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Contents lists available atScienceDirectChemical Engineering Journaljournal sonophotocatalysis and peroxymonosulfate activationover 3D urchin-like MoS2/C nanoparticles for accelerating levofloxacinelimination: Optimization and kinetic studyLibin Zenga,1, Shiyu Lia,1, Xinyong Lia,b ,, Jianan Lia, Shiying Fana, Xin Chena, Zhifan Yina,Moses Tadéb, Shaomin LiubaState Key Laboratory of Fine Chemicals, Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology,Dalian University of Technology, Dalian 116024, ChinabDepartment of Chemical Engineering, Curtin University, GPO Box U1987, Perth, WA 6845, AustraliaHIGHLIGHTSAn electron transition layer strategywas successfully introduced intoMoS2/C materials.The constructed 3D urchin-like MoS2/C core/shell structure was crucial tothe electron-hole separation effi-ciency.It displayed the highest antibiotic re-moval performance with0.0702 min1rate in PMS-assistedsonophotocatalytic reactor.The possible mechanism and electrontransfer path of this multi-couplingsystem were elaborated.GRAPHICAL ABSTRACTARTICLE INFOKeywords:Carbon buffer layerUrchin-likeSynergistic effectSonophotocatalytic systemPMS activationActive radicalsABSTRACTSolar response catalysts designed, sustainable wastewater-purification technology developed, possesses hugeadvantage in resolving the global issue of scarcity of drinkable water availably. In this work, a smart 3D core-shell urchin-like MoS2/carbon photocatalyst was developed with special hollow interior and the internal carbonlayer embedding, which could largely enhance the electrical conductivity of MoS2, accelerate the electrontransfer efficiency, and provide more active sites The effect of the pollutant concentration, the initial pH, co-existing anions, peroxymonosulfate (PMS) dose and ultrasonic intensity are carried out to explore the perfor-mance of sonophotocatalytic coupling system for levofloxacin degradation under visible light irradiation.Following the optimized condition, the MoS2/C catalyst presented excellent degradation rate (0.0702 min1)inPMS activated sonophotocatalytic system (synergy index reaching to 2.6). The active radical trapping, de-gradation intermediates and possible catalytic mechanism were investigated and proposed, respectively. Basedon these results, the unique development, fabrication, core/shell strategy, and high-efficiency coupling approachwould motivate new models of wastewater treatment technologies. 20 March 2019; Received in revised form 19 June 2019; Accepted 21 June 2019Corresponding author at: State Key Laboratory of Fine Chemicals, Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School ofEnvironmental Science and Technology, Dalian University of Technology, Dalian 116024, China.E-mail Li).1L. Zeng and S. Li contributed equally.Chemical Engineering Journal 378 (2019) 122039Available online 27 June 20191385-8947/ © 2019 Elsevier B.V. All rights reserved.T
1. IntroductionWith the accelerating development of pharmaceutical industry,antibiotics have been extensively used in thefields of human/veter-inary medicine and aquaculture[1,2]. As a member of broad-spectrumfluoroquinolone antibiotics, levofloxacin (LEV) plays a significant rolein oral and intravenous formulations because of its excellent bacter-icidal activity and tissue penetration[3,4]. Due to its high solubility(16.98 mg mL1at 298.15 K) and special two dissociation constants(pKa1= 5.70; pKa2= 8.00)[5], the residue of pharmaceutical industry,hospital discharges even the body excretion of livings, have resulted inlargely accumulating in underwater and surface water. Especially forrecent years, the negative effects of LEV on human beings and eco-system have been frequently reported. More unfortunately, it has raisedsevere problems, such as the persistent drug resistance, even super-bacteria breeding[6,7]. Consequently, developing an efficient route toeliminate LEV from wastewater is becoming notably urgent.Several techniques have been reported for the elimination of LEV inrecent decades, such as photocatalysis[8], electrocatalysis[9], micro-bial-based catalysis[4], Fenton[10], sonocatalysis[10], ozonation[11], etc. Among these methods, photocatalysis, as a type of advancedoxidation processes (AOPs), has become a pivotal strategy due to itsmild conditions, high oxidative radicals, and renewable solar energyutilized[1215]. However, limited utilization of considerable visiblelight and rapid recombination of excited electrons/holes seriously blockits extensively industrial application potential[16,17]. Therefore,constructing a robust photocatalyst for environmental LEV removal isstill necessary.Recent progress in graphene studies is inspiring tremendous interestin a series of two-dimensional (2D) semiconductor materials[18,19].Particularly, molybdenum sulfide (MoS2), with a suitable visible-light-excited band gap, is known as a promising photocatalyst. Because of itsintriguing rich active sites, earth-abundant, high electron mobility, andenvironmental compatibility[2022], it is gradually becoming poten-tial new-generation materials for widely applications especially in en-vironmental restoration[23,24]. However, the short charge transferpath, easily agglomeration and poor conductivity, which largely sup-press its practical applications[25,26]. In order to overcome itself in-trinsic shortcomings, numerous strategies have been developed toconstruct superior catalysts. For instance, depending on morphologyregulation, Lou et al. reported that the synthesized novel hierarchicalMoS2microboxes[27]highly possessed active sites and exposed largeaccessible surfaces. Afterward, his group further designed N-doped C/MoS2nanoboxes to ameliorate conductivity by electronics structureengineering[28], which presented excellent specific capacity of ap-proximately 1000 mA h g1and remarkable stability up to 200 cycles.Recently, hollow micro-/nanostructures strategy was universally em-ployed as basically synthetic concept to design advanced catalyticmaterials because of its special structure and intriguing properties, suchas efficient mass transport, high surface area and multi-step visible lightabsorption[29]. Chen et al. fabricated submicrometer-sized TiO2hollow spheres for the degradation of phenol[30], which largely en-hanced high photocatalytic activity relative to commercial P25 catalyst.Additionally, Xie and his co-workers developed silver-loaded yolk-shellTiO2hollow microspheres, which displayed prominent separation andtransfer efficiency of photo-excited carriers[31]. These unique nanos-tructures indicated significant synergistic effect and superior photo-catalytic performance for pollutants removal. Nowadays, based on 3Dsea urchin-like structure possessed high packing density and multi-electron channel, which has been extensively applied to constructNiCo2S4microsphere[32], porous Ni0.5Co0.5Se2[33],Fe3O4@PDA-Aghollow microspheres[34], TiO2@C composite[35], etc., indicating thatcoating with carbon layer is also a powerful strategy to enhance elec-trons transfer rate[36]. Also, recent works further explained thatcombined with MoS2with carbon layer could largely improve theelectrons move rapid[37,38]and enhance the catalytic performance[21,39]. In view of the above-mentioned considerations, the construc-tion of hollow hierarchical urchin-like MoS2wrapped by carbon layerseems to be more promising photocatalyst for pollutant elimination.With the progressive development of multi-technology coupling,many studies strongly revealed that the combination with two evenmultiple means is an effective strategy to strongly enhance the de-gradation efficiency of contaminant, such as photo-Fenton[40], pho-toelectrocatalysis[41], photocatalysis/O3[42], photocatalysis/biode-gradation[43] and sonophotocatalysis[44]. Among the above couplingsystem, sonophotocatalysis has drawn enormous attention due to itshigh synergistic effect [45,46], which is contributed to effectively ex-tending the transfer path of the photo-excited electron-hole pairs.Under sono-assisted conditions, the presence of ultrasound waveswould force the pyrolysis of oxygen and water molecules to directlygenerate numerous reactive oxygen species (ROS). Most importantly,because of its the acoustic cavitation phenomenon, it could dis-aggregate catalyst particles and enhance the mass transfer betweeninterfaces[47]. Recently, sonophotocatalytic technique opens up agreat potential to effectively deal with industrial wastewater. For ex-ample, sono-assisted photocatalysis was employed to mineralizestyrene-acrylic acid copolymer and displayed a significant enhance-ment in degradation rate (more than two times)[48]. Xiong et al. de-monstrated that a dual-frequency ultrasonic assisted (20/40 kHz) pho-tocatalytic system was designed to achieve efficient elimination abilityfor methylene blue, which the synergy index was up to 2.36[47].Furthermore, the higher oxidative group sulfate radical (SO4%, oxi-dation potential 2.53.1 V) activated by peroxymonosulfate (PMS) isgradually playing an emerging character in AOPs[49]. Similarly toFentons reaction, coupling transition metal photocatalytic or sonicactivated PMS is a remarkable method to activate and produce SO4%,which could rapidly degrade organic pollutants[5052]. However, tothe best of our knowledge, the research based on sonophotocatalyticsystem with activated PMS simultaneously is rarely reported to efficientremoval antibiotics.On basis of the above analysis, we designed a highly uniform 3Durchin-like MoS2/C core/shell photocatalyst by low-cost glucose as thecarbon source (Scheme 1). This unique urchin-like nanostructures dis-played large surface area and numerous 3D electron-conducting chan-nels, which improved the separation efficiency of photo-excited elec-tron-hole pairs under visible-light irradiation. Furtherly, the effects ofcatalytic conditions, including PMS dosages, LEV concentration andultrasonic power intensity, were investigated to optimize the LEV de-gradation performance in the ultrasound assisted photocatalytic cou-pled system, and the main reactive active species (%OH and SO4%)were also detected byin situelectron paramagnetic resonance technique(EPR). Importantly, the synergistic effect and possible proposed me-chanism of PMS activated sonophotocatalytic system was system-atically illustrated. Therefore, this work can provide deep insight intoconstructing advanced photocatalysts and offering an efficient electrontransfer platform to accelerate environmental pollutants removal.2. Experimental section2.1. Synthesis of urchin-like hierarchical silica templateUrchin-like hierarchical silica (UHS) template was synthesized by amodified method[53]. In details, the A solution including tetra-ethoxysilane (12.5 g), 1-pentanol alcohol (7.5 mL) and cyclohexane(150 mL) was added to an agitated B solution containing hex-adecylpyridinium bromide (5 g), urea (3 g) and deionized water(150 mL) to yield emulsion under stirring condition. The mixing solu-tion was vigorous stirred for 60 min to be aged and transferred to twoTeflon lined autoclaves with capacity of 250 mL. Then, the Teflon linedautoclaves were maintained at 120 °C for 6 h. After cooling to roomtemperature, the white sample was collected by centrifugation at8000 rpm for 5 min, washed several times with deionized water, andL. Zeng, et al.Chemical Engineering Journal 378 (2019) 1220392