In purified water, carbon dioxide dissociated to form a weak carbonic acid which reduced the capacity of anion exchange resins.
CO2 + H2O ⇔ H2CO3 ⇔ H+ + HCO3-
Impact on Laboratory Water Systems
Dissolved gases can impact the carbonic acid concentration as well as the formation of nitrogen or oxygen bubbles that could have negative effects on processes such as particle counting or spectrophotometry. In microbiological processes where purified water is used in open containers, the water will rapidly re-equilibrate with the gases in the air.
If your feedwater has higher levels of carbon dioxide, it will be worth adding an optional degasser. Speak to your local ELGA specialist for advice on the best solution for your feedwater.
Degassing in PURELAB® Chorus
Carbon dioxide (CO2) is one of the 3 principal gases, along with oxygen and nitrogen, which dissolve from the atmosphere into water. However, unlike the other two, which are uncharged, CO2 carries a slight negative charge and so behaves as a weak anion.
Effects of CO2 in water purification systems
CO2 will degrade the resistivity of purified water and since it exists in equilibrium with carbonic acid (H2CO3) when dissolved in water, lower the pH. Since it does behave as an anion, it will also use up capacity in the resin pack when it is removed. It is worth noting that at “normal” water pH range, RO is not able to remove it at all, so it passes across the membrane with the permeate.
How can CO2 be removed from water?
In essence, CO2 can be removed by either forcing it out of solution or by converting it into a form in which the Reverse Osmosis (RO) membrane can remove it. On large systems, forced-draught degassing towers are used, but on the smaller scale, a degassing membrane is more commonly used. The alternative “conversion method” involves dosing sodium hydroxide solution into the RO feed so that the pH is above 8.5, thus ensuring all the CO2 is converted to HCO3-, which the RO removes very effectively. Of course, a mixed bed resin pack will also remove it very effectively, but this may mean a more frequent pack exchange is needed if the CO2 concentration entering is high.
How ELGA choose to remove carbon dioxide gas from Lab Water
The ELGA LabWater approach is to use a degassing membrane since the flow rates we encounter are relatively low. For CO2 removal, it is usual to pass a sweep flow of low-pressure air across the membrane to encourage the CO2 to leave the water, but we prefer to apply an ejector-induced vacuum to the non-water side to “suck” the CO2 across the membrane.
This has the advantages of less pipework/connections and avoiding the need for compressed air (or a compressor!).
With a degassing membrane, we expect to always achieve well below 5 mg/l of CO2 and usually less than 1 mg/l in the purified water. This will be even lower after a DI polishing pack.
When should CO2 removal be used?
There is an argument for always using it, but it will demonstrate the most benefit (in terms of RO permeate quality and/or downstream DI pack life) on waters which have low pH (<6) and/or high bicarbonate alkalinity (>200 mg/l).
A cautionary note: The pH of RO permeate will always be lower than the feedwater pH. This is normal and is due to the removal of other buffering impurities by the RO, leaving a CO2– rich permeate. No acid has been added!