Seawater Alkalinity Research Articles

Seawater alkalization via an energy-efficient electrochemical process for CO2 capture

Electrochemical pH-swing strategies offer a promising avenue for cost-effective and energy-efficient carbon dioxide (CO2) capture, surpassing the traditional thermally activated processes and humidity-sensitive techniques. The concept of elevating seawater's alkalinity for scalable CO2 capture without introducing additional chemical as reactant is particularly intriguing due to its minimal environmental impact. However, current commercial plants like chlor-alkali process or water electrolysis demand high thermodynamic voltages of 2.2 V and 1.23 V, respectively, for the production of sodium hydroxide (NaOH) from seawater. These high voltages are attributed to the asymmetric electrochemical reactions, where two completely different reactions take place at the anode and cathode. Here, we developed a symmetric electrochemical system for seawater alkalization based on a highly reversible and identical reaction taking place at the anode and cathode. We utilize hydrogen evolution reaction at the cathode, where the generated hydrogen is looped to the anode for hydrogen oxidation reaction. Theoretical calculations indicate an impressively low energy requirement ranging from 0.07 to 0.53 kWh/kg NaOH for established pH differences of 1.7 to 13.4. Experimentally, we achieved the alkalization with an energy consumption of 0.63 kWh/kg NaOH, which is only 38% of the theoretical energy requirements of the chlor-alkali process (1.64 kWh/kg NaOH). Further tests demonstrated the system's potential of enduring high current densities (~20 mA/cm2) and operating stability over an extended period (>110 h), showing its potential for future applications. Notably, the CO2 adsorption tests performed with alkalized seawater exhibited remarkably improved CO2 capture dictated by the production of hydroxide compared to the pristine seawater.

Advancements in seawater immersion wound management: Current treatments and innovations.

With advancements in naval warfare, the number and severity of seawater injuries have skyrocketed, necessitating effective seawater immersion (SWI) wound management. The unique marine pathogens, salinity, low temperature and alkalinity of seawater are the main environmental factors that can influence SWI wound healing. The current treatment strategy for SWI wounds follows a standard protocol based on terrestrial wound conditions, neglecting seawater conditions. The key requirements for ideal SWI treatment include good adhesion to the wound surface to minimize further exposure to seawater, enhanced wound healing properties to minimize wound healing time and antibacterial properties to prevent infections from marine pathogens. Current SWI wound-specific treatments range from elaborate techniques like vacuum-sealed drainage and vacuum-assisted closure for severe blast injuries to simple application of hydrogels or collagen dressings for minor injuries. This review discusses the current status and development of various treatment modalities for SWI wounds. The development of these treatment strategies and an understanding of their mechanisms of action make us better prepared to manage and treat SWI injuries.

Carbon dioxide sequestration through mineralization from seawater: The interplay of alkalinity, pH, and dissolved inorganic carbon

Marine carbon dioxide removal technologies rely on the equilibration of the seawater with atmospheric CO2, where a pH increase can facilitate carbon dioxide removal either through increased absorption capacity of CO2, or precipitation of minerals. Here, we focus on explicitly defining the governing factors that influence seawater alkalinity with respect to CaCO3 mineralization and atmospheric CO2 uptake. We utilize an electrochemical setup and different water types, from near-shore Mediterranean seawater as well as Red Sea Salt water. The setup allows us to separate mineralization from alkalinity enhancement via an electrochemical membrane cell, and a second compartment with controlled environment, where the interplay of ion concentration, precipitation, pH, DIC concentration, and total alkalinity are evaluated. The effect of sparging additional carbon dioxide before, and after the precipitation of CaCO3 was evaluated while utilizing both a mild pH swing (from, ∼8.3 to ∼9.5), and a larger pH swing (from, ∼8.3 to ∼10.2). A thermodynamic model is presented defining the energy consumption to increase the pH, and the influence of Mg thereon is explicated. Furthermore, we detail theoretical aquatic chemistry simulations via PHREEQC to rationalize the observed intricate dynamics of the carbonate system in seawater. We end with a discussion of the implications of total alkalinity, pH, salt complexation, and carbon uptake capacity relevant to of all ocean alkalinity enhancement and marine based carbon dioxide removal systems, concluding that the carbon uptake capacity of water returned to the ocean must be verified in such technologies

Ocean acidification will not affect the shell strength of juveniles of the commercial clam species Chamelea gallina: Implications of the local alkalinization of seawater

Ocean acidification (OA) is expected to decrease the strength of bivalves’ shells, especially during the early stages of development, with negative consequences to the resilience of natural populations and the economy. The objectives of the present study were to assess the long-term effect of increasing pCO2 after 217 days of exposure under controlled conditions of pH of ∼8.2, 8.0, and 7.7 on the strength and integrity of shells of juveniles of the commercial striped venus clam Chamelea gallina. Shell strength was estimated through compression tests and integrity through scanning electron microscopy (SEM) and dispersive X-ray analyses (EDX). The results showed that under increasing pCO2 the shell strength of juveniles is unaffected, which could be related to the locally elevated total alkalinity of seawater with respect to other parts of the coastal lagoon. However, despite this, it was also observed that the juvenile clams exposed to elevated pCO2 decreased their shell thickness and increased the porosity of their prismatic layer. Under future OA conditions, these changes could eventually compromise the integrity of the shells, becoming more vulnerable to the attack of predators and breakable during fishing operations. Future studies should address the plasticity of the organisms and the effect of the alkalinization of seawater on the resilience of shellfish juveniles under global change conditions.

Potential of CO2 sequestration by olivine addition in offshore waters: A ship-based deck incubation experiment

Ocean alkalinity enhancement is considered as an effective atmospheric CO2 removal approach, but currently, little is known about the carbon sequestration potential of implementing olivine addition in offshore waters. We investigated the effect of olivine addition on the seawater carbonate system by carrying out a deck incubation experiment in the Northern Yellow Sea; the dissolution rate of olivine was calculated based on the increase in seawater alkalinity (TA), and the CO2 sequestration potential was evaluated. The results showed that the dissolution of olivine increased seawater TA and decreased partial pressure of CO2, resulting in oceanic CO2 uptake from the atmosphere through sea-air exchange; it also increased seawater pH and mitigated ocean acidification to a certain extent. The addition of 1 ‰ olivine had a more significant effect on the seawater carbonate system than 0.5 ‰ olivine addition. The average dissolution rate constant of olivine was 1.44 ± 0.15 μmol m−2 d−1. Assuming that olivine settles completely on the seabed due to gravity, the theoretically maximum amount of CO2 removed by applying 1 tonne of olivine per square meter area in the Northern Yellow Sea is only 2.0 × 10−4 t/m2. Therefore, when olivine addition is implemented in the offshore waters, it is necessary to consider reducing the olivine size, prolonging the settling time of olivine in the water column; and spreading olivine in well-mixed waters to prolong the residence time through repeated resuspension, thus increasing its potential in carbon sequestration.

Design and Implementation of Iron-Based Electrochemical System for Simultaneous Carbon Capture and Hydrogen Gas Recovery

The catastrophic results of climate change and energy crisis, such as the rise in wildfires, floods, and ecosystem disruptions highlight the critical need of urgent and innovative global-scale solutions for carbon dioxide (CO2) removal (CDR). Ocean-based CDR methods, leveraging the ocean's capacity as a large sink to sequester up to 30% of emissions, offer a great promise. Yet, implementing and testing marine CO2 removal (mCDR) techniques such as ocean iron fertilization (OIF) or ocean alkalinity enhancement (OAE) face significant challenges. To overcome these challenges, a novel self-operating electrochemical technology is designed and presented in this work. The proposed technology, namely electrochemical OIF (EOIF), does not only combine OIF and OAE, but also recovers hydrogen gas (H2) from seawater, offering a promising solution for achieving quantifiable and transparent large-scale mCDR. The EOIF design s focused on employing Fe/Fe-producing anodes to eliminate the production of acids at the anode in traditional electrochemical OAE systems and replace it with Fe release, where the cathode can be made from naturally abundant cost-effective materials, such as carbon. The obtained results show that the EOIF system can be implemented to increase both the concentration of ferrous iron (Fe+2) by 0-0.5 mg/L, and the seawater pH by 8% (i.e., 25% decrease in the hydrogen ions concentration). It also offers the flexibility to optimize the release of iron (Fe+2/Fe+3) by adjusting the magnitude of the electric current in the systems, as well as by optimizing the electrode material and geometry. In certain ocean regions, enhanced iron concentrations stimulate the naturally occurring biological carbon pump (BCP), leading to increased phytoplankton growth, CO2 uptake, and subsequent export of carbon to the deep ocean. Simultaneously, the system increases seawater alkalinity and the buffer capacity, enhancing CO2 solubility and storage in the shallow ocean through the solubility pump. The techno-economic assessment shows that the combined advantages of the EOIF system can lower the mCDR cost by more than 95% compared to the traditional methods. The obtained measurements demonstrate the scalability of EOIF and its ability to operate using solar energy at a lower cost. Overall, the proposed EOIF technology offers a practical, effective, and sustainable solution for addressing climate change and energy recovery on a large-scale.

The Influence of Fresh Submarine Groundwater Discharge on Seawater Acidification Along the Northern Mediterranean Coast of Israel

AbstractIn the oligotrophic Southeastern Mediterranean Sea (SEMS), it has been shown that dissolved inorganic nutrient (DIN) from fresh submarine groundwater discharge (FSGD) enhance primary production in coastal waters. In this study pH, Total Alkalinity (TA) and DIN of seawater and fresh water in a sea‐cave in the northern part of the Israeli Mediterranean coast and a nearby contact spring, respectively, were measured during October 2018–March 2020. The results show gradients of measured salinity, TA, pH and DIN along the cave axis year‐round, suggesting that they are influenced by FSGD. The seawater near the back of the cave was supersaturated with respect to atmospheric CO2 nearly year‐round and there is a strong positive divergence from its regional open‐water thermal dependence, which suggests that FSGD is also a source of atmospheric CO2 in this region. Comparison of TA, salinity and pCO2 from the back of the sea cave to their corresponding values from an abrasion platform monitoring site, ca. 3 km south of the cave, suggests that FSGD is occurring along the entire shoreline in this region. Thus, despite the increased productivity due to FSGD mediated nutrient enrichment of adjacent coastal waters of the oligotrophic SEMS, they are still a source of atmospheric CO2 nearly year‐round. Finally, the apparent trends of seawater acidification (ΔpH/Δt = −0.006 yrs−1) and pCO2 increase (+8 ppmV yr−1) observed at the nearby monitoring site since 2013 are explained by increased groundwater recharge and resulting FSGD total alkalinity compared to dissolved inorganic carbon inputs (ΔTA/ΔDIC = 1:1.2).

Electrochemical ocean iron fertilization and alkalinity enhancement approach toward CO2 sequestration

Achieving net-zero emissions by 2050 requires the development of effective negative emission techniques, including ocean-based approaches for CO2 sequestration. However, the implementation and testing of marine CO2 removal (mCDR) techniques such as ocean iron fertilization (OIF) or ocean alkalinity enhancement (OAE) face significant challenges. Herein, a novel self-operating electrochemical technology is presented that not only combines OIF and OAE, but also recovers hydrogen gas (H2) from seawater, hence offering a promising solution for achieving quantifiable and transparent large-scale mCDR. Experimental results show that the electrochemical OIF (EOIF) can not only increase the concentration of ferrous iron (Fe+2) by 0–0.5 mg/L, but also significantly increases the seawater pH by 8% (i.e., a 25% decrease in the hydrogen ions concentration). The release of iron (Fe+2/Fe+3) can be regulated by adjusting the magnitude of the electric current and its form (e.g., pulsed current and polarity reversal), as well as by optimizing the electrode material and geometry. In certain ocean regions, enhanced iron concentrations stimulate the naturally occurring biological carbon pump (BCP), leading to increased phytoplankton growth, CO2 uptake, and subsequent export of carbon to the deep ocean. Simultaneously, the system increases seawater alkalinity and the buffer capacity, enhancing CO2 solubility and storage in the shallow ocean through the solubility pump. The obtained measurements demonstrate the scalability of EOIF and its ability to operate using solar energy at a lower cost. Overall, the proposed EOIF technology offers a practical, effective, and sustainable solution for addressing climate change on a large scale.

Sea surface acidification events in the Andaman Sea associated with the last Toba volcanic activity

To date, little is known about the impact of super-eruptions on ocean biogeochemistry. Using boron isotopes ratios measured on planktonic foraminifera in the marine sediment core BAR94–25, we provide a high-resolution pH record in the Andaman Sea (North of Sumatra), spanning Marine Isotopic Stage 5 to 3. This transition encompasses the super-eruption of the Toba volcano, 74,000 years ago, making it possible to decipher the potential impact of the super-eruption emissions on the ocean pH for the first time. Our results show that inferred foraminiferal pH values generally follow those predicted by glacial-interglacial CO2 variations. However, several abrupt pH drops coincide with Toba ash deposition. This suggests the occurrence of acidification events possibly related to Toba volcanic sulphur emission episodes. These pH drops are followed by anomalous pH increases, possibly relating to localised increases in seawater alkalinity following the alteration of large ash deposits on land.

Mesocosm study of PAC-modified clay effects on Karenia brevis cells and toxins, chemical dynamics, and benthic invertebrate physiology

Modified clay compounds are used globally as a method of controlling harmful algal blooms, and their use is currently under consideration to control Karenia brevis blooms in Florida, USA. In 1400 L mesocosm tanks, chemical dynamics and lethal and sublethal impacts of MC II, a polyaluminum chloride (PAC)-modified kaolinite clay, were evaluated over 72 h on a benthic community representative of Sarasota Bay, which included blue crab (Callinectes sapidus), sea urchin (Lytechinus variegatus), and hard clam (Mercenaria campechiensis). In this experiment, MC II was dosed at 0.2 g L-1 to treat bloom-level densities of K. brevis at 1 × 106 cells L-1. Cell removal in MC II-treated tanks was 57% after 8 h and 95% after 48 h. In the water column, brevetoxin analogs BTx-1 and BTx-2 were found to be significantly higher in untreated tanks at 24 and 48 h, while in MC II-treated tanks, BTx-3 was found to be higher at 48 h and BTx-B5 was found to be higher at 24 and 48 h. In MC II floc, we found no significant differences in BTx-1 or BTx-2 between treatments for any time point, while BTx-3 was found to be significantly higher in the MC II-treated tanks at 48 and 72 h, and BTx-B5 was higher in MC II-treated tanks at 24 and 72 h. Among various chemical dynamics observed, it was notable that dissolved phosphorus was consistently significantly lower in MC II tanks after 2 h, and that turbidity in MC II tanks returned to control levels 48 h after treatment. Dissolved inorganic carbon and total seawater alkalinity were significantly reduced in MC II tanks, and partial pressure of CO2 (pCO2) was significantly higher in the MC II-only treatment after 2 h. In MC II floc, particulate phosphorus was found to be significantly higher in MC II tanks after 24 h. In animals, lethal and sublethal responses to MC II-treated K. brevis did not differ from untreated K. brevis for either of our three species at any time point, suggesting MC II treatment at this dosage has negligible impacts to these species within 72 h of exposure. These results appear promising in terms of the environmental safety of MC II as a potential bloom control option, and we recommend scaling up MC II experiments to field trials in order to gain deeper understanding of MC II performance and dynamics in natural waters.

Modelling and Simulation of a Wet Scrubber System

Shipping is a relatively clean transport method with low emissions per ton-mile compared with road transport. However, harmful emissions emitted in coastal areas are a concern, as these affect local air quality and health. To reduce sulphur oxide (SOX ) emissions, the International Maritime Organization (IMO) implemented a global sulphur cap of 0.5 wt% and the 0.1 wt% limit in emission control areas (ECAs). Ship owners can opt for either low sulphur fuels or wet scrubber systems. Wet scrubber systems are a reliable method for reducing SOX emissions with capture rates of up to 98%. These systems may use seawater alkalinity or caustic soda (e.g. closed-loop systems) to neutralise the SOX emissions. However, the dynamic loading of engines can cause large fluctuations in the exhaust flow conditions, and it is unknown how these affect the effectiveness of the scrubber. This study explores the impact of dynamic loads on the SOX removal efficiency of closed-loop wet scrubbers. A dynamic model of a closed-loop wet scrubber utilising fresh water and caustic soda is developed and verified using publicly available data. The model applies the two-film theory to model the gas-liquid interface. Billet and Schultes liquid hold-up theory is used to model the liquid film thickness in the packed bed. Maintaining scrubber efficiency with large load fluctuations or high-frequency fluctuations requires an increased liquid flow. The scrubber control system used a set-point of 75% of the equivalent compliance limit to ensure compliance with the 0.1% ECA limit during load fluctuations. The model and results can be used to develop a more advanced control system for improved scrubber operation and integration with a selective catalytic reduction (SCR) system to demonstrate compliance with the IMO NOX Tier III limit when using high-sulphur heavy fuel oil (HFO).

Assessing the technical aspects of ocean-alkalinity-enhancement approaches

Abstract. Ocean alkalinity enhancement (OAE) is an emerging strategy that aims to mitigate climate change by increasing the alkalinity of seawater. This approach involves increasing the alkalinity of the ocean to enhance its capacity to absorb and store carbon dioxide (CO2) from the atmosphere. This chapter presents an overview of the technical aspects associated with the full range of OAE methods being pursued and discusses implications for undertaking research on these approaches. Various methods have been developed to implement OAE, including the direct injection of alkaline liquid into the surface ocean; dispersal of alkaline particles from ships, platforms, or pipes; the addition of minerals to coastal environments; and the electrochemical removal of acid from seawater. Each method has its advantages and challenges, such as scalability, cost effectiveness, and potential environmental impacts. The choice of technique may depend on factors such as regional oceanographic conditions, alkalinity source availability, and engineering feasibility. This chapter considers electrochemical methods, the accelerated weathering of limestone, ocean liming, the creation of hydrated carbonates, and the addition of minerals to coastal environments. In each case, the technical aspects of the technologies are considered, and implications for best-practice research are drawn. The environmental and social impacts of OAE will likely depend on the specific technology and the local context in which it is deployed. Therefore, it is essential that the technical feasibility of OAE is undertaken in parallel with, and informed by, wider impact assessments. While OAE shows promise as a potential climate change mitigation strategy, it is essential to acknowledge its limitations and uncertainties. Further research and development are needed to understand the long-term effects, optimize techniques, and address potential unintended consequences. OAE should be viewed as complementary to extensive emission reductions, and its feasibility may be improved if it is operated using energy and supply chains with minimal CO2 emissions.

Electrochemical ocean alkalinity enhancement using a calcium ion battery

The oceans play a major role in moderating atmospheric CO2 levels. Enhanced CO2 uptake into ocean waters can be achieved by the provision of appropriate cations to the surface ocean, an approach known as ocean alkalinity enhancement (OAE). Here, we present a calcium ion battery approach that enhances alkalinity via electrochemical manipulation of seawater calcium concentrations. We demonstrate the efficacy of this approach using a potassium barium iron cyanide [K2BaFe(CN)6] (PBFC) electrode, a Prussian blue analogue, to move calcium ions from one reservoir of seawater to another. Using material and electrochemical characterization of the Ca2+ ion insertion and expulsion properties of PBFC in synthetic seawater, we determine the repeatability of Ca2+ ion insertion and expulsion from the PBFC electrode. Our analyses prove a 2.75 % increase in seawater alkalinity via the PBFC electrode, which yields 2.64 mg CO2 (0.72 mg C) uptake per liter of seawater. This proof-of-concept method offers a unique, low-cost, energy efficient electrochemical approach for atmospheric carbon dioxide removal that can combine with marine-based renewable energy to enable a new family of effective, scalable climate change solutions.

Globally enhanced calcification across the coccolithophore Gephyrocapsa complex during the mid-Brunhes interval

Evolutionary or adaptative changes in Noelaerhabdaceae coccolithophores occurred in parallel with major changes in carbonate export and burial during scenarios of low orbital eccentricity, with a ∼400 kyr recurrence, during the Pleistocene. Coeval with these conditions of enhanced proliferation, here we report that the calcification of specimens was enhanced at a global scale and across multiple species or morphotypes within the Gephyrocapsa complex during the Mid-Brunhes (MB) interval. This acme of increased production of organic and inorganic carbon by Gephyrocapsa, suggests that such global changes may originate from a common driver. Increased seawater alkalinity, with an appropriately long residence time, is proposed as environmental trigger on the selection of a wide variety of highly calcified and prolific Gephyrocapsa taxa. This new perspective highlights the role of orbital forcing in phytoplankton evolution or adaptation, via a global environmental driver in the form of seawater carbon chemistry. Our results fit with earlier proposals appealing for an intensified biological pump and respiration dissolution during this interval. We hypothesize that the Gephyrocapsa acme may play a double-edged role, by increasing shallow respiration dissolution rates, limiting the removal of alkalinity by burial, which may help to recycle alkalinity and maintain constant levels at the ∼400 kyr scale. This idea suggests the potential capacity of the Noelaerhabdaceae coccolithophore acmes to modify the typical behaviour of carbonate compensation in the ocean and that the changes in coccolithophore calcification may be indicative of changes in ocean carbonate chemistry and the operation of the global carbon cycle in the past.

Chemical Alteration of Riverine Particles in Seawater and Marine Sediments: Effects on Seawater Composition and Atmospheric CO2

Numerous studies have shown that riverine particles react with seawater. Reactions include dissolution of reactive silicate minerals (e.g., feldspars) and formation of authigenic clays and carbonates. Previous studies have either focused on mineral dissolution (marine silicate weathering) or authigenic phase formation (reverse weathering). A comprehensive study that assesses all processes affecting the marine alteration of riverine particle has -to our knowledge- not yet been conducted. Our contribution aims to fill this gap. We first quantify cation exchange between seawater and riverine particles that occurs when particles enter the marine realm and show that significant global cation fluxes are induced by this process (-1.3 Tmol Na yr-1, -0.2 Tmol K yr-1, -0.4 Tmol Mg yr-1, +1.2 Tmol Ca yr-1) where the positive sign indicates cation release into seawater while the negative sign denotes uptake on particles. We then use thermodynamic and kinetic modeling to investigate how much of the suspended particle load dissolves in contact with seawater and estimate corresponding global release rates for dissolved cations and silica assuming congruent dissolution (+0.06 Tmol Na yr-1, +0.15 Tmol Ca yr-1, +2.8 Tmol Si yr-1). Subsequently, we investigate rates of mineral dissolution and authigenic clay and carbonate formation in marine sediments applying reactive transport modeling, porewater data and mass balance calculations. Our best estimates for net fluxes across the sediment/water interface (dissolution–mineral formation) result as +1.5 Tmol Na yr-1, -2.5 Tmol K yr-1, -2.0 Tmol Mg yr-1, +2.5 Tmol Ca yr-1, and +1.9 Tmol Si yr-1 where most of the Na and Ca release is induced by plagioclase dissolution, K is taken up in authigenic clays and Mg is removed from solution by authigenic clay and carbonate formation. We conclude that the alkalinity of seawater is not significantly affected by marine silicate alteration since cation release fluxes (Na, Ca) are as high as cation uptake fluxes (K, Mg) on equivalent basis. Moreover, marine silicate weathering and reverse weathering are closely coupled since Al required for clay formation is mostly provided by feldspar dissolution while Al removal in authigenic clay promotes and maintains feldspar dissolution in marine sediments. Authigenic carbonate formation in anoxic subsurface sediments sequesters significant amounts of carbon (2.5 Tmol C yr-1) according to our estimates where most of the Ca and alkalinity required for carbonate formation are provided by the dissolution of Ca-bearing silicate minerals. This hidden sedimentary cycle provides a sink for dissolved inorganic carbon that may drive a slow draw-down of atmospheric CO2 on geological timescales. Marine silicate alteration has an even stronger effect on the geochemical evolution of seawater by generating large fluxes of dissolved K, Mg, Ca and Si.

A differential titration method for determining the dissociation characteristics of natural organic acids in the coastal zone

Characterizing the concentrations and equilibrium characteristics of organic acid/base pairs is essential for quantifying CO2-system components and their behaviors and interactions in organic-rich coastal environments. Relatively few studies have been devoted to characterization of titratable organics, and their contributions to total alkalinity are frequently implicitly or explicitly considered negligible. In this work, novel potentiometric titration procedures were developed to quantify the concentrations and dissociation constants of river-derived titratable organics and thereby their alkalinity (Org-Alk) contributions to the total alkalinity (TA) of coastal seawater. The overall approach was to compare the titration properties of two paired solutions of identical salinity (S ≈ 5): (a) a sample of “estuarine” water formulated as a mixture of organic-rich river water plus open-ocean seawater and (b) an organics-free “baseline” approximation of that estuarine water, formulated as a mixture of dilute aqueous sodium chloride plus open-ocean seawater. Acid was incrementally added to both solutions, and the resulting two titration curves were then compared to provide direct visualization and quantification of the contributions of organics to TA. In other words, at any given pH, the difference in acid added to the two solutions (with a small correction for silicate alkalinity) indicates acid uptake by titratable organics in the estuarine sample. This procedure was performed for four replicate samples at 15, 25, and 35 °C. Our titrations indicated the presence of three types of titratable organics. The dominant type had a concentration of approximately 200 μmol∙kg−1 and a well-defined dissociation constant (pK) that did not vary with temperature: pKOrg2(average) = 6.33 ± 0.01. The second type had a concentration of approximately 110 μmol∙kg−1 and a moderately well-defined and high pK: 8.56 ≤ pKOrg3 ≤ 9.32. The third type had a concentration of approximately 40 μmol∙kg−1 and a low, poorly defined pK: 4.1 ≤ pKOrg1 ≤ 6.1. Inference of this third type was required to eliminate systematic residuals that were otherwise present in our fitted titration data (residuals = direct observations – model predictions). These data allow for a summation of TA contributions from organic constituents that may contribute (especially in coastal waters) but are typically uncharacterized. Such data are required for rigorous CO2-system models that rely on inputs of the commonly used data pair of TA and dissolved inorganic carbon (DIC). Studies of CO2 system behavior in the coastal zone clearly require two types of alkalinity titrations – one that includes a summation of inorganic and organic components (i.e., conventional titrations), and a second CO2-free titration in which the concentrations and pK characteristics of organic acids are quantified.

Seawater alkalinity enhancement with magnesium hydroxide and its implication for carbon dioxide removal

For the first time we used lab and wind-wave tank experiments to prove the concept of using Mg(OH)2 for the ocean alkalinity enhancement and carbon dioxide removal approach (OAE-CDR). Experiment results showed up to 370 μmol kg−1 seawater total alkalinity (TA) increase without precipitation, stable enhanced TA, and OAE-CDR efficiency (ΔDIC/ΔTA) consistent with theoretical calculation. Based on the experimental results, we calculated the global Mg(OH)2 OAE-CDR efficiency and CO2 sink from this approach. The efficiency is in favor of lower initial DIC to TA ratio, lower temperature, and higher atmospheric CO2, and therefore is the lowest at the equator (0.7–0.8) and increases with latitude (1.0–1.2 above 70 N/S). However, factoring in the ocean's surface area, low latitude regions can absorb more atmospheric CO2. We conservatively estimate that 44. 4 × 109 ton of CO2 (∼ 3.3 times of current annual CO2 sink in the ocean) could be removed from the atmosphere with 175 μmol kg−1 Mg(OH)2 (equivalent to 350 μmol kg−1 of TA) added to the top 10 m of the ocean.

Electrolytic Seawater Mineralization and the Mass Balances That Demonstrate Carbon Dioxide Removal.

We present the mass balances associated with carbon dioxide (CO2) removal (CDR) using seawater as both the source of reactants and as the reaction medium via electrolysis following the "Equatic" (formerly known as "SeaChange") process. This process, extensively detailed in La Plante E.C.; ACS Sustain. Chem. Eng.2021, 9, ( (3), ), 1073-1089, involves the application of an electric overpotential that splits water to form H+ and OH- ions, producing acidity and alkalinity, i.e., in addition to gaseous coproducts, at the anode and cathode, respectively. The alkalinity that results, i.e., via the "continuous electrolytic pH pump" results in the instantaneous precipitation of calcium carbonate (CaCO3), hydrated magnesium carbonates (e.g., nesquehonite: MgCO3·3H2O, hydromagnesite: Mg5(CO3)4(OH)2·4H2O, etc.), and/or magnesium hydroxide (Mg(OH)2) depending on the CO32- ion-activity in solution. This results in the trapping and, hence, durable and permanent (at least ∼10 000-100 000 years) immobilization of CO2 that was originally dissolved in water, and that is additionally drawn down from the atmosphere within: (a) mineral carbonates, and/or (b) as solvated bicarbonate (HCO3-) and carbonate (CO32-) ions (i.e., due to the absorption of atmospheric CO2 into seawater having enhanced alkalinity). Taken together, these actions result in the net removal of ∼4.6 kg of CO2 per m3 of seawater catholyte processed. Geochemical simulations quantify the extents of net CO2 removal including the dependencies on the process configuration. It is furthermore indicated that the efficiency of realkalinization of the acidic anolyte using alkaline solids depends on their acid neutralization capacity and dissolution reactivity. We also assess changes in seawater chemistry resulting from Mg(OH)2 dissolution with emphasis on the change in seawater alkalinity and saturation state. Overall, this analysis provides direct quantifications of the ability of the Equatic process to serve as a means for technological CDR to mitigate the worst effects of accelerating climate change.

Direct Air Capture of CO2 through Carbonate Alkalinity Generated by Phytoplankton Nitrate Assimilation.

Despite the consensus that keeping global temperature rise within 1.5 °C above pre-industrial level by 2100 reduces the chance for climate change to reach the point of no return, the newest Intergovernmental Panel on Climate Change (IPCC) report warns that the existing commitment of greenhouse gas emission reduction is only enough to contain the warming to 3-4 °C by 2100. The harsh reality not only calls for speedier deployment of existing CO2 reduction technologies but demands development of more cost-efficient carbon removal strategies. Here we report an ocean alkalinity-based CO2 sequestration scheme, taking advantage of proton consumption during nitrate assimilation by marine photosynthetic microbes, and the ensuing enhancement of seawater CO2 absorption. Benchtop experiments using a native marine phytoplankton community confirmed pH elevation from ~8.2 to ~10.2 in seawater, within 3-5 days of microbial culture in nitrate-containing media. The alkaline condition was able to sustain at continued nutrient supply but reverted to normalcy (pH ~8.2-8.4) once the biomass was removed. Measurements of δ13C in the dissolved inorganic carbon revealed a significant atmospheric CO2 contribution to the carbonate alkalinity in the experimental seawater, confirming the occurrence of direct carbon dioxide capture from the air. Thermodynamic calculation shows a theoretical carbon removal rate of ~0.13 mol CO2/L seawater, if the seawater pH is allowed to decrease from 10.2 to 8.2. A cost analysis (using a standard bioreactor wastewater treatment plant as a template for CO2 trapping, and a modified moving-bed biofilm reactor for nitrate recycling) indicated that a 1 Mt CO2/year operation is able to perform at a cost of ~$40/tCO2, 2.5-5.5 times cheaper than that offered by any of the currently available direct air capture technologies, and more in line with the price of $25-30/tCO2 suggested for rapid deployment of large-scale CCS systems.

Natural photosynthetic microboring communities produce alkalinity in seawater whereas aragonite saturation state rises up to five

Bioerosion, resulting from microbioerosion or biogenic dissolution, macrobioerosion and grazing, is one the main processes involved in reef carbonate budget and functioning. On healthy reefs, most of the produced carbonates are preserved and accumulate. But in the context of global change, reefs are increasingly degraded as environmental factors such as ocean warming and acidification affect negatively reef accretion and positively bioerosion processes. The recent 2019 SROCC report suggests that if CO2 emissions in the atmosphere are not drastically reduced rapidly, 70%–99% of coral reefs will disappear by 2,100. However, to improve projections of coral reef evolution, it is important to better understand dynamics of bioerosion processes. Among those processes, it was shown recently that bioeroding microflora which actively colonize and dissolve experimental coral blocks, release significant amount of alkalinity in seawater both by day and at night under controlled conditions. It was also shown that this alkalinity production is enhanced under ocean acidification conditions (saturation state of aragonite comprised between 2 and 3.5) suggesting that reef carbonate accumulation will be even more limited in the future. To better understand the conditions of production of alkalinity in seawater by boring microflora and its possible consequences on reef resilience, we conducted a series of experiments with natural rubble maintained under natural or artificial light, and various saturation states of aragonite. We show here that biogenic dissolution of natural reef rubble colonized by microboring communities dominated by the chlorophyte Ostreobium sp., and thus the production of alkalinity in seawater, can occur under a large range of saturation states of aragonite, from 2 to 6.4 under daylight and that this production is directly correlated to the photosynthetic activity of microboring communities. We then discuss the possible implications of such paradoxical activities on reef resilience.