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A limited amount of hydrogen peroxide is a source of supplementary oxidising agent which could cause a high rate of degradation of persistent organic pollutants (POPs) until a certain optimum concentration (Chakinala et al., 2009). An excessive quantity of hydrogen peroxide could also serve as hydroxyl radical scavenger, thus promoting the recombination of the hydroxyl radical and consequently leading to a reduced degradation rate (Chakinala et al., 2008). The formation of hydroxyl radicals from hydrogen peroxide and its scavenging effect in the presence of hydrogen peroxide is presented in (6.i) and (6.ii).

𝑯_𝟐𝑶_𝟐 → 𝑯𝑶⦁ + 𝐇𝐎⦁………. (6.i) 𝑯_𝟐𝑶_𝟐 + 𝐇𝐎⦁ → 𝑯_𝟐𝑶 + HOO ….………… (6.ii)

The optimum amount of hydrogen peroxide needed for the degradation of a specific amount of acetaminophen in a jet loop hydrodynamic cavitation system must be determined to ensure the efficiency of the process. During the current studies, the concentration of hydrogen peroxide applied for the treatment of ACE solution in a jet loop hydrodynamic cavitation system was varied between 0.5 mg/ L and 10 mg/ L as described in Table 6.2.

PH20 was the untreated ACE solution while PH26, P5P6, 1P6, 2P6, 5P6 or 10P6 represents the application 0.0 mg/ L, 0.5 mg/ L, 1 mg/ L, 2 mg/ L, 5 mg/L or 10 mg/ L of hydrogen peroxide respectively in the jet loop hydrodynamic cavitation system system (pH 2, 400 kPa, 10 L of 10 mg/ L ACE, 4 mm orifice plate hole size, 60 minutes, n = 3). The UV spectra obtained after 60 minutes treatment of ACE solution in the jet loop hydrodynamic cavitation is presented in Figure 6.10. 200 220 240 260 280 300 0.0 0.5 1.0 1.5 2.0 2.5 2nd isosbestic point Ab so rb a n ce Wavelength (nm) PH20 PH26 P5P6 1P6 2P6 5P6 10P6 1st isosbestic point

Figure 6.8: UV spectra showing the effect of hydrogen peroxide application during the transformation of acetaminophen in the jet-loop hydrodynamic cavitation and showing the first isosbestic point at the application of 2 mg/ L hydrogen peroxide (400 kPa, pH 2, 4 mm orifice plate hole size, 10 L of 10 mg/ L ACE, n = 3)

It was observed at the end of the treatment, that the UV absorbance spectra of the experiments (PH26, P5P6 and 1P6) which represent the addition of 0 mg/ L, 0.5 mg/ L and 1.0 mg/ L respectively of hydrogen peroxide to ACE solution in the jet loop hydrodynamic cavitation

system (pH 2, 400 kPa, 10 L of 10 mg/ L ACE, 4 mm orifice plate hole size, 60 minutes, n = 3) were more intense than the UV specrum of the untreated ACE (PH20). This implies that the added hydrogen peroxide was only able to produce intermediates with an intense absorbance spectrum during the treatment in the jet loop hydrodynamic cavitation system. Conversely, the UV spectra at 60 minutes treatment of ACE solution when 2.0, 5.0 or 10 mg/ L hydrogen peroxide (2P6, 5P6 or 10P6) was applied show intersections with the spectrum obtained for the untreated ACE (PH20). These points of intersections are known as isosbestic points. They mark the wavelength at which the absorption of light by a mixed solution remains constant as the equilibrium between the components of solution changes. The appearance of isosbestic point in the current investigation is a justification of the existence of the equilibrium mixture of degraded ACE solution and the residual ACE solution, making up the two major chemical components in the treated solution. It was observed from the obtained spectra of treated ACE that the maximum intermediate products were generated in the first 10 minutes of treatment and there after, its transformation started. The spectra obtained when 0, 0.5 or 1.0 mg/ L of hydrogen peroxide was applied during the treatment of ACE solution in the jet loop hydrodynamic cavitation system is presented in appendix 3.

The obtained spectra show that after 60 minutes treatment, the application 0.5 or 1.0 mg/ L hydrogen peroxide in the jet loop hydrodynamic cavitation system was not able lower the λmax

below that which was obtained for the untreated ACE solution. Conversely, the application of 2.0, 5.0 or 10 mg/ L hydrogen peroxide in the jet loop hydrodynamic cavitation system was able to lower the λmax below that which was obtained for the untreated ACE solution after 30 minutes

of treatment (Appendix 4).

Based on this account, it can be reported that the substantial transformation of ACE solution leading to its degradation was achieved after 30 minutes of treatment when 2 mg/ L, 5 mg/ L or 10 mg/ L hydrogen peroxide (2P6, 5P6 and 10P6) was applied in the jet loop hydrodynamic cavitation system (pH 2, 400 kPa, 10 L in 10 mg/ L ACE, 4 mm orifice plate hole size, 60 minutes, n = 3). Therefore, the % degradation of ACE was extimated using kinetic of degradation equation that was described in Section 3.8.3. The bar chart of % degradation during the treatment of ACE in the jet loop hydrodynamic cavitation system is presented in Figure 6.11.

Figure 6.9: The effect of H2O2 concentration on % degradation during the treatment of ACE in the jet loop hydrodynamic cavitation system (pH 2, 400 kPa, 10 L of 10 mg/ L

ACE, 4 mm orifice plate hole size, n = 3).

The degradation of ACE became effective after 30 minutes treatment time with the application of 2 mg/ L or 5 mg/ L hydrogen peroxide in the jet loop hydrodynamic cavitation system (pH 2, 400 kPa, 10 L of 10 mg/ L ACE, 4 mm orifice plate hole size, 60 minutes, n = 3). Meanwhile, when the higher amount of hydrogen peroxide (10 mg/ L) was applied the scavenging effect was noticed after 50 minutes treatment. On the basis of this, an optimum degradation of ACE solution was achieved when 5 mg/ L hydrogen peroxide was applied in the jet loop hydrodynamic cavitation system as currently designed. Addition of a higher concentration of hydrogen peroxide up to 10 mg/ L did not produce an improvement in the extent of degradation at the end of the (60 minutes) treatment time. The benefits of effective application of hydrogen peroxide during the degradation of ACE solution in the jet loop hydrodynamic cavitation system includes the reduction of the energy demand as well as decreased degradation time. The reduction in effectiveness on the addition of excessive hydrogen peroxide (more than 5 mg/ L) is primarily due to the scavenging effects of hydrogen peroxide on the hydroxyl radical or the recombination reaction of the hydroxyl radical (Stocking et al., 2011). In the next section, the combination of hydrogen peroxide with Fenton catalyst (Iron (II) sulfate or gnZVI) was

0 10 20 30 40 50 60 70 40 50 60 % d e gr ad ation Time (minutes) 2P6 5P6 10P6

considered in order to ensure efficient degradation of ACE solution in jet loop hydrodynamic cavitation system.

6.5.5 Application of the optimum conditions during the degradation of acetaminophen in the jet