Captura ($13M to capture CO2 from the air using seawater)

Captura, an American cleantech company founded in 2021, leverages well-established electrodialysis technology to capture carbon dioxide (CO₂) directly from 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.

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 carbon removal system

The diagram below depicts Captura’s ocean-based carbon removal system.

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