Kun Yang ,Yuyu Liu *,Jiawen Liu ,Jinli Qiao

1 Institute of Sustainable Energy/College of Science,Shanghai University,Shanghai 200444,China

2 State Environmental Protection Engineering Center for Pollution Treatment and Control in Textile Industry,College of Environmental Science and Engineering,Donghua University,Shanghai 201620,China

3 Shanghai Jinyuan Senior High School,Shanghai 200333,China

Keywords:Multilayer structured electrode Nano-hollow sphere Antibiotics Electro-oxidation

A B S T R A C T In this study,electrodeposition and thermal decomposition were alternatively used for the fabrication of a series of novel multilayer-structured SnO2–Sb–Ce/Ti(SSCT)electrodes,and their physiochemical and electrochemical properties were investigated for electrochemical oxidation of tetracycline(TC)in aqueous medium.Experimentally,after the SnO2–Sb–Ce(SSC)composite was electrodeposited for 120 s on the titanium substrate in aqueous solution,the outer thermal coatings composed of SSC were synthesized by a hydrothermal method.Both influences of electrodeposition time(T ed)and thermal decomposition time(T td)were investigated to obtain the optimum preparation.It was found that when increasing T ed to a certain extent a longer lifetime of electrode can be achieved,which was attributed to a more solid interlayer structure.A notable SSCT T ed,T td electrode,i.e.,SSCT3,10,which was prepared through three times of 120 s'electrodeposition(T ed=3)and ten times of thermal decomposition(T td=10)obtained the highest oxygen evolution potential 3.141 V vs.SCE.In this selected electrode,when 10 mg·L-1 initial TC concentration was added to this wastewater,the highest color removal efficiency and mineralization rate of TC were 72.4%and 41.6%,respectively,with an applied electricity density of 20 mA·cm-2 and treatment time of 1 h.These results presented here demonstrate that the combined application of electrodeposition and thermal decomposition is effective in realization of enhanced electrocatalytic oxidation activity.

1.Introduction

Nowadays,large amounts of various antibiotics are used routinely for the prevention and control of diseases in animal husbandry and aquaculture industries[1,2].However,the widespread use of antibiotics around the world has resulted in extensive environmental accumulation[3,4].Various potential risks of antibiotics including increasing antibiotic resistance of pathogens,damaging the human organs,causing super infection,as well as contaminating aquatic environments,have increased special attention[5,6].Among all the antibiotics,tetracycline(TC)as a class of broad-spectrum antibiotics is commonly used in the treatment of a variety of bacterial infections,such as Gram-positive,Gram-negative aerobic bacteria and Rickettsia[7,8].TC is of particular interest,since it is being widely applied for human beings and livestock,especially it is fed to animals in large quantities as growth promoters[9,10].

During the last decades,numerous wastewater treatment technologies have been developed to reduce the environmental risks of antibiotics[9,11–16].At present,traditional biological methods require multiple processing units to reduce antibiotic toxicity,which inevitably lead to higher operating costs and unsatisfactory treatment effects[11,17].Other traditional wastewater treatment technologies such as adsorption[9],membrane processes[18],and activated sludge[19,20]may control the mobility and spread of antibiotics,but can not sufficiently remove antibiotics from sewage.In this regard,there is a great need to develop a highly efficient,maneuverable and environmental compatible treatment technique.

Electrochemical oxidation shows a remarkable ability to remove recalcitrant organic contaminants such as antibiotics,and therefore evokes great attention[10,21–26].The selection of electrode material is a decisive factor for the electrode performance.In recent years,dimensionally stable anodes with distinct advantages,such as high stability,high activity and low cost,have been widely investigated for electrochemical applications[27–29].As an illustration,Ti/SnO2–Sb is reported to destroy compounds completely[30,31],but two major limitations confined its large-scale application:(i)relatively short life span[30],and(ii)low oxygen evolution potential[32].The literature survey reveals extensive efforts have been devoted on the improvements of doping effect with different metal options,as well as the selection of loading method.However,hither to there are limited studies on exploring the importance of combined electrode preparation methods,in particular,the combination of hydrothermal synthetic methods instead of the traditional direct doping method.

Based on above considerations,with the aim of obtaining the durable and high overpotential electrode,we used both electrodeposition and thermal decomposition to prepare a series of multilayer structured SnO2–Sb–Ce/Ti(SSCT)electrodes.To illustrate,the interlayer fabricated by electrodeposition was expected to effectively protect the titanium substrate and prolong the service lifetime of electrodes.SnO2–Sb–Ce with a nano-hollow sphere structure prepared by a hydrothermal method was deposited as outer coatings.The main goal of this study is to investigate the influences of electrodeposition time(Ted)and thermal decomposition time(Ttd)to remediate antibiotics via electrochemical oxidation.The scanning electron microscope(SEM),X-ray diffraction(XRD)and linear sweep voltammetry(LSV)characteristics were investigated to optimize the preparation parameters in affecting the removal efficiency of TC.

2.Materials and Methods

2.1.Electrode preparation

2.1.1.Titanium surface treatment

Pure titanium sheets(99.9%,30 mm×30 mm×1 mm)were pretreated by the following steps:Initially,the sample sheets were polished on abrasive papers from 600-mesh to 1200-mesh accordingly,rinsed with deionized water,then degreased in 20%NaOH at 80°C for 60 min.Afterwards,the samples were etched in 30%HCl for about 180 min until the titanium surface reached a uniform gray tone.Finally,all the sheets were thoroughly washed with deionized water.

2.1.2.Preparation of SnO2–Sb–Ce electrode

In this study,an interlayer of SnO2–Sb–Ce was firstly electrodeposited on the titanium substrate in an aqueous solution containing 0.5 mol·L-1SnCl4·5H2O,0.07 mol·L-1SbCl3,0.005 mol·L-1Ce(NO3)3·6H2O,0.3 mol·L-1C6H12O6·H2O and 0.1 mol·L-1HNO3with a constant current density of 4 mA·cm-2for 120 s using a direct current(DC)power supply(PWS4721,Tektronix Ltd.United States).After electrodeposition,these electrodes were cleaned with deionized water and annealed in a muffle furnace at 500°C for 30 min.For comparison purpose,above procedures were repeated for two,three and four times,respectively.

Later,the SnO2–Sb–Ce nano-hollow spheres were hydrothermally prepared at 180°C for 24 h by in a deionized water and ethanol(1:1)solution containing 0.1 mol·L-1SnCl4·5H2O,0.014 mol·L-1SbCl3,0.001 mol·L-1Ce(NO3)3·6H2O and 0.3 mol·L-1C6H12O6·H2O.Then the products obtained were repeatedly washed with absolute acetone and deionized water for 15 to 20 times,and centrifuged at a speed of 11000 r·min-1, finally dried at 80 °C for 12 h.50 mg of citric acid monohydrate(CA)was firstly dissolved in 5 ml of ethylene glycol(EG)at 60°C to prepare glycol citrate and then 25 mg of above solid powders was added,well mixed to obtain the SnO2–Sb–Ce slurry.The slurry was carefully and uniformly coated on the surface of one pre-electroplated titanium plate(30 mm×30 mm)with a pipette.Then they were dried at 80°C for 10 min and calcinated at 550°C for 10 min.Such procedure was repeated for four,nine and fourteen times,respectively.Finally,crystallization of the aged films was induced by calcination at 550°C in air for 5 h.Hence,a total 12 electrodes were obtained and denoted as SSCTTed,Ttd(Ted:number of times for electrodeposition;Ttd:number of times for thermal decomposition).

2.2.Analytical methods

2.2.1.Physicochemical characterization

The surface microstructure and morphology of SSCTTed,Ttdelectrodes were characterized by SEM(Hitachi Industrial Equipment Systems Co.,Ltd.,Japan).The crystal structures of coatings and the compositions were studied by XRD using Cu Kαradiation(Rigaku Corporation,Japan).The XRD patterns were taken for 2θ angles from 10°to 80°at a scan rate of 0.02(°)·s-1.The accelerating voltage and the applied current density were 35 kV and 20 mA·cm-2,respectively.

2.2.2.Electrochemical measurement of the electrodes

LSV characterization of SSCTTed,Ttdelectrodes was carried out in an unstirred solution using an electrochemical analyzer(CHI760D,Chenhua Instrument Shanghai Co.Ltd.,China).A conventional three electrode cell system was used in this experiment with as-prepared electrode as the working electrode,a saturated calomel electrode(SCE)as the reference electrode and titanium plate as the counter electrode.This process was conducted in an aqueous solution of 0.5 mol·L-1H2SO4and 0.1 mol·L-1Na2SO4with a scan rate of 20 mV·s-1between 0.5 and 3.5 V(vs.SCE).Unless otherwise stated,the temperature was kept at 20°C during the whole measurement process.

2.2.3.Accelerated service life test

The accelerated service life tests were performed by anodic polarization of the SSCTTed,Ttdelectrodes in an aqueous solution of 0.5 mol·L-1H2SO4and 0.1 mol·L-1Na2SO4.The current density was controlled at the constant 100 mA·cm-2,which was little lower than that applied by Chen et al.[33,34].The electrode was deemed to be deactivated when the cell voltage reached by 5 V by a DC power supply(PWS4721,Tektronix Ltd.United States).

2.2.4.Tetracycline electrolysis

TC was selected as a model antibiotic with an initial concentration of 5 mg·L-1(0.5 mol·L-1H2SO4and 0.1 mol·L-1Na2SO4as electrolyte),and the volume of TC solution in a cylindrical electrolytic cell is 200 ml.The electrical current was provided by a DC power supply(Wuhan Landian Electron Co.,Ltd.,China)and the applied current density is 20 mA·cm-2.Absorbance of the liquid samples was recorded by a UV2200 UV–VIS spectrophotometer(Shanghai Sunny Hengping Scientific Instrument Co.,Ltd.,China)at the wavelength of 355.7 nm every 10 min.The degraded samples were taken into the TOC analysis after a 1-hour electrocatalysis.

3.Results and Discussion

3.1.SEM and XRD characterization

Fig.1 shows the scanning electron micrographs of the SSCTTed,Ttdelectrodes.It is easily observed from Fig.1(a)that the titanium substrates were not absolutely covered by Ted=1.As shown in Fig.1(b),there are also a few bare titanium substrates.However,from Fig.1(c),it can be found that the titanium substrates are totally covered by Ted=3.This indicates the preparation of the electrode does require more than Ted=3 at a constant current density of 4 mA·cm-2for 120 s,so that the titanium substrates can be effectively protected and hence,the service life of SSCTTed,Ttdelectrodes is lengthened.From a microscopic perspective,the SnO2coatings on SSCT3,10electrode reveal a nano-hollow sphere morphology,as shown in Fig.1(d).Fig.1(e)shows relatively uniform nano-hollow sphere SnO2with 6 μm diameter on a SSCT3,10electrode.While in the case of the SSCTTed,5electrodes,the diameter of hollow sphere SnO2is significantly decreased and non-spherical particles occur.This may be due to the hydrothermal coating did not swell completely in a short calcination period.Fig.1(f)shows the surface morphology of the SSCT3,15electrode.The aggregation of SnO2nano-hollow spheres is observed,which is most likely due to the excessive calcination of the hydrothermal coatings,or the oxide crystals of Sn,Sb and Ce aggregated.By comparing three different SSCT3,Ttdelectrodes,it is observed that the surface of SSCT3,10was roughest and crystals distributed uniformly.Based on the findings by An et al.[35],such structure may contribute more surface activity sites and higher specific surface area for the electrocatalytic reaction.Note that the surface microstructure and morphology of SSCT4,Ttdare quite similar to SSCT3,Ttd,thus not specifically described here.

Fig.1.SEM images of the SSCT1,5(a),SSCT2,5(b),SSCT3,5(c),SSCT3,10(d and e)and SSCT3,15(f)electrode surfaces.

The X-ray diffracto gram was employed to probe the crystal structure of the SSCT3,10electrode.The characteristic of SnO2cassiterite with a rutile type structure is clearly detected in Fig.2.The diffraction patterns can be indexed to SnO2and the characteristic 2θ valued at(110),(101)and(220).Notably,the(220)plane is the most intense peak which suggests the preferred orientation crystallographic direction.The strong peak intensities of other planes indicate the good crystallinity of SnO2for SSCT3,10electrode,which may due to the introduction of Sb and Ce.No diffraction peaks of antimony oxides and cerium oxides are detected.This is mainly due to the lattice doping in SnO2[36].Besides,no peak for TiO2is observed and only a few peaks of metal Ti can be found,which suggests that as-reported preparation strategy can effectively protect titanium substrate from being oxidized during the preparation process.

3.2.Electrochemical performance

Fig.2.X-ray diffraction pattern of the SSCT3,10 electrode.

Fig.3.Linear sweep voltammetry curves of the SSCT T ed,T td electrodes in 0.5 mol·L-1 H2SO4 and 0.1 mol·L-1 Na2SO4 aqueous solutions,scan rate:20 mV·s-1.

Fig.4.Comparison of the service life of SSCT T ed,T td electrodes in 0.5 mol·L-1 H2SO4 and 0.1 mol·L-1 Na2SO4 aqueous solution under a constant current density of 100 mA·cm-2.

Fig.3 shows the linear sweep voltammograms of prepared electrodes in the aqueous solution of 0.5 mol·L-1H2SO4and 0.1 mol·L-1Na2SO4with a scan rate of 20 mV·s-1.It is clearly observed in Table 1 that the oxygen evolution potential of the SSCT3,10electrode was 3.141 V,which is higher than the other electrodes with the oxygen evolution potentials ranging from 2.083 V to 2.479 V.It is likely that the doping of Sb and Ce increase the oxidation potential of the metal,especially,in situ coating induced by a hydrothermal synthesis could result in higher specific surface area,smaller particle size of the catalyst,and more active sites.It is well known that the oxygen evolution potential is a principal parameter that determines the performance of electrolysis[32,37,38],and higher oxygen evolution potential is desirable for organic oxidation.Above analyses authenticate that our composite multilayer electrode featured with nano-hollow sphere structure could inhibit the power from generating oxygen.

3.3.Accelerated life tests

The results of accelerated service life measurements are presented in Fig.4.The accelerated lifetime of SSCT3,15was 32.5 h,which is 3.6 times longer than that of the SSCT1,5electrode.It is found that the accelerated lifetime of the electrodes always lengthened with the increase of Ttd.The electrochemical stability of the electrode with Ted=3 is stronger than others.These results demonstrated that an increasing Tedcould improve the electrochemical stability of the SSCTTed,Ttdelectrodes.However,the accelerated lifetime of SSCT4,Ttddid not prolong with the increasing number of electrodeposition time.The difference in the electrode stability induced by Tedcan be explained by the presence of mechanical stress in the outer coatings,which is generated by the thermal expansion between the coating and the titanium substrate[39].To summarize,the durability of the SSCTTed,Ttdelectrodes follows the order as:SSCT1,5＜SSCT1,10＜SSCT1,15＜SSCT2,5＜SSCT2,10＜SSCT4,5＜SSCT2,15＜SSCT4,10＜SSCT4,15＜SSCT3,5＜SSCT3,10＜SSCT3,15.By comparing the accelerated lifetime of those electrodes,the SSCT3,10electrode was deemed to be the best-performing electrode under our experimental conditions.

3.4.Tetracycline electrolysis

Fig.5 shows the color removal efficiencies of TC on the SSCTTed,Ttdelectrodes after 60 min of electrolysis.As shown in Fig.5(a),when Tedwas increased from 1 to 3,the removal efficiency is significantly enhanced at the same Ttd.But when continuously increasing Ted,thecolor removal efficiency dropped slightly.This may be due to the formation of the interlayer with in sufficient times of electrodeposition.However,as the number of Tedincreases,electrodeposition crystals would grow over the substrate.Then it is almost possible to conduct electrons on the electrode at Ted=4,which means the interlayer was too thick.Fig.5(b)shows the removal efficiency has a trend from rise to decline with increasing Ttdwhen at the same Ted.TC could be most effectively decolorized on the SSCT3,10electrode,where the discoloration of TC could reach 72.4%.This may be due to the catalyst on the SSCTTed,10electrodes generated by hydrothermal synthesis process,which provides more active sites and results in a higher production of hydroxyl radicals from water oxidation.

Table 1Oxygen evolution potentials of different SSCT electrodes at 1 mA·cm-2 current density

The kinetics of TC electrolysis on SSCTTed,Ttdelectrodes at a constant current density were fitted with pseudo- first-order kinetics.The value of kinetic constant k is calculated and summarized in Table 2.It is clearly found that the maximum rate of discoloration observed at the SSCT3,10electrode was up to 2.3 times than that of the SSCT1,0electrode(1.24,1.05,1.40,1.24 and 1.11 times to SSCT3,5,SSCT3,15,SSCT1,10,SSCT2,10,and SSCT4,10ones,respectively).

Fig.6.TOC removals attained on different electrodes in 0.5 mol·L-1 H2SO4 and 0.1 mol·L-1 Na2SO4 solution.

Fig.6 shows the TOC removalobtains identical variation tendency as discoloration in general.The TOC depletions on the SSCT3,10electrode reveals the best performance,reaching mineralization efficiency high as 41.6%,while 17.7%is received on the SSCT1,5electrode.It can be also found that when Tedis further increased from 3 to 4,the TOC mineralization did not increase accordingly.This trend suggests that in the cases of identical outer most morphologies,the electric voltage rises with the increasing thickness of electrodeposited coatings,which would promote a competitive side reaction,i.e.,the oxygen evolution reaction.Moreover,similar appearances were found on the other SSCT electrodes with the same Ttd.It can be concluded that less number of Ttdis insufficient to provide effective catalytic interface.Furthermore,the differences in the mineralization ability of these electrodes might be due to the increasing number of Ttdthat could aggrandize the active sites to generate more hydroxyl,thus the TOC removal can be accelerated.

4.Conclusions

In this study,electrodeposition and thermal decomposition were alternately applied to fabricate a novel multilayer-structured SSCTTed,Ttdelectrode with nano-hollow sphere coatings.The prepared electrodes were used for electrochemical oxidation of TC in an electrolyte solution of 0.5 mol·L-1H2SO4and 0.1 mol·L-1Na2SO4.The lifetime and electrocatalytic activity of anodes were significantly improved with the increase of electrodeposition time and thermal decomposition time.However,the further increase of above treatment time may cause the aggregation of nano-hollow spheres and debase the electrocatalytic activity of the electrode.Among all prepared electrodes,the SSCT3,10electrode obtained the highest oxygen evolution potential,indicating that the optimal number of times for electrodeposition and thermal decomposition were 3 and 10,respectively.In addition,the best color removal efficiency and mineralization rate of TC were both achieved by the SSCT3,10electrode after 1 h.Our results suggest the formation of electrodeposition interlayer with thermal decomposition coatings achieved both favorable stability and catalytic activity,in particular,the performance of thermal coating materials can be improved by the hydrothermal synthetic method.

Table 2Rate constants for different SSCT electrodes

Acknowledgments

The author would like to thank the State Environmental Protection Engineering Center for Pollution Treatment and Control in Textile Industry,the College of Environmental Science and Engineering,Donghua University.