Matrix effect in the hydroxyl radical induced degradation of β-lactam and tetracycline type antibiotics

Due to the spread of antibiotic resistant bacteria, elimination of antibiotics from purified wastewater is a highly important task. Purified wastewater contains large variety of organic/inorganic compounds that strongly influence the efficiency of advanced oxidation processes (AOP). In this work, we investigate the radiation-induced degradation of selected antibiotic contaminants (oxacillin and cloxacillin from the β-lactams; tetracycline and chlortetracycline from the tetracyclines) in four matrices: pure water, tap water, synthetic wastewater and purified wastewater received from a wastewater treatment plant. Changes in technically important parameters, such as COD, TOC, BOD, OUR, acute toxicity and antibacterial activity, were investigated at 0.1 mmol dm−3 (40–48 mg dm−3) antibiotic concentration. None of the four antibiotics were biodegradable in any of the four matrices, however, after irradiation with relatively low doses the obtained products were biodegradable. Oxacillin and cloxacillin were not toxic in Vibrio fischeri test, while tetracycline and chlortetracycline showed toxicity that was strongly reduced by the irradiation treatment. Both COD and TOC decreased after irradiation, their ratio shifted towards TOC indicating an increase in the average oxidation state (AOS). The increase in AOS was lower in purified wastewater matrices: this matrix may contain various small, highly oxidized molecules in high concentration, which degrade very slowly in AOP. The antibacterial activity in most of matrices was greatly reduced or completely disappeared at around 2–4 kGy. However, in purified wastewater matrices some antibiotic activity remained even at 4 kGy. Here the degradation of antibiotic is slow (small ΔCOD/dose value) because a large fraction of the reactive radicals is scavenged presumably by small, highly oxidized molecules in the solution. Although the water radiolysis product H2O2 affected some of the bioassays, this phenomenon was absent in purified wastewater. The purified wastewater after ionizing radiation treatment can be safely released into the receiving lakes or rivers.