2.8 Pharmaceutical removal methods
2.8.4 High and low pH methods
The pH is an important parameter to consider for the degradation of pharmaceuticals. It has an impact on the degradation rate of the pharmaceuticals, however, the degradation of the pharmaceuticals due to a change in pH is dependent on the molecular structure of the pharmaceuticals (Yin et al., 2017).
Yin and co-workers (2017) investigated the degradation of five pharmaceuticals (atenolol, metoprolol, propranolol, fluoxetine, and venlafaxine) to evaluate the pharmaceutical degradation variability within a pH range of 2 - 12. The degradation of propranolol and venlafaxine increased with an increase in pH while atenolol, metoprolol and fluoxetine experienced the highest degradation at a pH of 7, 2 and 12, respectively. The half-lives of the pharmaceuticals varied significantly which were as follows: 5.7 - 28.5 d for propranolol,
29.6 - 78.2 d for metoprolol, 56.3 - 81.4 d for atenolol, 46.6 - 183.2 d for fluoxetine and 68.8 - 145.4 d for venlafaxine (Yin et al., 2017). It is inferred that the pharmaceuticals were
deprotonated at varying degrees due to the difference in the acid and basic functional groups (such as carboxylic acid, hydroxyl groups and amines) which are present in the structure of the pharmaceuticals (Hapeshi et al., 2010).
On the other hand, Mohammed-Ali (2012) found that tetracycline was less stable in an alkaline solution than an acidic solution. The absorbance of tetracycline was measured, and a linear degradation (with a positive slope) was observed over five days, thus, the degradation
of tetracycline progressed over time. The degradation of tetracycline in high pH occurred when tetracycline opened its ring to form isotetracycline (Mohammed-Ali, 2012).
Another study by Agrahari and co-workers (2015) found that tenofovir was less stable in strong acidic and alkaline environments. The approximate shelf-life, half-life and time required for a 90% degradation of tenofovir was 3.84, 25.3, and 84 h under acidic conditions, and 58.3, 385, and 1280 h under alkaline conditions. The degradation of tenofovir was more favourable in a strong acidic environment, since further hydrolysis in strong acidic conditions occurs due to non-chromophoric low molecular weight compounds that alter the bonds of the degradation products (Agrahari et al., 2015).
The change of pH influences other micropollutants apart from pharmaceuticals. The calcium oxide (CaO) treatment of A. suum eggs (which pose an infective hazard) was investigated for sewage sludge. A concentration of 10% (w/w) CaO (85%) was added to the sewage sludge.
The high pH (>12) destroyed the embryonate ability of the A. suum eggs (Eriksen et al., (1996).
Literature has shown that adjusting the pH of a solution (to become either basic or acidic) influences the degradation of the pharmaceuticals. Yin and co-workers (2017) showed that the degradation of each pharmaceutical under different pH conditions is varied. As such, Table 1 provides a summary of the degradation behaviour due to a change in pH of the pharmaceuticals investigated for this work.
Table 1: The degradation of pharmaceuticals due to a change in pH.
Pharmaceutical Degradation of the pharmaceutical due to a change in pH
Paracetamol Yang and co-workers (2008) discovered that the degradation of paracetamol (in aqueous solution) by TiO2 photocatalysis increased slowly between pH 3.5 and 9.5. However, the degradation rate decreased at a pH between 9.5 and 11.0 (Yang et al., 2008).
Salicylic acid Rao and co-workers (2009) used a ZnO catalyst to degrade salicylic acid. The study found that the degradation of salicylic acid was most effective at a neutral pH (Rao et al., 2009).
Diclofenac Bagal and Gogate (2013) used a combined process of hydrodynamic cavitation and heterogeneous photocatalysis to degrade diclofenac sodium. The optimal degradation pH was pH 4 (Bagal and Gogate, 2013).
Clopidogrel The study by Raijada and co-workers (2010) suggested that alkaline microenvironments need to be avoided for clopidogrel bisulphate since it causes the drug to change to an oily free base i.e., causes degradation (Raijada et al., 2010).
Chlorpheniramine maleate
Lv and co-workers (2015) showed that increasing the pH of the chlorpheniramine increased the degradation of the drug. A higher degradation rate was observed between pH 8 to 9 which might be due to the release of more OH radicals (Lv et al., 2015).
Zidovudine A study by Dunge and co-workers (2004) investigated the degradation behaviour of zidovudine under various conditions. Zidovudine hydrolyzed more in an acidic environment than an alkali environment (Dunge et al., 2004).
Lamivudine Wang and co-workers (2019) investigated the degradation of lamivudine using bicarbonate enhancing electrochemical degradation. The study showed that the initial pH did not have an effect in the degradation of lamivudine due to the bicarbonate enhancing electrochemical degradation (Wang et al., 2019).
Tenofovir According to Golla and co-workers (2016) the degradation of tenofovir alafenamide fumarate (TAF) and tenofovir disoproxil fumarate (TDF) increased with an increase in pH. TDF was more unstable than TAF at pH 5, 6.8 and 10 (Golla et al., 2016).
Nevirapine Bhembe and co-workers (2020) investigated the photocatalytic degradation of nevirapine. The study showed that nevirapine showed a higher degradation efficiency in an acidic environment than an alkaline environment (Bhembe et al., 2020)
Stavudine A study by Dunge and co-workers (2004) investigated the degradation behaviour of stavudine under various conditions. Stavudine hydrolyzed more in an acidic environment than an alkali environment (Dunge et al., 2004).
Abacavir sulfate A study by Ramesh and co-workers (2020) revealed that when abacavir sulfate was subjected to alkaline hydrolysis, the pharmaceutical was not degraded (Ramesh et al., 2020).