Activated carbon is able to adsorb chloramines and so a

combination of chlorination and adsorption on activated carbon can be applied for removal of ammonia.

The most likely reaction for chloramine on activated carbon is a surface oxidation:

C + 2NHCI2 + H20 <=> N2 + 4H+ + 4CI" + CO (8.2)

Furthermore, it is important to know that the CI2/NH3-N oxidized mole ratio is 2:1, for oxidation by this pathway.

The mono-chloramine reaction with carbon appears more complex. On fresh carbon the reaction is most probably:

After this reaction has proceeded to a certain extent, partial oxidation of mono-chloramine is observed, possibly according to the equation:

2NH2CI + CO <=> N2 + H20 + 2H+ + 2CI" + C (8.4) It has been observed that activation of fresh carbon is necessary before mono-chloramine can be oxidized.

However, the reaction of chlorine with ammonia or amino compounds presents a problem in the practice of chlorination of waste water containing such nitrogen compounds.

Fig. 8.1. Breakpoint chlorination.

Figure 8.1 shows the residual chlorine as a function of the chlorine applied. Between points 1 and 2 in the figure, mono- and di-chloramine are formed. The oxidation processes with chlorine occurring between points 2 and 3 give a decline in residual chlorine. Point 3 is called the breakpoint. It corresponds to a stoichiometric ratio of chlorine to ammonium-N of 7.6. It is sufficient to add this amount of chlorine for ammonium removal, provided that the waste water does not contain other components, that are oxidized by chlorine. A ratio of chlorine to ammonium-N of 8-10 is, however, required in most cases in practice. Addition of chlorine in this interval probably produces free nitrogen gas as the predominant product of oxidation. Fair et al (1968) even propose that the reaction involving the formation of NOH as an intermediate, followed by the formation of nitric oxide, NO, could explain the observations between points 2 and 3:

In total:

2NHCI2 + HOCI + 6HzO = 2NO + 5H30+ + 5CI" (8.7)

Further addition of chlorine beyond the breakpoint gives an increasing residue of free chlorine. Chlorine doses below the breakpoint requirement can be used to oxidize ammonia if chlorination is followed by contact with activated carbon (Bauer and Vernon, 1973).

When accidental overdosing of chlorine has occurred or after an intentional addition of large quantities of chlorine to accelerate disinfection, it will be desirable to remove the excess chlorine. This is possible with a reducing agent, such as sulfur dioxide, sodium hydrogen sulfite or sodium thiosulfate:

Oxidative degradation by chlorine is limited to a small number of compounds. Nevertheless, oxidation of these compounds contributes to overall reduction of BOD5 in wastes treated with chlorine. A disadvantage is that chlorinated organic compounds may be formed in large quantities. A variety of chlorine compounds is applied in waste water treatments. For these compounds the available chlorine can be calculated, and is generally expressed as percentage chlorine having the same oxidation ability. Data for the different chlorine-containing compounds are given in Table 8.1.

It can be seen that the actual percentage of chlorine in chlorine dioxide is 52.5, but the available chlorine is 260%. This is, of course, because the oxidation state of chlorine in chlorine dioxide is +4 which means that five electrons are transferred per chlorine atom, while Cl2 only transfers one electron per chlorine atom.

SOz + Cl2 + 2H20 = H2S04 + 2HCI NaHS03 + Cl2 + HzO = NaHS04 + 2HCI 2Na2S203 + Cl2 = Na2S406 + 2NaCI

Hypochlorite is obtained by the reaction of chlorine with hydroxide in aqueous solution:

Table 8.1

Actual and available chlorine in pure chlorine-containing compounds

Table 8.1

Actual and available chlorine in pure chlorine-containing compounds

Mol.

Chlorine equiv.

Actual chlorine

Available

Compound

mass

(moles of Cl2)

(%)

chlorine (%)

Cl2

71

1

100

100

Cl20

87

2

81.7

163.4

CI02

67.5

2.5

52.5

260

NaOCI

74.5

1

47.7

95.4

CaCIOCI

127

1

56

56

Ca(OCI)2

143

2

49.6

99.2

HOCI

52.5

1

67.7

135.4

Chlorinated lime, also called bleaching powder Is formed by reaction of chlorine with lime:

A higher content of available chlorine is present in calcium hypochlorite, Ca(OCI)2. Chlorine dioxide is generated in situ by the reaction of chlorine with sodium chlorite:

Was this article helpful?

0 0
Waste Management And Control

Waste Management And Control

Get All The Support And Guidance You Need To Be A Success At Understanding Waste Management. This Book Is One Of The Most Valuable Resources In The World When It Comes To The Truth about Environment, Waste and Landfills.

Get My Free Ebook


Responses

  • Rosina Bianchi
    Does activated carbon adsorb ammonium containing compounds?
    2 months ago
  • luke
    Does activated carbon remove nitrogen dioxide?
    1 month ago

Post a comment