Degradation of norfloxacin in aqueous solution using hydrodynamic cavitation: Optimization of geometric and operation parameters and investigations on mechanism

Abstract

Hydrodynamic cavitation (HC), as one of novel advanced oxidation processes (AOPs), is considered to be a promising technology for wastewater treatment because it has the advantages of wide application range, simple operation and no secondary pollution. In this work, the effects of different orifice plate geometric parameters such as orifice shape (circle, square and triangle), orifice plate thickness (2.00–6.00 mm), orifice number (n = 1–5) and orifice divergence angle (α = 0° and ± 45°) on the norfloxacin degradation by using HC were studied to obtain a strong HC effect. And the effects of various operating parameters such as solution pH (3.0–9.0) and initial norfloxacin concentrations (5.0–15 mg/L) on the norfloxacin degradation by using HC were discussed. The trapping experiments of free radicals (OH and O2−) were also carried out for exploring the mechanism of norfloxacin degradation. In addition, adopting the designed orifice plates with optimized geometric parameters, the norfloxacin degradation efficiencies by using HC combined with some added oxidants (H2O2, K2S2O8 and NaClO) were investigated. The Total Organic Carbon (TOC) analysis at the optimum conditions was also performed to identify the mineralization extent. The intermediate products during the norfloxacin degradation were detected by using Liquid Chromatograph-Mass Spectrometer (LC-MS). The experimental results indicated that, under 5.0 bar inlet pressure for 10 mg/L initial norfloxacin concentration at 40 °C operating temperature in pH = 3.0 solution, the degradation ratio of norfloxacin can reach up to 84.20% in the HC system and can reach up to 96.45% in the HC + H2O2 system by using the orifice plate with 4.00 mm thickness, three square orifices and α = 0° orifice divergence angle. The TOC test results show that the mineralization ratio can reach 71.57% in the HC system for 150 min successive cycling, which demonstrated that the HC can effectively mineralize norfloxacin. In addition, the LC-MS results manifested that norfloxacin moleculars can be degraded into a series of organic compounds with low molecular weight and further be oxidized into CO2, H2O and some inorganic ions during the HC process. Overall, the work has demonstrated that the HC is an effective strategy for a large-scale treatment of antibiotics wastewater.