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.


Introduction
In coastal areas where fossil-fuel-fired power stations, petrochemical works, and cement manufacturing plants are located, excessive atmospheric CO 2 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][5][6][7][8][9][10][11][12].In addition to increasing energy efficiency, switching to less carbon-intensive energy sources, and absorbing and utilizing CO 2 onshore for carbon reduction [13], turning to the marine carbon sink is also an important option [14][15][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 CO 2 by geological injection sequestration is estimated at 400-10,000 Â 10 8 tons, reaching about 1. 33-33.33 times the annual global atmospheric emission of 340 Â 10 8 tons [18][19][20].The concept of seabed sequestration was first proposed for injecting CO 2 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 CO 2 into subsurface porous rock formations [22][23][24][25].By this method, the injected CO 2 is mainly captured from the atmosphere or immediately from emission, instead of from the seawater directly.
Sequestration involving marine biological use can reduce the CO 2 concentration in the seawater directly.Corallines utilize a large amount of CO 2 in their photosynthesis and thus decrease CO 2 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 (CaCO 3 ) [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][31][32][33].The main advantage of this method is brought about by the abundant marine calcium ions (Ca þ ), which chemically react with the bicarbonate ions (HCO 3 À ) 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 þ and HCO 3 À 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 CaCO 3 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 CaCO 3 has been developed further since first being proposed in the 1940s [35][36][37][38].Regarding bulk carbon sequestration, however, its effect is neutral because the acidity created at the anode would increase the dissolution of CaCO 3 and release CO 2 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 CaCO 3 in the seawaterbut research on this technique is rarely reported.
Based on the electrode-separating technique, an improved method of electrochemical CO 2 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 CaCO 3 .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 CaCO 3 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 wavecurrent driven aerator.

Carbon mineralizing sequestration by electrode separation technique
Carbon mineralizing sequestration by cathodic precipitation of CaCO 3 is based on the electrochemical deposition technique.In this technique, the water molecule is firstly electrolyzed to generate OH À around the cathode, and then the abundant Ca þ ions and HCO À 3 ions in the seawater undergo a synthesis reaction with OH À ions to precipitate CaCO 3 and deposit it on the cathode [36][37][38].
In a normal marine environment where no extra oxygen is supplied, the electrolysis of water produces OH À ions and hydrogen gas (H 2 ) at the cathode.The electrolytic reaction is: Then the Ca 2þ ions locally combined by negative charge (e À ) undergo a reduction reaction with HCO À 3 and OH À to precipitate CaCO 3 : The reaction is: 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 OH À ions can be compensated for by continuous electrolysis of water (Equation ( 1)).
Simultaneously, the electrolysis of water produces H þ ions and oxygen gas (O 2 ) at the anode, and then these H þ ions dissolve in the seawater until they react with CaCO 3 to produce HCO À 3 ions.These two reactions are: 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 CaCO 3 in the seawater [36][37][38].In this study, however, the anode is separated and placed in its own container to limit the anodic H þ and HCO À 3 ions in the container; therefore, the net precipitation of CaCO 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 CaCO 3 : The reaction is [39]: Then, the reaction of the dissolved marine CO 2 and OH À ions is accelerated to generate HCO À 3 more quickly; this reaction is: Simultaneously, the precipitation rate of CaCO 3 is enhanced (Equation ( 2)).As a result, the speed of carbon reduction and the mineralizing sequestration of CO 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 OH À and no disturbance on the sea surface.Therefore, Equation ( 6) is limited by low dissolved CO 2 concentration and slow re-equilibration from the air.With the method proposed in this work, the aerator accelerates the creation of OH À 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 CO 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 CO 2 : Side views of the experimental set-up are shown in Figure 1a and b.
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 m 2 /g, a specific resistance of 720XÁcm, 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 cm 3 cubic block that was placed outside in a cubic container measuring 25 cm 3 .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).
Eight sets of experiments were conducted at an indoor temperature of 20 C. Three immersion ratios of the aerator under two atmospheric CO 2 conditions, and two sets of control tests without an aerator, were carried out (Table 1).The initial atmospheric CO 2 was set at an excessive concentration of C Co 2 ¼ 853ppm in the A test series and was set at a normal concentration of C Co 2 ¼ 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 CO 2 concentrations, respectively.Each experimental test was repeated  1.In the experimental test, the two holes on the top cover of the water flume were sealed.
The experimental water was taken from the East China Sea, and sediment particles of diameter greater than 55 lm were filtered out.The pH and composition of the collected seawater, including the concentrations of bicarbonate ions (HCO À 3 ), carbonate ions (CO 2À  3 ), and carbon dioxide (CO 2 ), are shown in Table 2.During the experimental tests, calcium chloride (CaCl 2 Þ was added into the water flume every 5 min to create a calcium ion solution and maintain the concentration of Ca 2þ within the range of 0.01-0.015mol/L.The seawater in the anode container was neutralized using an alkaline reagent (NaðOHÞ) 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.
The original pH of the experimental seawater was 8.1, indicating a healthy alkaline marine environment.Before every experimental test, CO 2 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 CaCO 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 CaCO 3 deposition were expected to increase over time in the experiment due to supplying extra oxygen and the continual conversion of CO 2 to CaCO 3 : But the conversion rate would vary with time because the CaCO 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 counterbalanced effects of the flow, wave, and aerator in squeezing out CO 2 and precipitating CaCO 3 in the seawater.Therefore, the pH variation and the CaCO 3 deposition were simultaneously observed in the experiments.In addition, the final atmospheric CO 2 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 CO 2 conditions.Note: the immersion ratio of the aerator (k) is defined as the ratio of the immersed height to the diameter of the aerator.

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 CO 2 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.Figures 3b and 4 show that under normal atmospheric CO 2 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 CO 2 conditions (Figure 3b) than at double atmospheric CO 2 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 CO 2 releases more H þ (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 CO 2 conditions, an improvement of 1.6%.Under normal atmospheric CO 2 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 CO 2 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 CaCO 3
The thickness of CaCO 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 CO 2 conditions.

Deposition of CaCO 3 in a carbon mineralization system with an aerator
The variation of cathodic CaCO 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 CO 2 condition, the sequestered CaCO 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.
Furthermore, Figure 5b and 6 show that under normal atmospheric CO 2 conditions, the CaCO 3 deposition is less than that under double atmospheric CO 2 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 CaCO 3 deposition increases with the immersion ratio of the aerator (Figures 5a and b and 6).Contrary to the marine pH improvement efficiency, carbon sequestration speed is higher under double atmospheric CO 2 conditions than that under normal atmospheric CO 2 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 CO 2 dissolved in the seawater, corresponding to the greater atmospheric CO 2 , which precipitates more CaCO 3 under the double atmospheric CO 2 condition.

Variation of atmospheric CO 2
The final atmospheric CO 2 concentration after 72 h in the experiments is shown in Figure 7.In tests A0-A3, the final atmospheric CO 2 concentration is decreased by 25.8-41.9%.In tests B1-B3, the final atmospheric CO 2 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 CO 2 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 CO 2 from the seawater.

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 CaCO 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 CaCO 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)) [38], the extra oxygen supplied in this study restricts the evolution of H 2 (Equation ( 5)).Therefore,  the less porous CaCO 3 may decrease the cathodic conductivity after deposition has covered a large part of the cathodic surface.2. The generated H þ or HCO 3 À 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 þ and HCO 3 À as well as their chemical production (CO 2 ) is more applicable outside the seawater [32][33][34][35][36][37][38][39][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 CO 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 CaCO 3 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 CaCO 3 occurs in the acidified seawater, which is accompanied by marine pH improvement due to consumption of CO 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 CaCO 3 deposition; 3. The carbon sequestration rate increases with the immersion ratio (k) of the aerator due to the increased oxygen supply, therefore the optimal installation of the aerator in the site should consider the maximum k 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 CO 2 on the active carbon fiber permanently, which is potentially beneficial for climate change mitigation and the global environment.

Figure 1 .
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.

Figure 3 .
Figure 3. Variation of marine pH over time.

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

Table 1 .
Properties of the aerators and initial atmospheric CO 2 concentration.

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