Chapter 4 Results and discussion 4-1 Ozonation at Variable pH Values
A series of experiments were conducted without phosphate buffer, and at three different ozone doses of 110, 60, and 20 mg/L as detected respectively in the inlet gas stream. The ozone dose was represented as [O3]/[DCF], and the values were 68, 37, and 12. Briefly, the ratios were 3 to 1. Furthermore, the residual diclofenac, total organic carbon (TOC), chloride, ammonia, and aldehyde were evaluated and discussed in the following sections.
4-1-1 Degradation of diclofenac
The degradation of diclofenac with time at variable pH values was shown in Figure 4-1. The diclofenac was almost vanished by ozonation within one hour. For the highest ozone dose, the diclofenac was rapidly oxidized and eliminated to almost zero within the first 5 min. Regarding to the other ozone doses, the concentration of diclofenac decreased rapidly to almost zero within the first 10 and 25 min at the ratio 2 and 1 respectively. As the ozone dose increased about three times from 60 mg/L to 20 mg/L, the time of reaching the same removal rate would be approximately equal to three. It could be concluded that higher ozone dose the diclofenac was removed faster and the removal rate of diclofenac was linearly correlated to the ozone dose.
On the other hand, after the diclofenac was removed, the ozone was still introduced consistently into the reactor and it started to dissolve in the water. As a result, the ozone dose detected in the liquid phase increased. The pH values detected with time decreased from 5.13-5.2 initially to 3.38-3.5 in the end of reaction of 60 min. The change of pH values indicated the not only the increasing dissolved ozone in the solution but also the possible production of acidic compounds during ozonation. Moreover, the removal conditions were also inferred by the data of ozone doses in the liquid phase, which were
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measured with time and presented in Figure 4-2. The concentration of ozone in the liquid phase remained zero when the diclofenac was still present in the solution, and then increased with time, as the concentration of diclofenac was no longer detectable by HPLC.
Fig. 4-1. Degradation of diclofenac as a function of time in the absence of buffer at various O3 doses; (●) O3:110 mg/L; (○) O3:60 mg/L; (▼) O3:20 mg/L.
Experimental conditions: pH0=5.2.
Reaction time (min)
0 10 20 30 40 50 60
DCF/DCF0
0.0 0.2 0.4 0.6 0.8 1.0
O3:110 mg/L O3:60 mg/L O3:20 mg/L
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Fig. 4-2. Ozone concentration in the solution in the absence of buffer at various O3
doses; (●) O3:110 mg/L; (○) O3:60 mg/L; (▼) O3:20 mg/L. Experimental conditions: pH0=5.2.
4-1-2 Degradation of total organic carbon (TOC)
Figure 4-3 shows the TOC removal rate of ozonation of diclofenac at three different ozone doses and without pH adjustment via phosphate buffer. For the highest ozone dose, the TOC removal was 41.59%, the best as expected. However, for the other two ozone doses, the removal efficiency were 32% and 33.74% respectively at ozone dose of 60 and 20 mg/L, respectively.
The TOC removal percentage at distinctive ozone inputs were supposed to be higher as the ozone dose increased. Although the best TOC removal efficiency occurred at the highest ozone dose, the other two ozone doses also yielded percentage TOC removal as expected. The TOC removal efficiency at other ozone doses varied slightly while one was twice of the other. The difference in TOC removal was likely brought by
Reaction time (min)
0 10 20 30 40 50 60
Ozone dosage :O3(liquid) (mg/L)
0 2 4 6 8
O3:110 mg/L O3:60 mg/L O3:20 mg/L
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the complex self-reaction of ozone. As ozone was introduced into the reactor, it would either soon decomposed into free radicals, such as OH radical, or maintained in its original molecular form, which will attack the chemical species in the solution. As for the high ozone dose, the concentration of radicals formed increased. Because free radical is reactive, the diclofenac decomposed easily, formed low-molecular-weight intermediates, and finally becomes mineralized. Therefore, as the ozone dose increased, the TOC removal increased. The two irregular results needed further investigation, which will be discussed in the following sections. This phenomenon perhaps indicates the low ozone dose might be beneficial to decompose diclofenac into short-chain organic compounds which are easily mineralized. In order to discuss more about the results and find the possible correlation, the last two ozone doses were taken as the main operative conditions with fixed pH values, which would be shown in section 4-2.
Fig. 4-3. Degradation of TOC removal as a function of time in the absence of buffer at various O3 doses; (●) O3:110 mg/L; (○) O3:60 mg/L; (▼) O3:20 mg/L.
Experimental conditions: pH0=5.2.
Reaction time (min)
0 10 20 30 40 50 60
TOC removed (%)
0 10 20 30 40 50
O3:110 mg/L O3:60 mg/L O3:20 mg/L
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4-1-3 Formation of chloride
Figure 4-4 shows the concentration of chloride formed during ozonation. The chloride was released consistently at the highest concentration of about 2.2 mg/L in the first 20 - 25 min, and then remained constant or slightly decreased with the last 40 - 35 min respectively at the two higher ozone doses. For the lowest one, the chloride continued to form during the whole process and reached about 2 mg/L finally.
The theoretical amount of chloride released during ozonation was predicted at a maximum of about 2.4 mg/L. It could be seen that the observed results were close to the maximal concentration, which indicated that the oxidation of diclofenac were almost completed in terms of dechlorination. Table 4-1 shows the percentage of chloride formation at the three ozone doses. The percentage of chloride formation was expressed in two terms, the maximum and the final concentration with respect to the theoretical value. In the present s study, the maximal concentration of chloride under one ozone dose was used to calculate the estimated percentage versus theoretical maximum concentration.
In Figure 4-4, the ozone doses seemed to show little correlation with the concentration of chloride formation. But the variance between the theoretical value and experimental data could still infer that ozone dose could affect the level of detected chloride concentration. The delayed time of maximum concentration of chloride and the lower concentration of chloride formation could indicate the minor ozone dose effect of chloride formation. It could be concluded that the lower ozone dose could render diclofenac degraded by various ways.
The chloride was formed as the ozone attacked the diclofenac, and this step was taken as the possible reaction pathway in the beginning. According to some studies, the formation of chloride somehow seems to have the connection with the TOC removal.
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The chloride, as the first step of the ozonation of diclofenac, was supposed to be totally released since the diclofenac had been disappeared in the semi-batch reaction. The above results indicated the lower ozone dose could probably produce the intermediates which might have contained chloride and have strong structure. These intermediates could not be decomposed or oxidized easily and contribute to the residual TOC.
Fig. 4-4. Evolution of Cl- as a function of time during ozonation in the absence of buffer at various O3 doses; (●) O3:110 mg/L; (○) O3:60 mg/L; (▼) O3:20 mg/L.
Experimental conditions: pH0=5.2.
4-1-4 Formation of ammonia
The possible forms of nitrogen compounds produced during the ozonation of diclofenac were monitored under each operational condition. Due to the detection limit of the ion chromatography technique, the nitrite and nitrate were not found in the reaction. Therefore, the ammonia formed from nitrogen of the diclofenac was detected and estimated as the main product.
Reaction time (min)
0 10 20 30 40 50 60
Cl- (mg/L)
0.0 0.5 1.0 1.5 2.0 2.5
O3: 110 mg/L O3: 60 mg/L O3: 20 mg/L
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The presence of ammonia indicated the ability of the oxidation of the nitrogen compounds. Under the condition of various pH values, as shown in Figure 4-5, the concentration of ammonia would reach approximately 0.122 mg/L after 40 minutes of ozonation even at different ozone doses. After 40 minutes of reaction, the decreasing concentration of ammonia at the highest ozone dose showed the better tendency of the oxidation of ammonia, while other two ozone doses showed almost the same results at the end of reaction. The results showed that the ozone dose had little effect on the oxidation of nitrogen in diclofenac when the pH values were not buffered.
The theoretical amount of nitrogen was about 0.47 mg/L in the ozonation of diclofenac. The percentage of ammonia formation was determined in two terms, the maximum and the final concentration with respect to the theoretical value. Moreover, the ozone dose showed little effect on the formation of ammonia. It could be concluded that the ozone dose could not control the pathway of the C-N cleavage.
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0 10 20 30 40
NH4+ (mg/L)
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
O3:110 mg/L O3:60 mg/L O3:20 mg/L
Fig. 4-5. Evolution of NH4+ as a function of time during ozonation in the absence of buffer at various O3 doses; (●) O3:110 mg/L; (○) O3:60 mg/L; (▼) O3:20 mg/L. Experimental conditions: pH0=5.2.
4-1-5 Summary
Table 4-1 compares results of ozone doses at various pH values on diclorfenac degradation. Due to the excess ozone injection, the removal all reached over 99%. The different level of ozone dose caused the time of the same removal at each curve increased by times. As a result, it could be inferred that the ozone dose increased the removal of diclofenac. For TOC degradation, the level of ozone dose showed insignificant effects. Though the highest ozone dose (110 mg/L) presented the best performance, the other ozone doses (60 and 20 mg/L) presented almost the same effectiveness. This phenomenon could also be demonstrated by the results of chloride release. For chloride formation, the results of ozone doses at 110 and 60 mg/L showed
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the same trend that the concentration of chloride reached the maximum at specific time then decreased. However, the concentration of chloride at the lowest ozone dose (20 mg/L) continued to increase during the reaction. It could be concluded that the low ozone dose could render diclofenac degraded into specific intermediates. These intermediates might have smaller structure as their parent compound, contain chloride, and difficult to degrade even after the breakdown of diclofenac. The occurrence of ammonia at three levels of ozone doses showed almost the same trend and reached the highest value at the same time. This may be concluded that the ozone dose barely affected the C-N cleavage, and it also was unrelated to the TOC degradation.
Table 4-1 Comparison of removal rate of DFC and TOC, and formation of chloride and ammonia during ozonation in different operational conditions.
O3:110 mg/L O3:60 mg/L O3:20 mg/L
DFC removal rate (%) >99 >99 >99
TOC removal rate (%) 41.5 32 33.7
Chloride formation rate (%)
Cmax/Cthe(%) 93.9 96.6 80.9
C60/Cthe (%) 90.8 91.0 80.9
Ammonia formation rate (%)
Cmax/Cthe(%) 26.7 29.7 31.9
C60/Cthe (%) 15.9 29.7 28.0
Cmax: The maxima concentration occurred in the reaction Cthe: The theoretical release concentration
C60: The concentration detected in the end of 60 min reaction
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4-2 Ozonation at fixed pH values
According to the results in the previous section, the ozone doses at 60 and 20 mg/L were chosen to investigate the effect of pH on the ozonation of diclofenac. Due to the large amount of residual ozone being introduced into the reactor at the ozone dose 110 mg/L, it was not taken into consideration as one suitable factor in experimental design of evaluating effect of adjusted-pH systems.
To maintain the stability of pH value, the buffer, in combination of monosodium and disodium phosphate, was used to adjust pH to specific values. The pH values were fixed at 5.5, 7.4, and 8.9, respectively. Two levels of ozone dose were also used to conduct the experiments for the degradation and formation of organic and inorganic by-products. The degradation of diclofenac and TOC, and the formation of chloride and ammonia were also detected.
4-2-1 Degradation of diclofenac
The degradation of diclofenac at fixed pH values (5.5, 7.4, and 8.9) at two levels of ozone dose (60 and 20 mg/L) is shown in Figures 4-6 and 4-7, respectively. At the higher ozone dose (60 mg/L), diclofenac degradation reached over 98% between 20 to 25 minutes at three pH values. For diclofenac removal at pH 5.5, 7.4 and 8.9, the results showed almost similar trend, especially at pH 5.5 and 8.9. Although it is insignificant, the diclofenac degradation at pH 7.4 was slightly faster than the other cases.
For the lower ozone dose (20 mg/L), the diclofenac removal reached over 99% at about 40 minutes under all three pH values. The diclofenac degradation obviously differed from 0 to 15 minutes more than the other ozone dose. As it can be seen that diclofenac degraded more rapidly at pH 7.4, and then followed by pH 8.9 and pH 5.5 in a decreasing order.
According to the previous studies (Vogna et al, 2004; Coelho et al, 2009), the rate
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constants would increase as the pH increased (5.0 to 7.0). Lower pH values inhibited the production of OH radical, which is considered as an active reactant that can undergo a series of fast reactions with target compound. Since the OH radical can be produced more extensively at higher pH values, the diclofenac degradation should be better than other lower pH values. For the pH 5.5 and 7.4, the results showed similar trend.
However, the results at pH 8.9 disagreed with what would be predicted.
The different ozone doses affected the diclofenac degradation even at the same pH values. For the delay of time of the turning points in the curves, it can be inferred that larger ozone dose could enhance the diclofenac removal. Besides, the result at larger ozone dose indicated the different levels of pH value got minor effect on the performance of diclofenac removal via ozonation.
Based on the observation mentioned above, it could be inferred that the larger ozone dose bring about more complicated reaction of ozone self-decomposition.
Moreover, the presence of phosphate buffer seemed to interfere more with the reaction of diclofenac at pH 8.9.
The effect of pH on diclofenac degradation
The effects of the presence of phosphate buffer on the degradation of diclofenac can be seen from the results shown in Figures 4-6, 4-7 and Figure 4-1. At the same ozone dose, the presence of phosphate buffer seemed to play a role as inhibitor of diclofenac degradation due to the time delay in reaching the same concentration. The result indicated that the control of pH in semi-batch would caused the diclofenac degrade slower than variable pH condition.
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Fig. 4-6. Degradation profile of diclofenac as a function of time in the presence of buffer at O3 doses 60 mg/L at different levels of pH values; (●) pH=5.5; (○) pH=7.4; (▼) pH=8.9.
Fig. 4-7. Degradation profile of diclofenac as a function of time in the presence of buffer at O3 doses 20 mg/L at different levels of pH values; (●) pH=5.5; (○) pH=7.4; (▼) pH=8.9.
Reaction time (min)
0 10 20 30 40 50 60
DCF/DCF0
0.0 0.2 0.4 0.6 0.8 1.0
pH=5.5 pH=7.4 pH=8.9
Reaction time (min)
0 10 20 30 40 50 60
DCF/DCF0
0.0 0.2 0.4 0.6 0.8 1.0
pH=5.5 pH=7.4 pH=8.9
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4-2-2 Degradation of total organic carbon (TOC)
The degradation of TOC at fixed pH values (5.5, 7.4, and 8.9) at two levels of ozone dose (60 and 20 mg/L) is shown in Figures 4-8 and 4-9, respectively. At the higher ozone dose (60 mg/L), TOC removal reached 43.6, 44.46, and 35.18% at pH values 5.5, 7.4, and 8.9, respectively. For TOC removal at pH 5.5 and 7.4 were almost similar in the first 10 minutes. As for the pH 8.9, the TOC degradation was the lowest and quite different from the other conditions.
For the lower ozone dose (20 mg/L), the TOC removal reached 25.97, 28.81, and 25.41% at pH 5.5, 7.4, and 8.9, respectively. The TOC removal at pH 5.5 was almost the same as at 8.9, while the TOC removal at pH 7.4 was slightly higher. Furthermore, the TOC degradation profiles at three levels of pH values were same, especially in the first 6 minutes.
Comparing the results of two ozone doses at the same pH value, the extent of TOC degraded apparently was more at higher ozone dose. On the other hand, different pH values affected TOC removal, significantly at larger ozone dose. Since the OH radical plays the major role in TOC degradation, the TOC removed should be greater under higher pH. However, the TOC degradation at pH 8.9 was relatively lower than that at other ozone doses, especially at 60 mg/L. This indicated that the OH radical contributed less to TOC degradation at higher ozone dose.
The profile of TOC degradation could be related to the diclofenac degradation in section 4-2-1. All in all, the best TOC removal and diclofenac degradation both occurred at pH 7.4 and two ozone doses. As for pH 5.5, the TOC removal was consistent with diclofenac degradation. For the smaller ozone dose, the TOC removal was consistent with that of diclofenac at pH 8.9. However, at pH 8.9 and larger ozone dose, although the profile of diclofenac degradation was similar that at pH 5.5, the TOC removal was
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lower than that of diclofenac by 8%. It could be concluded that the indirect reaction of OH radicals produced non-degradable intermediates that increased TOC degradation.
The effect of pH on TOC degradation
At ozone dose of 60 mg/L, the presence of phosphate buffer seemed to enhance the TOC removal. It could be inferred that pH control might prevent the indirect reaction and enhance the production of intermediates with smaller molecular weight or simple structure. Therefore, the extent of TOC degradation was greater at pH-controlled than that of pH-uncontrolled condition.
At ozone dose of 20 mg/L, the presence of phosphate exhibited contradictory results. The TOC removal was lower with pH control than that without pH control. This could be attributed to insufficient ozone dose. Since the ozone dose was small, the amount of OH radical formed was decreased due to the presence of phosphate buffer.
Therefore, the contribution of OH radical to TOC removal was reduced.
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Fig. 4-8. Degradation profile of TOC as a function of time in the presence of buffer at O3 doses 60 mg/L at different levels of pH values; (●) pH=5.5; (○) pH=7.4;
(▼) pH=8.9.
Fig. 4-9. Degradation profile of TOC as a function of time in the presence of buffer at O3 doses 20 mg/L at different levels of pH values; (●) pH=5.5; (○) pH=7.4;
(▼) pH=8.9.
Reaction time (min)
0 10 20 30 40 50 60
TOC removed (%)
0 10 20 30 40 50
pH=5.5 pH=7.4 pH=8.9
Reaction time (min)
0 10 20 30 40 50 60
TOC removed (%)
0 5 10 15 20 25 30 35
pH=5.5 pH=7.4 pH=8.9
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4-2-3 Formation of chloride
The formation of chloride at fixed pH values (5.5, 7.4, and 8.9) at ozone dose of 60 and 20 mg/L) is shown in Figures 4-10 and 4-11, respectively. At the higher ozone dose of 60 mg/L, chloride concentration reached about 1.9, 2.0 and 0.65 mg/L at pH values 5.5, 7.4, and 8.9, respectively. The chloride was continuously formed at a faster pace within the first 30 minutes, and then occurred at slower pace after 30 minutes. The amount of chloride formation at pH 5.5 and 7.4, were almost of the same value at about 2.0 mg/L. At pH 8.9, the chloride formation was the lowest comparing to that at the other pH values.
At the lower ozone dose of 20 mg/L studied, chloride concentration reached about 1.4, 1.6, and 0.8 mg/L at pH 5.5, 7.4, and 8.9, respectively. The chloride was formed continuously throughout the course of reaction. The chloride formation profile at pH 5.5 and 7.4 were similar and were relatively lower at pH 8.9.
Due to the high percentage of diclofenac degradation, the amount of chloride released was likely to be the theoretical value of 2.4 mg/L. The incomplete release of chloride indicated the abundant amount of intermediates formed, which was concident with the low TOC degradation. On the other hand, since pH 5.5 was an unfavorable for the formation of OH radical, the amount of chloride formation was small due to the higher chloride concentration at pH 5.5.
At pH 5.5 and 7.4, as the ozone dose decreased, the concentration of chloride decreased, too. In contrast, at pH 8.9, the concentration of chloride increased as the ozone dose decreased. Results demonstrated that ozone dose barely affected the chloride formation at higher pH values.
The effect of pH on chloride formation
Previously, Sein et al. (2008) investigated the presence of OH radical scavenger on