Optimization Of Process Parameters And Scale Effect Of Electrochemical Oxidative Degradation Of Antibiotics

Antibiotics are widely used in human and animals to prevent diseases.Great threaten has been shown for induced superbacteria because of the abuse of antibiotics,which was harmful for human health.Electrochemical oxidation system with Boron-doped Diamond(BDD)anode has shown great potential for antibiotics degradation because of its strong oxidation ability and remarkable corrosion stability,as well as its simple operation conditions.In the present study,the operating variables and degradation mechanism for antibiotics removal in BDD system were explored.What's more,the effects of mass transfer and the surface reaction rate coefficient of BDD anode were analyzed.The velocity field distribution of BDD anode reactor was simulated and the surface reaction rate coefficient of BDD anode was quantified,which was important for its practical application.The main results were summarized as follows:Sulfamethoxydiazine(SMD)was selected as the target pollutant in the present study.Response surface methodology(RSM)was applied to analyze the influence of different operating variables(current density,electrode spacing,flow rate and electrolysis time)on COD removal efficiency and specific energy consumption(Esp).Results showed that COD removal efficiency increased along with current density,flow rate and electrolysis time increasing,while decreased with the electrode spacing extending.Under the optimum degradation conditions of current density 23.42 mA·cm-2,electrode spacing 8.57 mm,flow rate 174.63 mL·min-1 and electrolysis time 2.8 h,COD removal efficiency was achieved as 76.25%with Esp of 361.22 kWh·kgCOD-1.Furthermore,degradation mechanism of SMD in BDD anode system was proposed based on the active sites identification by density functional theory(DFT)of Gaussian calculations and intermediates analyzation by high performance liquid chromatography quadrupole time-of-flight mass spectrometer(HPLC-Q-TOF-MS/MS).Results showed that hydroxylation of benzene rings,sulfur dioxide SMILEs rearrangement and extrusion,oxidation of the amine group and decomposition of SMD molecules were mainly four possible degradation mechanism during electrolysis.Based on the optimal operating conditions,mass transfer coefficient(km)of the system was calculated.km increased from 1.01×10-5 m·s-1 to 4.69 × 10-5 m·s-1 and the effectiveness factor(?)increased from 0.316 to 0.697 with flow rate speeded from 20 mL·min-1 to 180 mL·min-1.While km decreased from 3.67 × 10-5 m·s-1 to 2.79 × 10-5 m·s-1 and ? decreased from 0.780 to 0.619 with electrode spacing increased from 8 mm to 20 mm.Narrowing electrode spacing and increasing the flow rate would enhance the degradation capability.In addition,the surface reaction rate coefficient for BDD anode was calculated to be applied for the scale effect of reactor.A theoretical model for electrochemical oxidation of SMD in BDD anode system was built by computational fluid dynamics(CFD)software fluent with the velocity and mass fraction fields in the reactor obtained.Based on the experimental data,the surface reaction rates were determined as 18,45 and 69 min-1 under the current densities of 20,30 and 40 mA cm-2,respectively.According to these results,two larger BDD anode reactors(scaled up by 100 times and 400 times as the actual one)were built to explore the effects of scale effect on the capability of degradation.It was demonstrated that the scale effect had a great effect on the degradation capability in BDD system,the degradation rate of SMD reached 72.7%and 40.4%when the system scaled up by 100 times and 400 times after 60 minutes at the current densities of 20 mA·cm-2.The degradation capability decreased by 5.7%and 47.6%compared with the laboratory scale.And the degradation capability decreased slightly when the reactor was scaled up by 100 times,which was within the acceptable range.