Dechlorination with activated carbon

Remove chlorine with granular activated carbon (Dechlorination).

One of the main applications of activated carbon is the dechlorination, or removal of free chlorine from water.

This compound does not come from natural sources of supply, such as wells, rivers or lakes. Nor is it a pollutant, but rather a chemical that is added to water, mainly as a disinfectant, and sometimes to control odor and taste, control biological growth, or remove ammonia.

Dechlorination is a complicated mechanism that can follow different reaction paths in which activated carbon (AC) can intervene as a reactive or a catalyst. Free chlorine can be added to water in the form of liquid chlorine, sodium hypochlorite solution, or calcium hypochlorite tablets or pellets. In either case, the result is the same, with the chlorine dissolved in the form of hypochlorous acid (HOCl), a weak acid that tends to partially dissociate, as follows:

HOCl         H+ OCl

The distribution between hypochlorous acid and hypochlorite ion (OCl) depends on the pH and concentration of these species. Both molecular forms are defined as free chlorine. Both are strong oxidants that, when added to water, begin to react almost immediately with organic and inorganic impurities and are susceptible to oxidation. The chlorine that reacts in this stage is no longer free and becomes combined. The remainder requires some time, which can range from a few seconds to several tens of minutes, depending on its concentration, to exert a biocidal effect on microorganisms. The toxicity of free chlorine is believed to lie in its reaction with the enzymatic system of cells.

The chlorine involved in this disinfection stage is also combined and is no longer free. Once this stage is finished, it is necessary to eliminate residual free chlorine, not only because it is toxic to humans, but because it imparts a bad taste and odor to the water, interferes with industrial processes, and damages most ion exchange resins. used in softeners and demineralisers, and affects the membranes of Reverse osmosis RO.

Although several processes have been developed to lower free chlorine levels in water, fixed bed dechlorination of granular activated carbon (GAC) has been the most cost-effective, and therefore the most common. This is a vertical cylindrical tank with a bed of GAC through which water is circulated.

When the carbon is exposed to free chlorine, reactions take place in which the HOCl or OCl is reduced to chloride ion (Cl ). This reduction is the result of different possible reaction paths. In two of the most common ones, GAC acts as a reducing agent, according to the following reactions:

HOCl + C*    C*O + H+ Cl

2 HOCl + C*   C*O2 + 2 H + 2 Cl
Where C* represents activated carbon. C*O and C*O2 are surface oxides, which gradually take up space, which when blocked, no longer participate in the reaction. Some of these oxides are released into solution. This leaves spaces available again which therefore increase the capacity of the GAC for this reduction reaction. As for Cl, it also accumulates on the surface of the coal during the first moments of operation. As HOCl or OCl continues to reach the surface of the carbon, the reaction slows down a little, and then begins to released the Cl. This slowdown is due to the occupation of space by surface oxides. This occupation continues gradually, while the capacity for both adsorption and dechlorination of AC decreases.

In the above reactions, OCl , instead of HOCl, may be involved, with the difference that no H+is produced. It can be observed that the activated carbon reacts and therefore disappears. If there were no accumulation of surface oxides, the reaction would continue until the complete disappearance of the carbon.
Another reaction path, in which the carbon acts only as a catalyst, is as follows:

3 HOCl  C   HClO3 + 2 H+ + 2 Cl

This is favored when a significant percentage of the GAC surface is already saturated. On the other hand, there are many other possible reactions, some of which take place between free chlorine and surface oxides that were present in the carbon prior to its application. Each of them can form other more complex groups, with the subsequent release of H+ and Cl . An example of these is:

C*OH + OCl         C*OO + H+ + Cl

With all of the above, it can be seen that dechlorination is a complex operation, in which the CAG acts as a chemadsorbent. Several mathematical expressions have been developed that attempt to describe dechlorination in carbon beds, but none of them have been sufficiently accurate.

It should be clarified that at the same time that the CAG acts as a dechlorinator, it adsorbs the organic matter present in the water. Therefore, the greater the organic contamination, the shorter its life span as a dechlorinator, and vice versa. It should also be mentioned that even if the carbon continues to remove all the free chlorine, it may no longer be retaining organic matter. In other words, its capacity for physical adsorption of organic molecules ends sooner than its capacity to dechlorinate. Many water treatments companies whose water contains some organic contaminants mistakenly decide to change the carbon until they find traces of free chlorine in the effluent from the dechlorinator.

In well water treatment plants, water is chlorinated to a concentration of between 1 and 6 mg / l as a free chlorine. After the dechlorinator, the maximum permissible is usually less than 0.1 mg / l. The minimum capacity expected for a GAC in the most difficult cases of dechlorination, i.e. those 3 where organic matters is present in the water, is 400 m of treated water per kilogram of
carbon.

Conditions that affect dechlorination.

  1. By decreasing the particle size of the GAC, the diffusion velocity, and therefore the dechlorination velocity, increases considerably. As a consequence, the lifetime increases. Using the smallest possible particle size is the simplest and most effective way to achieve the highest GAC utilization.
  2. The pH of the influent controls the form of the free chlorine in the water. When its value is 7.6, half of the free chlorine is present as HOCl and half as OCl. The reaction of the HOCl with the activated carbon is much faster than that of OCl . The reaction of HOCl with activated carbon is much faster than OCl . Even at a pH of 4, almost everything is HOCl and at a pH of 10, almost everything is OCl . Therefore, the lower the pH, the faster the reaction, and the result is a longer operating time before free chlorine is detected in the effluent.
  3. The speed of dechlorination increases as the temperature increases, because the viscosity of the water decreases, making the diffusion of free chlorine towards the surface of the GAC faster. As a result, the life of the carbon is also extended.
  4. By increasing the concentration of free chlorine in the influent, the CAG is saturated in less time.
  5. Regardless of the value that the different conditions above may have, the GAC has a high dechlorination capacity in relation to the adsorption of organic contaminants. Therefore, in the case of dechlorinators, the operation in multiple columns in series is not justified, and the optimization should concentrate on the search for the most appropriate operating conditions for a single equipment.

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