Free chlorine removal
Activated carbon (AC) is frequently used in dechlorination processes, or removal of free chlorine from water. Said compound does not come from natural supply sources, like rivers, lakes or wells, and neither is it a contaminant. It is a chemical that is mainly added to water as a disinfectant, although on occasions it is used to control smell and flavour, biological growth or remove ammonia.
HOCl H+ + OCl-(1.1)
The distribution between hypochlorous acid and hypochlorite (CIO-) depends on the pH and the concentration of these species. Both molecular forms are defined as free chlorine and are strong oxidants because when they are added to water, they start by reacting nearly immediately with organic and inorganic impurities and those susceptible to oxidation. The chlorine that reacts at this stage is no longer free and becomes combined.
EThe chlorine that intervenes during this stage of disinfection also combines and is no longer free. Once this stage has ended, it is necessary to remove the residual free chlorine, not only because it is toxic for humans, but because it gives the water a bad flavour and smell, it interferes with industrial processes, harms the majority of the ion exchange resins used in softeners and demineralisers, and affects reverse osmosis membranes.
Although there are several processes for reducing the level of free chlorine in water, dechlorination in a packed bed of granular activated carbon (GAC) it is more cost effective and, therefore, more frequently used. In two of the most common, GAC acts as a reducing agent, in accordance with the following reactions:
HOCl + C* → C*O + H+ + Cl- (1.2)
2HOCl + C* → C*O2 + 2 H+ + 2 Cl- (1.3) where C* represents activated carbon. C*O y C*O2 are superficial oxides.
Shown below is another reaction path, in which carbon only acts as a catalyst:
3 HOCl → HClO3 + 2 H+ + 2 Cl- (1.4)
This is improved when a high percentage of the GAC surface area is already saturated. On the other hand, many other reactions are possible, some of which take place between the free chlorine and the oxides on the surface that existed in the carbon before it was applied. Each one of them can form other more complex groups, with the subsequent release of H+ and Cl-. Another example is:
C*OH + OCl- C*OO- + H+ + Cl- (1.5)
It is necessary to clarify that while GAC is acting as a dechlorinator, it adsorbs the organic matter in the water. Therefore, the higher the level of contamination, the less time it will last as a dechlorinator and, vice versa. It is also worth mentioning that even though the carbon is still removing all the free chlorine, it might not be retaining organic matter, meaning that its capacity to physically adsorb organic molecules finishes before its dechlorination capacity.