Sonochemical degradation of antibiotics from representative classes-Considerations on structural effects, initial transformation products, antimicrobial activity and matrix
In this work, the sonochemical treatment (at 354 kHz and 88 W L-120 ) of six relevant antibiotics belonging to fluoroquinolones (ciprofloxacin and norfloxacin), penicillins (oxacillin and cloxacillin) and cephalosporins (cephalexin and cephadroxyl) classes
was evaluated. Firstly, the ability of the process to eliminate them was tested,
showing that sonodegradation of these antibiotics is strongly chemical structure25
dependent. Thus, correlations among initial degradation rate of pollutants (Rd),
solubility in water (Sw), water-octanol partition coefficient (Log P) and topological
polar surface area (TPSA) were tested. Rd exhibited a good correlation with Log P
(i.e., the hydrophobicity degree of antibiotics). The considered penicillins had the
fastest elimination and from the constitutional analysis using Lemke method was
clear that the functional groups arrangement on these antibiotics made them highly
hydrophobics. The penicillins were degraded closer at cavitation bubble than the
fluoroquinolones or cephalosporins. The investigation of degradation products
showed that sonogenerated hydroxyl radical primary attacked the β-lactam ring of
cloxacillin and cephalexin, whereas on norfloxacin induced a decarboxylation. On
the other hand, the evolution of antimicrobial activity was also followed. It was
evidenced the process capacity to remove antimicrobial activity from treated
solutions, which was associated to the transformations of functional groups on
antibiotics with important role for interaction with bacteria. Additionally, degradation
of antibiotics having the highest (the most hydrophobic, i.e., cloxacillin) and lowest
(the most hydrophilic, i.e., cephadroxyl) Rd, was performed in synthetic matrices
(hospital wastewater and seawater). Ultrasound degraded both antibiotics; for cloxacillin in such waters higher eliminations than in distilled water were observed
(probably due to a salting-out effect exerted by matrix components). Meanwhile, for
cephadroxyl a moderate inhibition of degradation in hospital wastewater and
seawater respect to distilled water was found, this was related to competition by
hydroxyl radical of the other substances in the matrices. These results show the
quite selectivity of high frequency ultrasound to eliminate antibiotics form different
classes even in complex matrices