Carbon dioxide mineralization by electrode separation for quick carbon reduction and sequestration in acidified seawater

Abstract

Aiming to sequestrate the excessive carbon dioxide and convert the acidified seawater, an improved method of carbon dioxide mineralization is developed based on electrode separation mechanism and extra oxygen-supplying technique. By electrode separation the neutralizations of the anodic acidity and the cathodic alkalinity, as well as the precipitation and the dissolution of calcium carbonate (CaCO₃), are prevented. In addition, the extra-supplied oxygen prevents the evolution of hydrogen, which enhances the electric conductivity of the porous cathode and the deposition of CaCO₃. A series of indoor physical experiments were conducted and the results show that the acidified seawater was successfully converted to alkaline in 72h. The speed of carbon mineralizing sequestration is significantly enhanced by supplying extra oxygen. The carbon dioxide mineralization speed increases with the immerse ratio of the aerator due to the more reacted oxygen and the less hydrogen evolution, which gives more porous space in the cathode for more conductive seawater and more deposition of CaCO₃. The extra-supplied oxygen increases the CaCO₃ -deposition by 100-214% under excessive atmospheric- CO₂ conditions and 117-200% under normal atmospheric- CO₂ conditions, respectively. This method has an application potential for quick conversion of locally acidified seawater in emergent circumstances.

Keywords:

• CaCO₃ deposition rate
• oxygen supplying technique
• conversion of acidification

Introduction

In coastal areas where fossil-fuel-fired power stations, petrochemical works, and cement manufacturing plants are located, excessive atmospheric CO₂ usually impacts atmospheric and ocean environments [1]. Ocean mixing, upwelling, downwelling, and circulation can bring excessive carbon to some local ocean sites, which usually causes seawater acidification [2,3]. This will cause the decomposition of marine shells and a decline in fishery resources, thus endangering coastal and marine ecosystems [4–12]. In addition to increasing energy efficiency, switching to less carbon-intensive energy sources, and absorbing and utilizing CO₂ onshore for carbon reduction [13], turning to the marine carbon sink is also an important option [14–16]. Quick carbon reduction and sequestration at the locally acidified marine site is a crucial method of environmental protection and is essential for the recovery and sustainable development of the marine ecosystem.

As an efficient method of carbon reduction, marine carbon sequestration is developing rapidly because the marine carbon sink is the most significant on Earth, with a storage capacity of up to 93% of the global carbon [17]. There are three predominant methods of marine carbon sequestration: sequestration by geological injection, sequestration by biological utilization , and sequestration by ionic mineralization.

The capacity of the global marine realm to contain CO₂ by geological injection sequestration is estimated at 400–10,000 × 10⁸ tons, reaching about 1.33–33.33 times the annual global atmospheric emission of 340 × 10⁸ tons [18–20]. The concept of seabed sequestration was first proposed for injecting CO₂ into thermohaline currents and then being carried and spread into the deep ocean [21]. After that, various onshore and offshore geological sequestration techniques have been greatly improved with the additional step of injecting CO₂ into subsurface porous rock formations [22–25]. By this method, the injected CO₂ is mainly captured from the atmosphere or immediately from emission, instead of from the seawater directly.

Sequestration involving marine biological use can reduce the CO₂ concentration in the seawater directly. Corallines utilize a large amount of CO₂ in their photosynthesis and thus decrease CO₂ concentration and convert acidified seawater. Coral reefs are estimated to have the ability to sequester 900 million tons of carbon annually [26]. Reef formation is accompanied by a certain amount of mineral deposition of calcium carbonate (CaCO₃) [27,28]. However, in acidified marine environments, the corallines and coral reefs tend to die or become bleached [29], which will cause them to lose their ability to sequester carbon.

In recent years, marine carbon mineralizing sequestration has become an efficient method of carbon reduction and marine acidification conversion [30–33]. The main advantage of this method is brought about by the abundant marine calcium ions (Ca⁺), which chemically react with the bicarbonate ions (HCO₃⁻) and are sufficient in limiting the decrease in concentration so as not to cause significant negative effects on the marine ecosystem [20,31]. However, the reduction reaction of Ca⁺ and HCO₃⁻ is difficult to produce in acidified marine environments. It usually requires a huge amount of alkaline reagent [18,34], which limits its application due to the possible negative impact of the added alkaline reagents on marine ecology.

Compared with throwing alkaline reagents into the ocean, a preferable method is to electrolyze the seawater to create a strong local alkaline environment at the cathode. The cathode-precipitated CaCO₃ can be sequestered permanently and utilized as an excellent bio-cement or other construction material due to its merits of high strength, self-growth, and uniform thickness. Therefore, the electrochemical precipitation technique of CaCO₃ has been developed further since first being proposed in the 1940s [35–38]. Regarding bulk carbon sequestration, however, its effect is neutral because the acidity created at the anode would increase the dissolution of CaCO₃ and release CO₂ simultaneously. Theoretically, separating the anode from the marine cathode by placing it in its own container will prevent the neutralization of anodic acidity and cathodic alkalinity. Thus, this will lead to the precipitation and dissolution of CaCO₃ in the seawater – but research on this technique is rarely reported.

Based on the electrode-separating technique, an improved method of electrochemical CO₂ mineralization for carbon sequestration in acidified seawater is proposed in this study. The proposed method uses active carbon felt as the electrodes due to its high electrical conductivity and porous nature, allowing easy deposition of CaCO₃. The anode is placed in a separate container and connected with the marine cathode through a seawater corridor to prevent the neutralization of acidity/alkalinity of the seawater. In addition, an extra oxygen-supplying device, in the form of a wave-current-driven aerator, is introduced to increase the speed of carbon sequestration. The main function of the aerator is to prevent the evolution of hydrogen and thus to improve the electrical conductivity and the speed of CaCO₃ deposition on the cathode. Compared to previous work, the comprehensive utilization of the active carbon electrodes, the electrode-separation technique, and the oxygen-supplying device are new developments. Because the material of the electrodes is easily obtained, and because the electrode separation technique and the oxygen supplying technique are both easily applied in real ocean sites, this newly proposed method is expected to enhance the speed of carbon removal and sequestration in real acidified seawater. To investigate the effects and the efficiency of this method, a series of indoor experiments were conducted with a permeable cylinder installed underwater as the wave-current driven aerator.

Theoretical background

Carbon mineralizing sequestration by electrode separation technique

Carbon mineralizing sequestration by cathodic precipitation of CaCO₃ is based on the electrochemical deposition technique. In this technique, the water molecule is firstly electrolyzed to generate $O{H}^{-}$ around the cathode, and then the abundant Ca + ions and $HC{O}_3^{-}$ ions in the seawater undergo a synthesis reaction with $O{H}^{-}$ ions to precipitate CaCO₃ and deposit it on the cathode [36–38].

In a normal marine environment where no extra oxygen is supplied, the electrolysis of water produces $O{H}^{-}$ ions and hydrogen gas ($H_2$) at the cathode. The electrolytic reaction is: (1) $$2H_{2}O 2e H_2 2OH$$(1)

Then the $C{a}^{2+}$ ions locally combined by negative charge ($e^{-}$) undergo a reduction reaction with $HC{O}_3^{-}$ and $O{H}^{-}$ to precipitate $\mathit{CaC}{O}_3.$ The reaction is: (2) $$C{a}^{2+}+HC{O}_3^{-}+O{H}^{-}\to \mathit{CaC}{O}_3+H_{2}O$$(2)

It should be noted that the acidified marine water is still alkaline (pH > 7), which allows the above reactions (Equations (1)–(2)) to occur. And the $O{H}^{-}$ ions can be compensated for by continuous electrolysis of water (Equation (1)(1) $$2H_{2}O 2e H_2 2OH$$(1) ).

Simultaneously, the electrolysis of water produces $H^{+}$ ions and oxygen gas ($O_2$) at the anode, and then these $H^{+}$ ions dissolve in the seawater until they react with $\mathit{CaC}{O}_3$ to produce $HC{O}_3^{-}$ ions. These two reactions are: (3) $$2H_{2}O\to O_2+4H^{+}+4e^{-}$$(3) (4) $$H^{+}+\mathit{CaC}{O}_3\to C{a}^{2+}+HC{O}_3^{-}$$(4)

Consequently, in the situation where both electrodes are placed in one electrolyte of the seawater, the net reaction is neutral regarding marine pH and alkalinity, as well as precipitation and dissolution of $\mathit{CaC}{O}_3$ in the seawater [36–38]. In this study, however, the anode is separated and placed in its own container to limit the anodic $H^{+}$ and $HC{O}_3^{-}$ ions in the container; therefore, the net precipitation of $\mathit{CaC}{O}_3$ on the cathode in acidified seawater can occur.

Acceleration of reaction and deposition with extra oxygen supply

In a situation where extra oxygen is involved, the cathodic electrolysis reaction of the seawater prevents the evolution of hydrogen, which exposes more porous space of the active carbon cathode to the seawater. This therefore enhances the electrical conductivity of the cathode and the deposition rate of $\mathit{CaC}{O}_3.$ The reaction is [39]: (5) $$O_2+{2H}_{2}O+4e^{-}\to 4O{H}^{-}$$(5)

Then, the reaction of the dissolved marine CO₂ and $O{H}^{-}$ ions is accelerated to generate $HC{O}_3^{-}$ more quickly; this reaction is: (6) $$C{O}_2\mathrm{ }+O{H}^{-}\to HC{O}_3^{-}$$(6)

Simultaneously, the precipitation rate of $\mathit{CaC}{O}_3$ is enhanced (Equation (2)(2) $$C{a}^{2+}+HC{O}_3^{-}+O{H}^{-}\to \mathit{CaC}{O}_3+H_{2}O$$(2) ). As a result, the speed of carbon reduction and the mineralizing sequestration of $C{O}_2$ in acidified seawater increases. It should be noted that in normal acidified seawater and under normal air–sea exchange conditions, there is no quick creation of $O{H}^{-}$ and no disturbance on the sea surface. Therefore, Equation (6) is limited by low dissolved $C{O}_2$ concentration and slow re-equilibration from the air. With the method proposed in this work, the aerator accelerates the creation of $O{H}^{-}$ and improves the exchange of air–seawater, and thus enhances Equation (6).

Experimental set-up

The experimental set-up comprises a water flume, an individual anode container, a pair of electrodes, a set of direct electric current supply devices, and a set of aerators. The indoor rectangular recycling water flume measures 8 m long, 0.3 m wide, and 0.4 m high. Two water pools were installed, at the upstream and downstream ends of the water flume, to adjust and regulate the water level in the flume. The experimental water was driven by a pump installed in the recycling layer. To model excessive $C{O}_2$ atmospheric conditions, a 0.25 m high arched top cover was installed over the flume. The cover and the flume were bound with rubber bands. Two 0.05 m diameter holes were created in the cover to inject air with excess $C{O}_2.$ Side views of the experimental set-up are shown in Figure 1a and b.

Figure 1. (a) Schematic of the water flume in side view. (b) Photograph of the water flume. (c) Schematic of the water flume system, top view.

The electrolysis device comprises a pair of electrodes. The cathode was installed inside the water flume, and the anode was put in a container outside. The water flume with the cathode and the container with the anode were interconnected at the downstream end by a water corridor. The material of both electrodes was activated carbon fiber with a specific surface area of 1500 m²/g, a specific resistance of 720$\Omega$·cm, and a specific capacitance of 162 F/g. The cathode was made of a piece of felt 7.5 m long, 0.15 m high, and 0.05 m thick and was installed starting 0.5 m from the upstream end of the water flume and parallel to the side wall. A wood frame was set up to support the cathode felt. The anode comprised a 10 cm³ cubic block that was placed outside in a cubic container measuring 25 cm³. The water flume and the container were connected by a water corridor at the downstream end of the water flume (Figure 1c).

The aerator was designed in the form of a permeable cylinder, which is easily driven by marine waves and currents (Figure 2). The permeable cylinder has a diameter of 0.3 m and a length of 0.24 m (Figure 2). A series of 12 aerators were installed parallelly underwater along the side wall of the water flume, at uniform intervals of 0.3 m (Figure 1c).

Figure 2. Photographs of the aerator.

Eight sets of experiments were conducted at an indoor temperature of 20 °C. Three immersion ratios of the aerator under two atmospheric CO₂ conditions, and two sets of control tests without an aerator, were carried out (Table 1). The initial atmospheric CO₂ was set at an excessive concentration of $C_{{Co}_2}=853ppm$ in the A test series and was set at a normal concentration of $C_{{Co}_2}=410ppm$ in the B test series. Tests A1–A3 and B1–B3 were carried out to investigate the effects and efficiency of marine pH improvement and carbon mineralizing sequestration. Control tests A0 and B0 were used to investigate the effects and efficiency of the set-up without an aerator, under normal and excessive atmospheric CO₂ concentrations, respectively. Each experimental test was repeated three times, each time with a new cathode and anode. Accurate initial atmospheric $C_{{Co}_2}$ values for all the tests are shown in Table 1. In the experimental test, the two holes on the top cover of the water flume were sealed.

Table 1. Properties of the aerators and initial atmospheric $\mathrm{C}{\mathrm{O}}_2$ concentration.

Test	A1	A2	A3	B1	B2	B3	A0	B0
Number of aerators	23	23	23	23	23	23	0	0
Immersion ratio of the aerator ($\lambda$)	0.18	0.35	0.5	0.18	0.35	0.5	–	–
$C_{{\mathrm{CO}}_2}$ (ppm)	853	853	853	410	410	410	853	410

Note: the immersion ratio of the aerator ($\lambda$) is defined as the ratio of the immersed height to the diameter of the aerator.

The experimental water was taken from the East China Sea, and sediment particles of diameter greater than 55 μm were filtered out. The pH and composition of the collected seawater, including the concentrations of bicarbonate ions ($HC{O}_3^{-}$), carbonate ions ($C{O}_3^{2-}$), and carbon dioxide ($C{O}_2$), are shown in Table 2. During the experimental tests, calcium chloride ($Ca{Cl}_2)$ was added into the water flume every 5 min to create a calcium ion solution and maintain the concentration of ${Ca}^{2+}$ within the range of 0.01–0.015 mol/L. The seawater in the anode container was neutralized using an alkaline reagent ($Na(OH)$) every 20 min and was changed every hour. It should be noted that the calcium ions are abundant in the real ocean and thus did not need to be replenished to the cathode area artificially.

Table 2. Marine pH and composition of seawater (mol/L).

	$\mathrm{pH}$	$\mathrm{C}{\mathrm{a}}^{2+}$	$\mathrm{HC}{\mathrm{O}}_3^{-}$	$\mathrm{C}{\mathrm{O}}_3^{2-}$	$\mathrm{C}{\mathrm{O}}_2$	${\mathrm{O}}_2$
		(mol/L)	(×10^−3 mol/L)
Original values	8.1	0.012	2.03	0.21	0.025	0.239
During the tests		0.01–.015	Not measured

The original pH of the experimental seawater was 8.1, indicating a healthy alkaline marine environment. Before every experimental test, CO₂ was injected into the experimental water to decrease the marine pH to 7.5 and create an acidified marine environment.

For all the experiments, a direct electric current of 1.5 V was supplied to the electrodes, flow velocity was set at 0.35 m/s, and the amplitude of the seawater wave was 0.05 m. The total thickness of $\mathit{CaC}{O}_3$ deposited on two sides of the cathode was measured ultrasonically. Ten measurement points were set along the center line of the cathode fiber felt (Figure 1b). The interval between two neighboring measurement points was 0.068 m.

Marine pH and $\mathit{CaC}{O}_3$ deposition were expected to increase over time in the experiment due to supplying extra oxygen and the continual conversion of CO₂ to $\mathit{CaC}{O}_3.$ But the conversion rate would vary with time because the $\mathit{CaC}{O}_3$ covering the cathode would cause a change in the electric current. Therefore, marine pH was measured every 2 h during the first 12 h, and then measured every 4 h from the 12th hour to the 36th hour, and then every 6 h from the 36th hour to the 72nd hour.

Results

The carbon mineralization system performs carbon sequestration at different speeds under both conditions with and without an aerator. This process is accompanied by pH variation under the counter-balanced effects of the flow, wave, and aerator in squeezing out CO₂ and precipitating $\mathit{CaC}{O}_3$ in the seawater. Therefore, the pH variation and the $\mathit{CaC}{O}_3$ deposition were simultaneously observed in the experiments. In addition, the final atmospheric CO₂ in every experimental test was observed to investigate the mechanism of the electro-chemical carbon mineralization system in carbon reduction.

Marine pH variation

Marine pH variation of the seawater was observed under two aerator conditions (with and without an aerator) and two atmospheric CO₂ conditions.

Marine pH variation in a carbon mineralization system with an aerator

The variation in the marine pH of the six experimental tests of carbon mineralization with an aerator are shown in Figure 3a and b, and the rate of improvement in pH over 72 h is shown in Figure 4. Figures 3a and 4 show that under the double atmospheric CO₂ condition, marine pH increases from 7.5 to 7.81, 7.84 and 7.88 after 72 h at aerator immersion ratios of 0.18, 0.35 and 0.5, respectively. In addition, the marine pH improvement rate increases with the immersion ratio of the aerator, reaching 4.13%, 4.53%, and 5.07% at the three respective immersion ratios.

Figure 3. Variation of marine pH over time.

Figure 4. Final marine pH improvement rate after 72 h.

Figures 3b and 4 show that under normal atmospheric CO₂ conditions, the proposed method increases marine pH more significantly and thus converts the same originally acidified seawater to a more healthy alkaline environment. At the three aerator immersion ratios of 0.18, 0.35, and 0.5, marine pH increases from 7.5 to 7.92, 7.97, and 8.08 after 72 h, and the improvement rate of marine pH increases with the aerator immersion ratio as well, reaching 5.6%, 6.27%, and 7.73%, respectively.

A comparison of Figure 3a and b shows the speed of marine pH improvement is much higher under normal atmospheric CO₂ conditions (Figure 3b) than at double atmospheric CO₂ conditions (Figure 3a). The marine pH improvement rate increases by 36.6%, 38.4% and 52.5% at the three immersion ratios of 0.18, 0.35 and 0.5, respectively. The main reason is that the continually supplied excess CO₂ releases more $H^{+}$ (Equation (7)), which causes the slower alkalization of the acidified seawater.

Marine pH variation in carbon mineralization system without an aerator

Figure 3c shows the variation of marine pH by the normal electro-chemical carbon mineralizing technique without an aerator. Marine pH increases from 7.5 to 7.62 after 72 h under double atmospheric CO₂ conditions, an improvement of 1.6%. Under normal atmospheric CO₂ conditions, in contrast, marine pH increases from 7.5 to 7.73 after 72 h, improved by 3.1%. Although marine pH is improved under both atmospheric CO₂ conditions, the normal electrochemical carbon mineralizing technique occurs at a lower speed than that with the aerator (Figure 3a and b).

A comparison of Figure 3a–c indicates the introduction of an aerator increases the marine pH improvement speed significantly, increasing it by 158–216% under excessive carbon emission conditions and by 83–153% under normal carbon emission conditions.

Sequestration of $\mathit{CaC}{O}_3$

The thickness of $\mathit{CaC}{O}_3$ deposition on the cathode, which represents the carbon sequestration in the seawater, was observed under two aerator conditions (with and without an aerator) and two atmospheric CO₂ conditions.

Deposition of $\mathit{CaC}{\mathit{O}}_3$ in a carbon mineralization system with an aerator

The variation of cathodic $\mathit{CaC}{O}_3$ deposition with time is shown in Figure 5a–c, and the final thickness of deposition in 72 h is shown in Figure 6. Figure 5a and 6 show that under the double atmospheric CO₂ condition, the sequestered $\mathit{CaC}{O}_3$ reaches 1.4, 1.8 and 2.2 mm in 72 h at an aerator immersion ratio of 0.18, 0.35 and 0.5, respectively.

Figure 5. Variation of cathodic $\mathit{CaC}{\mathit{O}}_3$ deposition with time.

Figure 6. Final $\mathit{CaC}{\mathit{O}}_3$ deposition after 72 h.

Furthermore, Figure 5b and 6 show that under normal atmospheric CO₂ conditions, the $\mathit{CaC}{O}_3$ deposition is less than that under double atmospheric CO₂ conditions, reaching 1.3, 1.5, and 1.8 mm after 72 h at aerator immersion ratios of 0.18, 0.35 and 0.5, respectively. In addition, under both atmospheric conditions, the $\mathit{CaC}{O}_3$ deposition increases with the immersion ratio of the aerator (Figures 5a and b and 6).

Deposition of $\mathit{CaC}{\mathit{O}}_3$ in a carbon mineralization system without an aerator

Figures 5c and 6 show the variation of $\mathit{CaC}{O}_3$ thickness using the normal electrochemical carbon mineralization technique without an aerator. The deposited $\mathit{CaC}{O}_3$ reaches 0.7 mm after 72 h under double atmospheric CO₂ conditions, whereas it reaches 0.6 mm under normal atmospheric CO₂ conditions. A comparison of Figure 5a–c indicates the introduction of an aerator improves the carbon sequestration speed by 100–214% under double atmospheric CO₂ conditions, and by 117–200% under normal carbon atmospheric conditions.

Contrary to the marine pH improvement efficiency, carbon sequestration speed is higher under double atmospheric CO₂ conditions than that under normal atmospheric CO₂ conditions at three aerator immersion ratios and without an aerator (Figures 5a–c and 6). The higher sequestration might be caused by the higher amount of CO₂ dissolved in the seawater, corresponding to the greater atmospheric CO₂, which precipitates more $\mathit{CaC}{O}_3$ under the double atmospheric CO₂ condition.

Variation of atmospheric CO₂

The final atmospheric CO₂ concentration after 72 h in the experiments is shown in Figure 7. In tests A0–A3, the final atmospheric CO₂ concentration is decreased by 25.8–41.9%. In tests B1–B3, the final atmospheric CO₂ concentration is reduced by 0–0.79%, but in test B0, it increases slightly (by 0.79%). In both A and B test series, the atmospheric CO₂ reduction increases with the immersion ratio of the aerator. This indicates that the main function of the aerator is to supply extra oxygen to the seawater to participate in the electrolysis reaction of water, instead of squeezing out CO₂ from the seawater.

Figure 7. Final atmospheric CO₂ concentration after 72 h.

Discussion

The proposed carbon mineralization method in this study is validated as effective and efficient in carbon reduction and carbon sequestration in acidified seawater. To understand the mechanism and develop this method further, four points should be noted:

1. Both the marine pH improvement rate and the $\mathit{CaC}{O}_3$ deposition rate decrease after 12 h of experiment time (Figures 3 and 4). This might be caused by decreased electric current passing through the deposited cathode since the $\mathit{CaC}{O}_3$ is a poor conductor [37]. Although in the previous marine-site experiments, the cathode was not found to decrease its conductivity due to the generated $H_2$ creating pores on the cathode (Equation (1)(1) $$2H_{2}O 2e H_2 2OH$$(1) ) [38], the extra oxygen supplied in this study restricts the evolution of $H_2$ (Equation (5)(5) $$O_2+{2H}_{2}O+4e^{-}\to 4O{H}^{-}$$(5) ). Therefore, the less porous $\mathit{CaC}{O}_3$ may decrease the cathodic conductivity after deposition has covered a large part of the cathodic surface.

2. The generated $H^{+}$ or ${HC{O}_3}^{-}$ ions at the anode still need to be neutralized or utilized in the separated container. The electrode-separation technique limits the negative effects of the neutralization outside the seawater. In addition, the utilization of $H^{+}$ and ${HC{O}_3}^{-}$ as well as their chemical production ($C{O}_2$) is more applicable outside the seawater [32–40].

3. Because the speeds of pH improvement and carbon sequestration increase with the immersion ratio of the aerator, the optimized installation of the aerator should consider the maximum immersion ratio at which the aerator can be driven by marine waves and currents.

4. In real ocean application, the electrode material needed per ton of sequestered $C{O}_2$ will depend on the specific surface area, specific resistance and specific capacitances of the active carbon fiber, because these three factors affect the conductivity and, therefore, the rate and amount of CaCO₃ deposition. The ranges of these three factors are wide; therefore, how much cathodic material is needed has to be evaluated according to the specific materials in use.

The efficiency changes with deposition build-up due to changes in specific surface area, specific resistance, and specific capacitance of the active carbon fiber. Therefore, the efficiency also needs to be evaluated according to the specific materials when used in real ocean applications.

Conclusions

The proposed method is based on the comprehensive utilization of active carbon fiber electrode material, the electrode-separating technique, and the oxygen-supplying technique. This method is validated as effective for quick carbon reduction and sequestration in acidified seawater through indoor water-flume experiments. The main conclusions are the following:

1. By the proposed method, net deposition of $\mathit{CaC}{O}_3$ occurs in the acidified seawater, which is accompanied by marine pH improvement due to consumption of $C{O}_2,$ and thus the method is effective in carbon reduction and carbon sequestration.

2. The aerator enhances carbon sequestration significantly, with the fundamental mechanism being that the extra oxygen prevents the evolution of hydrogen and thus increases the electrical conductivity and the cathodic $\mathit{CaC}{O}_3$ deposition;

3. The carbon sequestration rate increases with the immersion ratio ($\lambda$) of the aerator due to the increased oxygen supply, therefore the optimal installation of the aerator in the site should consider the maximum $\lambda$ at which the wave and current can drive the aerator.

Due to its merits of quick removal of carbon and easy site implementation, the proposed method has application potential for quick carbon reduction and carbon sequestration in the emergent circumstances of locally acidified seawater. In addition, this method sequesters the marine $C{O}_2$ on the active carbon fiber permanently, which is potentially beneficial for climate change mitigation and the global environment.

Acknowledgements

This work is supported in part by the National Science Foundation Council under Grants 51479109 and 51479137.

Data availability statement

The authors confirm that the data supporting the findings of this study are available at 10.6084/m9.figshare.21501075.

Disclosure statement

No potential conflict of interest was reported by the authors.