Understanding the effects of mineral water matrix on degradation of several pharmaceuticals by ultrasound: Influence of chemical structure and concentration of the pollutants

Degradation of seven relevant pharmaceuticals with different chemical structures and properties: acetaminophen (ACE), cloxacillin (CXL), diclofenac (DCF), naproxen (NPX), piroxicam (PXC), sulfacetamide (SAM) and cefadroxil (CDX), in distilled water and mineral water by ultrasound was studied herein. Firstly, proper conditions of frequency and acoustic power were determined based on the degradation ability of the system and the accumulation of sonogenerated hydrogen peroxide (24.4 W and 375 kHz were found as the suitable conditions for the sonochemical treatment of the pharmaceuticals). Under such conditions, the pharmaceuticals degradation order in distilled water was: PXC > DCF ~ NPX > CXL > ACE > SAM > CDX. In fact, the initial degradation rate showed a good correlation with the Log P parameter, most hydrophobic compounds were eliminated faster than the hydrophilic ones. Interestingly, in mineral water, the degradation of those hydrophilic compounds (i.e., ACE, SAM and CDX) was accelerated, which was attributed to the presence of bicarbonate ions. Afterwards, mineral water containing six different initial concentrations (i.e., 0.331, 0.662, 3.31, 16.55, 33.1, and 331 µM) of selected pharmaceuticals was sonicated, the lowest concentration (0.331 µM) always gave the highest degradation of the pollutants. This result highlights the great ability of the sonochemical process to treat bicarbonate-rich waters containing pollutants at trace levels, as pharmaceuticals. Finally, the addition of ferrous ions to the sonochemical system to generate a sono-Fenton process resulted in an acceleration of degradation in distilled water but not in mineral water. This was attributed to the scavenging of sonogenerated HO• by bicarbonate anion, which decreases H₂O₂ accumulation, thus limiting the Fenton reaction.