How Chlorine Kills Pathogens in Pool Water: Understanding its Mechanism

Chlorine is one of the most widely used disinfectants for water treatment, thanks to its effectiveness in deactivating a wide range of microorganisms and its relatively low cost. This powerful oxidizer, when added to water, can destroy pathogens, including bacteria and viruses, by breaking their chemical bonds. Let's delve deeper into the fascinating process of how chlorine neutralizes these harmful microorganisms in pool water.

Chemical Reaction of Chlorine in Water

When chlorine is introduced into water, it forms hypohalous acids, primarily hypochlorous acid (HOCl), under specific pH conditions. The reaction can be summarized as follows:

Cl? H?O → HOCl HCl

Depending on the pH, hypochlorous acid can further dissociate into hypochlorite ions (OCl?) and hydrogen ions (H?), as shown:

HOCl H?O → H?O? OCl?

These hypohalous acids and hypochlorite ions then decompose into chlorine and oxygen atoms, releasing atomic oxygen, which is a potent oxidizer and a highly effective disinfectant.

Disinfection Mechanism of Chlorine

The disinfection process of chlorine in water is based on two key reactions: the oxidation of microorganism cell walls and the substitution reaction of enzymes in the cells. The oxygen atoms and hypochlorite ions play crucial roles in this process.

Free Chlorine Formation

When underchloric acid (HOCl) and hypochlorite ions (OCl?) combine, they produce 'free chlorine', which is the active form of the disinfectant. Free chlorine is responsible for the majority of the disinfection process because of its higher reactivity and effectiveness compared to hypochlorite ions. This free chlorine breaks down into hydrochloric acid (HCl) and atomic oxygen (O).

Penetration and Disruption of Pathogens

Pathogenic microorganisms have naturally negatively charged cell walls, which makes them more susceptible to the neutral hypochlorous acid (HOCl) than the negatively charged hypochlorite ions (OCl?). Hypochlorous acid effectively penetrates slime layers, cell walls, and protective layers of bacteria, thereby killing the pathogens.

Microorganisms are directly affected by the atomic oxygen released from the breakdown of hypochlorous acid. Atomic oxygen disrupts the integrity of cell walls and damages cellular components, leading to cell death or reproductive failure in the bacteria.

Conclusion

In summary, the effectiveness of chlorine in killing pathogens in pool water is underpinned by its powerful oxidizing properties and its ability to break down the cell structures of microorganisms. Whether it’s in the form of free chlorine or hypohalous acids, chlorine ensures that swimming pools remain safe and hygienic by neutralizing harmful pathogens.

References

[1] Chlorination and Disinfection

[2] Swimming Pool Water Pollution Control