Water guide - Carbon Dioxide

Carbon Dioxide

Introduction: What is Carbon Dioxide?

Carbon dioxide (CO₂) is a colorless gas with a faintly acidic odor. It has the molecular formula CO₂, an atomic mass of 44 g/mol, and a density of 1.98 g/cm³. Under normal atmospheric conditions, CO₂ exists as a gas; however, under high pressure, it becomes a liquid and can be stored in pressurized gas cylinders.

In water chemistry, carbon dioxide is a crucial component of the lime–carbonic acid equilibrium system, influencing pH balance and alkalinity.

The Carbon Cycle and Climate Change

Carbon dioxide naturally circulates between the atmosphere, hydrosphere, lithosphere, and biosphere, forming part of the global carbon cycle.

• Natural processes: CO₂ is produced through cellular respiration in bacteria, animals, and humans. In this process, oxygen is used to oxidize food, releasing CO₂ as a by-product. Conversely, plants absorb CO₂ during photosynthesis, converting it into sugars and oxygen.

• Human activities: Since the Industrial Revolution, atmospheric CO₂ concentrations have risen dramatically, primarily due to the burning of fossil fuels. This increase is a key driver of climate change and global warming.

Carbon Dioxide in the Oceans

In oceans, carbon dioxide exists mainly in dissolved form, as carbonic acid, bicarbonates, and carbonates. The solubility of CO₂ in seawater increases as temperature decreases, meaning cold water holds more dissolved CO₂. Cold, carbon-rich water sinks to deeper ocean layers, contributing to long-term storage.

The oceans act as a major carbon sink, absorbing about one-third of human-generated CO₂ emissions. However, excess dissolved CO₂ causes ocean acidification, which lowers alkalinity and harms marine life such as corals, shellfish, and plankton.

Carbon Dioxide in Water Treatment

Carbon dioxide plays a significant role in water purification and treatment systems:

• Reverse Osmosis (RO): CO₂, as a gas, can pass through RO membranes and cannot be removed by standard filtration. To manage this, special processes such as membrane degasification are used.

• Conversion to bicarbonates: By raising the pH (e.g., adding sodium hydroxide before RO), CO₂ can be converted into hydrogen carbonate ions, which are retained by RO membranes.

• Electrodeionization (EDI): When designing EDI systems downstream of RO, engineers must account for CO₂ and silica concentrations using the Feedwater Conductivity Equivalent (FCE).

• Ultrapure water production: For applications in microelectronics, laboratories, and pharmaceuticals, CO₂ must be carefully controlled. Dissolved CO₂ can quickly increase the conductivity of ultrapure water, compromising strict purity requirements. Solutions include CO₂ filters or nitrogen blanketing of ultrapure water tanks to prevent contamination.

Technical Applications of Carbon Dioxide

Beyond its environmental role, carbon dioxide has numerous industrial and commercial applications:

• Beverage industry: Carbonation of soft drinks and mineral water

• Food processing: Leavening agent in baking (released by yeast or baking powder)

• Packaging: Protective atmosphere for fruit and vegetable storage

• Safety: Fire extinguishing agent

• Refrigeration: Used as a refrigerant in cooling systems

• Manufacturing: Protective gas during welding

• Chemicals industry: Raw material for producing fuels, plastics, and other compounds

Conclusion

Carbon dioxide is not only a critical greenhouse gas in the carbon cycle but also a key factor in water treatment and industrial processes. Its careful management is essential to reducing environmental impact, ensuring sustainable water use, and maintaining ultrapure water quality in advanced applications.

By understanding the properties and behavior of CO₂, industries and researchers can better address challenges in climate change, water treatment, and resource sustainability.

FAQS

1. Why is carbon dioxide important in water treatment?

CO₂ affects pH balance, alkalinity, and conductivity in water treatment systems. It must be controlled in reverse osmosis and EDI to meet ultrapure water standards.

2. Can reverse osmosis remove CO₂?

No. Because CO₂ is a dissolved gas, it passes through RO membranes. Removal requires membrane degasification or conversion to bicarbonates using pH adjustment.

3. How does CO₂ affect ultrapure water?

Even small amounts of dissolved CO₂ can increase the conductivity of ultrapure water, causing it to fall outside strict purity specifications. This is why CO₂ filters or nitrogen blanketing are often used in storage tanks.

4. What is ocean acidification, and why is CO₂ responsible?

When excess CO₂ dissolves in seawater, it forms carbonic acid, lowering alkalinity. This process, called ocean acidification, harms corals, shellfish, and other marine organisms.

5. What are the main industrial uses of carbon dioxide?

CO₂ is widely used in carbonated beverages, food packaging, welding, refrigeration, firefighting systems, and as a chemical feedstock in various industries.