Zwitterionic monolayer grafted ceramic membrane with an antifouling performance for the efficient oil-water separation

Tianyu Zhang,Qian Wang,Wei Luan,Xue Li,Xianfu Chen,2,Dong Ding,Zhichao Shen,Minghui Qiu,Zhaoliang Cui,Yiqun Fan,*

1 State Key Laboratory of Materials-Oriented Chemical Engineering,National Engineering Research Center for Special Separation Membranes,College of Chemical Engineering,Nanjing Tech University,Nanjing 211816,China

2 Zhangjiagang Institute of Nanjing Tech University,Suzhou 215699,China

Keywords:Ceramic membrane Zwitterionic Anti-fouling Hydration layer Oil/water emulsion

ABSTRACT Enormous demands on the separation of oil/water(O/W)emulsions in various industries,such as petrochemical,food and pharmaceutical industries,are looking for high performance and energy-efficient separation methods.Ceramic membranes have been used to deal with O/W emulsions,for its outstanding characteristics of easy-operation,high-flux,and long-term stability.However,membrane fouling is still a challenge in the industrial application of ceramic membranes.Herein,antifouling ceramic membranes were fabricated by grafting zwitterions on the membrane surface via an environment-friendly two-step grafting method,which improves the antifouling property and permeability.Successful grafting of such zwitterion on the ceramic surface was assessed by the combination of FTIR and XPS characterization.More importantly,the hydration can be formed by electrostatic interactions layer on the modified membrane,which was confirmed by TGA characterization.The antifouling performance of prepared zwitterionic ceramic membranes in the separation of O/W emulsions was systematically tested.The results suggested that zwitterion can significantly improve the flux of ceramic ultrafiltration membrane,and can also improve antifouling property dramatically by reducing the irreversible fouling in the separation of O/W emulsions.Therefore,zwitterionic ceramic membranes hold promising potentials as an antifouling,highly efficient and green method in the practical purification of the O/W emulsions.

1.Introduction

Water shortage has become an urgent topic in the 21st century because of the population explosion [1,2] and tremendous water consumption in industry and agriculture [3].Therefore,purifying and recycling wastewater is of paramount significance for sustainable development.It is now widely recognized that membrane technology is a powerful panacea refreshing wastewater from various contaminants,accompanied by a series of advantages such as high efficiency,low energy consumption and chemical-free operation[4,5].As a typical kind of inorganic membrane materials,ceramic membranes possess some unparalleled merits such as high chemical durability,no swelling effect,excellent mechanical strength,and thus can be employed in harsh circumstances [6,7].Besides,ceramic materials are born with hydrophilicity,which provides ceramic membrane high water permeability [8,9].However,membrane fouling,which is caused by the adsorption and deposition of contaminants on the membrane surfaces and pore walls,significantly worsen the filtration performance,shorten membrane life,and dramatically confined membrane application[10-12].

Over the past decades,researchers have devoted themselves to developing antifouling membrane separation processes such as combining electrical fields and ultrasound[13]with membrane filtration or exploiting advanced antifouling membrane materials[14].However,more and more researchers have found that the surface properties of ceramic membrane will also have an important impact on the performance of ceramic membranes.There are many ways to modify the surface of ceramic membrane,the most widely used method to prevent fouling is grafting the membrane surface with special materials,that is surface grafting technology.Compared with coating,chemical vapor deposition,electroless plating and other methods,surface grafting technology is simple and feasible,low cost and easy for industrial production[13,15].The grafting reagents are usually chosen based on two principles:(1) long chain that can repel foulants with steric effect and (2) hydrophilicity that can form a hydration layer on the membrane surface to prevent foulants depositing on it.Therefore,some hydrophilic polymers such as poly (acrylic acid) [16],polyelectrolyte[17],poly(ethylene glycol)[18]and polyethylene glycol(PEG)derivatives[19]have been used to modify the ceramic membrane to design anti-fouling membrane.

Zwitterionic materials [20-22],which are made up of the cations and anions in the same organic compound,are wellknown to be effective in reducing fouling phenomena because of the formation of ‘‘free water” hydration layer surrounding the zwitterionic groups[23].It is reported that the zwitterion can bind more water molecules than PEG and thus possesses stronger repulsion to pollutant[24].The pioneering work was done by Jianget al.[25].They incorporated zwitterionic copolymer into polyacrylonitrile to fabricate ultrafiltration membranes and found that membrane fouling,especially irreversible membrane fouling,was remarkably reduced due to the presence of zwitterionic groups[26,27].The reported results have demonstrated that the zwitterion modified membrane could exhibit excellent anti-fouling performance.However,the preparation and application of zwitterionic ceramic membranes were barely reported.

In this study,we aimed at designing an antifouling interface based on ceramic membranes through the combination of zwitterionic monomers via surface modification.We were motivated to investigate if the zwitterionic monomers could react with normal ceramic membranes that are readily prepared by Al2O3.An environment-friendly two-step grafting method was proposed to graft zwitterions on the membrane surface.The characterizations of FTIR,XPS,and TGA were applied to verify the successful grafting of such zwitterion on the ceramic surface.The prepared zwitterionic ceramic membranes were also tested in the separation of O/W emulsions to systematically evaluate their separation,permeation,and anti-fouling performance.This work not only demonstrates the feasibility of grafting zwitterion on the surface of ceramic membrane,but also provides a way for future researchers to modify the surface of ceramic membranes.It also provides a new solution for the design and fabrication of antifouling ceramic membranes.

2.Experimental

2.1.Materials

Disc Al2O3ceramic membranes with an asymmetric porous structure are home-made,which have an average pore size of 100 nm,a diameter of 30 mm and a thickness of 2 mm.Unless otherwise specified,all chemicals and reagents were applied as received without any further purification.Soybean oil was purchased from Nanjing Chemical Plant. Sodium 2-bromoethanesulphonate (SBTS),(N,N-dimethyl-3-aminopropyl)trimethoxysilane (DMAPS),and ethanol were purchased from Sigma-Aldrich Company.The SBTS and soybean oil solutions were prepared with deionized water (DI water).

2.2.Fabrication of zwitterionic ceramic powder and zwitterionic ceramic membrane

The zwitterionic ceramic powder (ZCP) and zwitterionic ceramic membrane(ZCM)were fabricated by two-step grafting DMAPS and SBTS on ceramic powder and ceramic membrane,which was illustrated in Fig.1.In the first step,DMAPS was used to modify the ceramic membrane with the tertiary amino group.DMAPS was dropped into anhydrous ethanol slowly,and the mixture was stirred for 2 h to ensure well mixed.Then,ceramic powder and ceramic membrane were immersed in DMAPS ethanol solution at 25 °C for 12 h with stirring.After that,the obtained DMAPS grafted ceramic powder named ‘‘CP-DMAPS” and ceramic membrane named ‘‘CM-DMAPS” were then rinsed with ethanol and dried at 110 °C for 12 h.Herein,the various concentration of DMAPS was selected (2.5,5,10,15,20 and 25 mmol·L-1) to optimize the performance.

In the second step,SBTS was used to modify the CP-DMAPS and CM-DMAPS with the sulfonate group to synthesis zwitterions.Firstly,SBTS was added into DI water and stirred for 2 h to ensure well mixed,then CP-DMASP and CM-DMAPS were immersed in SBTS aqueous solution at 45 °C for 12 h with stirring.After that,the obtained powder and membrane named CP-DMAPS-SBTS and CM-DMAPS-SBTS individually were then rinsed with water and dried at 110 °C for 12 h.Herein,to determine the most suitable condition,the concentration of DMAPS was set as (0.02,0.06,0.1,0.2,0.4,0.6,0.8 mol·L-1).

2.3.Characterization of membranes

FITR spectroscopy (AVATAR-360,Nicolet,USA) is used to characterize the change of functional groups on the surface of ceramic powder.The spectra with a wave number ranging from 4000 to 400 cm-1were collected by cumulating 32 scans at a resolution of 4 cm-1.The elementary composition of the ceramic powder,CP-DMAPS and CP-DMAPS-SBTS surface was analyzed by X-ray photoelectron spectroscopy (XPS,ESCALAB 250,Thermo,USA),with a monochromatic Al Kα X-ray source.The thermal stability of ceramic powder and CP-DMAPS-SBTS was investigated using thermal gravity analysis (TGA,STA449F3,Netzsch,Germany) at a heating rate of 5 °C·min-1up to 800 °C in the atmosphere.After the unmodified and modified membranes were coated with gold powder by a vacuum electric sputter coater (SCD005,Bal-Tec,USA),the morphology of the membranes was observed by fieldemission scanning electron microscopy (FESEM,S 4800,HITACHI,Japan) under standard high-vacuum conditions.The fluorescence microscopy images of membranes were obtained by using an inverted microscope system (IX73,Olympus,Japan).The dynamic water contact angle of the membranes was determined using a DropMeter contact-angle system(A-100P,MAIST,China)in sessile drop mode by a video camera recording at 10 frames per second.

2.4.Membrane separation performance

The performances of the original and modified ceramic membranes were characterized by a cross-flow filtration system.Permeability of the membrane was calculated by the following Eq.(1):

whereVis the water or solution permeate volume through the membrane,Ais the effective filtration area (m2) and Δtis the permeation time (h),ΔPis the transmembrane pressure (MPa).The pure water permeability (PWP) was tested at 20 °C under a transmembrane pressure of 0.1 MPa,and was denoted asJ0.

For oil-in-water emulsions,soybean oil was employed as the oil phase.Span 80 was used as the emulsifier to obtain the stable 500 mg·L-1oil-in-water emulsions and the oil concentration was measured using a total organic carbon analyzer (TOC,liquid TOC II,America).The rejection of oil was defined in Eq.(2):

wherecfandcpare the oil concentration of the feed and permeate respectively.

Fig.1.The schematic illustration of the zwitterion grafting of the ceramic membrane.

2.5.Antifouling mechanism of membranes

The oil-in-water emulsion was quickly replaced in the stirred cell after half an hour permeation of pure water.The instant fluxJ0(L·m-2·h-1·bar-1,1 bar=0.1 MPa),was measured every 10 min based on the water quantity permeated through the membranes.After 2 h filtration of oil-in-water emulsions,theJ0was expressed asJ1and then the fouled membranes were washed with deionized water for 30 min and their pure water flux was re-evaluated.Consequently,the recovered water flux of cleaned membranes,J2(L·m-2·h-1·bar-1),was measured again.

To further analyze the fouling process in details,the flux recovery ratio (FRR),total fouling ratio (Rt),reversible fouling ratio (Rr)and irreversible fouling ratio(Rir)were calculated using the following equations [28],respectively:

3.Results and Discussion

Firstly,the ZCP was fabricated to prove the feasibility of this method and characterize the chemical component and stability of it.Then,the conditions of fabricating ZCM were optimized,and the anti-fouling property of it was proved.Finally,the ZCM was used to separate the O/W emulsion,which provides evidence that ZCM is antifouling by decreasing irreversible fouling.

3.1.Fabrication and characterization of zwitterionic ceramic materials

The mechanism of zwitterion grafting reaction can be explained into two steps:(1) the active tertiary amino group can be introduced to the surface of the membrane through the reaction between DMAPS and the hydroxyl on the ceramic powder,DMAPS is easily soluble in ethanol.Under 35°C and atmospheric pressure,the -OCH3group in the silane molecule is hydrolyzed under the action of the trace water of ethanol to form (N,N-dimethyl-3-aminopropyl) silanol.Due to the large number of hydroxyl groups on the surface and pores of the ceramic membrane,it will undergo a dehydration condensation reaction with the silanol in the modification solution to form a stable M-O-Si chemical bond;(2) grafting the -group on the CP-DMAPS by the reaction between tertiary amino of CP-DMAPS and SBTS.The tertiary amine on the CM-DMAPS replaces the Br in the SBTS to form a stable C-N-C chemical bond,thereby introducing the -group and obtain the CM-DMAPS-SBTS.To investigate whether the zwitterions have been grafted on the surface of ceramic membrane and the exiting condition of them,a series chemical characterizations of CMDMAPS-SBTS were conducted.Firstly,the original and modified ceramic powder were studied by FTIR (Fig.2a).IR band at 1430 and 2942 cm-1can be attributed to Al-O-Si bond formation[29].Peaks at 1200 cm-1are assigned to C-N stretching vibration of the quaternary ammonium group and at 1620 cm-1are attributed to O=S=O stretching vibration of sulfonic acid groups[30].The presence of these new peaks hints that DMAPS and SBTS molecules were effectively anchored to the alumina surface via a chemical reaction.

In addition to FTIR spectra,the occurrence of this modification was further studied by X-ray photoelectron spectroscopy.Various atomic concentrations of the ceramic powder,CP-DMAPS and CPDMAPS-SBTS were calculated in Table 1.With the atomic concentration increase of C (1s),Si (2p) and N (1s) as well as the appearance of the peaks(Fig.2b)at 100.1 eV(Si2p)and 398.4 eV(N1s),it can confirm that DMAPS molecules were grafted on the ceramic powder surface successfully.Moreover,the Si (2p) and N (1s)atomic concentration drop,while the C (1s),S (2p) and Na (1s)atomic concentration increase by the second step grafting.These results firmly demonstrate the successful grafting of sulfonic zwitterion on the ceramic powder surface.In addition,the detection depth of XPS is not more than 10 nm[31],while the Al atom is still detected,which indicates the modified zwitterion layer is relatively thin and almost has no effect on the morphology of ceramic membrane.Therefore,the feasibility of zwitterionic membranes fabrication is proved.

Table 1Chemical composition of alumina powders surface

The thermal stabilities of the modified membrane play a significant role in the membrane application system.Here,the thermal stabilities of ceramic powder and CP-DMAPS-SBTS powder were measured using TGA.As shown in Fig.3,the mass of ceramic membrane keeps stable with temperature increasing from 0 to 500 °C,because of the thermal stability of alumina.For CP-DMAPS-SBTS,there is about 10% mass loss with temperature increasing from 250°C to 300°C,which is due to the decomposition of the zwitterion.The result proves the thermal stability of CP-DMAPS-SBTS under 250 °C,but also indicates the successful fabrication of zwitterion on the ceramic membrane.Interestingly,CP-DMAPS-SBTS loses mass at 70 °C and accordingly there is a strong energy peak at 70 °C.Because of the existence of the hydration layer on the zwitterionic membrane [32],the water combined on the CPDMAPS-SBTS will lose mass when heated.Therefore,the zwitterionic ceramic membrane can be fabricated by two-step grafting and the antifouling property can be obtained for the hydration layer on it.

Fig.2.Chemical composition characterization of membranes.Reaction mechanism of(a)Overall infrared spectroscopy and(b)XPS wide-scan spectrum of CM-DMAPS-SBTS.

Fig.3.The TG curve and DSC curve curve of (a) CP-DMAPS-SBTS and (b) ceramic powder.

3.2.Optimization of grafting conditions

After proving the possibility that zwitterion can be grafted on the ceramic materials,the fabrication conditions are investigated to prepare optimal ZCM.Because ZCM is fabricated by two-step grafting,the concentration of DMAPS and SBTS are optimized successively.Grafting organic materials on the ceramic membrane surface have a significant effect on the water flux of modified membranes [33-35].Thus,the DMAPS concentration is investigated by normalized volumetric flux,J/J0,of CM-DMAPS.As shown in Fig.4(a),theJ/J0decreases from 100% to 47% with the DMAPS concentration increasing from 0 mmol·L-1to 10 mmol·L-1,which indicates that membrane surface grafting molecular density raises gradually.ThenJ/J0remains unchanged when DMAPS concentration of the reactant solution is above 10 mmol·L-1,which indicates that the grafting density of DMAPS is saturated when the concentration of DMAPS is 10 mmol·L-1.The results can be explained by the change of CM-DMAPS hydrophilicity,which can be proved by the water contact angle.As seen from Fig.4(a),the initial WCA increases from 28.2° ± 5° to 81.2° ± 5° with DMAPS concentration from 0 to 10 mmol·L-1,then keeps stable with concentration larger than 10 mmol·L-1.This phenomenon can be explained by the hydrophobic methyl on DMAPS that decreases the hydrophilicity of ceramic membrane.According to the analysis results of Fig.4(a),DMAPS concentration of 10 mmol·L-1was selected as the best concentration of DMAPS to provide necessary precondition for the next step grafting with less reagent.

On the basis of DMAPS grafting,the best SBTS concentration was investigated by testing theJ/J0of CM-DMAPS-SBTS in the second step.As shown in Fig.4(b),firstly theJ/J0of CM-DMAPS-SBTS raises from 47%to 113%with SBTS concentration increasing from 0 to 0.1 mol·L-1.Then it tends to keep stable with the increase in the concentration of SBTS.This result can be explained by the grafting density of SBTS.When the concentration of SBTS is less than 0.1 mol·L-1,the grafting density of zwitterion increases andJ/J0of the membrane increases continuously,with the increase of concentration of SBTS.Then,with higher SBTS concentration,the grafting density of SBTS is saturated,so the concentration of SBTS does not affect theJ/J0of ZCM.Interestingly,theJ/J0of ZCM is higher than 100%,which attributes to the hydrophilia of zwitterion,it can enhance the permeability of membrane [36].Similarly,the phenomenon can be proved by the change of WCA of ZCM too.The initial WCA decreases from 81.2°±5°to 18.6°±5°with the raise of SBTS concentration from 0 to 0.1 mmol·L-1,then keeps unchanged after that.The results showed that the PWF of CM-DMAPS-SBTS was higher than that of the ceramic membrane,and the best concentration of SBTS is 0.1 mol·L-1.

Fig.4.The influence of different concentrations of grafting reagent (a) DMAPS and (b) SBTS on the membrane permeability and water contact angle.

3.3.Membrane morphology and zwitterion distribution

Due to that membrane morphology affects the performance[37],the difference between ceramic membrane and CM-DMAPSSBTS is investigated by FE-SEM (Fig.5).Giving the fact that the Al2O3material is chemical inertia and the size of zwitterion molecules is much smaller than Al2O3particle,this modification process will not change the membrane morphology.From Fig.5,it can be discerned that the ceramic membrane remains its particle packing after modification.The similar results have also been reported in literature [38].Besides,EDX mapping (Fig.6) was employed to investigate the distribution of introduced elements.It was found that the Al,O,C,Si,N,S co-existed evenly,where Al and O were primary,which proved that zwitterions distribute on the ceramic membrane uniformly.Therefore,the grafted zwitterion did not change the morphology and existed on the ceramic membrane surface meanly.

3.4.Water wetting behavior and protein adsorption test

The hydrophilicity of membranes is a significant property,which can affect the permeability antifouling performance of membranes [39].The contact angle of ceramic membrane,CMDMAPS and CM-DMAPS-SBTS membranes were measured,as shown in Fig.7.The initial water contact angle of ceramic membrane,CM-DMAPS and CM-DMAPS-SBTS membranes were 28.2° ±5°,38.5°±5°and 18.6°±5°individually,for the-N(CH3)2improve the hydrophobicity,and the zwitterion is a hydrophilic group which can enhance the hydrophilicity of membranes [40].In addition,the time that the water drop needs to permeate the three membranes is in the sequence:CM-DMAPS ＞ ceramic membrane ＞ CM-DMAPS-SBTS.The result also proved the improved hydrophilicity of ceramic membrane.The increase of CM-DMAPS-SBTS hydrophilicity is the unique property of zwitterion,which can absorb water around them and form a hydration layer [41].

According to the research of Jianget al.[25] that the high hydrophilicity of membrane can enhance the antifouling property,therefore,the foulants adsorption property of CM-DMAPS-SBTS was conducted.The fluorescence protein adsorption on membranes was investigated(Fig.8),which presented the fluorescence microscopy images of original Al2O3powder and the zwitterion modified Al2O3powder after being exposed to BSA-FITC solution for 8 h.A large amount of fluorescence was observed for the original Al2O3powder,indicating a significant BSA-FITC adsorption on the Al2O3powder.Compared with the original Al2O3powder,the fluorescence intensities of the zwitterion modified Al2O3powder decreased obviously.Thus,the enhance of ceramic membrane hydrophilicity can decrease the static protein adsorption,and the dynamic antifouling property was investigated below [42].

3.5.Antifouling performance of zwitterionic membranes

Before investigating the antifouling performance of ZCM,the O/W emulsion filtration performance was tested.So,a soybean-inwater system filtration experiment was conducted,and the results were shown in Fig.9.The permeability of ZCM decreased from 93.8 to 56.8 L·m-2·h-1·bar-1,which is higher than the original membrane decreasing from 82.8 to 42.0 L·m-2·h-1·bar-1with performing time from 10 to 120 min.This result is contributed to three aspects:(1) more hydrophilic ZCM owns higher permeability;(2)the hydration layer of ZCM provides better antifouling property and (3) the steric hindrance formed by zwitterionic molecular brush.However,the rejection to oil of ceramic membrane is a little higher than ZCM especially in the initial stage,which is due to that the fouling of soybean on the ceramic membrane makes pore size narrow[43].In a sense,the little lower rejection of ZCM indicates a better antifouling property.

Fig.5.The difference between the morphology of (a) origin and (b) modified membrane.

Fig.6.The elemental distribution of CM-DMAPS-SBTS.

Fig.7.Dynamic wetting behavior of ceramic membrane,CM-DMAPS and CMDMAPS-SBTS.(a)The dynamic contact angle of the three membranes and(b-d)the initial water contact angle of the three membranes.

Fig.8.The fluorescence microscopy images of original and the zwitterion modified Al2O3 powder after exposed to BSA-FITC solution for 8 h.

Fig.9.The filtration performance of modified and original membranes for treating soybean oil emulsion.(1 bar=0.1 MPa)

After testing the filtration performance,the antifouling property and mechanism were investigated.The filtration experiment is designed into three stages.In the first step,the pure water permeability of ceramic membrane and ZCM was tested for 30 min.At the second step,O/W emulsion was filtrated by the two membranes for 1 hour.Finally,after 10 min back-washing,the PWP of the cleaned two membranes were tested for another 30 min.The typical time-dependent permeation of membranes is shown in Fig.10(a).It can be seen that during the pure water and O/W emulsion process,theJ/J0values for modified and original membranes were near.However,after the backwash process,theJ/J0values of modified and original membranes increased by about 60%in the soybean-in-water system.Next,Rt,Rr,RirandFRRof modified and original membranes were calculated in these two systems.As shown in Fig.10(b),the total fouling ratio of the original membrane was a little higher than that of the modified membrane.However,the irreversible fouling ratio of origin membrane is nearly 3 times as much as that of the virgin membrane in the soybean-in-water system.And the reversible fouling takes a significant role in total fouling,which indicates the better antifouling property of the modified membrane.

Fig.10.The antifouling properties of membranes.(a) The antifouling properties of original and modified membranes:time-dependent permeation flux of different membranes at 0.1 MPa during three steps:water flux for 30 min,soybean oil solution flux for 60 min,and water flux for 30 min after backwash;(b)flux recovery and fouling resistance ratios of various membranes in soybean oil-in-water system.

Fig.11.The SEM pictures of modified and original membranes filtrated by soybean oil solution (a),(c) before and (b),(d) after backwashing.

To further prove the explanation,the surface morphology of them were investigated(Fig.11).According to the figures,the fouling layer can be observed and that of the ceramic membrane is more than ZCM before backwashing.Then,almost all foulants on the ZCM was removed,and the morphology of the surface particles was fully revealed after backwashing.However,there are still foulants on the ceramic membrane surface after backwashing.In addition,the oil concentration of backwash water was detected,and the results were shown in Fig.12.The oil concentration of the backwash water of the modified membrane was much higher than that of the original membrane,which indicated that more oil was removed in the modified membranes.And the picture of backwashed membranes (Fig.12(b)) illustrated that the foulants of the modified membrane are easier to be removed vividly.

Fig.12.The backwash results of modified and original membranes.(a) The oil concentration of backwash water in soybean oil-in-water system and (b) the picture of modified and original ceramic membranes after backwashing.

4.Conclusions

In this work,a novel two-step grafting technology was proposed to graft zwitterion on the ceramic membrane for enhancing the antifouling property.The following conclusions can be drawn:

(1) The feasibility of the fabrication of zwitterionic ceramic membrane was proved by characterizing the composition of ceramic powder and ZCP with FTIR and XPS.The existence of hydrated layer on zwitterion was confirmed by the TGA test.

(2) To fabricate the zwitterionic ceramic membrane with optimal performance,the effect that concentration of DMAPS and SBTS has on the water permeate flux of membranes was investigated.

(3) The zwitterionic ceramic membrane showed higher performance than ceramic membrane during the O/W emulsion filtration test,which was explained by the static protein adsorption and dynamic water contact angle test.

(4) The antifouling property of zwitterionic ceramic membrane was contributed by the decrease of irreversible fouling,which was concluded by a typical time-dependent permeation of zwitterionic ceramic membrane and ceramic membrane.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (21921006,21706115),the National Key Research and Development Program of China(2017YFC0403702),the Project for Marine Science and Technology Innovation of Jiangsu Province(HY2018-10),and Jiangsu Students’Innovation and Entrepreneurship Training Program(201810291044Z).