Results
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Results
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Figure 9 contains the results from degradation and control experiments with methylene blue.
Figure 9: Degradation and control experiments for methylene blue. Figure 9 plots the solution concentration as a fraction of the initial concentration versus time. For each of the three experiments, the initial concentration was approximately 4.8 ppm. The pink squares represent a control experiment conducted with UV light and no catalyst. In 240 minutes there was approximately 10% degradation. The blue diamonds represent a control experiment conducted with the catalyst present but no UV light. In 240 minutes there was approximately 9% degradation. The green triangles represent the degradation experiment conducted with UV light and catalyst present. In 240 minutes there was approximately 21% degradation. This value is greater than those obtained in the control experiments. This shows that in the presence of both UV light and the PanaCat® catalyst the rate of methylene blue degradation increases significantly Figure 10 demonstrates the pseudo first-order decay for photocatalyzed methylene blue degradation.
Figure 10: Pseudo first-order decay for methylene blue degradation. Figure 10 plots the log of concentration over initial concentration versus time. There appears to be a very good linear correlation between the values, which is characteristic of a pseudo first-order decay. The first-order rate constant for methylene blue degradation is given by the absolute value of the slope of Figure 10, which is 0.0009 s-1. The half-life is 12.8 hours. Figure 11 contains the results from degradation and control experiments with congo red.
Figure 11: Degradation and control experiments for congo red Figure 11 plots the solution concentration as a fraction of the initial concentration versus time. For each of the three experiments, the initial concentration was approximately 7.5 ppm. The pink squares represent the control experiment conducted with UV light and no catalyst. In 240 minutes there does not appear to be any sign of degradation. The blue diamonds represent a control experiment conducted with the catalyst present but no UV light. In 240 minutes there was approximately 9.5% degradation. The green triangles represent the degradation experiment conducted with UV light and catalyst present. In 240 minutes there was approximately 32% degradation. This value is greater than those obtained in the control experiments. This shows that in the presence of both UV light and PanaCat® catalyst the rate of congo red degradation increases significantly. Figure 12 demonstrates the pseudo first-order decay for photocatalyzed congo red degradation.
Figure 12: Pseudo first-order decay for congo red degradation. Figure 12 plots the log of concentration over the initial concentration versus time. There appears to be a very good linear correlation between the values, which is characteristic of a pseudo first-order decay. The first-order rate constant for congo red degradation is given by the absolute value of the slope of Figure 12, which is 0.0015 s-1. The half-life is 7.7 hours. After conducting several experiments with both dyes, it became evident that small amounts of dye absorbed onto the walls of the tubing and to the surface of the reactor chamber. This would explain the degradation values for the control experiments for methylene blue, as shown in Figure 9, where the degradation remains unchanged between the two control experiments. This further indicates that the main mechanism for observed degradation in the control experiments is due to dye absorption onto solid surfaces within the reaction system. Furthermore, this behavior was observed in one of the control experiments for congo red, as shown in Figure 11. The degradation value observed when the catalyst is present while in the absence of UV light is very similar between the two experiments, which alludes to surface absorption. However, the control experiment in Figure 11 which uses UV light alone demonstrates a negligible level of degradation. This behavior can be explained in terms of the experimental set-up. Prior to conducting experiments, the entire reaction system is flushed with deionized water in order to remove any dye species which may have absorbed onto surfaces during previous runs. However, this was not the case for the UV control experiment in Figure 11. The system was not thoroughly flushed prior to experimentation. There remained some congo red dye absorbed within the system, possibly preventing anymore dye form depositing onto the surface and thus resulting in the observed negligible decay. Figure 13 shows a comparison in photocatalytic degradation between methylene blue and congo red dyes. Over a 240 minute period, congo red exibits 11% more degradation than methylene blue.
Figure 13: A comparison between congo red and methylene blue photocatalytic degradation It is clear that there exists a relationship between the type of dye used and the rate at which there is photocatalytic degradation. It is possible that the chemical structure of congo red lends itself more to oxidation by hydroxyl radicals, than does methylene blue. Moreover, it is also possible that congo red may absorb less UV light than methylene blue. This would make more photons available to impinge on the catalyst and promote the formation of hydroxyl radicals. However, it must be acknowledged that the degradation comparisons between the dyes are made on a mass basis. On a molecular basis the analysis is much different. Methylene blue and congo red have molecular weights of 320 and 697 respectively. This means that about twice as many molecules of methylene blue decay per ppm versus each ppm of congo red, which may mean that molecular degradation in methylene blue is higher than for congo red. Therefore, it is only safe to say that the rate of degradation for both dyes is significantly increased in the presence of UV light and the PanaCat® photocatalyst, and that the effect of specific dyes on degradation rates is debatable. Moreover, the initial dye concentration may also determine weather or not the catalyst demonstrates activity under water. When the initial dye concentration is high, a large amount of dye molecules agglomerate on the surface of the catalyst, which stain the catalyst. Figure 14 shows a comparison between a control experiment for congo red, at 27 ppm with the catalyst and no UV light , and the degradation experiment.
Figure 14: Congo red degradation and control experiments. Initial concentration 27 ppm. Here, there appears to be a similar level of degradation between the two experiments; however, the control experiment exhibits about 3% greater degradation. The presence of UV light and catalyst does not enhance the degradation rate. In both experiments, the high initial concentrations result in catalyst staining, which effectively keeps UV light from impinging on the catalyst surface. For both experiments, the observed degradation is most likely a result of the dye leaving the solution, and adhering to the catalyst surface. This would explain the high level of initial decay, which levels off over time. Similar results were observed with methylene blue at an initial concentration of 50 ppm. Further experimentation is required to determine a threshold for initial concentrations to optimize rate kinetics. Figure 15 shows the effects of catalyst density on methlyene blue degradation.
Figure 15: Catalyst density effects on methylene blue degradation. Identical degradation experiments were conducted at an initial concentration of approximately 5 ppm for two different catalysts. The higher density catalyst has an approximate 40% increase in mass over the lower density catalyst. A 9% increase in the degradation rate is observed with the higher density catalyst Figure 16 shows the reproducibility in methylene blue degradation experiments.
Figure 16: Reproducibility for methylene blue degradation. Both experiments were conducted under identical conditions. There is very good agreement between the two experiments. The repeat experiment shows 3% more degradation than the original experiment. This is within an acceptable range of error. Therefore, there appears to be an acceptable level of reproducibility for experiments conducted with this photocatalytic system. Similar reproducibility was observed with congo red.
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