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Potential impacts of effluent from accelerated weathering of limestone on seawater carbon chemistry: A case study for the Hoping power plant in northeastern Taiwan
Abstract
The Hoping power plant (HPP), located on the northeastern coast of Taiwan, is considered an ideal site to test the accelerated weathering of limestone (AWL) technique for fossil fuel CO2 mitigation because the required reactants, seawater and limestone, are readily available and inexpensive to obtain. In this study, we investigated the current status of chemical hydrography at two off-shore stations near the HPP, the results of which can provide a baseline for future evaluations of marine environmental changes arising from the potential implementation of AWL. Additionally, seawater samples collected around the HPP were used to evaluate the effectiveness of CO2 absorption for the One-step and the Two-step AWL reactors, which were designed by the Industrial Technology Research Institute of Taiwan. The results demonstrate that the Two-step reactor is more effective than the One-step reactor in terms of CaCO3(s) dissolution, thus implying conversion of more dissolved CO2 into HCO3− balanced by Ca2 +, which is environmentally benign and can be stored for the long-term in the ocean. The capacity of CO2(g) absorption for the current Two-step reactor was estimated to be 121 μmol kg− 1. However, more efficient conversion of CO2 to ocean alkalinity using this method is possible with optimizing reactor designs and operating procedures. The impacts of the AWL effluent solution on pH and carbonate saturation state (Ω) in the receiving seawater were further examined by conducting a simple mixing simulation. The simulated result indicates that a 10-fold dilution would be sufficient to maintain the pH and Ω changes within a range of 0.2 and 0.8, respectively, which are regarded as constraints for safely discharging wastewater into the ocean.
... The black lines indicate the gas flows and the blue lines indicate the process water flows. Table 1 shows the values for pCO2, AT, DIC, pH and Ωcalc in each of the four states for a representative case study, which is based on data reported from a two-step pilot reactor consisting of a separate gas-liquid and liquid-solid reactor (see Chou et al., 2015, and as further discussed below). The CO2 concentration of the gas stream was 15%, 110 while the pCO2 of the atmosphere is fixed at 420 ppm. ...
... ΔDICseq is the DIC that is added to the process water due 120 to dissolution from the gas stream and ΔDICcarb is the DIC added through the dissolution of CaCO3. The pCO2, AT and DIC values (indicated by #) are based on values measured in a two-step AWL pilot reactor (Chou et al., 2015). The values of AT, DIC, pH, and Ωcalc (indicated with *) are calculated using CRAN:AquaEnv (Hofmann et al., 2010) During step (i), the alkalinity remains invariant between state (1) and state (2) (vertical trajectory in Fig. 2). ...
... Some have remained at a conceptual model stage, while others have been tested in bench-top or pilot scale operations ( Table 2). As such, the technological readiness level is still limited and restricted to pilot scale applications (Chou et al., 2015;Kirchner 300 et al., 2020b). In this section, we will compare four different reactor designs: a one-step reactor (Caldeira and Rau, 2000;Chou et al., 2015), a two-step reactor (Chou et al., 2015), a slurry reactor (Kirchner et al., 2020b) and a buffered AWL reactor (Caserini et al., 2021). ...
To achieve climate stabilization, substantial emission reductions are needed. Emissions from industrial point sources can be reduced by applying carbon capture and storage (CCS) methods, which capture carbon dioxide (CO2) before it is released to the atmosphere. CCS applications typically target CO2 storage within geological reservoirs. Accelerated weathering of limestone (AWL) provides an alternative CCS approach, in which CO2 is stored as dissolved inorganic carbon in the ocean. At present, AWL technology remains at the pilot scale with no industrial implementation. Here, we review the proposed reactor designs for AWL, comparing them in terms of CO2 capture efficiency, CaCO3 dissolution efficiency, CO2 sequestration efficiency, and water usage. For this, we represent AWL as a four step process: (i) CO2 dissolution, (ii) CaCO3 dissolution, (iii) alkalinization (step only included in the case of buffered AWL), and lastly (iv) re-equilibration. AWL application is generally characterized by a large water usage and the need for large reactor sizes. Unbuffered AWL approaches show substantial degassing of CO2 back to the atmosphere after the process water is discharged. Buffered AWL compensates the unreacted CO2 by Ca(OH)2 addition, and hence prevents degassing, which substantially increases the CO2 sequestration efficiency. Yet, buffered AWL require a source of CO2-neutral Ca(OH)2. The need for process water can be reduced by increasing the CO2 fraction of the gas stream or increasing its pressure. Further optimization of the pulverized carbonate particles could reduce the amount of Ca(OH)2 needed to buffer the unreacted CO2. The anticipated CO2 sequestration efficiency of buffered AWL is comparable with that projected for large-scale CCS in geological reservoirs.
... The effectiveness of CO2 absorption for one-step and two-step AWL reactor configurations were evaluated in the previous study by Chou et al. [11]. The impact of AWL effluent solution on pH and saturation state in receiving seawater was also evaluated by using simulations. ...
... This study obtained higher CO2 removal efficiency compared to Haas et al. [10] but lower than that achieved in their prior work on the lab-scale reactors. Subsequently, higher TA achieved through circulating product water compared to Chou et al. [11] and Rau [1] but not at equilibrium due to an unexpected power plant shut down. Unfortunately, precipitation observed after 24 hours of stirring which contradicted the previous study by Rau [1] to equilibrate CO2 with the atmosphere. ...
... Comparing the calculated values of pH and Ω of an AWL reactor effluent to that obtained by experiment, Chou et al. [11] recorded significantly lower pH and Ω at 6.55 and 0.23 respectively, which was likely caused by incomplete CaCO3 dissolution observed during the experiment, as indicated by the measured total alkalinity, which was much lower than the predicted value. This may be expected given that acidity is what drives the reaction, and as CO2 is consumed the pH will rise, slowing the rate of reaction, leading to incomplete dissolution. ...
The present work explores accelerated weathering of limestone (AWL) as an option to combat the climate change problem posed by the still growing carbon footprint. AWL is a promising low-technology solution that sequesters CO2 through the dissolution of calcium carbonate (CaCO3) in water to form calcium bicarbonate. One aspect of the feasibility of its application is whether the newly formed bicarbonate will remain in solution for the long term. This paper examines the carbonate equilibria for various water bodies by combining Bjerrum plots with the calcite saturation value, omega.
The hydrochemistry software, aqion, was used to simulate a range of water compositions with different salinities at various conditions. The study compares saturation states of rainwater, limestone quarry water, seawater, and reverse osmosis concentrate, with their respective dissolved inorganic carbon (DIC) concentrations, across the full range of pH values. The areas of stability and instability can be observed from the generated plots in order to be able to design effective AWL processes. A key issue for successful implementation of AWL is operating within the CaCO3 saturation limit and ensuring dissolved material does not subsequently reprecipitate.
... The effectiveness of CO2 absorption for one-step and two-step AWL reactor configurations were evaluated in the previous study by Chou et al. [11]. The impact of AWL effluent solution on pH and saturation state in receiving seawater was also evaluated by using simulations. ...
... This study obtained higher CO2 removal efficiency compared to Haas et al. [10] but lower than that achieved in their prior work on the lab-scale reactors. Subsequently, higher TA achieved through circulating product water compared to Chou et al. [11] and Rau [1] but not at equilibrium due to an unexpected power plant shut down. Unfortunately, precipitation observed after 24 hours of stirring which contradicted the previous study by Rau [1] to equilibrate CO2 with the atmosphere. ...
... Comparing the calculated values of pH and Ω of an AWL reactor effluent to that obtained by experiment, Chou et al. [11] recorded significantly lower pH and Ω at 6.55 and 0.23 respectively, which was likely caused by incomplete CaCO3 dissolution observed during the experiment, as indicated by the measured total alkalinity, which was much lower than the predicted value. This may be expected given that acidity is what drives the reaction, and as CO2 is consumed the pH will rise, slowing the rate of reaction, leading to incomplete dissolution. ...
The present work explores accelerated weathering of limestone (AWL) as an option to combat the climate change problem posed by the still growing carbon footprint. AWL is a promising low-technology solution that sequesters CO2 through the dissolution of calcium carbonate (CaCO3) in water to form calcium bicarbonate. One aspect of the feasibility of its application is whether the newly formed bicarbonate will remain in solution for the long term. This paper examines the carbonate equilibria for various water bodies by combining Bjerrum plots with the calcite saturation value, omega.
The hydrochemistry software, aqion, was used to simulate a range of water compositions with different salinities at various conditions. The study compares saturation states of rainwater, limestone quarry water, seawater, and reverse osmosis concentrate, with their respective dissolved inorganic carbon (DIC) concentrations, across the full range of pH values. The areas of stability and instability can be observed from the generated plots in order to be able to design effective AWL processes. A key issue for successful implementation of AWL is operating within the CaCO3 saturation limit and ensuring dissolved material does not subsequently reprecipitate.
... Marine storage of CO 2 in the form of bicarbonate ions has the potential to last for geologic times, on the order of 10,000 years [12][13][14]. Rau and Caldeira [6,15] proposed a method called Accelerated Weathering of Limestone (AWL), consisting of the reaction of CO 2 from power plants' exhaust gas with seawater and calcium carbonate minerals (CaCO 3 ), namely CaCO 3 (s) + CO 2 (g) + H 2 O(l) → Ca 2+ (aq) + 2HCO 3 − (aq) (1) This method has progressed from the laboratory level [16] to a feasibility case study [17], to a pilot-scale reactor [18], and to modeling of local impacts on seawater carbonate chemistry [19]. An improvement of this method, named buffered accelerated weathering of limestone (BAWL), has been proposed by Caserini et al. [9]. ...
The dissolution of CO2 in seawater in the form of bicarbonate ions is an attractive alternative to storage in geological formations, on the condition that the storage is stable over long periods and does not harm the marine environment. In this work, we focus on the long-term chemical stability of CO2 absorbed in seawater as bicarbonate by monitoring the physico-chemical properties of the solutions (pH, dissolved inorganic carbon and alkalinity) in six different sets of experiments on both natural and artificial seawater lasting up to three months. The bicarbonate treatment of natural seawater consists of mixing it with pre-equilibrated solutions obtained from the reaction of CO2 and Ca(OH)2, with the same pH as natural seawater. This was achieved with a pilot plant working with tons of seawater, while small-scale laboratory experiments were carried out by adding sodium bicarbonate to artificial seawater solutions. If the increase in the overall carbon concentration in the final mixture does not exceed a critical threshold (about 1000–1500 μmol/L), the resulting bicarbonate-rich solutions are found to be stable for over three months.
... This method has progressed from the laboratory level [12] to the feasibility case study [13] and to a pilot-scale reactor, [14] as well as with modelling of local impacts on seawater carbonate chemistry. 2 [15] An improvement of this method, named buffered accelerated weathering of limestone (BAWL), has been proposed by Caserini et al. [16] With this approach, CO2 is used in stoichiometric excess with respect to the carbonate minerals, but calcium hydroxide [Ca(OH)2, also known as slaked lime, SL] is added in the final stages of the process to produce a buffered ionic solution at the same pH of the seawater. ...
The dissolution of CO2 in seawater in the form of bicarbonate ions is an attractive alternative to the storage in geological formations, on the conditions that the storage is stable over long times and does not harm the marine environment. In this work, we focus on the long-term chemical stability of CO2 absorbed in seawater as bicarbonate, by monitoring the physico-chemical properties of the solutions (pH, dissolved inorganic carbon and alkalinity), in six different sets of experiments on both natural and artificial seawater lasting up to three months. The bicarbonate treatment of natural seawater consists in pouring pre-equilibrated solutions obtained from the reaction of CO2 and Ca(OH)2. If the pre-equilibrated mixture does not exceed a critical threshold (ca. 1000-1500 mmol/L), the resulting bicarbonate-rich solutions can be stable for over three months.
... In addition, the released product solution of this process has been also used for the algae cultivation, to understand the influence to the downstream organisms in the ocean. The primary estimation of the influence to the ocean of the product solution in the lab-scale system has been reported by Chou et al. (2015). Fig. 1 shows the concepts of the main reactions used in this study. ...
... The storage of CO 2 in the form of bicarbonates has been proposed by Rau and Caldeira (1999), with a method called Accelerated Weathering of Limestone (AWL), consisting in the reaction of CO 2 from power plants exhaust gas with seawater and carbonate minerals (CaCO 3 ), calcite or aragonite, with a final discharge in the ocean of an ionic solution rich in bicarbonates. This method has progressed from the laboratory level (Rau, 2011) to the feasibility case study (Chou et al., 2015) and to a pilot-scale reactor (Kirchner et al., 2020a), as well as with modelling of local impacts on seawater carbonate chemistry (Kirchner et al., 2020b). ...
We present an evolution of the Accelerated Weathering of Limestone (AWL) method to store CO2 in seawater in the form of bicarbonates. Buffered Accelerated Weathering of Limestone (BAWL) is designed to produce a buffered ionic solution, at seawater pH, which derives from the reaction between a CO2 stream and a powder of micron-sized calcium carbonate particles in a long tubular reactor. Addition of calcium hydroxide to buffer the unreacted CO2 before the discharge in seawater is also provided. BAWL aims to overcome the main limitations of AWL, such as the high amount of water needed, the large size of the reactor, the risk of CO2 degassing back into the atmosphere, if the ionic solution is released into shallow waters, as well as the induced seawater acidification. This paper presents the chemical background of the technology and evaluates its feasibility by considering the chemical equilibria in the different phases of the process. The CO2 emitted for limestone calcination leads to a 24% CO2 penalty; a preliminary cost analysis assesses a storage cost of 100 € per tonne of CO2 from an external source. It finally discusses the main features to be considered for the design at the industrial scale.
... Although the concept of AWL is not new [6], it has only been performed in lab-scale or mid-scale applications for research purposes [7][8][9][10][11]. It has been shown that up to 97% of the CO 2 could be captured from a gas stream in a laboratory [9]. ...
The reduction in CO2 emissions is a major task for the coming decades. Accelerated weathering of limestone (AWL) can be used to capture CO2 from effluent gas streams and store it as bicarbonate in marine environments. We give an overview of the fundamental aspects of AWL, including associated CO2 emissions during the operation of AWL, characteristics of the accumulating bicarbonate-rich product water, and factors influencing the outgassing of CO2 from the ocean back into the atmosphere. Based on these aspects, we identify locations where AWL could be carried out favorably. The energy demand for AWL reduces the theoretical CO2 sequestration potential, for example, by only 5% in the case of a 100 km transport of limestone on roads. AWL-derived product water is characterized by high alkalinity but low pH values and, once in contact with the atmosphere, passive outgassing of CO2 from AWL-derived water occurs. This process is mainly driven by the difference between the ƒCO2 in the atmosphere and the oceanic surface layer, as well as the sea surface temperature at the discharge site. Promising sites for AWL may be in Florida or around the Mediterranean Sea, where outgassing could be prevented by injections into deep water layers.
... AWL effectively bypasses emission and drawdown to achieve ocean storage of flue gas CO 2 , but does not result in additional CO 2 drawdown to the ocean as the AWL discharge stream has to be degassed and diluted to be in equilibrium with surface mixed layer conditions. AWL is the most advanced OAE approach (TRL4 -bench scale system validation), having progressed from laboratory experiments (Rau 2011) to a feasibility case study for a coastal power plant in Taiwan (Chou et al. 2015), and most recently a pilot scale AWL reactor processing a 200 m 3 /hr flue gas slipstream from a coal-fired power plant at Wilhelmshaven, Germany (Kirchner et al. 2020b). AWL can be a low cost carbon capture option for coastal power plants and other emitters, particularly if there is an adequate local supply of waste limestone, for example from the cement or stone industries (Rau et al. 2007). ...
... Dissolution of carbonate minerals (e.g. CaCO 3 (s)) can be achieved by reacting them with waste flue gas CO 2 and seawater (Caldeira and Rau, 2000;Chou et al., 2015;Langer et al., 2009;Rau, 2011;Rau and Caldeira, 1999;Rau et al., 2007). This raises seawater pCO 2 to >0.51 kPa and lowers pH and CaCO 3 (aq) saturation state such that when contacted with solid calcium carbonate, reaction with CO 2 spontaneously occurs (Eqn 2 step 1). ...
The report provides an initial high-level review of twenty-seven proposed marine geoengineering techniques - with its potential subsets - for climate mitigation that focuses on their efficacy, practicality, side-effects, knowledge gaps, verification and potential environmental and socio-economic impacts.
... In addition to reducing CO 2 emissions, AWL-derived waters are characterized by elevated alkalinity and can be discharged into the marine environment to enhance the oceanic buffer capacity and counteract ocean acidification (Caldeira and Rau, 2000). Currently, only a few studies are available on the technical development of suitable reactors for small-and mid-scale applications (Rau et al., 2007;Rau, 2011;Haas et al., 2014;Chou et al., 2015). Most research is done on the general feasibility (including economical aspects) and the putative impact on climate change and oceanic carbonate system parameters on a global scale. ...
Reducing CO2 emissions is a key task of modern society to attenuate climate change and its environmental effects. Accelerated weathering of limestone (AWL) has been proposed as a tool to capture CO2 from effluent gas streams and store it primarily as bicarbonate in the marine environment. We evaluated the performance of the biggest AWL-reactor to date that was installed at a coal-fired power plant in Germany. Depending on the gas flow rate, approximately 55% of the CO2 could be removed from the flue gas. The generated product water was characterized by an up to 5-fold increase in alkalinity, which indicates the successful weathering of limestone and the long-term storage of the captured CO2. A rise of potentially harmful substances in the product water (NO2-, NO
x
-, NH4+, SO42-, and heavy metals) or in unreacted limestone particles (heavy metals) to levels of environmental concern could not be observed, most likely as a result of a desulfurization of the flue gas before it entered the AWL reactor. At locations where limestone and water availability is high, AWL could be used for a safe and long-term storage of CO2.
... In addition to reducing CO 2 emissions, AWL-derived waters are characterized by elevated alkalinity and can be discharged into the marine environment to enhance the oceanic buffer capacity and counteract ocean acidification (Caldeira and Rau, 2000). Currently, only a few studies are available on the technical development of suitable reactors for small-and mid-scale applications (Rau et al., 2007;Rau, 2011;Haas et al., 2014;Chou et al., 2015). Most research is done on the general feasibility (including economical aspects) and the putative impact on climate change and oceanic carbonate system parameters on a global scale. ...
Human activities are responsible for a > 45 % rise of atmospheric CO2 since the industrial revolution started; burning of fossil fuels being the largest source. Via accelerated weathering of limestone (AWL), CO2 can be captured from effluent gas streams and stored in the marine environment primarily in the form of bicarbonate. We studied the CO2 storage capacity and how AWL-derived water impacts the carbonate chemistry of the southern North Sea. Therefor, a three-dimensional hydrodynamic model was coupled with a sub-module to model the carbonate chemistry. We studied three scenarios: 1) scrubbing effluent gas streams of a combined heat and power plant (60 kW), 2) scrubbing 10 % or 3) 100 % of the flue gas of a coal-fired power plant (750 MW). Whereas, impacts on seawater carbonate chemistry due to AWL discharge were imperceptible in the first scenario, a maximum change in pHT and calcite saturation state of 0.1 and 0.6 was found in scenario 2. In scenario 3, the decrease in pHT exceeded 1 around the discharge site and the calcite saturation state reached 8 in large parts of the Jade Bay, posing the possibility of significant impacts on the marine ecosystem. Abiotic precipitation of calcite might occur around the discharge site. In all three scenarios, 50 % of the captured CO2 re-entered the atmosphere after the simulated time period of one year. This study shows that care is needed in siting, sizing and operating AWL facilities to maximize climate and ocean ecosystem benefits while minimizing negative environmental impacts.
... 10.1002/2016RG000533 1 atm. Chou et al. [2015] investigated the operation of a two stage AWL reactor in which gas-liquid equilibration occurs prior to solid-liquid equilibration and found lower carbon sequestration efficiencies of <50%. Despite this work, AWL research has been largely confined to small-scale experiments. ...
A recent paper in Reviews of Geophysics discussed increasing ocean alkalinity as an alternative method of carbon sequestration in response to climate change.
... In addition, the released product solution of this process has been also used for the algae cultivation, to understand the influence to the downstream organisms in the ocean. The primary estimation of the influence to the ocean of the product solution in the lab-scale system has been reported by Chou et al. (2015). Fig. 1 shows the concepts of the main reactions used in this study. ...
The present research has been shown the possibility of use of Georgian natural zeolites – mordenite and clinoptilolite modified H-forms for removal of nine volatile N-nitrosamines (VNA) and two tobacco-specific N-nitrosamines (TSNA) from tobacco mainstream smoke. Previously, adsorption properties of the above-mentioned zeolites modified H-forms were investigated towards genotoxic compounds as sorbates - nine volatile N-nitrosamines namely N-nitrosodimethylamine - NDMA, N-nitrosomethylethylamine - NMEA, N-nitrosodiethylamine - NDEA, N-nitrosodipropylamine - DPNA, N-nitrosodibutylamine - NDBA, N-nitrosopiperidine - NPIP, N-nitrosopyrrolidine - NPYR, N-nitrosomorpholine - NMPA, N-nitrosodiphenylamine – NDPA and two tobacco-specific N-nitrosamines 4-(methylnitrosamino)-1-(3- pyridyl)-1-butanone (NNK) and N′-nitrosonornicotine (NNN).
It was specially constructed dynamic type laboratory equipment for adsorption study of N-nitrosamines on zeolites modified H-forms which was composed of the following parts: 1.Quartz tube for burning tobacco; 2. Specially made glassware with bubbler on glacial bath for n-nitrosamine absorption; 3.Vacuum pump. The smoke from tobacco burning in quartz tube was conducted through organic solvent which absorbs all N-nitrosamine compounds without any loses. A new, rapid and effective analytical GC-MS method of quantitative determination of N-nitrosamines was developed and validated to control the concentrations of the above-mentioned toxic compounds in test solutions obtained from tobacco smoke.
Analytical data has been shown that studied mordenite modified H-form’s adsorptive capability is better than clinoptilolite H-form, more precisely; mordenite H-form decreases the content of volatile N-nitrosamines in tobacco smoke to 74 % and clinoptilolite H-form decreases to 63 %; mordenite H-form decreases the content of tobacco-specific N-nitrosamines in tobacco smoke to 95 % and clinoptilolite H-form decreases to 89 %. This phenomenon gives the perspective of creation “ant-nitrosamine” cigarette.
... 10.1002/2016RG000533 1 atm. Chou et al. [2015] investigated the operation of a two stage AWL reactor in which gas-liquid equilibration occurs prior to solid-liquid equilibration and found lower carbon sequestration efficiencies of <50%. Despite this work, AWL research has been largely confined to small-scale experiments. ...
Over the coming century humanity may need to find reservoirs to store several trillions of tons of carbon dioxide (CO2) emitted from fossil fuel combustion, which would otherwise cause dangerous climate change if it were left in the atmosphere. Carbon storage in the ocean as bicarbonate ions (by increasing ocean alkalinity) has received very little attention. Yet, recent work suggests sufficient capacity to sequester copious quantities of CO2. It may be possible to sequester hundreds of billions to trillions of tonnes of C without surpassing post-industrial average carbonate saturation states in the surface ocean. When globally distributed, the impact of elevated alkalinity is potentially small, and may help ameliorate the effects of ocean acidification. However, the local impact around addition sites may be more acute but is specific to the mineral and technology. The alkalinity of the ocean increases naturally because of rock weathering in which > 1.5 moles of carbon are removed from the atmosphere for every mole of magnesium or calcium dissolved from silicate minerals (e.g., wollastonite, olivine, anorthite), and 0.5 moles for carbonate minerals (e.g., calcite, dolomite). These processes are responsible for naturally sequestering 0.5 billion of CO2 tons per year. Alkalinity is reduced in the ocean through carbonate mineral precipitation, which is almost exclusively formed from biological activity. Most of the previous work on the biological response to changes in carbonate chemistry have focused on acidifying conditions. More research is required to understand carbonate precipitation at elevated alkalinity to constrain the longevity of carbon storage. A range of technologies have been proposed to increase ocean alkalinity (accelerated weathering of limestone, enhanced weathering, electrochemical promoted weathering, ocean liming), the cost of which may be comparable to alternative carbon sequestration proposals (e.g., $20 - 100 tCO2-1). There are still many unanswered technical, environmental, social, and ethical questions, but the scale of the carbon sequestration challenge warrants research to address these.
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.
This paper provides an industrial-scale technical assessment of absorption of CO2 in water to react into bicarbonate (HCO3⁻), with the goal of storing HCO3⁻ in the oceans as a carbon sequestration technology. A potential advantage of the process is that it will not require a CO2 transport and storage infrastructure that will be expensive for small scale and remote emission sources. Process simulations are utilized to estimate absorber column length and for mass flow estimations of water and base required for a target capture rate of 90%. The results indicate that the process is technically feasible under specific conditions, with pH regulation being highly important, although the demand for base represents a limiting factor. Yet, a potential niche for the process is CO2 capture at smaller plants emitting small amounts of CO2.
We investigate the availability of CO_2 ocean storage by means of chemical conversion of CO_2 to the dissolved inorganic carbon (mainly the bicarbonate ion) in seawater. The accelerated weathering of limestone (AWL) technique, which is accelerating the natural CO_2 uptake process through the chemical conversion using limestone and seawater, was proposed as an alternative method for reducing energy-related CO_2 emission. The method presented in this paper is slightly different from the AWL method. It involves reacting CO_2 with seawater and quicklime obtained from alkaline wastes to produce the bicarbonate-rich solution over 100 times more than seawater, which could be released and diluted into the ocean. The released dense bicarbonate-enriched water mass could subside into the deeper layer because of the density flow, and could be sequestrated stably in the ocean.
To assess the impact of rising atmospheric CO2 and eutrophication on the carbonate chemistry of the East China Sea shelf waters, saturation states (Ω) for two important biologically relevant carbonate minerals – calcite (Ωc) and aragonite (Ωa) – were calculated throughout the water column from dissolved inorganic carbon (DIC) and total alkalinity (TA) data collected in spring and summer of 2009. Results show that the highest Ωc (∼9.0) and Ωa (∼5.8) values were found in surface water of the Changjiang plume area in summer, whereas the lowest values (Ωc = ∼2.7 and Ωa = ∼1.7) were concurrently observed in the bottom water of the same area. This divergent behavior of saturation states in surface and bottom waters was driven by intensive biological production and strong stratification of the water column. The high rate of phytoplankton production, stimulated by the enormous nutrient discharge from the Changjiang, acts to decrease the ratio of DIC to TA, and thereby increases Ω values. In contrast, remineralization of organic matter in the bottom water acts to increase the DIC to TA ratio, and thus decreases Ω values. The projected result shows that continued increases of atmospheric CO2 under the IS92a emission scenario will decrease Ω values by 40–50% by the end of this century, but both the surface and bottom waters will remain supersaturated with respect to calcite and aragonite. Nevertheless, superimposed on such Ω decrease is the increasing eutrophication, which would mitigate or enhance the Ω decline caused by anthropogenic CO2 uptake in surface and bottom waters, respectively. Our simulation reveals that, under the combined impact of eutrophication and augmentation of atmospheric CO2, the bottom water of the Changjiang plume area will become undersaturated with respect to aragonite (Ωa = ∼0.8) by the end of this century, which would threaten the health of the benthic ecosystem.
Accelerated weathering of limestone appears to provide a low-tech, inexpensive, high-capacity, environmentally friendly CO2 mitigation method that could be applied to about 200 fossil fuel fired power plants and about eight cement plants located in coastal areas in the conterminous U.S. This approach could also help solve the problem of disposal of limestone waste fines in the crushed stone industry. Research and implementation of this technology will require new collaborative efforts among the crushed stone and cement industries, electric utilities, and the science and engineering communities.
To assess the impact of rising atmospheric CO2 and
eutrophication on the carbonate chemistry of the East China Sea shelf
waters, saturation states (Ω) for two important
biologically-relevant carbonate minerals, calcite (Ωc)
and aragonite (Ωa) were calculated throughout the water
column from dissolved inorganic carbon (DIC) and total alkalinity (TA)
data collected in spring and summer of 2009. Results show that the
highest Ωc (~9.0) and Ωa (~ 5.8)
values were found in surface water of the Changjiang plume area in
summer, whereas the lowest values (Ωc=~2.7 and
Ωa=~1.7) were concurrently observed in the bottom water
of the same area. This divergent behavior of saturation states in
surface and bottom waters was driven by intensive biological production
and strong stratification of the water column. The high rate of
phytoplankton production, stimulated by the enormous nutrient discharge
from the Changjiang, acts to decrease the ratio of DIC to TA, and
thereby increases Ω values. In contrast, remineralization of
organic matter in the bottom water acts to increase the DIC to TA ratio,
and thus decreases Ω values. The projected result shows that
continued increases of atmospheric CO2 under the IS92a
emission scenario will decrease Ω values by 40-50% by the end of
this century, but both the surface and bottom waters will remain
supersaturated with respect to calcite and aragonite. Nevertheless,
superimposed on such Ω decrease is increasing eutrophication,
which would mitigate or enhance the Ω decline caused by
anthropogenic CO2 uptake in surface and bottom waters,
respectively. Our simulation reveals that under the combined impact of
eutrophication and augmentation of atmospheric CO2, the
bottom water of the Changjiang plume area will become undersaturated
with respect to aragonite (Ωa=~0.8) by the end of this
century, which would threaten the health of the benthic ecosystem.
Model studies suggested that human-induced increase in nutrient load may
have stimulated primary production and thus has enhanced the
CO2 uptake capacity in the coastal ocean. In this study, we
investigated the seasonal variations of the surface water's partial
pressure of CO2 (pCO2sw) in the highly
human-impacted Changjiang-East China Sea system between 2008 and 2011.
The seasonality of pCO2sw has large spatial
variations, with the largest extreme of 170 ± 75 μatm on the
inner shelf near the Changjiang Estuary (from 271 ± 55 μatm in
summer to 441 ± 51 μatm in autumn) and the weakest extreme of
53 ± 20 μatm on the outer shelf (from 328 ± 9 μatm
in winter to 381 ± 18 μatm in summer). During the summer
period, stronger stratification and biological production driven by the
eutrophic Changjiang plume results in a very low CO2 in
surface waters and a very high CO2 in bottom waters on the
inner shelf, with the latter returning high CO2 to the
surface water during the mixed period. Interestingly, a comparison with
historical data shows that the average pCO2sw on
the inner shelf near the Changjiang Estuary has decreased notably during
summer, but it has increased during autumn and winter from the 1990s to
the 2000s. We suggest that this decadal change is associated with
recently increased eutrophication. This would increase both the
photosynthetic removal of CO2 in surface waters and the
respiratory release of CO2 in bottom waters during
summertime, thereby returning more CO2 to the surface during
the subsequent mixing seasons and/or episodic extreme weather events
(e.g. typhoons). Our finding demonstrates that increasing anthropogenic
nutrient delivery from a large river may enhance the sequestration
capacity of CO2 in summer but may reduce it in autumn and
winter. Consequently, the coastal ocean may not necessarily take up more
atmospheric CO2 in response to increasing eutrophication, and
the net effect largely depends on the relative time scale of air-sea gas
exchange and offshore transport of the shelf water. Finally, the case we
reported for the Changjiang system may have general ramifications for
other eutrophic coastal oceans.
The methane concentration and pCO2 in surface waters and the overlying marine air were continuously surveyed along the pathway of the Kuroshio, from the eastern coast of Honshu to Taiwan, and then across the eastern part of the East China and South China Seas in September of 1994. Off Honshu, the CH4 content was controlled by the confluence of the relatively CH4-poor waters of the Kuroshio and the Oyashio and the CH4-rich Tsugaru Warm Current, the latter carrying water into the Pacific Ocean with a methane content more than twice the equilibrium value with the atmospheric CH4 partial pressure. Along the Kuroshio, the surface water was supersaturated in methane with respect to the atmosphere by 10–15% and appears considerably enriched relative to open Pacific surface waters at same latitudes. The northeastern part of the South China Sea, part of the deep basin of this marginal sea, showed CH4 concentrations similar to those found in open-ocean waters. In contrast, highly variable oversaturations up to 700% were observed along the northwestern coast of Borneo, most probably related to known seepage from oil and gas deposits in this area.The pCO2 of surface water was higher than the atmospheric pCO2 throughout the area surveyed. However, the ΔpCO2 of the surface waters varied from close to 0 to more than 60 μatm. The observed oversaturation in areas influenced by the Kuroshio confirm that, during a short period in late summer, the surface waters of this current between Taiwan and Japan act as a moderate source for atmospheric CO2.
Depth distributions of pH, dissolved oxygen, dissolved inorganic carbon (DIC), total alkalinity (TA), and delta 13CDIC in the water column across the Luzon Strait from the South China Sea to the west Philippine Sea were investigated thoroughly to attest whether the South China Sea subsurface water outflow could act like a ``shelf pump'' to export the carbon from the interior of the South China Sea into the open Pacific. Results show that the outflow is capable of transporting 17.6 +/- 9.0 Tg C a-1 in DIC form out from the South China Sea to the western Pacific, a quantity equivalent to ~35 +/- 18% of the annual export production of the entire South China Sea. Furthermore, owing to the input of this South China Sea outflow, the subsurface waters of the Kuroshio Current become enriched in DIC/TA ratio but depleted in delta 13CDIC. Such a change in seawater carbon chemistry might further attenuate the capacity of CO2 sequestration and hamper the use of delta 13CDIC data as a tracer to estimate anthropogenic CO2 uptake rate in seawaters around the Kuroshio main path. More importantly, since these modifications can make all their ways northward along with the Kuroshio Current, the effect may reach even as far as to the higher-latitude region in the northwestern Pacific.
Various methods have been proposed for mitigating release of anthropogenic CO2 to the atmosphere, including deep-sea injection of CO2 captured from fossil-fuel fired power plants. Here, we use a schematic model of ocean chemistry and transport to analyze the geochemical consequences of a new method for separating carbon dioxide from a waste gas stream and sequestering it in the ocean. This method involves reacting CO2-rich power-plant gases with seawater to produce a carbonic acid solution which in turn is reacted on site with carbonate mineral (e.g., limestone) to form Ca2+ and bicarbonate in solution, which can then be released and diluted in the ocean. Such a process is similar to carbonate weathering and dissolution which would have otherwise occurred naturally, but over many millennia. Relative to atmospheric release or direct ocean CO2 injection, this method would greatly expand the capacity of the ocean to store anthropogenic carbon while minimizing environmental impacts of this carbon on ocean biota. This carbonate-dissolution technique may be more cost-effective and less environmentally harmful, and than previously proposed CO2 capture and sequestration techniques.
This paper reviews the thermodynamic basis of two approaches that are used to measure the total hydrogen ion concentration of sea water, potentiometry using a glass electrode and spectroscopy using an indicator dye. Both of these methods depend ultimately on measurements made using the classical hydrogen/silver-silver chloride cell for their calibration and thus provide equivalent pH scales. As a result of recent advances in measurement techniques and calibration, we should expect to see a revival in the popularity of pH measurements and a renewed understanding of the importance of this parameter in interpreting acid-base processes in sea water; particularly those involving the geochemically important carbon dioxide system.
In recent years the total alkalinity (TA) of seawater has been measured with high precision (∼±2 μmol kg−1) in the Atlantic, Pacific, and Indian oceans. In this paper we have analyzed the surface alkalinity of the major ocean basins using these measurements as well as those obtained during the GEOSECS and TTO studies. The salinity normalized alkalinity (NTA=TA×35/S) in subtropical gyres between 30°S and 30°N is remarkably invariable except in upwelling areas (e.g., the Eastern Equatorial Pacific). The NTA increases toward high latitudes (>30°) and is inversely proportional to sea surface temperature (SST). This increase in NTA with latitude (or decreasing temperature) is attributed to the upward transport of deep waters with higher NTA due to the dissolution of CaCO3(s). The distribution of surface NTA in the major ocean basins shows that the major basins can be divided into regions where different trends of NTA are observed and boundaries between the regions are similar to those of the major ocean currents. The linear behavior of NTA (∼±5 μmol kg−1) with respect to SST makes it possible to provide regional maps of NTA. These maps can be used to estimate TA in surface waters in large areas of the ocean from values of SST and salinity (S). By combining the estimates of TA using SST and S (from the Climatological Atlas of the World Ocean) with underway fCO2 measurements (by ships, moorings, and satellites), it is possible to map the detailed distribution of TCO2 for surface waters over a large area of the ocean. Calculations of TCO2 from measurements of fCO2, SST, and S in the subtropical Pacific Ocean agree with the coulometrically measured values to ±5 μmol kg−1.
A coral reef represents the net accumulation of CaCO3 produced by corals and other calcifying organisms. If calcification declines, then reef-building capacity also declines. Coral reef calcification depends on the saturation state of the carbonate mineral aragonite of surface waters. By the middle of next century, increased CO2 concentration will decrease aragonite saturation state in the tropics by 30%, and biogenic aragonite precipitation by 14–30%. Coral reefs are particularly threatened, since reef-building organisms secrete metastable forms of CaCO3, but the biogeochemical consequences on other calcifying marine ecosystems may be equally severe.
Anthropogenic elevation of atmospheric carbon dioxide (pCO(2)) is making the oceans more acidic, thereby reducing their degree of saturation with respect to calcium carbonate (CaCO3). There is mounting concern over the impact that future CO2-induced reductions in the CaCO3 saturation state of seawater will have on marine organisms that construct their shells and skeletons from this mineral. Here, we present the results of 60 d laboratory experiments in which we investigated the effects of CO2-induced ocean acidification on calcification in 18 benthic marine organisms. Species were selected to span a broad taxonomic range (crustacea, cnidaria, echinoidea, rhodophyta, chlorophyta, gastropoda, bivalvia, annelida) and included organisms producing aragonite, low-Mg calcite, and high-Mg calcite forms of CaCO3. We show that 10 of the 18 species studied exhibited reduced rates of net calcification and, in some cases, net dissolution under elevated pCO(2). However, in seven species, net calcification increased under the intermediate and/or highest levels of pCO(2), and one species showed no response at all. These varied responses may reflect differences amongst organisms in their ability to regulate pH at the site of calcification, in the extent to which their outer shell layer is protected by an organic covering, in the solubility of their shell or skeletal mineral, and in the extent to which they utilize photosynthesis. Whatever the specific mechanism(s) involved, our results suggest that the impact of elevated atmospheric pCO(2) on marine calcification is more varied than previously thought.
A coral reef represents the net accumulation of calcium carbonate (CaCO3) produced by corals and other calcifying organisms. If calcification declines, then reef-building capacity also declines. Coral reef calcification depends on the saturation state of the carbonate mineral aragonite of surface waters. By the middle of the next century, an increased concentration of carbon dioxide will decrease the aragonite saturation state in the tropics by 30 percent and biogenic aragonite precipitation by 14 to 30 percent. Coral reefs are particularly threatened, because reef-building organisms secrete metastable forms of CaCO3, but the biogeochemical consequences on other calcifying marine ecosystems may be equally severe.
A lab-scale seawater/mineral carbonate gas scrubber was found to remove up to 97% of CO(2) in a simulated flue gas stream at ambient temperature and pressure, with a large fraction of this carbon ultimately converted to dissolved calcium bicarbonate. After full equilibration with air, up to 85% of the captured carbon was retained in solution, that is, it did not degas or precipitate. Thus, above-ground CO(2) hydration and mineral carbonate scrubbing may provide a relatively simple point-source CO(2) capture and storage scheme at coastal locations. Such low-tech CO(2) mitigation could be especially relevant for retrofitting to existing power plants and for deployment in the developing world, the primary source of future CO(2) emissions. Addition of the resulting alkaline solution to the ocean may benefit marine ecosystems that are currently threatened by acidification, while also allowing the utilization of the vast potential of the sea to safely sequester anthropogenic carbon. This approach in essence hastens Nature's own very effective but slow CO(2) mitigation process; carbonate mineral weathering is a major consumer of excess atmospheric CO(2) and ocean acidity on geologic times scales.
CO2 released from combustion of fossil fuels equilibrates among the various carbon reservoirs of the atmosphere, the ocean, and the terrestrial biosphere on timescales of a few centuries. However, a sizeable fraction of the CO2 remains in the atmosphere, awaiting a return to the solid earth by much slower weathering processes and deposition of CaCO3. Common measures of the atmospheric lifetime of CO2, including the e-folding time scale, disregard the long tail. Its neglect in the calculation of global warming potentials leads many to underestimate the longevity of anthropogenic global warming. Here, we review the past literature on the atmospheric lifetime of fossil fuel CO2 and its impact on climate, and we present initial results from a model intercomparison project on this topic. The models agree that 20–35% of the CO2 remains in the atmosphere after equilibration with the ocean (2–20 centuries). Neutralization by CaCO3 draws the airborne fraction down further on timescales of 3 to 7 kyr.
Over the period from 1750 to 2000, the oceans have absorbed about one-third of the carbon dioxide (CO2) emitted by humans. As the CO2 dissolves in seawater, the oceans become more acidic and between 1750 and 2000, anthropogenic CO2 emissions have led to a decrease of surface-ocean total pH (pH T) by ~0.1 units from ~8.2 to ~8.1 (see Chapters 1 and 3). Surface-ocean pHT has probably not been below ~8.1 during the past 2 million years (Hönisch et al. 2009). If CO2 emissions continue unabated, surface-ocean pH T could decline by about 0.7 units by 2300 (Zeebe et al. 2008). With increasing CO2 and decreasing pH, carbonate ion (CO32–) concentrations decrease and those of bicarbonate (HCO-3) rise. With declining CO32– concentration ([CO32–]), the stability of the calcium carbonate (CaCO3) mineral structure, used extensively by marine organisms to build shells and skeletons, is reduced. Other geochemical consequences include changes in trace metal speciation (Millero et al. 2009 ) and even sound absorption ( Hester et al. 2008 ; Ilyina et al. 2010 ). Do marine organisms and ecosystems really ‘care’ about these chemical changes? We know from a large number of laboratory, shipboard, and mesocosm experiments, that many marine organisms react in some way to changes in their geochemical environment like those that might occur by the end of this century (see Chapters 6 and 7). Generally (but not always), calcifying organisms produce less CaCO3, while some may put on more biomass. Extrapolating such experiments would lead us to expect potentially significant changes in ecosystem structure and nutrient cycling. But can one really extrapolate an instantaneous environmental change to one occurring on a timescale of a century? What capability, if any, do organisms have to adapt to future ocean acidification which is occurring on a slower timescale than can be replicated in the laboratory? Simultaneous changes in ocean temperature and nutrient supply as well as in organisms’ predation environment may create further stresses or work to ameliorate the effect of changes in ocean chemistry.
The pH, alkalinity (Alk), and dissolved inorganic carbon (DIC) in estuarine waters of several rivers of Georgia in the southeastern United States are reported. Although they discharge into a narrow area in the South Atlantic Bight, the rivers along the coast of Georgia differ significantly in their drainage area (piedmont rivers vs. coastal plain rivers), chemical composition (contents of carbonate and humics), and discharge rates. Large differences in pH, DIC, and Alk between these rivers clearly reflect the differences of material inputs to the rivers. Dramatic pH increases in the early stage of mixing (more obvious for the coastal plain rivers) reflect the low buffering capacity of the river waters despite the high content of humic substances. Flux of riverine DIC to the South Atlantic Bight is estimated to be 52.5 x 109 mol yr-1. We suggest that the presently available world total riverine DIC flux to the ocean could have much uncertainty if the small fiver fluxes are important. Calculated pCO2 values from the pH and DIC measurements are extremely high in low-salinity areas (1,000 to >6,000 μatm at salinity <10) as are the corresponding CO2 fluxes to the atmosphere (20 to >250 mol m-2 yr-1 at salinity <10). Data presented suggest that the most likely causes that sustain the high pCO2 values and high water-to-air fluxes in the estuaries are CO2 inputs from organic carbon respiration in the tidally flooded salt marshes and groundwater.
The reaction of a mineral carbonate, such as limestone, with water and CO2 to form bicarbonate in solution, is explored as a CO2 mitigation strategy. Initial cost estimates for such a process range from 128 per tonne CO2 sequestered, with an energy penalty of about 8% and with relatively low environmental impact. The regional availability and transport of water and mineral carbonate appear to be the primary determinants of the strategy’s cost and applicability. The bicarbonate-rich waste effluent would be released into rivers or coastal waters, ultimately adding a small amount to the existing, very large bicarbonate reservoir in the ocean. For many applications, this form of ‘marine’ carbon sequestration appears to be less costly, less affected by national and international regulations, more environmentally friendly and more effective over the long term than direct CO2 injection into the ocean.
Several analytical problems in the determination of nitrate using flow injection analysis (FIA) coupled with an on-line Cd reductor have been studied. It was found difficult to prepare a nearly 100%-efficient copperized Cd reductor which maintains its efficiency over a lengthy period. Instead, the use of a narrow and lower efficiency Cd coil is recommended because it is more stable and therefore more suitable for FIA. Since the conversion of nitrate to nitrite is not quantitative, results for nitrate tend to be over-estimates when nitrite is also present. This problem has been solved by using a simple correction scheme to compensate for the effect of nitrite, thus enabling the correct nitrate concentration to be evaluated. The validity of the correction procedure has been confirmed by running a series of known standards containing both nitrate and nitrite with three types of FIA manifolds. Results for nitrate were accurate for fresh and saline waters even when the co-existing nitrite concentrations were high.
Spectrophotometric open-ocean seawater pH measurements are simple, fast and precise. The sulfonephthalein indicator m-cresol purple (mCP) is recommended for open-ocean surface-to-deep pH measurements. The salinity- and temperature-dependence of such measurements (293⩽T⩽303 and 30⩽S⩽37) is given as: on the total hydrogen ion concentration scale ([H+]T = [H+]f + [HSO4−]), in units of mol·(kg-soln)−1. R is the ratio of indicator absorbances at molar absorptivity maxima (i.e. R + 578A/434A). The at-sea analytical precision of the technique, evaluated on two recent NOAA cruises, is approximately 0.0004 pH units.
To explore the effect of atmospheric forcing on the CO2 system in the subtropical northwest Pacific Ocean, which is oligotrophic and nitrogen limited, total alkalinity (TA), dissolved inorganic carbon (DIC), fugacity of CO2 (fCO2), and other pertinent data (i.e. temperature, salinity, and concentrations of nitrate and chlorophyll a (Chl a)) were collected from 7 cruises during the spring Asian dust storm (ADS) periods of 2007 and 2008. In contrast to the reported substantial fCO2 decrease following dissolved iron addition in the “high-nutrient-low-chlorophyll” region during the mesoscale iron enrichment experiments, the present results show that no significant drawdown of fCO2 was found following an ADS event, despite the fact that an approximately 3-fold increase of Chl a was observed. This may be attributed to the fact that nutrients from the wind-induced entrainment of subsurface water, rather than atmospheric deposition, were the major source stimulating biological production. The entrained nitrate not only comes with it high CO2 but also may have rendered an unfavorable environmental condition for nitrogen fixers to compete with other picophytoplanktons. Consequently, even if the Fe and/or P deposition may have increased, nitrogen fixation, a mechanism favoring CO2 sequestration in the oligotrophic region, cannot take place during the ADS period. A model simulation further confirms that the increase of fCO2 caused by CO2 inputs from the subsurface water can nearly be compensated for by the fCO2 decrease resulting from the accompanying cooling effect and the enhancement of biological production. Accordingly, although a previous study revealed that the elevated biological production may enhance particulate organic carbon export during the ADS period, our results suggest that it may not contribute much to the sequestration of atmospheric CO2 in the oligotrophic subtropical northwest Pacific Ocean. Our work further suggests that sea surface TA and perhaps DIC can be predicted from salinity in this low production area.
1] Total alkalinity (TA) distribution and its relationship with salinity (S) along the western North Atlantic Ocean (wNAO) margins from the Labrador Sea to tropical areas are examined in this study. Based on the observed TA‐S patterns, the mixing processes that control alkalinity distribution in these areas can be categorized into a spectrum of patterns that are bracketed by two extreme mixing types, i.e., alongshore current‐dominated and river‐dominated. Alongshore current‐dominated mixing processes exhibit a segmented mixing line with a shared mid‐salinity end‐member. In such cases (i.e., Labrador Sea, Gulf of Maine, etc.), the y‐intercept of the high salinity segment of the mixing line is generally higher than the local river alkalinity values, and it reflects the mixing history of the alongshore current. In contrast, in river‐dominated mixing (Amazon River, Caribbean Sea, etc.), good linear relationships between alkalinity and salinity are generally observed, and the zero salinity intercepts of the TA‐S regressions roughly match those of the regional river alkalinity values. TA‐S mixing lines can be complicated by rapid changes in the river end‐member value and by another river nearby with a different TA value (e.g., Mississippi‐Atchafalaya/Gulf of Mexico). In the wNAO margins, regression intercepts and river end‐members have a clear latitudinal distribution pattern, increasing from a low of ∼300 mmol kg −1 in the Amazon River plume to a high value between ∼500–1100 mmol kg −1 in the middle and high latitude margins. The highest value of ∼2400 mmol kg −1 is observed in the Mississippi River influenced areas. In addition to mixing control, biological processes such as calcification and benthic alkalinity production may also affect ocean margin alkalinity distribution. Therefore, deriving inorganic carbon system information in coastal oceans using alkalinity‐salinity relationships, in particular, those of generic nature, may lead to significant errors.
The apparent dissociation constants of carbonic acid in seawater were determined as functions of temperature (2-35°C) and salinity ( 19-43%) at atmospheric pressure by measurement of K'1 and the product K', K',. At 35sa salinity and 25°C the measured values were pE1 = 6.600 and pK'2 = 9.115; at 35% and 2°C the measured values were pK'1 = 6.177 and pKPz = 9.431.
Measurements of dissolved inorganic carbon (DIC), pH, total alkalinity (TA), and partial pressure of CO2 (pCO2) were conducted at a total of 25 stations along four cross shelf transects in the East China Sea (ECS) in January 2008. Results showed that their distributions in the surface water corresponded well to the general circulation pattern in the ECS. Low DIC and pCO2 and high pH were found in the warm and saline Kuroshio Current water flowing northeastward along the shelf break, whereas high DIC and pCO2 and low pH were mainly observed in the cold and less saline China Coastal Current water flowing southward along the coast of Mainland China. Difference between surface water and atmospheric pCO2 (ΔpCO2), ranging from ~ 0 to − 111 μatm, indicated that the entire ECS shelf acted as a CO2 sink during winter with an average flux of CO2 of −13.7 ± 5.7 (mmol C m− 2 day− 1), and is consistent with previous studies. However, pCO2 was negatively correlated with temperature for surface waters lower than 20 °C, in contrast to the positive correlation found in the 1990s. Moreover, the wintertime ΔpCO2 in the inner shelf near the Changjiang River estuary has appreciably decreased since the early 1990s, suggesting a decline of CO2 sequestration capacity in this region. However, the actual causes for the observed relationship between these decadal changes and the increased eutrophication over recent decades are worth further study.
The use and impacts of accelerated weathering of limestone (AWL; reaction: CO2+H2O+CaCO3→Ca2++2(HCO3−) is explored as a CO2 capture and sequestration method. It is shown that significant limestone resources are relatively close to a majority of CO2-emitting power plants along the coastal US, a favored siting location for AWL. Waste fines, representing more than 20% of current US crushed limestone production (>109 tonnes/yr), could provide an inexpensive or free source of AWL carbonate. With limestone transportation then as the dominant cost variable, CO2 mitigation costs of 4/tonne appear to be possible in certain locations. Perhaps 10–20% of US point–source CO2 emissions could be mitigated in this fashion. It is experimentally shown that CO2 sequestration rates of 10−6 to 10−5 moles/sec per m2 of limestone surface area are achievable, with reaction densities on the order of 10−2 tonnes CO2 m−3day−1, highly dependent on limestone particle size, solution turbulence and flow, and CO2 concentration. Modeling shows that AWL would allow carbon storage in the ocean with significantly reduced impacts to seawater pH relative to direct CO2 disposal into the atmosphere or sea. The addition of AWL-derived alkalinity to the ocean may itself be beneficial for marine biota.
The published experimental data of Hansson and of Mehrbach et al. have been critically compared after adjustment to a common pH scale based upon total hydrogen ion concentration. No significant systematic differences are found within the overall experimental error of the data. The results have been pooled to yield reliable equations that can be used to estimate for seawater media a salinities from 0 to 40 and at temperatures from 2 to 35°C.
Current velocity, measured by Shipboard Acoustic Doppler Current Profiler (Sb-ADCP) during 1991–2000, was used to study the upper-ocean (<300 m) currents around Taiwan. The collected data were debugged, calibrated, grid, and averaged to compose a three-dimensional current-velocity distribution. The validity of the composite current velocity was supported by 12 sets of moored current-velocity time series. Qualitative agreement was obtained. The moored time series also indicated that the seasonal variation of current around Taiwan was generally weak except for the shallow-water regimes.The composite and moored currents revealed a branch of the Kuroshio that intruded steadily and persistently into the South China Sea. Part of the intruded Kuroshio flowed out of the South China Sea through the northern Luzon Strait and re-united with the main stream Kuroshio. The Kuroshio had two velocity maximum cores southeast of Taiwan, but gradually combined into one as the Kuroshio flowed north. The Kuroshio was deflected by the I-Lan Ridge east of Taiwan and the zonal-running shelf break northeast of Taiwan. At the shelf break, the Kuroshio split, with one branch intruding onto the shelf.West of the Luzon Strait, the Kuroshio intruded into the South China Sea. Some water flowed northward into the Taiwan Strait and re-joined the Kuroshio. Currents in the Taiwan Strait flowed primarily in a northward direction, except for the southward current near the coast of Mainland China. North of the Taiwan Strait, a branch of the northward flow followed the northern coast of Taiwan to join the Kuroshio.The composite current varied consistently from season to season. There was generally poor correlation between currents and local winds, especially in the deep-water regime. Remote forces were important in the currents around Taiwan.
The effects of variation of acidity and molybdate concentration on the spectrophotometric determination of phosphate by the method of Murphy and Riley were studied. The [H+]:[Mo] ratio was found to be a crucial parameter which not only influences the form of the final reduced complex, but also plays a key role in controlling the reaction kinetics. Normal colour formation of the reduced phosphoantimonylmolybdenum blue complex was only observed with a ratio between 60 and 80; the reaction was faster at low acidities. Silicate is a possible source of interference at high temperatures, but the formation of the silicoantimonylmolybdenum blue complex was found to be much slower than that of the corresponding phosphorus species.
The reaction of a mineral carbonate, such as limestone, with water and CO2 to form bicarbonate in solution, is explored as a CO2 mitigation strategy. Initial cost estimates for such a process range from 128 per tonne CO2 sequestered, with an energy penalty of about 8% and with relatively low environmental impact. The regional availability and transport of water and mineral carbonate appear to be the primary determinants of the strategy’s cost and applicability. The bicarbonate-rich waste effluent would be released into rivers or coastal waters, ultimately adding a small amount to the existing, very large bicarbonate reservoir in the ocean. For many applications, this form of ‘marine’ carbon sequestration appears to be less costly, less affected by national and international regulations, more environmentally friendly and more effective over the long term than direct CO2 injection into the ocean.
Chlorophyll a concentration, primary production, and environmental conditions over the entire shelf of the subtropical East China Sea (ECS) were studied extensively during four seasonal cruises between December 1997 and October 1998. Nutrient concentrations in the northwestern half of the shelf were enriched all year-round, but primary production showed high seasonal variations. Intensive primary production was mostly observed in summer at about 939 mg C m−2 d−1. On average, the value in summer was about 3 times higher than that in other seasons. Annual primary production was 155 g C m−2 y−1. In the southeastern half of the shelf, on the other hand, nutrient concentrations were seasonally variable, but primary production showed only slight seasonal variations with a mean value of 395 mg C m−2 d−1. Annual primary production was 144 g C m−2 y−1. The annual variations in shelf-averaged primary production can be well described with a normal distribution curve. For the entire shelf of the ECS, annual primary production was 145 g C m−2 y−1. The rate of primary production was regulated by seawater temperature from winter to early spring. The rate of primary production was, in turn, regulated by the availability of nutrients, especially phosphate, from summer to autumn. In addition, turbidity might also play a role in the regulation of primary production in the waters of the inner shelf.
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