Waterguide - Carbon dioxide

What is carbon dioxide?

In normal conditions, carbon dioxide (symbol CO2, atomic mass 44 g/mol, density 1.98 g/cm3) is a colourless gas with an acidic odour. Under high pressure, CO2 is liquid and can be stored in pressurised gas cylinders. The CO2 molecule is a compound consisting of carbon and oxygen. In water chemistry, CO2 is an important component for the lime-carbonic acid system.

Carbon cycle and climate change

On Earth, carbon dioxide can be found in the atmosphere, the hydrosphere, the lithosphere and the biosphere. The interaction between these spheres forms part of the carbon cycle and is mainly carried out through carbon dioxide. The concentration of carbon in the atmosphere has kept fluctuating over the course of Earth’s history, but has been increasing dramatically since the start of industrialisation, in particular due to the burning of fossil fuels.

CO2 is also created through the cell respiration process of bacteria, animals and humans, who use oxygen to oxidise food and breathe out CO2. Plants, on the other hand, remove CO2 from the air and produce sugar and oxygen during photosynthesis.

Carbon dioxide in the oceans

CO2 is only present in oceans in a dissolved form and as carbonic acid with hydrogen carbonates and carbonates. As the solubility of CO2 in seawater increases the lower the temperature, the amount of dissolved CO2 depends on the time of year and the water temperature. Cold, CO2-rich water sinks to greater depths of the ocean.

The oceans represent an important carbon sink and absorb around a third of anthropogenic carbon dioxide emissions. However, increased amounts of dissolved carbon dioxide in salt water reduces its alkalinity. Ocean “acidification” has a negative effect on many sea dwellers, especially on corals and shellfish.

Carbon dioxide in water treatment

As a gas, carbon dioxide can pass through a membrane and therefore cannot be retained in reverse osmosis. It is possible to separate CO2 using a special process (membrane degasification) or to turn it into hydrogen carbonate by increasing the pH value (by adding caustic soda before reverse osmosis), which can be retained by the RO membrane.

If electrodeionisation (EDI) is to be connected downstream of the reverse osmosis, you must remember to factor in the concentration of CO2 in the design process. The EDI is designed using the so-called Feedwater Conductivity Equivalent (FCE), which is calculated using both conductivity and the concentration of CO2 and SiO2.

If e.g. ultrapure water with very low conductivity (considerably below 1 µS/cm) is needed in microelectronics or for lab or pharmaceutical purposes, you not only need to remove the CO2 (e.g. through membrane degasification), but also factor in the dissolution of CO2 in ultrapure water tanks. CO2 that dissolves from the air can cause the conductivity of ultrapure water to increase fast, meaning that the specifications are no longer adhered to. This is why ultrapure water tanks are sometimes fitted with CO2 filters, or, if the thresholds are very strict, a nitrogen coating is applied, which completely prevents this dissolution process.

Technical application of carbon dioxide

  • Carbonation in soft drinks or mineral water
  • Leavening agent for baking (released by yeast or baking powder)
  • Protective atmosphere for storage/fruit and vegetable packaging
  • Extinguishing agent
  • Refrigerant
  • Protective gas during welding
  • Raw material in the chemicals industry

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