Characterization of oil component and solid particle of oily sludge treated by surfactant-assisted ultrasonication

Zuhong Lin,Fushuai Xu,Lili Wang,Liyang Hu,Lingfu Zhu,Jie Tan,Zhifeng Li,Tingting Zhang,*

1 Department of Environmental Science and Engineering,Research Centre for Resource and Environment,Beijing University of Chemical Technology,Beijing 100029,China

2 Sinopec Petroleum Exploration Research Institute,Beijing 100083,China

Keywords: Oily sludge Surface Morphology Ultrasonic treatment Surfactants

ABSTRACT The ultrasonic technique has been demonstrated to be a promising method for the disposal of hazardous oily sludge.However,the separation of oil from the surfaces of the solid particles is still difficult due to the strong interaction between the oil and solid particle.In this study,three types of surfactants were used to assist the ultrasonic treatment of oily sludge.The oil component,surface composition,and structure of the solid particle were determined.The results showed that different surfactants had different oil removal abilities.In the three surfactant-assisted sonication systems,the oil removal rate increased during the starting reaction period and then decreased with longer sonication time.The results of four components analysis suggested that surfactant easy to be ionized in water posed a better removal effect on resins,while the amphiphilic surfactant preferred saturates,aromatics and asphaltenes.The morphology analysis indicated that particle size was shattered into smaller ones by the ultrasonic process,and the wettability of the solid surface also changed during this treatment.The characterization of the oil component and solid particle during surfactant-assisted ultrasonication treatment will help to better understand the separation of oil from oily sludge and improve the oil recovery efficiency from oily sludge.

1.Introduction

Oily sludge is a special hazardous waste originated from oil production,transportation storage,and refining,and it usually in the form of water-in-oil (W/O) [1].There are plenty of hazardous substances in this stable system,such as various hydrocarbons[2,3],some pathogenic bacteria and heavy metals[4,5].The mass fraction of total petroleum hydrocarbons in oily sludge is commonly in the range of 15%and 50%[6].There is an urgent need to recover oil and treat hazardous waste in China.Therefore,an efficient method for recycling oil from oily sludge has become imperative.

A series of methods have been used to treat oily sludge,such as pyrolysis,incineration,solidification/stabilization,solvent extraction,ultrasonic treatment and thermochemical cleaning[6,7].Among them,ultrasonic treatment is considered an effective method[8–12],which was caused by the cavitation effect of ultrasonic waves and would produce a large number of cavitation bubbles.Cavitation bubbles can cause strong turbulence in the surrounding fluid;this process allows the fluid to enter the solid particle interface where it is difficult for conventional agitation to intervene;thus,the processing efficiency is increased[13].At the same time,surfactants have also shown a good auxiliary effect during ultrasonic treatment.The surfactant solution penetrates the boundary between the solid and the oil,thereby reducing the interfacial tension.The surfactant concentration is higher than the critical micelle concentration,and a large number of micelles are produced,through which the oil components are dissolved[14–17].However,the ultrasound process will change in the properties of the solid particle,which may lead to the recombination of the removed oil with the solids[18].

Moreover,the oil in oily sludge consists of four components:saturates,aromatics,resins and asphaltenes (SARA) [19].The content of the four components determines the type of crude oil in the oily sludge and the difficulty of the treatment of the sludge.Saturates and aromatics are light components in the oil that are easy to treat.Resins and asphaltenes are difficult to remove due to their high molecular weight and strong polarity [20].The addition of surfactants and ultrasound can lead to an overall reduction in the four components[9].

Previous research found that low-frequency ultrasound is more conducive to the treatment of the oily sludge,and the addition of a complex surfactant solution is conducive to further oil-solid separation of the oily sludge[8–11].Gao et al.[21]used an ultrasonic method and complex surfactant solutions to treat oily sludge on a pilot scale,which significantly removed the four components of the oil.He et al.[22]found that ultrasound waves could affect the particle size and even increase the proportion of light hydrocarbon components in the oil.Huang et al.[2]studied the effect of the particle surface characteristics of oily sludge on oil recovery during the centrifugation and they found that the particles in the oily sludge showed high hydrophobicity and resulted in a bad recovery rate.Different types of surfactants were used in this experiment,and the property change in the solid particles in the oily sludge during ultrasonic treatment and the removal of the oil components in an optimal time were investigated.However,most of the current studies focused on the optimization of ultrasonic parameters and surfactants.Few studies have explored the mechanism of the ultrasonic separation and surfactant action on the solid particle in the oily sludge.Both the size and wettability of the solid particle and the type of surfactant have a great influence on the oil removal performance during ultrasonic treatment[3,20].

This paper focuses on the variation of solid and residual oil composition before and after ultrasonic treatment.The solid particle surface and oil component were characterized by various instruments,which aimed to better understand the separation mechanism of oil from oily sludge and improve the oil recover efficiency from oily sludge.

2.Materials and Methods

2.1.Materials

Oily sludge sample was collected from Northern China in September 2018.The sample appeared ash black and loose-dry structures with pungent smells.It consisted of 19.2%water,72.5%solids,and 8.3%oil(by mass).The major components of oil in oily sludge include saturates,aromatics,resins and asphaltenes (SARA),and in the original oily sludge,the component mass contents were 50.8%,29.2%,16.7%,and 3.3%,respectively.

The anionic surfactant fatty alcohol polyoxyethylene sodium sulfate(AES)and nonionic surfactant polyoxyethylene sorbitan fatty acid ester(Tween 80)were purchased from Jiangsu Haian Petrochemical Plant.Carbon tetrachloride,hexane,isoamyl alcohol,hydrogen peroxide,concentrated hydrochloric acid,potassium bromide and the cosurfactant sodium carbonate (Na2CO3) were purchased from Beijing Chemical Company.The specific chemical properties are shown in Table 1.The other chemicals were analytically pure and used without further purification.

Table 1Surfactant information

2.2.Experimental design

The sonication experiment was carried out in an ultrasonic device(CR-1021i,Chun Rain,China).The size of this device was 60 cm×30 cm×35 cm,as illustrated in Fig.1.The frequency of the device was 28 kHz,and the power was 440 W.Previous studies have shown that 28 kHz is an optimal frequency for the ultrasonic treatment of oily sludge[9].The ultrasonic direction used in this experiment was from the bottom to the top.In this way,solid particles can be fully suspended in the solution,which is conducive to the deoiling of oily sludge[8].

Fig.1.Schematic of the ultrasonic device.

To investigate the influence of different types of surfactants on the oil component removal performance,the batch ultrasonic experiments were carried out.Solid–liquid mass ratio is an important operating parameter in oily sludge washing process.The increase of sludge content could result in increased viscosity of the slurry which then could impede the formation and collapse of cavitation bubbles [9,10,13].With the decrease of solid–liquid mass ratio,the decrease in viscosity by dilution improved its mobility and ultrasonic intensity and the oil removal rate increased[10].Consequently,by considering oil recovery and the quality of the recovered oil,the effective sludge/water ratio was 1:2 when using the combined treatment process.Anionic surfactant AES,nonionic surfactant Tween 80,and the cosurfactant Na2CO3were selected to treat the oily sludge.The dosage of the surfactants was 6‰.AES and Tween 80 had similar hydrophilic–lipophilic balance(HLB) value (14–15).They are easier to form micelles and disperse the oil into the water to form a stable state resulting in the detachment of the oil from the surface of the solid particles[21–23].In each experiment,20 g oily sludge was mixed with a 40 ml surfactant solution in six beakers.Then these beakers were placed in the ultrasonic device with an electric timing agitator (RW 20,IKA,Germany).These reactions were carried out under the conditions of ultrasonic frequency of 28 kHz,power of 440 W,and stirring speed of 100 rpm.Samples were collected at interval sonication time of 15,30,45,and 60 min.The obtained samples were then freeze-dried using a vacuum freeze drier(LGJ-10,Songyuan,China)for further analysis and characterization.All experiments were carried out at 25 °C.No obvious emulsion was observed in the investigated system.

To study the influence of the surfactants on the morphology of the oily sludge particles,the two optimized liquid anionic and nonionic surfactants were directly mixed with the oily sludge.The content of the surfactants was 12‰.Then,the mixture was kept in darkness for 24 h and freeze-dried for further characterization.

2.3.Oil content and component analysis

The contents of the water,oil and solid in the oily sludge were analyzed via gravimetric methods [24].A certain amount of oily sludge samples was taken and recorded as m0.Then,the samples were completely freeze-dried,and these samples were recorded as m1.The same mass of the oily sludge was ultrasonically extracted by carbon tetrachloride at a ratio of 1:5 for 20 min.The solid–liquid separation was carried out by a centrifuge (CenLee16K,Cence,China)for 5 min at a speed of 5000 r·min–1.The extraction process was conducted several times until the extract liquor was colorless.Then,the solid after extraction was dried completely and recorded as m2.The following formula was used to calculate the content of oil,water and solid in the oily sludge.

where ηwater,ηoiland ηsolidsare the water,oil and solid contents in the oily sludge(wt%),respectively.m0is the original sample(g),m1is the original sample after drying(g)and m2is the dry extracted sample(g).

The contents of saturates,aromatics,resins and asphaltenes(SARA)of the oil in the oily sludge were analyzed following the Chinese standard method SY/T 5119-2008 using thin-layer chromatography equipped with a flame ionization detector (MK6s,Iatroscan,Japan)[25].Briefly,1 g oily sludge was extracted with 50 ml trichloromethane for 30 min using Dean-Stark extraction.Then the residues were collected for the analysis of asphaltenes,and the extract was filtered for the analysis of saturates,aromatics and resins.The extract passed through the activated silica gel column and eluted by petroleum ether,benzene and benzene-ethanol mixture(v/v=1:1),respectively.After sample preparation in the rod thin-layer,a mixture of dichloromethane and ethanol(v/v=1:4)was applied to separate the three components of saturates,aromatics and resins.Finally,the silica gel chromatographic rod was evaporated at room temperature for 2 min,and then transferred to the rod thin layer flame ionization detector for the detection of the three components.

2.4.Characterization of solid particles

Thermal field scanning electron microscopy(SEM,Gemini SEM 300,Carl Zeiss,Germany)was used to observe the morphology of the solid particles obtained from the oily sludge.The particle size distribution was analyzed using a laser granulometry(Mastersizer 2000,Malvern,United States of America).The crystal phase composition of the sludge particles was characterized by X-ray diffraction (XRD,D8 Advance,Bruker-AXS,Germany).The functional groups of the oily sludge particles before and after treatment were analyzed by Fourier transform infrared spectrometry(FTIR,Nicolet 6700,Thermo Fisher,USA).The static contact angle of the solid particles at various stages was observed using an optical contact angle measuring instrument(OCA50AF,Dataphysics,Germany).

3.Results and Discussion

3.1.Oil removal efficiency of different surfactants

Fig.2 shows the deoiling performance of different surfactants during a certain period of sonication time.The deoiling effect of the three surfactants increased in the first 40 min and then decreased.The anionic surfactant AES had a relatively better effect;the residual oil content in the solid was 2.2%at 45 min.The nonionic surfactant Tween 80 reduced the oil content to 4.1%at 30 min.For the oily sludge treated with the cosurfactant Na2CO3,the residual oil content was 4.7%at 30 min.

Generally,anionic and nonionic surfactants were used to remove oil from oily sludge[26].Soil solids have a high sorption for cationic surfactants,which is not conducive to the deoiling of the oily sludge[17].Anionic surfactants will dissociate into amphiphilic anions and cations in aqueous solution,and the critical micelle concentration of ionic surfactants is generally higher than that of nonionic surfactants.Good deoiling performance can be achieved when the surfactant concentration is higher than the critical micelle concentration[26,28].During the first 20 min,the cosurfactant Na2CO3plays a major role in dispersing the oil in the water.Cosurfactants are usually alkaline agents that can disperse the oil into the water and enhance the surfactant ability[14,27].When the process proceeded to 30 min,the nonionic surfactant showed better performance than the anionic surfactant AES because nonionic surfactants were not readily ionizing in the aqueous solution since their hydrophilic groups are non-dissociable.Moreover,it is relatively easier for nonionic surfactants to form micelles because their aggregation is mainly due to the hydrophobic attraction between nonpolar chains.Alternatively,hydrophilic chains are easily separated in the aqueous phase,thus causing the nonionic surfactants to generally have a lower critical micelle concentration [15],thereby allowing them to more easily form micelles.Also,the nonionic surfactant can form a stable emulsion in water,and the stability of this emulsion is hardly destroyed by other substances,such as salt,thereby causing a higher deoiling efficiency of the nonionic surfactant within a certain range.At 40 min,the anionic surfactant AES worked better than the other surfactants.Because some ethoxy groups are present in AES chains,the ethoxy group is a nonionic hydrophilic group.As a result,AES has both anionic and nonionic surface activities and showed a better effect.

Fig.2.The effect of surfactant type on oil removal(solid–liquid mass ratio:1:2,surfactant concentration: 6‰,ultrasonic frequency: 28 kHz,ultrasonic power: 440 W and temperature:25°C).

The ultrasonic method can mix liquid and oily sludge particulate solids.During the initial ultrasonic process,the oily sludge and surfactant solution come into contact.The reason for the high deoiling rate at this time was the uniform mixing of each phase and the transformation of the solid surface property to hydrophilic[22].After 30 min,the oily sludge particles became small,which was beneficial for deoiling.However,as the particle size decreased,the microcracks and the width of the pores in the solid blocks decreased.This effect might result in the enhanced interaction between the particles,which prevented the solution and ultrasonic vibrations from penetrating the particle pores [13].Additionally,a diminutive particle size and an excessive time will lead to the recombination of the oil and particles[11].

3.2.The oil component variation in the oily sludge particles

Fig.3 shows the variation of SARA four components of the oily sludge treated by ultrasound and surfactants.Fig.3a shows the SARA component variation conditions.Via the ultrasonic-surfactant treatment,all the SARA components decreased.The residual SARA components treated by the anionic surfactant AES were 9.7%,9.8%,6.1%,and 1.0%,respectively.After treatment with the nonionic surfactant Tween 80,the SARA components decreased to 16.8%,15.5%,14.5%,and 1.8%,respectively.The cosurfactant Na2CO3reduced the SARA components to 18.0%,18.1%,10.2%,and 3.0%,respectively.

The removal rates of SARA are shown in Fig.3b.The anionic surfactant AES removed 81.0%,66.5%,63.6%,and 69.6%of SARA components,respectively.The effect of nonionic surfactant Tween 80 treatment was slightly lower than AES,and the removal rate of SARA components reached 67.0%,66.5%,13.2%and 45.1%.For the cosurfactant Na2CO3,the removal rates of the SARA components were 64.6%,38.0%,38.9%and 8.3%,respectively.

Fig.3.(a)The content of SARA component in the oily sludge before and after treated by different surfactants,(b)the removal rates of SARA component in the oily sludge treated by different surfactants.

The anionic surfactant AES showed no selectivity for the removal of SARA components;on the contrast,the nonionic surfactant Tween 80 and the cosurfactant Na2CO3showed selectivity for the removal of the heavier components.For the nonionic surfactant Tween 80,the component removal efficiency for the saturates and aromatics was 5.1 and 3.6 times that of the resins,while for cosurfactant Na2CO3,the removal efficiency for the saturates and aromatics was nearly 7.8 and 4.6 times that of asphaltenes,respectively.From the structure and character of the surfactant,the anionic surfactant AES and cosurfactant Na2CO3can ionize in the aqueous solution;these surfactants showed a heavier component(resins)removal effect than those surfactants that cannot ionize in the solution.However,for the heaviest component(asphaltenes),the anionic surfactant AES and nonionic surfactant Tween 80,which had amphipathic properties,performed better than the surfactants without amphipathic properties.Therefore,in future applications,surfactants can be selected according to the SARA result of crude oil in the oily sludge.If the resins are high,then the ionization surfactants can be used.If the asphaltenes are high,then surfactants with amphipathic properties can be chosen.This method also provides a guide for the compounding principle of complex surfactants that are commonly used today.

Generally,resins and asphaltenes are wrapped together,and resins cover the asphaltenes [28],mainly at the bottom of the oily layer.There were more metal ions inside,and their viscosity was higher than that of other components[29].The saturates and aromatics were mainly concentrated in the middle and at the surface of the oily layer(Fig.4a).The anionic surfactant AES and the cosurfactant Na2CO3in water can produce an ionization effect and make the negatively charged ions in the system come into being(Fig.4b and d).Solid particles are negatively charged as well,which form a mutual repulsion effect in the whole system,and the resin can be separated[11,21].Since Tween 80 will not ionize in the water,the removal effect of resins was low(Fig.4c).For the saturates,aromatics and asphaltenes,an amphipathic surfactant could significantly reduce the interfacial tension,and the less polar components,such as saturates and aromatics,can be significantly removed.For the asphaltenes,a natural surfactant was considered in some studies; although it had no obvious hydrophilic and hydrophilic ends,it still had some surface activity[13,30].Many studies have shown that different surfactant combinations further reduce the surface tension [14].Therefore,an endogenous natural surfactant asphaltene and an exogenous surfactant will further reduce the surface tension and produce a better removal effect on the asphaltenes(Fig.4b and c).However,Na2CO3did not exhibit amphipathic properties;therefore,it's difficult to remove asphaltene.

Fig.4.The schematics of(a)the interaction between oil components and solid particles,and the removal effect of(b)AES,(c)Tween 80 and(d)Na2CO3 of different oil components.

Fig.5.SEM images of(a and d)the original oily sludge,(b and e)oily sludge mixed with AES and(c and f)oily sludge mixed with Tween 80.

3.3.The morphology of the oily sludge particles

Fig.5 showed the SEM images of the oily sludge before and after mixing with surfactants.The original oily sludge samples were covered with oil and looked smooth(Fig.5a and d).When mixed with AES,the sludge surface was full of bubbles(Fig.5b and e).The oil film formed on the solid particles tended to move off of the surface [31].Similarly,Fig.5c and f present images of Tween 80 mixed with the oily sludge.The mixture showed a state of expansion,and the solid particles were covered by the surfactant-oil mixed film.

Both surfaces of the surfactant-sludge mixture were rougher than those of the original oily sludge,and some of the oil was easily separated from the particles.The negatively charged groups and solid particles in the anionic surfactants can form electrostatic repulsion,which made the oil separate[26].Moreover,anionic surfactants tended to produce large amounts of stable foam,which was also a desirable attribute in cleaning operations.AES has good foaming properties[32],and the formation and cracking of bubbles introduce air and leave some small holes in the oily sludge.As a result,the oil components could desorb from the oily sludge particles when treated by AES.Tween 80 has a strong oil solubility effect.In general,solid particles with SiO2as the main component in the oily sludge are connected with crude oil by hydrogen bonds,which leads to a strong interaction between the crude oil and solid [33].Nonionic surfactants dissolve in the aqueous phase by forming intermolecular hydrogen bonds between the hydrophilic groups and water molecules and they have a higher solubilization ability in the solution.By reducing the surface tension,the hydrogen bonds between the crude oil and solid can open further,resulting in the dissolution of the oil into the water phase[26].Both of the effects allow the oil to easily separate from the solid.

Fig.6.SEM images of(a and c)the original oily sludge and(b and d)oily sludge after ultrasonic treatment.

Fig.7.Particle size-frequency distribution of solid particles before and after treatment.

Fig.6 shows SEM images of oily sludge before and after ultrasonicsurfactant treatment.Fig.6a and c show that the surface of the original solid was smooth and large.After ultrasonic treatment,their surfaces became rough,and the particle size became small.This behavior was caused by the continuous ultrasonic action on the solid particles for a certain period,resulting in structural damage and dispersion of the solid particles into smaller ones.When the ultrasonic wave propagated in the oily sludge medium,a certain positive pressure and negative pressure were applied in the liquid due to the transfer of energy,thereby causing a series of cavitation bubbles to grow,expand and collapse in successive cycles.This process produces continuous shock waves resulting in high-velocity microturbulence and intense local mixing[34].This high-velocity microturbulence acted on solid surfaces and increased the surface area through surface pitting[21].This phenomenon is shown in Fig.6d.Moreover,the turbulence caused by the ultrasonic treatment can also cause the particles to collide.Even for some difficult-to-break solids,ultrasound creates a shock wave that causes the liquid to enter the solid interior due to capillary action[18].The surface of Fig.6d has no obvious oil film.Similar changes in the rock and its surface organic matter were observed by Ghamartale[35].The results proved that ultrasonic-surfactant treatment was good for the oily sludge.

Fig.8.The FTIR spectra of oily sludge(a)with cleaned solid and treated by(b)AES,(c)Tween 80,(d)Na2CO3.

Fig.9.The XRD diffractograms of solid particles.

Fig.7 shows the particle size-frequency distribution of the oily sludge particles before and after the treatment.The frequency distribution was in the range of 100 to 500 μm(average:200 μm)before treatment.After treatment,the particle size distribution was in the range of 15 to 80 μm (average: 67 μm).Certain fine particles were peeled off from the original soil particles,and the collision between particles resulted in a reduction of the overall particle size.This finding was in agreement with the SEM image shown in Fig.6d.There is a mutual interference between different factors such as particle-size distribution,solids concentration,oil and additives in crude oil exploitation.The particle-size distribution directly influences the packing ability of particles in suspensions[36].There was an increase in shear stress and apparent viscosity values as the size of the silica particles increased in water suspensions.The solid particle size should be paid more attention to when ultrasonic technology is applied to treat the oily sludge.

3.4.The structure and surface analysis of oily sludge particles

Fig.8 shows the FTIR spectrum of the functional groups on the solid before and after treatment.The typical peaks of the alkyl chain were assigned at 2930 cm?1,2960 cm?1and 3030 cm?1,which included-CH2-and-CH3-[12,14,37].Additionally,earlier research studies suggested that the peaks were aliphatic and aromatic.All of them represent petroleum hydrocarbon compounds.Fig.8a shows that the solid after complete extraction with carbon tetrachloride exhibited almost no fluctuation in this range.The solid was cleaned by comparison with the original sample.Figs.8b,c and 9d showed that the petroleum hydrocarbon stretch intensity was reduced by different surfactant treatments.The stretch intensity indicates that the anionic surfactant AES demonstrated the best performance among the three surfactants.The nonionic surfactant Tween 80 showed better performance than the cosurfactant Na2CO3.

The C=C stretching vibrations near 1400 cm?1and 1600 cm?1represented the benzene skeleton[32].This result showed that there were still some heavy components of the oil adsorbing on the treated solid surface.This finding also confirmed that the ultrasonic-surfactant process mainly removed the light components.This result was the same as that shown in Fig.3.The peaks near 1000 cm?1and 500 cm?1represent Si-O-Si or Si-O stretching[12,37],which might confirm that the solid consisted of clay or quartz[38].This result was further confirmed by the XRD results of the solid(Fig.9).The main solid component of the oily sludge was quartz,but a small amount of albite and muscovite were found.

The contact angle can be used to judge the wettability of the solid surface[22].The results of different surfactant treatments are shown in Fig.10.The contact angle of the original sample surface was 125.2°(Fig.10a).Because the raw oily sludge particles were covered with crude oil,the water did not disperse significantly on its surface,resulting in obviously hydrophobic surface characteristics[39].Fig.10b,c,and d show the contact angles of the solid treated by the anionic surfactant AES,the nonionic surfactant Tween 80 and the cosurfactant Na2CO3during the ultrasonic process,respectively.The contact angle of the solid surface treated by AES and Tween 80 showed a trend of decreasing first and then increasing along with the ultrasonic process.The minimum values for the contact angle are 23.0° and 19.9° at 40 and 30 min,respectively.The contact angle of the solid treated with Na2CO3was at a minimum after 20 min of treatment and then gradually increased with time.This increase was related to the reduction in the particle size and the recombination of the oil and particles[22].Although the decrease in particle size was beneficial for the separation of the oil,the excessive treatment time increased the probability of recombination of the oil and solid,which led to an increase in the contact angle[40].

Fig.10.The static contact angle of(a)the original sample and treated by(b)AES,(c)Tween 80,(d)Na2CO3.

4.Conclusions

Different types of surfactants had different treatment efficiencies of the oily sludge components and required different treatment times.The anionic surfactant AES had an excellent foaming property,which was good for oil separation from solid surfaces,while the nonionic surfactant Tween 80 can solubilize the oil at the solid surface,and this property can help remove the oil from the solid as well.The optimal treatment times for AES,Tween 80 and Na2CO3were 40 min,30 min,and 20 min,respectively.The cosurfactant Na2CO3and anionic surfactant AES can ionize anions in water,which was conducive to the removal of resins to 38.9%and 64.6%,respectively.While the nonionic surfactant Tween 80 cannot ionize in water,the removal efficiency of resins was low,only 13.2%.For the removal of saturates,aromatics and asphaltenes,the amphiphilic anionic surfactants AES and nonionic surfactants Tween 80 perform better than the cosurfactants.The ultrasonic process can change the morphological structure of oily sludge solids; therefore,some surface properties can change as well.Through a series of additional effects brought by ultrasonic cavitation,solid particles were dispersed into smaller particle sizes,which was beneficial for the deoiling of the oily sludge within a certain time range,but can increase the recombination probability of the oil and solid over time.

Declaration of Competing Interest

No potential conflict of interest was reported by the authors.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China(Nos.41977142 and 41807133),the special fund of State Key Joint Laboratory of Environment Simulation and Pollution Control(No.18K05ESPCT).