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3.16(106)

3.16(10-7)

in all pH ranges up to less than 7.5. At exactly pH 7.5, the concentrations of HOCl and OCl- are equal and above this pH, OCl- predominates over all chlorine disinfectant species. This reality is more than just a theoretical interest, because HOCl is 80 to 100% more effective than OCl- as a disinfectant (Snoeyink and Jenkins, 1980). We now conclude that the optimum pH range for chlorination is up to 7.0. Beyond this range, OCl- predominates and the disinfection becomes less effective.

The three species Cl2(aq), HOCl, and OCl- are called free chlorine. Although Cl2(aq) is a stronger oxidizer than the other two, it is not really of much use, unless the chlorination is done under a very acidic condition.

Example 17.3 At pH 6.0, calculate the mole fraction of HOCl.

Solution: At pH 6.0 the mole fraction of Cl2(aq is practically zero. From Table 17.4, the mole ratio of HOCl to OCl- at this pH is 31.62.

Therefore,

mole fraction of HOCl =

Expression of chlorine disinfectant concentration. Now that we have detailed the various reactions of the chlorine disinfectants, it is time to unify the concentrations of the chlorine species. By convention, the concentrations of the three species are expressed in terms of the molecular chlorine, Cl2. The pertinent reactions are written as follows:

From these reactions,

fO mg/L NaOCl = nNcOci = 2-32fi;:5 = 0^5 mg/L Cl2 (17.19)

1.0 mg/L Ca(OCl)2 = CC^ = 440.1 = a99 mg/L Cl2 (1720)

Whenever a concentration of a chlorine disinfectant is mentioned, the above equations are implicitly referred to, and this concentration is the equivalent Cl2 concentration. Of course, the equivalent Cl2 concentration of the chlorine gas disinfectant is Cl2.

Reaction mediated by sunlight. Aqueous chlorine is not stable in the presence of sunlight. Sunlight contains ultraviolet light. This radiation provides the energy that drives the chemical reaction for breaking up the molecule of hypochlorous acid. The water molecule breaks up, first releasing electrons that are then needed to reduce the chlorine atom in HOCl to chloride. The overall reaction is as follows:

The O2 comes from the break up of the water molecule, oxidizing its oxygen atom to the molecular oxygen.

The previous reaction in the presence of sunlight is very significant. If the disinfectant is to be stored in plastic containers, then this container must be made opaque; otherwise, the chlorine gas will be converted to hydrochloric acid, and the hypochlo-rites will be converted into the corresponding salts of calcium and sodium.

Example 17.4 A solution of sodium hypochlorite containing 50 kg of NaOCl is stored in a transparent plastic container. It had been stored outdoors for quite some time and then used to disinfect a swimming pool. How effective is the disinfection? What material is used instead for the disinfection? And how many kilograms is it?

Solution: Because the solution is stored outdoors and in a transparent container, the following reaction occurs:

From this reaction, no disinfectant exists in the container and the disinfection is not effective. Ans

The material used instead for disinfection is the salt NaCl. Ans The amount of sodium chloride used to disinfect is

Reactions with inorganics. Reducing substances that could be present in the raw water and raw wastewater and treated water and treated wastewater are ferrous, man-ganous, nitrites, and hydrogen sulfide. Thus, these are the major substances that can interfere with the effectiveness of chlorine as a disinfectant. The interfering reactions are written as follows: with ferrous:

with manganous:

with nitrites:

with hydrogen sulfide:

For activated sludge plants that produce only partial nitrification, it is a common complaint of operators that a residual chlorine cannot be obtained at the effluent. The reason for this is the reaction of nitrites with the chlorine disinfectant producing nitrates as shown in Reaction (17.30).

Example 17.5 An activated sludge plant of a small development is out of order, and a decision has been made following approval from a state agency to discharge raw sewage to a river. The effluent was found to contain 8 mg/L of ferrous, 3 mg/L manganous, 20 mg/L nitrite as nitrogen, and 4 mg/L hydrogen sulfide. Calculate the mg/L of HTH needed to be dosed before actual disinfection is realized. What is the chlorine concentration?

Solution: Examining Reactions (17.22) to (17.33) reveals that the number of reference species is equal to two moles of electrons except Reaction (17.33), which has eight moles of electrons. By considering all the other reactions, the number of milliequivalents of HOCl needed

■ + .. + „„ ... + ____=______... + ,. „ ... + -

2Fe/2 Mn/2 NO2 / 2 H2S /8 2( 55.8 )/2 54.9/2 (14+32)/2

Therefore, n fi™ un!( Ca (OCl) 2^ 0f 40.1 + 2( 16 + 35.5) mg/L of HTH = 54.°8 ip-^HOC--) = 54.0^2>(1^-+16-+^^.5;

From Equation (17.20), the chlorine concentration = 73.69(0.99) = 72.95 mg/L

Reactions with ammonia and optimum pH range for chloramine formation.

Effluents from sewage treatment plants can contain significant amounts of ammonia that when disinfected, instead of finding free chlorine, substitution products of ammonia called chloramines are found. In addition, in water treatment plants, ammonia are often purposely added to chlorine. This, again, also forms the chloramines. Chloramines are disinfectants like chlorine, but they are slow reacting, and it is this slow-reacting property that is the reason why ammonia is used. The purpose is to provide residual disinfectant in the distribution system. In other words, the formation of chloramines assures that when the water arrives at the tap of the consumer, a certain amount of disinfectant still exists.

The formation of chloramines is a stepwise reaction sequence. When ammonia and chlorine are injected into the water that is to be disinfected, the following reactions occur, one after the other in a stepwise manner.

First, it is to be noted that the reaction is expressed in terms of HOCl. By the equivalence of reactions, however, the above reactions can be manipulated if the equivalent amount of the other two species is desired to be known. In monochloram-ines and dichloramines, therefore, the chlorine is combined in ammonia; they are called combined chlorines. As will be shown in subsequent discussions, the concentration of trichloramine is practically zero during disinfection; thus, it is not included in the definition of combined chlorine.

Reaction (17.34) indicates that at the time when one mole of HOCl is added to one mole of NH3, the conversion into monochloramine is essentially complete. In view of the relationship of HOCl and OCl- as a function of pH, however, this statement is not exactly correct. From previous discussions, at pH 7.5, hypochlorous acid and the hypochlorite ion exist in equal mole concentrations, but beyond pH 7.5, the hypochlorite ion predominates. OCl- does not directly react with NH3 to form the monochloramine, but must first hydrolyze to produce the HOCl before Reaction (17.34) proceeds. Thus, when the pH is above 7.5, addition of one mole of HOCl to one mole of ammonia does not guarantee complete conversion into NH2Cl. At these pH values, the one mole of HOCl added becomes lesser, because of the predominance of the hypochlorite ion. HOCl, however, exists at practically 100 concentrations at pH's below 7.0; hence, at this range, a mole for mole addition would essentially guarantee the aforementioned conversion into monochloramine.

Now, Reactions (17.35) and (17.36) indicate that by adding two moles of HOCl and three moles of HOCl, the conversion into dichloramine and the trichloramine are, respectively, essentially complete. For the same reason as in the case of the conversion into monochloramine, these two and three moles are not really these values, because the resulting concentrations depend upon the pH of the solution. Above pH 7.5, the conversions are not complete.

Let us have more discussion regarding the formation of dichloramine. The oxidation state of nitrogen in NH2Cl from where the dichloramine comes from is -1. The oxidation state of the nitrogen in NHCl2, itself, is +1. This means that in order to form the dichloramine, two electrons must be abstracted from the nitrogen atom. Now, the other substances that have been observed to occur, as the amount of hypochlorous acid added is increased, are the nitrogen gas and nitrates. Take the case of the nitrogen gas. The oxidation state of the N atom in the N2 molecule is zero. This means that in order to form the nitrogen gas from NH2Cl only one electron needs to be abstracted from the nitrogen atom; this is an easier process than abstracting two electrons. It must then be concluded that before the dichloramine is formed, the gas must have already been forming, and that for the dichloramine to be formed, more HOCl is needed than is needed for the formation of the gas.

The reaction for the formation of the nitrogen gas may be written as follows:

Let us discuss the formation of the monochloramine versus the formation of the nitrogen gas. The oxidation state of the nitrogen atom in ammonia is -3. And, again, its oxidation state in NH2 Cl is -1. Thus, forming the monochloramine from ammonia needs the abstraction of two electrons from the nitrogen atom. Now, again, the oxidation state of nitrogen in the nitrogen gas is zero, which means that to form the nitrogen gas from ammonia needs the abstraction of three electrons; this is harder than abstracting two electrons. Thus, in the reaction of HOCl and NH3, the monochloramine is formed rather than the nitrogen gas, and the gas is formed only when the conversion into monochloramine is complete by more additions of HOCl.

Consider the formation of the nitrate ion. The oxidation state of nitrogen in the nitrate ion is +5. Thus, this ion would not be formed from ammonia, because this would need the abstraction of eight electrons. If it is formed from the monochloramine, it would need the abstraction of six electrons, and if formed from the dichloram-ine, it would need the abstraction of four electrons. Thus, in the chloramine reactions with HOCl, the nitrate is formed from the dichloramine. We will, however, compare which formation forms first from the dichloramine: trichloramine or the nitrate ion. The oxidation state of the nitrogen atom in trichloramine is +3. Thus, to form the trichloramine, two electrons need to be abstracted from the nitrogen atom. This may be compared to the abstraction of four electrons from the nitrogen atom to form the nitrate ion. Therefore, the trichloramine forms first before the nitrate ion does.

The reaction for the formation of the nitrate ion may be written as follows:

Now, let us discuss the final fate of trichloramine during disinfection. In accordance with the chloramine reactions [Reactions (17.34) to (17.36)], by the time three moles of HOCl have been added, a mole of trichloramine would have been formed. This, however, is not the case. As mentioned, while the monochloramine decomposes in a stepwise fashion to convert into the dichloramine, its destruction into the nitrogen gas intervenes. Thus, the eventual formation of the dichloramine would be less; in fact, much, much less, since, as we have found, formation of the gas is favored over the formation of the dichloramine. In addition, monochloramine and dichloramine, themselves, react with each other along with HOCl to form another gas N2O [NH2Cl + NHCl2 + HOCl — N2O + 4H+ + 4Cl-]. Also, there may be more other side reactions that could occur before the eventual formation of the dichloramine from mono-chloramine. Overall, as soon as the step for the conversion of the dichloramine to the trichloramine is reached, the concentration of dichloramine is already very low and the amount of trichloramine produced is also very low. Thus, if, indeed, trichloramine has a disinfecting power, this disinfectant property is useless, since the concentration is already very low in the first place. This is the reason why combined chlorine is only composed of the monochloramine and the dichloramine. Also, it follows that since dichloramine is, itself, simply decomposed, it is not the important combined chlorine disinfectant; the monochloramine is. If the objective is the formation of the disinfecting chloramines, it is only necessary to add chlorine to a level of a little more than one mole of chlorine to one mole of ammonia in order to simply form monochloramine. Beyond this is a waste of chlorine.

Now, let us determine the optimum pH range for the formation of the monochloramine. The key to the determination of this range is the predominance of HOCl. Hypochlorous acid predominates over the pH range of less the 7.0; therefore, the optimum pH range for the formation of monochloramine is also less than 7.0.

Example 17.6 In order to provide residual disinfectant in the distribution system, chloramination is applied to the treated water. If two moles of HOCl have been added per mole of ammonia, calculate the moles of nitrogen gas produced.

Solution: So many intervening reactions may be occurring during chloramination that it is not possible to determine exactly the amount of resulting species. Experimentally, a sample may be put in a closed jar and chloramination performed. The liberated nitrogen gas may then be measured; but in this problem the moles of nitrogen produced simply cannot be calculated. Ans

Reactions with organic nitrogen. Chlorine reacts with organic amines to form organic chloramines. Examples of the organic amines are those with the groups -NH2, -NH-, and -N =. Parallel to its reaction with ammonia, HOCl also reacts with organic amines to form organic monochloramines and organic dichloramines by the chloride atom simply attaching to the nitrogen atom in the organic molecule. For example, methyl amine reacts with HOCl as follows:

CH3Cl + HOCl ^ CH3NHCl (an organic monochloramine, monochloromethyl amine) + H2O (17.43)

As in the conversion of monochloramine to dichloramine, monochloromethyl amine converts to dichloromethyl amine in the second step reaction as follows:

CH3NHCl + HOCl ^ CH3NCl2 (an organic dichloramine, dichloromethyl amine) + H2O (17.44)

Other nitrogen-containing organic compounds are the amides which contain the group -OCNH2 and -CNH-. The ammonia and organic amine molecules have basic properties. They react readily with HOCl, which is acidic. The organic amides, on the hand, are less basic than the amines are; thus, they do not react as readily to form organic chloramides with hypochlorous acid. They consume chlorine, however, so organic amides as well as organic amines are important in chloramination. Although the organic chloramides and organic chloramines have some disinfecting power, they are not as potent as the ammonia chloramines; thus, their formation is not beneficial. Organic chloramides and organic chloramines are also combined chlorines.

Example 17.7 Show the half reaction that will exhibit the property of organic chloramines as disinfectants.

Solution: A characteristic property of chlorine disinfectant is the conversion of the chlorine atom in the disinfectant into the chloride ion. Thus, in portraying the chemical reaction, the formation of the chloride should always be shown. Let CH3NHCl represent the organic chloramines. Therefore, its half reaction as a disinfectant is as follows:

CH3NHCl + 4H+ + 4e- (from organism disinfected) ^ Cl- + NH+ + CH4

As this half reaction shows, the disinfectant grabs four mole electrons from the organism disinfected per mole of the disinfectant, disabling the organism. Ans

Breakpoint reactions. Figure 17.1 shows the status of chlorine residual as a function of chlorine dosage. From zero chlorine applied at the beginning to point A, the applied chlorine is immediately consumed. This consumption is caused by reducing species such as Fe2+, Mn2+, H2S, and NO-. The reactions of these substances on HOCl have been discussed previously. As shown, no chlorine residual is produced before point A.

In waters and wastewaters, organic amines and their decomposition products such as ammonia may be present. In addition, ammonia may be purposely added for chloramine formation to produce chlorine residuals in distribution systems. Also, other organic substances such as organic amides may be present as well. Thus, from point A to B, chloro-organic compounds and organic chloramines are formed. Ammonia will be converted to monochloramine at this range of chlorine dosage.

Beyond point B, the chloro-organic compounds and organic chloramines break down. Also, at this range of chlorine dosage, the monochloramine starts to convert to the dichloramine, but, at the same time, it also decomposes into the nitrogen gas and, possibly, other gases as well. These decomposition reactions were addressed previously.

Chloramines and Presence of

Chloramines and Presence of

FIGURE 17.1 Chlorine residual versus applied chlorine.

As the curve continues to go "downhill" from point B, the dichloramine converts to the trichloramine, the conversion being complete at the lowest point indicated by "breakpoint." As shown, this lowest point is called the breakpoint. In addition, nitrates will also be formed from the dichloramine before reaching the breakpoint. In fact, other substances would have been formed as decomposition products from monochloramine and dichloramine, as well as other substances would have been formed as decomposition products from the chloro-organic compounds and organic chloramines.

As shown by the downward swing of the curve, the reactions that occur between point B and the breakpoint are all breakdown reactions. Substances that have been formed before reaching point B are destroyed in this range of dosage of chlorine. In other words, the chloro-organics that have been formed, the organic chloramines that have been formed, the ammonia chloramines that have formed, and all other substances that have been formed from reactions with compounds such as phenols and fulvic acids are all broken down within this range. These breakdown reactions have been collectively called breakpoint reactions.

The breakpoint reactions only break down the decomposable fractions of the respective substances. All the nondecomposables will remain after the breakpoint. This will include, among other nondecomposables, the residual organic chloramines, residual chloro-organic compounds, and residual ammonia chloramines. As we have learned, the trichloramine fraction that comes from ammonia chloramines has to be very small at this point to be of value as a disinfectant. All the substances that could interfere with disinfection and all decomposables would have already been destroyed, therefore, any amount of chlorine applied beyond the breakpoint will appear as free chlorine residual.

Important knowledge is gained from this "chlorine residual versus applied chlorine" curve. We have learned that all the ammonia chloramines practically disappear at the breakpoint. We have also learned that the organic chloramines are not good disinfectants. Therefore, as far as providing residual disinfectant in the distribution system is concerned, chlorination up to the breakpoint should not be practiced. Since the maximum point corresponds to the maximum formation of the ammonia monochloramine, the ideal practice would be to chlorinate with a dosage at this point. Note that, in Figure 17.1, appreciable amounts of combined residuals still exists beyond the breakpoint; however, these combined residuals mainly consist of combined chloro-organics, which have little or no disinfecting properties, and combined organic chloramines, which have, again, little or no disinfecting properties. Trichloramine, as we have mentioned, will be present at a very minuscule concentration.

The practice of chlorinating up to and beyond the breakpoint is called super-chlorination. Superchlorination ensures complete disinfection; however, it will only leave free chlorine residuals in the distribution system, which can simply disappear very quickly.

Note: If superchlorination is to be practiced to ensure complete disinfection and it is also desired to have long-lasting chlorine residuals, then ammonia should be added after superchlorination to bring back the chlorine dosage to the point of maximum monochloramine formation.

Example 17.8 Referring to Figure 17.1, if a dosage of 1.8 mg/L is administered, determine the amount free chlorine residual that results, the amount of combined residual that results, and the amount of combined ammonia chloramine residual that results. Also, determine the amount of organic chloramine residual that results.

Solution: From the figure, the concentration of residual chlorine at a dosage of 1.8 mg/L = 0.38 mg/L. The concentration of the residual at the breakpoint = 0.20 mg/L. Therefore, free chlorine residual = 0.38 - 0.20 = 0.18 mg/L Ans amount of combined residual = 0.20 mg/L Ans amount of combined ammonia chloramine — 0 Ans amount of organic chloramine cannot be determined Ans

Reactions with phenols. Chlorine reacts readily with phenol and organic compounds containing the phenol group by substituting the hydrogen atom in the phenol ring with the chlorine atom. These chloride substitution products are extremely odorous. Because phenols and phenolic groups of compounds can be present in raw water supplies as a result of discharges from industries and from natural decay of organic materials, the formation of these odorous substances is a major concern of water treatment plant operators.

Figure 17.2 shows the threshold odor as a function of pH and the concentration of chlorine dosage. Figure 17.2a uses a concentration of 0.2 mg/L and, at a pH of 9.0, the maximum threshold odor concentration is around 28 ^g/L. When the pH is reduced to 8.0 this threshold worsens to around 20 ^g/L, and when the pH is further reduced to 7.0, the threshold concentration becomes worst at around 13 ^g/L. Thus, chlorination at acidic conditions would produce very bad odors compared to chlo-rination at high pH values. This is very unfortunate, because HOCl predominates at the lower pH range, which is the effective range of disinfection.

In Figure 17.2b the concentration of chlorine has been increased to 1.0 mg/L. For the same adjustments of pH, the maximum threshold concentrations are about the same as in Figure 17.2a; however, in the cases of pH's 7.0 and 8.0, the threshold odors practically vanish at approximately 3 to 5 h after contact as opposed to greater than 60 h when the dosage was only 0.2 mg/L. Thus, increasing the dosage produces the worst nightmare for odor production.

Figure 17.3 shows the reaction scheme for the breakdown of phenol to odorless low molecular weight decomposition products using HOCl. The threshold odor concentrations of the various chloride substituted phenolic compounds are also indicated in brackets. Note that the worst offenders are 2-monochlorophenol and 2,4-dichlorophenol, which have an odor threshold of 2.0 ^g/L. In order to effect these breakdown reactions, superchlorination would be necessary, which would also mean that the odor had increased before it disappeared.

Example 17.9 In the reaction scheme of Figure 17.3, what atom has been displaced in ortho chlorophenol by the chlorine atom to form 2,6-dichlorophenol?

Solution: The hydrogen atom in the phenol ring has been displaced by the chlorine. Ans

Time, hr

FIGURE 17.2 Threshold odor from chlorination of phenol: (a) chlorine 0.2 mg/L, initial phenol 5.0 mg/L; (b) chlorine 1.0 mg/L, initial phenol 5.0 mg/L; all at 25°C and threshold odors are concentrations in Jg/L. (From Lee, G. F. (1967). Principles and Applications of Water Chemistry. S. D. Faust and J. V. Hunters (Eds.). John Wiley & Sons, New York. With permission.)

Time, hr

FIGURE 17.2 Threshold odor from chlorination of phenol: (a) chlorine 0.2 mg/L, initial phenol 5.0 mg/L; (b) chlorine 1.0 mg/L, initial phenol 5.0 mg/L; all at 25°C and threshold odors are concentrations in Jg/L. (From Lee, G. F. (1967). Principles and Applications of Water Chemistry. S. D. Faust and J. V. Hunters (Eds.). John Wiley & Sons, New York. With permission.)

Formation of trihalomethanes. Reactions of chlorine with organic compounds such as fulvic and humic acids and humin produce undesirable by-products. These by-products are known as disinfection by-products, DBPs. Examples of DBPs are chloroform and bromochloromethane; these DBPs are suspected carcinogens. Snoeyink and Jenkins (1980) wrote a series of reactions that demonstrate the basic steps by which chloroform may be formed from an acetyl-group containing organic compounds. These reactions are shown in Figure 17.4.

Note that the initial reaction involves the splitting of the hydrogen atom from the methyl group using the hydroxyl ion. The hydroxyl ion is again used in (3), (5), and (7). Because the hydroxyl is involved, this would mean that chloroform formation is enchanced at high pH. To prevent formation of the chloroform, all that is necessary

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