Removal of Ammonia from Water by Ozone ... - ACS Publications

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Removal of Ammonia from Water by Ozone Microbubbles Snigdha Khuntia, Subrata Kumar Majumder, and Pallab Ghosh* Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati−781039, Assam ABSTRACT: Ammonia is a major source of water pollution. One of the most common methods for removal of ammonia from water is oxidation. In this work, ozonation of ammonia using microbubbles was studied in a pilot plant. The experimental results indicate that ozone microbubbles were quite effective in oxidizing ammonia. Oxidation of ammonia was effective at high pH and high ozone generation rates. Ozonation was found to occur by direct reaction of ozone with ammonia at the higher pH. However, the hydroxyl radicals were also involved at the lower pH. Bromide ions acted as a catalyst in the ozonation process, and a faster rate of oxidation of ammonia and lower yield of nitrate was observed. The volumetric mass transfer coefficient of ozone in water was determined. It increased with the increasing rate of ozone generation and the pH of the medium.

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chlorine remains in water, which requires further separation.11 Use of ozone is a well-known method for the oxidation of ammonia. The reaction of ammonia with ozone can be expressed as12,13

INTRODUCTION Ammonia is present in variable concentrations in surface and groundwater. The presence of ammonia in water is undesirable due to several reasons. Ammonia is rapidly oxidized by bacteria to nitrite and nitrate, thereby consuming oxygen dissolved in water. Ammonia is a source of nitrogen, which is a nutrient for algae and other forms of plant life, and thus contributes to eutrophication.1 The total ammonia content of water is the sum of that present as NH4+ (at low pH) and that as NH3 (at high pH). Un-ionized ammonia (viz. NH3) is the more toxic form, because it is a neutral molecule, and thus is able to diffuse across the epithelial membranes of aquatic organisms much more readily than the charged ammonium ion. Ammonia can block oxygen transfer in the gills of fish. Fish suffering from ammonia poisoning appear sluggish, and come to the surface of water gasping for air. In marine environments, the safe level of ammonia is below 1 mg/dm3.2 The presence of ammonia reduces the efficiency of common water purification methods, such as chlorination, inasmuch as a part of chlorine is consumed by ammonia. In addition, ammonia is a major source of undesirable odor in sewage and wastewater. Conventional methods of removal of ammonia from wastewater are biological denitrification, stripping, ion exchange, break-point chlorination, and chemical precipitation.3,4 Some of the newer methods are photocatalytic and electrochemical oxidation.5−7 Each of these methods has its own limitation. For example, the biological denitrification process involves a series of reactors for nitrification, denitrification, BOD decomposition, and solid−liquid separation.8 The bacteria used in this process are very sensitive and cannot withstand wide ranges of pH and temperature, halogen compounds, cyanides, and other heavy metals present in ammonia-containing water. The low nitrogen loading rate is another limitation of this process.9 The air-stripping method creates additional air pollution when ammonia is converted from liquid to gas phase.10 Besides this, it is applicable for higher ammonia concentrations (e.g., 1000 mg/dm3), and its efficiency is reduced at low ammonia concentrations. The breakpoint chlorination is frequently used for ammonia removal. However, a large amount of chlorine is required for ammonia oxidation. After the oxidation process, the unused © 2012 American Chemical Society

NH3 + 4O3 → H+ + NO−3 + H 2O + 4O2

(1)

The lifetime of ozone in water is quite short (the half-life is ∼15 min at 298 K at pH = 7),14 and its rate of reaction with ammonia is slow.15 Several works have been reported in the literature12,13,15−25 on oxidation of ammonia by ozone millibubbles. In recent years, ozone microbubbles (viz. bubbles of diameter less than 50 μm) have been extensively used for water treatment.26 The use of microbubbles significantly increases the efficiency of the ozonation process due to the larger gas−liquid interfacial area, slow rising velocity with longer lifetime, and smaller amount of ozone required due to generation of hydroxyl radicals responsible for the oxidation of pollutants. Microbubble-aided ozonation is a faster process as compared to the biological degradation of ammonia, which takes days to complete.27 It can be used for water treatment in large scale.28 In addition, it can be applied at a wider range of pH than the biological nitrification processes. After the ozonation is complete, ozone ultimately self-decomposes to oxygen without introducing any pollutant to water; only the products of oxidation are left in the treated water. So far, hardly any work has been reported in the literature on the removal of ammonia from water using the microbubble technology. In this work, the ozonation of ammonium salts in a pilot plant using a commercial microbubble generator has been reported. The effects of ozone feed rate and pH of the reaction medium on ozonation have been studied. Treatment using oxygen microbubbles has also been reported and compared with that using ozone microbubbles. The mechanism of oxidation of ammonia by microbubbles of these gases has been investigated in detail. The catalytic effect of bromide ions on ozonation efficiency has also been studied. Mass transfer of Received: Revised: Accepted: Published: 318

August 17, 2012 December 11, 2012 December 11, 2012 December 11, 2012 dx.doi.org/10.1021/ie302212p | Ind. Eng. Chem. Res. 2013, 52, 318−326

Industrial & Engineering Chemistry Research

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of pressure. The mean diameter of the microbubbles was 25 μm. The gas intake capacity of the MBG was 1.7 cm3/s. The MBG operated in a recirculation mode and continuously generated the microbubbles. The aqueous phase reaction mixture, contained in a polycarbonate reactor of 20 dm3 capacity, was recirculated through the MBG, as illustrated in Figure 1. The excess gas, which was not dissolved in the aqueous phase, was passed through an ozone destructor [make: Oz-Air (India), model: Dest-50], which catalytically converted the ozone to oxygen and released it to atmosphere. The gas issuing from the reactor was also passed through this ozone destructor. The temperature of the reaction medium increased from 298 to 308 K during ozonation of ammonia by the microbubbles. To keep the temperature of the reaction mass under control, the reactor was placed in a water bath and the temperature of the bath was controlled by an external circulator [make: Jeio Tech (Korea), model: RW-2025G]. The temperature of the aqueous phase within the reactor was controlled at 298 ± 1 K by this method. Samples were withdrawn from the reactor by pipet. The ammonium concentration in the reactor was analyzed by an ion meter [make: Eutech (Singapore), model: Ion Meter 2700] with ammonium-specific electrode [model: NH415XX], which was operable up to pH = 10. In every sample, an ionic strength adjuster was added, which completely converted all ammonia to ammonium. At every 900 s interval, samples were taken out of the reactor and the total ammonia concentration was measured under continuous magnetic stirring. The concentration of ozone in the reactor was measured by a colorimeter [make: Eutech (Singapore), model: C 105] using the ozone-specific reagent (viz. the DPD method).29 The pH of the aqueous phase was measured by a pH meter [make: Eutech (Singapore), model: pH 2700, electrode: 93X218819]. Continuous circulation of the aqueous phase and the release of microbubbles in the reactor ensured a uniform concentration of the ammonium salt and ozone in the reactor. This was verified by randomly withdrawing samples from different parts of the reactor and measuring the ammonium and ozone concentrations. The concentration of nitrate, which was formed by the oxidation of ammonia as per the reaction described by eq 1, was determined by a UV−visible spectrophotometer [make: Thermo Electron (India), model: UV 2300]. The absorbance was measured at 220 nm as described in the literature.29 The volumetric mass transfer coefficient was determined in the nonreacting systems (i.e., in absence of ammonia). Ozone microbubbles were introduced into the reactor by the MBG, as described earlier, at the required pH and ozone generation rate. The pH of the aqueous phase was adjusted to the desired value before supplying ozone. The concentration of ozone in the aqueous phase was measured, as described earlier, by using the ozone-specific reagent. Ozone self-decomposes in water, and the rate of this decomposition increases with the increasing pH of the aqueous phase.30 Therefore, the supply of ozone was continued until the dissolved ozone concentration reached a constant value, which corresponds to the steady state ozone concentration in water. Experimental Conditions. All experiments were performed in an air-conditioned room where the temperature was maintained at 298 K. The variation of temperature in the room was controlled at 298 ± 1 K.

ozone in water by the microbubbles has been studied. The volumetric mass transfer coefficient has been determined, and its variations with the ozone generation rate and the pH of the aqueous medium have been investigated.

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EXPERIMENTAL SECTION Materials Used. Oxygen (>98% purity) was isolated from air. A portion of this oxygen was converted to ozone. The details of these procedures are described in the next section. Ammonium chloride (99.8% assay), potassium nitrate (>98% assay), ammonium sulfate (99.5% assay), sodium carbonate (99.9% assay), and hydrochloric acid (35% assay) were purchased from Merck (India). Sodium hydroxide (>98% assay) was purchased from Rankem (India) and sodium bromide (99% assay) was purchased from Titan Biotech (India). Standard solutions for the ammonium and pH meters were purchased from Eutech (Singapore) and Oakton (USA). Aqueous solutions of the ammonium salts were prepared by using filtered tap water after removing iron from it. Oxidation of Ammonia by Microbubbles. Oxygen was isolated from air by an oxygen concentrator [make: Oz-Air (India), model: HG 03]. It employs the pressure swing adsorption technique to generate high-purity oxygen (>98% by volume) from air. This oxygen was fed to the ozonator [make: Oz-Air (India), model: ISM 10 Oxy EC], which converted the oxygen to ozone by the corona-discharge method. The ozonator had a provision to control the rate of generation of ozone in the range of 0−3 × 10−6 kg/s. The output flow rate of oxygen from the oxygen concentrator was controlled in the range of 30−70 cm3/s. This range was recommended by the manufacturer of the ozonator for its stable operation. The flow rate of the gas mixture (i.e., oxygen and ozone) coming out of the ozonator was measured by a rotameter, which had a range of 8−80 cm3/s. The percentage of ozone in the gas mixture was varied in the range of 0.7−2%. The experimental setup is schematically shown in Figure 1. Some experiments were performed with oxygen microbubbles, in which the gas coming out of the oxygen concentrator was directly used.

Figure 1. Schematic of the experimental setup.

Tap water was filtered through an iron removal cartridge [make: Eureka Forbes (India), model: Iron-Nil]. All ammonium salt solutions were prepared using this water. The gas containing ozone and oxygen was passed into the microbubble generator (MBG) [make: Riverforest Corporation (USA), model: AS MK-III]. The gas was dissolved in water (inside the MBG) by applying a high pressure. After dissolution, the microbubbles were generated by the release 319

dx.doi.org/10.1021/ie302212p | Ind. Eng. Chem. Res. 2013, 52, 318−326


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RESULTS AND DISCUSSION Effect of pH on Ozonation of Ammonia. The dissociation of NH4Cl in water leads to an equilibrium between the ammonium ion (NH+4 ) and free ammonia (NH3), which can be expressed as, NH3 + H 2O ⇌ NH+4 + OH−

Table 1. pH of Some Common Industrial Effluents Containing Ammonia

(2)

As mentioned in the Introduction, the existence of these two species in aqueous solution depends on the pH of the solution. At pH < 7, the ammonium form (NH+4 ) dominates, whereas at pH > 7, the amount of free ammonia (NH3) increases.20 The fraction of free ammonia at a given pH can be calculated from the equation19 c NH3 c NH3 + c NH+4

=

10 pH − 14 Kb + 10 pH − 14

source of ammonia

pH

spent brine23 diluted chemical industrial wastewater31 raw leachate32,33 mine effluents34 anaerobic digestion effluent wastewater35 aquaculture system36

7 7.32 ± 0.17 8.22, 7.53 ± 0.51 7−8 8.5 7.49−7.77

However, the efficiency of the process may be improved by an alkali treatment. During the reaction of ammonia with ozone, the pH of the aqueous medium decreases (as shown later), and the final treated effluent would have a lower pH. The pH plays an important role in the decomposition of ozone into hydroxyl radicals.30 The formation of free radical is a chain reaction which involves the initiation, propagation, and termination steps. Takahashi et al.37 have shown that hydroxyl radicals are formed from ozone microbubbles under strongly acidic conditions in the presence of mineral acids (i.e., HCl, H2SO4, and HNO3). The extreme accumulation of ions around the ozone−water interface (i.e., at the surface of the collapsing microbubbles) transforms ozone to the ·OH radicals. The hydroxyl radicals are stronger oxidizing species as compared to ozone, which is reflected by their standard redox potentials (viz. 2.8 and 2.07 V for ·OH and O3, respectively).30 However, as their occurrence depends on the pH of the medium, the mechanism of oxidation of ammonia can occur either by direct oxidation or via the hydroxyl radicals. According to Hoigné and Bader,18 direct oxidation of ammonia occurs at pH < 9. In the experiments reported in this work, pH of the solution was varied from 6 to 9. Direct oxidation is likely to occur in this range of pH. At low pH, the concentration of free ammonia was practically negligible, and therefore, the rate of oxidation of ammonia was very slow. As the pH was increased, more free ammonia was formed, and the availability of free ammonia enhanced the rate of oxidation. A few methods are available for detecting the role of hydroxyl radicals in the oxidation of ammonia. Carbonate and bicarbonate ions, whenever present in a concentration comparable to that of NH3, may protect it from oxidation by reducing the hydroxyl radical.18,30 The reaction of hydroxyl radicals with carbonate ions yields the carbonate ion radical according to the reaction30

(3)

where Kb is the ionization constant, and its value is 1.774 × 10−5 at 298 K. Equation 3 indicates that the fraction of free ammonia drastically increases with increasing pH. For example, the fraction of free ammonia at pH = 6 is only 6 × 10−4, which increases to 0.4 at pH = 9. According to Hoigné and Bader,18 ozone cannot oxidize NH+4 , but it can oxidize free ammonia either directly or indirectly. This fact has been corroborated by Kuo et al.19 The effect of pH on the ozonation of ammonia at different ozone generation rates is shown in Figure 2. It is observed from

Figure 2. Variation of concentration of ammonia with time in the reactor at different pH and ozone generation rates.

·OH + CO32 − → CO3− ·+OH−

this figure that the increase in pH from 6 to 9 favored the removal of ammonia. With increasing pH, the percentage of ammonia in the ammonia/ammonium mixture increased. At pH = 6, only 19% ammonia could be removed in 7.2 ks. However, a remarkable increase in the removal of ammonia was observed at pH = 9 at all ozone generation rates. The oxidation of ammonia can be very fast when pH > 9, because of the conversion of ammonium to free ammonia. However, the MBG used in this study was operable in the pH range of 6−9. The manufacturer of the MBG recommended this range considering the materials of construction of the internals of the equipment. Therefore, the experiments were conducted in the aforementioned pH range. The pH of industrial effluents containing ammonia generally lies between 7 and 8.5.23,31−36 Some of the common effluents and their pH values are listed in Table 1. It is observed from this table that ammonia oxidation can be effectively carried out for many effluents without adding alkali.

(4)

Figure 3 shows the effect of carbonate on the oxidation of ammonia in the pH range of 6 to 9. At pH = 6, only a very small amount of ammonia was oxidized. However, the addition of carbonate inhibited the reaction almost completely. At pH = 7, the rate of oxidation decreased due to the protective effect of carbonate, whereas at pH = 8 and 9, the effect of addition of carbonate was negligible, and the concentration profiles of ammonia were unaltered. This shows that at pH = 6 and 7, there was generation of free radicals, which oxidized ammonia. At such low pH, where the fraction of free ammonia was small, the oxidation was due to both direct reaction of ammonia with ozone as well as by the reaction with hydroxyl radicals. This also shows that at pH = 8 and 9, oxidation of ammonia was mainly due to the direct reaction with molecular ozone. A similar effect of carbonate ions was observed at the other ozone generation rates. The pH of the aqueous solution inside the 320



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Figure 3. Effect of sodium carbonate (100 mg/dm3) on the oxidation of ammonia at different pH at the ozone generation rate of 5.6 × 10−7 kg/s.

Figure 5. Effect of pH on oxidation of ammonia (at low concentration) by ozone microbubbles at the ozone generation rate of 1.1 × 10−6 kg/s.

reactor slightly decreased with time, as shown in Figure 4, due to the conversion of ammonia to nitrate and formation of the

and oxygen is passed into water in the form of microbubbles, ozone preferentially dissolves in water. The equilibrium concentration of ozone in the aqueous phase can be described by the Henry’s law, which is given by39 ⎛ ρRT ⎞ cg ⎟ cO*3 = ⎜ ⎝ M ⎠H (5) where c*O3 is the equilibrium concentration of ozone in the aqueous phase, ρ is the density of water, R is the gas constant, T is temperature, M is the molecular weight of water, cg is the concentration of ozone in the gas phase, and H is Henry’s law constant. The Henry’s law constant, H, depends on pH and temperature, which is given by40 0.035 − exp( − 2428/ T ) H = 3.84 × 107cOH

(6)

where cOH is the hydroxide ion concentration. At low ozone generation rate, the concentration of ozone in the gas mixture, which formed the microbubbles, was low due to the presence of a large proportion of oxygen in the gas. Therefore, the concentration of ozone in the aqueous phase was low. The velocity of the gas fed to the MBG was the same in all experiments; only the concentration of ozone in the gas varied. The effect of ozone generation rate on ammonia oxidation can be observed from Figure 2. When the ozone production rate was high (e.g., 1.7 × 10−6 kg/s), the concentration of ozone in the aqueous phase was also high, due to which the rate of reaction considerably increased. The concentration profiles of ozone in the reactor at pH = 9 are shown in the Figure 6. Ozone has a short lifetime in water,14 and it spontaneously decomposes. Thus, the profiles shown in Figure 6 depict the amount of ozone, which was available in the reactor after reacting with ammonia, and after its self-decomposition. Ozonation of Ammonium Sulfate. Ammonium chloride was used as the source of ammonia in the work described in the previous sections. The chloride ion has the potential to be oxidized to chlorine by ozone. In addition, chlorine is also capable of oxidizing ammonia. To investigate whether the chloride ion had any role in the ammonia oxidation process, another ammonium salt (viz. ammonium sulfate) was used. Synthetic ammonia solutions were prepared using (NH4)2SO4, and the ozonation experiments were carried out as described in the section entitled Oxidation of Ammonia by Microbubbles. Figure 7 shows the comparison of oxidation of ammonia at pH = 8 using ammonium chloride and ammonium sulfate. It is −

Figure 4. Variation of pH in the reactor during ozonation at different ozone generation rates.

H+ ions (eq 1). Because pH plays such an important role in the ozonation of ammonia, the reduction in pH would slow down the rate of reaction by lowering the amount of free ammonia. The allowable limit of ammonia in water is usually below 1 mg/dm3.2 Therefore, a study was undertaken using a 1 mg/dm3 feed solution of ammonium chloride, and the concentration profiles of total ammonia with time were studied. These are shown in Figure 5. It is observed from this figure that the ozone microbubbles effectively oxidized ammonia and the total ammonia concentration was considerably reduced. Effect of Ozone Generation Rate on Oxidation of Ammonia. In this pilot-plant study of removal of ammonia by ozone microbubbles, the concentration of ozone in the binary gas mixture (i.e., mixture of oxygen and ozone) was small (viz. 0.7−2% by volume). This was necessary as per the design of the ozonator so that a continuous stable operation was possible. Many commercial ozonation plants for water treatment use a similar concentration of ozone in the feed gas. Increase in the concentration of ozone in the gas fed to the MBG increased the rate of oxidation of ammonia due to the enhanced concentration of ozone in the aqueous phase. The solubility of ozone in water is several times higher than that of oxygen. To illustrate, the mole fraction solubility of oxygen in water is 2.3 × 10−5 and the same for ozone is 9.1 × 10−5 at 298 K and atmospheric pressure.38 Therefore, when a mixture of ozone 321



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Figure 8. Effect of pH on oxidation of ammonia by oxygen microbubbles at the oxygen flow rate of 4.2 × 10−5 m3/s.

Figure 6. Ozone concentration profiles in the reactor at different ozone generation rates at pH = 9.

the pH of the medium needs to be increased for the availability of free ammonia. However, this would diminish the generation of free ·OH radicals from the oxygen microbubbles. The results presented in this section clearly indicate that the oxidation of ammonia in natural waters by oxygen would be very slow, and practically unimportant. Therefore, the oxygen demand exerted by ammonia would be insignificant. Ozonation of Ammonia using Bromide Catalyst. Equation 1 shows that ammonia reacts with ozone and forms nitrate. Nitrate is hazardous for humans and animals.42 According to Yang et al.,24 there is a possibility of formation of nitrite by direct reaction of ammonia with ozone. However, nitrite further reacts with ozone, and finally nitrate is formed. These reactions are shown below.

Figure 7. Comparison of ozonation of NH4Cl and (NH4)2SO4 at pH = 8 at three ozone generation rates.

observed from this figure that ozonation of ammonia was practically the same for both the ammonium salts. No significant variation was observed at other pH as well. This confirms that neither chloride nor sulfate had any role in the ozonation of ammonia. Oxidation of Ammonia using Oxygen Microbubbles. It has been reported in the literature41 that free hydroxyl radicals are generated from collapsing oxygen microbubbles under strongly acidic conditions. These free radicals have been detected by using the electron-spin-resonance method. At strongly acidic conditions (e.g., pH = 2), generation of ·OH radicals was enhanced by the use of copper catalyst. Therefore, the possibility of oxidation of ammonia by the free radicals generated from oxygen microbubbles was explored in this work. Experiments were conducted at pH = 6, 7, 8, and 9. The concentration profiles of ammonia at these values of pH are shown in Figure 8. It is observed from this figure that a small amount of ammonia was oxidized at pH = 7. However, there was hardly any oxidation at pH = 9. Practically, there was no change in the concentration of ammonia with time at this pH. On the other hand, at pH = 6 and 8, there was a small decrease in the ammonia concentration. These results indicate the possibility of oxidation of ammonia by the hydroxyl radicals at low pH. At pH = 6, the amount of free radical is likely to be more than that at pH = 8, but the free ammonia available for oxidation would be very small. It appears that the most favorable condition was attained at pH = 7. The oxidation of ammonia using oxygen microbubbles, therefore, cannot be considered as an effective process. For oxidation of ammonia,

NH3 + 3O3 → NO−2 + 3O2 + H+ + H 2O

(7)

NO−2 + O3 → NO−3 + O2

(8)

13,15,16,21,24,25

Several works have demonstrated that nitrate, thus formed, can be converted to nitrogen in the presence of a bromide salt. Some types of water that are subjected to ozonation often contain small amounts of bromide. For example, the concentration of bromide found in surface water is in the range of 0−0.8 mg/dm3,43 and in groundwater it is present in the range of 0−2 mg/dm3.44 The average bromide concentration in seawater is about 70 mg/dm3.16,45 The reaction of bromide ion with ozone follows the following sequence of reactions.46 O3 + Br − → OBr − + O2 −

−

O3 + OBr → Br + 2O2 −

2O3 + OBr →

BrO−3

H+ + OBr − ⇌ HOBr

+ 2O2

(9) (10) (11) (12)

It is observed from eq 11 that ozonation of bromide yields bromate, BrO−3 , which is carcinogenic in nature. According to the World Health Organization (WHO), the maximum permissible concentration of bromate in drinking water is 25 μg/dm3.47 It has been reported in the literature46 that even though the ozonation of Br− forms BrO−3 , however, this conversion is inefficient due to the cyclic regeneration of Br−, which is formed by an intermediate, OBr−, as shown by eqs 9 and 10. HOBr (sometimes termed “active bromine”) is the important intermediate, which can brominate ammonia. The active bromine forms slowly in common drinking water 322



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treatment practice. Further oxidation to BrO−3 is slow at low pH. According to Tanaka and Matsumura,16 formation of BrO−3 does not occur if ammonia is present in water. However, the production of bromate increases with increasing pH because a higher pH favors the formation of OBr−, which enhances the formation of BrO−3 , as shown by eq 11. The following reactions are involved when ammonia reacts with ozone in presence of bromide.16,23 HOBr + NH3 → NH 2Br + H 2O

(13)

HOBr + NH 2Br → NHBr2 + H 2O

(14)

HOBr + NHBr2 → NBr3 + H 2O

(15) −

+

Figure 9. Concentration profiles of ammonia and nitrate in the reactor in presence of sodium bromide.

2H 2O + NHBr2 + NBr3 → N2 + 3Br + 3H + 2HOBr (16)

Tanaka and Matsumura23 have suggested three pathways of ozonation of ammonia. The first pathway is described by eqs 13−16, in which hypobromous acid reacts with ammonia, and intermediates such as NH2Br, NHBr2, and NBr3 are formed. Finally, these compounds are converted to nitrogen, as depicted by eq 16. A different reaction for the formation of N2 (which may be viewed as an alternative of eq 16) has been proposed by Yang et al.24 This is given by NH 2Br + NHBr2 → N2 + 3Br − + 3H+

the nitrate concentration profile shifted to the higher times under these conditions. Therefore, a low bromide/ammonia molar ratio was effective in oxidizing ammonia to nitrate, and subsequently to nitrogen. The conversion of ammonia to nitrate was faster as well. Mass Transfer of Ozone by Microbubbles. The microbubbles are efficient for the transfer of ozone from the gas phase to the aqueous phase.26 Mass transfer of ozone in the microbubble systems was studied in the pH range of 6−9 at three ozone generation rates. The concentration of ozone dissolved in water increased with time, and asymptotically reached a constant value, which corresponds to the steady state concentration of ozone in water (css). The concentration profiles of ozone in water at different ozone generation rates are depicted in Figure 10. It is observed from Figure 10 that the

(17)

The overall reaction of ammonia with active bromine, as per eqs 13−15, is given by16 3HOBr + 2NH3 → N2 + 3Br − + 3H+ + 3H 2O

(18)

The second pathway is described by the following reaction, in which nitrate is formed by the reaction of NH2Br with ozone. 3O3 + NH 2Br → NO−3 + Br − + 2H+ + 3O2

(19)

The third pathway involves direct reaction of ammonia with ozone, and the formation of nitrate, as given by eqs 7 and 8. The hypobromous acid and hypobromite are in equilibrium with each other. The former reacts very slowly with ozone, and does not contribute significantly to bromate formation. It has been reported by Gunten and Hoigné47 that ammonia causes a lag time for the formation of bromate. For the molar ratios of cHOBr/cNH+4 > 1, NHBr2 is formed, which reacts four times more slowly with ozone than NH2Br.15 Further reactions in the bromine−ammonia system involve NHBr2 and NBr3, as described by eq 16. This reaction leads to a further delay in the bromate formation. In this work, ozonation of ammonia in presence of bromide was carried out by keeping the concentrations of the ammonia and bromide in the molar ratio of 18:1. As per eq 18, the molar ratio of ammonia and bromide for the complete reaction is 2:3. But after the complete oxidation of ammonia, the bromate formation can be significant at the high bromide concentrations. To avoid the additional bromate formation, the experiments were carried out at a low ratio of bromide to ammonia. Figure 9 shows the depletion of ammonia with time. It is observed from this figure that ammonia was removed at a faster rate than that shown in Figure 2 (i.e., in absence of bromide). The concentration of nitrate first increased, and subsequently decreased, inasmuch as it was converted to nitrogen. At lower pH and lower ozone generation rates, the concentration of nitrate was higher, which indicates that the conversion to nitrogen was less. Furthermore, the maximum of

Figure 10. Variation of ozone concentration in the reactor in absence of ammonia at different ozone generation rates.

saturation concentration of dissolved ozone increased with increasing ozone generation rate. From eq 5, it can be observed that the value of c*O3 depends on cg, and hence, on the ozone generation rate. Therefore, from the foregoing discussion, it is apparent that the higher ozone generation rate increased the value of css and c*O3. For the determination of the volumetric mass transfer coefficient of ozone, the rate of self-decomposition of ozone needs to be considered. It has been reported in the literature30,40,48 that the decomposition of ozone in water depends on pH and temperature, and that it follows a firstorder kinetics. When ozone is absorbed into water from a gas mixture and it simultaneously undergoes decomposition in a completely mixed reactor, the mass balance equation is given by 323


Industrial & Engineering Chemistry Research dcO3 dt

= kla(cO*3 − cO3) − kdcO3

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Table 2. Volumetric Mass Transfer Coefficient (kla) at Different pH and Ozone Generation Rates

(20)

volumetric mass transfer coefficient, kla × 103 (s−1)

where cO3 is the concentration of ozone in the aqueous phase, kla is the volumetric mass transfer coefficient, and kd is the rate constant for decomposition of ozone. The diffusivity of ozone in the gas phase is much larger than that in water, so the resistance to mass transfer in the gas phase is negligible, compared to that in the liquid phase. The equilibrium concentration of ozone in the aqueous phase (cO*3) is related to its steady state concentration (css) by the following equation.40

⎛ kla ⎞ css = ⎜ ⎟cO* ⎝ kla + kd ⎠ 3

Ġ O3 = 5.6 × 10−7 Ġ O3 = 1.1 × 10−6 pH (kg/s) (kg/s) 6 7 8 9

dt

= (kla + kd)(css − cO3)

2.4 2.8 3.6 4.0

2.5 3.2 7.8 13.3

CONCLUSIONS The following conclusions were reached based on the experimental studies and the analysis of the results. (1) Ammonia present in water is effectively oxidized by ozone microbubbles. The oxidation is effective under alkaline conditions (i.e., at pH > 7). The effectiveness of ozonation of ammonia increases with increasing pH. Even for low-ammonia feed concentrations (e.g., 1 mg/ dm3), ozonation is quite effective. (2) At pH ≥ 8, ozonation of ammonia seems to occur by the direct reaction with molecular ozone. On the other hand, at pH = 6 and 7, hydroxyl radicals also contribute in the oxidation process. (3) The rate of ozone generation has a significant effect on the ammonia oxidation. The concentration of ozone in the gas coming out of the ozonator (and fed to the microbubble generator) increases with increasing ozone generation rate. As a result, the concentration of ozone in the aqueous phase increases, and the rate of ozonation is increased. (4) During the ammonia ozonation process, the pH of the reaction medium decreases with time due to the formation of H+ ions. (5) The anions of the ammonium salts do not have any effect on the ozonation of ammonia. No difference was found between ammonium chloride and ammonium sulfate in this regard. (6) Oxygen microbubbles are not effective in oxidizing ammonia. (7) The rate of oxidation of ammonia by ozone becomes faster in presence of bromide ions. The advantage of the use of bromide is that the nitrate formed by the oxidation of ammonia is ultimately converted to nitrogen. Water containing bromide salts can take advantage of this fact. (8) As the concentration of ozone in the gas phase increases, the equilibrium ozone concentration in the aqueous phase increases. This results in a higher solubility of ozone in water. The volumetric mass transfer coefficient increases with increasing ozone generation rate. (9) The volumetric mass transfer coefficient increases with increasing pH as a result of the faster rate of selfdecomposition of ozone at the high pH, which enhances the rate of mass transfer.

(21)

(22)

Equation 22 can be integrated with the boundary condition that at t = 0, cO3 = 0. This gives ⎛ c ⎞ ss ⎟⎟ = (kla + kd)t ln⎜⎜ ⎝ css − cO3 ⎠

1.9 2.2 3.2 3.7

kd × 104 (s−1)48

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From eqs 20 and 21, the mass balance equation for ozone can be written as dcO3

1.7 1.9 2.1 2.6

Ġ O3 = 1.7 × 10−6 (kg/s)

(23)

Equation 23 predicts that a plot of ln[css/(css − cO3)] versus t would be straight line passing through the origin (see Figure 11), and the slope of this line would be (kla + kd). From the slope, the value of kla can be computed, if the value of kd is known.

Figure 11. Determination of volumetric mass transfer coefficient of ozone in water by microbubbles.

The values of kla are presented in Table 2. These values agree well with those reported in the literature49−51 for similar systems. It is observed from Table 2 that kla increased with increasing ozone generation rate. Increase in the ozone generation rate increased the rate of mass transfer from the gas phase to the aqueous phase, which is evident from the concentration profiles shown in Figure 10. Increase in pH also increased the value of kla. With the increasing pH, the rate of decomposition of ozone increased, which enhanced the rate of mass transfer of ozone. The time taken to reach the steady state concentration decreased with increasing pH.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91.361.2582253. Fax: +91.361.2690762. Notes

The authors declare no competing financial interest. 324



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ACKNOWLEDGMENTS We thank the Department of Science and Technology (Water Technology Initiative), Government of India, for the financial support of this work, through the grant number: DST/TM/ WTI/2k10/266/(G), dated September 5, 2011.

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NOTATION cg = concentration of ozone in the gas phase (mol/m3) cHOBr = concentration of hypobromous acid in the aqueous phase (mol/m3) cNH3 = concentration of ammonia in the aqueous phase (mol/m3) cNH+4 = concentration of ammonium in the aqueous phase (mol/m3) cO3 = concentration of ozone in the aqueous phase (mol/m3) cO*3 = equilibrium concentration of ozone in the aqueous phase (mol/m3) cOH− = concentration of hydroxide ion in the aqueous phase (mol/m3) css = steady state concentration of ozone in the aqueous phase (mol/m3) Ġ O3 = ozone generation rate (kg/s) H = Henry’s law constant (Pa/mol fraction) kd = first-order decomposition rate constant of ozone (s−1) kla = volumetric mass transfer coefficient of ozone (s−1) Kb = ionization constant of ammonia M = molecular weight of water (g/mol) R = gas constant (J mol−1 K−1) t = time (s) T = temperature (K)

Greek Letters

ρ = density of water (kg/m3) Abbreviations

BOD = biological oxygen demand MBG = microbubble generator WHO = World Health Organization

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