Study On The Removal Of Tetracycline By The Reactive Electrochemical Membrane Coupled With Ozonation

In recent years,many drugs such as antibiotics,estrogens andβ-blockers have been released into the water environment.Some of these drugs have high ecological toxicities,and their ecological safeties have attracted worldwide attention.Conventional water treatment processes are difficult to remove drugs effectively.Current researches show that advanced oxidation technologies are effective to remove drugs from water,which are represented by the ozonation and electrochemical oxidation.However,the ozonation has relatively low organic mineralization efficiency and is difficult to degrade small-molecule organic acids such as oxalic acid effectively,and the electrochemical oxidation is limited by the flow-by operation mode of conventional electrodes and may has disadvantages such as slow mass transfer rate,low current utilization and relatively low energy consumption.Therefore,it is still necessary to develop more efficient processes to eliminate drugs from water.In this study,a relatively low-cost,stable and electrochemical satisfied Ti/Ti₄O₇ flat plate anode was developed for the electrochemical degradation of drugs,so as to evaluate the characteristic of Ti₄O₇ anode material for drug removal in water.Using high-purity Ti₄O₇ powder as materials,a tubular porous Ti₄O₇ reactive electrochemical membrane（REM）was developed,which could enhance the electrochemical mass transfer via membrane filtration.The complete drug electrochemical oxidation pathway was identified by analyzing the degradation intermediates,and the toxicities of these intermediates were assessed by quantitative structure activity relationship（QSAR）analysis to facilitate the understanding of the measured toxicity change during drug electrochemical oxidation.Moreover,the technology of the Ti₄O₇ REM coupled with ozonation was developed,in which process the ozonation,membrane filtration and electrochemical oxidation were combined to synergistically removal typical drugs in water.Optimal parameters of the composite process were proposed.Thus,a new water treatment technology with high drug removal efficiency and mineralization efficiency was developed so as to ensure the safety of water ecosystems.The main contents and conclusions are summarized as follows:（1）Study on the removal of tetracycline by electrochemical oxidation using the Ti/Ti₄O₇ anode.In this study,a Ti/Ti₄O₇ anode（a new kind of‘non-active’anode）was prepared by the plasma spraying technology,a Ti/SnO₂-Sb anode（a typical‘non-active’anode）was prepared by the sol-gel technique and a Ti/RuO₂-Ir anode（a typical‘active’anode）was prepared by the thermal decomposition method.By comparing the surface morphologies,electrochemical properties,degradation and mineralization of tetracycline in water,anode stability and energy consumption,the characteristics of TC electrochemical oxidation with the Ti/Ti₄O₇ anode was analyzed.The electrochemical degradation kinetics was determined by investigating the effects of various factors on TC removal with the Ti/Ti₄O₇ anode.The anode efficiency of the Ti/Ti₄O₇ anode was also investigated by analyzing the mineralization efficiency,inorganic ion concentration and toxicity change during TC electrochemical oxidation.The Ti/Ti₄O₇ anode prepared by plasma spraying had compact coating layer on the anode surface,relatively high oxygen evolution potential（OEP）,satisfactory stability and high activity in the degradation and mineralization of TC in water.Compared with the Ti/SnO₂-Sb and Ti/RuO₂-Ir anode,the Ti/Ti₄O₇ anode had compact coating layer with high purity and satisfactory stability,relatively high OEP（2.6 V vs.SCE）in H₂SO₄solution,high TC removal efficiencie（95.8%）and TOC removal efficiencie（75.7%）,long service life,and low energy consumption(271.3 KWh·kg^-1 TOC).These satisfactory performances made it more suitable for TC electrochemical oxidation.Following a pseudo-first-order kinetics（R²>0.97）,the TC electrochemical oxidation with the Ti/Ti₄O₇ anode was favored by the high current density,low anode-cathode distance and low initial TC concentration.Under the optimum conditions of 15mA·cm^-2 current density,10 mm anode-cathode distance and 5 mg·L^-1 initial TC concentration,the TOC removal efficiency gradually increased and the mineralization current efficiency gradually decreased;the concentrations of NH₄⁺,NO₃^-and NO₂^-in solution increased with the reaction time,and the increasing trend of NH₄⁺was significantly higher than of NO₃^-and NO₂^-;the bioluminescence inhibition ratio slightly increased from 54.53%to 55.41%and finally decreased to 16.78%.（2）Study on the removal of tetracycline by electrochemical oxidation using the Ti₄O₇ REM.In this study,a tubular porous Ti₄O₇ REM was developed by the extrusion-calcination method and its structure and properties were characterized.The electrochemical degradation kinetics was determined by investigating the effects of various factors on TC removal with the Ti₄O₇ REM.The anode efficiency of the Ti₄O₇REM was also investigated by analyzing the mineralization efficiency,inorganic ion concentration,toxicity change and energy consumption during TC electrochemical oxidation.The Ti₄O₇ REM prepared by extrusion-calcination enhanced the electrochemical mass transfer through membrane filtration,and TC was electrochemical oxidized into three-,two-and one-ring intermediates and eventually carboxylic acids over the Ti₄O₇REM,the production of which delayed the toxicity decrease during TC electrochemical oxidation.The Ti₄O₇ REM had high purity and uniform distribution of active constituent Ti₄O₇.The inside and outside surfaces of the Ti₄O₇ REM were smooth,flat with no cracks.The pore size was about 1⁶μm.The Ti₄O₇ REM had relatively high OEP（2.7 V vs.SCE）in H₂SO₄ solution,which was close to those of the Ti/Ti₄O₇ and BDD anodes.Following a pseudo-first-order kinetics（R²>0.94）,the TC electrochemical oxidation with the Ti/Ti₄O₇ anode was favored by the high current density,high membrane flux and low initial TC concentration.Under the optimum conditions of 15mA·cm^-2 current density,40 L·h^-1·m^-2 membrane flux and 5 mg·L^-1 initial TC concentration,the TOC removal efficiency reached 76.8%;the concentration changes of NH₄⁺,NO₃^-and NO₂^-in solution were consistent with those of the Ti/Ti₄O₇ anode;the energy consumption was 121.5 KWh·kg^-1 TOC.（3）Study on the intermediates and pathway of the TC electrochemical oxidation.The complete TC electrochemical oxidation pathway was identified by analyzing the degradation intermediates detected by HPLC-MS and GC-MS.The toxicities of these intermediates were assessed by QSAR analysis to facilitate the understanding of the measured toxicity change during TC electrochemical oxidation.For the first time,a complete pathway of TC electrochemical oxidation was proposed based on 25 identified intermediates,categorized as primary intermediate（（1）,m/z=461）,secondary intermediates（（2）,（3）and（4）,m/z=432,477 and 509）,tertiary intermediates（（5）,（6）and（7）,m/z=480,448 and 525）,key downstream intermediate（（8）,m/z=496）,downstream intermediates（（9）to（15）,m/z=412,451,396,367,298,351and 253）and short-chain carboxylic acids（oxalic acid,formic acid,carbamic acid and2-oxo-malonic acid）.These intermediates possessed different toxicities according to QSAR estimates,and intermediates（1）,（2）and（3）showed the highest toxicity.The production of these toxic intermediates delayed the toxicity decrease during TC electrochemical oxidation,fitting well with the results of toxicity tests.（4）Study on the removal of TC by the Ti₄O₇ REM coupled with ozonation.In this study,the technology of the Ti₄O₇ REM coupled with ozonation was developed,in which process the ozonation,membrane filtration and electrochemical oxidation were combined to synergistically removal TC from water.Via the analysis of the degradation intermediates,the changes of the type and concentration of degradation intermediates were explored.Taking into account the O₃ dosage,TOC removal efficiency and current density,the optimum conditions for TC degradation and mineralization were proposed.The effects of the Ti₄O₇ REM coupled with ozonation on the removal of other typical drugs such as metoprolol,ciprofloxacin,and sulfamethoxazole were also investigated.This study found that the electrochemical activity of the Ti₄O₇ REM coupled with ozonation had a high activity for the degradation and mineralization of TC and other drugs in water.When TC was removed by the Ti₄O₇ REM coupled with ozonation,ozonation was first performed in the non-energized Ti₄O₇ REM(5 mg·L^-1 initial TC concentration)and thus ozonation was coupled with membrane filtration.At the optimal conditions of 2 mg·min^-1 of O₃ dosage for 20 min,the TC removal efficiency reached above 97%,the TOC removal efficiency was 9.1%,and TC is oxidized into large numbers of macromolecule organics and small molecule organics such as carboxylic acids.After that,O₃ was stopped and the TC solution was further treated by electrochemical oxidation over the Ti₄O₇ REM.At the optimal conditions of 2 mA·cm^-2of current density for 60 min,the TOC removal efficiency reached 77.2%and the type and intensity of degradation intermediates were significantly reduced.Compared to ozonation coupled with membrane filtration and electrochemical oxidation over the Ti₄O₇ REM,the Ti₄O₇ REM coupled with ozonation had high TC removal efficiencie（120 min,97.2%）and TOC removal efficiencie（120 min,77.8%）,low energy consumption(TOC 77%,101.5 KWh·kg^-1).Under the optimum conditions of 2mA·cm^-2 current density and 2 mg·min^-1 O₃ dosage for 80 min,the TOC removal efficiency reached 77.2%;the concentrations of NH₄⁺,NO₃^-and NO₂^-in solution increased slightly after 20 minutes reaction and then increased significantly;the bioluminescence inhibition ratio changed little during the first 20 minutes reaction（54.53%to 50.55%）and then decreased significantly（50.05%to 14.47%）.The removal of metoprolol,ciprofloxacin,and sulfamethoxazole by the Ti₄O₇ REM coupled with ozonation achieved high drug（98.8%,96.2%and 92.5%）and TOC removal efficiencies（77.8%,81.1%and 75.1）.The technology of the Ti₄O₇ REM coupled with ozonation could be used in the research and practice of other drug removal in water.In this study,high-performance Ti/Ti₄O₇ flat anode and tubular porous Ti₄O₇reactive electrochemical membrane were respectively prepared by plasma spraying and extrusion-calcination,combined with structural and performance analysis.For the first time,a complete pathway of TC electrochemical oxidation was proposed based on 25identified intermediates,and the toxicities of these intermediates were assessed by QSAR analysis to facilitate the understanding of the measured toxicity change during TC electrochemical oxidation.The Ti₄O₇ REM was coupled with ozonation to develop a new type of water treatment technology with relatively high organic material removal efficiency and mineralization efficiency,and relatively low energy consumption.