i.e. $\Omega_{CaCO_3}$ > 19)²². In coastal areas, higher suspended particulates may increase nucleation. Early research suggests there is a relationship between increased alkalinity loss due to precipitation and higher TSS in the receiving water body.²³ In the effluent pipe, pipe roughness can also increase potential nucleation sites. Thus, the risk of secondary precipitation is most pronounced in the effluent pipe, mixing zone and coastal domain, where the carbonate chemistry perturbation and potential nucleation sites are largest.

Limiting pH and the aragonite saturation state has been shown to be effective at avoiding this result, and laboratory research to characterize the critical thresholds that trigger precipitation under close-to-natural conditions are ongoing.^24,25,26,27 Furthermore, precipitation dynamics occur on a timescale between minutes to hours,^24,26 which suggests that dilution could be an effective risk mitigation strategy.²⁸

An example avoidance strategy is setting a pH threshold, with consideration of carbonate chemistry variables, TSS and dilution at the site, and continuous monitoring of carbonate chemistry variables and TSS to ensure that conditions for secondary precipitation are avoided. In some cases, secondary precipitation can be identified by an observed increase in turbidity. Monitoring of turbidity is recommended, however it may be difficult to isolate a signal from secondary precipitation over natural fluctuations.

Biotic Calcification

Increases in biotic calcification can cause CO₂ outgassing. The carbonate chemistry conditions promoted by Electrolytic Seawater Mineralization could promote calcification due to the lowered H⁺ and/pr elevated saturation state.^29,30,31,32 Alkaline feedstock dissolution may release trace metals which has the potential to fertilize blooms of calcifiers¹⁰.

Early stage research manipulating Total Alkalinity with the aim of simulating OAE has found no significant increase in biologically produced calcium carbonate at elevated alkalinity in the ocean.^33,13 However, the Black Sea, a naturally elevated alkalinity environment, harbors extensive blooms of the coccolithophores ^34,35, a major group of calcifying plankton. This is thought to be due to the favorable carbonate chemistry promoted by the elevated alkalinity regime.³²

This is still an area where more research is needed, particularly through mesocosm and field trials, albeit there is a rich body of literature on lab and mesocosm scale species-specific responses to changing seawater carbonate chemistry.³⁶ The risk of alkalinity loss due to biotic calcification may be Project and location specific. Recently published meta-analyses synthesizing data from ocean acidification studies for OAE support this claim that species and functional group specificity is likely.³⁷ Coastal areas with significant benthic calcification of CaCO₃ sediments may be especially susceptible to this feedback.

A recommended avoidance strategy is setting thresholds on pH and TA (e.g., $pH < 9$, $\Delta TA < 1,000\ μmol/kg$ in effluent pipe prior to discharge, which are lower perturbations than in previous studies showing no significant increase in biologically produced CaCO₃, or as determined for the specific deployment site),³³ and monitoring for changes in ocean biota.

Interactions with Sediments

Early research suggests that ocean alkalinity enhancement may reduce natural alkalinity fluxes from sediments³⁸. This risk may be exacerbated by Projects with settling particles which result in alkalinity enrichment in sediments, and the potential impacts on the net removal calculation is highly uncertain at this time. More research in this area is needed and the Protocol will be updated with future advancements.

A recommended avoidance strategy for Electrolytic Seawater Mineralization Projects is to limit accumulation of alkalinity on the sea bed through careful design of discharge rates and infrastructure. Potential monitoring approaches may include sediment sampling, measuring particle settling and accumulation rates. Alternatively, acceptable evidence for quantification could include numerical modeling of particle transport in receiving waters, sediment sampling, sediment transport modeling, measuring benthic alkalinity fluxes, measuring changes in net calcification at the sea bed and sediment diagenesis modeling.

Discharge of Undissolved Alkalinity

Brucite is formed at the cathode and redissolved during catholyte processing. Alkaline solids may also be released to the effluent stream during anolyte processing. Any undissolved alkalinity which is released in the effluent as TSS will dissolve in the open ocean and enhance local alkalinity. This will result in increased pH, total alkalinity (TA), and potentially facilitate additional CO₂ uptake via air-sea gas exchange if the alkaline enriched waters remain in contact in the atmosphere.

Biological Fertilization

Incomplete Acid Neutralization

If the anolyte stream is released to the ocean prior to neutralization, the acid in the anolyte would lead to outgassing of CO₂ in the ocean and counteract the CDR occurring within the closed system. Spillages of unneutralized anolyte and/or unneutralized acid by-products (such as potential acidic slurry) must be reported, quantified and treated as a loss against CDR.

An alkalinity/acidity budget is recommended on the anolyte stream to demonstrate the acid neutralization capacity of the alkaline feedstock is consistent with observed changes in pH in the anolyte, and TA loss through precipitation in the catholyte is replenished.

Within the Electrolytic Seawater Mineralization system, there are several points throughout the process where direct measurement of the carbonate system and associated parameters may be observed (see Section 11.1 for full list of monitoring locations). Measurements along these locations are recommended as checks that the facility and process are operating as expected. For example, these measurements can be used to:

• validate the carbonate system and associated parameters along each step of the Electrolytic Seawater Mineralization process
• conduct a Ca and Mg ion budget throughout the system
• conduct a DIC budget throughout the system
• conduct an alkalinity/acidity budget throughout the system
• validate the mineral phase distribution and CO₂ content in collected solids grab samples

Project Proponents should describe any additional validation measurements they are collecting throughout the facility in the PDD, particularly if they increase confidence in the efficacy of the system.

DIC concentration is directly measured at several points throughout the Electrolytic Seawater Mineralization process. DIC can also be calculated using carbonate system solvers such as CO2SYS and PHREEQC. Given any two of pH, TA and pCO₂, DIC can be solved through the equations of chemical thermodynamics. Using pH and TA will give a more accurate estimate of DIC than using pH and pCO₂, however more frequent measurements can be made with pCO₂ due to the existence of autonomous pCO₂ instruments. Project Proponents should corroborate the DIC directly measured for the calculation of $\Delta CO_2e_{DIC,\ RP}$ in Equation 3 with calculated DIC values.

There are two options available for quantifying $CO_2e_{CarbonateMinerals,\ RP}$, which are described in Section 8.3.2. Both Option 1 (solid phase measurements) and Option 2 (liquid phase measurements) are expected to result in the same estimate for $CO_2e_{CarbonateMinerals,\ RP}$, assuming full dissolution of brucite and negligible carbonate mineral precipitation and dissolution upon recombination. Undissolved brucite detected by solids characterization must be subtracted from the total amount of solids. Project Proponents should check that the results of both options for calculating $CO_2e_{CarbonateMinerals,\ RP}$ agree within one standard deviation.

The sum of CO₂ uptake at point C2 represents the flux of atmospheric carbon into seawater as DIC or carbonate minerals during catholyte processing. This term can be calculated as:

$$CO_2e_{Catholyte,\ RP} = \Delta t \sum_{i}^{T} m_{atm, i} \times (C_{in,i}-C_{out,i})$$

Equation 6

Where

• $CO_2e_{Catholyte,\ RP}$ represents the maximum uptake of atmospheric $CO_2$ into seawater as DIC or carbonate minerals, in tonnes $CO_2e$.
• $\Delta t$ is the time period over which measurements are averaged, in minutes.
• $T = RP/\Delta t$, which is the total number of time intervals in the Reporting Period $RP$ to be summed over.
• $m_{atm}$ represents the average mass flow rate of air at catholyte processing, in kg/min. This term can be measured in L/min and converted to kg/min using the density of air.
• $C_{in}$ represents the concentration of $CO_2$ in the air stream as it enters the catholyte processing chamber over time interval $i$, in %wt of $CO_2e$.
• $C_{out}$ represents the concentration of $CO_2$ in the air stream as it exits the catholyte processing chamber over time interval $i$, in %wt of $CO_2$.

Note that this term does not include CO₂ fluxes that occur during anolyte processing. If there is no net ingassing or outgassing during anolyte processing, then $CO_2e_{Catholyte,RP}$ can serve as a check on the sum of $\Delta CO_2e_{DIC,\ RP}$ + $CO_2e_{CarbonateMinerals,\ RP}$, calculated according to Section Section 8.3. However, if $\Delta CO_2e_{DIC,\ RP}$ + $CO_2e_{CarbonateMinerals,\ RP}$ > $CO_2e_{Catholyte,\ RP}$, there is net ingassing at the anolyte, whereas if $\Delta CO_2e_{DIC,\ RP}$ + $CO_2e_{CarbonateMinerals,\ RP}$ < $CO_2e_{Catholyte,\ RP}$ indicates net outgassing at the anolyte. It is recommended that an audit of the CO₂ on the anolyte is conducted to close any discrepancies in the carbon mass balance.

For Electrolytic Seawater Mineralization, the counterfactual is typically considered to be 0 if all applicability criteria are met and conditions outlined in Section 4 are also met. The applicability criteria, combined with required quantification of potential ocean losses (Section 8.3.3), are necessary to ensure additionality above any changes in natural ocean uptake induced by the Project.⁴⁰

$CO_2e_{Emissions, RP}$ is the total GHG emissions associated with a Reporting Period, ${RP}$. This can be calculated as:

$$CO_2e_{Emissions,\ RP} = CO_2e_{Establishment,\ RP} + CO_2e_{Operations,\ RP} + CO_2e_{End-of-Life,\ RP} + CO_2e_{Leakage,\ RP}$$

Equation 7

Where

• $CO_2e_{Emissions,\ RP}$ represents the total GHG emissions for a Reporting Period, in tonnes of $CO_2e$.
• $CO_2e_{Establishment,\ RP}$ represents the GHG emissions associated with project establishment, represented for the Reporting Period, in tonnes of $CO_2e$, see Section 8.5.1.
• $CO_2e_{Operations,\ RP}$ represents the total GHG emissions associated with operational processes for a Reporting Period, in tonnes of $CO_2e$, see Section 8.5.2.
• $CO_2e_{End-of-Life,\ RP}$ represents GHG emissions that occur after the Reporting Period and are allocated to a Reporting Period, in tonnes of $CO_2e$, see Section 8.5.3.
• $CO_2e_{Leakage,\ RP}$ represents GHG emissions associated with the Project’s impact on activities that fall outside of the system boundary of a Project, over a given Reporting Period, in tonnes of $CO_2e$, see Section 8.5.4.

The following sections set out specific quantification requirements for each variable.

GHG emissions associated with project establishment should include all historic emissions incurred as a result of project establishment, including but not limited to the SSRs set out in Table 1.

Project establishment emissions occur from the point of project inception to the moment before the first removal activity takes place. GHG emissions associated with project establishment may be amortized over the anticipated project lifetime, or per output of product. Requirements for amortization are outlined in Section 7 of the GHG Accounting Module.

GHG emissions associated with $CO_2e_{Operations,\ RP}$ should include all emissions associated with operational activities, including but not limited to the SSRs set out in Table 1.

$CO_2e_{Operations,\ RP}$ emissions must be attributed to the Reporting Period in which they occur. Allocation may be permitted in certain instances, on a case by case basis in agreement with Isometric.

$CO_2e_{End-of-Life,\ RP}$ includes all emissions associated with activities that are anticipated to occur after the Crediting Period until the end of the Project Commitment Period. This includes activities related to ongoing monitoring for Reversals.

$CO_2e_{End-of-Life,\ RP}$ must be estimated upfront and allocated in the same way as set out for calculation of $CO_2e_{Establishment,\ RP}$.

Given the uncertain nature of $CO_2e_{End-of-Life,\ RP}$ emissions, assumptions must be revisited at each Reporting Period and any necessary adjustments made.

$CO_2e_{Leakage,\ RP}$ includes emissions associated with a Project's impact on activities that fall outside of the system boundary of a Project. It includes increases in GHG emissions as a result of the Project displacing emissions or causing a knock on effect that increases emissions elsewhere. As an example, creating a market for feedstocks may generate new revenue in the source sector that alters producer behavior in ways that result in additional GHG emissions.

It is the Project Proponent's responsibility to identify potential sources of leakage emissions, however, for an Electrochemical Marine CO₂ Removal Project the following must be considered as a minimum:

• feedstock replacement
• consumables replacement

$CO_2e_{Leakage,\ RP}$ emissions must be attributed to the Reporting Period in which they occur. Allocation may be permitted in certain instances, on a case by case basis in agreement with Isometric.

GHG emissions accounting must be undertaken in alignment with the GHG Accounting Module v1.1, which ensures a consistently rigorous standard in how GHG emissions are quantified and reported between different CDR Projects and approaches. This includes:

• Requirements for data quality, including a detailed data quality hierarchy for activity data and emission factors;
• Consideration of materiality in emissions accounting;
• Emissions amortization requirements;
• Co-product allocation requirements. For ESM projects, this will apply to the production of co-products such as hydrogen;
• By-product accounting relating to inputs to the process that are by-products; and
• Waste input accounting relating to inputs to the process that are wastes.

The Energy Use Accounting Module v1.3 provides requirements on how energy-related emissions must be calculated for The Project so that they can be subtracted in the net CO₂e removal calculation. It sets out the calculation approach to be followed for intensive facilities and non-intensive facilities and acceptable emission factors.

Energy emissions are those related to electricity or fuel usage. Examples of electricity usage may include, but are not limited to:

• operation of process equipment (i.e., pumps, mixers, blowers, flow control, measurement instruments)
• operation of electrolyzers, catholyte and anolyte processing

Examples of fuel consumption may include, but are not limited to:

• handling equipment, such as fork trucks or loaders
• fuel consumption of sampling vessels

The GHG Accounting Module v1.1 provides requirements on how transportation and embodied emissions must be calculated for The Project so that they can be subtracted in the net CO₂e removal calculation.

Embodied emissions are those related to the life cycle impact of equipment and consumables. They may include, but are not limited to:

• feedstock, consumables and associated equipment produced, constructed and utilized explicitly for the CDR Project
• electrolyzer, catholyte and anolyte processing equipment
• process equipment (i.e., pumps, mixers, blowers, flow control, measurement instruments)
• environmental monitoring equipment and consumable materials such as batteries, sensors, buoys, instruments and cleaning supplies
• raw materials and equipment used in the fabrication, assembly and construction of plant
• equipment related to waste handling and disposal

Transportation emissions are those related to transportation of products and equipment. They may include, but are not limited to:

• transportation of equipment, feedstock and consumables to the facility
• transportation of separated solid carbonate minerals to storage site
• transportation and shipping related to collection and analysis of samples for environmental monitoring

This Protocol provides multiple options for durable storage of CO₂. The Project Proponent can choose from the following available options, including using a combination of the below options, when submitting their Project for verification.

The primary storage reservoirs of the CO₂ removed through Electrolytic Seawater Mineralization is Dissolved Inorganic Carbon (DIC) in the ocean. The Durability and reversal risks of this storage reservoir are discussed in the Ocean DIC Storage Module.

Some of the stored carbon from Electrolytic Seawater Mineralization may be discharged to the ocean as solid carbonate minerals that will disperse as suspended solids and ultimately settle in sediments. Depending on the saturation state of the carbonate mineral in the local environment, solid carbonate minerals may undergo dissolution. This Protocol makes the conservative assumption that no solid carbonate mineral dissolution will occur. This assumption must be reflected in the total carbonate accounting and net CDR calculation.

Two core considerations relevant to the durability of marine carbonate minerals are:

1. The saturation state of the carbonate mineral phases in the dispersal area, and
2. The fate of the carbonate mineral if these phases experience under-saturation and dissolution.

The carbonate mineral phases that will likely be abundant in discharged suspended solids will include calcium carbonate CaCO₃ (i.e., calcite and aragonite), and hydrous magnesium carbonate (e.g., nesquehonite MgCO₃ $\cdot$ 3H₂O, lansfordite MgCO₃ $\cdot$ 5H₂O, hydromagnesite Mg₅(CO₃)₄(OH)₂ $\cdot$ 4H₂O, dypingite Mg₅(CO₃)₄(OH)₂ $\cdot$ 5H₂O, and others)^1,41.

The surface ocean tends to be super-saturated with respect to both primary calcium carbonate phases (calcite and aragonite), indicating a likelihood of mineral stability. The saturation depth, which is the depth at which the saturation index $\Omega=1$, varies spatially in the world's oceans ⁴². Above the saturation depth, $\Omega>1$, which indicates that seawater is supersaturated with respect to calcite or aragonite. Below the saturation depth, $\Omega<1$ indicates that seawater is undersaturated with respect to calcite and aragonite and that dissolution is favored. For calcite, super-saturated conditions tend to be present throughout the water column in the Atlantic Ocean, to greater than 3 kilometers depth in the Indian and Southern Oceans, greater than 2 kilometers depth in the Southern Pacific Ocean, and the topmost several hundred meters in the Northern Pacific Ocean. For aragonite, saturation depths are shallower. Super-saturated conditions are present throughout the water column in the North Atlantic, to greater than 1 km depth in the South Atlantic Ocean and South Indian Ocean, and in the topmost several hundred meters elsewhere⁴².

In contrast to calcite and aragonite, hydrous magnesium carbonate minerals are generally not observed in the water column or marine sediment, and can therefore be assumed to be under-saturated everywhere in the global ocean. Therefore, calcium carbonate phases are likely to persist as minerals in shallow near shore environments where $\Omega>1$, whereas magnesium carbonate phases are likely to dissolve upon discharge. Carbonates buried in sediment will be subject to elevated CO₂(aq) and lower pH due to remineralization of organic matter. After sufficient sedimentation and isolation from the sediment-water interface, conditions will tend towards mineral dissolution⁴³.

Carbonate minerals will dissolve according to the following formula:

$$XCO_3 + CO_2(aq) \Leftrightarrow X^{2+} + 2HCO_3^-$$

Equation 9

where

• $X$ represents either $Ca$ or $Mg$.

Equation 9 shows that for every unit Ca- or Mg-carbonate that dissolves, two units of alkalinity and one unit of dissolved inorganic carbon (DIC) will be added. Under average ocean pH conditions, divalent alkalinity such as $Ca^{2+}$ and $Mg^{2+}$ will stabilize approximately 1.7 molecules of $CO_2$ as dissolved inorganic carbon³³. Therefore, dissolution of carbonate phases will tend to stabilize 70% more $CO_2$ than if the mineral had not dissolved (with values being lower in more isolated, low pH sedimentary environments). Any resulting marine DIC will have an average lifetime in the ocean of 10,000+ years (see Section 9.1 above).

Taken together, carbonate minerals released into the marine environment may be stabilized under super-saturated conditions, or dissolve under under-saturated conditions. Mineral dissolution will lead to increased carbon dioxide removal and storage as marine DIC. In either case, carbonate accounting based on the amount of carbon stored in discharged carbonate solids provides a conservative estimate of the total carbon removed, with a minimum durability of 10,000 years.

Carbonate-bearing minerals produced during Electrolytic Seawater Mineralization may be separated and stored on land. The requirements for open and closed system carbonated materials storage are provided in the Carbonated Material Storage and Monitoring Module.

Carbonate-bearing minerals produced during Electrolytic Seawater Mineralization may be separated and stored as additives to concrete. The requirements for storage of carbonates in the built environment are provided in the CO₂ Storage via Carbonation in the Built Environment Module.

As outlined in Risk of Reversal Section of the Isometric Standard, the Buffer Pool is a mechanism used to insure against Reversal risks that may be observable and attributable to a particular Project through monitoring.

Projects using this Protocol must determine the appropriate Risk of Reversal and buffer pool size (see Appendix 2: Risk of Reversal Questionnaire).

Based on present understanding, reversals in the global ocean Dissolved Inorganic Carbon (DIC) reservoir will not be directly observable with measurements and attributable to a particular Project. Reversals of DIC in the ocean is a system-wide storage uncertainty that should be addressed through further scientific research. Carbon stored as DIC in the ocean is classified as having No Observable Risk of Reversal according to the Isometric Standard Risk Assessment Questionnaire. This results in a 0% ++buffer pool for Projects using this Storage Module. This reversal risk will be reassessed at the renewal of the Crediting Period++, or when new scientific research and knowledge are produced.Carbonate minerals that are released into the marine environment may end up as DIC if they dissolve, and thus will have the same Reversal Risk and Buffer Pool as DIC in the ocean.

Projects utilizing the Carbonated Material Storage and Monitoring Module v1.2 must assess the reversal risk of the specific storage site following the Module's requirements to determine an appropriate buffer pool (typically 0% due to No Observable Risk) and reversal risk for carbonate mineral storage.

Projects utilizing the CO₂ Storage via Carbonation in the Built Environment Module v1.1 may submit documentation to conduct a Project-specific reversal risk assessment and determine an appropriate buffer pool and uncertainty discount. This documentation may include information on regional geology, hydrology, climatic conditions and project operations (see Section 11.4 of the CO₂ Storage via Carbonation in the Built Environment Module v1.1 for a full list of documents that may be appropriate). If no documentation is available for a Project-specific reversal risk assessment, Projects storing carbonate minerals in the built environment are subject to a default uncertainty discount of 6%.

If a Project uses more than one storage Module, then the uncertainty discount should be allocated based on the amount of carbon stored in each reservoir using the formula below:

$$UD_{RP} = CO_2e_{Removal,\ RP} \times \sum_{i}^{N} \Big[ UD_i \times p_i \Big]$$

Equation 10

Where:

• $UD_{RP}$ is the total uncertainty discount allocation for the Reporting Period, RP, in tonnes of $CO_2e$.
• $N$ is the number of storage sites used
• $UD_i$ is the uncertainty discount allocation for storage site i, in %.
• $p_i$ is the proportion of $CO_2$ stored in storage site i, in %.
• $CO_2e_{Removal,\ RP}$ represents the total net $CO_2e$ removal for Reporting Period, RP, in tonnes of $CO_2e$.

Example Calculation

An Electrolytic Seawater Mineralization Project removes 1,000 tonnes of CO₂e, where 90% of the removed carbon is stored as DIC in the ocean, and 10% is stored as carbonated materials on land. Following a site specific risk assessment of the terrestrial carbonated materials storage site, the Carbonated Materials Storage Module gives a 5% uncertainty discount for terrestrial carbonated materials storage.

The total uncertainty discount for the Reporting Period is therefore:

$UD_{RP} = 1000 \times \Big[ (2\% \times 90\%) + (5\% \times 10\%)\Big] = 23$ tonnes $CO_2e$

For more details on Reversals, refer to the Isometric Standard.

• G-X20Z-0Project Proponents must develop a conceptual model of the discharge plume transport and dispersion, such as through producing a mixing zone model (which is usually a standard requirement in ocean discharge permits). The mixing zone model will estimate initial dispersal of the discharge, and will dictate the recommended reach of the monitoring efforts. Sensitivity analysis is required to demonstrate adherence to water quality limits at the boundary of the mixing zone under varying environmental conditions. Long-term changes which are likely to affect transport and dispersion of discharge must be described.

• G-GPWM-0Project Proponents must have sufficient plans for restoring water quality conditions prior to discharge.

• Rocks and minerals utilized in the anolyte stream must be sufficiently well characterized following Isometric’s Rock and Mineral Feedstock Characterization Module.

• quantify net carbon removal, including redundant measurements for reconciliation calculations,
• validate processes occurring throughout the facility,
• demonstrate permit compliance,
• monitor environmental conditions, and
• establish processes for adaptive management to ensure that Projects stop if negative impacts are identified.

For an Electrolytic Seawater Mineralization Project, monitoring is required both within the facility as well as in the ocean. This Protocol describes required and recommended monitoring measurements at the following locations, which are illustrated in Figure 4 (note: a full list of monitoring requirements and recommendations provided in Table 4):

1. Seawater intake (A): the intake pipe where seawater is drawn in. Measurements taken here must be after any preprocessing (e.g. filtering) of the seawater and before seawater enters the electrolyzer.
2. Catholyte after electrolysis (B1): the catholyte stream immediately after electrolysis, before any atmospheric CO₂ equilibration has occurred.
3. Atmospheric injection inlet and outlet (C1): the inlet and outlet of atmospheric gas that is used as the source of CO₂ during catholyte processing.
4. Post Catholyte Processing (D1): the catholyte stream after atmospheric equilibration and solids separation has occurred.
5. Anolyte after electrolysis (B2): the anolyte stream immediately after electrolysis.
6. Atmospheric outgassing at anode (C2): the outlet of the anode or anolyte holding chamber that is used to monitor CO₂ outgassing.
7. Post anolyte processing and heavy metals remediation (D2): the anolyte stream after anolyte processing and heavy metals remediation has occurred.
8. Pre-outflow (E): the chamber containing the mixture of fully processed catholyte and anolyte before discharge.
9. Separated solids: the solid carbonate minerals that are precipitated in the catholyte, separated from the processed seawater, and transported off site.
10. Throughout facility: general measurements for marine biota and human health safety.
11. Edge of mixing zone: the mixing zone is the area near the discharge where initial dilution takes place and is a regulatory concept that describes the area where water quality criteria can be exceeded. Beyond the mixing zone, it may be difficult to detect measurable signals from the Project, so in-ocean measurements are concentrated at the edge of the mixing zone. See Section 11.3.1.
12. Deployment area: certain measurements, such as for monitoring ecological impacts, may occur outside the mixing zone. Monitoring that occurs outside the mixing zone but still in the vicinity of the Project is referred to as within the “deployment area”.
13. Control site (not shown in Figure 4): An area which is similar to the project site in all regards except the CDR activity. The use of a control site is determined on a site by site basis as outlined in the PDD and is not universally required.

Figure 4 Monitoring locations in the electrolytic CO₂ removal facility as well as in the ocean. Monitoring locations referred to throughout this Protocol are shown in green.