Magnetic biochar generated from oil-mill wastewater by microwave-assisted hydrothermal treatment for sonocatalytic antibiotic degradation

The pollution risks of biomass-containing and antibiotics-containing wastewater mean that it must be treated before discharge into the environment. In a process following circular economy principles, oil-mill wastewater (OMWW) was treated in a microwave-assisted hydrothermal process and with simultaneously fabricated magnetic biochar (MBC), which was used as a catalyst for the sonocatalytic degradation of antibiotics metronidazole (MET) and ciprofloxacin (CIP) in aqueous solutions. FeCl3·6H2O, FeCl2·4H2O, and KOH were added to the OMWW, which was then treated in a SynthWave microwave reactor under either a nitrogen or air atmosphere. The UV254 and COD removal efficiencies of the OMWW reached 42.3% and 60.6%, respectively, after treatment at 250 °C for 3 h. The MBC obtained from the OMWW treatment with air exhibited the highest adsorption and catalytic activity for antibiotics degradation. The synergistic effects of combining adsorption and sonolytic removal were in the ranges of 0.76–3.07 (MET) and 0.75–3.17 (CIP), resulting in significant degradation rate constants of pseudo-first-order kinetics (k1) that ranged from 0.010 to 0.086 min−1 (MET) and from 0.018 to 0.050 min−1 (CIP). Hydrophobic MET underwent faster degradation than hydrophilic CIP. The •OH is crucial for removing the model antibiotics. Initial antibiotics concentration, reaction volume, ultrasonic frequency, and power had clearer impacts on antibiotics degradation than bulk temperature. Furthermore, the possible sonocatalytic degradation pathways have also been proposed herein. The absorbents and catalysts were derived during the purification of OMWW and subsequently used for antibiotics removal from water, becoming an innovative case of circular economy and sustainable waste treatment.