Dechlorination, or the elimination of free chlorine from water, is one of the principal uses of activated carbon. This substance is not obtained from natural sources like lakes, rivers, or wells. It is not a contaminant either; instead, it is a chemical that is added to water, primarily as a disinfectant but also occasionally to control taste and odour, limit biological growth, or get rid of ammonia.

Dechlorination with activated carbon

Activated carbon (AC) can operate as a reactive or a catalyst during the complex dechlorination process, which can take several different courses.

What are the Stages of dechlorination with activated carbon?

1: Liquid chlorine, sodium hypochlorite solution, calcium hypochlorite pills, or calcium hypochlorite pellets, can all be used to add free chlorine to water. Hypochlorous acid (HOCl), a weak acid with a tendency to partially dissociate, is used to dissolve the chlorine in both cases, and the outcome is the same:

OCl     =    H+ OCl–

2: The pH and concentration of these species affect how, hypochlorous acid and hypochlorite ion (OCl-) are distributed. Free chlorine is a term that describes both molecular types. Both substances are potent oxidants that, when added to water, start to interact with both organic and inorganic contaminants almost immediately.

In this stage of the reaction, the chlorine is no longer free but has mixed. The remainder takes some time to produce a biocidal impact on microorganisms; this amount of time might range from a few seconds to several tens of minutes, depending on its concentration.

3: Free chlorine is thought to be hazardous because of how it interacts with cells' enzyme machinery. In this stage of disinfection, chlorine is mixed and is no longer free. Remaining free chlorine must be removed when this stage is complete, since it is hazardous to humans, gives water a foul taste and odour, obstructs industrial processes, and harms the majority of ion exchange resins. It impacts the reverse osmosis RO membranes and components of softeners and demineralizers.

Although, numerous methods have been developed to reduce the amount of free chlorine in water, fixed bed dechlorination using granular activated carbon (GAC) has proven to be the most practical and widespread. This is a vertical, cylindrical tank that circulates water across a GAC bed.

4: When free chlorine is present, processes occur in which the HOCl or OCl- are reduced, to chloride ions (Cl–). Different probable reaction routes have led to this decline.

According to the following reactions, GAC functions as a reducing agent in two of the most frequent ones:

HOCl + C = CO + H+ Cl–

2 HOCl + C   = CO2 + 2H + 2 Cl–

Surface oxides CO and CO2 increasingly occupy space and, when blocked, cease to participate in the reaction. These oxides release some of their compounds into solution. By making spaces available once more, the GAC's capacity for this reduction response is increased.

5: During the initial stages of operation, Cl- also builds up on the coal's surface. The reaction slows down a little as HOCl or OCl- gets closer, to the carbon's surface and then starts to release Cl-. Surface oxides' occupy of space is the cause of this delay. The ability for both adsorption and dechlorination of AC declines, as this occupation gradually continues.

6: Instead of HOCl, OCl- may be involved in the reactions mentioned above, with the exception that no H+ is generated. It is evident that the activated carbon reacts and eventually vanishes. The process would go on until all traces of the carbon were gone, if there was no build-up of surface oxides.

3 HOCl  =   HClO3 + 2 H+ + 2 Cl–

7: When a sizable portion of the GAC surface is already saturated, this is preferred. On the other hand, there are numerous more potential reactions, some of which include surface oxides that were already present on the carbon, before it was applied.

Each of them is capable of forming further, more complicated groups, which releases H+ and Cl- as a result. As an illustration of these:

C*OH + OCl–      =   COO– + H+ + Cl–

With everything mentioned above, it is clear that dechlorination is a difficult process in which the GAC serves as a chemadsorbent. Dechlorination in carbon beds has been attempted to be described mathematically, using a number of different equations, but none of them have been found to be sufficiently accurate.

Elements that have an impact on dechlorination

· Velocity

The GAC's diffusion velocity and, consequently, its dechlorination velocity both significantly increase, as the GAC's particle size is reduced. As a result, the lifetime lengthens. The simplest and most efficient technique to obtain the highest GAC usage, is to use the lowest particle size feasible.

· pH

The form of free chlorine in the water is regulated by the influent's pH. Half of the free chlorine is found as HOCl and half as OCl, when its value is 7.6. HOCl and activated carbon react with each other much more quickly than OCl-. HOCl reacts with activated carbon significantly more quickly than OCl-.

Nearly everything is HOCl even at a pH of 4, and nearly everything is OCl- at a pH of 10. Therefore, the faster the reaction, the lower the pH, and the longer the working period until free chlorine is found in the effluent.

· Temperature

Because, water viscosity reduces as temperature rises, free chlorine diffuses more quickly towards the GAC's surface, speeding up the de-chlorination process. As a result, the carbon's lifespan is likewise prolonged.

GAC is saturated faster by raising the influent's free chlorine content

Regarding the adsorption of organic pollutants, the GAC has a high dechlorination capability, regardless of the importance of the various parameters. Dechlorinators should not be operated in numerous columns in series as a result, and optimization efforts should instead focus on finding the ideal operating conditions, for a single piece of equipment.

It should be made clear that the GAC adsorbs the organic materials present in the water, while simultaneously acting as a dechlorinator. Its lifespan as a dechlorinator will therefore be shorter the more organic contamination there is, and vice versa. It should be noted that even if the carbon keeps eliminating all free chlorine, it might stop maintaining organic stuff.

In other words, it loses its ability to physically adsorb organic molecules before it loses its ability to dechlorinate. Many water treatment firms make the error of changing the carbon, until they notice evidence of free chlorine in the dechlorinator's effluent, when their water contains certain organic pollutants.

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