Captura (Captura Corp) is a ocean-based carbon removal company headquartered in Pasadena, California. Captura leverages well-established electrodialysis technology to capture carbon dioxide (CO₂) directly from the seawater rather than from the air. This approach is highly scalable and cost-effective, and it avoids many of the technical engineering challenges that face land-based Direct Air Capture (DAC) technologies. Captura has been awarded $1 million by XPRIZE for capturing carbon dioxide from ocean water to combat climate change.
Challenges: Ocean-based carbon removal
Carbon emissions
Since the early 1900s, carbon dioxide (CO₂) levels in the atmosphere have increased by 50% due to human activities. When fossil fuels (such as coal, oil, and natural gas) are burned for energy production, transportation, and industrial processes, CO₂ is released into the atmosphere. This excess CO₂ acts as a greenhouse gas, trapping heat and causing the air and ocean temperatures to rise. CO₂ emissions play a crucial role in driving climate change.
This warming effect has caused the global average temperature to rise by about 1.1 ºC since the pre-industrial period. This has led to rising in the frequency and intensity of extreme weather events, melting of polar ice caps and glaciers and rising sea levels, shifts in species ranges and increased risk of species extinction, agriculture and food security, and ocean acidification.
To mitigate these impacts, the Paris Agreement aims to limit global warming to well below 2 ºC above pre-industrial levels. The Intergovernmental Panel on Climate Change (IPCC) estimates that a “carbon budget” of about 500 GtCO₂, which corresponds to about ten years at current emission rates, provides a 66% chance of limiting global warming to 1.5 ºC.
Ocean carbon sequestration
The oceans cover more than 70% of the earth’s surface. They store a lot of CO₂. They are the largest carbon sink on the planet, absorbing about 40% of the CO₂ emitted by human activities. They are an important buffer in climate change.
At its current average pH of 8.1, seawater contains 150 times more CO₂ than an equal volume of the air. The seawater locks the atmospheric CO₂ in the form of ions (HCO₃⁻ and CO₃²⁻) and solid precipitates (CaCO₃ and MgCO₃) according to the following reversible chemical reactions:
CO₂ + H₂O ⇆ H₂CO₃
H₂CO₃ ⇆ H⁺ + HCO₃⁻
HCO₃⁻ ⇆ H⁺ + CO₃²⁻
CO₃²⁻ + Ca²⁺ ⇆ CaCO₃↓
CO₃²⁻ + Mg²⁺ ⇆ MgCO₃↓
As CO₂ emissions increase, the ocean absorbs more CO₂, forming more carbonic acid and lowering the ocean’s pH, making it more acidic. As the oceans absorb more CO₂ than they can handle, it could lead to several potential consequences, such as ocean acidification. Ocean acidification can have negative effects on marine life, particularly organisms with calcium carbonate shells or skeletons, such as corals, mollusks, and some plankton species.
Ocean-based carbon removal companies
Several companies are developing ocean-based carbon removal technology, such as Equatic, Planetary Technologies, and Ebb Carbon.
Equatic company has developed a transformative electrolytic method for CO₂ removal that leverages the high concentration of CO₂ in seawater and the enormous abundance of Ca²⁺ and Mg²⁺ cations. The in-situ alkalization of seawater in electrolytic flow reactors forces CO₂ mineralization via reactions between dissolved CO₂ and Ca²⁺ and Mg²⁺ to permanently lock CO₂ as stable carbonate solids and/or as aqueous bicarbonates. The process also produces green hydrogen (H₂) that can be used to fuel the process during intermittency or sold to generate revenue.
Planetary Technologies has developed an approach of Ocean Alkalinity Enhancement by electrochemically producing magnesium hydroxide (Mg(OH)₂) substance via an electrolyzer and safely adding Mg(OH)₂ to seawater by using a floating platform. Planetary’s electrolyzer system electrolyzes sodium sulfate (Na₂SO₄) electrolyte to produce sulfuric acid (H₂SO₄) and sodium hydroxide (NaOH) base in order to convert MgSO₃ minerals into Mg(OH)₂. Planetary has developed a floating platform for safely dispersing Mg(OH)₂ in the ocean for CO₂ sequestering.
Ebb Carbon company has developed an Ocean Alkalinity Enhancement system that uses renewable energy and an electrodialysis technology to produce NaOH base solution. The base solution is added to the seawater in a controlled manner and safely increases the local pH to create a natural chemical reaction that removes CO₂ from the air.
Captura Technology
Captura has developed an ocean-based carbon removal technology that uses renewable energy and the well-established electrodialysis technology. The electrodialysis uses seawater to produce hydrochloric acid (HCl) and sodium hydroxide (NaOH) base solutions. The acid solution is used to acidify the seawater in a tank, causing HCO₃⁻ and CO₃²⁻ ions to decompose into CO₂ gas. CO₂ is then captured and stored. The base solution is used to neutralize the acidified decarbonized seawater to a pH level that is safe for reintroduction to the oceans.
Captura ocean carbon removal system
The diagram below depicts Captura’s ocean-based carbon removal system.
The system mainly comprises a seawater storage tank, liquid-gas membrane contactors, precipitation reactor, ion exchanger, electrodialysis stack, acid/seawater reactor, CO₂ storage tank, and neutralization reactor.
- Seawater storage tank
The seawater storage tank stores seawater which is filtered by a screen.
- Liquid-gas membrane contactors
There are two liquid-gas membrane contactors which are used to remove gas from liquid streams. The first one removes dissolved nitrogen and oxygen gas from the seawater stream before the seawater enters the electrodialysis stack and acid/seawater reactor. The second one removes released CO₂ from acidified seawater.
- Precipitation reactor
The precipitation reactor precipitates CaCO₃ and MgCO₃ from the seawater by adding NaOH base solution produced by the electrodialysis stack, leaving NaCl-based salt water.
- Electrodialysis stack
Electrodialysis stack uses NaCl-based salt water to produce HCl and NaOH solutions.
- Acid/seawater reactor
The acid solution from the electrodialysis acidifies the seawater, causing HCO₃⁻ and CO₃²⁻ ions to decompose into CO₂ gas.
- Neutralization reactor
The neutralization reactor neutralizes the acidified decarbonized seawater by using the NaOH solution produced by the electrodialysis to a pH level that is safe for reintroduction to the oceans.
How does Captura technology work?
As depicted in the diagram above, a water pump pumps a stream of screen-filtered seawater (pH 8.1) from the seawater storage tank. The seawater stream is further filtered by a cartridge filter before passing through the first liquid-gas membrane contactors. The liquid-gas membrane contactors remove dissolved nitrogen and oxygen gasses from the seawater by a vacuum pump.
From the contactor membranes, the degassed seawater stream is divided into two streams.
A large fraction of the seawater stream is introduced into the acid/seawater reactor for decarbonization. The remaining small fraction (<1%) is sent to the electrodialysis stack to produce acid and base solution streams.
Before entering the electrodialysis stack, the seawater is treated in a precipitation reactor, where the addition of NaOH base solution precipitates CaCO₃ and MgCO₃ from the seawater via the following chemical reactions:
Ca²⁺ + HCO₃⁻ + OH⁻ → CaCO₃↓ + H₂O
Mg²⁺ + HCO₃⁻ + OH⁻ → CaCO₃↓ + H₂O
This precipitation pretreatment prevents the precipitation formation within the bipolar membrane electrodialysis stack. The precipitates are then filtered, and the solution passes through an ion exchanger to generate NaCl-based salt water, which is then introduced into the electrodialysis stack.
The electrodialysis stack produces hydrochloric acid (0.4 M HCl) and sodium hydroxide (0.4 M NaOH) base solutions, as described in detail below. The acid solution is introduced into the decarbonization reactor, where the acidification causes seawater to release CO₂ gas according to the chemical reactions:
H⁺ + CO₃²⁻ → HCO₃⁻
H⁺ + HCO₃⁻ → CO₂↑ + H₂O
The acidified seawater (pH is around 4) passes through the second liquid-gas membrane contactors, where the CO₂ gas is removed by a vacuum pump, resulting in acidic decarbonized seawater. The pure CO₂ is stored in a tank for sequestration or industrial applications.
The acidified decarbonized seawater stream (pH > 4) output from the membrane contactors is then introduced into the neutralization reactor, where it is combined with a fraction of the concentrated NaOH stream to raise the pH of the acidified decarbonized seawater to near levels normally found in the ocean.
How does Captura electrodialysis work?
The diagram below illustrates Captura’s electrodialysis stack for producing acid and base solutions. Note that Captura’s electrodialysis stack differs from that of Ebb Carbon company, which develops Ocean Alkalinity Enhancement technology to remove carbon in the oceans.
Captura’s electrodialysis stack comprises a series of stacked electrodialysis cells between a cathode and an anode. Each cell includes three chambers: a salt chamber, an acid chamber, and a base chamber. Anion exchange membranes separate adjacent salt and acid chambers, bipolar membranes separate adjacent acid and base chambers, and cation exchange membranes adjacent base and salt chambers.
The electrode solution (ES) contains a reversible redox couple, sodium ferro/ferricyanide (Na₃/Na₄[Fe(CN)6]), and is re-circulated to minimize any polarization losses associated with concentration overpotentials at the electrodes. Two cation-exchange membranes are employed to selectively transport sodium ions (Na⁺) from the anolyte or towards the catholyte, respectively.
During operation, the flow control system directs salt streams through salt chambers, acid chambers, and base chambers, where dilute salt solution, acid, and base are respectively produced. The acid solution is introduced into the acid/seawater reactor. The base solution is divided into two streams. The precipitation reactor receives one base stream, while the neutralization reactor receives the other. The dilute salt solution is also sent to the neutralization reactor.
The diagram below illustrates the working mechanism of Captura’s electrodialysis stack.
The applied electric field causes chloride ions (Cl⁻) to pass from the salt chambers through the anion-exchange membranes into the acid chambers, and also causes sodium ions (Na⁺) to pass from the salt chambers through the cation-exchange membranes into the adjacent base chambers. In the bipolar membrane, water is dissociated into protons (H⁺) and hydroxide ions (OH⁻). H⁺ transfers into the acid chamber, where it combines with Cl⁻ to form acid (HCl), while OH⁻ transfers to the base chamber, where it combines with Na⁺ to form base (NaOH).
Consequently, the concentrations of acid (HCl) in the acid stream and base (NaOH) in the base stream increase. In other words, the acid and base streams leaving the electrodialysis stack have a higher concentration of acid and base substance, respectively, than the acid and base streams entering the stack. Therefore, the properties of concentration and pH change as each acid/base fluid stream passes through the electrodialysis stack.
The electrode reactions in the cell are a one electron, reversible redox reaction as the following:
Cathode: [Fe(CN)₆]³⁻ + e⁻ → [Fe(CN)₆]⁴⁻
Anode: [Fe(CN)₆]⁴⁻ → [Fe(CN)₆]³⁻ + e⁻
Captura Patent
- US20220144673A1 Electrodialyzer and electrodialysis system for co2 capture from ocean water
Captura Products
Carbon offset credit market
The market value of carbon offset credits varies widely. In current carbon markets, the price of one carbon credit can range from a few cents per metric ton of CO₂ emissions to $15/mtCO₂e (metric tons of CO₂ equivalent) or even $20/mtCO₂e. However, the voluntary carbon offset market, which was worth about $2 billion in 2021, is projected to grow to $10-40 billion by 2030, transacting 0.5-1.5 billion tons of CO₂ equivalent, as opposed to the current 500 million tons. The total value of carbon credits produced and sold to help companies and individuals meet their de-carbonization goals could approach $1 trillion as soon as 2037.
Captura pilot system
The first pilot system was equipped with sensors and instruments to constantly monitor performance and was capable of removing one ton of CO₂ from the atmosphere per year. It began ocean trials in August 2022 at Caltech’s Kerckhoff Marine Laboratories in Newport Beach, California.
The most recent system is a 100-fold scale-up from the company’s first pilot. The new system has already been successfully operating end-to-end in the company’s lab in Pasadena. At AltaSea, Captura’s team conducts technology development and ocean modeling work, which enable the company to validate and improve the efficiency of the pilot DOC system and guide feasibility studies for commercial facilities.
Captura Funding
Captura has raised a total of $13M in funding over 2 rounds, including
Their latest funding was raised on Jan 12, 2023 from a Series A round.
Captura Investors
Captura is funded by 6 investors, including
Hitachi and Caltech are the most recent investors.
Captura Founder
Chengxiang Xiang and Harry Atwater are Co-Founders.
Captura CEO
Steve Oldham is CEO.
Captura Board Member and Advisor
Ed Phillips is Board Member.
