R 2 CHNH 2
4.5 Ozone initiated oxidation of 2-chloroethanol in aqueous medium
4.5.1 Effect of pH on the ozonation of 2-chloroethanol
It is widely known that pH plays a major role in the decomposition of ozone in solution.
Kurniawan et al.136 reported that in alkaline conditions (pH >8), once the reaction of free radical was initiated by ozone, a series of oxidative degradation reactions that completely oxidized the recalcitrant organic material into CO2 and H2O occurred in the solution.
Therefore, the effects of pH on the treatment performance of ozonation were studied by varying the solution pH from 4 to 10, while keeping ozone dose concentration and temperature of the reaction mixture constant.
To evaluate the effect of solution pH on the conversion of 2-chloroethanol, 100 μg mL-1 of ozone gas was constantly bubbled through 20 mL aliquots of 10 % 2-chloroethanol in water at pH 4, 7 and 10 respectively. Gas chromatographic analysis of the product mixture was performed after each time interval. Peaks for AcAlde and AA eluted at their respective retention times with no other new product peak appearing. Table 4.12 and Figure 4.20 summarize the reaction data for the conversion of 10 % 2-chloroethanol in water at pH 4 and selectivity profiles of AcAlde, AA and the unknown products.
Table 4.12 Conversion and selectivity data for ozonation of 10 % 2-chloroethanol in water at pH 4 as a function of time
Ozonation
Time (h) Conversion
(%) Selectivity (%)
AcAlde AA Unknown
3 10.9 63.7 31.1 5.2
6 20.4 62.0 30.1 8.0
9 25.1 57.3 28.8 13.9
12 29.3 55.6 27.2 17.2
* Oxygen flow rate = 200 mL min-1 * Ozone concentration = 100 g mL-1
An observation of the data in Table 4.12 and Figure 4.20 shows that percent conversion of 2-chloroethanol increases with ozonation time. Percentage conversion of the substrate increases from 10.9 % after 3 h ozonation to 29.3 % after 24 h ozonation. It is evident
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that the conversion rate of 2-chloroethanol increases sharply from 3 h to 6 h and thereafter increases gradually up to 12 h.
Figure 4.20 Percent conversion of 10 % 2-chloroethanol in water at pH 4 and selectivity profiles of AcAlde, AA and unknown versus ozonation time
This pattern of m-xylene conversion indicates that ozone dissolution increases with increasing contact time with the substrate solution. This is in agreement with Ozomax Ltd who reported that the maximum number of moles of O3 transferred to the solution may be calculated from:
Mozone = NA x S x t Where: Mozone = number of moles O3 transferred
NA = Avogadro’s number
S = total bubble surface area, and t = contact time.
This equation reveals that ozone dissolution in the substrate solution increases with increasing contact time. The results show that the selectivity’s towards AcAlde and AA is high after 3h ozonation and steadily decreases with an increase in ozonation time.
This is due to the oxidation of AcAlde to AA, implying the existence of consecutive reactions for these products at longer ozonation times.
0 10 20 30 40 50 60 70
2 3 4 5 6 7 8 9 10 11 12
Conversion and Selectivity (%)
Ozonation time (h)
conversion - 2-CE S - AcAlde S - AA S - unknown
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The effect of increasing the pH of the substrate solution to neutral (pH = 7) shows a marginal increase in conversion of 2-chloroethanol as ozonation time is increased.
Table 4.13 and Figure 4.21 summarize the reaction data for the conversion of 10 % 2-chloroethanol in water at pH 7 and selectivity profiles of AcAlde, AA and unknown
products.
Table 4.13 Conversion and selectivity data for ozonation of 10 % 2-chloroethanol in water at pH 7 as a function of time
Ozonation
Time (h) Conversion
(%) Selectivity (%)
AcAlde AA Unknown
3 14.5 61.7 32.5 5.9
6 22.7 57.5 34.1 8.4
9 30.8 54.7 33.5 11.7
12 32.8 52.7 41.4 5.9
* Oxygen flow rate = 200 mL min-1 * Ozone concentration = 100 g mL-1
Figure 4.21 Percent conversion of 10 % 2-chloroethanol in water at pH 7 and selectivity profiles of AcAlde, AA and unknown versus ozonation time
0 10 20 30 40 50 60 70
2 3 4 5 6 7 8 9 10 11 12 13
Conversion and Selectivity (%)
Ozonation time (h)
conversion - 2-CE S - AcAlde S - AA S - unknown
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An observation of the data in Table 4.13 and Figure 4.21 shows that percent conversion of 2-chloroethanol increases with ozonation time. Percentage conversion of the substrate increases from 14.5 % after 3 h to 32.8 % after 12 h when treated with 100 g mL-1 ozone. It is evident that the conversion rate of 2-chloroethanol increases sharply during the first 9 h of ozonation and thereafter increases gradually up to 12 h. The oxygenated product AcAlde showed the highest selectivity, starting at 61.7 % after 3 h ozonation and thereafter decreasing to 52.7 % after 12 h ozonation. This pattern of product formation indicates that AcAlde is readily oxidized to its corresponding carboxylic acid in aqueous medium at neutral pH.
Table 4.14 and Figure 4.22 summarize the reaction data for the conversion of 10 % 2-chloroethanol in water at pH 10 and selectivity profiles of AcAlde, AA and unknown products. The effect of increasing the pH of the substrate solution to alkaline shows a rapid increase in conversion of 2-chloroethanol as ozonation time is increased.
Table 4.14 Conversion and selectivity data for ozonation of 10 % 2-chloroethanol in water at pH 10 as a function of time
Ozonation
Time (h) Conversion
(%) Selectivity (%)
AcAlde AA Unknown
3 20.6 66.0 32.1 1.9
6 26.1 54.8 41.5 3.7
9 33.5 50.7 44.1 5.2
12 39.9 43.8 53.9 2.3
* Oxygen flow rate = 200 mL min-1 * Ozone concentration = 100 μg mL-1
An observation of the data in Table 4.14 and Figure 4.22 shows that percent conversion of 2-chloroethanol increases with ozonation time. Percentage conversion of the substrate increases from 20.6 % after 3 h of ozone treatment to 39.9 % after 12 h of ozone treatment. It is evident that the conversion rate of 2-chloroethanol increases steadily during the 12 h of ozonation.
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Figure 4.22 Percent conversion of 10 % 2-chloroethanol in water at pH 10 and selectivity profiles of AcAlde, AA and unknown versus ozonation time
The oxygenated product AcAlde showed the highest selectivity, starting at 66.0 % after 3 h ozonation and thereafter decreasing to 43.8 % after 12 h ozonation. Selectivity towards AA increased steadily from 32.1 % after 3 h to 53.9 % after 12 h. This pattern of product formation indicates that AcAlde is readily oxidized to its corresponding carboxylic acid, AA, in aqueous medium at pH 10.