Photolytic and photocatalytic removal of a mixture of four veterinary antibiotics

Abstract

The removal of a mixture of four veterinary antibiotics (VA) – tetracycline (TET), ciprofloxacin (CIP), sulfadiazine (SDZ) and sulfamethoxazole (SMX) – via photo-degradation (UVC) and photocatalysis with TiO2 (UVC/TiO2) was investigated in a batch reactor under different initial concentrations (20, 100, 500 and 1000 μg/L per antibiotic). Ultra-high performance liquid chromatography coupled to a mass spectrometry (UHPLC-MS/MS) was used to determine the removal of these veterinary antibiotics. The removal of all antibiotics via photolysis was around 98–99% after 100 h for TET, 122 h for CIP, 212 h for SDZ and 240 h for SMX. Nevertheless, the removal of all antibiotics via photocatalysis was around 99–100% after 4.2 h for TET, 3.5 h for CIP, 7.1 h for SDZ and 16.5 h for SMX. The photolysis for the four veterinary antibiotics followed a first-order irreversible kinetic model. The photocatalysis of TET, CIP and SDZ followed a Langmuir-Hinshelwood kinetic model, and adsorption was considered the limiting step. SMX followed a first-order irreversible kinetic model. The photolytic degradation rate constant (k1) was 0.00073 min−1 for TET, 0.00055 min−1 for CIP, 0.00031 min−1 for SDZ and 0.00027 min−1 for SMX. While for photocatalysis k1 was 0.0044 min−1 for SMX; kL-H was 0.0284 min−1 for TET, 0.0379 min−1 for CIP and 0.0141 min−1 for SDZ. The VA degradation was enhanced by the use of a catalyst. Additionally, electrical energy per order (EEO) was assessed to estimate the electrical energy efficiency of each process. EEO values for photolysis were 339.06 kWh/m3/order for TET, 449.84 kWh/m3/order for CIP, 795.31 kWh/m3/order for SDZ and 897.71 kWh/m3/order for SMX. On the other hand, EEO values for photocatalysis were 14.96 kWh/m3/order for TET, 12.07 kWh/m3/order for CIP, 20.39 kWh/m3/order for SDZ and 62.10 kWh/m3/order for SMX. The energy consumption for photocatalysis was considerably lower than for photolysis. This study determined an overall degradation rate constant for a wide range of TET, CIP, SDZ and SMX concentrations. Furthermore, when working with a pH of 8 (a typical pH from wastewater from livestock farms) and a VA mixture whose concentrations resemble the characteristics of real water samples, that photolysis and photocatalysis are potential processes for wastewater treatment with low energy consumption.