As expected, tenofovir is not shown in the results of the current work as the sample solutions were stabilized with calcium hydroxide, raising the pH (˃12). Tenofovir degraded almost immediately at a high environment (refer to section 4.1). Despite the limited number of pharmaceuticals used for the current work, the results of this work agree with literature and add to the existing knowledge and understanding of the degradation of pharmaceuticals due to a one-time dosage of hydrogen peroxide for urine stabilized with calcium hydroxide. The combined effect of the hydrogen peroxide in a high pH environment was inefficient for the degradation of the pharmaceuticals investigated for this work. Furthermore, the urea conservation of degradation method was 87.4% (refer to section 4.6).
Figure 21: Hydrodynamic cavitation results: Basic sample 1 (A), Basic sample 2 (B), Acidic sample 1 (C), Acidic sample 2 (D). The respective abbreviations of the pharmaceuticals labelled under each graph are as follows: paracetamol (PARA), chlorpheniramine maleate (CHL), stavudine (STA), lamivudine (LAM) and zidovudine (ZID).
The chromatograms (in Figure 22) for the HC system operated at pH 12.4 showed a slight decrease of the peaks representing the pharmaceuticals even though there was a slight increase in the degradation products identified for paracetamol. Therefore, the HC system operated at a high pH was ineffective in the degradation of the pharmaceuticals over a 30- minute period. The y-axis is not shown, since the graph shows the relative comparisons of the samples at three different time intervals. The chromatograms were given to illustrate the degradation of the pharmaceuticals over time, and not to quantify the degradation from the chromatograms.
Figure 22: Chromatograms for hydrodynamic cavitation at pH 12.4: sample 1 (A) and sample 2 (B). The colour coded chromatograms correspond to the time during the HC system as follows: 0 minutes (red); 15 minutes (green) and 30 minutes (blue). The respective abbreviations of the pharmaceuticals represent the following pharmaceuticals: paracetamol (Para), chlorpheniramine maleate (Chloro), stavudine (Sta), lamivudine (Lami) and zidovudine (Zido).
Contrary, the chromatograms (shown in Figure 23) represent the degradation of the pharmaceuticals at a low pH (pH 2) over a 30-minute duration. Furthermore, there was an increase in the peaks of the degradation products. Although the degradation products were not identified, the increase in their concentration confirmed that the primary pharmaceuticals were more degraded by the HC system operated at pH 2 when compared to the HC system operated at pH 12.4.
Figure 23: Chromatograms for hydrodynamic cavitation at pH 2: sample 1 (A) and sample 2 (B). The colour coded chromatograms correspond to the time during the HC system
as follows: 0 minutes (red); 15 minutes (green) and 30 minutes (blue). The respective abbreviations of the pharmaceuticals represent the following pharmaceuticals: paracetamol (Para), chlorpheniramine maleate (Chloro), stavudine (Sta), lamivudine (Lami) and zidovudine (Zido).
The results from the HC experiments showed potential in the degradation of pharmaceuticals from the hydroxyl radicals generated from the cavitation process. The HC system operated at a low pH (pH 2) resulted in a higher degradation than the system operated at a high pH (12.4).
It was postulated that the bonds in the pharmaceutical structures are weaker at a low pH since hydroxyl radicals are produced for both the hydrogen peroxide and the HC system.
However, the HC was less effective at a high pH. Therefore, a low pH was ideal for the oxidation of the pharmaceuticals by the hydroxyl radicals.
The difference in the degradation of the pharmaceuticals at different pH environments is explained by the fact that each solution has an optimum pH at which pharmaceuticals can degrade (Rajoriya et al., 2016). This proves that the degradation of pharmaceuticals improved when the pH was optimized to pH 2. The results from the HC degradation method at a high pH also showed that hydroxyl radicals were ineffective in degrading pharmaceuticals at a high pH. Nevertheless, the results from the optimization of the pH for the HC system suggest that the optimization of the other parameters that influence the degradation of pharmaceuticals using the cavitation system might further improve the degradation efficiency of the cavitation system. Based on the findings from literature, the optimization of inlet pressure, temperature and cavitation devices improve the degradation efficiency of a HC system (Rajoriya et al., 2016).
An inlet pressure of 200 kPa was initially applied for this work, but literature showed that the optimum inlet pressure for the removal of various dyes was within the range of 480 – 490 kPa (Mishra and Gogate, 2010; Gogate and Bhosale, 2013). This explains the low pharmaceutical degradation efficiency at a pressure of 200 kPa. The temperature was not monitored for the initial HC experiments, however there is a consensus from literature that a temperature of 50˚C is optimal for the degradation of micropollutants (Wang et al., 2009; Šarc et al., 2017).
In addition, Saharan (2013) found that a circular venturi performs better than an orifice. It can be assumed that the cavitation device did not have an influence on the degradation of the pharmaceuticals since a circular venturi was used for the initial HC experiments.
The HC experiments required larger sample volumes. For this reason, a simplified urea-water solution was used instead of real urine. This compromised the use of all the eleven selected pharmaceuticals for the current work since the pharmaceuticals were acquired through donations. This is because the pharmaceuticals were expensive, therefore, the quantity of the pharmaceuticals which could be used was restricted. As a result, only five pharmaceuticals (two OTCs and three ARVs) were used for the HC experiments. Nonetheless, the results were valid since studies for the degradation of pharmaceuticals are often focused on one specific micropollutant instead of a mixture of micropollutants. Furthermore, the five
pharmaceuticals were representative of the eleven pharmaceuticals (five OTCs and six ARVs) chosen for the current work since two OTCs and three ARVs were used.
The optimization of the HC system was considered for this work since it also conserved more than 90% of urea (refer to section 4.6). Besides being a new pharmaceutical removal technology, it was less invasive than the optimization of the hydrogen peroxide method which would require more equipment and a new setup for the combined hydrogen peroxide/ultraviolet system. Furthermore, the optimization of GAC was not considered because it already had a high pharmaceutical removal efficiency (˃94%). Additionally, changing the dosage of calcium hydroxide would not be beneficial since a dosage of 10 g L-1 was an overestimate for the stabilization of urine (Randall et al., 2016).