Carbon dioxide (CO2) is a gas that can readily dissolve in water, influencing a variety of natural systems and human-engineered processes. Understanding how CO2 enters aquatic environments is important because it plays a fundamental role in biological and geological cycles. The presence and behavior of dissolved CO2 in water are shaped by both naturally occurring phenomena and deliberate human activities.
Natural Pathways of CO2 Absorption
Atmosphere-ocean exchange is a significant natural pathway for CO2 to enter water bodies. Carbon dioxide gas from the atmosphere dissolves into surface waters, driven by differences in the partial pressure of CO2 between the air and the water. This gas exchange works towards a balance between atmospheric and oceanic CO2 concentrations. Water temperature and salinity also influence how much CO2 can dissolve; colder and less saline waters generally absorb more CO2.
Aquatic organisms contribute to the CO2 content in water through respiration. Plants, animals, and microbes living in aquatic environments release CO2 as a byproduct of their cellular processes. For instance, fish excrete CO2 into the surrounding water. Submerged aquatic plants also respire, especially in darkness, releasing CO2 into the water.
The decomposition of organic matter is another natural source of CO2 in water. When dead plants, animals, and other organic materials break down, microorganisms like bacteria and fungi consume them. This decomposition process releases CO2 into the water as one of its end products. This occurs in aquatic sediments and the water column.
Subsurface sources also introduce CO2 directly into water. Volcanic activity and hydrothermal vents on the ocean floor release CO2 from the Earth’s interior into the surrounding water.
Human-Driven Methods of CO2 Introduction
Carbonation of beverages is a common method, where CO2 gas is intentionally dissolved into drinks like soda water, soft drinks, and beer. Cooling the liquid and infusing it with pressurized CO2 maximizes gas absorption and creates the characteristic fizz.
In industrial settings, CO2 is used for pH control in water and wastewater treatment. Carbon dioxide is injected into water to lower its pH, providing a safer alternative to traditional chemicals. It is also used in chemical manufacturing processes where it may be intentionally or incidentally introduced into water.
Aquaculture, the farming of aquatic organisms, sometimes utilizes CO2 injection. This can control pond water pH or enhance algae growth. CO2 influences water chemistry to optimize conditions for farmed species.
Carbon capture and storage (CCS) technologies involve dissolving captured CO2 into water for transport or storage in geological formations. This method sequesters CO2 from industrial sources, such as power plants, by injecting it into deep saline aquifers or depleted oil reservoirs. Dissolving CO2 in water for injection aids long-term storage and reduces potential leakage.
The Chemistry of CO2 in Water
When carbon dioxide gas (CO2(g)) enters water, it physically dissolves into the aqueous phase, forming dissolved carbon dioxide (CO2(aq)). The amount of CO2 that dissolves depends on temperature and pressure; colder water typically holds more dissolved gas.
After dissolution, dissolved CO2 reacts with water molecules (H2O) to form carbonic acid (H2CO3). This reaction is reversible, allowing carbonic acid to convert back into dissolved CO2 and water. The formation of carbonic acid is a foundational step for subsequent chemical changes.
Carbonic acid is a weak acid that dissociates in a stepwise manner. First, carbonic acid releases a hydrogen ion (H+) and forms a bicarbonate ion (HCO3-). This reaction can be represented as H2CO3 ⇌ H+ + HCO3-. The bicarbonate ion then undergoes a second dissociation, releasing another hydrogen ion and forming a carbonate ion (CO3^2-). This second step is HCO3- ⇌ H+ + CO3^2-.
The release of hydrogen ions (H+) from these reactions directly influences the water’s pH. An increase in hydrogen ion concentration leads to a decrease in pH, making the water more acidic. This chemical transformation of CO2 in water helps explain its impact on aquatic environments.